Basic Aerodynamic Theory

Question 1

What is free stream static pressure

Answer 1

A term in aerodynamics to describe the air conditions which exist ahead of a body moving through the air and as yet unaffected by it.

Question 2

What is dynamic pressure and what does 1/2pV^2 mean?

Answer 2

Pressure caused by movement of air against a solid object.

P = density (RHO)

Dynamic energy = 1/2pV^2 (V= velocity of the airstream)

Question 3

Explain the principle of airspeed indication, and indicate the relationship between indicated, calibrated, equivalent and true airspeeds (IAS, CAS, EAS, and TAS)

Answer 3

Measured by comparing current dynamic pressure against current free stream static pressure

Total pressure = dynamic pressure + static pressure

ICED T

PCD

Question 4

What’s the relationship between IAS/TAS

Answer 4

TAS - actual speed of aircraft
IAS - speed available to pilot

ASI is calibrated to read IAS under ISA conditions. Only under ISA will IAS = TAS.

When different IAS will differ from TAS. But with IAS constant, TAS increases in altitude because aircraft must fly faster for ASI to read the same indication.

Question 5

With respect to aerofoils, describe the meanings of the following terms: section, leading edge, trailing edge, chord, chord line, thickness, thickness/chord ratio, camber.

Answer 5

Leading edge - edge facing into airstream
Trailing - edge at downstream side
Chord line - straight line joining trailing edges and leading edges
Chord - distanced between the leading and trailing edge measured along the chord line.
Thickness - depth of aero foil. (About 30-40% of chord)
Thickness/chord ratio - max thickness of aerofoil expressed as a percentage of the chord.
Camber - curve of a line drawn between the upper and lower surfaces.

Question 6

Distinguish between high-lift, general purpose (GP) and high-speed aerofoil sections. (%)

Answer 6

(15-17% for high lift, general purpose 10-12% and high speed is 7%)

Question 7

Define relative airflow and angle of attack

Answer 7

Angle of attack - angle between chord line and relative airflow

Relative airflow - speed and direction of air movement past aerofoil.

Question 8

State Bernoulli’s theorem in simple terms, and describe streamline flow, turbulent flow, and the application of Bernoulli’s theorem to the streamline flow around an aerofoil.

Answer 8

Theorem - In streamline flow of an ideal fluid, the sum of the energy of position, plus energy of motion will remain constant. (Dynamic energy + static pressure = a constant )

Streamline - if succeeding particles of air in an airstream follow the same steady and predictable path then this path can be represented by a streamline.

Turbulent - if airflow is required to change direction too abruptly and flow breaks down and becomes unpredictable.

Application - can be applied in streamline airflow with a velocity of up to 250kts. Higher velocities airflow is compressed.

Question 9

Describe a venturi and explain venturi effect.

Answer 9

Venturi - converging diverging duct.

Venturi effect - velocity of a fluid passing through a Venturi will increase and its static pressure will decrease.

Question 10

Explain the changes to the airflow and pressure distribution around a typical aerofoil in a low- subsonic speed airflow as α is increased from the zero-lift angle to beyond the stalling angle

Answer 10

low AoA, relatively little disturbance of the airflow pass the aerofoil.

As AoA is increased, airflow must increasingly deviate from its path and accelerate to follow the contour of the upper surface. Resulting with the upper area of the lower pressure moving forward.

At higher angles, the area of high pressure forward of the LE spreads toward the rear until it eventually covers the whole of the lower surface.

Beyond the stall angle, the change in direction around the LE and forward upper surface becomes too abrupt and the airflow can no longer conform. So, the airflow separates from most of the upper surface, and the turbulent wake behind the aerofoil becomes greatly enlarged.

When the Critical angle is exceeded, the low pressure envelope over the upper surface virtually collapses and becomes unpredictable and pressure below the aerofoil continues to increase with angle of attack.

Question 11

Explain the terms upwash and downwash in an airflow and how they’re created.

Answer 11

Upwash - when the streamlines indicate that the airflow turns upward ahead of the aerofoil and are more prominent as AoA increases therefore greater pressure differential between upper and lower surfaces.

Created by small pressure disturbances ahead of the aerofoil which cause air particles to move towards lowest pressure area. In effect, the air is able to sense the approach of an object travelling to it and move to place of least resistance.

Downwash - as the airflow passes the aerofoil, its turned downward with respect to free stream direction.

