Basics

Question 1

Moment Coefficient equation

Answer 1

Cm = M / q.S.L

Question 2

What is aerodynamics

Answer 2

The behaviour of air flow around an object and the effect it has on it

Question 3

List things to think about when designing in 2D

Answer 3

1. Assumes infinite wing span
2. Inaccurate for finite wings
3. Approximation is invalid at wing tips
4. Approximation is okay for wings with large AR
5. For swept wings, the section and the coefficients need to be transformed accordingly

Question 4

Reynolds number =

Answer 4

Re = pVl / u

p - density
u - viscocity
l - reference length

Question 5

Mach number =

Answer 5

M = V/a

a - speed of sound

Question 6

If 2 geometries are similar, the air flows are said to be …

Answer 6

Dynamically similar

Question 7

List properties of Flow similarity

Answer 7

Cl1 = Cl2
Re1 = Re2
M1 = M2

• Streamline patterns are the same

Question 8

Why is flow similarity useful

Answer 8

• Can do scaled tests in wind tunnel (cheaper)

Question 9

To match Re of real sized objects, smaller models have to either …

Answer 9

1. Have a higher density/pressure
2. Lower temp/viscosity
3. Higher velocity

Question 10

List compressibility categories and there related Mach numbers

Answer 10

Incompressible - M<0.3

Subsonic - 0.3

Question 11

For inviscid compressible flow what is Cl a function of

Answer 11

Cl = f(Mach)

Question 12

For viscid incompressible flow what is Cl a function of

Answer 12

Cl = f(Reynolds)

Question 13

What is the centre of pressure

Answer 13

The location where the resultant of a distributed load effectively acts on the body

Question 14

Equation for Centre of Pressure

Answer 14

Xcp = - Mle / n

n - normal force
Mle - Moment

For small incidence:
Xcp = Mle / L

Question 15

How does centre of pressure move with reducing lift

Answer 15

Xcp moves downstream as lift reduces (can be outside aerofoil)

Question 16

What is the aerodynamic centre

Answer 16

Fixed point about which the moment is independent of incidence (about 25% of chord length behind leading edge)

Question 17

How do thickness and viscous effects change the aerodynamic centre

Answer 17

moves aerodynamic centre upstream

Question 18

How does compressibility effects change the aerodynamic centre

Answer 18

moves aerodynamic centre downstream

Question 19

Equation for vorticity

Answer 19

Vorticity = ∇ x V(velocity vector)

Question 20

What does vorticity equal if flowfield is irrotational

Answer 20

vorticity = 0

Question 21

Define circulation

Answer 21

the negative line integral of the velocity vector around a curce

Question 22

For an irrotational flowfield, the circulation of a closed curve equals =

Answer 22

zero (contained in the flowfield)

Question 23

What are the boundary conditions for inviscid flow

Answer 23

for inviscid flow, the flow cannot penetrate the solid surface i.e. velocity vector is tangent to the surface

Question 24

What is Kelvin’s circulation theorem

Answer 24

circulation around a closed curve consisting of the same fluid elements remains constant as the fluid elements move through the flow

Question 25

What is Kutta’s condition

Answer 25

Due to viscous effects, the flow has to leave the trailing edge smoothly. This results in circulation and therefore lift

Question 26

The lift generation per unit span on an aerofoil is given as =
(Kutta-Joukowski theorem)

Answer 26

l = ρVΓ

Γ - circulation

Question 27

What is the origin of the circulation around the aerofoil

Answer 27

1. Before aerofoil moves there is no circulation
2. As aerofoil movies a vortex rolls up and around the sharp trailing edge and is flushes downstream
3. This starting vortex has a circulation with the same magnitude but opposite sign as that around the aerofoil

Question 28

What is the thin aerofoil theory

Answer 28

A thin aerofoil can be simulated by a vortex sheet places along the camber line

Question 29

Equations for a symmetrical thin aerofoil

Answer 29

Cl = 2πα
dCl/dα = 2π
Xcp = Xac = Xc/4

Question 30

Equations for a chambered thin aerofoil

Answer 30

Cl = 2π(α-α(l=0))
dCl/dα = 2π
Xac = Xc/4

Question 31

Explain panel methods to solve for the incompressible inviscid flow field

Answer 31

1. Aerofoil surface divided into ‘panels’
2. Elementary solutions of unknown strength are linearly distributed on these panels
3. velocity/boundary conditions give n+1 equations and n+1 unknowns
4. Solutions of linear system gives functions for the potential flow fields
5. From this, work out velocity vector and then pressure from Bernoulli;s equation
6. Deduct forces and moments from surface pressure

Question 32

What application can the panel method have

Answer 32

used for 3D geometries

Question 33

Explain digits in NACA-4-series

Answer 33

NACA2412
2 - 2%chord length is the max camber
4 - 40%chord length is the position of max chamber
12 - 12%chord length is the thickness

Question 34

Explain digits in NACA-6-series

Answer 34

NACA65-218
6 - indicates 6 series
5 - 50%c is the minimum pressure
2 - 0.2 is the design lift coefficient
18 - 18%c is the thickness

Question 35

Describe stall

Answer 35

• due to viscous effects a boundary layer forms on the aerofoil surface
• as the streamwise adverse pressure gradient increases the boundary layer reduces
• at certain point the boundary layer separates
• after separation from the upper surface, the lift drops and drag increases
• this is known as stall

Question 36

As Re increases Cl …

Why

Answer 36

Clmax increases

This is because at high Re, the boundary layer is more resistant to separation

Question 37

As Camber increases Cl …

Answer 37

Clmax increase

• shifts curve up and left
• stall angle reduces
• increased Cdmax too

Question 38

As thickness increases Cl …

Why

Answer 38

Clmax increases up to a certain point, then reduces again.
This is because thin aerofoil have smaller leading edge radius, leading to higher adverse pressure gradient
Drag penalty with increased thickness