Pilot's Handbook of Aeronautical Knowledge
Chapter 5: Aerodynamics of Flight
Forces Acting on the Aircraft
Understanding how the four forces (thrust, drag, lift, and weight) work, and knowing how to control them with the use of power and flight controls, are essential to flight. Design, weight, load factors, and gravity affect an aircraft during flight maneuvers.
- Thrust is the forward force produced by the powerplant/propeller or rotor. It opposes or overcomes the force of drag.
- Drag is a rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. As a general rule, drag opposes thrust.
- Lift is a force that is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the flight path. Lift opposes weight.
- Weight is a force that pulls the aircraft downward because of the force of gravity applied to the aircraft's mass. Weight opposes lift.
There can be no unbalanced forces in steady, straight flight — flying level, when climbing, or when descending. Newton’s Third Law states that for every action or force there is an equal, but opposite, reaction. The sum of the opposing forces is always zero. Adding total thrust reduces the effect of total drag, and adding lift always reduces the effect of weight.
In straight, level, unaccelerated flight, the opposing lift/weight forces are equal. Note however, that some thrust can contribute to lift, and some drag can contribute to weight. In glides, some of the weight vector contributes to thrust. Therefore, rather than stating "lift vs. weight," the accurate statement is the sum of all upward forces vs. the sum of all downward forces are equal in straight, level, unaccelerated flight.
In order to maintain a constant airspeed, thrust and drag must be equal. In order to maintain a constant altitude, lift and weight must be equal.
When thrust is reduced, airspeed slows until thrust and drag are equal, at which point the aircraft flies at a constant airspeed. The opposite is true when thrust is increased.
Angle of Attack (AOA) is the angle between the chord line of the airfoil and the direction of the relative wind.
Lift varies with thrust and AOA. A high AOA at a low airspeed can produce level flight. A low AOA at a high airspeed also can result in level flight. The speed regimes are grouped as low- speed flight, cruising flight, and high-speed flight. At very high speeds and level flight, it is even possible to have a slightly negative AOA. If speed decreases enough, the required AOA will increase to the critical AOA. Any further increase in the AOA will result in the wing stalling.
As the AOA increases, lift increases (all other factors being equal).
For a given airfoil, lift (L) is determined through the relationship of:
- the air density (p)
- the airfoil velocity (V)
- the surface area of the wing (S)
- the coefficient of lift (CL) .
Thus:
(p V S CL) / 2 = lift
Lift is proportional to the square of the aircraft's velocity. For example, an airplane traveling at 200 knots has four times the lift as the same airplane traveling at 100 knots, if the AOA and other factors remain constant. Pilots have direct control over angle of attack and airspeed. They can change the wing with deployment of flaps. Air density cannot be changed, except if a pilot selects a different altitude or route that offers different conditions.
For every AOA there is a corresponding airspeed required to maintain altitude in steady, unaccelerated flight. The only method of increasing lift is by increasing velocity if the AOA is held constant.
Lift varies directly with wing area. A wing twice as large (of same design) as a standard wing will provide twice as much lift.
The lift-to-drag ratio (L/D) is the amount of lift generated by a wing or airfoil compared to its drag. It is a measure of airfoil efficiency. It is determined by dividing the coefficient of lift by the coefficient of drag.
If the aircraft is operated in steady flight at L/D Max, the total drag is at a minimum. This is considered to be where the aircraft will have its best fuel efficiency, or "maximum range."
Drag is the force that resists movement of an aircraft through the air. Parasite drag provides no benefit to flight and is an unwanted byproduct that can be reduced through streamlining. Induced drag is a byproduct of lift.
Parasite drag can be broken down into three categories:
- Form drag is the portion of parasite drag generated by the aircraft due to its shape and airflow around it. As air separates around the engine cowling, antennas, etc., it will seek to rejoin. Form drag is a result of this inefficiency.
