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Aviation History
1950
1950 - 0441.PDF
FLIGHT. 2 March 1950 295 SAILPLANE DESIGN and OPERATION Some Concise Comments by R.Ae.S. Graduates* HIGH-PERFORMANCE GLIDERS By F. G. IRVING, B.Eng., D.I.C., Grad.R.Ae.S. ALTHOUGH allowance must be made for the effects of , **• the war, no glider has yet been made in this country with a performance better than that of the German Weihe of about 13 years ago. The Weihe has a profile drag coefficient of 0.016, and uses a Gottingen 549 wing section. In these days of low-drag sections, it should be possible to improve that figure, and by cleaning up the wing-fuselage junction, deleting external aileron horns and so forth, something of the order of 0.012 should be feasible. A logical design procedure would then be to aim for the best mean cross-country speed for some chosen '' average thermal strength." Hpw strong? Probably, for this country, not more than 5ft/sec true, assuming, say, 0.5ft/sec "down-draught" between thermals. Proceeding in the manner explained by K. G. Wilkinson in his lecture [R.Ae.S. Graduate and Student Section, September 28th, 1948], the optimum wing loading for these conditions can be found, the assumed value of Cuo. and the highest aspect ratio which is structurally reasonable. Given suitable structure-weight data, a suitable span can then be chosen. Apart from the usual assumptions about weight of pilot, and so on, it would be wise to allow for about three times the amount of miscellaneous equipment considered by any previous designer. Oxygen, batteries, rotary converters and radio should not be accompanied by prickings of con- science about all-up weight. It seems probable that the final answer would involve a span of about 65ft, an aspect ratio of 20 or 22, a wing loading of about 4.5 lb / sq ft and the use of flaps when circling in thermals. One would also like a higher rate of roll than is at present usual, more directional stability, a rudder which does not over-balance in a side-slip, 3m more elbow- room and a maximum aero-towing speed of about 80 m.p.h. It must also be at least as easy as the Weihe to rig, and a good ideal more accessible for adjustment of turn buckles and so forth. Some method of construction, such as that called for by H. Kendall in the design which won the <IB.G.A. two-seater design competition would help to keep the price practicable. * Published in t ie Graduate and Student Section of the " THE ICING OF SAILPLANES By P. RIVERS, B.Sc.(Eng.), Grad.R.Ae.S. "\X7HILE strenuous efforts are being made to protect all '" aircraft likely to fly in icing conditions, high-flying sailplanes, which in summer are more likely to meet icing than any aircraft, are unable to carry any general protec- tion whatsoever. The lack of a power supply rules out all forms of wing protection, mechanical, fluid, or thermal, but so far no major aerodynamic trouble has been caused by ice- formation, as the low speed of the sailplane renders the actual catch of water small, and keeps the ice- formation down to a strip a few inches wide along the leading edge. The only danger in this direction lies in the possible jamming of the controls by ice forming on exposed control horns or on the tips of horn balances; the former can be shielded fairly easily, while horn balances are going out of favour on advanced machines. The icing-up of air brakes is more serious, since these are the sailplane pilot's stand-by if he reaches a dangerous high speed condition while flying blind; they must be kept free at all costs. They can be prevented from icing in the closed position by gentle exercising of them every minute or so, but they ice up rapidly when out, so that a pilot may have to land with the brakes jammed open. The main concern of pilots is to keep their blind-flying instruments operating. For this reason electrically driven turn-and-slip indicators are used in preference to venturi- driven ones. The air-speed indicator is still vulnerable, however, as it must operate from a venturi or pitot tube, which will ice up. Pitot tubes rarely block during icing, as the ice forms a continuation of the tube, but during descent the ice thaws and runs back into the tube. Heated pitot- static heads for power aircraft consume 70-100 watts, so are impracticable for sailplanes; most of this heat is required to keep the complete head ice-free. The cockpit static pressure is accurate enough for sail- plane use, so the problem becomes one of protecting only the tip of the pitot tube. Recently a number of people have concentrated their attention on this point, and it seems possible that soon this bogy also will be laid. Journal of the Royal Aeronautical Society," February, 1950 HERMES IN THE AIR (Continued from page 289) only needs to be held for a few seconds. Wheel retraction seemed to have no appreciable effect on trim, and the flaps are linked with the elevator tabs and automatically com pensate for trim change when moving up or down. For example, the control wheel itself moves-backwards a little as flaps come in, and the tabs alter the trim. The flap lever is gated and has a maximum lift (40 deg) setting and a half-way setting between this and fully (80 deg) down. In spite of its size (and assuming, of course, that no emergencies arise to require the attention of an engineer) the pilot of the Hermes has all the essential controls at his finger-tips and could fly the aircraft on solo circuits. After the take-off I flew the aircraft around- for a few minutes to "cool off and build up," and then settled down at about 1,400ft for a final circuit and landing. The air- craft needed persuading down to a speed of 180 kt, even when well throttled back, but once the wheels had been lowered quite high power—2,400 r.p.m. and 30-32U1 boost —was required round the last half of the circuit to main- tain 150-160 kt. For the last turns I used 40 deg of flap and kept the speed at 140-150 kt. There was plenty of drift on the final approach, and with full flap and a speed °f 115 kt, between 25 and 30m of boost were required to maintain speed and a steady descent. If speed is allowedto get too low the sink is pronounced and a good deal of. power is required ior recovery. . Over-the-hedge speed of 105 kt land I cut things pretty fine to get a short touch-down) was adequate. Both the drift and a flock of birds—at least one of which we hit with a pronounced thud—attempted to upset my landing, but the Hermes touched down firm, fair and square without much assistance from me, and showed no tendency to bounce or swing. It had required a good strong pull and plenty of aft trim to keep the tail low for touch-down, and it was simplest to let the nose wheel touch-down very shortly afterwards. I did not use the powerful brakes until fairly late on the run but was able to stop smoothly and in plenty of time to turn off. The pilot's view on the approach is good, and even on the final turn it is possible to keep the runway well in view. The natural approach is, however, rather long and shallow, and compared with some contemporary American airliners the Hermes would be considered under-flapped. It is a matter of regret to everyone concerned that the Hermes IVs are not already in service, but even so they are. expected to prove useful members of the B.O.A.C. fleet for several years to come. Above all, the Hermes is a very British aircraft in conception, appearance, and particularly in the way it handles There are still all too few British airliners flying on world routes and too many foreign .air- craft flying for British operators. (Delivery of the first Hermes is reported as we go to press.)
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