1. Basic Aerodynamic Theory
ATC Chapters 1 & 6

2. Aim
To introduce basic aerodynamic theory to a sufficient level to be able to understand chapter 6 of ATC BAK

3. Objectives State the how lift is produced
State the lift formula and its proportional values
Name the control surfaces and the axis around which the aircraft rotates
Show arrangement of forces during S & L
State the meaning of, and factors affecting stability

4. P+V=C 1. Lift Bernoulli's Theorem
“In the streamlined flow of an ideal fluid, the sum of all the energies remains constant”
At low airspeeds air acts like a fluid therefore we can say…
Static pressure + Dynamic pressure = Constant
Dynamic pressure is caused by movement of an object therefore we can say…
Pressure (pressure energy) + Velocity (Kinetic energy) = Constant
P+V=C

5. 1. Lift Bernoulli's Theorem P+V=C P+V=C P+V=C
To prove the theory we can look at a venturi. A venturi is a converging, diverging duct As air flows through a venturi it’s speed increases. Since energy is being conserved it’s pressure decreases. As it passes into the divergent duct pressure increases, velocity decreases
If we look at the shape of the bottom half of the venturi it looks like the top of our wing
P+V=C
P+V=C
P+V=C

6. 1. Lift Bernoulli's Theorem
As air flows over the top surface of an aerofoil it is accelerated, therefore pressure is…
Reduced
The pressure difference between the low pressure on the top of the wing and relatively higher pressure on the bottom of the wing creates an aerodynamic force that we call Lift

7. 2. Lift Formula
The factors that affect the aerodynamic force (Lift) produced by our aircraft can be seen in the lift formula
L = CL . 1/2.ρ.V2 . S
Where:
CL - Co-efficient of lift
ρ (Rho) – Free stream air density
V – True airspeed (TAS)
S – Plan view wing surface area

8. 2. Lift Formula CL - Co-Efficient of lift L = CL . 1/2.ρ.V2 . S
CL refers to the lifting ability of the wing
Its made up of a number of factors including:
Angle of Attack (AoA)
Camber
Aspect Ratio
Surface condition
L = CL . 1/2.ρ.V2 . S

9. 2. Lift Formula Angle of Attack L = CL . 1/2.ρ.V2 . S
Is defined as the angle between the relative airflow (RAF) and chord line of an aerofoil
Lift
Chord Line
L.E.
AoA
T.E.
RAF
As AoA increases lift increases
L = CL . 1/2.ρ.V2 . S

10. 2. Lift Formula Camber L = CL . 1/2.ρ.V2 . S
Mean Camber is the curvature of a line drawn equidistant between the upper and lower surfaces of the wing
Lift
Chord Line
Line of mean camber
AoA
RAF
As camber increases lift increases
L = CL . 1/2.ρ.V2 . S

11. 2. Lift Formula Camber L = CL . 1/2.ρ.V2 . S
High camber aerofoils can be found on aircraft that require high lift at low airspeeds
Medium camber (general purpose) aerofoils can be found on light training aircraft
Low camber aerofoils can be found on aircraft that travel at high airspeeds
L = CL . 1/2.ρ.V2 . S

12. 2. Lift Formula Aspect Ratio L = CL . 1/2.ρ.V2 . S
Aspect ratio is the ratio of wing span to chord
Its is measured by:
As Aspect Ratio increases lift increases
High aspect ratio wings can be seen on gliding aircraft
Light training aircraft typically have medium Aspect Ratio wings
Low aspect ratio wings can be seen on aerobatic aircraft
L = CL . 1/2.ρ.V2 . S

13. 2. Lift Formula ρ - Air density L = CL . 1/2.ρ.V2 . S
Ambient density of the free stream air (air not being disturbed by the passage of the aircraft)
If density is increased, lift will increase
L = CL . 1/2.ρ.V2 . S

14. 2. Lift Formula V - True Airspeed (TAS) L = CL . 1/2.ρ.V2 . S
The aerodynamic force produced is directly proportional to the airspeed squared
The faster the airspeed, the more lift produced
L = CL . 1/2.ρ.V2 . S

15. 3. Control Surfaces S - Plan surface area L = CL . 1/2.ρ.V2 . S
The size of wing area is directly proportional to the aerodynamic force produced
A larger wing area, will interact with a larger volume of air and therefore produce more lift
L = CL . 1/2.ρ.V2 . S

16. 3. Control Surfaces Control Surfaces
The three primary control surfaces are:
Ailerons
Ailerons