- Interference drag comes from the intersection of airstreams that creates eddy currents, turbulence, or restricts smooth airflow. The most interference drag is observed when two surfaces meet at perpendicular angles. Fairings are used to reduce interference drag.
- Skin-friction drag is the aerodynamic resistance due to the contact of moving air with the surface of an aircraft. Every surface has some roughness, even at a microscopic level. The area between the wing and the free-stream velocity level is about as wide as a playing card and is called the boundary layer. Aircraft designers will remove protrusions and use flush-mount rivets to reduce skin-friction drag. Paint and dirt also contribute, so very efficient airplanes will use well-polished aluminum surfaces.
Induced drag is an aerodynamic penalty, based on the inefficiency of the wing's generation of lift. At the wingtips, the air pressure above and below the wing tends to equalize. The lateral flow outward from underneath the wing imparts a rotational velocity, creating vortices that trail behind the airplane. The downwash flow behind the trailing edge is the source of induced drag. The downwash over the top of the airfoil at the tip bends the lift vector rearward, creating induced drag.
Induced drag increases proportionally with increased AOA. Low airspeeds require higher AOA, and thus have more induced drag. Induced drag varies inversely with the square of the airspeed, while parasite drag increases as the square of the airspeed.
Gravity pulls all objects toward the center of the earth. An aircraft's weight is concentrated at the center of gravity (CG), while lift forces occur at the center of pressure (CP) Aircraft designers fix the aft limit of the CG in front of the CG to maintain equilibrium in flight.
Wingtip Vortices
Wingtip vortices create wake turbulence. Aircraft should avoid encountering wake turbulence from other aircraft. This requires pilots to not fly directly across another plane's flight path, and also not to follow another airplane too closely. When landing after a large aircraft, small airplanes should remain above the glideslope and touch down after the preceding aircraft's point of landing. When departing, the small aircraft should rotate and climb prior to the preceding aircraft's rotation point. At multi-runway airports, pilots also should take precautions to avoid vortices that drift across runways. Vortices will drift 1,000 FPM in 10 knots of wind.
Ground Effect
Ground effect the result of air trapped between the runway and the wing when the airplane is operating close to the ground. This is because the ground interferes with the airflow patterns around the wing — resulting in a reduction of upwash, downwash, and wingtip vortices. Less thrust and AOA are required to maintain lift in ground effect. Thus, an aircraft can become airborne in ground effect with less airspeed than is required to climb out of ground effect.
Ground effect extends up to the the length of its wingspan, where reduction in drag is minimal (1.4%). Induced drag is markedly more reduced at one-quarter of the aircraft's wingspan (23.4%) and one-tenth the wingspan (47.6%). The manufacturer's recommended takeoff speed is necessary to provide adequate initial climb performance. The aircraft becoming airborne is not a positive indication of flight out of ground effect. When landing, any excess airspeed may create a "float" effect beyond the intended touchdown point. A reduction in power is necessary to offset the increase in lift caused by ground effect.
Axes of an Aircraft
The aircraft's three axes — longitudinal, lateral, and vertical — pass through the aircraft at the center of gravity (CG) When an aircraft changes its attitude or position in flight, it rotates around one or more of the axes. [5-18]
Moment and Moment Arm
The mathematical measure of an aircraft's tendency to rotate about its CG is called a moment. It is the product of a force applied and the distance from which the force is applied. A "moment arm" is the distance from a reference point ("datum") to the to the applied force.
Aircraft Design Characteristics
Stability is the inherent quality of an aircraft to correct for conditions that may disturb its equilibrium and to return to or to continue on the original flight path. Static stability can be broken into three categories, or qualities:.
- Positive static stability: The initial tendency of the aircraft to return to its original state of equilibrium after being disturbed.
- Neutral static stability: The initial tendency of the aircraft to remain in a new condition after its equilibrium has been disturbed.
- Negative static stability: The initial tendency of the aircraft to continue away from the original state of equilibrium after being disturbed.