Consequence for generating lift, mass of air must be move in a given direction to produce a lift force in opposite direction. When aerofoil is producing lift downwash must exceed upwash, and thus increase with AoA.

Question 12

Explain the term centre of pressure (CP); and describe typical movement of the CP
with increasing angle of attack (α).

Answer 12

Term - the pressures existing at various points around an aerofoil that can be measured and compared with the free stream static pressure.

The CoP sits at about 40% mean aerodynamic chord when the angle of attack is about 2° . it moves progressively forward as the angle of attack increases reaching its forward most point of 15% at the critical angle of attack, As the angle of attack is increased above the stall, the centre of pressure moves quickly rearwards

Question 13

What is angle of incidence

Answer 13

The angle at which an aerofoil is attached to the fuselage. And is the angle between the chord line and the longitudinal axis.

Question 14

Define the total aerodynamic reaction force (TR) of an aerofoil;

Answer 14

The individual vectors which are combined into a single resultant which indicates the magnitude and direction of the aerodynamic force o the aerofoil under different conditions.

Question 15

Describe how TR varies with increasing angle of attack (α).

Answer 15

TR force increases with AoA and becomes more titled towards the rear. As stalling AoA is passed TR force suddenly reduces and becomes more tilted rear.

Question 16

Define the TR components lift and drag.

Answer 16

Lift is the component of TR at a right angle to the relative airflow.

Drag is parallel to relative airflow and opposing motion.

Question 17

Identify the factors affecting lift (low-subsonic speed airflow). (6)

Answer 17

Free stream air density (p∞)
Free stream air velocity (v∞)
Size of the wing
Shape of the wing
Condition of the wing
AoA

Question 18

State the lift formula, and the three basic functions contained within it.

Answer 18

L = Cl1/2pV^2S

Cl = coefficient of lift
1/2pV^2 = dynamic pressure
S = wing area

Question 19

Describe the meaning of the term, coefficient of lift (CL).

Answer 19

A multiplier dependent on the shape and condition of the wing and varies as AoA is changed

Question 20

Given a typical CL versus α curve for a GP-type aerofoil, identify:

(a) the zero lift angle;
(b) the angle for maximum CL (CLmax).

Answer 20

Zero lift angle - AoA where an aerofoil will produce no net lift and is generally -4º.

Max Cl - 16º AoA aka the stalling AoA.

Question 21

Explain the meaning of a high CLmax.

Answer 21

Has a greater lifting capacity and is able to produce more lift overall all of its normal operating AoA.

Will be able to fly at slower speed without stalling and more manoeuvrability.

Question 22

With respect to the CL curve, describe the effect of: (a) increased camber;
(b) surface roughness (e.g. contamination).

Answer 22

(A) increased camber means Cl is increased over all normal operating AoA. As it increases the angle at which the wing will stall may also change.

(B) any roughness of the surface of a wing encourage flow breakaway which brings early onset of stall and reduction of Cl max.

Question 23

Describe three-dimensional flow over a wing and explain how wingtip and trailing edge vortices are formed.

Answer 23

Airflow along the wing from root to tip called the spanwise flow.

The wingtip vortices are the major effect and are caused by the air ‘spilling’ from high pressure (through atmospheric) to low pressure around the wingtips. The core of each vortex spins at high speed, dragging more air from its surroundings with it and growing as it extends back from the wingtip.

trailing edge vortices are the result of the airflow meeting at the trailing edge at slightly different angles. They are less pronounced and less stable than the wingtip vortices and generally become absorbed in the turbulent and unpredictable flow of the wake from the trailing edge.

Question 24

Explain the effect of induced downwash on α.

Answer 24

Induced down wash produced behind the wing affects the overall average angle of the airflow over the wing, tilting it downward.

The geometric angle of attack (between remote RAF and chord line) is reduced by the downwash angle to what is called the effective AoA.

Question 25

Define aspect ratio (AR) and describe the effect of AR on CL.

Answer 25

Aspect ratio is the ratio of the wingspan to chord of a wing.

Higher AR, lower amount of downwash. High AR wing, chord wise flow has little to develop as air crosses the wing so less air spills over the wingtip and the angle at which the air meets at the trailing edge is small and the area affected by downwash is a relatively small proportion of the total area behind the wing

Low AR, much larger proportion of the total flow is spilt around the wingtip and the angle which airflow meets is greater and the area behind the wing affected by downwash is greater.

So it decreases Cl.