Ace The Technical Pilot Interview 2/E Flashcards
1
Q
What are the forces acting on an aircraft in flight?
A
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. When 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.
2
Q
What produces the maximum glide range?
A
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.
3
Q
What is the effect of weight on the glide range?
A
The glide range does not vary with weight, provided that the aircraft is flown at its optimal angle of attack and speed for that weight, because the glide range is proportional to the lift-drag ratio, which does not vary with 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.
4
Q
What is rate of climb/ descent?
A
Rate of climb/ descent is the vertical component of the velocity of an aircraft and 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.
5
Q
What is the effect of weight on rate of descent?
A
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.
6
Q
What is an aerofoil?
A
An aerofoil is a body that gives a large lift force compared with its drag when set at a small angle to a moving airstream, e.g., aircraft wings, tailplanes, rudders, and propellers.
7
Q
What is an aerofoil chord line?
A
The chord line is a straight line from the leading edge to the trailing edge of an aerofoil.
8
Q
What is the mean chord line?
A
The mean chord line is the wing area divided by the wing span (sometimes referred to as the standard mean chord).
9
Q
What is the mean chamberline?
A
The mean chamberline is a line from the leading edge to the trailing edge of equidistance on the upper and lower surfaces of an aerofoil.
10
Q
What is the angle of incidence?
A
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.)
11
Q
What is angle of attack?
A
Angle of attack is the angle between the chord line of an aerofoil and the relative airflow.
12
Q
What is washout on a wing?
A
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.
13
Q
What is dihedral?
A
Dihedral is the upward inclination of a wing from the root to the tip.
14
Q
What is anhedral?
A
Anhedral is the downward inclination of a wing from the root to the tip.
15
Q
What is lift?
A
Lift is the phenomenon generated by an aerofoil due to pressure differences above and below the aerofoil.
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 farther 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.
16
Q
What is the formula for lift?
A
½ R V2 S CL ½ R = half the value of the air density V2 = airflow velocity squared S = wing plan area CL = coefficient of lift
The combined values of these properties determine the amount of lift produced.
17
Q
What is coefficient of lift (CL)?
A
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.
18
Q
Describe center of pressure.
A
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.
Center of pressure position/ angle of attack. 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.
19
Q
Describe the lift-weight pitching moments.
A
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.
A center of gravity forward of the center of pitch has a nose-down pitching moment. A center of gravity aft of the center of pitch has a nose-up pitching moment. The center of pitch 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.
20
Q
Describe aspect ratio.
A
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
21
Q
During what phase of flight is lift the greatest?
A
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:
- Configuration (flap setting)
- Speed of airflow over the wing
- Angle of attack (which is optimized during the takeoff stage of flight) plus
- Air density
22
Q
What is direct lift control?
A
The elevator/ stabilizer provides the direct lift control. The elevator and stabilizer 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. On some specific aircraft types, direct lift control also refers to an automatic varying of spoiler deployment to maintain a constant pitch attitude on the approach to land.
23
Q
What are high lift devices?
A
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/ delay the separation of the airflow boundary layer and therefore produce an increase in the coefficient of lift maximum.
24
Q
What is drag?
A
Drag is the resistance to motion of an object (aircraft) through the air.
25
Define the two major types of drag and their speed relationship.
Profile and induced drag = total drag Profile drag is also known as zero-lift drag and is comprised of 1. Form or pressure drag
2. Skin-friction drag
3. Interference drag
26
Tell me more about profile 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.
27
Tell me more about Induced Drag
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 incidence associated with higher speeds create smaller wing-tip trailing vortices that have a lower value of energy loss.
28
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, i.e., 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.
29
Describe the drag curve for a piston/ propeller aircraft.
For a piston-engined 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-engined 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.
30
Describe the drag curve on a jet aircraft.
The drag curve on a jet aircraft is the same as for 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
1. 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.
2. 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-engined 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.)
3. 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.
31
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.
32
What are high-drag devices?
The following devices increase the drag penalty on an aircraft:
1. Trailing edge flaps (in high-drag/ low-lift position)
2. Spoilers a. In flight detent, used as a speed brake b. On the ground, used as lift dumpers
3. Landing gear
4. Reverse thrust (ground use only)
5. Braking parachute
33
What causes/ are wing-tip vortices?
