FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1941
1941 - 0157.PDF
JANUARY I6TH, 1941. 49 \\ PER ARDUA AD ASTRA" (Continued) these guns are used. Owing to the great cold at high altitudes, special precautions would have to be taken either to keep the guns warm so as to prevent the oil freezing, or to use a type of oil which would remain liquid under these conditions. It should also be noted that guns which are being fired will get much hotter, or heat up faster, at high altitudes than they do at ground level. But I shall have more to say about this cooling problem when discussing engines. The pressure in the cabin would be kept up by an engine- driven pump, which could conveniently be of the centri- fugal type. The size of this pump, in Service aircraft, demands special consideration, for it should be ample to keep up pressure when the cabin has been damaged by enemy fire. It is impossible to take too many precautions to ensure that this latter contingency does not become disastrous. Power Required We have now to get an idea of how much power will be reo tired for an aeroplane tlying at high altitudes. It is nit within the scope of this article to go deeply into the design of such a machine, so a number of approximations will be used to simplify the calculations. In order to do this it is convenient to start by assuming we are dealing with an aeroplane which has modern characteristics and that it is proposed to endow it.with enough power at a height of 45,000ft. to enable it to have a reasonable per- formance at this height. We will assume, therefore, that the weight of the machine is 12,500 lb., that its engines provide 1,500 horse-power normally, that it has a wing- loading of 27 lb. per square foot, and that its normal top speed is about 250 miles an hour. As a first approximation it is assumed that the best climb occurs at a CL of 1, and it follows that the best climbing speed at ground level is about 105 miles an hour. Approximately again, it is assumed that the best climb at all heights always occurs at the same CL, SO that if the machine is at 45,000ft. its best climbing speed will be 105 x -t=- The value of <r appropriate to our chosen height is 0.192, so that the value of 1/ vV is 2.27. So our required speed works out at 105 x 2-27, or 238 miles an hour, or 350 foot- seconds. The net power required to fly at this height under these conditions will then be 350 x 890 foot-pounds per second. I should say that the figure of 890 lb. is ob- tained by assuming that the L/D of the aeroplane at a CL cf 1 is 14—another approximation. The net horse-power ^•equired to fly is then: — 350x890 =570 h.p. 550 But it is not sufficient to assume that the machine should only have sufficient power to fly at this height. We must have something in reserve, and in order to allow for this it is assumed that it has sufficient additional power to climb at a rate of 500ft. per minute. This is about 8.3ft. per second, so that the additional power required for the climb is: — 8 3 x 12,500 J ° — =187 h.p. Therefore, the total horse-power at this height it willbe necessary to provide is 570+187, or 757. It will be assumed that the propeller efficiency is 0.75, so the actualpower that the engine will have to produce will be 757 divided by 0.75, or 1,010 h.p. For a rough approximationof the time to height we will assume that such an aero- plane should reach 20,000ft. in 17 minutes. This is a meanclimbing speed of 1,180ft. per minute. Guessing, then, that a fair approximation of the climbing speed from thisheight to 45,000ft. would be 750ft. per second, the time tor this remaining 25,000ft, would be, say, 33 minutes. Sothat the total time to this height would be 50 minutes. A climbing rate of this order might be a reasonable figurefor a long-distance bomber which could remain in the air for many hours and which might be able to afford 50minutes to get up to its operating height, but would be clearly an impossible figure for a fighter carrying the normalfuel allowance for this type, for it would have no sooner reached this height than the pilot would have to thinkseriously of going home again. It is obvious, then, that the problem of designing a fighter suitable for such a heightis more difficult of solution than that of designing a bomber, for in the former case the proportion of the low-level horse-power which must be developed ^fiejght is much greater than in the latter. In other words, it would seem that theproblem of attack from great heights is easier than the problem of defence. From what has been said it will be clear that the problemof the high-altitude fighting aeroplane is very largely an engine problem. Aeroplane design difficulties certainlyexist, but methods of getting over them can be suggested and there does not seem to be any difficulty which cannotbe solved by experience and careful design. Eventually, beyond doubt, the engine difficulties will also be Solved,but they seem to involve much more serious problems than anything to do with the aeroplane. The Supercharger Problem Let us start by making an approximate shot at the super-charging problem, bearing in mind that we have found that if our standard type of aeroplane is to fly satisfactorilyat 45,000ft., it must have engines which can produce about 1,000 horse-power at that height, compared with 1,500horse-power at ground level. As the ratio of 1,000/1,500 is about 0.67, we will assume roughly that if we can pro-duce an induction pipe pressure of 0.67 of the atmospheric pressure at ground level we shall get somewhere about ourrequired power. This means an induction pjffe pressure of about 10 lb. per square inch. Now, as the atmosphericpressure at 45,000ft. is 1.85 lb. per square inch, the compression-ratio we shall require from the superchargeris 10/1.85, or 5.4. Noting that the standard atmospheric temperature at our chosen height is —55 deg. C, and work-ing the temperature rise out by the usual adiabatic formula, making at the same time an allowance for the efficiency ofthe supercharger, we shall find that we are faced with a temperature of the air at the supercharger delivery ofabout 430 deg. C. absplute, which is equivalent to 157 deg. C. We can reas/wfably deduct 20 deg. from this as anallowance for the heat abstracted by the latent heat of the fuel, leaving 137 deg., which will have to be reducedsomehow to, say, 20 deg. C. before it is allowed to enter the engine if the latter is to develop the expected power.The quantity of air which it will be necessary to cool can be roughed out as follows. Suppose our engine has thevery reasonable fuel consumption of 0.5 lb. per horse-power per hour (and remembering that the best mixture meansan air/fuel ratio of 14), then the amount of air it will be necessary to cpoT per hour will be 0.5 x 14 x 1,000, or 7,000lb. per hour. This amounts to 117 1b. per minute. This amounts again to about 2,300 cubic feet per minute at thelower temperature. No easy problem ! The next thing it is necessary to look at is the relativeeffect of the same radiator at ground-level and at 45,000ft. Let it be assumed that the cooling effect of anyradiator is proportional to: — . (T - T) x V-7 x a- At the height we are proposing to fly we have the advan-tage of a higher value of V and a greater temperature dif- ference, but <T. of course, is considerably less. Substitut-ing the actual values in this formula it beoermes at ground level 221 and at 45,000ft. 137, so that the ratio, or therelative effect of the same radiator at these two heights and under these conditions, becomes 137/221, or about 0.62.But as the ratio of the horse-power required at ground level and at 45,000ft. is 1,000/1,500, or 0.67, it wouldseem that the radiator would have to be increased in size about 8 per cent. Engine cooling would thus not presentany particular difficulty and air-cooled types could be used
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
