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Aviation History
1944
1944 - 1241.PDF
JUNE 15TH, 1944 FLIGHT 647 M. CORRESPONDENCE TheEdvor does not hold himself responsible'for the views expressed by co, respondents. The names and addresses of the wnters not necessarily for publication, must in all cases accompany tetters. THREE FAMOUS "GLADS" Information Wanted WONDER if any of your readers could furnish me with details about the camouflage colouring, squadron markings and identification numbers of the three Gladiators Faith, Hope and Charity, of Malta fame? C. RUPERT MOORE. I W IDENTIFICATION DIFFICULTIES Figures Open to Doubt ITH reference to the letter from " E. B.," in your issue dated May 24th, I cannot let the statement that forty-four transports were lost due to the fire from our own Naval guns go unanswered. The figures given came from art unofficial source, and, therefore, in themselves are open to doubt, but * the wording "our own Naval guns" would imply that the guns were fired from vessels of the Royal Navy. The Royal Navy has for many years taken a very real interest in aircraft recognition, and the Naval vessels present on the occasion in question were a mixture of all the Allied Navies. R.N.R. A SUPRA-NATIONAL AIR FORCE Problem to be Faced YOUR interesting editorial comment (Flight, May 4th) on the Labour Party's proposal to create a Supra-national Air Force should be a salutary reminder to many of us that we must face the problem of international power. If the new- League of Nations to be set up is to have any authority and power to prevent war, it must have some military force directly under its own control. To outline the organisation that is required I would suggest the creation of an international force consisting of: (1) An International General Staff, developed from the Com bined Chiefs of Staff Committee, whose chief executives should be drawn from the three Powers. (2) A Supra-national Air Force, including airborne troops. (3) National armies and navies as contingents or quotas in this international force. R. FULL TAMES. JET PROPULSION FOR AIRCRAFT There is No " Unbalanced Force " MAY I, in turn, butt into this interesting correspondence? The box with a hole in it of your very able correspondent E. Burke (Flight, May 25th) is excellent for conveying to be ginners an idea of how J.P. works, but even beginners should never be allowed to forget than an "unbalanced force" does not exist any more than a rod with one end. What really balances the pressure on the area corresponding to the hole is the force required to accelerate enormously the mass of gas ejected from the hole. The true facts are more nearly repre sented if we replace the box by a truncated cone with the small end open It is then evident that the gases arc being accelerated all the way from the combustion point to the open ing and, therefore, that it is not even "very nearly " true that the pressure is equal all over the inside. It must fall off all along the length of the cone as the gases are accelerated. The mass of gas at any moment being accelerated (viz., the contents of the truncated cone) may be calculated, the force pushing it is known (viz., the pressure maintainable by the impeller multiplied by the area of the opening) and hence from first principles (P=mf) we can get the actual acceleration, given the length, and the velocity of ejection. I should be surprised if this velocity, to give a thrust suitable for driving an aircraft, turned out to be less than the speed of sound, even in a very hot gas. As to the thrust augmentor, I do not feel very sure that this is not a revival o*f the old bogy, so many times laid, about the jet having to push on something after emerging torn the opening. I rather think that once the jet has "jetted" its useful work is finished. Your correspondent says "the motor .delivers a jet into the throat of the thrust augmentor and draws air through it due to entrainment." Certainly, but that air has to be picked up from rest and accelerated to, say, 400 m.p.h. in the wrong direction before it comes into play. And what about the extra drag introduced? Again,^1 cannot agree that "it is easily seen that P10 is less than P,." What about the disturbance created by dragging this hour-glass-shaped object through the air at 400 m.p.h.? It seems to me that P9 and PJ0 are incalculable. They might perhaps be determined experimentally in a wind tunnel. The expanding nozzle looks to me rather like an attempt to neutralize the effects of a wrongly shaped expansion chamber. If the truncated cone (as I have called it) is of the right shape and nicely streamlined inside, the gases will be ejected so nearly in the right direction—astern—and at such speed and so low a pressure that they will not have time to expand side ways enough to impinge on the sloping sides of a nozzle of any practicable length before theit speed and momentum carried them clear. Moreover, gases ejected into a divergent nozzle are retarded, not accelerated, as they pass along it because the amount of room a unit volume of exhaust gas has to fill increases as the square of the increasing diameter, and the moving gases cannot fill this increasing space and retain theii original speed. The momentum of the gas thus tends to form a vacuum behind it which gives a thrust in the wrong direction for J.P. This principle was used in the " trumpet " exhausts of racing motors to "suck" the exhaust out of the cylinders L. SHELFORD BIDWELL. INVASION TACTICS Should Bridges be Destroyed T WOULD like to bring to the notice of readers of Flight a -*- point which I find very disturbing. In the great Allied air offensive over Northern France. Belgium and Holland, we read that as many as possible ol the road and rail bridges have, and are being, destroyed by Allied bombing. Surely this will have a detrimental effect on the advance of the Allied armies it or when an invasion of t ra west coast of Europe is launched. When the Geimans overran the Low Countries and France it was largely due to the fact that the Luftivajfe avoided bombing bridges during the Allied retreat, and so left tlutn intact for the swift drives of their armoured columns You may think that the bridges would be destroyed by the Germans after they had retreated across thein, but in the case of a disorgani~ed army retreating under heavy air and artillery blows, and hotly pursued by the Allied armies, the blowing up of budges by them might not be possible. In support of this last argument the French army in their retreat in 1940 left many of their bridges intact behind them, and many Allied aircraft were shot down in a gallant attempt to accomplish what their supporting armies failed to do, namely, the destruction of the bridges. I would be glad to hear the opinion of other readers on this matter. R. HUDSON. SORTING OUT THE SNAGS " Indicator " Replies M R. BLACKBURN certainly seems to be a very determined carrier of a chip on his shoulder. In his letter of May 18th, he " refuses to believe " that ait pressure (or, incidentally, structure flexing) can cause the failure of undercarriage indications, and claims that the " dec trical contact type of tell-tale has eliminated this snag." Absolute nonsense! Such trouble is common, in a small way, to all aircraft, and I can name three very well-known aire in which trouble from this cause has been pronoaa^iaantl repeated^ in spite of all the efforts of gr^H^ffstarl with test n^ As for blind-flying instruments,i^(^"not doubt that the panel assemblies are s»bjected^ta^Slnsportation vibration and that this is the rnajojcausty^Wrailure on actual air test. The fact remains that thmffa0&\[, and that no system is yet avail able for testing in ^^preparatory to flight. I am sure that " Test Pilot " is as aware as I am that such instruments are sealed and that panels are usually fitted en bloc—but they can be changed, as separate instruments or otherwise. Mr. Blackburn '' would like to feel that ' Test Pilot "... has a little more confidence in his associates than he would have us believe." I daresav that " Test Pilot " would have this extra confidence, if people of Mr. Blackburn's type were
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