17. 3. Control Surfaces Control Surfaces
The three primary control surfaces are:
Elevator

18. 3. Control Surfaces Control Surfaces
The three primary control surfaces are:
Rudder

19. 3. Control Surfaces
The control surfaces work by deflecting the trailing edge of the surface, increasing the effective AoA, therefore increasing…
Lift
Lift
RAF
Chord Line
Chord Line
Chord Line
RAF
Lift
RAF

20. 3. Control Surfaces The three axes the aircraft rotates about are:
1. Lateral axis - Pitch
Lateral axis

21. 3. Control Surfaces The three axes the aircraft rotates about are:
Longitudinal axis - Roll
Longitudinal axis

22. 3. Control Surfaces The three axes the aircraft rotates about are:
3. Normal axis - Yaw
Normal axis

23. 4. Forces in Straight and Level
Straight and level flight is defined as flight at a constant heading altitude and power setting with the aircraft in balance.
In this state, the motion will not change. Therefore, it must have a constant ...
velocity.
Constant velocity means nil acceleration therefore all forces must be in a state of ...
equilibrium.

24. 4. Forces in Straight and Level
There are four main forces acting in S & L flight:
LIFT
THRUST
DRAG
WEIGHT

25. 4. Forces in Straight and Level
These forces do not act from the same point
Lift - Is produced by the wings and acts upwards through the centre of pressure.
LIFT
Weight - Acts straight down through the centre of gravity to the centre of the earth.
Thrust - Is provided by the engine through the propeller.
CoG
DRAG
THRUST
CoP
Drag - Is the resistance to motion felt by all bodies within the atmosphere.
WEIGHT

26. 4. Forces in Straight and Level
Because the forces are not acting from the same point they create a couple
A couple is two equal and opposite forces acting about a pivot point creating a torque or turning moment
The two couple’s generate opposing pitching moments
LIFT
L / W Couple = Nose DOWN moment
T / D Couple = Nose UP moment
DRAG
THRUST
WEIGHT

27. 4. Forces in Straight and Level
We said that the forces must be in equilibrium, therefore:
LIFT =
WEIGHT
(L / W Couple)
THRUST =
DRAG
(T / D Couple)
For the aircraft to fly S & L the nose down moment must equal...
the nose up moment.

28. 4. Forces in Straight and Level
WEIGHT
LIFT
DRAG
THRUST
If the moments are not equal, the tailplane makes up the difference.
In a correctly loaded aircraft the tail plane will create a small force…
Downwards
Force
The forces are now in equilibrium

29. 5. Stability
Stability describes the properties of a body’s motion after it is displaced from equilibrium.
Static Stability refers to the initial reaction of a body after being displaced from a position of equilibrium
Dynamic Stability becomes apparent after a body shows static stability. It refers to the subsequent motion of the disturbed body

30. 5. Stability
Positive Static Stability refers to a body which will return to equilibrium after being displaced.

31. 5. Stability
Neutral Static Stability refers to a body which will maintain its displacement from equilibrium.

32. 5. Stability
Negative Static Stability refers to a body which will increase its displacement from equilibrium.

33. 5. Stability
To have dynamic stability the body must have Static Stability
If an aircraft has Positive Static and Positive Dynamic Stability it will eventually return to its original attitude without pilot input
Positive static stability
Positive Dynamic Stability
Returns to original attitude
Aircraft is disturbed
Original attitude

34. 5. Stability
If an aircraft has Positive Static and Neutral Dynamic Stability it will oscillate about the original attitude without pilot input
Positive static stability
Neutral Dynamic Stability
Aircraft is disturbed
Original attitude
Will continue to oscillate about original attitude

35. 5. Stability
If an aircraft has Positive Static and Negative Dynamic Stability it will diverge from its original attitude without pilot input
Positive static stability
Diverges from original attitude
Aircraft is disturbed
Original attitude
Negative Dynamic Stability

36. 5. Stability Longitudinal stability
Stability along the longitudinal axis
Stability about the lateral axis
Design features that affect longitudinal stability include longitudinal dihedral 4˚ 2˚

37. 5. Stability Lateral stability Stability along the lateral axis
Stability about the longitudinal axis
Design features that affect lateral stability include tail/fin surface area, wing position, pendulum effect and lateral dihedral

38. 5. Stability Directional stability Stability along the normal axis
Stability about the normal axis
Most important factor affecting is surface area aft of CoG

39. Questions?