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Aviation History
1947
1947 - 2036.PDF
584 FLIGHT NOVEMBER 2OTK, 1947 Engine-off Landings effective pull-up, because if we consider theright-hand end of the thick curve, where it crosses the slanting line, it is obviousthat although the pull-up cannot be con- tinued at the same high thrust, it could• piite well be continued at a lower thrust, in which case the helicopter would havestarted to go up again before blade stalling occurred. In a typical landing in which the pull-upis done at constant thrust, the motion of. the pitch lever would be as shown in Fig. 9,where the sharp increase in pitch at the beginning of the pull-up corresponds to thesudden increase of thrust to the value which is- afterwards held constant. (The figures111 this diagram are calculated in a rather crude way, and are intended to indicatemotion of the pitch lever rather than abso- lute values of bletde pitch, which is showntoo small by about a degree.) The sudden decrease of pitch at the endof the pull-up would then be necessary to prevent the helicopter from rising again. Onthe other hand, we have already noted that .the thrust in a pull-up ought to be high atthe beginning and low at the end, so that in the most effective pull-up the initial sharpincrease in pitch would be even more marked than in Fig. 9, while there ought to be aless marked reduction in pitch at the end of the pull-up. In fact, this diagram sug- "ijests that the most efficient pull-up is one in which the pitch is suddenly increasedby about 10 degrees and then left alone until it is wanted for final adjustments as thehelicopter settles on to the ground. This action of the pitch lever could, of course,be obtained automatically by pressing a button, but I do not think automatic devicesof that kind should ht encouraged. Ground Effect and Air Inertia Now if the pull-up had been performed inthe most effective manner, the original heli- copter could have stopped quite easily with-out the blade weight being increased at all, and at this point I was prepared to let thematter rest for the time being, because it was already clear that engine-off landings inpure vertical flight are possible within the range of our current design practice. But,as I mentioned, not only had the ground effect been neglected in hovering, but alsothe inertia of the air had been neglected during the pull-up, and this latter featureof the estimates has since been the subject of quite severe criticism on the grounds thatthey are too pessimistic. Now the assumption of constant rotorthrust during the pull-up implies that at the beginning of the pull-up the thrust isinstantaneously increased to n times the weight of the aircraft, and the reason whythese estimates are pessimistic is that to use the curve of Fig. 5 in the way it wasused is equivalent to saying that the velocity of the downward-induced flow is also in-creased instantaneously ~to n times its normal value. But the induced flow is not some-thing which happens only in the plane of the rotor—on the contrary, to speed it upentails altering the whole flow pattern around the rotor, and the inertia of the airinvolved in this flow pattern is considerable, so that an appreciable time is required toestablish a. change in the induced velocity. I do not know of any reliable method ofestimating th^s'time, but certainly in a pull- up the induced velocity would not increaseto the value appropriate to the vortex-ring state through which the helicopter isassumed to pass in the estimates! we have considered. Also, if th.e' pull-up is quick, itwould in practice be performed close to the ground, which would further interfere withthe build-up of induced velocity, and in these circumstances I think it is not un-reasonable to suppose that the increase in induced velocity is small. If this is so, the effect on the performanceof the pull-up is very considerable, as shown in Fig. 10, which illustrates the performanceof the original helicopter with light blades, assuming the induced velocity remainsthroughout the pull-up at the value appro- priate to steady vertical descent. Now thisdiagram and the previous estimate for this helicopter represent two extreme possibilities,and the truth lies somewhere between them. For instance, an unaltered induced velocitycannot reasonably be accepted in the case of the right-hand curve, which represents apull-up started at 100ft and lasting over 4» seconds. In this case the previous esti-mate of a final velocity of 12 ft/sec is prob- 130 • INITIAL ROTOR gPEED> HEIGHT OF PULL-UP TIME OF PULL-UP , HOVERING TWE ilWITH GROUND EFFECT) n; TOTAL TlfriE .455467 486 (SECSK 72 TIME t (SECONDS) Fig. 10. Helicopter A. Induced velocity unaltered by pull-up. ably somewhat nearer the truth. On theother hand, it is very