Dynamic stability refers to the aircraft response over time when disturbed from a given pitch, yaw, or bank, also with three subtypes:
- Positive dynamic stability: The motion of the displaced object decreases in amplitude over time. The object displaced returns toward the equilibrium state.
- Neutral dynamic stability: Once displaced, the displaced object neither decreases nor increases in amplitude.
- Negative dynamic stability: Over time, the motion of the displaced object increases and becomes more divergent.
Stability affects both maneuverability and controllability. A maneuverable aircraft can be maneuvered easily and withstand the stresses imposed by maneuvers. A controllable aircraft responds to the pilot's control, especially with regard to flight path and attitude.
Longitudinal stability refers to the aircraft's pitching motion — the quality that makes an aircraft stable about its lateral axis. An aircraft with longitudinal instability can be difficult and dangerous to fly. Static longitudinal stability depends upon aircraft design — specifically, the location of the wing and tail surfaces relative to the CG, and the size of the tail surfaces.
Most aircraft are designed slightly nose-down. The horizontal stabilizer is set with a negative AOA to counter this. Downwash from the wings also creates a downward pressure on the horizontal stabilizer. At slow airspeeds, the downwash is reduced, and the aircraft will tend to pitch down. As speed increases in a nose-low attitude, the downwash once again increases the pitch. This exchange in forces creates oscillation. If the oscillations eventually cease, the aircraft has positive longitudinal stability.
Lateral stability refers to the aircraft's longitudinal axis, which is rotated during a roll. Positive lateral stability dampens the roll effect of the aircraft in straight-and level flight, achieved via aircraft design:
- Dihedral places the outer tips of the wings in a higher position than at the wing root. In a sideslip, the low wing will have a higher AOA, while the high wing will have a lower AOA.
- Sweptback wings and high wings contribute to dihedral, and thus lateral stability.
- Keel effect is created by high wings, which cause the fuselage to behave like the keep on a ship. The weight of the fuselage acts like a pendulum.
- Weight distribution: When an aircraft's keel area is mostly above the CG, lateral stability is enhanced.
The vertical stabilizer and fuselage contribute to the aircraft's directional stability, which is around the yaw axis. These act like a weather-vane, keeping the aircraft pointed into the wind.
Dutch roll is a coupled instability, with directional and lateral oscillations. The aircraft can make a figure-eight pattern with the nose, created by oscillations in both roll and yaw. Because dutch roll is undesirable in aircraft design, most have spiral instability.
Spiral instability creates an over-banking tendency, as the high wing in a slip gains speed, and thus more lift At the same time, directional stability will cause the aircraft to pitch down. This is easily controlled by the pilot, but also why an aircraft cannot be flown "hands-off" without wing-leveler or autopilot technology. Failure to recover from spiral instability (the "graveyard spiral") can have catastrophic results.
Effect of Wing Planform
Planform is the shape of the wing as viewed from directly above and deals with airflow in three dimensions. Aspect ratio, taper ratio, and sweepback are factors in planform design.
An increase in aspect ratio (the length of the wing compared to its width) provides less drag, but also adds weight. Conversely, a wing with smaller aspect ratio will have more drag but weigh less. Most aircraft design find a compromise in wing aspect ratio that balances the negative effects of weight and drag.
Tapering the wing's aspect ratio — the length of the chord line — from the root to the tip, also increases lift, reduces drag, and reduces weight. Training airplanes typically have high aspect ratios and are forgiving a slow speeds. Faster aircraft with smaller wings and sweepback are more difficult to fly.
Elliptical wings are the most efficient, but also difficult to construct. Elliptical wings also have undesirable control and stall characteristics. Rectangular and tapered wings can employ twist and variation to achieve similar low-drag efficiencies, and they have better stall characteristics for GA training and flight operations.
Aerodynamic Forces in Flight Maneuvers
In a turn, lift acts in the direction of the turn (centripetal force). Lift is separated into vertical and horizontal components. Centrifugal force is the equal and opposite reaction of the horizontal component of lift.
