1. West Point Aviation Club Private Pilot Ground Instruction
Lesson 3: Aerodynamics

2. Four Forces of Flight Lift Weight Thrust Drag
Lift is the opposing force of weight and thrust is the opposing force of drag. To fly, lift must overcome weight, and thrust must overcome drag in order fly.
Acceleration and direction are caused by different variations of these four forces.
If thrust is greater than drag, the aircraft is accelerating.
When drag is greater, the aircraft decelerates.
When lift is greater than weight, the airplane gains altitude.
When weight is greater, the airplane looses altitude.
forces are equilibrium
* The four forces are in equilibrium in unaccelerated flight
meaning lift = weight and thrust = drag

3. How is Lift Created? Lift – Opposes Weight
Bernoulli’s Principle
Apply Bernoulli’s Principle to an Airfoil:
Newton’s Third Law: For every action, there is an equal and opposite reaction.
Together…these forces produce the total amount of lift
Bernoulli’s Principle: as the velocity of a fluid (air) increases, it’s internal pressure decreases
Newton’s Third Law: for every action, there is an equal and opposite reaction

4. Angle of Attack
Parts of Airfoil
Chord line, relative wind, camber
Angle of attack
Critical Angle of Attack: The angle at which an aircraft stalls, decreasing the lift.

5. Angle of Attack Facts
The angle of attack at which an airplane stalls remains the same regardless of gross weight.
As altitude increases the indicated airspeed-at which a given airplane stalls in a particular configuration will remain the same regardless of altitude.
Regardless of weight and altitude.
Critical angle of attack can be found in aircraft operators manual.

6. Spins 1. In order to spin, an airplane must be stalled
2. During a spin, both wings
are stalled. One wing has a greater angle of attack than the other.
One wing has a little more lift than the other

7. Frost Frost will disrupt smooth flow of air over the wing.
Less lift capability
May require additional speed to become airborne
Frost on wings will disrupt the smooth flow of air over the wing. This will adversely affects its lifting capability.
Weight added.
Frost that has not be removed from the wings of an airplane before flight may prevent the airplane from becoming airborne at a normal takeoff speed.

8. Weight The opposing force of lift Always directed to center of earth
Magnitude depends on mass of airplane parts, fuel, and payload
Now its time to introduce weight and how it interacts with lift…

9. Center of Gravity
The center of gravity is the average location of the weight of the aircraft.
The aircraft rotates around the center of gravity. (To be extrapolated upon later)
The weight is distributed throughout the airplane. But we can often think of it as collected and acting through a single point called the center of gravity.
Balance pencil on finger for example to find the balancing point (center of gravity; average location of weight)
In flight, the airplane rotates about the center of gravity

10. Center of Pressure
The center of pressure is the concentrated point of all lifting forces.
Center of gravity is the average of the weight while center of pressure is the concentration of all lifting forces. Weight vs. Lift; they are opposite forces, so CG is NOT the same as CP.
CG changes due the location of weight. While the weight of airplane parts will generally remain the same, the CG can be move forward or aft based upon the weight and location of the fuel and payload.
Tail down force keeps nose up, like a kid on a seesaw.
CG must be in front of CP

11. Stability
When the center of gravity is located near the center of pressure, the aircraft is longitudinally stable
Aircraft require less effort to control when inherently stable
Loading an aircraft to the most AFT CG limit will make the airplane less stable at all speeds
If an airplane is loaded aft of the aft CG limit, the pilot might experience difficulty in recovering from a stalled condition
Changes in the center of pressure of a wing affect the aircraft’s aerodynamic balance and controllability
Describe what longitudinal means. Show lateral axis (the axis along the wings) and vertical axis (up and down).
Flying encompasses two major problems; overcoming the weight of an object by some opposing force, and controlling the object in flight. Both of these problems are related to the object's weight and the location of the center of gravity; this determines the aircraft’s stability.
With airplane model, show how when the load is aft of CG limit, it will be difficult to recover from a stall. (Can’t get nose down to recover airflow over top of wing when the weight in the tail keeps the nose up; the aircraft also rotates around center of gravity.)
The weight is actually distributed throughout the airplane, and it is important to know the distribution in order to pack loads in a way that the airplane will remain stable in flight.
During a flight, an airplane's weight constantly changes as the aircraft consumes fuel.
Because distribution of the weight and the center of gravity changes due to fuel consumption, the pilot must constantly adjust the controls to keep the airplane balanced.

