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MEC324 - Aerodynamic Design > Part II > Flashcards

Part II Flashcards

1
Q

How are aerodynamic forces generated

A

generate from the action of pressure and shear stresses

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2
Q

wetted surface area means

A

Surface area that is exposed to air

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3
Q

List four types of drag

A

Skin friction
wave drag
pressure drag
induced drag

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4
Q

Skin friction drag explained

A

caused by viscous shear stresses in the boundary layer

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5
Q

Wave drag explained

A

pressure forces caused in supersonic flow (shock waves)

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6
Q

Pressure drag explained

A

Pressure forces in the boundary layer

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7
Q

induced drag explained

A

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

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8
Q

Vorticity is defined as

A 
w = ∇ x V
w = 2Θ

where Θ = rotation of a fluid element

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9
Q

How can vorticity be spread through a fluid

A

only spreads through in an incremental fashion by diffusion or advection

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10
Q

Define vortex line

A

always tangent to the vorticity

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11
Q

Define vortex tube

A

closed curve with points indicating vortex line

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12
Q

vortex filament

A

fluid contained in a vortex tube

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13
Q

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

2, Vortex cannot end inside the fluid it must:

A
  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
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14
Q

In reality what happens to a vortex

A

in reality vorticity is diffused through the action of viscosity

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15
Q

Equation for induced angle of attack

A

αi = α - αeff

or

αi = /w\ / V∞

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16
Q

Define geometric wing twist

A

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

Rotate down = washout (common)

Rotate up = wash in

17
Q

Define aerodynamic wing twist

A

aerofoil would change shape as move further from the fuselage

18
Q

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

A
  • predicts infinite velocity at the wing tips
  • assumes lift per unit area is constant
  • fails to accurately predict induced drag
19
Q

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

A

Superimposes horseshe vortices

Good: moderate/high AR

Bad: Low AR, swept wings

20
Q

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

A

αi = Cl / πAR

21
Q

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

A

Cdi = Cl^2 / πAR

22
Q

List three properties about elliptical wings

A
  • has the minimum induced drag for given lift
  • Cl is constant
  • αi is constant
23
Q

Define a planform

A

the shape and layout of a wing

24
Q

Give equation for the taper ratio

A

Chord tip / Chord root

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

25
Q

Give general equation for Cdi

A

Cdi = Cl^2 / π.e.AR

26
Q

What are winglets used for

A

used to smooth out the pressure discontinuity.

This leads to smaller trailing vortex wasting less energy and decreased lift-induced drag

27
Q

What is the result of having winglets

A
  • lower induced drag
  • decrease fuel consumption (3-5%)
  • smaller wings possible
28
Q

Define sweep angle

A

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

29
Q

What shape are delta wings and when are they used

A

triangle - used for supersonic flow

30
Q

Give benefits of delta wings

A
  • 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
31
Q

Give disadvantages of delta wings

A
  • 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

MEC324 - Aerodynamic Design (6 decks)

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