Wing-tip vortices are created by spanwise airflow over the upper and lower surfaces of a wing/ aerofoil that meet at the wing tips as turbulence and therefore induce drag, especially on a swept wing. spanwise 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 equalized 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 spanwise pressure gradient, i.e., pressure changing along the wing span.
34
What are the effects of spanwise airflow over a wing?
1. Creates wing-tip vortices.
2. Reduced aileron (wing control surface) efficiency.
3. Reversed spanwise airflow increases disturbed airflow on the wing’s upper surface at the tip, contributing to a wing-tip stall.
35
What are the effects of wing-tip vortices?
1. Create 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).
2. Vortices create turbulence, which may affect the safety of other aircraft within approximately 1000 ft below or behind the aircraft.
3. Downwash affects the direction of the relative airflow over the tailplane, which affects the longitudinal stability of the aircraft.
36
How do you prevent spanwise 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.
37
What is the purpose of vortex generators/ fences?
To reduce spanwise airflow and thereby reduce its effects. One of the effects of spanwise 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 and so denser airflow perpendicular to the wing, which helps to maximize the effectiveness of the control surfaces, especially the ailerons. Fences also help to maximize the effectiveness of the control surfaces in a similar yet cruder manner. However, they are used normally to reduce the reverse spanwise 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).
38
What are winglets, and how do they work?
Winglets are aerodynamic efficient surfaces located at the wing tips. They are designed to reduce induced drag. They dispense the spanwise airflow from the upper and lower surface often at different points, depending on the particular design, thus preventing the intermixing of these airflows that otherwise would create induced drag vortices.
39
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.
40
What are the effects of excessive aircraft weight?
If the limiting weight of an aircraft is exceeded, the following effects are experienced:
1. 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.
2. Stalling speed is increased.
3. Maneuverability is reduced.
4. Wear on tires and brakes is increased.
5. Structural safety margins are reduced.
41
Describe center of gravity.
The center of gravity (C of G, CG) is the point through which the total weight of a body will act.
42
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.
43
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 × arm
44
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.
Note: Lift forward of weight has a nose-up pitching moment, which is counterbalanced by the downward deflection of the horizontal tailplane, which creates a nose-down counterpitch. 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.
45
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.)
46
What are the reasons/ effects of keeping a center of gravity inside the forward position of its limits?
The forward position of the center of gravity is limited to
1. Ensure that the aircraft is not too nose heavy so that the horizontal tailplane has a sufficient turning moment (in the pitching plane) available to overcome its natural longitudinal stability.
2. Ensure that the aircraft’s pitch control (rotation and flare) is not compromised, with high stick forces (tailplane turning moment), by restricting the aircraft’s tailplane arm forward center of gravity limit. (Remember, tailplane moment (stick force) = arm × weight.) Note that this is particularly important at low speeds (i.e., takeoff and landing), when the elevator control surface is less effective.
3. Ensure a minimum horizontal tailplane deflection, which produces a minimal download airforce on the tailplane and 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 maneuverability is maintained. The aft position of the center of gravity is limited to
47
What are the reasons/ effects of keeping a center of gravity inside the aft position of its limits?
The aft position of the center of gravity is limited to
1. Ensure that the aircraft is not too tail heavy so that the horizontal tailplane has a sufficient turning moment available to make the aircraft longitudinally stable.
2. Ensure that enough pitch control stick forces (tailplane turning moment) are adequately felt through the control column by guaranteeing the aircraft’s tailplane arm to an aft center of gravity limit. (Remember, moment (stick force) = arm × weight.)
3. Ensure a minimum horizontal tailplane deflection, which produces a minimal upload airforce on the tailplane and 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 maneuverability is maintained.
48
What are the effects of a center of gravity outside its forward limits (range)?