In a turn, since total lift is divided between horizontal and vertical components of lift, the vertical component of lift is reduced, and the aircraft will descend. AOA can be increased to restore the vertical component of lift and maintain altitude in the turn. Additional thrust must be applied to maintain a constant airspeed.
The horizontal component of lift is proportional to the angle of bank. Since a higher AOA will reduce airspeed, additional thrust is required to maintain airspeed. This additional thrust also is proportional to the angle of bank.
An increase in airspeed in a turn will increase the turn radius. Thus, the slower the airspeed in a turn, the smaller the turn radius.
Centrifugal force acts proportionally opposite of the horizontal component of lift. As airspeed increases in a turn, the turn radius increases, and centrifugal force increases..
In a slipping turn, the aircraft is yawed to the outside of the turning flight path, while the aircraft is pulled to the inside of the turn. Horizontal lift exceeds centrifugal force. In a skidding turn, the aircraft is yawed to the inside of the turn, while the aircraft is pulled to the outside of the turn, and centrifugal force exceeds horizontal lift. [5-35]
A standard-rate turn is 3-degrees per second. The angle of bank varies with airspeed, with greater angles of bank required at higher airspeeds. A steep turn and standard-rate turn are virtually identical at 400 mph.
While an aircraft will have an increase in AOA upon takeoff, during a steady-state climb-out AOA, the wings generate the same amount of lift as in level flight, while airspeed is diminished unless additional thrust is applied. The downward weight component contributes to total drag until steady-state climb is achieved, when thrust and drag are equal. [5-36] [5-37]
Climb is dependent upon the aircraft's excess thrust that can overcome drag and a component of weight (or all weight, in a straight-up climb). When an aircraft no longer has excess thrust to climb, it has reached its absolute ceiling.
In a descent, at first the AOA is decreased momentarily. Airspeed increases as the flight path changes. To descend at the same airspeed as straight-and-level flight, power must be removed.
An aerodynamic stall occurs when the critical AOA is exceeded and there is a separation of airflow across the wing's surface. The does not completely stop generating lift, but it cannot sustain level flight.
The Component of Lift (CL) approaches its maximum (CL-max) as AOA increases and then rapidly falls off. In straight-wing aircraft, the stall originates at the wing root so that aileron effectiveness is maintained. This is possible because of the wing design, which may use wing-twisting at the root or stall strips.
Because CL is aft of CG in GA aircraft, stalls will result in a nose-low attitude and restoration of lift. Improperly loading an aircraft, particularly with the CG too far aft of the CL, may make recovery from a stall impossible.
Stalling speed can vary, but a wing always stalls at a given AOA. Based on aircraft design, critical AOA can range between 16 and 20 degrees.
Critical AOA is most commonly exceeded at low speed, high speed, and turns.
In recovery from a high-speed dive, the increase in AOA can cause a stall at a higher airspeed than normal.
In level turns, centrifugal force increases the aircraft's weight, requiring additional lift to maintain level flight. .
In a stall, downwash on the horizontal stabilizer becomes ineffective. This, combined with the CG forward of the CL, causes the aircraft's nose to pitch down, decreasing AOA and restoring lift.
Ice, snow, and frost will cause the boundary layer to separate from the wing at a higher airspeed than the stall speed over a smooth wing. Ice also increases weight and drag.
Angle of Attack Indicators
The FAA is promoting AOA indicators as one of many safety initiatives to reduce loss-of-control accidents. Reliance on the airspeed indicator and 1G stall speed does not account for the range of flight conditions in which a stall can occur. 1G stalls occur in level, coordinated flight at max-gross weight. Without an AOA indicator, stall speed is invisible.
Basic Propeller Principles
A propeller is two twisted airfoils that provide thrust. A fixed-pitch propeller cannot have its pitch adjusted by the pilot and is most efficient in cruising speed.
