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Aviation History
1928
1928 - 0298.PDF
APRIL 19, 1928 was produced at the Royal Aircraft FactoryJ(as it then was). That screwoperated quite successfully, fitted to a BE.2C machine, and demonstrated some of the advantages to be gained by the use of the variable-pitch airscrew.He believed that both Mr. Lynam and Capt. Forsyth were concerned with it. Little was done until after the war, when a number of people tried to producevariable pitch airscrews, some of which achieved a measure of success and some none at all. He believed it was correct to say that the airscrew developedby Dr. Hele Shaw and Mr. Beacham had proved so far the most successful. It had certainly gone farther than any of the others. Mr. Lynam (who is in charge of the Airscrew Section at the R.A.E.) saidthe problem of the variable-pitch airscrew was an extraordinarily fascinating one, but the obstacle in the way of its utilisation was that of its weight.If one took into account—as one must—the weight of the airscrew, the gloss was taken off the proposition. In heavier-than-air craft weight was offundamental importance, and the weight of the variable pitch airscrews constructed so far had been so great as practically to nullify the advantagesto be obtained. In lighter-than-air craft weight was less important, and there he considered that a good case could be made out for its use. But hewas certainly not convinced that a very strong case could be made out for its use on heavier-than-air craft. In some of them it was possible to makeout a case, but on most of them it was not. Referring to the diagrams shown by the lecturer, he said that the fixedand variable-pitch curves coincided at the one condition for which they were designed, but in other conditions the efficiency of the fixed screw, of course,fell below that of the variable. The reason given was not strictly that to which he wished to attribute it—namely, that with coarse-pitch airscrews,with large pitch-diameter ratio and large surface, a very much greater gain was to be. effected by varying the pitch than was the case with finer pitchscrews. Therefore, the variable-pitch airscrew (for brevity we shall call this the v.p. airscrew—ED.) stood its best chance on geared engines, where oneused airscrews of large pitch-diameter ratio and large area. Unfortunately, by getting more power from the engine by varying the pitch, one could notuse that power so efficiently because in reducing the pitch so as to get higher engine speed and more power, one decreased the pitch-diameter ratio andthe maxmiuzii efficiency. Secondly, one was putting more power through an airscrew of the same diameter, and therefore increasing the slipstream loss. On supercharged engines one would expect that the v.p. airscrew had avery much greater chance of improving performance, but he believed it could be accepted that unless one were supercharging to very great heightsindeed, certainly to about 20,000 ft., little advantage would be gained. Even at 20,000 ft. it was doubtful whether the v.p. airscrew represented anysensible advantage when the weight had been taken into account. There were qualifications that had to be made. For instance, if the performanceat low altitude, or at medium altitude below the operational altitude, was not improved, the v.p. airscrew lost its value. Unless one could obtainmore power from the engine, and unless the additional power from the engine was gained at the rate of, say, 1 h.p. for every 2 lbs. weight, one would dobetter by putting the weight into the engine and using a fixed pitch screw. Mr. Lynam said he was responsible for the production of airscrew blades,and he was frequently told that the blades themselves were heavy. They were, and we had to find means of producing very much lighter blades thanthose produced to-day. While admitting the weight of the blades, he did not agree that they were the sole cause of the great weight of the wholev.p. airscrew. The Gloster airscrew fur the " Jupiter " weighed about 180 lbs. while the weight of a fixed pitch airscrew of the samesize and with similar Duralumin blades was about 100 lbs., so that there was an 80 per cent, increase in weight by the introduction of themechanism for varying the pitch. He believed that in the case of the " Condor " engine the ratio was much the same. He advocated hollow blade?except for the very small airscrews used on racing aircraft, and the use of higher grade materials. Also the blades should be hinged to the centre,since by doing that the blades became lighter because relieved of bending, and the centre became appreciably lighter for the same reason. By use ofhigher .grade materials and hinged blades, one might get the weight of the v.p. airscrew for the "Jupiter" down to 80 or 90 lbs. Discussing the particular automatically-controlled airscrew which Dr. HeleShaw had evolved, he thought the automaticity introduced a great deal of mechanism, and he could not believe that anything which containedso much mechanism could be as reliable as something with less mechanism. In his opinion only one type of machine required the automatically-operatedv.p. airscrew, namely, the single-seater fighting scout with supercharged engine. For large aeroplanes and seaplanes he thought it was agreed thatthere was no reason why one should not use the very much simpler manually- operated gear. Col. the Master of Sempill asked Mr. Lynam if he could deal with theresults of the trials which were being