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Aero ME-445 > Definitions and nomenclature > Flashcards

Definitions and nomenclature Flashcards

1
Q

What is dimensional analysis?

A

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

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2
Q

What is buck-pi and how is it used?

A

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.

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3
Q

What is dynamic similarity

A

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

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4
Q

When are flow situations dynamically similar?

A
  1. The bodies and solid boundaries are geometrically similar.
  2. The similarity gourps are the same (pi-groups)
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5
Q

What is the reynolds number?

A

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

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6
Q

What is the Froude number?

A

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

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7
Q

definition of Euler Number

A

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

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8
Q

definition of Cauchy Number

A

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

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9
Q

definition of Mach Number

A

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

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10
Q

definition of Strouhal Number

A

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

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11
Q

definition of Weber Number

A

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

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12
Q

What are the aerodyanmic forces acting on an object?

A
  • 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
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13
Q

What are the aerodynamic moments acting on an object?

A 
Rolling moment (x-axis).
Yawing moment (z-axis).
Pitching moment (y-axis) Positive when it tends to want to pitch nose up.
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14
Q

definition ofCoefficient of lift?

A

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

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

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15
Q

definition of Coefficient of drag?

A

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

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

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16
Q

definition of Moment Coefficient?

A

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

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

17
Q

definition of Coefficient of pressure?

A

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

18
Q

Airfoil nomenclature?

A

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

19
Q

What is Chord?

A

Distance from leading edge to trailing edge.

20
Q

What is Camberline?

A

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

21
Q

What is Thickness ratio?

A

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

22
Q

What is meant by Spanwise direction?

A

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

23
Q

What is Aspect Ratio?

A

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

24
Q

What is Wingspan

A

length of the wing, tip-to-tip.

25
Q

What does NACA stand for?

A

National advisory committe of aviation

26
Q

NACA 4-digit series

A

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

27
Q

NACA 5-digit series

A

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

28
Q

How is lift generated?

A

long answer

29
Q

What is Aerodynamic centre?

A

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

30
Q

What is Centre of pressure

A

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

31
Q

What is Centre of gravity

A

The point over which the aircraft would balance.

32
Q

How should AC, X_cp, CG be arrenged?

A

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

33
Q

Pros and cons of Sweep angle

A

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.

34
Q

Pros and cons of Dihedral angle

A 
Angle between xy-plane and wing.
gamma>0 Dihedral wing 
gamma<0 Anhedral wing
\+ Roll stability (fbd why)
- Performance loss
35
Q

Pros and cons of Thickness Ratio

A

+ Structural reasons

-In general increased C_d,min with increasing thickness

36
Q

Pros and cons of large Aspect Ratio?

A

+Higher aerodynamic efficiency

  • less manoverable
  • Structurally challenging.
37
Q

Pros and cons of Wing taper

A

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

38
Q

Pros and cons of Wing twist

A

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)

39
Q

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

A

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

Aero ME-445 (6 decks)

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