A propeller blade is shaped like an airfoil, with an upper and lower camber, with the area of decreased pressure in front of the propeller. Blade angle determines the mass of air handled by the propeller.
Propeller efficiency is the ratio of thrust horsepower and brake horsepower. Geometric pitch is the theoretical distance a propeller should advance in one revolution, while effective pitch is the distance it actually advances. Effective pitch includes propeller slippage in the air.
The twist of the propeller accounts for the fact that the outer portion of the propeller travels faster than the inward portion. The twist permits relatively constant AOA along the length of the propeller in cruising flight, which equalizes thrust along the length of the propeller.
Constant-speed propellers use low blade-angles (low AOA) during takeoff, thus handling a smaller mass of air per revolution. This permits the engine to turn at high RPM. Despite the smaller mass of air, the high RPM permits a climb. At higher speeds, a higher blade angle (high AOA) keeps the propeller efficient with respect to the relative wind, as a higher mass of air is handled per revolution. In cruise, RPM and fuel consumption are decreased.
Left-turning tendency at high power is caused by torque reaction, spiraling slipstream, gyroscopic precession, and P-factor.
- Torque reaction accounts for the longitudinal axis moving opposite the effect of the propeller and crankshaft on rollout (per Newton's Third Law of Physics). In flight, this will induce a roll to the left. (In United States aircraft, the propeller rotates clockwise, as viewed from the pilot's seat, counter-clockwise as viewed from the front of the aircraft.)
- Spiraling slipstream wraps over the top of the wings and under the fuselage, striking the left side of the vertical stabilizer.
- Gyroscopic precession refers to the deflection of a spinning rotor when force is applied to its rim. The resulting force appears 90 degrees ahead of the rotation. This is most prominent in tailwheel aircraft. Pilots must use both elevator and rudder to counter adverse pitching and yawing.
- Propeller factor (P-factor) refers to the fact that the downward blade of the propeller takes a bigger bite of air than the upward blade of the propeller, when operating at high AOA. Also referred to as "asymmetric loading."
Load Factors
Load Factor is a measure of force applied to a body when it is accelerated. Any force applied to an aircraft to deflect it from flight in a straight line will produce stress upon the structure. This force is measured in "Gs", with 1G representing the effect of gravity on a body at rest. GA aircraft are designed to withstand G-loading, to certain limits.
"Load Limit Factors" are the highest load factors that a given aircraft can be expected to experience under normal operating situations. Regulations require that aircraft be designed to withstand 150% of their load limit factors, which is known as the "Factor of Safety". This strength reserve should not be explored by pilots.
Maneuvering Load Factors are split into three categories, by certification.
- Normal: 3.8g to -1.52g
- Utility: 4.4g to -1.76g
- Acrobatic: 6g to -3g
For the above load limit factors, a 150% factor of safety is applied. These categories do not apply to aircraft build before the categories were introduced. Typically, older airplanes without an operational-category placard are comparable to the utility category if below 4,000 lbs.
In turns, load factor is generated by a combination of centrifugal force and weight [5-52]. Load factor increases rapidly after the bank has reached 50 degrees. At steeper banks (60 = 2g, 80 = 5.76g), the wing must produce more lift to maintain level flight. (Note: Load factor begins to increase beyond 1g at 30 deg of bank.)
An aircraft's stall speed increases in proportion to the square root of the load factor. Thus, an airplane that stalls at 35 knots will stall at 70 knots in a 4g turn. [5-54] Pilots should be aware of the risk of stalling the aircraft with increased load factors. Pilots also should not stall the aircraft above its maneuvering speed, which would impose a tremendous load factor.
Design Maneuvering Speed (Va) is the speed at which a single flight control can be moved to its operational limit, in smooth air, without risk of damaging the airplane. Va is established the Airplane Flight Manual (AFM) or Pilot's Operating Handbook (POH). In older GA airplanes, VA typically is 1.7 times normal stalling speed.
