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Post by Br. Marius on Apr 16, 2015 12:41:46 GMT
Justify the following:
(a) Why does flutter happen?
(b) Quasi-steady aerodynamics appears adequate to predict flap-lag flutter.
(c) A rotor resulted into limit cycle oscillations (instability) at a high altitude.
(d) Miller's approximation for near shed wake modeling is a very useful simplification for rotor analysis.
(e) Dynamic inflow is of no value for the prediction of vibratory loads.
(f) Fixed wing people always talk of wing stall (static), whereas rotary wing people always relate to dynamic stall.
(g) At a high forward speed, slowing the rotor reduces the compressibility effect, which appears beneficial, but rarely practiced. Discuss both performance and dynamic consequences of slowing the rotor. \(\newcommand{\dd}{\; \mathrm{d}} \newcommand{\sstar}{\;\star\star} \newcommand{\Star}[1]{\stackrel{\star}{#1}} \newcommand{\SStar}[1]{\stackrel{\sstar}{#1}} \newcommand{\ihat}{\boldsymbol{\;\hat{\imath}}} \newcommand{\jhat}{\boldsymbol{\;\hat{\jmath}}} \newcommand{\khat}{\boldsymbol{\;\hat{k}}}\)
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Post by matthorr on Apr 17, 2015 0:45:35 GMT
(a) Flutter is caused by the interaction or coupling of unsteady aerodynamic, structural and intertial forces (such as Coriolis Forces) to produce a self-sustained oscillation. (b) Quasi-Steady aerodynamics (c) A rotor entering into limit cycle oscillations at high altitude is likely do the the increased aerodynamic coupling as a result of the high blade pitch angles required to maintain thrust at high altitudes (low air density) and/or the blades stalling. (d) Miller’s approximation is a useful simplification since it states that only the shed wake from 15 degrees to 45 degrees behind the blade is significant. The remained of the shed wake can be neglected to reduce the amount of computation. For frequencies that are integer multiples of the rotor frequency, a simple expression for lift deficiency can be used. (e) Dynamic inflow models are simplified too much to be of use in predicting structural vibratory loads. Dynamic inflow represents a the low-frequency change in inflow across the entire rotor disk, not the local high-frequency changes needed for vibration analysis. For that application, more detailed lifting line or CFD (full unsteady aerodynamic) models are used. (f) In fixed wing, the wing is generally subject to a uniform flow field and the loss of lift proportional to the increased angle of attack. In helicopters, the blades are subject to a varying flow field and experience pitching motions that induce hysteresis in the blade lift and moment, creating a dynamic stall. (g) Slowing a rotor at high speed - reduce structural lag damping - move closer to struc- tural modes (fan plot) - affect all other normalized frequencies (affects ideal pre-cone, eigen analysis for a.e. stability) - increase effects of retreating blade stall - increase power/torque requirements for given thrust
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Post by matthorr on Apr 17, 2015 0:46:20 GMT
Anyone have a good answer for (b)? Other than "it's what you showed us in class, so it had better be good enough"?
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Post by Br. Marius on Apr 17, 2015 17:30:00 GMT
Yeah, right? I must've gone over that one a half-dozen times. I ended up saying that all the aerodynamic forces and their coupling effects that contribute to the potential for flap-lag flutter can be extracted from a quasi-steady analysis because contributing effects (e.g., wake interactions, Coriolis forces, flapping, etc) occur with a frequency of NbΩ. This relatively low frequency enables the use of a quasi-steady aerodynamic analysis to diagnose flap-lag flutter. A fully unsteady aerodynamic analysis is only necessary for assessing vibratory loads.
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Post by matthorr on Apr 17, 2015 17:46:19 GMT
That sounds like a good answer.
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