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Aviation History
1951
1951 - 0215.PDF
FLIGHT, 1 February 1951 139 mately 200 cyc/min), and also multiples and fractions thereof.In order to explore the practical implications of the low- frequency band of disturbances, an investigation had been under-taken by the B.E.A. Helicopter Unit on an S-51 machine,; in particular, explorations covered the frequencies and amplitudesin various directions and at various points in the structure under a variety of e.g. and flight conditions; they also covered the effectof change in such variables as main and tail rotor out of balance and out of track. The frequency band explored was up to 1,500cyc/min, and it was discovered that the predominant disturb- ances were of frequencies: — Approximately 200 cyc/min .... 1st main rotor order -„ 600 cyc/min 3rd „ „ „ „ 1,200 cyc/min 1st tail „ „ The records showed clearly that frequencies having values inthe region of 15/18, 30/33 and 56/60 cyc/min were present. Two important points which were brought to light by theinvestigations were, first, that a main rotor blade out-of-track was the most critical variable (one blade three inches out of trackwas sufficient to more than double the first- and third-order main rotor amplitudes). Secondly, when the S-51 was in its standardform in respect of balance and tracking, and flying at a speed of^ 67 m.p.h., the level of disturbance in the nose associated withthe third order was just within the limit of comfort denned by Constant Aircraft Vibration). At 90 m.p.h. the level was doubleConstant's threshold value. The level in the cabin at the cruising condition was approximately on Constant's threshold. The lec-turer went on to recommend the provision to the operator of an improved means of main rotor blade tracking, and added thatthe findings showed that there was no doubt that low-frequency disturbances were present and had to be catered for.The author then discussed flexible mountings and said that there was a need for development of a special type of flexiblemounting for the isolation of low-frequency disturbances from equipment. On the subject of acceptable vibration levels, Mr. McClementsreferred to the work of a variety of investigators, in particular a report by Goldman, of the Naval Medical Research Institute,Maryland (A Review of Subjective Responses to Vibratory Motion of the Human Body in the Frequency Range 1 to 70cyc/sec). There was still a good deal of speculation about acceptable threshold values tor the helicopter. In conclusion, the author stressed the need for more factualdata on acceptable noise and vibration levels; he believed that a good way of defining those levels was to allow the public tomake the decisions. B.E.A. now carried-helicopter passengers and could play a part by obtaining passenger and general-publicreaction to noise. Already there had been indications that valu- able information could be obtained in this way. During theB.EJV. passenger service last summer there were passenger com- plaints about the vibration level when the machine was knownto be rough in respect of the rotor third-order level of disturb- ance, but no complaints when the machine was known to benormal. This was an example of passengers detecting the mechanical vibration sensation threshold—which, incidentally,tended to agree with Constant's threshold. SOURCES of VIBRATIONM R. J. SHAPIRO (Cierva Autogiro Co., Ltd.), in makinghis "Survey of the Practical Approach to Character- istic Vibration Sources in Helicopters," dealt first withrotor unbalance. Rotor unbalance, he stated, was a once-per- rev impulse, and was thus normally a case for separation ofimpulse and response. The unbalance was a rotating force, the origin of which could be mass unbalance of the blades or geo-metric asymmetry in their distribution; the latter could be caused by (a) unequal pitch ; (b) errors in geometry of articu-lation; and (c) unequal damper settings. Response depended on rotor mountings. Rotor r.p.m. had to be removed from thenatural frequency of the hub on its mountings, and both sub- critical and super-critical mountings were practicable. In considering natural frequency, it had to be borne in mindthat a hub with rotating blades was a system with several degrees of freedom. If to a system with one degree of freedom..,. a second degree was added, the effect was that of introducing a second natural frequency, and the lower of the two was lessthan the lowest individual natural frequency. The sum total of the effect of rotating articulated blades was that of a further degreeof freedom, and the natural frequency of the system was iower than with equivalent masses representing the blade. Naturalfrequency could be computed if the dynamic and elastic characteristics of hub mountings were known. The naturalfrequency of the combined system was about 80 per cent of the stationary system. Of vibration due to asymmetry in forward flight, the lecturerstated that, in the maze of inter-relations required for a full i description, a useful approximation could