Part II

Question 1

How are aerodynamic forces generated

Answer 1

generate from the action of pressure and shear stresses

Question 2

wetted surface area means

Answer 2

Surface area that is exposed to air

Question 3

List four types of drag

Answer 3

Skin friction
wave drag
pressure drag
induced drag

Question 4

Skin friction drag explained

Answer 4

caused by viscous shear stresses in the boundary layer

Question 5

Wave drag explained

Answer 5

pressure forces caused in supersonic flow (shock waves)

Question 6

Pressure drag explained

Answer 6

Pressure forces in the boundary layer

Question 7

induced drag explained

Answer 7

caused by vortex induced velocity when lift is generated (only in 3D models)

Question 8

Vorticity is defined as

Answer 8

w = ∇ x V
w = 2Θ

where Θ = rotation of a fluid element

Question 9

How can vorticity be spread through a fluid

Answer 9

only spreads through in an incremental fashion by diffusion or advection

Question 10

Define vortex line

Answer 10

always tangent to the vorticity

Question 11

Define vortex tube

Answer 11

closed curve with points indicating vortex line

Question 12

vortex filament

Answer 12

fluid contained in a vortex tube

Question 13

Helmholtz theorems:
1. Strength of Γ vortex is _______ along its length

2, Vortex cannot end inside the fluid it must:

Answer 13

1. Strength of Γ vortex is constant along its length

2, Vortex cannot end inside the fluid it must:

• extend to + or - ∞
• end at a solid boundary
• form a closed loop

Question 14

In reality what happens to a vortex

Answer 14

in reality vorticity is diffused through the action of viscosity

Question 15

Equation for induced angle of attack

Answer 15

αi = α - αeff

or

αi = /w\ / V∞

Question 16

Define geometric wing twist

Answer 16

sideways on, the aerofoil of the wing would rotate as you moved away from the fuselage

Rotate down = washout (common)

Rotate up = wash in

Question 17

Define aerodynamic wing twist

Answer 17

aerofoil would change shape as move further from the fuselage

Question 18

Why is a single horseshoe vortex a poor approximation for an aerofoil

Answer 18

• predicts infinite velocity at the wing tips
• assumes lift per unit area is constant
• fails to accurately predict induced drag

Question 19

What does Prandtl lifting theory do and what is it good/bad at predicting

Answer 19

Superimposes horseshe vortices

Good: moderate/high AR

Bad: Low AR, swept wings

Question 20

For an elliptical wing, what is the equation for induced angle of attack

Answer 20

αi = Cl / πAR

Question 21

For an elliptical wing, what is the equation for Coefficient of lift-induced drag

Answer 21

Cdi = Cl^2 / πAR

Question 22

List three properties about elliptical wings

Answer 22

• has the minimum induced drag for given lift
• Cl is constant
• αi is constant

Question 23

Define a planform

Answer 23

the shape and layout of a wing

Question 24

Give equation for the taper ratio

Answer 24

Chord tip / Chord root

Tapered wings are generally as good as elliptical ones and are easier to manufacture

Question 25

Give general equation for Cdi

Answer 25

Cdi = Cl^2 / π.e.AR

Question 26

What are winglets used for

Answer 26

used to smooth out the pressure discontinuity. | This leads to smaller trailing vortex wasting less energy and decreased lift-induced drag

Question 27

What is the result of having winglets

Answer 27

- lower induced drag - decrease fuel consumption (3-5%) - smaller wings possible

Question 28

Define sweep angle

Answer 28

angle between line drawn from 25%c of wings, and the lateral axis

Question 29

What shape are delta wings and when are they used

Answer 29

triangle - used for supersonic flow

Question 30

Give benefits of delta wings

Answer 30

- structurally efficient - low wave drag - large chord means high fuel volume - sufficient leading edge sweep can produce positive vortex lift - stable in pitch so tail is not required - relatively inexpensive to build - high stall angle

Question 31

Give disadvantages of delta wings

Answer 31

- low speeds require high angle of attack - vortex lift means increased drag - more powerful engine required to maintain low speed - more viscous drag i.e. low L/D ratio - flaps are harder to integrate