Aerodynamics: Forces & Weight Flashcards

Question

Describe the lift-weight pitching moments.

Answer 1

If lift and weight do not act through the same point, they create a moment causing a nose-up or nose-down pitch.

Answer 2

The ratio of the wing’s span to its geometric chord.

Answer 3

In general, during 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 of flight) plus 4. Air density

Answer 4

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.

Answer 5

The following devices increase the lift force produced by the wings: 1. Trailing edge flaps (Fowler flaps) increase lift at lower angles of deflection. 2. Leading edge flaps (Krueger flaps) and slats increase lift by creating a longer wing chord line, chamber, and area. 3. Slots (boundary layer control) prevent/delay the separation of the airflow boundary layer and therefore produce an increase in the coefficient of lift maximum.

Answer 6

Drag is the resistance to motion of an object (aircraft) through the air.

Answer 7

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 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 incidence 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. 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.

Answer 8

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 straightwinged 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.

Answer 9

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.

Answer 10

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. Figure 1.7 Thrust-drag couple pitching moments. 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.

Answer 11

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

Answer 12

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. Figure 1.8 spanwise airflow/vortices on the 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.

Answer 13

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.

Answer 14

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.

Answer 15

Fences and vortex generators. These items direct the airflow over the wing’s upper surface perpendicular to the leading edge.

Answer 16

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).

Answer 17

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.

Answer 18

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.

Answer 19

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.

Answer 20

The center of gravity (C of G, CG) is the point through which the total weight of a body will act

Answer 21

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 noseup 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.

Answer 22

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

Answer 23

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.)

Answer 24

When the pitching moment of the lift-weight couple is not balanced perfectly (see Q: Describe the lift-weight pitching moments, page 5), 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.

Answer 25

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 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

Answer 26

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. (See Q: What is longitudinal stability? page 33.) 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. (See Q: How does a forward center of gravity af ect 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 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. 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. (See Q: What is longitudinal stability? page 33.) 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.

Answer 27

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

Answer 28

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. Figure 1.10 Forward center of gravity position effect on stall speed. Conversely, the opposite is true: A center of gravity aft of the center of pressure will cause a lower stall speed.

Answer 29

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 controllability.

Answer 30

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.

Answer 31

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.

Answer 32

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. (See Qs: What is the ef ect 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 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.

Answer 33

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/second 2)

Answer 34

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).