Maneuvers that carry a risk of excessive load factor include turns, stalls, spins, high-speed stalls, chandelles, and lazy eights. Rough air also carries a risk of excessive G-loading. In extremely rough air, airspeed should be reduced to below Va.
Limit load is a force applied to an aircraft that causes a bending of the aircraft structure that does not return to the original shape. Ultimate load is the load factor applied to the aircraft beyond the limit load and at which point the aircraft material experiences structural failure.
A VG diagram presents the flight operating strength of the aircraft on a scale with airspeed and load factor metrics. [5-55]
The Rate of Turn (ROT) is the number of degrees (expressed in degrees per second) of heading change that an aircraft makes. When airspeed is increased and bank angle is not changed, the rate of turn is reduced — the aircraft will take longer to complete a 360-degree turn.
The Radius of Turn is directly linked to the ROT. If airspeed increases and the bank angle is not changed, the radius of turn also increases — the area of the turn becomes larger.
Aircraft are certificated for weight and balance so that the effect of weight applied to the structure is known, and so that the effect of weight on flight characteristics is known.
Commercial Pilot & Flight Instructor Test Questions
The angle between the chord line of an airfoil and the relative wind is known as the angle of attack.
The angle between the chord line of the wing and the longitudinal axis of the aircraft is known as the angle of incidence.
If a pilot applies right rudder to a stable airplane, the tail deflects left and the nose moves right.
A line drawn from the leading edge to the trailing edge of an airfoil and equidistant at all points from the upper and lower contours is called the mean camber line.
Lift produced by an airfoil is the net force developed perpendicular to the relative wind.
During flight with zero angle of attack, the pressure along the upper surface of the wing would be less than atmospheric pressure.
— This is because the shape of the airfoil still creates a reduction in pressure.
The angle of attack of a wing directly controls the distribution of positive and negative pressure acting on the wing.
That portion of the aircraft's total drag created by the production of lift is called induced drag, and is greatly affected by changes in airspeed.
As airspeed increases in level flight, total drag of an aircraft becomes greater than the total drag produced at the maximum lift/drag speed because of the increase in parasite drag.
— Parasite drag increases as the square of the airspeed.
As airspeed decreases in level flight, total drag of an aircraft becomes greater than the total drag produced at the maximum lift/drag speed because of increase in induced drag.
— Higher angles of attack create more induced drag.
The resistance, or skin friction, due to the viscosity of the air as it passes along the surface of a wing is called profile drag.
Which relationship is correct when comparing drag and airspeed? Induced drag varies inversely at the square of the airspeed.
The airspeed that allows a pilot to glide for the greatest distance in still air occurs at the angle of attack which produces the maximum lift over drag (L/D max).
The force which imparts a change in the velocity of a mass is called thrust.
If an increase in power tends to make the nose rise, this is the result of the line of thrust being below the center of gravity.
—The point of application of the force that pulls an airplane through the air is its thrust line.
How can a pilot increase the rate of turn and decrease the radius at the same time? Steepen the bank and decrease airspeed.
As the angle of bank is increased, the vertical component of lift decreases and the sink rate increases.
— This is true if angle of attack remains unchanged.
What action is necessary to make an aircraft turn? Change the direction of lift.
— Angled lift has both vertical and horizontal components.
When considering the forces acting upon an airplane in straight-and-level flight, at a constant airspeed, which statement is correct? Weight always acts vertically toward the center of the earth.
During a steady climb, the rate of climb depends upon excess power.
During a steady climb, the angle of climb depends upon excess thrust.
Which statement is true regarding the forces acting on an airplane in a steady-state climb? The sum of all upward forces is equal to the sum of all downward forces.
— Note that G-loading is not present in steady-state climb.
Which statement describes the relationship of the forces acting on an aircraft in a constant-power and constant-airspeed descent? Thrust is equal to drag; lift is equal to weight.
