1 - Aerodynamics Flashcards
Forces/Aerofoil
What are the forces acting on an aircraft in flight?
Drag, thrust, lift, and weight.
When thrust and drag are in equilibrium, an aircraft will maintain
a steady speed.
For an aircraft to accelerate, thrust must exceed the
value of drag. vVhen lift and weight are in equilibrium, an aircraft will
maintain a steady, level attitude.
For an aircraft to climb, lift must exceed the weight of the aircraft.
In a banked turn, weight is a constant, but lift is lost due to the effective reduction in wing span. Therefore, to maintain altitude in a
banked turn, the lift value needs to be restored by increasing speed and/or the angle of attack.
Forces/Aerofoil
What produces the maximum glide range?
A maximum lift-drag ratio.
Obtained by the aircraft being flown at its
optimal angle of attack and corresponding minimum drag speed (VIMD) , produces an aircraft’s maximum glide range.
Forces/Aerofoil
What is the effect of weight on the glide range?
The glide range does not vary with weight.
Because the glide range is proportional to the lift-drag ratio. Which does not vary with weight.
Provided that the aircraft
is flown at its optimal angle of attack and speed for that weight.
Therefore, if a heavy aircraft were flown at the correct angle of attack
and speed, it would glide the same distance as a lighter aircraft.
However, the heavier aircraft would have a higher airspeed than the lighter aircraft, and therefore, although it would glide the same distance,it would take less time to do so.
Forces/Aerofoil
What is rate of climb/descent?
Rate of climb/descent is the vertical component of the velocity of an aircraft.
Also determines the time it will take to either climb or descend from a given height.
It is normally expressed in terms of feet per minute.
Forces/Aerofoil
What is the effect of weight on rate of descent?
The heavier the aircraft, the greater its rate of descent.
This is so because a heavy aircraft will fly at a higher airspeed for a given angle of attack, and so its rate of descent will be increased.
(See Qs: How does weight
affect an aircraft’s flight profile descent point? page 18; Why does an
aircraft descend quicker when it is lighter? page 283.)
Forces/Aerofoil
What is an aerofoil?
An aerofoil is a body that gives a large lift force compared with its drag when set at a small angle of attack to a moving air stream.
e.g., aircraft wings, tail planes, rudders, and propellers.
Forces/Aerofoil
What is an aerofoil chord line?
The chord line is a straight line from the leading edge to the trailing edge of an aerofoil.
Forces/Aerofoil
What is the mean chord line?
The mean chord line is the wing area divided by the wing span.
(sometimes referred to as the standard mean chord).
Forces/Aerofoil
What is the mean chamber line?
The mean chamber line is a line from the leading edge to the trailing edge equidistante on the upper and lower surfaces of an aerofoil.
Forces/Aerofoil
What is the angle of incidence?
The angle of incidence is the angle between the aerofoil’s chord line and the aircraft’s longitudinal datum.
It is a fixed angle for a wing but may be variable for a tailplane.
(It is sometimes called rigging incidence.)
Forces/Aerofoil
What is angle of attack?
Angle of attack is the angle between the chord line of an aerofoil and the relative airflow.
Forces/Aerofoil
What is washout on a wing?
Washout is a decrease in the angle of incidence from the wing root to the tip.
This compensates for the early stall due to the higher levels of loading experienced at the wing tips.
Forces/Aerofoil
What is dihedral?
Dihedral is the upward inclination of a wing from the root to the tip.
Forces/Aerofoil
What is anhedral?
Anhedral is the downward inclination of a wing from the root to the tip.
Lift
What is lift?
Lift is the phenomenon generated by an aerofoil due to pressure differences above and below the aerofoil.
Note: An aerofoil is cambered on its topside and flat on its bottom side.
Therefore, the airflow over the top of the aerofoil has to travel further and thus faster than the airflow below the aerofoil.
This causes the pressure below the aerofoil to be greater than above, creating a pressure difference, which results in an upward lift force.
Lift
What if the formula for lift?
1/2R + V2 + S + CL
1/2R = half the value of the air density V2 = airflow velocity squared S = wing span area CL = coefficient of lift
The combined values of these properties determine the amount of lift
produced.
Lift
What is coefficient of lift (CL)?
Coefficient of lift (CL ) is the lifting ability of a particular wing.
It depends on both the shape of the wing section (fixed design feature) and the angle of attack.
Lift
Describe center of pressure.
The center of pressure is represented as a single point acting on the wing chord line at a right angle to the relative airflow, through which the wing’s lifting force is produced.
The position of the center of pressure is not a fixed point but depends on the distribution of pressure along the chord, which itself depends on the angle of attack.
Thus, for a greater angle of attack, the point of highest suction (highest air pressure value) moves toward the leading edge.
The distribution of pressure and center of pressure point thus will be further forward the higher the angle of attack and further aft the lower the angle of attack.
Lift
Describe the lift-weight pitching moments.
If the forces of lift and weight are not acting through the same point (line), then they will set up a moment causing either a nose-up or nose-down
pitch depending on whether the lift is acting in front of or behind the center of gravity point.
Note: A center of gravity forward of the center of pressure has a nose-down pitching moment.
A center of gravity aft of the center of pressure has
a nose-up pitching moment.
The center of pressure moves if the angle of attack changes, and the center of gravity moves as the weight changes (mainly due to fuel being
used).
Therefore, their positions will vary during a flight.
Lift
Describe aspect ratio.
Aspect ratio is the ratio of the wing’s span to its geometric chord, e.g., 4:1.
High aspect ratio = high lift (gliders)
Low aspect ratio = lower lift but capable of higher speeds
Lift
During what phase of flight is lift the greatest?
In general, the takeoff.
Note: Lift is caused by a pressure difference above and below the wing, and the size of the difference determines the amount of lift produced.
(See Q: What is lift? page 3.)
The difference in pressure experienced is affected by the functions of
lift, which are
1. Configuration (flap setting)
2. Speed of airflow over the wing
3. Angle of attack (which is optimized during the takeoff stage offlight)
plus
4. Air density
Lift
What is direct lift control?
The elevator/stabilizer provides the direct lift control.
The elevator and stabiliser are aerofoils that by their positions create an upward or downward balancing force that controls the direct lift force from the main aerofoils (wings), thus determining the attitude of the aircraft around the lateral axis.
Lift
What are high lift devices?
The following devices increase the lift force produced by the wings:
- Trailing edge flaps (Fowler flaps) increase lift at lower angles of deflection.
- Leading edge flaps (Krueger flaps) and slats increase lift by creating a longer wing chord line, chamber, and area.
- Slots (boundary layer control) prevent/delays the separation of the airflow boundary layer and therefore produce an increase in the coefficient of lift maximum.
Drag
What is drag?
Drag is the resistance to motion of an object (aircraft) through the air.
There are 2 types of Drag.
Profile + Induced drag = Total Drag
Drag
Define the two major types of drag and their speed relationship.
Profile & Induced drag = Total Drag
PROFILE DRAG is also known as zero-lift drag and is comprised of
- Form or pressure drag
- Skin-friction drag
- Interference drag
Profile drag increases directly with speed because the faster an aircraft moves through the air, the more air molecules (density) its surfaces encounter, and it is these molecules that resist the motion of
the aircraft through the air. This is known as profile drag and is
greatest at high speeds.
INDUCED DRAG is caused by creating lift with a high angle of attack that exposes more of the aircraft’s surface to the relative airflow and is associated with wing-tip vortices.
A function of lift is speed, and therefore, induced drag is indirectly related to speed, or rather the lack of
speed.
Thus induced drag is greatest at lower speeds due to the high angles of attack required to maintain the necessary lift.
Induced drag reduces as speed increases because the lower angles of attack associated with higher speeds create smaller wing-tip trailing vortices that have a lower value of energy loss.
Minimum drag speed (VIMD) is the speed at which Induced and Profile drag values are equal.
Drag
What is Minimum drag speed (VIMD)?
Minimum drag speed (VIMD) is the speed at which Induced and Profile drag values are equal.
It is also the speed that has the lowest total
drag penalty:
VIMD = minimum drag speed
Therefore, this speed also represents the best lift-drag ratio (best aerodynamic efficiency) that will provide the maximum endurance of the aircraft.
Drag
Describe the drag curve for a piston/propeller aircraft.
For a piston-engine propeller aircraft read straight-winged. It has a typical total drag curve comprised of a well-defined steep profile drag curve at high speeds.
This is so because the wing is not designed for
high speeds, and therefore, as speed increases, profile drag increases as a direct result.
It also has a well-defined induced drag curve at low
speeds.
This is so because the straight-winged aircraft has a higher CL value, and with induced drag being proportional to lift, the lower the speed, the greater is the angle of attack required to achieve the necessary
lift, and therefore, the greater is the associated induced drag component.
It also has a well-defined bottom VIMD (minimum drag speed) point and is capable of a lower stall speed than a jet.
Flight below VIMD in a piston-engine aircraft is very well defined by the steep increase in the drag curve in flight as well as on paper.
Speed is not stable below VIMD, and because of the steep increase of the curve
below VIMD, it is very noticeable when you are below VIMD.
That is, below VIMD, a decrease in speed leads to an increase in drag that
causes a further decrease in speed.
(See Q: Explain speed stability.
page 22; see also Fig. 1.5, Total Drag Curve, page 7.)
Drag
Describe the drag curve on a jet aircraft.
The drag curve on a jet aircraft is the same as for a a piston aircraft in that it is comprised of Induced drag, Profile drag, and a VIMD speed, but its speed-to-drag relationship is different.
This is so because the jet aircraft has swept wings, which are designed to achieve high cruise speeds.
But as a consequence has poorer lift capabilities, especially at low speeds.
Therefore, because profile drag is a function of speed and induced drag is proportional to lift, the drag values against speed are different on a jet/swept-winged aircraft.