Summary:
Stability- More
Stick Forces - More
Drag - More
If the center of gravity is outside its forward limit, the aircraft will be nose heavy, and the horizontal tailplane will have a long moment arm (tailpipe to center of gravity point) that results in the following:
1. Longitudinal stability is increased because the aircraft is nose heavy.
2. The aircraft’s pitch control (rotation and flare) is reduced or compromised because it experiences high stick forces due to the aircraft’s long tailplane moment arm. [Remember, tailplane moment (stick force) = arm × weight.]
3. A large balancing download is necessary from the horizontal tailplane by deflecting the elevator or stabilizer. 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 download on the horizontal tailplane, which increases 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 maneuverability 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.
Generally, the aircraft is heavy and less responsive to handle in flight and requires larger and heavier control forces for takeoff and landing.
49
What are the effects of a center of gravity outside its aft limits (range)?
Summary:
Stability- Less
Stick Forces - Less
Drag - Less
If the center of gravity is outside its aft limit, the aircraft will be tail heavy, and the horizontal tailplane will have a short moment arm (tailplane to center of gravity point) that results in the following:
1. The aircraft is longitudinally unstable because it is too tail heavy for the horizontal tailplane turning moment to balance.
2. The aircraft’s pitch control (rotation and flare) is increased (more responsive) because it experiences light stick forces due to the aircraft’s short tailplane arm. [Remember, tailplane moment (stick force) = arm × weight.] This lends itself to the possibility of overstressing the aircraft by applying excessive g forces.
3. A large balancing upload is necessary from the horizontal tailplane by deflecting the elevator or stabilizer. 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 maneuverability 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.
Generally, the aircraft is effectively lighter and more responsive to handle in flight and requires smaller and lighter control forces for takeoff and landing.
50
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 stabilizer 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 stabilizer 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.
51
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 stabilizer 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.
52
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. 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 controllability.
53
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. The three reasons for a change in the aircraft’s weight are
1. 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.
2. Passenger movement.
3. High speeds. This is so because the greater the speed, the greater 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, or in other words, by reducing the downward force on the tailplane. This reduced downward force on the tailplane reduces the effective weight of the tailplane section and thereby of the aircraft.
54
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 magnitude 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-engined aircraft in all its handling maneuvers, climbs, descents, and turns because of its greater weight and velocity.
55
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, remember, momentum = mass × velocity. Therefore, for a constant indicated air speed (IAS) or Mach number (i.e., its V/ MMO), the heavier aircraft will have to maintain a shallower rate of descent to check its momentum. 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 100-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.
56
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.
Note: G force is a unit of measurement that is equivalent to the acceleration caused by the earth’s gravity (32.174 feet/ secondSq).
57
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).
58
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 many interactive compromises that culminate in directly fulfilling the wing’s high-speed requirement and inversely the wing’s lift and structural requirements.
59
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 normal (perpendicular) (AC) to the leading edge a shorter distance than the chordwise resultant (AB). Since the wing is responsive only to the velocity vector normal to the leading edge, the effective chordwise 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 chordwise component becomes sonic, and thus the critical number is raised.
60
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.
61
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
(2) stability in turbulence.
1. 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 straight-winged aircraft experiences sonic disturbed airflow, resulting in a loss of lift at relatively low speeds. Therefore, a different wing design was required for the aircraft to be able achieve higher cruise speeds. 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.
2. 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.
62
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:
1. Poor lift qualities are experienced because the sweep-back design has the effect of reducing the lift capabilities of the wing.
2. Higher stall speeds are a consequence of the poor lift qualities of a swept wing.
3. 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.)
4. 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.
63
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 spanwise 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: 1. Greater chamber at the tip; this increases airflow speed over the surface, which delays the stall. 2. 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.
64
Explain speed stability.
Speed stability is the behavior of the speed after a disturbance at a fixed power setting. The behavior 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:
1. An increase in speeds leads to an increase in drag, thus causing a return to the original speed.
2. A decrease in speed leads to a decrease in drag, thus causing a return to the original speed.
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:
1. A decrease in speed leads to an increase in drag, which causes a further decrease in speed, thus causing a negative speed divergence.
2. An increase in speed leads to a decrease in drag, which causes a further increase in speed, thus causing a positive speed divergence.
65
What is Mach number?