12. Thrust
Forward force produced by the propeller which propels the airplane through the air
It is the opposite force of drag
Thrust: Forward force produced by the engine which propels the airplane through the air.

13. How Propeller Produce Thrust
Propeller is another airfoil
(Newton and Bernoulli's)
The prop is twisted, so it pushes through the air like a screw.
(3:56-6:31)

14. Drag
The air resists the motion of the aircraft and the resistance force is called drag.
It is the opposite force of thrust
Have you ever stuck your hand out of a moving car’s window? The fast moving air particles push your hand back; this is also drag.

15. Types of Drag Types of Drag Parasite: caused by non-lifting surfaces
Interference: when objects on aircraft meet; intersections.
Skin Friction: the actual roughness of the skin.
Any place where air particles collide into aircraft.
Induced: caused by wind pushing down on wings when lift is produced
Drag: Backward or retarding force which limits the speed of the aircraft. Caused by a combination of forces to include lift (believe it or not).
Two types of Drag: Parasite and Induced Drag
Parasite drag: Talk about hand out of window
Induced Drag (or Drag due to Lift): is generated by the airflow circulation around the wing as it creates lift. The circulating air pushes down on wing.

16. Wingtip Vortices Pressure Differential at wing tip causes vortices
Higher pressure on bottom of wing and lower pressure on top.
Higher pressure air particles move up around wing to the lower pressure.
The particles pushing down on the wing pushes the plane down in which lift must overcome.
This is induced drag. Some lift is lost in producing lift.
Pressure Differential at wing tip causes vortices

17. Total Drag
Induced drag is lessened when the airplane is moving slower because there is less air particles pushing down on wing at a given time.
Parasite drag is increased with speed: air particles collide harder at higher speed.
Hand out of car moving 100 vs 5 mph.
Is there ever a way to have both high induced drag and high parasite drag simultaneously?

18. Primary Flight Controls
Throttle
manipulate the four forces using flight controls.
Move in 3 dimensions
Airplanes rotate in three dimensions: roll, pitch, and yaw.
Roll rotates on longitudinal axis pitch on lateral axis yaw on normal axis.
The plane will always rotate around center of gravity
To manipulate the four forces, we use ailerons, elevators, rudder
Throttle

19. Ailerons and Roll Ailerons control roll. controlled by the stick
The wing with aileron up will be lowered and the wing with aileron down will raise.
Look at chord line and angle of attack. Less angle of attack = less lift
Wind pushing on ailerons
The plane rolls around the center of gravity.

20. Elevators and Pitch
Air hitting under the elevator forces the nose downward
Air hitting the top of the elevator forces the nose upward
The elevators controls pitch. This also adjusts the angle of attack.
The elevators
wind pushes on elevators
The airplane will pivot around center of gravity, like a seesaw.

21. Rudders and Yaw The rudder controls yaw.
The rudders are controlled by pedals on the floor.
Wind pushing on elevators
The aircraft will always yaw around the center of gravity.
Apply a little right rudder to adjust for the p-factor (next lesson)

22. Wing Flaps
Flaps enable the pilot to make steeper approaches (increasing the angle of descent) to a landing without increasing airspeed.
Flaps vs ailerons
steeper angle of decent
Short distance landing

23. How Flaps Work Lift effected by angle of attack.
more parasite drag and lift
fly a slower approach
at less angle of attack (improving visibility over the nose).
fly a steeper approach without speeding up.

24. Vortices (Wake Turbulence)
Are created when aircraft is developing lift
Wake turbulence vortices circulate outward, upward, and around each wing tip. They travel below the aircraft producing the lift.
Can be dangerous for smaller aircraft when from a larger one
3. small aircraft around.
Avoid path of larger aircraft
light , quartering tail winds
4:30
4. The greatest vortex strength occurs when the aircraft is heavy, clean, and slow

25. Ground Effect
Ground effect is the result of the interference of the surface of the Earth with the airflow patterns about an airplane.
Induced drag decreases and lift increases
Aircraft fly more efficiently in ground effect-it floats
Ground effect occurs within 1 wingspan from the ground
1. Slower speed = less resistance on aircraft = less induced drag
2. Ground keeps pressure under wings
air hits bottom of airplane (Newton’s third law).