The three main differences are:
- Flatter total drag curve because
a) . Profile drag is reduced especially against higher speeds.
b) . Induced drag is reduced (flatter drag curve) because the swept wing has very poor lift qualities, especially at low speeds.
These factors combined give rise to a smaller total drag range
against speed, which results in a flatter total drag curve.
- The second difference is a consequence of the first because of the relative flatness of the drag curve, especially around VIMD.
The jet aircraft does not produce any noticeable changes in flying qualities other than a vague lack of speed stability, unlike the piston-engine aircraft, in which there is a marked speed-drag difference.
(Speed is unstable below VIMD, where an increase in thrust has a greater drag penalty for speed gained, thus with a net result of losing speed for a given increase in thrust.)
- VIMD is a higher speed on a jet aircraft because the swept wing is more efficient against profile drag, and therefore, the minimum drag speed is typically a higher value.
Drag
Describe the pitching moment associated with the thrust-drag couple.
If the forces of thrust and drag are not acting through the same point (line), then they will set up a moment causing either a nose-up or nose-down pitch depending on whether the thrust is acting above or below the dragline.
Therefore, a change in thrust (increase or decrease) in straight and level flight can lead to a pitching tendency of the aircraft.
Likewise, an increase or decrease in drag also can lead to a pitching tendency of the aircraft.
For example, an increase in thrust on an aircraft with engines mounted under the wing, with a higher dragline, will cause a nose-up pitch as thrust is increased.
Drag
What are high-drag devices?
The following devices increase the drag penalty on an aircraft:
1. Trailing edge flaps (in high-dragllow-lift position)
- Spoilers
a. In fight , used as a speed brake
b. On the ground, used as lift dumpers - Landing gear
- Reverse thrust (ground use only)
- Braking parachute
Drag
What causes/are wing-tip vortices?
Wing-tip vortices are created by span-wise airflow over the upper and lower surfaces of a wing/aerofoil that meet at the wing tips as turbulence and therefore induces drag, especially on a swept wing.
Span-wise airflow is created because a wing producing lift has a lower static pressure on the upper surface than on the lower surface.
At the wing tip, however, there can be no pressure difference, and the pressure is equalised by air flowing around the wing tip from the higher pressure on the lower surface to the lower pressure on the upper
surface.
There is therefore a span-wise pressure gradient, i.e., pressure changing along the wing span.
Drag
What are the effects of span-wise airflow over a wing?
- Creates wing-tip vortices.
- Reduced aileron (wing control surface) efficiency.
- Reversed span-wise airflow increases disturbed airflow on the wing’s upper surface at the tip, contributing to a wing-tip stall.
Drag
What are the effects of wing-tip vortices?
- Creates aircraft drag (induced drag because the vortices induce a downward velocity in the airflow over the wing, causing a change in the direction of the lift force so that it has an induced-drag component; therefore, it creates a loss of energy).
- Vortices create turbulence, which may affect the safety of other
aircraft within approximately 1000 ft below or behind the aircraft. - Downwash affects the direction of the relative airflow over the tail plane, which affects the longitudinal stability of the aircraft.
Drag
How do you prevent span-wise airflow on a wing, especially a swept wing?
Fences and vortex generators.
These items direct the airflow over the wing’s upper surface perpendicular to the leading edge.
Drag
What is the purpose of vortex generators/fences?
To reduce span-wise airflow and thereby reduce its effects.
One of the effects of span-wise airflow over the wing is reduced effectiveness of the ailerons due to the diagonal airflow over the control
surfaces.
Vortex generators are located on the upper surface of a wing to create a slightly disturbed airflow perpendicular to the leading edge of the wing, which helps to maximize the effectiveness of the control
surfaces, especially the ailerons.
Fences also help to maximize the effectiveness ofthe control surfaces in a similar yet cruder manner.
However, they are used normally to reduce the reverse span-wise airflow on the upper wing surface from reaching the wing tips, thus reducing the airflow, which contributes to
a wing-tip stall.
Vortex generators are used in other areas of the aircraft where a disturbed airflow is required (disturbed air tends to be denser and of a slower velocity), such as inlets to some types of auxiliary power
units (APUs).
Drag
What are winglets, and how do they work?
Winglets are aerodynamically eflicient surfaces located at the wing tips. They are designed to reduce induced drag.
They dispense the span-wise airflow from the upper and lower surfaces at different points, thus preventing the intermixing of these airflows that otherwise would create induced-drag vortices.
Weight and Aircraft Momentum
What limits an aircraft’s structural weight?
The main force generated to balance the aircraft’s gross weight is the lift force, and if the lift cannot equal the aircraft’s weight, then the aircraft
cannot maintain level flight.
Therefore, the aircraft weight is directly restricted by the lift capabilities of the aircraft.
Note: The lift force generated is limited by the size (design) of the wing, the attainable airspeed (airspeed is limited by the power available
from the engine/propeller), and the air density.
Weight and Aircraft Momentum
What are the effects of excessive aircraft weight?
If the limiting weight of an aircraft is exceeded, the following effects
are experienced:
- Performance is reduced.
a. Takeoff and landing distance is increased.
b. Rate of climb and ceiling height are reduced.
c. Range and endurance will be reduced.
d. Maximum speed is reduced. - Stalling speed is increased.
- Maneuverability is reduced.
- Wear on tires and brakes is increased.
- Structural safety margins are reduced.
Weight and Aircraft Momentum
Describe center of gravity.
The center of gravity (C ofG, CG) is the point through which the total weight of a body will act.
Weight and Aircraft Momentum
Describe a component arm.
The definition of a component arm is the distance from the datum to the point at which the weight of a component acts (center of gravity point).
By convention, an arm aft of the datum, which gives a nose-up moment, is positive, and an arm forward of the datum, which gives a nose-down moment, is negative.
Therefore, for a constant weight, the longer the arm, the greater is the moment.
Weight and Aircraft Momentum
Describe center of gravity moment.
The moment is the turning effect/force of a weight around the datum.
It is the product of the weight multiplied by the arm:
Moment = weight X arm
Weight and Aircraft Momentum
How is the pitching moment of the lift-weight couple balanced?
When the pitching moment of the lift-weight couple is not balanced perfectly, extra forces are provided by the horizontal tailplane to center the aircraft’s pitching moment.
(see Q: Describe the lift-weight pitching moments? page 3)
Note: Lift forward of weight has a nose-up pitching moment, which is counterbalanced by the downward deflection of the horizontal tail plane, which creates a nose-down counter pitch.
Therefore, lift aft of weight requires the opposite balance.
The tailplane force has a turning moment in the pitching plane (nose up or nose down) about the lateral axis at the center of gravity point.
Its effectiveness depends on its size and the length of its moment arm from the center of gravity.
Weight and Aircraft Momentum
Describe the center of gravity range.
The center of gravity range relates to the furthest forward and aft center of gravity positions along the aircraft’s longitudinal axis, inside which the aircraft is permitted to fly.
This is so because the horizontal tailplane can generate a sufficient lift force to balance the aircraft’s
lift-weight moment couple so that it remains longitudinally stable and retains a manageable pitch control.
(Center of gravity range or envelope is listed in the aircraft’s flight manual, and accordance is mandatory.)
(See Q: What is longitudinal stability? page 33.)
Weight and Aircraft Momentum
What are the reasons/effects of keeping a center of gravity inside the forward position of the center of gravity is limit?
- Ensure that the aircraft is not too nose heavy so that the horizontal tailplane has a sufficient turning moment available to overcome its natural longitudinal stability.
- Ensure that the aircraft’s pitch control (rotation and flare) is not compromised, with high stick forces (tail plane turning moment), by restricting the aircraft’s tail plane arm forward center of gravity limit. [Remember, tailplane moment (stick force) = arm X
weight.
Note that this is particularly important at low speeds
(i.e., takeoff and landing), when the elevator control surface is less effective.
- Ensure a minimum horizontal tail plane deflection, which produces a minimal downward force on the tailplane that is required to balance the lift-weight pitching moment.
Therefore, the stabilizer and/or the elevator is kept streamlined to the relative airflow, which results in
a. Minimal drag. Therefore, performance is maintained.
b. Elevator range being maintained. Therefore, the aircraft’s pitch manoeuvrability is maintained.
Weight and Aircraft Momentum
What are the reasons/effects of keeping a center of gravity inside the Aft position of the center of gravity is limit?
- Ensure that the aircraft is not too tail heavy so that the horizontal tail plane has a sufficient turning moment available to make the aircraft longitudinally stable.
- Ensure that enough pitch control stick forces (tailplane turning moment) are adequately felt through the control column by guaranteeing the aircraft’s tail plane arm to an aft center of gravity limit.
[Remember, moment (stick force) = arm X weight.] - Ensure a minimum horizontal tail plane deflection, which produces a minimal upwards force on the tail plane and is required to balance the lift-weight pitching moment.
Therefore, the stabiliser and/or the elevator is kept streamlined to the relative airflow, which results in
a. Minimal drag. Therefore, performance is maintained.
b. Elevator range being maintained.
Therefore, the aircraft’s pitch manoeuvrability is maintained.
Weight and Aircraft Momentum
What are the effects of a center of gravity outside its forward limit?
Generally, the aircraft is heavy and less responsive to handle in flight.
And requires larger and heavier control forces for takeoff and landing.
Nose heavy, and the horizontal tailplane will have a long moment arm (tailpipe to center of gravity point) that results in the following:
- Longitudinal stability is increased because the aircraft is nose heavy. (See Q: What is longitudinal stability? page 33.)
- The aircraft’s pitch control (rotation and flare) is reduced or compromised because it experiences high stick forces due to the aircraft’s
long tail plane moment arm. [Remember, tail plane moment (stick force) = arm X weight.] - A large balancing download is necessary from the horizontal tailplane by deflecting the elevator or stabiliser.