conducted under his supervision, to which Mr. Lynam replied that he would come to that later. He agreedas to the reliability attained by large hydraulic gear, but would ask the lecturer whether he had any experience of small hydraulic machinery. Itwas important that the pilot should have control over the governor. When taking off he would almost certainly want to utilise the maximum revolutions,but he would pot cruise at anything like maximum revolutions. The range would have to be greater than provided at present. The pilot would want,say, 2,100 r.p.m. when taking off, and probably 1,600 r.p.m. for cruising. Blade nutter had been made out worse than it really was. Dealing with the chairman's request for information concerning the trials,he was not sure he was permitted to give the information, as the official report had not yet been sent to the Air Ministry. He could, however, endorseDr. Hele-Shaw's statement that the airscrew had been in flight for over 30 hours, and that no breakdown whatever had occurred. As to its operation,those- at the R.A.E. were not satisfied, and had never been satisfied, but under the circumstances could not detail their grievances. Wing-Commander T. R. Cave-Browne-Cave said he did not know whatalteration of pitch angle was possible with the mechanism described. For airship work one wanted about 15° from neutral to astern, 15° from neutralto ahead, and 90° from neutral to the position of minimum drag. He doubted whether the gear described could be made to give this large angularrange. He thought the authors had net stated the whole case for the v.p.airscrew for heavier-than-air craft. It was very desirable to be able to reduce the drag of an airscrew whose engine had stopped, and he thoughtit was rather a special advantage of the v.p. airscrew to enable this to be done. Flight-Lieut. Capon said it was somewhat dangerous to quote actualfigures as to the improvement obtainable from a v.p. airscrew, but he had attempted to evaluate the improvement and bad obtained animprovement of 4 per cent, at a pitch-diameter ratio of 0-6, 9 per cent. at 0-9, and 12 per cent, at 1-2. The improvement at the higher pitch-diameter ratio was notable, and it was difficult to imagine that any reasonable increase in weight would annul that. In the case of a single-seater fighter there would be a definite advantage because such a machine when fighting would be continually diving and rising. The variation ofpitch would give it a considerably increased glide. Wing Commander Hynes intimated that he belonged to the " anti "school. On a basis of thrust horse-power per pound weight of power plant, under the various conditions, the advantages of the v.p. airscrew wereto say the least, very slight. Col. Fell pointed out that the application of. the v.p. airscrew to thesuper-compression engine for long-range work was very important. One worked just on the detonation point, and throttled the engine on theground, which meant that the power when taking off was very seriously reduced. If the v.p. propeller could be used in connection with such anengine, it seemed to him that that difficulty would be overcome. Mr. H. L. Milner, while agreeing that the question of weight was offundamental importance, suggested that Mr. Lyman was not quite fair in criticising, on the score of weight, the first model that was produced.Naturally, in producing the first, one played for safety, as one could not afford to take the risk of a breakdown. Also, the authors of the paper hadnot the resources of the R.A.E. behind them. Designs had now been produced which indicated that the full equipment of the v.p. gear forthe " Condor" engine could be provided with an increase of weight of 50 lbs. only. If that weight were spread over the power of the engine, itamounted to about 0-07 lb./h.p. Mr. Burroughes said the subject of the value of the v.p. airscrew, in aidingthe take off of a heavily-loaded aeroplane or seaplane, might well be elaborated. He pointed out that the value depended upon what the weightof the variable pitch gear would be. That information had not been obtained yet, but Mr. Milner had indicated that considerable savings in weight hadbeen effected. If the weight could be still further decreased, the value of the gear would be correspondingly increased. Mr. W. O. Manning said that some years ago, when he was experimentingwith a small hydraulic mechanism, not for airscrews, he had difficulty due to air getting into the small pump and preventing it working. He wouldlike to know the extra cost of the v.p. airscrew. Weight also was obviously of importance, and no definite conclusions could be formed unless accurateinformation was available. Mr. Bramson said that at the present time a good deal of trouble, expenseand weight was regarded as permissible in order to provide a supercharger, but asked what was the use of the supercharger if one had to throttle downat height because the revolutions became higher than the engine would stand. To him it seemed essential to have a v.p. airscrew to absorb thepower which one had taken such trouble to obtain. He also suggested the possibility of using the v.p. airscrew for braking. Captain Boothby related his experience, in 1912, with v.p. propellers onairships. The blades were very thin, and he was rather afraid of them. In his view, v.p. propellers would become essential in airships. Dr. H. C. Watts said he would have liked to disagree thoroughly withMr. Lynam, but, unfortunately, he was afraid he was in entire