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Aviation History
1942
1942 - 0203.PDF
JANUARY 22ND, 1942 FLIGHT 79 Correspondence The Editor does not hold himself responsible for the mews expressed by correspondents. The names and addresses of the writers, not necessarily for publication, must in all cases accompany letters. TURBO-SUPERCHARGING Effect of Exhaust Back Pressure YOUR interesting"article, "Turbo-Supercharging,", in your issue of January 1st, 1942, gives me the opportunity to deal with views expressed there by the writer on this matter. We read there : " It is not providing something for nothing, but is doing a certain difficult job in an efficient manner. In fact, it represents a considerable mechanical advance." Yes, in general it is true, yet certain disadvantages of the applied system are worthy of mention. There is a difference between free exhaust and that in which the exhaust gases have certain work to do before they leave the engine and discharge into the surrounding atmosphere. In the latter case the pressure of exhaust gases at the inlet into the nozzle ring of a -turbine will be higher than that of free exhaust due to the resistance encountered. The braking rffect on the gases creates conditions in which more heat is ,-««*rven up to the walls of, the exhaust manifolds, and thus increased pressure seems to be inevitable inside them. This increase of pressure, in comparison to the free exhaust, must have its influence on efficiency due to the somewhat obstructed exhaust, and the actual output of power will be less than the theoretical one. The same has been observed by running an engine with manifolds and long exhaust pipes, and without these items, when testing; in the latter case a certain definite increase of power has been noticed. But in spite of these the advantages of the application of a turbo-supercharger to aircraft engines are obviously great, and the whole work is a nice technical achievement. W. JAWORSKI. CAMERA CURIOSITY Shutter Acceleration^ FURTHER to letters by recent contributors on the points raised in "Camera Curiosity," I should like to introduce a further point and that is the question of camera shutter move ment: I must confess that I am far from an expert on photo graphy, but examination of the camera that I normally use shows that the film is exposed from bottom to top, the shutter opening from bottom to top and closing top to bottom. This may vary in different cameras, but the principle of the follow ing argument is unaltered. (3) SLOW REVOLUTION (b) FASTER REVOLUTION Taking the simplest case, i.e., a head-on view, in which the airscrew is revolving in an anti-clockwise direction, we will assume that snapshots are taken at two different engine speeds. The results will appear as (a) and (b). The shutter takes time to open and so apparently the bottom of the plate is ex posed for a slightly longer period than the top, sufficient to allow the bottom blade to move through nearly 120 deg. or one-third of a rev. in case (b). Thus the lower blade appears distorted and mote curved than the upper in this particular case. Where the reverse is true (i.e., when the upper blade is the more curved), my theory *ould indicate that the film has been exposed from top to bottom. Thus the "curved distortion" of revolving blades appears to be a question of shutter movement. Perhaps some photographer would care to comment on this? PROP. POSN. ! • X — TWO BLAOED -*r HEAD ON 2 2 3-QUARTER 9 II 2 1 C&5 SIDE 8 3 2 THREE BLADED Tt._ HEAD ON 5 3 2 3-QUARTER 40 21 43 sioe 6 3 II PR OR POSN X -< Y The main question raised by "puzzled schoolboy," namely the apparent "standardised" propeller position in photo graphy, is largely a question of attitude. As C. Rupert Moore showed, the probability that the airscrew will appear vertical is greater than the probability that it will appear horizontal, but it is by no means a hard and fast rule. The table is compiled fr&m a study of photographs in about 50 back numbers of Flight. The number of times that the airscrew appeared in positions indicated by the small diagrams in the two extreme columns is shown for three different views for both two and three-bladed airscrews. R. H. WARRING. AIRSCREW TERMINOLOGY Need for Standard Expressions 'XT'OUR recent articles on propellers have prompted me to -A- write this letter, since there appears to be considerable confusion in recent propeller terminology, not only in Great Britain but also in America. No two people will agree as to whether propellers on the same shaft in the tandem relationship should be called dual rotation, counter-rotation or contra-propellers. There is also some discussion as to what fixed vanes behind a propeller, to reduce rotation.in the slipstream, should be called, but the majority apparently agree on contra-vanes. In addition, there is the question as to what to call propellers on a two-engine aircraft where the propellers, as viewed from the slipstream, rotate in clockwise and counter-clockwise directions. The writer prefers the term "oppositely rotating propellers," but to be strictly correct it would be best to refer to these pro pellers as left- and right-hand propellers, viewed from the slipstream. As to the controversy regarding the tandem propellers on the same shaft, there is a definite leaning toward the use of the term " dual rotation " around here, the total number of blades being prefixed before the designation. That is, two four-blade propellers in tandem would be called an eight-blade dual- rotation unit. I think you will agree that there is definite need lor standard isation in propeller terminology, leaving out, of course, the age-old question whether it is "airscrew" or "propeller," in order to eliminate existing confusion. Perhaps your readers also have some definite thoughts in this connection, which would prove interesting. May I take this opportunity to state that 1 have read Fligltt over the past several years and have always found it very interesting and informative. RICHARD C. MOLLOY (Aeronautical Engineer, Research Division). United Aircraft Corp., East Hartford, Connecticut. BARREL ENGINES Rotary Valves Recommended HE very interesting article on wobbler-plate engines does not appear to have aroused much comment from your readers—perhaps it was too convincing to argue about. Your contributor seems to have overlooked the most obvious T
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