Mach number (MN) is a true airspeed indication, given as a percentage relative to the local speed of sound; e.g., half the speed of sound = 0.5 Mach.
66
What is the critical Mach number (Mcrit)?
Mcrit is the aircraft’s Mach speed at which the airflow over a wing becomes sonic—critical Mach number. The aircraft’s Mach speed is lower than the airflow speed over a wing. A typical Mcrit 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.
67
Describe the characteristics of critical Mach number (Mcrit)?
1. Initial Mach buffet, caused by the shock waves on the upper surface of the wing as the aircraft approaches Mcrit, is usually experienced.
2. 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.
3. A nose-down change in attitude (Mach tuck) occurs at or after Mcrit.
4. A possible loss of control.
68
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:
1. 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 chordwise and causes a rearward shift in the center of the lift (center of pressure).
2. 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.
69
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 (see preceding question), which induces the aircraft to pitch down (or the aircraft’s nose to tuck).
70
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.
71
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 signaling a proportional upward movement of the elevator or variable-incidence stabilizer 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.
72
What are the effects of compressibility?
Compressibility 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. There are two main effects of compressibility:
1. Compressibility error on dynamic pressure reading flight instruments; e.g., air speed indicator shows an overread error that is greater the higher the aircraft’s speed.
2. 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.
73
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-engined 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-engined propeller aircraft enjoys a relatively large margin between VNO and VDF and has very little overspeed 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 overspeed tendency. Therefore, the speed margin on a jet aircraft is very significant.
74
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 1.2/ 3 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.
75
What is coffin corner?
Coffin corner occurs at an aircraft’s absolute ceiling, where the speeds at which Mach number buffet and prestall 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.
Mach number and the slow speed stall buffet are coincident at coffin corner because a stall is a function of indicated air speed (IAS) 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 IAS decreases with altitude due to the decreasing LSS. To prevent the IAS decreasing to its stall speed, the Mach number must be increased, which results in an increasing IAS.
For a constant IAS, the Mach number increases with altitude due to a decreasing LSS and temperature to a point where the IAS exceeds Mcrit. To prevent the Mach number 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 attainable altitude by the aircraft.
76
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 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 and/ or move the control column forward).
77
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.
1. Weight
a. Actual weight
b. Load factor, g in a turn
c. Effective weight/ center of gravity position
2. Altitude
3. Wing design/ lift
4. Configuration
5. Propeller engine power
78
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.
79
What wing design areas delay the breakup of airflow (stall)?
1. Wing slots are the main design feature that delays/ suppresses stall speed. A slot is a form of boundary layer control that reenergizes 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.
2. Lower angle of incidence and a greater chamber for a particular wing section, e.g., wing tips.
80
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 unstalled 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.
81
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 1. 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. 2. Compressibility error on the IAS/ ASI( R). The compressibility correction that forms part of the difference between the indicated airspeed (IAS) and airspeed indicator (reading) [ASI( R)] (which is uncorrected) and equivalent airspeed (EAS, which is IAS corrected for compressibility and position instrument error) is larger in the EAS to IAS/ ASIR direction due to the effect of the Mach number, resulting in a higher IAS stall speed.
82
What is a superstall?
A superstall 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:
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.
2. An ineffective tailplane:
An ineffective tailplane makes the aircraft incapable of recovering from the stall condition, which is due to 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 tailplane’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.
83
What systems protect against a stall?
Stall warner’s and stick pushers. Stall warners are either an artificial audio warning and/ or a stick shaker, which usually are activated at or just before the onset of the prestall buffet. Stick pushers are normally used only on aircraft with superstall qualities and usually activate after the stall warning but before the stall, giving an automatic nose-down command. Both systems normally receive a signal from an incidence-measuring probe.
84
What is Dutch roll?
Dutch roll is an oscillatory instability associated with swept-wing jet aircraft. It is the combination of yawing and rolling motions. 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.
85
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 uncommanded 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 of these phenomena will create an increased airflow speed over the wing’s upper surface, which produces more lift and increases its angle of attack. At the same time, the inner wing will travel slower and, in effect, become more swept relative to the airflow, and both these phenomena will reduce