This results in
a. An increased wing angle of attack resulting in higher induced drag, which reduces the aircraft’s overall performance and range.
b. Increased stalling speed due to the balancing
down force on the horizontal tail plane, which increases the aircraft’s effective weight. (See Q: How does a forward center of gravity affect the stall speed, and why? page 16.)
c. Also, if the elevator is required for balance trim, less elevator is available for pitch control, and therefore, the manoeuvrability of the aircraft to rotate at takeoff or to flare on landing is reduced.
d. In-flight minimum speeds are also restricted due to the lack of elevator available to obtain the necessary high angles of attack required at low speeds.
Weight and Aircraft Momentum
What are the effects of a center of gravity outside its Aft limit?
Generally, the aircraft is effectively lighter and more responsive to handle in flight and requires smaller and lighter control forces for
takeoff and landing.
If the center of gravity is outside its aft limit, the aircraft will be tail heavy, and the horizontal tail plane will have a short moment arm (tail plane to center of gravity point) that results in the following:
- The aircraft is longitudinally unstable because it is too tail heavy for the horizontal tail plane turning moment to balance. (See Q: What is longitudinal stability? page 33.)
- The aircraft’s pitch control (rotation and flare) is increased (more responsive) because it experiences light stick forces due to the aircraft’s
short tail plane arm.
[Remember, tailplane moment (stick force) = arm X weight.]
This lends itself to the possibility of over stressing
the aircraft by applying excessive g forces. - A large balancing upwards force is necessary from the horizontal tail plane by deflecting the elevator or stabiliser. This results in
a. A decreased wing angle of attack, resulting in lower induced drag, which increases the aircraft’s overall performance and range.
b. A lower stalling speed due to the balancing upload on the horizontal tailplane, which decreases the aircraft’s effective weight.
c. Also, if the elevator is required for balance trim, less elevator is available for pitch control, and therefore, the manoeuvrability of the aircraft to recover from a pitch-up stall attitude is reduced.
d. In-flight maximum speeds are also restricted due to the lack of elevator available to obtain the necessary low angles of attack required at high speeds.
Loading
If you were loading an aircraft to obtain maximum range, would you load it with a forward or aft center of gravity (forward or aft cargo hold)?
An aft center of gravity position hold loading, for aircraft (especially jet/swept wing) with a nose-up en route attitude will allow it to achieve its maximum possible range.
An aft center of gravity position, normally is accomplished by using the aft cargo hold, which gives the aircraft its nose-up en route attitude naturally; therefore, the stabiliser can remain streamlined to
the airflow and produce no relevant drag (e.g., aft center of gravity = 6° nose-up attitude
= 0° elevator/stabilizer deflection).
Thus the aircraft can be operated at its optimal thrust setting to obtain its maximum range without having to use excessive engine thrust to compensate for drag.
Note: It is beneficial even when the center of gravity is aft of its optimal position because the stabiliser would produce a greater lift force (to produce a downward pitching moment of the nose to gain its en route attitude).
This is beneficial to the aircraft’s overall performance
because it increases the aircraft’s overall lift capabilities,
whereas a forward center of gravity has a detrimental effect on the aircraft’s performance.
However, a few aircraft with a nose-down en route attitude would require a forward center of gravity position.
Loading
How does a forward center of gravity affect the stall speed, and why?
A center of gravity forward of the center of pressure will cause a higher stall speed.
This is so because a forward center of gravity would cause a natural nose-down attitude below the required en route cruise attitude for best performance.
Therefore, a downward force is induced by the stabiliser to obtain the aircraft’s required attitude.
However, this downward force is in effect a weight and thereby increases the aircraft’s overall effective weight.
Weight is a factor of the stall speed, and the heavier the
aircraft, the higher is the aircraft’s stall speed.
Conversely, the opposite is true: A center of gravity aft of the center of pressure will cause a lower stall speed.
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Why does a jet aircraft have a large center of gravity range?
A jet aircraft needs a large center of gravity range because its center of gravity position can change dramatically with a large change in its weight during a flight. (See Q: What causes center of gravity movement? page 17.)
Therefore, to accommodate a large center of gravity movement, the aircraft has to have a powerful horizontal tailplane to balance the large lift-weight pitching moments so that the aircraft remains
longitudinally stable and retains its pitch control ability.
(See Q: What are the four reasons for a variable incidence tailplane? page 43.)
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What causes center of gravity movement?
The center of gravity is the point through which weight acts. Therefore, movement of the center of gravity is due to a change in weight.
The distribution of the aircraft’s weight can change for three reasons and thus cause the center of gravity position to move.
- Fuel burn. - The most common reason for center of gravity movement on a swept-wing aircraft is its decrease in weight as fuel is used in flight.
It should be remembered that because of the sweep, the wing and the fuel tanks housed inside cover a distance along the aircraft’s longitudinal axis.
Therefore, as fuel/weight is reduced progressively
along this axis, the weight-distribution pattern changes across the aircraft’s length.
- Passenger movement. - People eating and going to toilets.
- High speeds - This is so because the greater the speed, the greater is the lift created. To maintain a straight and level attitude, the aircraft adopts a more nose-down profile, which is accomplished by creating lift at the tailplane. This lift on the tail plane effectively reduces the weight of the tail plane section of the aircraft.
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How does VMCG/A vary with center of gravity position?
An aft center of gravity position requires a higher VMCG/A.
The turning moment acts around the center of gravity, and if the center of gravity is in the aft position, the vertical tailplane (rudder) moment arm will be shorter, and therefore, the vertical tailplane turning
moment is less for a given airspeed. Thus the aircraft requires a higher minimum control speed (VMCG/A) with an aft center of gravity position.
Turning moment = rudder to center of gravity arm X speed (VMCG/A)
Conversely, the opposite is same for a forward center of gravity position.
A forward center of gravity will have a longer arm, and therefore, the
vertical tail plane turning moment is greater for a given speed, and thus the aircraft can have a lower “‘MCG/A.
Loading
Describe the effects of an aircraft’s momentum.
Momentum of a body is the product of the mass of the body and its velocity, which enables the body (aircraft) to remain on its previous direction and speed for a quantity of time after an opposing force has been applied.
The momentum of a jet aircraft is significantly greater than that of a piston-engine aircraft in all its handling manoeuvres, climbs,descents, and turns because of its greater weight and velocity.
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How does weight affect an aircraft’s flight profile descent point?
The heavier the aircraft, the earlier is its required descent point. The heavier the aircraft, the greater is its momentum.
Remembering:
momentum = mass X velocity.
Therefore, for a constant indicated air speed (lAS) or Mach number, the heavier aircraft will have to maintain a shallower rate of descent to check its momentum.
(See Qs: What is the effect of weight on rate of descent? page 2; Why does
an aircraft descend quicker when it’s lighter? page 283.)
The shallower the rate of descent, the greater is the ground speed, and because an aircraft’s descent is a function of rate of descent (ROD), the aircraft will cover a greater distance over the ground per lOO-ft descent or per minute.
Therefore, total descent is measured against distance over the ground, which is a function of ground speed, which depends on momentum, which depends on weight. Thus the greater the aircraft’s
weight, the earlier is its required descent point.
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What is positive g force?
Positive g force is the influence of the force of gravity on the normal terrestrial environment beneath it.
This is perceived as the normal weight of any body, including ourselves, in the terrestrial environment,
i.e., 1G
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What is negative g force?
Negative g force is the opposite to positive g force
(i.e., the influence of the normal terrestrial environment above the force of gravity, 1G).
Swept Wing
Describe how you would design a high-speed aircraft wing.
Thin, minimal-chamber, swept wings.
In designing a high-speed wing, you need to consider first the requirement for economical high-speed performance in the cruise configuration.
However, you also have to consider the restraints on the design of the need to keep airfield performance within acceptable limits and the need to give the structural people a reasonable task.
There are several interactive design areas of a wing.
Some are purely for lift, some are a compromise between lift and speed, and some are purely for speed. For the high-speed requirements of a wing, the design would focus on sweep, thickness, and chamber.
The degree of sweep, thickness, and chamber used for the final high speed wing design depends on their any interactive compromises that culminate in directly fulfilling the wing’s high-speed requirement and inversely the wing’s lift and structural requirements.
Swept Wing
How does a swept wing aid the increase in its critical Mach number (Mcrit) speed?
The swept-wing design increases its Mcrit speed because it is sensitive to the (airflow) airspeed vector normal to the leading edge for a given
aircraft Mach number.
A swept wing makes the velocity vector
(perpendicular) to the leading edge a shorter distance than the chord-wise resultant .
Since the wing is responsive only to the velocity vector normal to the leading edge, the effective chord-wise
velocity is reduced (in effect, the wing is persuaded to believe that it is flying slower than it actually is).
This means that the airspeed can be increased before the effective chord-wise component becomes sonic, and thus the critical number is raised.
Swept Wing
Describe how you would optimize the lift design on a swept wing.
To optimize the lift design on a swept wing, you would need to:
(1) examine and develop the lift design areas of the clean wing and
(2) add high lift devices to the clean wing to a degree that satisfies our main lift concern, that of adequate airfield performance.
Swept Wing
What advantages does a jet aircraft gain from a swept wing?
The advantages a jet aircraft gains from the swept wing are
(1) high Mach cruise speeds and
(2) stability in turbulence.
- High Mach cruise speeds - The swept wing is designed to enable the aircraft to maximize the high Mach speeds its jet engines can produce.
*The swept-wing design delays the airflow over the wing from going supersonic and, as such, allows the aircraft to maximize the jet engine’s potential for
higher Mach cruise speeds.
*Additionally, the swept wing is also designed with a minimal chamber and thickness, thereby reducing
profile drag, which further increases the wing’s ability for higher speeds.
(See Q: Describe how you would design a high-speed aircraft wing, page 19.)
- Stability in turbulence - Ironically, a disadvantage of the swept wing is its poor lift qualities, which lends itself to an advantage in that it is more stable in turbulence compared with a straight-winged aircraft.
