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Aviation History
1927
1927 - 0926.PDF
DECEMBER 8, 1927 and calculations (although he did not say so), the lecturer concluded that a flying-boat could be designed suitable for operating under most conditions likely to be met with in which flying might be considered reasonably free from risk. He also called attention to the importance of good seamanship, and stated that a seaworthy boat might be lost or badly damaged due to bad seamanship. Thus the importance of training pilots in the art of seamanship could not be too strongly emphasised. The effects on time and run to take-off of taking-off speed and horse-power loading were dealt with next. For a machine of 27,500 lb. gross weight, fitted with three Rolls-Royce " Condor " engines, curves were derived showing that if the take-off speed was increased from 45 to 55 knots, the time to take off was increased by 10 seconds, and the length of run by 100 per cent. Looked at in another way this meant that in a rough sea twice as many waves would be encountered during take-off, thus doubling the risk of damage. Assuming these curves to be reasonably accurate, it was seen that for the same power loading the time to take-off was nearly proportional to the square of the take-off speed, and the length of run to the cube of the take-off speed. Taking as an example the same machine as above, the effect of power loading was examined at a take-off speed of 50 knots, and it was found to be approximately the same as that of take-off speed. For a power loading of 13-7 lb. h.p. the time was 22 seconds, and the run 1,100 ft. For a power loading, of 16-1 1b. h.p. the time was 38 seconds, and the run 2,000 ft. Air Performance Major Rennie stated that the ever-increasing demand for longer range had very naturally been followed up by an equally insistent demand for high cruising speed in order to reduce duration of flight for a given range. It had, he said. been stated that this might be attained solely by the process of " cleaning up " a design. Defining cruising speed as " the speed at which air miles per gallon are a maximum," the lecturer pointed out that this occurred when ' w 375 X was a maximum ; where ij = the propeller efficiency at cruising speed ; L D = the lift/drag ratio at this speed ; \V = the weight of aircraft ; f(p) the b.h.p. altitude correction factor ; p the density at altitude ; and p = the rate of fuel consumption at sea level For purposes of illustration Major Rennie took a three-engined biplane flying-boat of gross weight 17,000 lbs., fitted with three air-cooled engines developing 450 h.p. at 1,700 normal r.p.m., carrying 500 gallons of fuel and having a stalling speed of 55 knots, with R.A.F. 28 wing section. He then examined the effect on range and cruising speed of aspect ratio, take-off speed, and increase in top speed, due to " cleaning up." Three sets of curves illustrating these effects were shown. The first of these showed horse-power required against speed for variou> aspect ratios, ranging from 4 to 12, the speed corresponding to the former being 68 knots, and to the latter 62 knots, while the respective minimum horse-powers required were 620 and 400. The next illustrated the relation of range t<> cruising speed at various aspect ratios, the range being about 690 sea miles at 98 knots for aspect ratio 4 and 860 sea miles at 84 knots for aspect ratio 12. (Intermediate values were given on the graph.) In the third figure of this series curves of aspect ratios were plotted on speed against L/D of machine. A maximum L/D of nearly 9 was reached for aspect ratio 12 at 70 knots, and L D of just over 6 for aspect ratio of 4 at 85 knots. Leaving now the 17,000 lbs. boat and returning to the previous example taken, but with a gross weight of 27,500 lbs. and 900 gallons of petrol, Major Rennie gave curves illustrat- ing effect of take-off speed on range and cruising speed, using take-off speeds of 45, 50 and 55 knots. First curves of range against throttled level speed for the three take-off speeds, the maximum, range being 1,230 sea miles at 63 knots for 45-knot take-off speed, 1,190 miles at 67 knots for 50 knots take-off speed, and 1,130 miles at 73 knots for 55-knot take-off speed. Then curves of range against r.p.m. and curves of r.p.m. and cruising speed against take-off speed. And, finally, TJ L D and TJ against speed for the three take-off speeds. The curves showed that as the take-off speed increased from 45 to 55 knots the cruising speed increased by about the same amount, but the range at the higher speed decreased about 8 per cent. On the other hand, the r.p.m. had gone up from 1,500 to 1,670. The curves of range against throttled level speed were fairly flat, and the lecturer pointed out that to reduce duration of flight it