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Aviation History
1948
1948 - 0213.PDF
FEBRUARY 12TH, 1948 FLIGHT 185 011 dynamic stability tests are given in Fig. 7. These havebeen selected for approximately the same initial- displacement and illustrate the development of the phugoid motion undervarious conditions. The dynamic stability characteristics are given in terms ofdamping factor and periodic time of oscillation in Figs. 8 (a) and (b)" respectively for the power-on conditions. It will beseen that there are large changes in the stability with speed. The most surprising effect is the sudden change at about 35m.p.h. from a rapidly divergent oscillation to an equally rapidly damped one within a speed range of a few m.p.h. The auto-rotation results are given in terms of the dampingfactor and the period of oscillation in Figs, 9 (a) and (b). In auto-rotation there is little change in the dynamic stabilitywith speed, an improvement taking place as the speed increases. The experimental points for the various centre of gravitypositions are shown in the illustrations. No difference in the behaviour of the helicopter can be detected with change inthe centre of gravity position. There is no difference in the motion set up from an initial forward displacement of the Xa. O t O-1O z a. O-1O 2O 30 4Q AIRS#EED-M.P ti -* 5O 6O X FOR? INITIAL DISPLACEMENT • BACK? .. 20 3O AIRSPEED-M.P.H PERIODIC TIME OF OSCILLATION 6O Fig. 9, a and b. Auto-rotation characteristics in terms of damping factor (a) and period of oscillation (b). stick compared with that from an initial backward displace-ment. It is particularly noticeable in flying a helicopter thatatmospheric gusts have very much less effect than that usually experienced on fixed-wing aircraft. This is a feature of theflapping-hinge blade system which may be regarded to some f tent as an "automatic gust-alleviator."1 theoretical investigation of the effect of gusts on theflapping-bladed rotor shows this effect to be of an entirely different nature from that produced by longitudinal stick dis-placement. Whereas a. fore-and-aft movement of the stick produces a longitudinal tilting of the disc, the main effect offlying straight into a gust gives a direct effect on the lift and a lateral tilt of the disc. This is, of course, due to the phasedifference between the change in incidence of a hinged blade and the position of maximum blade displacement. There is also atendency to produce a longitudinal tilting of the disc, but as this is only of the order of i/ioth Of the lateral moment,and the pitching moment of inertia is about ten times that in roll, the longitudinal effect may be considered as negligiblein comparison with the lateral effect. Stated in general and simple form, the effect of a fore-and-aft stick movementchanges the attitude of the helicopter, while flying straight into a gust causes the aircraft to roll. Blind-Flying Controllability Now that helicopters have come^into commercial use, oneof the most important considerations from the point of view °f successful operation is the ability to fly in practically anyweather conditions, particularly in limited visibility and at night. Such operation may be divided into two main sub-jects; first, the ability to maintain satisfactory control, using instruments and without reference to external visual aids; arM, secondly, navigational problems. Only the former is con-sidered here. „ Before discussing the instruments in particular, it cannotbe emphasized too strongly that ease of blind flying is funda- mentally associated with satisfactory stability and controlcharacteristics and, in particular, with the ability to trim out any forces in the pilot's controls. The main difference in blind-flying a helicopter comparedwith a fixed-wing aircraft is in thelongitudinal b e - haviour; laterallyand directionally, both types show very similar havio\ir. be- ATTITUDE' Fig. 10. Comparisons of climb and dive attitudes between helicopter and fixed- wing aircraft. Fjg. 10 gives acomparison of the helicopter attitudewith that of the fixed - wing air-craft. A low wing- loading has beenassumed for the latter to givespeeds comparable •with those of thehelicopter. A simi-' lar comparison canbe made with an aircraft of higherwing loading but the speeds would" - be so high that the comparison would have to be made generally in terms ofattitudes at top speed, cruising speed, climbing conditions and so on. The fundamental difference of the two types is shownby the lines of constant speed, where the helicopter attitude remains substantially constant for any rate of climb or descent,whilst that of the fixed-wing aircraft varies considerably. For example, the helicopter climbing at 1,000 ft/min at 60 m.p.Ti(11 deg angle of climb) and then descending at 100 m.p.h. and 2,000 ft/min has to change its attitude by about 5 deg.On the fixed-wing aircraft the change is about 35 deg. It is early in the stage of helicopter blind-flying development to provide definite conclusions, but it would appear that satisfactory blind flying can be done with existing in-struments with a modified flying technique. The most serious defect at the moment is the inability to give the pilot areliable method of relating the attitude of the helicopter to the speed, and some better form of longitudinal indication isrequired. The airspeed indicator is becoming more important and is considered as the master instrument for the longi-tudinal control of the helicopter. The angle-of-bank indi- cation of the artificial horizon and the directional gyro remainthe master instruments in the other planes. Under these conditions, flying in cloud and " under thehood," and night flying, have been done quite successfully on the Sikorsky R-4B, but considerable concentration is re-quired from the pilot because of the unsatisfactory stability and control characteristics of this type of helicopter. In his concluding remarks, the lecturer observed that itis in stability and control that helicopter development is most backward. No one can claim that the flying characteristicsof the helicopter are at the moment satisfactory. The development of a helicopter which can be flown with thesame ease and lack of fatigue and concentration as that now- associated with the fixed-wing aircraft may take some time.Only the simultaneous advancement of the stability and con- trol flight work with the theoretical treatment of the subjectcan show where these improvements are to be found. The helicopter as we know it at the moment is completelyunsatisfactory dynamically under stick-free conditions, and although conditions are improved with the stick fixed, the'stability characteristics cannot be said to approach anywhere near the desired standard. The vital thing is for somemeans of accurately trimming-out the forces in the pilot's stick. This, and the divergent movement of the stick whenit is freed, have led to the fitting of irreversible controls on recent American types. While it may be of considerable assistance to the pilot at the present stage of development, it has several disadvantages and may not be the final solution to the unsatisfactory stick- free behaviour. The ability to fly the helicopter on instru- ments in poor visibility, or at night, makes even higher demands for the improvement of stability and control charac- teristics.
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