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Post by Br. Marius on May 7, 2015 15:15:52 GMT
Justify the following:
(a) Pitch and roll motions are important for air resonance instability.
(b) Roll mode plays more important role in air resonance instability than pitch mode.
(c) In hover, ground resonance analysis for a 2-bladed rotor is quite involved.
(d) Flap frequency is an important design parameter for air resonance instability.
(e) Stiff-inplane rotors can be quite safe from ground and air resonance instability, but rarely implemented in three or more bladed rotors. \(\newcommand{\dd}{\; \mathrm{d}} \newcommand{\sstar}{\;\star\star} \newcommand{\Star}[1]{\stackrel{\star}{#1}} \newcommand{\SStar}[1]{\stackrel{\sstar}{#1}} \newcommand{\ihat}{\boldsymbol{\;\hat{\imath}}} \newcommand{\jhat}{\boldsymbol{\;\hat{\jmath}}} \newcommand{\khat}{\boldsymbol{\;\hat{k}}}\)
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Post by Igor2 on May 11, 2015 2:19:11 GMT
(a) Pitch and roll motions are important for air resonance instability.
Air resonance is a phenomena with hingeless rotors. When the rotor tilts, hub moments develop giving rise to airframe oscillations. Low frequency lag and flap modes couple with the body pitch and roll modes. Therefore one needs to understand the pitch and roll to avoid air resonance instability.
(b) Roll mode plays more important role in air resonance instability than pitch mode. Roll mode of motion is likely to have frequency that is close to the lagging frequency of the blade. Usually the motion of the helicopter has fast rotor precession independent of the motion of the fuselage and slow precession where roll of fuselage has influence on the damping and frequency. The body of the helicopter appears to rock under the rotor a lot in roll mode exhibiting this “slow gyroscopic mode” that leads to air resonance instability.
(c) In hover, ground resonance analysis for a 2-bladed rotor is quite involved. See problem 2 of this homework assignment.
(d) Flap frequency is an important design parameter for air resonance instability. Low frequency flap mode is an important design parameter for air resonance. When the fuselage rolls and pitches it affects blade flapping so the corresponding Coriolis terms must be included as well as gyroscopic forces. Now blade flapping and fuselage motion are interacting with the lagging motion of the blade.
(d) Stiff-inplane rotors can be quite safe from ground and air resonance instability, but rarely implemented in three or more bladed rotors. Stiff-inplane rotors have lag frequency greater than 1 but they have higher loads and there is much more undesired vibration present. That is why soft in plane hingeless hubs offer reduced weight and maintenance to reduced vibration and loads when more than two blades are used.
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Post by Br. Marius on May 11, 2015 12:59:34 GMT
Where did you get (b) from? It makes sense to me why it would be true, but did you just intuit it or find it somewhere?
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Post by matthorr on May 11, 2015 13:06:19 GMT
I believe the answer to (b) is because of the difference between the Moments of Inertia - My >> Mx (Pitch MOI is much greater than Roll MOI). Pitch would also have more damping from the tail if that were included in the analysis.
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Post by Br. Marius on May 11, 2015 13:59:58 GMT
OK--that's a good deal. Thanks!
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Post by Deleted on May 11, 2015 15:03:10 GMT
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Post by matthorr on May 11, 2015 15:47:33 GMT
What does he know about it? I have one of his books, but have never used it. It's one of the classics in help dynamics. Good find.
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