*3A Determining Performance Limitations - Aerodynamics

Question 0

1. What are the four dynamic forces that act on an airplane during all maneuvers?
   (FAA-H-8083-25)

Answer 0

Lift—the upward acting force
Gravity—or weight, the downward acting force
Thrust—the forward acting force
Drag—the backward acting force

Question 1

2. What flight condition will result in the sum of the opposing forces being equal?
   (FAA-H-8083-25)

Answer 1

In steady-state, straight-and-level, unaccelerated flight, the sum of the opposing forces is equal to zero. There can be no unbalanced forces in steady, straight flight (Newton’s Third Law). This is true whether flying level or when climbing or descending. This simply means that the opposing forces are equal to, and thereby cancel the effects of, each other.

Question 2

3. What is an airfoil? State some examples.

FAA-H-8083-25

Answer 2

An airfoil is a device which gets a useful reaction from air moving over its surface, namely LIFT. Wings, horizontal tail surfaces, vertical tail surfaces, and propellers are examples of airfoils.

Question 3

4. What is the “angle of incidence”?

FAA-H-8083-25

Answer 3

The angle of incidence is the angle formed by the longitudinal axis of the airplane and the chord of the wing. It is measured by the angle at which the wing is attached to the fuselage. The angle of incidence is fixed and cannot be changed by the pilot.

Question 4

5. What is a “relative wind”?

FAA-H-8083-25

Answer 4

The relative wind is the direction of the airflow with respect to the wing. When a wing is moving forward and downward the relative wind moves backward and upward. The flight path and relative wind are always parallel but travel in opposite directions.

Question 5

6. What is the “angle of attack”?

FAA-H-8083-25

Answer 5

The angle of attack is the angle between the wing chord line and the direction of the relative wind; it can be changed by the pilot.

Question 6

7. What is “Bernoulli’s Principle”?

FAA-H-8083-25

Answer 6

Bernoulli’s Principle—The pressure of a fluid (liquid or gas) decreases at points where the speed of the fluid increases. In the case of airflow, high speed flow is associated with low pressure and low speed flow with high pressure. The airfoil of an aircraft is designed to increase the velocity of the airflow above its surface, thereby decreasing pressure above the airfoil. Simultaneously, the impact of the air on the lower surface of the airfoil increases the pressure below. This combination of pressure decrease above and increase below produces lift.

Question 7

8. What are several factors which will affect both lift and drag?

Answer 7

Wing area—Lift and drag acting on a wing are roughly proportional to the wing area. A pilot can change wing area by using certain types of flaps (i.e., Fowler flaps).
Shape of the airfoil—As the upper curvature of an airfoil is increased (up to a certain point) the lift produced increases. Lowering an aileron or flap device can accomplish this. Also, ice or frost on a wing can disturb normal airflow, changing its camber, and disrupting its lifting capability.
Angle of attack—As angle of attack is increased, both lift and drag are increased, up to a certain point.
Velocity of the air—An increase in velocity of air passing over the wing increases lift and drag.
Air density—Lift and drag vary directly with the density of the air. As air density increases, lift and drag increase. As air density decreases, lift and drag decrease. Air density is affected by these factors: pressure, temperature, and humidity.

Question 8

9. What is “torque effect”?

FAA-H-8083-25

Answer 8

Torque effect involves Newton’s Third Law of Physics—for every action, there is an equal and opposite reaction. Applied to the airplane, this means that as the internal engine parts and the propeller are revolving in one direction, an equal force is trying to rotate the airplane in the opposite direction. It is greatest when at low airspeeds with high power settings and a high angle of attack.

Question 9

10. What effect does torque reaction have on an airplane on the ground and in flight?
    (FAA-H-8083-25)

Answer 9

In flight—torque reaction is acting around the longitudinal axis, tending to make the airplane roll. To compensate, some of the older airplanes are rigged in a manner to create more lift on the wing that is being forced downward. The more modern airplanes are designed with the engine offset to counteract this effect of torque.
On the ground—during the takeoff roll, an additional turning moment around the vertical axis is induced by torque reaction. As the left side of the airplane is being forced down by torque reaction, more weight is being placed on the left main landing gear. This results in more ground friction, or drag, on the left tire than on the right, causing a further turning moment to the left.

