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Post by Br. Marius on Apr 28, 2015 18:50:46 GMT
Justify the following:
(a) Ground resonance is a mechanical instability.
(b) Through a quasistatic torsion model, the important coupling terms are retained to predict flap-lag flutter
(c) A faulty lag damper can lead to disaster.
(d) Taking-off from a ship deck requires a lot of attention.
(e) Reducing rotor inertia may help with the ground resonance problem, but is not recommended in a rotor design.
(f) For a rotor test in the wind tunnel, the characteristics of the balance are very important and are determined carefully using the shake test prior to rotation of blades. (Ground resonance point of view).
(g) One has to be careful to take-off a helicopter during cold weather.
(h) A small multi-bladed (5 or more blades) bearingless rotor (such as MD 900) may not be practical for civil applications. \(\newcommand{\dd}{\; \mathrm{d}} \newcommand{\sstar}{\;\star\star} \newcommand{\Star}[1]{\stackrel{\star}{#1}} \newcommand{\SStar}[1]{\stackrel{\sstar}{#1}} \newcommand{\ihat}{\boldsymbol{\;\hat{\imath}}} \newcommand{\jhat}{\boldsymbol{\;\hat{\jmath}}} \newcommand{\khat}{\boldsymbol{\;\hat{k}}}\)
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Post by Br. Marius on May 1, 2015 0:24:23 GMT
(a) Ground resonance is a mechanical instability.
Ground resonance is an interactions between the cyclic momentum imparted to the rotor hub by an unbalanced lag mode and the helicopter’s landing gear suspension. It does not require aerodynamics to exist.
(b) Through a quasistatic torsion model, the important coupling terms are retained to predict flap-lag flutter
Since the inertial and damping terms from torsion and generally small at high torsional frequencies, they can be neglected. The important coupling terms from torsion (static stiffness terms relating flap and lag motion to torsion) are retained in order to provide the necessary coupling effects.
(c) A faulty lag damper can lead to disaster.
Without damping, the lag frequency of the rotor blades is excited by rotor motion, causing extreme hub moments and vibration.
(d) Taking-off from a ship deck requires a lot of attention.
Since ground resonance is a mechanical instability and is related to the characteristics of the landing gear’s suspension system, a flexible deck changes the effective properties of that suspension system and can enable ground resonance unless preventative measures are taken.
(e) Reducing rotor inertia may help with the ground resonance problem, but is not recommended in a rotor design.
Reducing rotor inertia also decreases the amount of kinetic energy that can be stored by the rotor, decreasing the factor of safety in autorotation.
(f) For a rotor test in the wind tunnel, the characteristics of the balance are very important and are determined carefully using the shake test prior to rotation of blades. (Ground resonance point of view).
An off-balance rotor would create a similar resonance effect as a rotor that was off-balance in lag. Because of this, it is of great interest to ensure that one’s rotor is balance prior to testing as, particularly in a wind tunnel test (where damping is minimal), it could mean destruction of the rotor and dispersion of its components at high speed into the tunnel facility.
(g) One has to be careful to take-off a helicopter during cold weather.
In colder weather, mechanical dampers (such as those often used on landing gear) are not as effective, increasing the chances of ground resonance.
(h) A small multi-bladed (5 or more blades) bearingless rotor (such as MD 900) may not be practical for civil applications.
While such a rotor could, potentially, increase performance, a multi-bladed bearingless rotor would require a significant maintenance cost per flight hour. Hence, a slightly larger articulated or teetering design could reduce maintenance cost due to their design (e.g., less expensive parts and easier maintenance) and lesser number of blades.
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