agreement with him on most of the points raised. Concerning the diagrams takenfrom his book, Dr. Watts stated that it should be pointed out that the gains shown were obtained on the assumption that all the linear dimensions hadbeen varied. The actual gain obtained by each variation of pitch was one- half or one-third of that shown. Concerning the subject of weight,Dr. Watts said that the screw undergoing trials at Farnborough was one with hollow-steel blades made by Metal Propellers, Ltd. Whatever one's viewsof the v.p. propeller, the authors of the paper had helped considerably in solving one of the biggest problems o'f its design, i.e., the difficulty ofoperation. Major Mayo said it seemed to him that the v.p. propeller had been judgedbefore it had had a proper trial. The authors had been very discreet on the subject of weight. Mr. Lynam had suggested that for every 2 lb. puton the propeller one should get 1 extra h.p. Presumably he was arguing that the weight of the power plant should be as low as 2 lb./h.p. He(Major Mayo) knew of no power plant weighing as little as 21b.'h.p. If one took a comprehensive view, the average weight was more nearly 4 Ib.'h.p.It seemed to him that in judging the v.p. airscrew, the weight of petrol that was carried should be taken into account. That would make a very con-siderable difference in the balance sheet. In a long-range machine it might well be that the saving in petrol to be carried would make up for the wholeof the increase in weight due to the v.p. gear. If that were so, the advantage of the v.p. airscrew would be obtained for nothing so t*> speak. It was accepted that we must have geared engines for commercial flying,and thus conditions would be favourable for the application of the v.p. propeller. Again, it might almost be said that the limiting factor was theload with which a machine would get off. If the v.p. propeller could give an appreciable advantage in regard to take-off it would pay to carry the extraweight. From the point of view of commercial aviation, he would place the requirements of the v.p. propeller in the following order :—1, reliability ; 2,light weight ; and 3, reasonable cost. The Chairman (Colonel the Master of Sempill) said he was impressed bythe difference in the tone of the discussion. Usually the discussions ran not in an optimistic direction. He agreed that the wave of pessimism wasunnecessarily severe, as they could pleadl that the v.p. propeller was yet in its early stages, and that its weight to-day was no indication of whatits weight would be when due encouragement had been given for its develop- ment. He would have thought that more stress would have been laid on theadvantages of the variable pitch screw on fighting machines, and on heavily supercharged engines that would be developed in the near future. Also,he would have thought that the fact that the gear was automatic, and yet was under the control of the pilot constituted a very desirable feature. Mr. Beacham, replying to the discussion, said that most of the criticism hadbeen directed against the subject of weight. In designing the first gear they had no idea as to what forces were necessary to turn the blades. Inconsequence, they had allowed for greater forces than were necessary. As a matter of fact, the pump and the whole hydraulic system was capable ofwithstanding a pressure of something like 800 or 1,000 lbs. per square inch, whereas the pressure in actual flight was only about 200 lbs./sq. in. On thespinning tower the pressure went up to 600 lb., and it looked as if the engine vibration destroyed almost all the friction in the mechanism. It had nowbeen found that the whole of the ram mechanism could be put inside the shell which surrounded the propeller. Other details had also been dealt with,and as a result, the propeller, instead of weighing 100 lbs. more than the ordi- nary, Weighed only about 50 lb. more. On the subject of air in the hydraulicsystem, the condition for successful working was that the air should be expelled at a higher rate than that at which it could leak in. He was not quite surewhat angular movement could be obtained. In ships' steering gear, there was a range of 45 degrees each way, and he thought that, if necessary, therange demanded by Col. Cave-Browne-Cave could be provided. Dr. Hele-Shaw, who also replied, said that when Mr. Beacham and he hadhad the presumption to invent a new form of v.p. airscrew they had appreciated that they were attacking a problem which had already been the object ofstudy by a great many minds, and very acute minds—many of them at Farnborough (laughter). Commenting upon the criticisms that had been madehe said it was his experience that if everyone spoke well of a man and his invention, they did so out of sheer good nature. They felt that the poor fellowhad missed the bull's eye, and that a little kindness would soften his failure. In a lecture he had delivered many years ago he had pointed out that thehistory of ail progress was from simplicity to complexity, and not from comp'exity to simplicity. He quoted examples of this, such as the greatercomplexity of the " Jupiter " than that of a simple steam engine, and so forth. It was the same with the turbine. He welcomed the comments made on thev.p. airscrew. He and Mr. Beacham had not expected to be patted on the back for having done what Farnborough had not done, i.e., producejan airscrewwhich did all that was required of it. 270
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