Question 10

11. What are the four factors that contribute to torque effect?
    (FAA-H-8083-25)

Answer 10

“11. What are the four factors that contribute to torque effect? (FAA-H-8083-25)
Torque reaction of the engine and propeller. For every action there is an equal and opposite reaction. The rotation of the propeller (from the cockpit) to the right, tends to roll or bank the airplane to the left.
Gyroscopic effect of the propeller. Gyroscopic precession applies here: the resultant action or deflection of a spinning object when a force is applied to the outer rim of its rotational mass. If the axis of a propeller is tilted, the resulting force will be exerted 90° ahead in the direction of rotation and in the same direction as the applied force. It is most noticeable on takeoffs in taildraggers when the tail is raised.
Corkscrewing effect of the propeller slipstream. High-speed rotation of an airplane propeller results in a corkscrewing rotation to the slipstream as it moves rearward. At high propeller speeds and low forward speeds (as in a takeoff), the slipstream strikes the vertical tail surface on the left side pushing the tail to the right and yawing the airplane to the left.
Asymmetrical loading of the propeller (P-Factor). When an airplane is flying with a high angle of attack, the bite of the downward moving propeller blade is greater than the bite of the upward moving blade. This is due to the downward moving blade meeting the oncoming relative wind at a greater angle of attack than the upward moving blade. Consequently there is greater thrust on the downward moving blade on the right side, and this forces the airplane to yaw to the left.

Question 11

12. What is “centrifugal force”?

FAA-H-8083-25

Answer 11

Centrifugal force is the “equal and opposite reaction” of the airplane to the change in direction, and it acts “equal and opposite” to the horizontal component of lift.

Question 12

13. What is “load factor”?

FAA-H-8083-25

Answer 12

Load factor is the ratio of the total load supported by the airplane’s wing to the actual weight of the airplane and its contents. In other words, it is the actual load supported by the wings divided by the total weight of the airplane. It can also be expressed as the ratio of a given load to the pull of gravity; i.e., to refer to a load factor of three as “3 Gs.” In this case the weight of the airplane is equal to 1 G, and if a load of three times the actual weight of the airplane were imposed upon the wing due to curved flight, the load factor would be equal to 3 Gs.

Question 13

14. For what two reasons is load factor important to pilots?
    (FAA-H-8083-25)

Answer 13

a. Because of the obviously dangerous overload that it is possible for a pilot to impose on the aircraft structure.
b. Because an increased load factor increases the stalling speed and makes stalls possible at seemingly safe flight speeds.

Question 14

15. What situations may result in load factors reaching the maximum or being exceeded?
    (FAA-H-8083-25)

Answer 14

Level Turns—The load factor increases at a terrific rate after a bank has reached 45° or 50°. The load factor in a 60°-bank turn is 2 Gs. The load factor in a 80°-bank turn is 5.76 Gs. The wing must produce lift equal to these load factors if altitude is to be maintained.
Turbulence—Severe vertical gusts cause a sudden increase in angle of attack, resulting in large loads which are resisted by the inertia of the airplane.
Speed—The amount of excess load that can be imposed upon the wing depends on how fast the airplane is flying. At speeds below maneuvering speed, the airplane will stall before the load factor can become excessive. At speeds above maneuvering speed, the limit load factor for which an airplane is stressed can be exceeded by abrupt or excessive application of the controls or by strong turbulence.

Question 15

16. What are the different operational categories for aircraft and within which category does your aircraft fall?
    (FAA-H-8083-25)

Answer 15

The maximum safe load factors (limit load factors) specified for airplanes in the various categories are as follows:
Normal: +3.8 to -1.52
Utility (mild aerobatics including spins): +4.4 to -1.76
Aerobatic: +6.0 to -3.00”

Excerpt From: Michael D. Hayes. “Private Oral Exam Guide.” Aviation Supplies and Academics, Inc., 2012-05-25. iBooks.
This material may be protected by copyright.

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Question 16

17. What effect does an increase in load factor have on stalling speed?
    (FAA-H-8083-25)

Answer 16

As load factor increases, stalling speed increases. Any airplane can be stalled at any airspeed within the limits of its structure and the strength of the pilot. At a given airspeed the load factor increases as angle of attack increases, and the wing stalls because the angle of attack has been increased to a certain angle. Therefore, there is a direct relationship between the load factor imposed upon the wing and its stalling characteristics. A rule for determining the speed at which a wing will stall is that the stalling speed increases in proportion to the square root of the load factor.

Question 17

18. Define the term “maneuvering speed.”

FAA-H-8083-3

Answer 17

Maneuvering speed is the maximum speed at which abrupt control movement can be applied or at which the airplane could be flown in turbulence without exceeding design load factor limits. When operating below this speed, a damaging positive flight load should not be produced because the airplane should stall before the load becomes excessive.

