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Aviation History
1945
1945 - 2312.PDF
NOVEItTBER 22ND, T945 FLIGHT 385 CORRESPONDENCE fuel consumption for a given thrust. In the case of the air- screw no such effect will occur, the airscrew providing the same change of momentum and the same thrust as before. The magnitude of the efficiency-increase in the turbo-jet will probably depend on the intake ducting. Having never seen ny relevant data I cannot say; perhaps others may like to xpress an opinion. R. E. T. HACK. A A Fundamental Principle Involved S another non-technician, I hav.e read with interest the argu- ment put forward by your correspondent " Tyro " (Flight, November ist) that wince the forward movement of a jet- propelled aircraft is due to pure reaction, a headwind would not have the same effect on ground speed as in the case of an aircraft driven by an airscrew. It appears to me that the fallacy in this argument lies in the fact that "Tyro" has ignored the fundamental principle that Slur a state of equilibrium thrust must be equal to drag. Con- <sider a jet-propelled aircraft travelling in still air, both ground- speed and airspeed being 400 m.p.h. By some magic a 40 m.p.h. headwind suddenly springs up, increasing the air- speed to 440 m.p.h. This immediately increases drag by exactly the same amount as in the case of a conventional aircraft, and to maintain the new airspeed (which must be done to avoid •educing groundspeed) a corresponding increase in thrust is required. If more thrust is not provided airspeed must fall to the original figure of 400 m.p.h. so as to reduce the drag to its former value of parity with thrust, and groundspeed becomes 360 m.p.h. just as if an engine and airscrew had been the means of turning the energy of the fuel into propulsive effort. It would have been as reasonable to suggest that the speed of a bullet (which, like the "jet," is not provided with an airscrew) would be unaffected whether the projectile was travel- ling against a headwind, in still air, or even in vacuo. Without the least knowledge of ballistics it seems obvious to me that ihe increased drag involved by a higher muzzle velocity rela- tive to the air would cause the bullet to lose groundspeed more quickly and fall to earth in a shorter distance. Apart from ihe fact that a bullet derives its motion from a momentary reaction of burning gases and is thus subject to continuous deceleration, whereas a jet- or rocket-propelled aircraft in flight is in a state of equilibrium, there is no difference between the two cases. LINDSAY R. GLEGG. Ingenious but Fallacious Argument I'M afraid that "Tyro" has fallen into the old trap andconfused air speed and ground speed, though I admit that he has done it quite ingeniously! I should like to try to puthis mind at rest. Let us consider the case of two similar aircraft, one orthodoxand one reaction-propelled, having the same weight and the same lift and drag characteristics. When the effort needed to drag an aircraft through the airus the same as the thrust developed by the propelling agent, ^h aircraft is in equilibrium (as far as its speed in level flight is"concerned) and the air speed will remain steady. _ If the thrust increases, the air speed will remain until the drag of the air-frame and aerofoils once more exactly equals the thrust, and the air speed remains steady at a higher figure. Therefore, ifboth our hypothetical aircraft are being driven by a thrust of 2,000 lb., they will both attain an air speed such that thedrag of their airframes is equal to 2,000 lb., and since we have chosen two aerodynamically similar aircraft, they will attainthe same air speed. If we now subject them to a head wind, their ground speeds will be reduced equally. Obviously, to-have the same ground speed, they must have the same air speed. If the jet aircraft had a greater ground speed (andtheiufore a greater air speed) we should be getting something fef^nothing because the drag of the aircraft would have risenwifn the increased air speed without any increase of thrust. The fallacy of "Tyro's" statement that the orthodox air-craft is " slowed down " because it has to claw its way into a head wind is shown by the logical extension of that argument,which would hold that its air speed would increase in a tail wind. And is it reasonable, "Tyro," that-two aircraft flying in abeam wind could have the same air speed and drift and that their ground speeds would differ if they altered course intowind? If that were so, our two aircraft under discussion