26. Problems Caused by Ground Effect
Take off: May result in becoming airborne before reaching recommended take off speed
Landing: As a result of ground effect, induced drag decrease. A pilot must be aware, therefore, that any excess speed at the point of the flare may cause considerable floating.
Near the ground when take off and landing
1. taking off before recommended take off speed = Using more runway
2. When coming into land…
Flare with too much speed = “porpoise.”
Flare with not enough speed
Porpoising can be dangerous if all airspeed is bled out at considerable hight

27. Questions?

28. Written Exam Questions
The four forces actin on an airplane in flight are
A. lift, weight, thrust, and drag
B. lift, weight, gravity, and thrust
C. lift, gravity, power, and friction

29. Written Exam Questions
When are the four forces that act on an airplane in equilibrium?
A. During unaccelerated flight
B. When the aircraft is accelerating
C. When the aircraft is at rest on the ground

30. Written Exam Questions
The acute angle A is the angle of
A. incident
B. attack
C. dihedral

31. Written Exam Questions
The term “angle of attack” is defined as the angle
A. between the wing chord line and relative wind
B. between the airplane’s climb angle and horizon
C. formed by the longitudinal axis of the airplane and the chord line of the wing

32. Written Exam Questions
What is the relationship of lift, drag, thrust, and weight when the airplane is in straight-and-level flight?
A. Lift equals weight and thrust equals drag
B. Lift, drag, and weight equals thrust
C. Lift and weight equal thrust and drag

33. Written Exam Questions
How will frost on the wings of an airplane affect takeoff performance?
A. Frost will disrupt the smooth flow of air over the wing, adversely affecting its lifting capability
B. Frost will change the camber of the wing, increasing its lifting capability
C. Frost will cause the airplane to become airborne with a higher angle of attack, decreasing the stall speed

34. Written Exam Questions
An airplane is said to be inherently stable will
A. be difficult to stall
B. require less effort to control
C. not spin

35. Written Exam Questions
What determines the longitudinal stability of an airplane?
A. The location of the CG with respect to the center of lift
B. The effectiveness of the horizontal stabilizer, rudder, and rudder trim tab
C. The relationship of thrust and lift to weight and drag

36. Written Exam Questions
What is the purpose of the rudder on an airplane?
A. To control yaw
B. To control overbanking tendency
C. To control roll

37. Written Exam Questions
One of the main functions of flaps during approach and landing is to
A. decrease the angle of descent without increasing airspeed
B. permit a touchdown at a higher indicated airspeed
C. increase the angle of descent without increasing the airspeed

38. Written Exam Questions
What is one purpose of wing flaps?
A. to enable the pilot to make steeper approaches to a landing without increasing the airspeed
B. to relieve the pilot of maintaining continuous pressure on the controls
C. to decrease wing area to vary the lift

39. Written Exam Questions
As altitude increases, the indicated airspeed at which a given airplane stalls in a particular configuration will
A. decrease as the true airspeed decreases
B. decrease as the true airspeed increases
C. remain the same regardless of altitude

40. Written Exam Questions
An airplane has been loaded in such a manner that the CG is located aft of the aft CG limit. One undesirable flight characteristic a pilot might experience with this airplane would be
A. a longer takeoff run
B. difficulty in recovering from a stalled condition
C. stalling at higher-than-normal airspeed

41. Written Exam Questions
Loading an airplane to the most aft CG will cause the airplane to be
A. less stable at all speeds
B. less stable at slow speeds, but more stable at high speeds
C. Less stable at high speeds, but more stable at low speeds

42. Written Exam Questions
In what flight condition must an aircraft be placed in order to spin?
A. partially stalled with one wing low
B. in a steep diving spiral
C. stalled

43. Written Exam Questions
During a spin to the left, which wing(s) is/are stalled?
A. Both wings are stalled
B. Neither wing is stalled
C. Only the left wing is stalled

44. Written Exam Questions
The angle of attack at which an airplane wing stalls will
A. increase if the CG is moved forward
B. change with an increase in gross weight
C. remain the same regardless of gross weight

45. Written Exam Questions
What is ground effect?
A. the result of the interference of the surface of the Earth with the airflow patterns about an airplane
B. the result of an alteration in airflow patterns increasing induced drag about the wings of an airplane

46. Written Exam Questions
Floating cause by the phenomenon of ground effect will most be realized during an approach to land when at
A. less than the length of the wingspan above the surface
B. twice the length of the wingspan above the surface
C. a higher-than-normal angle of attack

47. Written Exam Questions
What must a pilot be aware of as a result of ground effect?
A. wingtip vortices increase creating wake turbulence problems for arriving and departing aircraft
B. Induced drag decreases; therefore, any excess speed at the point of flare may cause considerable floating