* This is so because the swept wing produces less lift and therefore is less responsive to updraughts, which allows for a smoother, more stable ride in gusty conditions.
Swept Wing
What disadvantages does a jet aircraft suffer from a swept wing?
Because the swept wing is designed for high cruise speeds, it suffers from the following disadvantages as a consequence:
- Poor lift qualities are experienced because the sweep-back design has the effect of reducing the lift capabilities of the wing.
- Higher stall speeds are a consequence of the poor lift qualities of a swept wing.
- Speed instability is the second consequence of poor lift at lower speeds for the swept-wing aircraft.
Speed is unstable below minimum drag speed (VIMD) because the aircraft is now sliding up the back end of
the jet drag curve, where power required increases with reducing speed.
This means that despite the higher coefficient of lift (CL ) associated with lower speeds, the drag penalty increases faster than the lift; therefore, the lift-drag ratio degrades, and the net result is a tendency to progressively lose speed.
Thus speed is unstable because of the drag penalties particular to the swept wing.
(See Q: Explain speed stability, page 22.)
- A wing-tip stalling tendency is particular to a swept-wing aircraft mainly because of the high local CL loading it experiences.
Uncorrected (in the design), this effect would make the aircraft longitudinally unstable, which is a major disadvantage.
(See Q: Where does a swept wing stall first, and what effect does this have on the aircraft’s attitude? page 21.)
Swept Wing
Where does a swept wing stall first, and what effect does this have on the aircraft’s attitude?
A simple swept and/or tapered wing will stall at the wing tip first if not induced/controlled to stall at another wing section first by the designer.
This is so because the outer wing section experiences a higher aerodynamic loading due to the wing taper, which causes a greater angle of incidence to be experienced to a degree where the airflow stalls at the wing tips.
The boundary layer span-wise airflow, also a
result of sweep, further contributes to the airflow stalling at the wing tips.
A stall at the wing tip causes a loss of lift outboard and therefore aft (due to the wing sweep), which moves the center of pressure inboard and therefore forward; this produces a pitch-up tendency that continues
as the wing stalls progressively further inboard.
A wing-tip stall is resolved in the wing design with the following better aerodynamic stalling characteristics:
- Greater chamber at the tip; this increases airflow speed over the surface, which delays the stall.
- Washout or twist, which creates a lower angle of incidence at the wing tips and delays the effect of the outboard wing loading that causes the stall.
Speed
Explain speed stability.
Speed stability is the behaviour of the speed after a disturbance at a fixed power setting.
The behaviour of an aircraft’s speed after it has been disturbed is a consequence of the drag values experienced by the aircraft frame.
Speed is said to be stable if after it has been disturbed from its trimmed state it returns naturally to its original speed. For example:
- An increase in speeds leads to an increase in drag, thus causing a return to the original speed.
- A decrease in speed leads to a decrease in drag, thus causing a return to the original speed.
Speed
Explain speed instability.
Speed is said to be unstable if after it has been disturbed from its trimmed state the speed divergence continues, resulting in a negative speed stability.
For example:
- A decrease in speed leads to an increase in drag, which causes a further decrease in speed, thus causing a negative speed divergence.
- An increase in speed leads to a decrease in drag, which causes a further increase in speed, thus causing a positive speed divergence.
Speed
What is Mach number?
Mach number (MN) is a true airspeed indication, given as a percentage relative to the local speed of sound. ie: half the speed of sound = 0.5 Mach. (See Q: Describe Mach number, page 122.)
Speed
What is the critical Mach number (Mcrit)?
(Mcrit) is the aircraft’s Mach speed at which the airflow over a wing becomes sonic
.
The aircraft’s Mach speed is lower than the airflow speed over a wing. A typical Merit speed of 0.72 M experiences sonic Mach 1 airflow speed over the upper surface of the wing.
Subsonic aircraft experience a rapid rise in drag above the critical Mach number, and because the aircraft’s engines do not have the available power to maintain its
speed and lift values under these conditions, the aircraft suffers a loss of lift.
Speed
Describe the characteristics of critical Mach number (Merl.)?
- Initial Mach buffet - Caused by the shock waves on the upper surface of the wing as the aircraft approaches (Mcrit) is usually experienced.
- An increase in drag - Because of the breakdown of airflow causes the stick force to change from a required forward push to a neutral force and then a required pull force as the aircraft approaches and
passes (Mcrit). - A nose-down change in attitude (Mach tuck) - Occurs at or after (Mcrit).
- A possible loss of control
Speed
Describe the changes in the center of pressure as an aircraft speed increases past the critical Mach number (Mcrit).
The center of pressure moves rearward on a swept wing as the aircraft passes its (Mcrit) for two reasons:
- The shock waves on the wing’s upper surface occur toward the leading edge because of the greater chamber, which creates the greatest airflow velocity to be experienced at this point.
This upsets the lift distribution chord-wise and causes a rearward shift in the (center of pressure).
- The swept wing tends to experience the shock-wave effect at the thick root part of the wing first, causing a loss of lift inboard, and therefore, the lift force now predominantly comes from the outboard part of the swept wing, which is further aft because of the wing
sweep.
(See Q: Describe center of pressure, page 4.)
Speed
What is Mach tuck?
Mach tuck is the nose-down pitching moment an aircraft experiences as it passes its critical Mach number (Mcrit).
Mach tuck is a form of longitudinal instability that occurs because of the center of pressure’s rearward movement behind the center of gravity which induces the aircraft to pitch down (or the aircraft’s nose to tuck).
Speed
What is the purpose of a Mach trimmer?
The purpose of a Mach trimmer is to automatically compensate for Mach tuck (longitudinal instability) above “Mcrit”. (See Q: What is Mach tuck? page 24.)
Speed
What is a Mach trimmer, and what is it used for?
A Mach trimmer is a system that artificially corrects for Mach tuck above the aircraft’s “Mcrit”
By sensing the aircraft’s speed and signalling a proportional upward movement of the elevator or variable-incidence stabiliser to maintain the aircraft’s pitch attitude throughout its speed range up to its maximum Mach demonstrated flight diving speed (MDF).
Note: Mach trimmers allow for an aircraft’s normal operating speed range to be above its “Mcrit” In the event of a Mach trimmer failure, there is usually an imposed reduced Mach maximum operating speed (MMO) value so that a margin is retained below the Mach speed at which the onset of instability occurs.
Speed
What are the effects of Compress-Ability?
Compress-ability is the effect of air being compressed onto a surface (at a right angle to the relative airflow), resulting in an increase in density, and thus dynamic pressure rises above its expected value.
It is directly associated with high speeds.
(See Chapter 5, “Atmosphere and Speed, page 115.)
There are two main effects of Compress-ability:
- Compress-ability error on dynamic pressure reading flight instruments: ie: ASI shows an over-read error that increases the faster the aircraft’s speed.
(See Q: Describe equivalent air speed [EAS], page 121.) - Compressed air is experienced on the leading edge of the wing, which disturbs the pressure pattern on the wing and causes the disturbed air shock-wave/drag effect at the critical Mach number.
(See Q: What is the critical Mach number [Mcrit]? page 23.)
Speed
What is the main influence on Mach number?
Mach number is a function of the (LSS-Local Speed of Sound), which is influenced by temperature. Therefore, Mach number is influenced by temperature.
As temperature decreases, the local speed of sound (LSS) decreases.
LSS = 38.94 / absolute temperature (Kelvin)
If the LSS decreases for a constant true air speed (TAS), then the Mach number must rise.
MN = TAS / LSS
Speed
Explain speed margins.
A speed margin is the difference between the aircraft’s normal maximum permitted operating speed and its higher certified testing speed.
For a piston-engine propeller aircraft:
VNO is the normal operating maximum permitted speed.
VNE is the higher, never exceeded operating speed.
VDF is the maximum demonstrated flight diving speed, established during design certification flight trials.
The piston-engine propeller aircraft enjoys a relatively large margin between VNO and VDF and has very little over-speed tendencies.
Therefore, the speed margin for a piston-engined propeller aircraft is not very significant.
For the jet aircraft:
VMO/MMO is the maximum indicated operating speed in knots or Mach number.
This is the normal maximum operating speed, which
ensures an aircraft’s structural integrity and adequate handling qualities.
VDF/MDF is the maximum demonstrated flight diving speed in knots or Mach number established during the design certification flight trials.
This flight diving speed incurs reduced aircraft structural integrity and often a lower level of handling qualities.
The jet aircraft’s margin between VMO/MMO and VDF/MDF is relatively small, and because of its low cruise drag and the enormous power available from its jet engines, especially at low altitudes, the jet
aircraft has a distinct over-speed tendency.
Therefore, the speed margin on a jet aircraft is very significant.
Speed
Explain maneuverability margins/envelope.
Maneuverability margin/envelope is contained by its upper and lower speed limits, which are either
(1) between the aircraft’s stall speed (Vs) at the bottom end of its speed range and its VDF/MDF speed at the
top end of its speed range or…
(2) between 1.66 of Vs (representing a safe operating limit above the stall) at the bottom end of its speed range and VMO/MMO at the top end of its speed range.
Speed
What is coffin corner?
Coffin corner occurs at an aircraft’s absolute ceiling, where the speeds at which Mach number buffet and pre-stall buffet occur are coincident, and although trained for, in practice, they are difficult to distinguish
between.
Therefore, a margin is imposed between an aircraft’s operating and absolute ceiling. (See Q: What is the Absolute vs Maximum service ceiling? page 209.)
Mach number buffet and the slow-speed stall buffet are coincident at coffin corner because a stall is a function of indicated air speed (lAS) and Mach number is a function of the local speed of sound (LSS), which
itself is a function of temperature.
For a constant Mach number (which is the normal mode of speed management), the lAS decreases with altitude due to the decreasing LSS. To prevent the lAS from decreasing to its stall speed, the Mach
number must be increased, which results in an increasing lAS.
