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Aviation History
1942
1942 - 0522.PDF
208 Topics of the Day FLIGHT \ X d> MARCH 5TH, 1942 QUITE HABITUAL A Mixed Causerie on Constant-speed Airscrews and Human Reflexes—With a Little Feathering Thrown in IT is, perhaps, a very mundane thing to say, but it never fails to surprise me how the extraordinary can so easily and quickly become the ordinary. Whether we are thinking of strange faces, strange ideas, or strange devices, the result is always the same; the face ceases to be strange after a week or, two, the idea ceases to surprise you, and the queer device becomes a commonplace. I well remember thinking, when the constant-speed airscrew first made its appearance, that this was an odd and even complicated affair the real usefulness of which hardly merited such complication. A two-speed or, at the most, a manually variable screw seemed to be all that was needed in any circumstances, and even when the fac totums of a big airscrew factory had spent many hours drumming the advantages of the c.s. unit into my addled brain I was still rather at a loss to see what all the fuss was about. Question of Familiarity Nowadays I am so used to the c.s. airscrew and its ' effects that I take it for granted and suffer considerably on the rare occasions when I am flying some older or un modified type with two-position or manually controlled airscrews. Earlier in this war I remember taking an old Tiger Whitley from one place to another; it was fitted' with two-position airscrews, and the effect of pulling t&e plugs was such that I almost thought the engines J4ad stopped. A fellow pilot, who was ferrying a second m/rHiine of the same type, actually landed it again beoiyfee he thought something really was the matter. MojpVe'cently—v I have flown machines which had not been^omoletply 1 finished and on which the revolutions have! haflVto I be manually adjusted to suit the varying conditions: ok V^e- off, climb, and level flight. The effort involved Was s\icf that I wondered how we ever managed without r^ie little mechanisms which so effectively keep the engine "revolu tions at a constant and efficient figure. Even now, there are plenty of semi-technical people on the ground who just cannot be made to see that a constant- speed airscrew really does just that—and if it doesn't, then it isn't working properly, or has the controls adjusted so that the mechanism, so to speak, has gone "over the edge" when the controls are in the take-off position. Of course, in quite a number of aircraft there is, in fact, a variation in engine speed during and after the take-off. The control, for instance, may have been set to give, say, 2,800 r.p.m. in the fully fine position, but on the run-up the engine may only give 2,600; the poor old c.s. unit is doing its very best and has wound the blades as far into the fine pitch angle as they will go in an endeavour to give the right engine speed in the very difficult circum stances of a static run. Nor is any mechanical device quite perfect, and there is inevitably some degree of lag in the operation of any c.s. airscrew—the revolutions momentarily jump when the throttle is opened, or vary a little in very bumpy air. Not very long ago I suffered an experience which pro vided me with the best possible lesson in the operation of a c.s. unit. One engine of a twin failed in the air through fuel starvation. Believe it or not, it was quite half a minute before I realised that the engine actually had failed. My first diagnosis was that the port c.s. unit had packed up and that the airscrew had gone into its fully coarse position. The boost remained more or less right at +1 or + ij lb. and the revolutions only dropped a couple the performanc from A to B acteristics. ing and thin fortable I had a of hundred or so. It was the increasing ache in my right leg while holding rudder (this one was not much because the good engine was at the time only giving about half its 'full power) which eventually told me the true story. Now, with fixed-pitch airscrews, the port engine revo lutions would have dropped off the clock and the boost , would have gone right down. As it was, nobody had tote? the port constant-speed unit that there was no need to do any more work and it was busily trying to keep the revolutions up at the chosen figure ; it did its best, and if it, had had an infinite range of blade movement it would probably have succeeded. As it was, the 2,000 r.p.m. or so which it managed to maintain while windmilling was enough to keep the blower in fair action and consequently to keep the boost up. Nobody had told the blower, either, that it was merely wasting its time pushing neat air through the engine with the throttle position unchanged. I am wondering now-whether „ if I had opened the throttle fully, and thus-fedQced intake/clrag; both the boost and the revo lutions would have b«n maintained. **"* The aching leg wM an interesting thing in itself and shows how often on«[ies entirely by reflexes. On another occasion I flew a Machine in which trie rudder tab had been incorrectly ^justed. Xks I wasn'11 very interested in the machine, and jwas merely going 1 iLdidnfa bother aboui its handling char- er BtMut a^|uarter of an/ hour of map-read- g abojft other things, I began to feel uncom- ing about my body *x the seat; perhaps flying boot or I had unevenly tightened the Sutipn h/^Jess. I didn't know wjfet it was until the dis- n to take the defuifte form of an ache, or a ofJEramp, in my righ^eg. For twenty minutes I had ieerwnolding on righrfrrudder without knowing it. I flew |he#est of the wjj^with the right wing down and my feet <jn#:he floor^frest them. Feathering Facilities While writing around the somewhat grisly subject of engine failure, I remember that I used to think that another airscrew refinement was by way of being an unnecessary and pansy innovation—the feathering business. Ad-" mittedly the majority of people are a little afraid of tB|| device even now. Maybe it won't unfeather, and then where are you? Well, if an engine stops you won't be needing it again anyway, and the difference in single- engined flying with and without the airscrew feathered must be felt to be believed. And I imagine that the ability to feather an airscrew has saved the freedom and even the lives of quite a number of bomber crews. With no bomb load a twin-engined aircraft will carry along quite happily on one good engine at its cruising boost, and the only worry for the crew lies in the business of getting all the available fuel to this good engine. Quite apart from the fact that the machine can usually be held on the rudder trim alone once the airscrew has been feathered, there is less drag, less boost has to be used to maintain the same speed, and there is, consequently, a better chance of getting all tip way back. I heard of one rather over-enthusiastic bom Met pilot who actually flew to his objective with his bomb load and made several rather crabwise runs over the target, all on one engine from somewhere about the other side of the Channel. With the rest of his crew to be considered, this was not, perhaps, a very wise thing to do—but it is the way in which wars are won. "INDICATOR.'
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