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Aviation History
1927
1927 - 0122.PDF
FEBRUARY 24, 1927 fingined machine lay in the fact that it rould glide for a greater distance in thecacu of one engine failure than could the single cngined machine. The lecturer then gave some figures to show how a twin-engined machinemight be expected to perform in the event of one engine failing. Taking a machine weighing 13,500 lbs. and fitted with two water-cooled geared-downengines developing a total of 850 h.p. at maximum revolutions, it might be assumed that the thrust horse-power at maximum speed would be about660 h.p., or 77*5 per cent, of the maximum, but as the air speed was reduced the thrust horse-power fell off very seriously, partly owing to the reduction inrevolutions per minute, and partly to loss of propeller efficiency. Taking the case where the machine was climbing at full throttle, the minimum thrusthorse-power required to keep the machine in the air would be about 270 h.p. or 32 per cent, of the maximum, but at the low air speed at which this occurredthe loss of thrust horse-power available would be so great that it was more efficient to climb at a higher speed. For the machine in question it mightbe taken as about 1 -4 times the minimum speed. At this climbing speed the thrust horse-power required to maintain height would be about 315 h.p.In other words, more than 5 per cent, of the maximum brake horse-power of the engines would be wasted by climbing at this higher speed. It would befound that the brake horse-power of the engine had fallen by about 8 per cent., owing to a reduction in revolutions per minute, and that the efficiency of thepropellers had fallen to about 68 per cent., so that the net thrust horse-power available was only 530 h.p., or 62'5 per cent, of the maximum power of theengines. This left a balance of about 215 h.p. available for climb, which would result in a rate of climb of 525 ft. per minute. In the event of one enginefailing the machine would be flown under closely similar conditions. The machine would be kept at approximately the best climbing speed, when thethrust horse-power available would l)e about 265. The thrust horse-power required to maintain height would be increased by the extra drag of thestopped engine and by the drag due to setting the rudder over. The power absorbed in that way would be about 25 h.p., so that the thrust horse-powerrequired to maintain height would be 340 b.h.p. Thus, there was a deficiency of some 75 h.p., which would result in the machine losing height at the rateof about 185 ft. per minute. Assuming that the height at which the engine failure occurred was 3,000 ft., the pilot would have 16 minutes in which toreach a landing-ground. The gliding range would be about 18 miles in calm air and 22 miles with a following wind of 20 m.p.h. In the case of a similarsingle-engined machine the rate of descent in case of engine failure would not be less than 800 ft. per minute, which would give the pilot 3-| minutes from3,000 ft. and a range of four miles in calm air or 4i miles in a following wind of 20 m.p.h. The twin-engined machine considered was assumed to be fitted with ordinarypropellers of constant pitch. By the use of variable-pitch propellers a great improvement could be effected. In the example taken the loss due to reduc-tion in revolutions per minute amounted to 8 per cent., and the loss due to flying at a higher speed than that at which minimum power was requiredamounted to 5 per cent. With the variable-pitch propeller the position would be that the thrust horse-power required to maintain height at climbing speedwould be reduced to 270 h.p., while the thrust horse-power available would be increased to 595, assuming a propeller efficiency of 70 per cent. With bothengines running the rate of climb would be nearly 800 ft. per minute as against 525 ft. per minute. With one engine out of action the thrust horse-powerrequired and the thrust horse-power available would just about balance, so that the machine would be just able to maintain its height. At this point thelecturer referred to the article by Mr. C. C. Walker, published in The Aircraft Engineer of January 27, 1927. Maj. Mayo thought that we should not have to wait long before an efficientand suitable variable-pitch propeller was available. As soon as such a propeller was available the position of the twin-engined machine would be considerablyaltered, and he thought it quite possible that the type would then have a new lease of life. Although the introduction of variable-pitch propellers wouldmean that radial air-cooled engines could be used to very much greater advantage, they would not, he thought, be really suitable for use in twin-engined machines until they had been fitted with reduction gears. He thought the only serious objection to the use of reduction gears was the extra noiseentailed. Having shown that the twin-engined machine had greater possibilities thanwas often supposed, the lecturer pointed out that there could be no doubt that the three-engined machine was essentially much more reliable than the twin-engined. It was not to be expected that all three-engined machines would have a rate of climb with one engine out of action. Machines of the typedesigned to meet the performance requirements for the European services should, when fitted with geared-down engines, have a small reserve