Definitions and nomenclature

Question 1

What is dimensional analysis?

Answer 1

Using the fact that a physical problem needs to be dimensionally consistent

Question 2

What is buck-pi and how is it used?

Answer 2

Explain how it is used in practice. A complex physical relation which is dependent on many parameters can be reduced to a relation that is dependent on dimensionless groups.

Question 3

What is dynamic similarity

Answer 3

If two flows are dynamically similar we can directly compare one with the other. Allows for reduced or englarged prototype testing.

Question 4

When are flow situations dynamically similar?

Answer 4

1. The bodies and solid boundaries are geometrically similar.
2. The similarity gourps are the same (pi-groups)

Question 5

What is the reynolds number?

Answer 5

Re=rhoUL/mu
DensityFreestream Velocitycharacteristic lengths scale/ Dynamic viscosity

The ratio of inertia Forces divided by viscous forces.

Generally important for all types of fluid problems

Question 6

What is the Froude number?

Answer 6

Fr=U/sqrt(g*L)
The ratio of inertia forces to gravitatonal forces.
Relevant in problems with flow with a free surface

Question 7

definition of Euler Number

Answer 7

Eu=p/rho*U^2
Ratio of pressure forces to intertia forces
Relevant in problems where pressure or pressure differences are of interest.

Question 8

definition of Cauchy Number

Answer 8

Ca=rho*U^2/E_v (E_v=Bulkmodulus)
Ratio of inertia forces and compressibility forces.
Flows in which compressibility of fluid is important.

Question 9

definition of Mach Number

Answer 9

Ma=U/a (a speed of sound)
Ratio of inertia forces and compressibility forces.
Flows in which compressibility of fluid is important.

Question 10

definition of Strouhal Number

Answer 10

St=omega*l/U
Local intertial forces/Convective inertia forces
For unsteady flow with a characteristic oscillation frequency.

Question 11

definition of Weber Number

Answer 11

We=rhoU^2L/sigma (sigma surface tension)
Inertia forces/Surface tension forces.
Problems in which surface tension is important.

Question 12

What are the aerodyanmic forces acting on an object?

Answer 12

• Lift= perpendicular to free stream
• Drag= Parallell to free stream
• Normal force= perpendicular to chord
• Axial force= Parallell to chord.
• Cross wind force (y-direction).
• Weight
• Thrust

Question 13

What are the aerodynamic moments acting on an object?

Answer 13

Rolling moment (x-axis).
Yawing moment (z-axis).
Pitching moment (y-axis) Positive when it tends to want to pitch nose up.

Question 14

definition ofCoefficient of lift?

Answer 14

C_L=L/(1/2rhoU^2*S)

C_l=L/(1/2rhoU^2*c) (lift/meter wing)

Question 15

definition of Coefficient of drag?

Answer 15

C_D=D/(1/2rhoU^2*S)

C_d=D/(1/2rhoU^2*c)

Question 16

definition of Moment Coefficient?

Answer 16

C_M=M/(1/2rhoS*c_bar) (c_bar =mean chord)

C_m=M/(1/2rhoc*c_bar)

Question 17

definition of Coefficient of pressure?

Answer 17

C_p=p-p_inf/(1/2rhoU^2)

Question 18

Airfoil nomenclature?

Answer 18

Chord, Camberline, Thickness ratio, Wingspan, Aspect Ratio, Incidence

Question 19

What is Chord?

Answer 19

Distance from leading edge to trailing edge.

Question 20

What is Camberline?

Answer 20

Take half the distance between upper and lower surface. Measure this point to the chordline. The camberline is the locus of all these points.

Question 21

What is Thickness ratio?

Answer 21

Divide the distance from the upper surface to the lower surface by the chord

Question 22

What is meant by Spanwise direction?

Answer 22

The direction of on the wing from tip-to-tip (y)

Question 23

What is Aspect Ratio?

Answer 23

b^2/S (b- span width) The ratio of span length to chord length.

Question 24

What is Wingspan

Answer 24

length of the wing, tip-to-tip.

Question 25

What does NACA stand for?

Answer 25

National advisory committe of aviation

Question 26

NACA 4-digit series

Answer 26

1st digit: Max camber (percent of chord length) 2nd digit: Location of max camber (tenths of chord) 3&4 digit: Max thickness ratio.

Question 27

NACA 5-digit series

Answer 27

Ex: NACA LPSTT L= Single digit indicating max C_L at optimal alpha (CL=0.15*L) P= Max camber point (x=0.05*P) S= if 0 simple, if 1 camber reflexion point exists. TT= Max thickness ratio in percent of chord.

Question 28

How is lift generated?

Answer 28

long answer

Question 29

What is Aerodynamic centre?

Answer 29

The AC is the chordwise position where the pitching moment is independent of aplha dM/d(alpha)=0

Question 30

What is Centre of pressure

Answer 30

The centre of pressure, X_cp, is the chordwise position at which the pitching moment is zero.

Question 31

What is Centre of gravity

Answer 31

The point over which the aircraft would balance.

Question 32

How should AC, X_cp, CG be arrenged?

Answer 32

We want the moment about the CG to be positive for all positive lifts. AC behind CG.

Question 33

Pros and cons of Sweep angle

Answer 33

Angle between wing and centerline normal. + delays high mach effects since it reduces the effective free stream velocity - performance loss at low speeds. -Tip stalling problem.

Question 34

Pros and cons of Dihedral angle

Answer 34

``` Angle between xy-plane and wing. gamma>0 Dihedral wing gamma<0 Anhedral wing + Roll stability (fbd why) - Performance loss ```

Question 35

Pros and cons of Thickness Ratio

Answer 35

+ Structural reasons | -In general increased C_d,min with increasing thickness

Question 36

Pros and cons of large Aspect Ratio?

Answer 36

+Higher aerodynamic efficiency - less manoverable - Structurally challenging.

Question 37

Pros and cons of Wing taper

Answer 37

+Reduced weight -Bad stalling characteristics + Reduced tip vortex strength, look at dimensional lift, reduce c and lift decreases towards wingtip => smaller wing tip vorticies.

Question 38

Pros and cons of Wing twist

Answer 38

Change of alpha along span. + Main reason is to make sure that the flow over the control surfaces dont deattach first. We want to the wing to stall close to the fueselage first (alpha_root>alpha_tip)

Question 39

What is the parallell axis-theorem and how can it be applied for moving moment coefficients?

Answer 39

Babar. M_b=M_a+BAxR Or with nondimensional coeff. C_mb=C_ma+((b/c)-(a/c))*C_n