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Aviation History
1947
1947 - 1976.PDF
FLIGHT NOVEMBER 13TH, T947 Engine-off Landings First Thorough Examination of an Aspect of Helicopter Flight Hitherto Somewhat Neglected SOME work with Rotating-wing Air-craft" was the title of a lecturedelivered to the Helicopter Asso- ciation of Great Britain on October 25th by Mr. O. L. L. Fitzwilliams, B.A. The lecturer is helicopter engineer at West- land Aircraft, Ltd., where preparations are now being made for the production under licence of the Sikorsky S-51. He was, during the war, at the Airborne Forces Experimental Establishment at Beaulieu, and the first part of his lecture dealt with the fun and games they had there when a dismantled Focke-Achgelis Fa 330 rotating-wing glider arrived. With no instructions as to how it worked, they had to resort to theoretical esti- mates and logical reasoning before fly- ing the kite from a trailer. Fortunately the estimates proved reasonably correct, and Mr. Robert Kronfeld was able to fly it successfully. In fact, it turned out to be quite docile. Mr. Fitzwilliams also gave some in- teresting particulars of the Focke- Achgelis Fa 223 twin-rotor helicopter which was the first helicopter to cross the Channel (in September, 1944), piloted by its German crew. Another section of the lecture dealt briefly with the activities of the West- land-Sikorsky S-51, and was followed by a film illustrating some of the points made in what was the main purpose of the Paper: landing a helicopter without using engine power. The film showed a Hoverfly I making engine-off landings at Beaulieu in 1945. So far as we know, this subject has not previously been dealt with in pub- lished form other than very superficially, and as it is of great importance in the future development of helicopters, it de- serves to be made available to the widest possible circle of readers. We under- stand that the lecture is to be published in full-in the Journal of the Helicopter Association, and later there may be a dis- cussion, for which there was no time on October 25th. Following is an almost verbatim report of this part of Mr. Fitzwilliams's lecture. There is still a good deal of argumentabout engine-off landings, and in discuss- ing them with all sorts of people I find adiversity of views and experience which is extraordinary, considering the length oftime that helicopters have been in operation. The subject is made up of a number of verysimple considerations which most people consider te be obvious, but, probably forthis reason, nobody seems to have bothered to analyze them in detail or to set them outin a logical sequence. Piloting Technique Our present confusion arises partly be-cause the requirements of an engine-off landing have not received adequate atten-tion in the design of our first genera- tion of helicopters, and partly because veryfew pilots have been trained ab initio on helicopters. Thus the early pilots havefound that they can pull oft forced land- ings in a helicopter with the aid of tech-niques previously learnt from the aeroplane and the Autogiro. These techniques arefamiliar, and their successful modification to suit the helicopter has enabled us toanswer a lot of awkward questions about what happens when the engine stops, butwhen one watches the modern pupil being taught to practise engine-off landings bydoing violent llare-outs or high-speed dives, followed by long floats over the ground,one cannot help thinking that an essential screw somewhere wants tightening. In practice, these techniques work re-markably well, but they are nowhere near what is required before the helicopter canbe considered suitable for large-scale use as a common means of personal transport. Alsotheir fanuliarity and apparent naturalness have led to their limitations being accepttdas inherent limitations of the helicopter, and it is now up to the designer to exposethis fallacy by providing helicopters which, in the event of engine failure, can be landedas helicopters instead of as imitation aero- planes or Autogiros. Although opinion among designers maynot be unanimous on this point, I believe the facts are sufficiently obvious to ensurean early improvement in the ease with iO 2030405060 7080 MPH. fL 11200 20 60 80 100 120 FT/SEC MIN. RATE OF DESCENT ROTOR P|< ROTORONLC 4 LINEA ENERGY USED TO AJTCEST LINEA ONLY 20 50 60 70 80" AIRSPEED W APPROACH GUDE(HPH) Fig. I. The use of linear energy in engine- off landing of a Hoverfly I (Sikorsky P-4B). which engine-off landings can be made, butin general it will take some time to produce new helicopters and to modify existing ones,to meet the new requirements. In the mean- time, before our confusion gets worse con-founded by current training programmes, I think there is a need to set out and examinethe considerations governing the perform- ance of an engine-off landing, so that weran at least have a common basis for dis- cussion of our present practice, and a com-mon understanding of the changes which may shortly be expected.To begin with, the most obvious thing about any method of performing an engine-off landing is that its primary purpose is to eliminate the downward velocity of theapproach glide. Therefore, during some part of the landing manoeuvre, the vertical com-ponent of the total air force acting on the aircraft must exceed the weight for longenough to allow the downward velocity to vanish. During the approach glide the aircraftis in equilibrium at a constant speed so that the total air force acting on it is ver-tical and equal to the weight, and the energy required to produce this force is suppliedby the potential energy which the aircraft is steadily losing by virtue of its descent. "I he supply of potential energy is, however,cut off at the same time as the descent is arrested, while the force required to do thisis at the same time greater than that which was steadily maintained in the approachglide. The necessary considerable supply of energy must, therefore, be tapped fromsome other source, and in an engine-off land- ing the only other sources are the kineticenergy which the helicopter possesses by virtue of its speed along the glide path,and the kinetic energy stored in the ro by virtue of their angular velocity. ttherefore follows that, while the downward velocity of a helicopter is being arrestedthere will be a reduction of the speed along the flight path or of the angular velocity ofthe rotors or of both; these three ways of using the kinetic energy which is availablecorrespond to the three types of possible engine-off landings. Employment of Kinetic Energy The considerations which govern the man-ner in which the kinetic energy is used are illustrated in Fig. i. This graph may lookrather complicated at first, but I think it will be quite easily followed if we start withthe thick arrow in the upper part of the graph, which is a polar diagram. The thickarrow represents the motion of a helicopter in steadv flight because its length indicatesthe speed of the helicopter and its down- ward inclination is the actual slope of theglide path since you will notice that the horizontal and vertical scales inside the bor-der of the diagram are marked off identically in ft/sec. The arrow rests on a curve which repre-sents approximately the glide performance of the Hoverfly I (Sikorsky R-4B) helicopter,and you will notice that if the tip of the arrow were moved from left to right alongthe curve, the length of the arrow would increase and its inclination would get lessuntil we arrive at the point A, where the arrow would indicate the gliding angle andflight path speed corresponding to the mini- mum rate of descent. After this the arrowwould get rapidly longer for small changes in the gliding angle, until at B it would be-come tangent to the curve and would then indicate the flight path speed for best glidingangle, which occurs near the top of the speed range for the Hoverfly I. Now if it is desired to eliminate the down-ward velocity of the approach glide by means of the kinetic energy of forwardmotion alone, the helicopter must be brought in at a fast glide, and the rotor incidencemust then be increased beyond the amount which is correct for steady flight. Therotor will then produce an excess thrust, and the aircraft will commence to do a nor-mal pull-out. Also the speed of the aircraft will fall during the pull-out, and the rotorincidence relative to the flight path will have to be continually increased so that foreach speed it is always more than the amount which would be correct for steadyflight. In this way the kinetic energy of forward motion is exchanged for the tem-porary excess thrust required to perform a pull-out. The result i« that, in general, thedownward velocity lias been eliminated at the expense of forward speed. If we consider a helicopter which is glidingfast so that the arrow tip is resting on B, then if the rotor incidence is increased bya small amount and the aircraft is subse- quently allowed to settle down in steadyflight, it will do so in a condition indicated by an arrow resting on some such pointas A, and the arrow will then be much shorter than before, indicating that a smallincrease of rotor incidence corresponds to a considerable loss of speed in steady flight
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