would obviously pay to fly at a throttled speed considerably higher than the cruising speed, provided the engines could maintain the required higher revolutions for long periods. For example, in the case of the 55 knots take-off speed, the cruising speed could be increased from 73 knots to 80 or 85 knots, with little loss in range, if the engines could main- tain 1,750 and 1,900 r.p.m. respectively. In order to examine the effect of a reduction in body drag the lecturer continued the calculations for the case of a take- off speed of 50 knots, and assumed the body drag reduced by 50 per cent., a figure not easily attained in practice and thus setting an upper limit. The effect of this reduction in body drag was to increase the top speed from 100 knots to 119 knots, the cruising speed from 67 knots to 75 knots, and the range by about 30 per cent. Major Rennie now turned from the subject of aircraft design to that of the effect of any reductions in fuel consumption which might be possible by improvements in the power plant. He pointed out that the efficiency, as far as range and cruising speed were concerned, could be measured in terms of fuel con- sumption throttled and the ability to run for long periods at at least 90 per cent, of normal revolutions. Comparing aver- age consumptions without mixture control with consumption given by high-compression Rolls-Royce " Condor" engine it was found that there was little change in cruising speed, but the range for the same fuel capacity was increased by about 20 per cent., or about the same improvement as was likely to be obtained by aerodynamic means. Another figure showed that if the specific consumption could be reduced from 0-5 pts. b.h.p. hr. to 0-45 pts. b.h.p. hr., the range could be increased by nearly 20 per cent, for the same fuel capacity (900 gallons). ' The lecturer said that little information was available as to the service likely to be obtained from an engine running for lengthy periods at from 90 per cent normal revolutions to normal revolutions. It would be helpful if engine constructors would come forward with some definite information on this point. The concluding sections of Major Rennie's paper dealt with propellers, of variable pitch or otherwise, ease of control to avoid fatigue, fuelling on the water, and, finally, twin-engined or three-engined boats, the general uses of the flying-boat in commerce. He concluded that, other things being equal, the success or otherwise of any proposed flying-boat line would depend almost entirely on the correct choice of take-off speed and power loading for the known conditions. The Discussion. THE CHAIRMAN (COL. THE MASTER OF SEMPILL) said he would heartily endorse Major Rennie's plea for support for the neglected flying-boat. He was glad that from a technical point of view the position was so good in this country, and that was in a large measure due to people like the lecturer. He felt sure that if there was a future for Imperial air routes by heavier-than-air craft, then that future would lie with the flying-boat. MAJOR BUCHANAN expressed his appreciation of the excel- lent work which Major Rennie was doing, and said he did not wish to be misunderstood when he offered certain critic- isms. He was, personally, a strong supporter of the flving- boat, and thought the time would undoubtedly come when it could hold its own as against the landplane. That time, however, was not yet. He had, perhaps, greater opportunity than any other man in this country for examining in great detail the design of all types of aircraft, and as a result of this intimate knowledge, he had to point out that the nying-l>oat, in any size hitherto built, was heavier than the landplane, and was also less efficient. No good purpose was served by overstating the case for the flying-boat. Concerning the statement made by the lecturer, that the old " F " boats were more seaworthy than modern flying-lx>ats, he saw there were a number of flying-lxiat pilots present that evening, and they would probably have something to say on that subject. To him it was rather strange that Major Rennie should, in spite of this view, have designed in the " Iris " a boat which differed as much as possible from the " F " l>oats. Reference had been made to the use of Iandplanes for long overseas flights, such as transatlantic flights, etc. That was probably due not so much to a preference for the landplane as to the fact that the flying-boat cost a great deal more. In conclusion, he ex- pressed the view that the future of the large machine would lie with the flying-boat. MR. PIERSON spoke on the relative efficiency of large land- planes and large flying-boats. The land machine was handi- capped by various things such as having to have a lower 834
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