Part II Flashcards
How are aerodynamic forces generated
generate from the action of pressure and shear stresses
wetted surface area means
Surface area that is exposed to air
List four types of drag
Skin friction
wave drag
pressure drag
induced drag
Skin friction drag explained
caused by viscous shear stresses in the boundary layer
Wave drag explained
pressure forces caused in supersonic flow (shock waves)
Pressure drag explained
Pressure forces in the boundary layer
induced drag explained
caused by vortex induced velocity when lift is generated (only in 3D models)
Vorticity is defined as
w = ∇ x V w = 2Θ
where Θ = rotation of a fluid element
How can vorticity be spread through a fluid
only spreads through in an incremental fashion by diffusion or advection
Define vortex line
always tangent to the vorticity
Define vortex tube
closed curve with points indicating vortex line
vortex filament
fluid contained in a vortex tube
Helmholtz theorems:
1. Strength of Γ vortex is _______ along its length
2, Vortex cannot end inside the fluid it must:
- 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
In reality what happens to a vortex
in reality vorticity is diffused through the action of viscosity
Equation for induced angle of attack
αi = α - αeff
or
αi = /w\ / V∞
Define geometric wing twist
sideways on, the aerofoil of the wing would rotate as you moved away from the fuselage
Rotate down = washout (common)
Rotate up = wash in
Define aerodynamic wing twist
aerofoil would change shape as move further from the fuselage
Why is a single horseshoe vortex a poor approximation for an aerofoil
- predicts infinite velocity at the wing tips
- assumes lift per unit area is constant
- fails to accurately predict induced drag
What does Prandtl lifting theory do and what is it good/bad at predicting
Superimposes horseshe vortices
Good: moderate/high AR
Bad: Low AR, swept wings
For an elliptical wing, what is the equation for induced angle of attack
αi = Cl / πAR
For an elliptical wing, what is the equation for Coefficient of lift-induced drag
Cdi = Cl^2 / πAR
List three properties about elliptical wings
- has the minimum induced drag for given lift
- Cl is constant
- αi is constant
Define a planform
the shape and layout of a wing
Give equation for the taper ratio
Chord tip / Chord root
Tapered wings are generally as good as elliptical ones and are easier to manufacture
Give general equation for Cdi
Cdi = Cl^2 / π.e.AR
What are winglets used for
used to smooth out the pressure discontinuity.
This leads to smaller trailing vortex wasting less energy and decreased lift-induced drag
What is the result of having winglets
- lower induced drag
- decrease fuel consumption (3-5%)
- smaller wings possible
Define sweep angle
angle between line drawn from 25%c of wings, and the lateral axis
What shape are delta wings and when are they used
triangle - used for supersonic flow
Give benefits of delta wings
- 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
Give disadvantages of delta wings
- 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