Adverse yaw during a turn entry is caused by decreased induced drag on the lowered wing and increased induced drag on the raised wing.
When rolling out of a steep-banked turn, what causes the lowered aileron to create more drag than when rolling in to a turn? The wing's angle of attack is greater as the rollout is started.
The tendency of an aircraft to develop forces which restore it to its original condition, when disturbed from a condition of steady flight, is known as stability.
The quality of an aircraft that permits it to be operated easily and to withstand the stresses imposed on it is maneuverability.
The capability of an aircraft to respond to a pilot's inputs, especially with regard to flightpath and attitude, is controllability.
The initial tendency of an aircraft to develop forces that further remove the aircraft from its original position, when disturbed from a condition of steady flight, is known as negative static stability.
If the aircraft's nose initially tends to return to its original position after the elevator control is pressed forward and released, the aircraft displays positive static stability.
If the aircraft's nose initially tends to move farther from its original position after the elevator control is pressed forward and released, the aircraft displays negative static stability.
If the aircraft's nose remains in the new position after the elevator control is pressed forward and released, the aircraft displays neutral static stability.
The most desirable type of stability for an aircraft to possess is positive dynamic stability.
If the airspeed increases and decreases during longitudinal phugoid oscillations, the aircraft is maintaining a nearly constant angle of attack.
— Phugoid oscillations have long cycles (10 seconds to 100 seconds) and usually can be trimmed out. Short-period oscillations introduce the risk of structural failure if not brought under control.
If an aircraft has negative dynamic and positive static stability, this will result in divergent oscillations.
— Negative dynamic stability will cause static corrections caused by positive static stability to increase.
An airplane would have a tendency to nose up and have an inherent tendency to enter a stalled condition when the center of pressure is forward of the center of gravity.
A swept-wing airplane with weak static directional stability and increased dihedral causes an increase in dutch roll tendency.
Changes in the center of pressure of a wing affect the aircraft's aerodynamic balance and controllability.
The purpose of an aircraft wing dihedral angle is to increase lateral stability.
— Lateral stability refers to the airplane's longitudinal axis, and thus its roll tendency. The stability is engineered along the lateral axis.
Which aircraft characteristics contribute to spiral instability? Strong static directional stability and weak dihedral effect.
— When a gust causes a sideslip, static stability yaws the nose into the relative wind, causing a bank. A weak dihedral effect will then fail to return the plane to level flight.
Maximum gliding distance of an aircraft is obtained when induced drag and parasite drag are equal.
Your flight takes you in the path of a large aircraft. In order to avoid the vortices you should fly above the flight path of the large aircraft.
If severe turbulence is encountered, the aircraft's airspeed should be reduced to maneuvering speed.
If an airplane's gross weight is 3,250 pounds, what is the load acting on this airplane during a 60-degree banked turn? 6,500 pounds.
— A 60-degree turn is a 2 G maneuver.
An airplane has a normal stalling speed of 60 knots but is forced into an accelerated stall at twice that speed. What is the maximum load factor that will result from this maneuver? 4 Gs.
— This question may require a chart for reference.
The angle of attack at which an airplane stalls remains constant regardless of gross weight.
As altitude increases, the indicated airspeed at which a given airplane stalls in a particular condition will remain the same as at a low altitude.
— The airplane at altitude will stall at the same indicated airspeed as at sea level. However, the true airspeed will be greater.
Which statement is true relating to the factors which produce stalls? The stalling angle of attack is independent of the speed of airflow over the wings.
The critical angle of attack at which a given aircraft stalls is dependent upon the design of the wing.
Which action will result in a stall? Exceeding the critical angle of attack.
Which statement is true concerning the aerodynamic conditions which occur during a spin entry? After a full stall, the wing that drops continues in a stalled condition, while the rising wing regains and continues to produce some lift, causing the rotation.
Which characteristic of a spin is not a characteristic of a steep spiral? Stalled wing.