For a constant IAS, the Mach number increases with altitude due to a decreasing LSS and temperature to a point where the Mach number exceeds “Mcrit”- To prevent the Mach number from exceeding “Mcrit”, the
IAS must be reduced, which results in a decreasing Mach number.
Therefore, there comes a point at the aircraft’s absolute ceiling where the aircraft can go no higher. This is so because it is bounded on one side by the low-speed buffet and on the other by the high-speed
buffet because the stall IAS and the “Mcrit” values are equal.
This is coffin corner, and this effect restricts the altitude attainable by the aircraft.
Stalls
Explain why an aircraft stalls.
An aircraft stalls when the streamlined/laminar airflow (or boundary layer) over the wing’s upper surface, which produces lift, breaks away from the surface when the critical angle of attack is exceeded, irrespective of airspeed, and becomes turbulent, causing a loss in lift.
(i.e., the turbulent air on the upper surface creates a higher air pressure than on the lower surface).
The only way to recover is to decrease the angle of attack.
(i.e., relax the back pressure of controls and/or move the control column forward).
Stalls
What properties affect an aircraft’s stall speed?
An aircraft will stall at a constant angle of attack (known as the critical angle of attack).
Because most aircraft do not have angle of attack indicators (except “eyebrows” on some electronic flight instrument system displays), the pilot has to rely on airspeed indications.
However, the speed at which the aircraft will stall is variable depending on the effects of the following properties.
- Weight
a. Actual weight
b. Load factor, G’s in a turn
c. Effective weight/center of gravity position - Altitude
- Wing design/lift
- Configuration
- Propeller engine power
Stalls
How does the stall speed vary with weight?
The heavier the aircraft, the higher is the indicated speed at which the aircraft will stall.
If an aircraft’s actual weight is increased, the wing must produce more lift (remember that the lift force must equal the weight force), but because the stall occurs at a constant angle of attack, we can only increase lift by increasing speed.
Therefore, the stall speed will increase with an increase in the aircraft’s actual or effective weight.
The stall speed is proportional to the square root of the aircraft’s weight.
Stalls
What wing design areas delay the breakup of airflow (stall)?
- Wing slots are the main design feature that delays/suppresses stall speed.
A slot is a form of boundary layer control that re-energizes the airflow to delay it over the wing from separating at the normal stall speed.
The wing therefore produces a higher coefficient of lift
(CL ) and can achieve a lower speed at the stall angle of attack.
- Lower angle of incidence and a greater chamber for a particular wing section, e.g: wing tip washout.
Stall
What changes the aircraft’s angle of attack at the stall?
The movement of the center of pressure point at the stall causes a change in the aircraft’s angle of attack.
Normally, a simple swept or tapered wing is designed so that the center of pressure will move rearward at the stall.
This is so because the stall normally is induced at the wing root first, where the center of pressure is at its furthest forward point across the wing span.
Therefore, the lift produced from the un-stalled part of the wing, toward the tips and therefore aft, is behind the root with an overall net result of the
center of pressure moving rearward, which results in a stable nose down change in the aircraft’s angle of attack at the stall.
Stall
What happens to the stall speeds at very high altitudes, and why?
The stall speed increases at very high altitudes, which the jet aircraft is capable of, because of…
- Mach number compressibility effect on the wing
At very high altitudes, the actual equivalent airspeed (EAS) stall speed increases because the Mach number compressibility effect on the wing disturbs the pressure pattern and increases the effective weight on the wing,
resulting in a higher EAS stall speed.
- Compressibility error on the lAS.
The compressibility correction that forms part of the difference between the indicated airspeed
(lAS) and airspeed indicator (which is uncorrected) and equivalent airspeed (EAS, which is IAS corrected for compressibility and position instrument error) is larger in the EAS to IAS direction due to the effect of the Mach number, resulting in a higher IAS stall speed.
Stall
What is a superstall?
A super stall also may be referred to as a deep stall or a Locked in stall condition, which, as the name suggests, is a stall from which the aircraft is unable to recover.
It is associated with rear-engined, high-T-tail, swept-wing aircraft, which because of their design tend to suffer from an increasing nose-up pitch attitude at the stall with an ineffective recovery pitching capability.
The BEA Trident crash in 1972 at Slough, England,
is probably the most famous and tragic outcome of a superstall.
A superstall has two distinct characteristics:
1. A nose-up pitching tendency
2. An ineffective tailplane
- The nose-up pitching tendency at the stall is due to
a. Near the stall speed, the normal rooftop pressure distribution over the wing chord line changes to an increasing-leading-edge peaky pattern because of the enormous suction developed by the
nose profile.
At the stall, this peak will collapse.
b. A simple virgin swept- or tapered-wing aircraft will stall at the wing tip first (if the wing has not been designed with any inboard stall properties) mainly due to the greater loading experienced, leading to a higher angle of incidence that causes the wing tip to stall.
Because of the wing sweep, the center of pressure moves inboard to a point where it is forward of the center of gravity, therefore creating an increasing pitch-up tendency.
c. The forward fuselage creates lift, which usually continues to increase with incidence until well past the stall.
This destabilizing effect has a significant contribution to the nose-up pitching tendency of the aircraft.
However, these phenomena themselves are not exclusive to high T-tail, rear-engined aircraft and alone do not create a superstall.
For a superstall to occur, the aircraft will have to be incapable of recovering from the pitch-up tendency at the stall.
- An ineffective tail plane makes the aircraft incapable of recovering from the stall condition, which is due to
a. The tailplane being ineffective because the wing wake, which has now become low-energy disturbed/turbulent air, passes aft and immerses the high-set tail when the aircraft stalls.
This greatly reduces the tail plane’s effectiveness, and thus it loses its pitching capability in the stall, which it requires to recover the aircraft.
This is so because a control surface, especially the elevator, requires clean, stable, laminar airflow (high-energy airflow) to be aerodynamically effective.
Instability
What is Dutch roll?
Dutch roll is an oscillatory instability, combining yawing and rolling motions, associated with swept-wing jet aircraft.
When the aircraft yaws, it will develop into a roll. The yaw itself is not too significant, but the roll is much more noticeable and unstable.
This is so because the aircraft suffers from a continuous reversing rolling action.
Instability
What causes Dutch roll?
Swept wings.
Dutch roll occurs when a yaw is induced either by a natural disturbance or by a commanded or an un-commanded yaw input on a swept-wing aircraft.
This causes the outer wing to travel faster and to
become more straight on to the relative airflow (in effect, decreasing the sweep angle of the wing and increasing its aspect ratio).
Both these phenomena will create more lift.
At the same time, the inner wing will travel slower and, in effect, becomes more swept relative to the airflow, and both these phenomena will reduce its lift.
Therefore, a marked bank occurs to the point where the outer, upward-moving wing stalls and loses all lift, and therefore the wing drops, causing a yaw to the stalled wing and thus leading to the sequence being repeated in the opposite direction.
This sequence will continue and produce the oscillatory instability around the longitudinal axis we know as Dutch roll.
Pitch fluctuations only occur with an extreme degree of Dutch roll.
Instability
What is the recovery technique from Dutch roll?
For a pilot to recover an aircraft suffering from Dutch roll, he or she would apply opposite aileron to the direction of the roll, assuming that the yaw dampers are not serviceable.
Although the root cause of Dutch roll is the yawing motion, application of a correcting rudder input by the pilot normally would worsen the
situation.
This is so because the yawing motion in the oscillatory cycle happens extremely quickly, and the pilot’s reaction would not be quick enough to catch the yaw, which already has developed into a
roll and dissipated.
Therefore, a rudder input to correct the initial yaw (which has since dissipated) would in fact aggravate the roll effect further into a side slip.
Aileron control therefore is employed because the roll cycle is of sufficient duration to allow the pilot to apply the correct opposite aileron control.
A severe Dutch roll may require two or three aileron inputs to dampen the oscillation gradually.
Instability
What prevents Dutch roll?
Yaw dampers prevent Dutch roll on swept-wing aircraft.
A basic reason for the Dutch-rolling tendency of an aircraft (apart from the wing sweep, of course) is the lack of effective fin and rudder area to stop it.
The smaller fin and rudder area is a design compromise
that makes the aircraft spirally stable to a degree.
Therefore, the effectiveness of the fin area must be increased in some other way to prevent Dutch roll.
This is achieved with yaw dampers.
Instability
What is directional stability?
Directional stability is the tendency for an aircraft to regain its direction (heading) after the aircraft has been directionally disturbed (e.g: an induced yaw) from its straight path.
This is achieved naturally because the fin (vertical tailplane) becomes presented to the airflow at a greater
angle of incidence, which generates a restoring aerodynamic force.
Instability
What is spiral stability and instability?
Spiral stability (or a spirally stable aircraft) is defined as the tendency of an aircraft in a properly coordinated banked turn to return to a laterally level flight attitude on release of the ailerons.
Spirally stable aircraft have dominant lateral surfaces (e.g., wings).
Spiral instability or a spirally unstable aircraft will see a banked turn increase fairly quickly, followed by the nose falling into the turn, leading to the aircraft entering into a spiral dive when the ailerons are
released in a coordinated turn.
Spirally unstable aircraft have dominant
(too large) vertical surfaces (e.g., tailplane). What happens is that as the aircraft starts to slip into the turn on release of the ailerons and before the rolling moment due to the side-slip can take effect, the
rather dominant fin jumps into play.
This is so because the fin/tailplane area (outside) becomes exposed to the relative airflow, which exerts two forces on the aircraft:
1. Around the vertical axis, which straightens the aircraft directionally
2. Around the longitudinal axis, which increases the bank.
This accelerates the outer (upper) wing and causes the bank to be increased further.
The increased bank causes another slip, which the
fin again straightens.
This sequence repeats, and the turn is thus made steeper.
Once the bank angle exceeds a given type-specific amount (say, 300), the nose falls into the turn, the speed increases as the roll increases, and the aircraft enters into a spiral dive.
