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Aviation History
1934
1934 - 0822.PDF
824 FLIGHT. AUGUST 9, 1934. 100 these things, but when the variables are so many and so interwoven it must be admitted that a variety of inter- pretations is possible. In comparing the basic machine with Case 4, the latter has been given a retractable undercarriage and the former has not. It might therefore be supposed that these two cases do not represent a difference between what can be done with and without C.P. This, however, is only partially true, for it is unlikely that the basic machine would be fitted with a retractable undercarriage and cer- tain that the Case 4 machine would be so fitted, and since the aeroplane of Case 4 is only made possible by C.P., it is of interest to make this comparison. It may be suggested that this machine could alterna- tively have been produced by fitting bigger engines and using fixed pitch. An aircraft of this kind would carry a much smaller load per h.p. at a higher maximum speed, and might have to take off throttled if reasonably efficient propellers were fitted, thus reducing the load still further. A hopeless tangle would soon be reached, however, by varying too many factors at the same time. By keeping to one power unit, one take-off ability, and one range, there should emerge a reasonably clear comparison. It has been assumed that machines fitted with C.P. propellers can be cruised at 75 per cent, of the power developed at maximum permissible r.p.m. This assump- tion does not mean much until the type test conditions are denned, but it can be taken as meaning that more power can be used for cruising if the r.p.m. can be held down by C.P. The basic aeroplane which has been taken for comparison is of mode- rately high speed. The value of C.P. in- creases as the design speed of the aero- plane is 1 n creased. Since i t would be i m possible in a short space to display the results to be expected for various design -'* . "... speeds, the way in which the value of C.P. propellers varies with speed is shown in Fig. 2 as the increase of thrust during take-off obtainable by a C.P. propeller designed rather for climb than speed plotted against design speed of the aircraft. The minimum pitch setting is adapted to 100 m.p.h. throughout. If a lower speed had been taken some gain would have been shown for the slower machines. One or two questions of general interest can now be answered. Is it worth while fitting C.P. propellers to existing transport aeroplanes? The answer would in general be a decided negative, for the result is a decrease of paying load and an increase in cost caused by the weight and cost of the propellers. Against this is the ability to cruise at a higher speed by using such extra power as the engine maker may sanction while keeping the r.p.m. down by increased pitch. It is, of course, assumed that the aeroplane is already satisfactory as regards take-off, ability to fly with one engine out, etc. Is it worth while fitting C.P. to a light aeroplane of low speed? Again the answer is obviously a decided negative, but it must be qualified by saying that if a new design with a gear reduc- tion were made, a substantially smaller engine could be installed to do the same job with C.P. This advantage <so 4-0 io I % INCREASE OF MEAN TAKE -OFF THRUST BY FITTING CP. PROPELLER FlG.2. / / * /Y 1/ / / / MAx^reeo M.BH. 100 IIS I5O could be obtained by the combination of gear reduction with C.P., but not by gear reduction alone. A word is necessary as to the validity of the various statements made and figures given in the course of these remarks. So far as the properties of the propellers are concerned, the information is mainly derived from N.A.C.A. Reports based on full scale tests in a large wind tunnel with the propeller driven by its own engine installed in an aeroplane. There does not seem to be room for large errors in the deductions from these. The de Havilland Company wished to make a sort of spot check on the interpretation of these figures recently and, there- fore, designed two wooden propellers for top speeds of 230 and 165 m.p.h. respectively to be tried on a Leopard Moth (" Gipsy Six "). Some preliminary trials have shown that leaving off the whole of the paying load with the high-speed propeller does not allow of as short an unstick distance as in the case of the 165 m.p.h. propeller with full load, the ratio of the total weight for equal unstick being 1.28:1. This, of course, is not the same thing as clearing a barrier, and the fact that the two propellers are not of quite the same diameter, etc., prevents direct application of results to C.P. It is, however, a quite adequate check on the harmful results of big pitch diameter ratio on take-off and a very clear indication of the gain obtainable by an alteration of pitch by an amount which would be realisable in practice with C.P. propellers. Figures have been given showing the relative cost of performing a passenger mile. It must be explained that these figures take into account obsolescence, insurance, maintenance of engine and aircraft, petrol, oil and piloting. The variations shown are based on the assumption that the cost of petrol is 36 per cent, of the total of the above- mentioned charges on the basic aeroplane. There is, unfortunately, no space to deal adequately with that type of military aeroplane which is not concerned with transport of the horizontal kind. There is no take- off limitation as a rule in such cases, as the excess of b.h.p. near the ground is so great that the low thrust horse power may be no drawback. The various points dealt with may be summed up as follows: — Ability to get a better static and climbing thrust from a given engine opens up all kinds of possibilities. The general result is that C.P. enables greater loads to be carried, and enables good loads, not otherwise obtainable, to be carried on high-speed aircraft. It was shown above that the paying load (Case 3) could be increased by 58 per cent, (say from 10 passengers to 16) at a lower speed, or that the same load could be carried 47 m.p.h. faster and at a lower cost using the same engines Very little can be obtained by fitting C.P. to existing aircraft beyond the ability to use more cruising power and the means of preventing abuse of the engine by low r.p.m. at full throttle on the ground. The use of special fuels can also be avoided or devoted to more useful ends. The real advantages have to be designed for in the first place, and very deceptive results can be obtained by tests on aeroplanes of low weight per horse-power, because the advantages of C.P., being all at the low speed end of the scale are easily masked in such machines. It must be admitted that it is extremely difficult to give cut and dried comparisons which are not open to question in a matter such as this where so many variables are involved, but the only way to examine the question is to suppose new designs created to utilise new facilities. The C.P. propeller is to the aeroplane what the gear box is to the car. It can be imagined that the application of a gear box to a car designed to do the best it could with a single gear would be the least favourable method of making use of the combination. In any transport aeroplane the maximum permissible power which can be utilised for short periods should be available for the take-off. It is required for no other pur- pose. It is to be hoped that there will be no military legacies to civil aviation in any future legislation which affects the use of C.P. propellers.
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