Collar’s Triangle: What is it and what does it describe?
-> Described the relationship between the forces relevant for dynamic aeroelasticity and the special cases between to forces.
Plot cL against alpha and descrive what happens before, during and after stall.
Describe the dependency of L and M(AC) on theta and on x (vertical movement). Describe every parameter on the L and M(AC) equations (with defelction).
With aileron:
Equations:
Aerodynamic center?
Center of pressure?
Point at which moment is constant wrt AoA
Point of application of aerodynamic pressure field resultant (Changes with lift)
Fill stiffness matrix:
And derive for theta.
How can we derive for theta here?
Describe divergence physically. Give expressions for qD. How does it depend on aileron deflection angle.
Structural defelction due to aerodynamic loading, which enhances further the aerodynamic loading itself.
Detailed:
Lift in front of the elatstic axis leads to torsional nose up moment.
Wing is elastic and therefore moment leads to defelction
defelction angle increases the AoA and therefore the lift again -> leads to further increase of deflection angle and so on..
this happens till equilibrium between aerodynamic and elastic forces is reached or structure fails
-> The aileron deflection can lead to a decrease of divergence, because additional lift is created at the aileron, which leads to a nose down moment and decreases deflection angle.
Describe aileron reversal (physically and expressions) and propose real life solution.
Aileron reversal is a phenomena, which can occur while the aileron is defelected. Normally a delta > 0 would increase the total lift of the airfoil. When aileron reversal occurs the opposide happens and lift decreases:
additional lift leads to nose down moment which changes deflection angle, which influences AoA
decrease AoA leads to less total lift of airfoil
If the decrease of lift due to nose down moment is bigger than increase of lift due to the increased camber, the total lift decreases instead of increase -> aileron reversal.
Solution:
-> High and low speed ailero
low: lower dynamic pressure, low torsional moment, bigger moment arm
high: high dynamic pressure, high torsional moment, small moment arm -> less deflection angle because stiffness bigger close to fuselage
Choose the correct diagramm fors sweep and qD.
What is flutter physically? If Im(p) /= 0 , when do we have flutter?
Instability due to coupling of two or more modes of an elastic body exposwed to a fluid flow.
Flutter if Re(p)>0 of one mode
-> Independent to Im(p)
Mode diagramm given with damping.
Identify where flutter occurs.
Design the same diagramm without damping.
Describe k (rad frequ.). explain it physically.
What happens if k is too high?
dimensionless frequency
tells how much time a particle spends close to an airfoil while getting through an oscillation
-> Period of oscillation
-> wm = harmonic motion of airfoil at frequency wm
-> Interaction time
How much is the particle influenced by the socillation?
low ( k << 1)
quais steady behavior
flow can adapt to chanfe in AoA without much delay
middle (k = 1)
airfoil experiences unsteadyness in the flow field
responds less instantaneously to change of AoA
high (k >> 1)
airflow doesn’t have enough time to adjust to changes
unsteady effects, easier flow separation
less lift anss more drag
-> dynamic stall !!
Give two assumptions made in the u-g-method.
System always in flutter condition
Puts fake structural damping in the systemm which represents flutter condition
If real danping us equal to the fake struktural damping, system flutters
Three diagrams from u-g model for a specific k.
Choose correct one explain why.
-> Have closer look to lecture
Give two excitation methods and compare their pros and cons.
Control surface impulses
+ no extra equipment necessary
- repeatability difficult
+ can be done during flight
- pilot may not provide good level of excitation
Thrusters
- needs extra equipment
+ repeatability easy
- just single shot devices
+ excitation of proper amplitiudes
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