Instability
What is lateral stability?
Lateral stability is the tendency for an aircraft to return to a laterally level position around the longitudinal axis on release of the ailerons in a side-slip.
There are two principal features that make an aircraft naturally laterally stable, namely,
1. Wing dihedral. The airflow due to a sideslip causes an increase in the angle of attack (lift) on the lower (leading) wing and a decrease in angle of attack on the raised wing because of the dihedral angle.
The lower wing thus produces an increase in lift because of the increased angle of attack, and the raised wing produces less lift.
The difference in lift causes a rolling moment that tends to restore the wing to its laterally level position.
2. Side loads produced on the keel surface. When the aircraft is side-slipping, a side load will be produced on the keel surface, particularly the fin.
This side load will produce a moment to roll the
aircraft laterally level, which in general terms is stabilizing.
The magnitude of this effect depends on the size of the fin, but regardless, its effect is small compared with other laterally stabilizing effects.
Instability
What is longitudinal stability?
Longitudinal stability is an aircraft’s natural ability to return to a stable pitch position around its lateral axis after a disturbance.
When an aircraft is in equilibrium, the tail plane in general will be producing an up or down load to balance the moments about the center of gravity.
(It is assumed that throughout the elevator remains in its original position during any disturbance in pitch.)
If the aircraft is disturbed in pitch (say, nose-up), there will be a temporary increase in the angle of attack.
The increase in tailplane angle of attack produces an increase in tail plane lift, which will cause a nose-down pitching moment.
(The tail plane is thus able to produce a stabilizing moment due to a displacement in pitch as long as the center of gravity remains within its limits.)
The wings also experience this increase in angle of
attack, resulting in the wings producing an increase in lift.
The moment and the direction of the moment produced by this lift will depend on the relative positions of the center of pressure and the center
of gravity.
Instability
Describe stability at high altitudes.
Longitudinal, lateral, directional, and oscillatory stability in general are reduced at high altitudes, in terms of dynamic stability, mainly because aerodynamic damping decreases with altitude.
The aircraft will feel and is less stable except for spiral stability, which improves with altitude, whereas oscillatory stability deteriorates very rapidly with altitude.
This is so because for a constant indicated airspeed (lAS), the fin suffers a smaller angle of incidence
and therefore has a smaller restoring force the higher the altitude.
Therefore, the fin is less dominant, which is detrimental to oscillatory stability but as a consequence means that the aircraft’s lateral surfaces (wings) become more dominant. This improves the aircraft’s spiral stability qualities
(spiral stability always opposes oscillatory stability, and vice versa).
Instability
What are the four flying-quality penalties experienced at very high altitudes?
- Restricted operating speed range (See Qs: Explain speed margins? page 25; What is coffin corner? page 26.)
- Reduced maneuverability (See Q: Explain maneuverability margin/ envelope, page 26.)
- Reduced aerodynamic damping
- Reduced stability (See Q: Describe stability at high altitudes, page 34.)
Control Surfaces
What are the primary/main flight controls?
Elevator
Ailerons
Rudder
Control Surfaces
What is the elevator, and how does it work?
The conventional elevator is a hinged control surface at the rear of the horizontal tail plane (stabilizer) that is controlled by the pilot’s control column.
As the elevator control surface is deflected, the airflow and thus the aerodynamic force around the elevator (horizontal tailplane) changes.
Moving the control column back deflects the elevator up, causing an increase in the airflow speed and thus reducing the static pressure on the underside of the elevator control surface.
In addition, the topside of the elevator faces more into the relative airflow, which causes an increase
in the dynamic pressure experienced.
These effects create an aerodynamic force on the elevator (horizontal tailplane) that rotates (pitches) the aircraft about its lateral axis.
That is, back control column movement
moves the elevator control surface upward, producing a downward aerodynamic force that pitches the aircraft up.
Thus the opposite is also true: Forward control column movement moves the elevator control surface downward, producing an upward aerodynamic force that pitches the aircraft down
Control Surfaces
What are ailerons, and how do they work?
Ailerons are control surfaces located at the trailing edges of the wings that control the aircraft’s motion around its longitudinal axis, known as roll.
The ailerons are controlled by left and right movement of the control column, which commands the ailerons in the following manner:
Moving the control column to the left commands the left aileron to be raised, which reduces the lift on the wing, and the right aileron is lowered, which increases the lift generated by this wing, thereby rolling the aircraft into a banked condition, which causes a horizontal lift force (centripetal force) that turns the aircraft.
The ailerons normally are powered (hydraulically) powered on heavy/fast aircraft because of
the heavy operating forces experienced at high speeds. (See Q: What are spoilers, and how do they work? page 37.)
Control Surfaces
What is the rudder and how does it work?
The rudder is a hinged control surface at the rear of the fin (vertical tail plane) that is controlled by the pilot’s rudder pedals.
As the rudder control surface is deflected, the airflow and thus the aerodynamic force around the rudder (vertical tailplane) changes.
Moving the left rudder pedal deflects the rudder to the left, causing an increase in the airflow speed and thus reducing the static pressure on the right-hand side of the rudder control surface.
In addition, the left side of the rudder faces more into the relative airflow, which causes an increase
in the dynamic pressure experienced.
These effects create an aerodynamic force to the right on the rudder (vertical tailplane) that rotates (yaws) the aircraft about its vertical/normal axis at its center of gravity point to the left.
Control Surfaces
How does the effectiveness of the control surfaces vary with speed?
The control surf’aces become more effective at higher speeds.
This results in a requirement for large control movements at low speeds and smaller control movements at high speeds to produce the same
control force.
Control Surfaces
What is elevator reversal?
Elevator reversal occurs at high speeds when the air loads/forces are large enough to cause a twisting moment on the deflected elevator surface to either a neutral or opposite position that results in sudden
reversal of the aircraft’s pitch attitude.
Control Surfaces
What is adverse yaw?
Adverse yaw is a yawing motion opposite to the turning/rolling motion of the aircraft.
Adverse yaw is caused by the drag on the down-going aileron being greater than that on the up-going aileron.
This imbalance in drag causes the yawing motion around the normal/vertical axis.
Since this yaw is adverse (i.e., in a banked turn to the left, the yaw is to the right), it is opposing the turn, which is detrimental to the aircraft’s performance.
Adverse (aileron) yaw is corrected in the design by the use of either differential ailerons or “Frise ailerons”.
Control Surfaces
What is aileron reversal (adverse), and when is it likely to occur?
Aileron reversal occurs at high speeds when the air loads/forces are large enough that they cause an increase in lift.
Because most of this lift is centered on the down-going aileron at the rear of the wing, a nose-down
twisting moment will be caused.
This will result in a decrease in the incidence of the wing to the extent that the loss of lift due to
the twisting cancels the lift gained from the aileron.
At this point the aileron causes no rolling moment, and if the wing twisting is exaggerated (which a down-going aileron can do), the rolling motion around the longitudinal axis can be reversed, hence an adverse rolling motion.
Control Surfaces
What is a yaw-induced adverse rolling motion, and when is it likely to occur?
The rudder inducing the aircraft to yaw one way can cause another form of adverse rolling motion in the opposite direction.
This happens at high speeds (above VMO/MMO) because the deflected rudder experiences
a sideways force that causes the aircraft to roll in the opposite direction; i.e., right rudder experiences a sideways force from right to left, causing a rolling moment to the left.
Control Surfaces
What are spoilers, and how do they work?
Spoilers consist of opening panels that extend from the upper surface of the wing and have the effect of spoiling/disturbing the airflow over the wing (drag), thereby reducing the lift.
For roll control:
The spoilers are raised on one wing and not the other, which creates an imbalance of lift values that produces a rolling moment.
The spoilers are connected to the normal aileron controls, and they work in tandem with each other for roll control.
Spoilers are in fact a more efficient roll-control surface than ailerons.
The disadvantage of roll control spoilers is that they cause an overall loss of lift, which may cause a loss of height and is particularly undesirable when flying close
to the ground.
As air speed brakes:
The spoilers are raised symmetrically on both wings to a flight detent position (using the speed brake lever), which causes a large increase in drag that slows down the speed of the aircraft.
Note. Buffet is usually experienced with spoiler (speed brake) deployment.
As ground lift dumpers:
The spoilers are raised systematically on both wings to the ground detent position (greater angle than the flight detent position), which causes a large increase in drag that…
(1) decreases lift over the wing, causing the aircraft to sink to the ground, and (2) acts as ground speed brakes to slow down the speed of the aircraft.
Control Surfaces
What are the three purposes of spoilers?
- Roll control (usually in combination with the ailerons). Note that the primary purpose of spoilers is roll control.
- Air speed brakes
- Ground lift dumpers
Control Surfaces
Describe differential and non-differential spoilers.
The difference between differential and non-differential spoilers is in how they provide lateral roll control when already extended as speed brakes.
Non-differential spoilers - When already partly extended as a speed brake, the spoilers will extend further on one side but will not retract on the other side in response to a roll command.
When already fully extended as a speed brake, both sides remain in the extended speed brake position, and therefore, the spoilers do not provide any roll control.
Differential spoilers - When already partly extended as a speed brake, the spoilers will extend further on one side and retract on the other side in response to a roll command.
When already fully extended as a speed brake, the spoilers will remain extended on one side and retract on the other side in response to a roll command.
Control Surfaces
Give six reasons for spoilers.
Four of the reasons why spoilers are needed on aircraft are to provide a degree of roll control.
This is so because the ailerons have the following
inadequacies:
1. The ailerons are limited in size and therefore effectiveness.
2. On a thin swept wing, ailerons that are too large will experience a high degree of air loading/lift, resulting in the wing twisting at high speeds that can produce aileron reversal (removes aileron roll control), which is very detrimental. 3. Ailerons tend to lose effectiveness at high speeds due to the span-wise diagonal airflow across the aileron, which is less effective than a perpendicular airflow. 4. High-speed swept-winged aircraft cause a strong rolling moment with yaw, known as adverse rolling moment with yaw.