Question 18

19. Discuss the effect on maneuvering speed of an increase or decrease in weight.
    (FAA-H-8083-25)

Answer 18

Maneuvering speed increases with an increase in weight and decreases with a decrease in weight. An aircraft operating at a reduced weight is more vulnerable to rapid accelerations encountered during flight through turbulence or gusts. Design limit load factors could be exceeded if a reduction in maneuvering speed is not accomplished. An aircraft operating at or near gross weight in turbulent air is much less likely to exceed design limit load factors and may be operated at the published maneuvering speed for gross weight if necessary.

Question 19

20. What causes an airplane to stall?

FAA-H-8083-25

Answer 19

The direct cause of every stall is an excessive angle of attack. Each airplane has a particular angle of attack where the airflow separates from the upper surface of the wing and the stall occurs. This critical angle of attack varies from 16° to 20° depending on the airplane’s design, but each airplane has only one specific angle of attack where the stall occurs, regardless of airspeed, weight, load factor, or density altitude.

Question 20

21. What is a “spin”?

AC 61-67

Answer 20

A spin in a small airplane or glider is a controlled (recoverable) or uncontrolled (possibly unrecoverable) maneuver in which the airplane or glider descends in a helical path while flying at an angle of attack greater than the critical angle of attack. Spins result from aggravated stalls in either a slip or a skid. If a stall does not occur, a spin cannot occur.

Question 21

22. What causes a spin?

AC 61-67

Answer 21

The primary cause of an inadvertent spin is exceeding the critical angle of attack while applying excessive or insufficient rudder, and to a lesser extent, aileron.

Question 22

23. When are spins most likely to occur?

AC 61-67

Answer 22

A stall/spin situation can occur in any phase of flight but is most likely to occur in the following situations:

a. Engine failure on takeoff during climbout—pilot tries to stretch glide to landing area by increasing back pressure or makes an uncoordinated turn back to departure runway at a relatively low airspeed.
b. Crossed-control turn from base to final (slipping or skidding turn)—pilot overshoots final (possibly due to a crosswind) and makes uncoordinated turn at a low airspeed.
c. Engine failure on approach to landing—pilot tries to stretch glide to runway by increasing back pressure.
d. Go-around with full nose-up trim—pilot applies power with full flaps and nose-up trim combined with uncoordinated use of rudder.
e. Go-around with improper flap retraction—pilot applies power and retracts flaps rapidly resulting in a rapid sink rate followed by an instinctive increase in back pressure.

Question 23

24. What procedure should be used to recover from an inadvertent spin?
    (AC 61-67)

Answer 23

a. Close the throttle (if not already accomplished).
b. Neutralize the ailerons.
c. Apply full opposite rudder.
d. Briskly move the elevator control forward to approximately the neutral position. (Some aircraft require merely a relaxation of back pressure; others require full forward elevator pressure).
e. Once the stall is broken the spinning will stop. Neutralize the rudder when the spinning stops.
f. When the rudder is neutralized, gradually apply enough aft elevator pressure to return to level flight.

Question 24

25. What causes “adverse yaw”?

FAA-H-8083-25

Answer 24

When turning an airplane to the left for example, the downward deflected aileron on the right produces more lift on the right wing. Since the downward deflected right aileron produces more lift, it also produces more drag, while the opposite left aileron has less lift and less drag. This added drag attempts to pull or veer the airplane’s nose in the direction of the raised wing (right); that is, it tries to turn the airplane in the direction opposite to that desired. This undesired veering is referred to as adverse yaw.

Question 25

26. What is “ground effect”?

FAA-H-8083-3

Answer 25

Ground effect is a condition of improved performance the airplane experiences when it is operating near the ground. A change occurs in the three-dimensional flow pattern around the airplane because the airflow around the wing is restricted by the ground surface. This reduces the wing’s upwash, downwash, and wingtip vortices. In order for ground effect to be of a significant magnitude, the wing must be quite close to the ground.

Question 26

27. What major problems can be caused by ground effect?

FAA-H-8083-3

Answer 26

During landing, at a height of approximately one-tenth of a wing span above the surface, drag may be 40 percent less than when the airplane is operating out of ground effect. Therefore, any excess speed during the landing phase may result in a significant float distance. In such cases, if care is not exercised by the pilot, he/she may run out of runway and options at the same time.
During takeoff, due to the reduced drag in ground effect, the aircraft may seem capable of takeoff well below the recommended speed. However, as the airplane rises out of ground effect with a deficiency of speed, the greater induced drag may result in very marginal climb performance, or the inability of the airplane to fly at all. In extreme conditions, such as high temperature, high gross weight, and high-density altitude, the airplane may become airborne initially with a deficiency of speed and then settle back to the runway.