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Aviation History
1936
1936 - 0530.PDF
FEBRUARY 27, 1936 13 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT 2 2 ac quirements for a small range of aircraft with speeds of 140 to 200 m.p.h. Above 200 m.p.h. it falls well below the ideal curve. The two-pitch coupled with two gears con tinues to do well up to 300 m.p.h. Beyond this the field is held alone by the constant-speed type coupled with two gears. Above 400 m.p.h. even this combination shows sjans of falling away. I have not thought it necessary to do farther, but should mankind ever fly at over 500 m.p.h. for a pitch 0.7Z) was 87 per cent, of the ideal, for this supercharged 20,000 feet case it is 77 per cent, of the ideal. There is therefore a stronger case for the V.P. air screw for low pitches. Further, as before at these low pitches, any of the combinations give much the same result which for practical purpose equals the ideal. Nevertheless, it still remains true that the gain at these low pitches is relatively small compared with the value of variable pitch a demand will probably arise for three gears, if the present type of airscrew has not by that time been replaced by some other form of jet propulsion. Let me make it clear at this point that my statements are only true and general when I confine myself to talking in terms of pitch/ diameter ratio. I am translating these ratios into terms of miles per hour to give a more readily appreciated meaning of my remarks. The speeds given infer that normal tip speeds are between 800 and 900 feet per second. For engines geared to give lower airscrew tip speeds the general conclusions still apply in terms of pitch diameter ratio, but the figures in m.p.h. must be suitably altered. Speaking, therefore, in terms of pitch diameter ratio, I would sum up generally for the non-supercharged or very mildly supercharged engine as follows: — If the aircraft under consideration is or would be fitted with an airscrew having a maximum pitch less than its dia meter, the value of fitting a variable pitch screw is very doubtful. If it has a pitch greater than its diameter it is worth while, and becomes more and more essential as the pitch increases. If the maximum pitch is greater than 1.3D a two-speed gear in the engine becomes desirable. (b) Supercharged Engine So far my remarks have been limited to non-super charged or moderately supercharged and ground boosted engines. I now proceed to the case of the supercharged en gine, and here again it has been found necessary, because of the variation in degrees of supercharging, to assume a hypothetical engine which I think does not so greatly differ from the average supercharged engine as to invalidate my remarks. The engine assumed is supercharged to give at 20,000 feet the same power as at sea level. Tht- airscrews considered are designed to run at low maxi mum engine r.p.m. flying level at 20,000 feet. Fig. 13 snows the thrust curves for an airscrew with a pitch of 2.5, and Fig. 14 for t 3 diameter. The climbing speed at sea kyel is assumed as 0.45 of the top speed at 20,000 feet. &• I5 shows the static thrust at sea level plotted against Pl en, and as in Fig. 12 a scale of speed assuming a tip speed of 850 feet /sec. is added. The thrust is expressed terms of the thrust in the designed conditions, i.e., in ms of the thrust at maximum r.p.m. at maximum level for n,at 20,00° feet- The first thing to note is that, whereas tne uosupercharged engine the fixed pitch static thrust on the higher pitch screws. I think my previous conclusion still remains true that for pitches below 10.D it is doubtful whether the cost of the V.P. is worth the gain. Above this pitch, or 140 m.p.h. at 20,000 feet, this doubt dis appears. The two-pitch continues to give a static thrust closely approaching the ideal up to about 300 m.p.h., and the constant speed up to 330 m.p.h. Above this the two- speed gear with the constant-speed variable pitch becomes desirable, but the two-speed gear with the two-pitch air screw is inferior to the constant speed with a single gear. Again, note the ineffectiveness of the two-speed or variable , gear with a fixed pitch airscrew. As compared with the non-supercharged engine, the main points to note are that variable pitch in some form becomes desirable at much the same pitch, i.e., approximately 150 m.p.h., but the two-speed gear does not become worth while until 300 m.p.h. or over, as compared with about 200 for the non-supercharged. In Fig. 16 I have con nected up the two cases, unsupercharged and super charged, to 20,000 ft. The curves show for various altitudes of supercharging the ratio of static thrust with each of the various combinations to fixed pitch static thrust. The airscrew used for illus trations has a pitch of one and a half diameters, cor responding approximately to 250 to 280 m.p.h at the various altitudes. The main purpose of this Fig. is to show that there is an altitude of supercharging at which the value of the two-speed gear disappears altogether. For this particular airscrew it is approximately 18,000 feet. Finally, I would say that it is one of the difficulties and dangers of talking on the performance of airscrews that there are so many variables and so many permutations and combinations. It is impossible in a limited set of curves to cover the whole range, and yet such a limited con sideration leaves room for misunderstanding and criticism. I trust, therefore, that in your criticism you will bear in mind the difficulties of presentation in a paper so limited in length. MilCr. ALTITUDE
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