A rectangular wing, as compared to other wing planforms, has a tendency to stall first at the wing root providing adequate stall warning.
— Aileron control remains effect as the stall develops.
Which subsonic planform provides the best lift coefficient? Elliptical wing.
— Each square foot of an elliptical wing produces the same lift pressure.
On which wing planform does the stall begin at the wingtip and progress inward toward the wing root? Sweepback wing.
A wing with a very high aspect ratio (in comparison with a low aspect ratio wing) will have a low stall speed.
— High-aspect-ratio wings also have better control qualities at low airspeeds, and they have less drag at higher angles of attack (negative examples of both qualities are mentioned as distractors).
At a constant velocity in airflow, a high aspect ratio wing will have (in comparison with a low aspect ratio wing) decreased drag, especially at a high angle of attack.
The use of a slot in the leading edge of the wing enables an airplane to land at a slower speed because it delays the stall to a higher angle of attack.
— A slot conducts air into the boundary layer on the upper surface of the wing, delaying airflow separation.
Which type of flap creates the greatest change in pitching moment? Fowler flap.
Which type of flap creates the least change in pitching moment? Split flap.
Which type of flap is characterized by large increases in lift coefficient with minimum changes in drag? Fowler flap.
An airplane leaving ground effect will experience a decrease in stability and a nose-up change in moments.
— Additionally, an increase in AOA will be required to maintain the lift coefficient, while induced drag will increase, requiring more thrust.
An airplane is usually affected by ground effect at what height above the surface? Less than half the airplane's wingspan above the surface.
— Ground effect exists up to the length of the wingspan above the surface, but at that height it is negligible.
If the same angle of attack is maintained in ground effect as when out of ground effect, lift will increase, and induced drag will decrease.
It is possible to fly an aircraft just clear of the ground at a slightly slower airspeed than that required to sustain level flight at higher altitudes. This is the result of interference of the ground surface with the airflow patterns about the aircraft in flight.
— The ground alters wingtip vortices, reducing induced drag, allowing for increased lift without increasing AOA.
Which statement is true regarding propeller efficiency? Propeller efficiency is the ratio of thrust horsepower to brake horsepower.
Blade angle of a propeller is defined as the angle between the chord line and the plane of rotation.
Propeller slip is the difference between the geometric pitch and effective pitch of the propeller.
The distance a propeller actually advances in one revolution is the effective pitch.
The reasons for variations in geometric pitch (twisting) along a propeller blade is that it permits a relatively constant angle of attack along its length when in cruising flight.
— The blade angle decreases from root to tip.
As a result of gyroscopic precession, it can be said that any yawing around the vertical axis results in a pitching moment.
— The propeller acts as a gyroscope. A pitching action will produce a yawing moment as well.
With regard to gyroscopic precession, when a force is applied at a point on the rim of a spinning disc, the resultant force acts in which direction and at what point? In the same direction as the applied force, 90 degrees ahead in the plane of rotation.
A propeller rotating clockwise, as seen from the rear, creates a spiraling slipstream that tends to rotate the aircraft to the left around the vertical axis, and to the right around the longitudinal axis.
— The rolling action is due to increased angle of attack on the left horizontal stabilizer.
Which is the best technique for minimizing the wing-load factor when flying in severe turbulence? Set power and trim to obtain an airspeed at or below maneuvering speed, maintain wings level, and accept variations of airspeed and altitude.
If an aircraft has negative dynamic and positive static stability, this will result in divergent oscillations.
— If an airplane has positive static stability, the initial tendency of the airplane is to return to the original state after it is disturbed. With negative dynamic stability, this movement creates divergent oscillations, which, instead of dampening, become progressively larger..
Airflow from two adjacent surfaces that merge and create eddy currents, turbulence, or restrict airflow is called interference drag.
When the angle of attack of a symmetrical airfoil is increased, the center of pressure will remain unaffected.