Remaining two reasons , the spoilers are needed to counteract (brake) the aircraft’s high speed in the air and on the ground.
5. Because the aircraft has low drag and the engines have a slow lag response rate, there is a need for high-drag devices in flight to act as a brake when the aircraft is required to lose speed and/or height quickly.
This is achieved by the use of the spoilers on both wings being raised simultaneously to the flight detent position, which creates a drag force opposing thrust and therefore reduces the aircraft’s speed and/or height.
6. On landing or during a rejected takeoff, there is a need to dump the lift off the wing and onto the wheels to assist in stopping the aircraft.
This is achieved by the use ofthe spoilers on both wings being raised simultaneously to the ground or up detent position in a similar manner as the in flight speed brake.
This position has a greater angle of deployment than the flight detent and/or uses more spoiler panels, therefore creating a greater drag force.
Control Surfaces
What limits the use of spoilers, and why do spoilers blow back?
Spoilers are limited by very high speeds (VDF/lVIDF), which cause them to blow back.
At very high speeds, the spoilers will be blown back to or near to their fully retracted position.
This occurs because the high air loads experienced on the spoilers surfaces at high speeds are greater than their design limit.
Obviously, the force experienced is a function of airspeed and angle of deflection.
Control Surfaces
How is spoiler blow-back prevented?
Spoilers are designed not to blow back in the normal operating speed range of the aircraft.
Therefore, correct speed management of the aircraft
will prevent the spoilers from blowing back.
Control Surfaces
How do you correct for spoiler blow-back?
In flight, reduce speed by reducing thrust to a speed where the spoilers will operate normally, and then recycle the speed brake lever.
Note: Spoiler blow-back will only occur when the aircraft’s speed is excessive (i.e., VDF/MDF), which itself should be experienced only in a non-normal flight condition, e.g., spiral dive, etc., when the recovery
drill incorporates reducing speed by closing the throttles.
Control Surfaces
What do leading-edge slats do?
Leading-edge slats increase the wing’s chamber area and mean aerodynamic chord (MAC), thereby increasing its coefficient of lift (CL) maximum, which reduces the aircraft’s stall speed.
Control Surfaces
What are Krueger flaps?
Krueger flaps are leading-edge wing flaps used to increase the wing chamber and therefore increase the coefficient of lift maximum.
Control Surfaces
What are Fowler flaps?
Fowler flaps are trailing-edge wing flaps (usually triple slotted) used to increase the wing area and chamber, which increases the coefficient of lift maximum for low flap settings, e.g., 1 to 25°.
High flap settings increase drag predominately more than lift and therefore are used to lose speed and/or height, most commonly during an approach to land.
Control Surfaces
What is the primary use of flaps on a jet aircraft?
The primary use of flaps, especially on a jet aircraft, is to increase lift by extending the geometric chord line of the wing, which increase its chamber and area.
Control Surfaces
What are the effects of extending flaps in flight?
Lowering the flaps in flight generally will cause a change in the pitching moment.
The direction and degree of the change in pitch depend on the relative original position of the center of pressure and the center of gravity.
The factors that contribute to this are…
1. The increase in lift created by the increased wing area and chamber will lead to a pitch-up moment if the center of pressure remains in front of the center of gravity.
2. If the associated rearward movement of the center of pressure is behind the center of gravity, then this will produce a nose-down pitch.
3. The flaps will cause an increase in the downwash, and this will reduce the angle of attack of the tail plane, giving a nose-up moment.
4. The increase in drag caused by the flaps will cause a nose-up or nosedown moment depending on whether the flaps are above or below the lateral axis.
The overall change and direction in the pitching moment will depend on which of these effects is predominant.
Normally, the increased lift created by extending the wing chord line when the flaps are extended is dominant and will cause a nose-up pitching tendency
because the center of pressure normally remains in front of the center of gravity.
Control Surfaces
What are the effects of raising flaps in flight?
The raising of flaps in flight, if not compensated for by increasing speed and changing attitude, will result in a loss of lift.
Control Surfaces
How do flaps affect takeoff ground run?
Flaps set within the takeoff range. A higher flap setting, within the takeoff range, will reduce the takeoff ground run for a given aircraft weight.
The use of flaps increases the maximum coefficient of lift of the wing due to the increased chord line for a low drag penalty, which reduces the stall speed Vs) and consequently the rotation (Vr) and takeoff safety
(V2 ) speeds.
This provides good acceleration until it has sufficient kinetic energy to reduce the takeoff ground run.
Typically, various flap settings from the first to the penultimate flap setting are available for takeoff
(i.e., takeoff range).
The higher the flap setting within this range, the less is the takeoff run required because the drag is not significantly increased because the angle of attack is low.
However, the drag increment is higher when the aircraft is in flight and out-of-ground effect because of the aircraft’s angle of attack is much higher.
Note: Initial and second-segment climb performance thus will be reduced with a high takeoff flap setting.
(See Q: How does the use of flaps affect the aircraft’s takeoff performance? page 197.)
Flaps set outside the takeoff range. A high flap setting outside the takeoff range will result in a large drag penalty that will reduce the aircraft’s acceleration, and therefore, the takeoff run will be greatly increased before VR is attained.
No or a very low flap setting outside
the takeoff range on takeoff will result in a low coefficient of lift produced by the wing for a given speed, and thus a higher unstick (Vr) speed is required to create the required lift for flight.
Therefore, an increased takeoff run to attain the higher Vr is required.
Control Surfaces
What are the purposes of yaw and roll dampers, and how do they work?
The purpose of a yaw damper is to:
(1) prevent Dutch roll and (2) coordinate turns.
The purpose of a roll dampers is to:
(1) damp/remove Dutch roll, (2) provide roll damping in turbulence, and (3) provide spiral stability.
The yaw damper’s main purpose is to prevent Dutch roll. When the fin area is insufficient to provide a natural oscillatory stability, the effective fin area must be increased in some other way.
This is accomplished
on power-operated rudders with the yaw damper.
(See Q: What causes Dutch roll? page 31.)
A yaw damper is a gyro system that is sensitive to changes in yaw, and it feeds a signal into the rudder, which applies opposite rudder to the yaw before the roll occurs, thus preventing Dutch roll.
The roll damper works through the aileron controls and can be used to…
1. Supplement a yaw damper for Dutch roll control by damping out the roll once it has been established (especially common on oscillatory unstable aircraft).
2. Purely for roll damping in turbulence. 3. Control spiral stability. (See Q: What is spiral instability? page 32.)
A roll damper normally is associated with providing spiral stability.
Control Surfaces
Describe parallel yaw dampers.
Parallel yaw dampers (early type) apply rudder control through the same control run as the pilot, and their activity is reflected in the rudder bar activity (moves the rudder pedals).
While this provides a visual indication of the yaw dampers’ serviceability, it does increase the rudder loads experienced by the pilot.
To prevent this making matters worse in the event of an engine out failure on takeoff or a crosswind landing, the
damper can be switched off for takeoffs and landings.
Since this damper in effect parallels the pilot’s actions, it became known as the parallel yaw damper.
Control Surfaces
Describe series yaw dampers.
The series yaw damper is a development of the parallel yaw damper and is found commonly in modern jet aircraft.
This system is attached to the rudder control circuit at the back of the aircraft, and as such, it does not move the pilot’s rudder pedals when it moves the rudder.
This means that the rudder foot forces are not increased and therefore allows the series yaw damper to be used for engine-out takeoffs and landings.
Control Surfaces
What is a stabilizer/variable-incidence tailplane?
A stabilizer/variable-incidence tailplane is an all-moving horizontal tailplane control surface (i.e., not fixed in one position).
(See Q: What is the purpose of a stabilizer? page 42.)
Normally, an all-moving horizontal tail plane is called a stabilizer when it is solely responsible for longitudinal balancing and it has a separate elevator with its own controls and movement range for pitch maneuverability.
A stabilizer normally is moved by its own independent
stab trim system that can be either a manual or an automatic device.
Normally, an all-moving tailplane is called a variable-incidence tailplane when it does not have an elevator surface.
Therefore, the variable-incidence tailplane provides pitch maneuverability by control column and longitudinal balancing by the trim system.
Control Surfaces
What is the purpose of a stabilizer?
The purpose of the horizontal stabilizer is to provide a longitudinal balancing force to the aircraft.
Thus the elevator range and aircraft’s pitch maneuverability are not compromised and remain available to be used solely to control the pitch of the aircraft.
The stabilizer covers large and small pitching moments; e.g., a single person who moves from the aft to the forward cabin will displace the overall weight balance.
Control Surfaces
What are the four reasons for a variable-incidence tailplane/stabilizer especially on a jet aircraft?
The four reasons for use of a variable-incidence tailplane or stabilizer are…
1. To provide a balancing force for a large center of gravity range.
2. To provide a balancing force for a large speed range.
3. To cope (longitudinally) with large trim changes as a result of position changes to the wing leading and trailing edge high-lift devices configuration changes)
4. To reduce elevator trim drag to a minimum While any of these requirements in isolation might not demand a variable-incidence tailplane, in combination they certainly do.
Once the need has been established for one of the requirements, then advantages also occur in the other areas.
Control Surface
What is the best center of gravity position with a stuck stabilizer, and why?
With a stuck stabilizer, the best center of gravity position is aft.
This can be accomplished by moving passengers to the rear of the aircraft and/or by moving fuel to outer wing tanks if possible.
Control Surface
What is the required action with a stuck stabilizer?
If a stabilizer has stuck while the aircraft is in a substantially trimmed-out condition, then as long as you maintain the speed at which the tail jammed, you will remain substantially in trim and therefore stable.