ofpower with one engine out of action, but if fitted with ungeared engines it would have to be of good aerodynamic design to be able to hold itsheight. A modern three-engined machine with ungeared air-cooled engines could be expected to maintain height on two engines, but ability to climb ontwo engines was only to be expected from machines of considerably better performance than that required for the European services, the de Havillaud" Hercules " being an example of such machines. In order to achieve the high performance it had been necessary to sacrifice a certain amount of pay-load, but without such sacrifice the service on the Cairo-Karachi route could not have been operated with the degree of safety and reliability that was nowassured. Although modern three-engined machines represented a great advance in regard to safety and reliability, their virtues would be enormouslyenhanced if their respective engines were fitted with reduction gears and variable-pitch propellers. A combination of these two improvements wouldrevolutionise the performance of such machines. Maj. Mayo did not consider the three-engined machine the end of thedevelopment, but regarded it only as a stage in the development of the multi-engined machine. The four-engined machine had advantages overthe three-engined machine, but the five- or six-engined machine had greater advantages over either, and a five- or six-engined machine, employingair-cooled engines with reduction gears and variable-pitch propellers, appeared to him to be the logical development which must come. Maj. Mayo expressed the opinion that the advent of the really reliableaeroplane would greatly reduce the scope and utility of the seaplane and flying-boat, and said that if it became possible to fly from point to pointwith almost absolute reliability there would be little reason for employing marine types, except in certain special cases. Turning to the subject of progress in regard to reliability of power plant,Maj. Mayo said the outstanding development during the last few years had been the world-wide movement in favour of the air-cooled engine forcommercial work. This type of engine had certain definite advantages in regard to reliability, such as elimination of defects in the water-coolingsystem, which had been one of the most prolific causes of trouble, and greater accessibility, which facilitated inspection and maintenance. Apart fromthis definite development, progress in reliability of power plants had been mainly in the direction of the gradual elimination of the detail weaknessesin the engines themselves and in their installations. There were still points in the design of engines and their accessories which were far from satisfactory,including such important items as magnetos, sparking plugs, engine instru- ments, etc., and engine manufacturers would do well to remember that anengine was not reliable unless its accessories were reliable also. As for controllability, the lecturer said that this most important question,haa been receiving much more attention during the. last year or two than ever it did before. The general standard of controllability was distinctlyabove that of the earlier machines. Greater attention had been paid in particular to control at low speeds, and this had been of special importancein the case of the large three-engined machines now in service. By careful balancing of control surfaces it had been possible to provide sufficient controlfor large machines, but the lecturer thought that any further increase in size of commercial aircraft would necessitate the use of relay controls operatedby servo-motor. So far as this country was concerned there had been no practical application to transport aircraft of the results of the work on theHandley Page slot and aileron control. Slot and aileron control could only be satisfactorily incorporated in a new design ; its application to existingmachines would involve considerable practical difficulties. The placing of a contract for a new type of commercial aircraft was a somewhat rare event,but he could assure the audience that a watchful eye was being kept on this and other developments. Among the " other " developments heincluded the good control achieved by Mr. Fokker in his thick-wing mono- plane types. On the subject of structural strength and detail design, Maj. Mayo praisedthe standard of general structural strength of British commercial aircraft, but thought it was true to say that the strength of detail parts and fittingsin commercial aircraft was not up to the standard of strength of the main structural members. The rest of Maj. Mayo's paper dealt briefly with the following subjects :instruments and equipment, fire prevention, general air service, air survey and exploration, transport of plant and machinery, taxi-flying, and cluband private flying, and the paper concluded with a general review of the situation. Maj. Mayo concluded his lecture by saying that he had hopedto be able to make a statement on the " Open International Safe Aircraft Competition," sponsored by the trustees of the Daniel Guggenheim Fundfor the promotion of aeronautics, of which Maj. Mayo is the English repre- sentative. The date of the paper, however, came just too soon for thatto be possible, but he could inform the audience that the object of the compe- tition would be to achieve a real improvement from the safety point of