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Aviation History
1942
1942 - 2120.PDF
4oo FLIGHT OCTOBER 8TH, 1942 Correspondence The Editor does not hold himself responsible for the views expressed by correspondents. The names and addresses of the writers, not necessarily for publication, must in all cases accompany letters. * BOMBING POLICY Use of Fast Light Aircraft T HE news of Mosquitoes in action for the first time is of much interest. It points to a technique which was advocated earnestly by the opposers of big four-engined bombers—i.e., that of a very fast, comparatively small bomber which would compare favourably with any fighter, thus making it especially useful in daylight raids. That the R.A.F. should adopt such a policy is pleasing, for the attacks of Mosquitoes, combined with those of American Fortresses, should enable us to increase greatly our daylight offensive against Germany and the Low Countries. Similar to the Hawker Hart, which gave the fighters of that time a headache when it first appeared, the Mosquito looks like prcrving lo be not " jusi. another aircraft." " ASTRA." INTERCHANGEABLE POWER PLANTS Latitude Required in Cowling Fasteners WHILE reading your excellent article on "Interchangeable Power Plants'' in your September 24th issue, I found myself questioning your statement that '' all engine cowlings are interchangeable, as they are made on jigs and checked on reference jigs," from which I presume is to be inferred that this method of production assists in the "saving of much valuable time in getting grounded aircraft back into the air again," a point which undoubtedly plays a very important part in operational work. Is this strictly correct? Surely, to be classed as interchangeable, a part should be capable of being readily detached and replaced with its mating part. In spite of being jig drilled and checked on reference jigs, I wonder how far removable engine cowlings are actually interchangeable once a machine has been in flight. From my experience, it would appear that there is consider able room for improvement in the method of fastening engine cowlings, and in my opinion a considerable amount of time and expense could be saved, both in production and mainten ance, if more attention were paid to the type of fastener used in fixing these cowlings, in making certain that the fastener provided sufficient tolerance and latitude to allow for distortion which, 1 understand, often takes place after a flight. It would be interesting to have readers' views on this small but, I suggest, important point. JOHN F. FLUDE. FUEL CONSERVATION The " Mating " Aspect of Running-in AFTER reading your correspondent "Student" in Flight for September 24th, there are one or two points I should like to raise. The running-in of an aircraft engine involves something more than " wearing off the tightness " ; indeed with bearing clear ances as they are at present, particularly in high powered engines, there is no "tightness" in the accepted sense of the word. What is important is the gradual warming up of parts "mated" for the first time; of parts, such as pistons, which are shortly to be exposed to higher temperatures than hitherto in their " lives." If a proceedure such as oulined by "Student" were to be adopted, i.e., "motoring over" by means of electricity, it would merely prolong the time the engine would have to be held on the test bed, as, after it was completed, it would still be necessary to run the engine under its own power, starting at low r.p.m., b.h.p. and temperature, and gradually increas ing all three in exactly the same manner as that criticised. Again, " Student " says: " Is it really necessary to continue to run an engine at more or less full throttle for hours on end, simply for running-in ? '' Here he is quite wrong. An aircraft engine is seldom, nowadays, run for hours on end during run ning-in. isieither is it "at more or less full throttle," as it only reaches cruising conditions, which is far from full throttle in the case of a supercharged engine, at the end of its running-in period. It is, of course, possible to use electric dynamometers, but these are more expensive and delicate, and probably more difficult to obtain, than the faithful water brake. "Student" has become a little confused when he says that " The engine could undergo its normal dynamometer tests and then the storage cells could be used ... to drive the engine during its running-in period." Presumably he means drive the next engine during its running-in period. There is one method, a time-honoured one, of running-in which "Student" has overlooked, i.e., by means of town gas. This has the advantage of simplicity, though of course, it uses fuel, albeit gas instead of petrol. In any case "Student" may rest assured that every effort is being made to conserve precious high octane petrol, and to dispense with anything that tends to waste fuel or time. M. C. GREY, A.R.Ae.S. Allisons Tested with Airscrews I WAS interested in the views of your correspondent "A. N. I." Flight, September 10th) and the fact that electric regeneration is now a common practice in the U.S.A. Of that 1 was not aware. I gather, however, that my main point—advantages of hangar test bed over dynamometer for air-cooled engines (chiefly on the score of quick production) is accepted. I should like to quote verbatim a passage from the July Aircraft Engineering, under the heading " Summary of Papers Read During the S.A.E. Conference, March I2th-i3th, 1942," and sub-title, "Production Testing Facilities of Allison Division of General Motors." " When Allison Division of General Motors entered the production field, the usual lush occurred to design and con struct equipment and buildings The ground rules then laid down, and rigidly adhered to since, specified that the greatest simplicity be maintained and little deviation could be made from known practice. '' The first decision reached pertained to the method of absorbing the power. Consideration was given to the use of electric dynamometer feeding back into power lines, to water brakes, to wooden test clubs and to controllable pitch air screws. '' Controllable pitch airscrews were selected because it was felt that operating conditions would more closely approach those of flight. It was felt that dynamometer operation would not show up defects in that the engine would not be subjected to lateral movements and torsional vibration as in an aircraft. This was correctly proved by defects which appeared in engines tested with flight airscrews after a quantity had been production-tested on a dynamometer. "Other factors entering into a decision against dynamo meter equipment included the relative ease of obtaining air screws instead of dynamometers; the ease of maintenance of airscrews and their record of long life in aircraft operation; the difficulty of designing or obtaining couplings that could be relied on for continuous use; the great quantities of cooling water required for electric eddy current absorption dynamo meters or water brakes; and, finally, the ease and simplicity of installation and operation of the engine." It would be interesting to know if the decision would have been the same had a shortage of electric power, tantamount to that existing in this country, been thrown into the balance. No doubt as the matter has been now raised in the House, the advocates of electric regeneration will have their chance in this country. As to your correspondent's remarks on the average power of a test plant, I would point out that I referred to a period of " six hours on any busy production stand." This takes into account "hooking" and "unhooking" the motor(s). Also it refers to Final Test which, in the case considered, being the result of an investigation a few years back, included the tuning of a jet-type carburettor still used in large numbers to-day. Although the regulations for aircraft engine testing issued by the Air Ministry apply to ali types of engine, the routine to fulfil them may vary widely. Fcr example, the use of carbu rettors which can fie tuned on an air-flow rig will greatly cut down the running necessary on the test bed and will tend to make what running is done of a less spasmodic nature. R. P. G. J.
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