However, because the aircraft will have to depart from the cruise speed in preparation to land, the following steps will reduce the main reasons for having a variable-incidence tail plane in the first place and
also reduce the effects of a stuck stabilizer.
(See Q: Describe the effects of a stuck stabilizer? page 43.)
This helps to maintain the aircraft’s longitudinal stability and maneuverability in pitch during its approach
and landing.
Generally, the following procedure would be followed:
1. Divert to a nearby airfield (so that the center of gravity movement due to the aircraft's change in weight is not excessive over a prolonged flight).
2. Move the center of gravity to an aft position. This will...
a. Longitudinally balance the aircraft at low speeds and thereby reduce the stick forces.
b. Reduce the elevator demand for the landing flare. (See Q: What is the best center of gravity position with a stuck stabilize; and why? page 43.)
3. Reduce speed as late as possible to minimize the length of time a balancing force, 'With its associated high stick forces, is required from the elevator.
4. Plan a long final approach, and make configuration changes, gear, and flaps earlier than usual to give time to sort out the aircraft before the next change is due.
5. Use a reduced flap setting for landing, which will reduce the landing flare required.
This allows you to maintain a higher approach
speed, which reduces the divergence of the aircraft from its cruise trimmed speed, reduces the balancing force required from the elevator, and thus reduces the stick forces experienced.
Having exercised these measures in response to this failure, the aircraft will retain enough scope to maintain a longitudinally stable condition and enough elevator pitch maneuverability to adjust the aircraft’s approach and landing attitude, which should not produce too many problems for the pilot.
Control Surface
Describe a runaway stabilizer condition and required action.
A stabilizer typically is held in its trimmed position by a series of brakes for both manual and autopilot modes.
If these brakes should fail, then the stabilizer will experience back-pressure from the airflow, which
will rotate the stabilizer to its maximum upward or downward mechanical stops, thereby inducing a marked out-of-trim unstable condition.
A stabilizer runaway, whether auto or manual trim, should not go undetected for too long, and when detected, decisive action should be employed.
Each aircraft type has its own drill, but in general terms:
1. Hold the control column firmly.
2. Autopilot (if engaged), disengage.
3. Stab trim cutout switches to cut out.
If the runaway continues,
4. Stabilizer trim wheel, grasp and hold.
Continue your flight using manual trim, and adopt early airspeed and configuration conditions.
Control Surface
Describe the effects of a jammed/degraded elevator.
A degraded or jammed elevator will result in less effective elevator maneuverability in pitch control.
Control Surface
What is the best center of gravity position with a Jammed/degraded elevator?
The best position for the center of gravity with a degraded/jammed elevator is aft.
This can be accomplished by moving the passengers to the rear of the cabin and/or by moving fuel to the outer wing tanks if possible.
An aft center of gravity position lessens the need for large pitch-control demands, especially during the approach and landing flare.
Control Surface
What is the required action for a jammed/degraded elevator?
If there is no elevator control (jammed), then the stabilizer trim can be used for pitch control.
If the elevator control is reduced (degraded), then it should be assessed if there is still sufficient elevator range available to land safely and used to do so if applicable.
If not, then the condition should be regarded as having no elevator control.
With a degraded or jammed elevator, several actions can be taken to minimize the need for major pitch changes and/or to improve the handling and management of the aircraft:
1. If possible, move the center of gravity rearward; this will reduce the need for large elevator angles, especially during the landing.
2. Plan a long final approach, and make configuration changes, gear, and flaps earlier than usual to allow more time to sort out the aircraft before the next change has to be made.
3. Restrict the flap angle for landing to reduce the flare demanded.
Control Surface
Describe the effects of a failure/reduction in elevator feels.
Artificial feel systems (normally duplicated) are employed on powered controls, especially the elevators.
They meet the requirement of progressive
feel against control surface deflection at constant speed and against a constant angle at varying speed based on our old friend
1/2R V2
Whenever the feel on an elevator control is significantly reduced, great care must be exercised in its use.
The control must be moved slowly and smoothly over minimum angles to avoid over-stressing the control surface structure but enough to maintain the flight path.
Over-stressing the control surface with a lack of feel is a significant problem, and for this reason, turbulence should be avoided.
Control Surface
What is the best center of gravity position with a reduced or failed elevator feel system?
The best position for the center of gravity with a reduced or failed elevator feel system is forward.
This can be accomplished by moving the passengers to the front of the cabin and/or by moving the fuel into
forward tanks if possible.
This increases the aircraft’s natural longitudinal stability and renders the pitch control less sensitive and feeling heavier, therefore making the aircraft less responsive to small elevator movements.
Thus the chances of the pilot over stressing the elevator control surface are minimized, although still possible.
Control Surfaces
Describe the effects of the air loads on a control surface and how these effects are managed?
If a control surface is deflected, the dynamic pressure/aerodynamic loads on it will increase and act as a lift force through its center of pressure point.
When multiplied by the control surface arm, it gives
the size of the moment trying to rotate the control surface back to its neutral position.
This moment is known as the hinge moment or air
load force.
That is, Hinge moment (air loads force) = lift force (air load) X arm.
Note: Where the lift force is a design product of the size of the control
surface and the magnitude ofthe lift force experienced depends on
(1) airspeed and
(2) angle of deflection of the control surface.
That is, the lift force increases dynamically in flight with either an increased control angle of deflection at a constant speed or at a constant angle of deflection with an increased speed.
And where the arm is a design
product of the distance between the center of pressure point and the hinge line.
They produce a hinge moment/air load force that tries to return the surface to its neutral position.
Control Surfaces
What is a tab surface, and what can it be used for?
A tab is a small hinged surface found on a primary flight control surface. Usually the tab forms part of the trailing edge on a manual control surface.
A tab can be used to provide
1. Trimming
2. Control balance
3. Servo operation of a control surface
to reducelbalance the opposing hinge moment (air load force) on the
associated control surface.
Control Surfaces
What is a balance tab?
A balance tab is a form of aerodynamic control balance on a control surface.
A control balance tab balances the main aerodynamic lift force load on a control surface with an opposing force, which thereby reduces the overall hinge moment (air load force).
Hinge moment (air loads) = lift force X arm
This is reflected by the stick control force that the pilot experiences being reduced to a manageable level.
(See Q: Describe the effects of the air loads
on a control surface and how these effects are managed, page 46.)
Control Surfaces
What is a hinge/horn balance?
A setback hinge is another form of aerodynamic control balance on a control surface, whereby the design of the control surface sets the hinge line back into the control surface, thus reducing the center of pressure to hinge line arm, which results in reducing the control surface hinge moment.
[Remember, hinge moment (air loads) = lift force X arm.]
Thus the stick control force that reflects the overall hinge moment experienced on the control surface is reduced to a manageable level for the pilot.
(See Q: Describe the effects of the air loads on a control surface and how these effects are managed, page 46.)
Another form of aerodynamic control balance on a control surface is a horn balance. A horn balance is a protruding control surface that produces a balancing lift force in the opposite direction of the main lift force and reduces the overall hinge moment/air load force.
They are common on elevator/stabilizers. Thus the stick control force that reflects the overall hinge moment experienced on the control surface is reduced to a manageable level for the pilot.
Note: A horn balance and setback hinge typically are used in tandem on a control surface.
Control Surfaces
What is a mass balance and what is it used for?
A mass balance is another form of aerodynamic balance control on a control surface.
The hinge moment/air load force experienced by a
deflected control surface tries to rotate the control surface back to its neutral position, but it is balanced by a mass weight that keeps it in its deflected position.
Thus the stick control force that reflects the overall air load force/hinge moment experienced on the control surface is reduced to a manageable level.
Control Surfaces
Why are control surfaces hydraulically operated on large aircraft?.
On large, fast aircraft, especially modern jets, it is found that the control forces required to move a control surface are simply beyond the strength of the pilot and are also too great to be controlled by pure
aerodynamic designs, e.g., balance tabs.
This is so because the shear sizes and weights of the control surface arms in question and the aerodynamic
airflow lift forces (load) generated on the deflected control surface are too great.
For modern large, fast jet aircraft, the answer lies in the powered control surface, typically hydraulic-powered systems, because they generate enough power to cope with the full air load force (i.e., not balanced) experienced on the control surface.
Control Surfaces
Why does a powered controlled surface need an artificial feel system?
An artificial feel system is required because power-operated flying controls are irreversible, i.e., they do not feed back to the pilot any sensory information about how hard the control surface is and thus what aerodynamic air forces it is coping with.
Therefore, there is a need to give this information to the pilot so that he or she is aware of the control angles being applied and their effect on the aircraft, in short, keeping the pilot in the sensory loop, which allows him or her to guard against over-stressing the control surface.
Control Surfaces
How does an artificial feel system work?
The simplest form of artificial feel consists of a spring box fitted into the control NM. This provides a feel and self-centering action, but the stick forces are constant and therefore are only suitable for aircraft types with a limited altitude and speed range.
Control Surfaces
What is Q feel, and where is it used?
Q feel is a sophisticated computer-based artificial feel system based on 1/2R V2 that is felt by the pilot through the control column and rudder pedals and is used commonly on aircraft with powered flight controls,
i.e., elevator, rudder, and ailerons.
It meets the requirements of progressive feel to match variable control surface deflection at a constant speed and/or for a constant angle of deflection at varying speeds. (See Q: How does an artificial feel system work? page 48.)
Control Surfaces
What are the inputs to Q feel?
- Static and dynamic pressure
2. Control surface angle of deflection
Control Surfaces
What are active controls?
An active control is a surface that moves automatically/actively in response to non-direct inputs.
For example, balance tabs actively/automatically
move in response to their associated control surfaces being moved.
Auto slats actively/automatically move to their full extend position to provide a better coefficient of lift (C L) on a B737 -300 ifthe aircraft senses a particular flight condition;
1 - trailing-edge flaps set 1 to 5 positions.
2 - auto slats in normal position.
3 - aircraft close to the stall.