might fly from Manchester to London in formation, but arrive atdifferent times! D. J. MASTERS, Fit. Lt. LONG-RANGE ROCKETS Square-cube Law and Acceleration TT is refreshing to be able at last to see authentic details of•*• the V.2 in print (Flight, November 8th), but the calcu- lations of Mr. Perring with regard to winged rockets appearvery far-fetched. He seems to take little account of the tre- mendous stresses involved in fitting wings to a rocket, or ofthe stability problems which arise in considering vertical flight with wings. The gliding angle of the "New York rocket,"1 in 80, is ridiculous, and it is noticeable that the other projects have much steeper flights downward. On the other hand, he seems unduly pessimistic about therange of the ordinary V.2 with boosters. It seems to me reasonable to take it that boosters can be designed up to aweight of about 90 tons with a similar proportion of structure weight, fuel weight and " payload." But the weights of theengine and accessories will be much less, thus offsetting the weight of the parts earning the V.2. inside. Owing to thesquare-cube law drag will go down to less than half the value for the V.2, so that a steady acceleration to about 4,500 m.p.h.at a height of 38 miles is possible. From here the V.2 takes over in almost dragless conditionsand reaches a final velocity of rather over 9,000 m.p.h., 150 miles along its flight path at an altitude of 133 miles, pointingat 45 degrees to the vertical for maximum range, and touching ground 1,050 miles from starting point, allowing for drag onentering the lower atmosphere. If the 2,150 lb. warhead is replaced by a 400 1b. atom bomb and its apparatus and theremainder made up with fuel, the initial speed goes up to nearly 10,500 m.p.h., and the range to 1,450 miles. It is worth noting that, with the high a now achieved, thereason for the great change in range corresponding to a small change in o lies more in the reduction of load carried thanin the increase of the acceleration period. Above all the range varies as the square of the maximum speed. Perhaps some reader will explain the effect of the earth'scurvation on range as I am not sure how great this is. Account must, of course, be taken of centrifugal force.I. G. HENRY. ECONOMICS OR EXTRAVAGANCE ? What to Do With Surplus Parachutes /""^ERTAIN sections of the tax-paying community are begin- >—' ning to feel very concerned about the disposal oi Govern- ment surplus equipment, judging by remarks made by the public at large and some of your correspondents in particular. Not an unnatural instinct to display, is it ? From an aero- nautical aspect it is especially provoking to visualise the loss of equipment to private and club flying which could actually constitute a better and cheaper form of subsidy than perhaps the Ministries realise. As has been proved by "Indicator" and others, this country reaped a dividend from the efforts of amateur aviators in this last war out of all proportion to the meagre benefits so generously ( ? ) distributed in earlier days. Will history repeat itself ? I do not intend to pursue the ramifications of this theme ad lib. for obvious reasons. Rather would I attempt to draw attention to one small but important feature I would consider worth driving for in any debate involving equipment disposal. The inconsistency of our law demands mechanically perfect aircraft on the grounds of safety, but does not compel the addi- tion of a parachute. I am not suggesting it should become compulsory, but I do think it should be encouraged. It seems probable that amongst the different Services there must be a considerable number of serviceable but redundant parachutes. An effort should be made, therefore, to earmark a quantity of these, NOT for manufacture into ladies' "wotnots," etc., but for free distribution to the light aeroplane and soaring clubs. The majority of these clubs could not afford to buy such necessities before the war, and are hardly likely to change now. Hence the manufacturers will not lose any customers, and it would be a fine gesture of mutual confidence if the Ministries involved were to adopt such a measure which would assuredly repay big dividends—intangible perhaps in a material sense—but of value to the community nevertheless in so many ways. Neither should it prove difficult to assess a fair method of distribution, based on the needs and integrity of the clubs making application. As the B.B.C. would say: "Think on these things." G. A. CHAMBERLAIN.
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