unlikely that anyone would start a pull-up at iooft, and, whenwe consider the more usual height of 4o-5oft, it is clear that the pull-up would be overin approximately two seconds and would take place largely within the ground cushion.In these circumstances there will probably not be any large build-up of the inducedflow, and 1 believe the figures quoted in this diagram are close to the truth for typicalpull-ups at constant thrust in actual future landings. But we have already noticed that therotor thrust should be high at the begin- ning of a pull-up and low at tbe end formaximum effectiveness, and this is very clearly indicated in the figure, which showsthat, in the first half-second of a pull-up, the pilot has at his command a brake ofalmost unlimited power, within the range which he is likely to require, and whichcosts practically nothing in rotor energy. The slight flaring of the rotor when thepitch is increased is a phenomenon which must occur whenever the thrust of an auto-rotating rotor is suddeny increased without a correspondingly sudden increase in theinduced flow. The phenomenon is very short-lived in vertical flight, because it canonly occur while air is flowing upwards through the rotor, whereas the extra thrustvery quickly reduces the rate of descent to ' less than the velocity of the induced flow,after which the flow through the rotor is downward. In practice, the phenomenonwould not produce any significant increase in rotor speed, but it would be noticeableas a small time-lag between the extra thrust and the falling off of rotor speed.When allowance is made for the inertia of the induced flow, the corresponding esti-mate of the motion of the pitch lever during the landing will also be altered, and theinitial sharp increase in pitch shown in a previous diagram would correspond to amuch larger increase in thrust, so that the collective pitch control may be expected tnbe more sensitive and more powerful than was originally estimated. The speeds and decelerations shown in this,diagram for a typical pull-up from, say, 50ft, are praciiically the same as in the extra floatat the end of an ordinary engine-off landing, namely, am initial speed of 30 m.p.h. and amean deceleration of slightly more than a half of iG/ Considering the extra brakingpower which is available, if required, and the allowance of, say, two seconds of hover-ing time for correction of mistakes, it does not appear that a landing of this kind wouldbe very difficult, unless it can be shown that judgment of height and speed in verticalflight is fundamentally different from that in horizontal motion. Personally, I donltthink there will be any difficulty of tM| kind, and I believe that if a pilot approachesthe vertical landing gradually in the course of his training he will find it fairly easy. Glides at Minimum Airspeed The vertical landing is certain to be animportant feature of a pilot's training, be- cause it is typical of all landings from glidesat the minimum airspeed. In the event of engine failure over built-up areas, or over-wooded or uneven ground, a glide at the minimum airspeed is attractive because itallows the pilot to steepen his glide path immediately he sees a clear space for land-ing, without waiting for the completion ol the pull-up which would intervene at higherspeeds. Also, when descending steeply, lie has complete freedom to use the azimuthstick in changing his flight direction without incurring changes in the flight path speedwhich would upset his judgment. Where the helicopter must be accurately positionedfor a pull-up in a confined space, this free- dom and directness in the approach will bevaluable features of a helicopter which is capable of vertical engine-off landing. The normal engine-off landing of such ahelicopter would naturally be done in the easiest circumstances, and these will corre-spond to a glide speed of 30 to 40 m.p.h. where the landing will include a little pull-out, a little flare-out, and plenty of pull-up, all combined in a manoeuvre which will beso easy, and will take such a long time anyway, that the power of the engine couldnot very well be used, even if it were still available. When we have got to this stage,we shall be able to return to the practice of the pre-war flying clubs, where we weretaught that every landing should be done as though it were a forced landing in nor-mally easy circumstances. I think this is a very wise teaching, and I hope that in thecourse of the next few years the use of engine power will largely disappear from thenormal landing of a helicopter. NOT AN ENGINE-OFF LANDING . Alan Bristow recently visited the Lympne Country Club and decided to land the Westland Sikorsky S5I on the 27 yd x 30 d iawn rather than on the nearby airfttld.
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