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Aviation History
1919
1919 - 1555.PDF
DECEMBER 4. J9i9 The gain in speed for a given loading and the gain in loading at a given speed show up very clearly. To further elucidate the matter Fig. 3 has been prepared. It shows these gains as direct percentage increments. It is interesting to note that the speed gain at given loading is approximately constant at 16 per cent., about half the per cent, increase in speed between sea-level and 14,000 ft. when flying at a constant L/D. The loading gain increases from 37 per cent, to nearly 60 per cent., being of far more importance the higher the speed concerned. THE PROPELLER. A technical consideration intimately uniting the engine and aeronautical designer on the subject of supercharging is that of propeller design. If a machine having a supercharger engine be fitted with a propeller suitable for low altitudes, the engine will over rev. at her working height. Two pro posals have been put forward for getting over this :— (a) To provide the propeller with blades, the pitch of which can be increased at heights so as to utilise the greater horse power available with the supercharger engine as compared with the ordinary motor. (6) To provide a variable-speed gear-box so that the engine revs, can be kept approximately constant, those of the propeller being varied. Both solutions are subject to disadvantages, the variable- pitch propeller offers the sweeter working mechanism, though the space into which the gear has to be fitted is small and the parts will be subject to centrifugal stresses. The gear-box will always present the difficulty of changing gears and is likely to be cumbrous. So far as the writer is aware, it has not got further than the patent specification stage, but the variable propeller has been got into the air. In discussions on the variable propeller, lie pitch is usually regarded as being increased at high altitudes, and it is con tended that this will cause a loss of efficiency large enough to counterbalance the gain due to the use of the supercharger engine. This attitude in regard to the problem is misleading, and comes from the excessive value given to climb by war con ditions. Any propeller designed for a supercharger engine will be designed to give its best at the working height selected, and the question is not that of any loss of speed and efficiency at heights, but of sacrifice of climb low down. The variable propeller will have a normal pitch at high altitudes and a fine pitch at low. Its diameter will have to be greater than that of the ordinary type, but there will be no loss of top speed, only of climb. So also the variable-speed screw will have a normal number of revolutions at heights and a lower rate of revolutions near the ground. In both cases the loss of climb will be due to the propeller not having so good a form for working at climbing speeds as the normal air screw. The real disadvantages to be faced, then, are the increase of propeller diameter and weight and the loss of climb in volved. Practically the most serious consideration is the loss of climb. To give some quantitative opinion on this point the writer has assumed the extreme case of a supercharger machine fitted with a fixed propeller designed to utilise the engine horse-power at 14,000 ft. On coming down to ground level the revolutions of, and horse-power supplied to, the propeller have been assumed to drop and the forward speed to be reduced all in such proportion as to keep the angle of attack of the blades constant. Thus the condition of maximum efficiency at working altitude has been combined with a condition at sea level much worse than that which would be realised in practice by either variable pitch or variable-speed propeller. The results of the calculations are as follow :— Weight of machine .. .. .. 600 lbs. Characteristics those of curve Fig. 1. B.h.p., normal at sea-level .. .. 400 Supercharger at 14,000 ft. .. 380 Performance. Normal. Supercharger. ., 105 m.p.h. 122 m.p.h. .. 725 ft./min. — 474 ft./min. 580 .. 605 .. 21.5 mins. .. 26 mins. Height, top speed. Sea-level 14,000 ft. Climb— Sea-level 14,000 ft. Time to 14,000 ft. It will thus be seen that under very disadvantageous con ditions the supercharger does not lose excessively. In prac tice better figures will be realised for the initial climb, with a corresponding reduction in the time to working height. It is not improbable that for machines with fight power loading the conditions assumed in the foregoing case may be accepted ; for commercial work it is a question of balancing the length of flight and time of cl.mb to the working height. Propeller mathematics are lengthy, involved and liable to error, and for that reason no attempt has been made to give anything more than an indication based on extreme limits. For commercial work the important part of the gain is in the direction of speed, climb being of secondary importance so long as enough is available to enable the machine to handle well when getting off the ground and keep the time taken to get to working height within reasonable limits. 60 So 40 %30 ID ^20 10 Joo /OS SPEEO M.P.H. US ISO 12s /30 M.P.H. y*^ Fl 1 ft • 6.3. j &S& SA 1_ &2$ ^*r \mo GAIH 1 __i AT SA. 1 J.. >e%2>**^ % GA//V FROM SUPERCHAftGtttG «r« i ^ 1 • • • _. • // A3 IS 17 /9 21 23 LOA.D/HG 25 lbs VHP. 1557
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