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Aviation History
1944
1944 - 0568.PDF
294 FLIGHT MARCH I6TH, 1944 CORRESPONDENCE the Jungmann, an aircraft designed as a trainer, as manoeuv rable as the Auster, designed with ultra-manoeuvrability in viewi1 Secondly, the German pilots were not A.O.P. officers, to whom the dodging of fighters is considered as part of the day's work, but inexperienced trainees. "TWO BRIGHT SPARKS." Lysanders were Also Manoeuvrable! I TRUST you will forgive this further intrusion into your space, prompted as it is by "Junior" (R.C.A.F. Pilot)'s reply in Flight, March 2nd, to my letter of January 20th. At the risk of being wearisome I should like to mention that I then expressed my opinion on two matters as follows: — («) That a slow manoeuvrable aircraft is practically defence less against a fast fighter; (6) That the more sordid aspects of warfare, necessary in a total war, should not be publicised as something to be applauded, or as amusing incidents. The reply of "Chief Test Pilot," dealing with point (a) is so full of controversial matter that I hesitate to become further involved by replying to all his points. However, although I have never flown a Typhoon, and as one whose business it is to know how an aircraft will fly from a designer's rather than a pilot's point of view, I should like to make the following few points. That Me ioo,Es (Daimler-Benz motors) with a speed of approximately 350 m.p.h. did fight in the Spanish War; that all engagements are not necessarily tail-chasing matches (an assumption which Chief Test Pilot seems to have made the basis of his argument), that Typhoons can and do fly low very successfully to attack railways locomotives (admittedly easier targets than a manoeuvrable Jungmann); and that the Auster which escaped from nine Messerschmitts did so by flying up and down a defile. So far, nothing which has been put forward against my con tention has done more than prove that a Jungmann can turn inside the turning circle oi a Typhoon. The plain fact remains, however, that an encounter between two such air craft takes on much of the aspect of a fighter v. bomber engagement. The Typhoon could break off the action and renew it at will at periods determined by the time necessary for it to flick up on to its back and come down in a dive on its target. The Jungmann would obviously not be sitting await ing its return or even moving at a slow speed in a straight line, as would a tiaifl boi it would be very limited in its movements compared with an aircraft that could acquire altitude four times as fast Frankly, I disbelieve the story that the aircraft concerned flew in tail-chasing circles. The obvious method of attack would be in the vertical plane, in which ease the differ ence in manoeuvrability would not be so marked. Incidentally, as one who knows something about the manoeuvrability of Lysandeis, from practical experience, I cannot help remembering how they were practically massacred in France With regard to "Junior" and point [b), might 1 suggest a more carelul perusal of my original letter? IAN S, ALEXANDER (Stud. R.Ae.S.). THE FLYING BOAT Values Not Sufficiently Stressed REFERRING to the article "The Flying Boat" published in your issue of February 24th, together with your editorial comment, might I point out the principal values of the flying boat, which do not seem to have been stressed sufficiently to make it quite clear that the flying boat stands in a category of its own ? In the first place, many countries and islands are so moun tainous in formation that airfields are not available anywhere, in which case the flying boat is the only form of aircraft that can be used. Secondly, the flying boat, when flying over water, always has an "airfield" beneath it for forced land ings. Thirdly, no undercarriage has yet been designed to carry a load above about 60 tons, so that all developments above that tonnage must of necessity be flying boats—or shall we say "Hying ships"?—to make a distinctive classification. The Editor might settle this classification once and for all, to the mutual advantage of all. The "flying ship" classification should be above the tonnage that it is possible to laud on an air field. I have personally designed a flying ship of 300 tons which is more efficient by 25 per cent, than any other form of aircraft. Th'S tonnage could only be landed on water. I should also like to point out that a flying ship could always run alongside £* quay for loading, like any other ship. J, R. CROULD (Major, late R.A.F.). THE DUCTED RADIATOR Thrust from Increased Air Velocity Y OUR correspondent Mr. R. D. Morrell (Flight, January 27th) requests an explanation of " Dragless Cooling," and it is possible that the following straightforward survey may meet his requirement, although avoiding any exact discussion of duct design. The purpose of a duct is to collect the cooling air, slow it down, cause it to pass through the matrix and take up heat, at the same time losing pressure, and finally accelerate it to join the main air stream in a manner which will avoid breaking up its streamline flow. This process obviously produces a rearward force (drag) on the radiator matrix, but this is largely counterbalanced by the air forces developed on the duct itself, which is of aerofoil section. These forces give a resultant in the direction of motion of the aircraft, as can be seen from the diagram, which is a two-dimensional representation of a duct, and it is to be observed that the force systems shown should be summed round the whole external and internal surfaces of the three- dimensional scheme- in other words "ring" forces. from XX to Y,Y, the air slows down, building up a pres sure in accordance with Bernoulli's theorem. This pressure, acting freely in all directions, can be represented by a resultant " ring " pressure Pi normal to the walls of the containing duct. When resolved perpendicular to and along the direction of motion a component is apparent, acting as a forward thrust on the aircraft. The same conception can be applied from Y2Y, to ZZ but here the adverse pressures involved are of a lower magnitude. The rearward ring pressure Pc is therefore smaller than Pi and we are left with a net force in the direction of motion of the aircraft. Furthermore, there are air forces on the outer sur face of the aerofoil duct contributing an additional " riug " resultant component to these other forces. The practical existence of the forward force can be shown by a freely suspended Townend ring or, in this case, a light mock-up of a duct, on the face of which impinges the blast of a fan. The ring will move forward towards the fan. Fortunately we need not evaluate all these forces in assessing the drag of a ducted radiator since they are all taken into account in the statement that the net drag is the rate of change of momentum of the air which passes through the duct, as .r'^ be shown by a detailed mathematical analysis. Herein lies the explanation of the fact that the net drag may in fact be negative (i.e., a thrust). For the heat energy given to the air by the radiator may be greater than the losses it suffers in passing through the radiator and duct, so that it finishes up at a higher velocity than that at which it started, thus giving a thrust, a la jet propulsion. The higher the internal velocity the smaller is the fraction of the original energy which is represented by the losses, and it can be shown that at aircraft speeds of the order of 450 m.p.h. the net result should be a thrust. It should be noted, however, that our arguments have not taken into account the form drag of the duct walls, and it is debatable whether the above ideal has in reality been reached. There is no doubt, however, that the development work carried out by Rolls-Royce, Ltd., in their Installation Department at Hucknall, contributed largely to the narrow margin of superiority possessed by our fighters at the beginning of the war. The position could have been reversed if Germany had our knowledge of such tilings as duct cooling, carburettor intake ram and ejector thrust from exhausts. C. L. HININGS, B.Sc, A.F.R.Ae.S., Superintendent, Aero Engine Instruction, Rolls-Royce, Ltd.
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