viewin the aerodynamic characteristics of heavier-than-air craft, without any serious sacrifice of the good practical qualities of the normal present-daytype. The competition would be open to competitors of any nationality, and very substantial prizes would be offered. THE DISCUSSION-. The Chairman (Colonel the Master of Sempill) then called upon Mr. HandleyPage to open the discussion. Mr. Handley Page agreed with the lecturer that military flying accidentswere more liable to occur than accidents to commercial aircraft, and thought the public did not differentiate between the two types of accidents. Hewas interested in Maj. Mayo's remarks concerning night flying, and quite agreed with the importance of this. He hoped that Imperial Airways hada programme in mind. He also agreed with the lecturer as to the value of gearing and variable-pitch airscrews, and what he personally would likewould be a form of propeller in which it was possible to vary the diameter, pitch, and number of blades. (Laughter.) He thought the radial air-cooledengine showed a falling-off in power on climb quite apart from the causes for this which the lecturer had mentioned. He found himself unable toagree with the lecturer on the question of seaplanes. This type of aircraft offered great advantages in that it often enabled them to alight on and startfrom the heart of a city, where frequently there were wide rivers available. He also pointed out that very often aerial surveys were only possible byoperating from rivers, and thought that the marine aircraft would always have a place in civil aviation. Mr. Handley Page could not share the lecturer'sfaith in machines with six or seven engines. Up to a point there was a gain in reliability by fitting more than one engine, but he thought such amultiplicity of engines would merely have the effect of making a breakdown certain. He was not aware of a fact to which the lecturer had referred,viz., that there was any difficulty about fitting slots and ailerons on existing machines, and could not quite imagine why this should be so. He concludedby saying that Maj. Mayo had admitted to disagreeing with the views he (Maj. Mayo) expressed four years ago, at the Air Conference, so that perhapsit was permissible to hope that in another four years' time something would, be done with the slot and aileron type of control. Mr. Bramson said he had one or two questions to ask. For instance,concerning the variable-pitch propellers, he would like to know whether it was proposed that these should be automatic in action. If they were to beoperated by the pilot, he could see that unfortunate individual having a lively time adjusting some six or seven propellers in addition to carryingout his other duties. He thought that with the variable loads on machines what a pilot required was an instrument which woul d tell him exactlywhat incidence he was flying, in order that he might fly at the most econo- mical angle. Concerning three-engined machines, he thought the man inthe street had by now got the idea that the three-engined type was always safe. Some three-engined machines would not fly on two engines, andtherefore it was almost a matter of obtaining money under false pretences to carry paying passengers in such machines. Mr. C. C. Walker pointed out that in multi-engined machines such as thefive or six-engined type to which the lecturer had referred, there was likely to be considerable interference with the aerodynamic qualities of the aeroplane,and he personally thought the twin-engined type more promising. Maj. Bulman referred to Maj. Mayo's statement that the only objectionto reduction gears was the noise. He could not quite agree with that view,, since satisfactory reduction gears were difficult to design and, generallyspeaking, were a nuisance. They meant extra weight and complication, but he was afraid there was no-help for that, and that the gearing must be produced. Capt. OHey thought the advantage of having more than one engine wasmainly that it enabled a pilot to get to a suitable landing-ground. Capt. Hill agreed with Mr. Bramson that an instrument for indicatingthe actual angle of incidence was required. Sqdn.-Ldr. Haig did not agree with the lecturer on the subject of thonecessity for servo-motors in large machines. These added complications, and it was possible to provide control surfaces which would do the work o;the servo-motor without their complication. Mr. Savage thought it would be necessary to go back to the geared enginein order to get the efficiency, and he hoped that the pusher type would be developed, because in that the noise and smell of the engine was largelyleft behind. Mr. Nicholson did not agree with the suggestion to go back to gearing.He thought what was required was to improve the propellers. In marin work development in propellers had enabled high speeds to be used whic!.had previously been thought impossible. As regards the lecturer's statemen about marine aircraft, he could certainly not agree. Three-quarters of tbworld's surface was water, and he would even go so far as to take the exactly opposite view of the lecturer and say that the commercial machine of thofuture would be a flying boat. rMr. Lankester Parker also put in a good word for the seaplane, and pointer out that modern flying boats were more efficient than land machines. 102
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