be employed withi. some profit. In this, the blade was replaced by its centrifugal f force, imagined to be acting along the blade axis. The mentalpicture thus obtained gave a few useful rules; (a) first harmonic of flapping produced no fluctuating forces and moments in athree-blade rotor ; (b) second harmonic of flapping produced no fluctuating force other than a third harmonic fluctuating momentwith flapping hinge offset in a three-blade rotor; (c) third har- i monic of flapping produced third harmonic force and no move-ment in a three-blade rotor. The effect of forward flight asym- metry was therefore almost invariably a third harmonic. Inthis connection, the approximate rules which applied to flapping motion should be remembered, viz.,, the first harmonic wasproportional to tip-speed ratio" and, therefore, speed. The second harmonic of flapping was proportional to the square ofthe tip-speed ratio, and the third harmonic was proportional to its fourth power. The higher order of vibrations thereforemounted quickly with forward speed, and to reduce impulses arising from forward speed it was advisable to have a low order• of flapping hinge offset and a fast rotor with high inertia. '• Hinge motion was due to flapping forces of the same fre-: quency, and these forces resulted in a bending moment which induced elastic oscillation. It was usually the third harmonicwhich caused an unacceptably high order of elastic oscillation, because in most blades it was the nearest order to a naturalfrequency of the rotating blade. In blades of more or less current design it was sufficient to investigate the first threeclastic modes, but in unconventional blades any modes giving frequencies up to the fifth or sixth order should be examined.Blade motions about the drag hinge were of vital concern in several vibration systems. The lecturer stated that he was heredealing with forced oscillations due to impulses arising directly and indirectly from airflow asymmetry in forward flight. Directeffects were drag variations which caused movement about the hinge, and indirect effects were Coriolis forces which arose whenflapping took place in the presence of coning (fundamentally due to the radial component of flapping motion). First-order flap-ping produced, among others, a second-order Coriolis accelera- tion and, hence, motions. In motion about the drag hinge, itwas also useful to employ a substitute picture of the blade re- placed by centrifugal force acting along the blade axis. Combi-nations of three blades gave the following results: no fluctuating forces or moments arose from first-harmonic motions. Third-harmonic motion would produce no forces other than a third-harmonic moment which became a torque oscillation.Second-harmonic motion produced no moment but a fourth- harmonic motion on the hub. Many claims had been advanced for a rotor with rigidlymounted blades and a floating hub. A near relative was the two-bladed " semi-rigid " rotor which had no higher-order flap-ping, no free coning, and no drag freedom. It was blessed with mechanical simplicity, but afflicted with the peculiar vices oftwo-bladed rotors. One of these was a second-harmonic vibra- tion due to drag fluctuations, particularly at given forward speeds.On the whole, this form of rotor had probably the most difficult set of residual-force vibrations; yet among the machines inwhich this system was applied there were some with an excel- lent reputation for smoothness. This had been achieved byadjusting the suspension of the rotor both to take care of the residual impulse and to isolate the rotor from the machine. Infact, the success of this procedure had led to a general rule that rigid blades required flexible rotor-mountings, whilst rigidmountings could be iised only with articulated and/or very flexible blades. Dealing with forced vibration due to abnormal aerodynamicstates, Mr. Shapiro observed that tip-stalling due to high inci- dences at the lip of the rotating blade could give rise to acritical increase in impulse magnitude equal in frequency to r.p.m. multiplied by the number of blades. In a machine withan otherwise acceptable general vibration-level, excessive vibra- tion had been experienced when the analytically determinedaerodynamic incidence at the tip of the retreating blade reached an angle 4 deg above the stalling angle of the profile. It wasvisualized that, in view of the non-stationary nature of the stall, its effect was delayed. By means of blade twist, the use ofcambered profiles, and high tip speed, tip-stalling could be made to occur at higher forward speed. Steep descent in conditionsin which the rotor continu3usly worked in disturbed air (some- times known as the " vortex-ring state ") gave rise to oscillatoryimpulses of higher magnitude. There was some evidence to suggest that a once-per-rev impulse was possible, but this matterwas largely unexplored. In connection with forced vibrations due to engine torqueimpulses, i.e., torsional vibrations, there appeared to be no
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