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Aviation History
1920
1920 - 0210.PDF
FEBRUARY 19, 1920 CORRESPONDENCE [The Editor does not hold himself responsible for opinions expressed by correspondents. The names and addresses of the writers, not necessarily for publication, must in all cases accompany letters intended for insertion in these columns.] PARACHUTES—POSITIVE AND NON-POSITIVE [1995] In his letter to you of February 5, my old con- troversialist kindly draws attention to my work, possibly with the idea of " drawing " me. Well, I am not an unwilling victim especially if the benefit of a little practical experience can be made use of by the ingenious colonel. With regard to positive opening, for which term I hear I am responsible, it must not be lost sight of that this has two entirely different aspects. The first is what I term registration of opening. I have observed that the length of free fall of positive- opening parachutes is in direct proportion to the load com- pounded with the air-speed. They can be relied upon with precision to open within a few feet of a calculated distance. Knowing this I jumped with a positive opening parachute from the Tower Bridge, only 153 ft., and have since jumped three times from aeroplanes at heights estimated by their pilots at less than 250 ft. If I had not known this do you suppose I should have been such a fool at my age with responsibilities as to have attempted these drops which people who know nothing about registration of action imagine were done out of dare-devilry ? Believe me, sir, what ever we may have been at 20, we are not dare-devils at 40. The second point is psychological. Quite apart from a predetermined limited depth of free- fall, the knowledge that the parachute is forcibly opened is most reassuring at the moment of jumping. With parachutes having problematical opening, like the common kite-balloon type, the uncertainty of the free-fall is such that no one ever dreams of making a practice drop with it at less than 2,000 ft., whereas the majority of the couple of hundred jumps which have been made with the Guardian Angel parachute—the only positive opener with which human drops have been made—have been done from less than 1,000 ft., and at least twenty of them have been done from less than 500 ft. The kite-balloon Spencer parachute has done wonderful work in the war, only three men in 600 being killed by total failure of the parachute to open. A good many observers have jumped with it from their balloons at less than 1,000 ft. and Lieuts. Bacchus and Mendl from only 350 ft. ; but its opening was always uncertain and the depth of free-fall quite independent of the load, etc. It might be thought that this does not matter, nor does it much, if you have " tons " of height, but if you haven't and you touch the ground before the free-fall has come to an end you will find yourself in one of those perfectly pink palaces with perfectly purple parks that Mr. Vale Owen tells us about in the Weekly Despatch every Sunday. I know all about it. I fell an independently calculated distance of 784 ft. out of 1,200 ft. at Roehampton. The parachute opened exactly three seconds before I became a candidate for the pink palace. No amount of argument will convince any one who has made - J— '•" ' ' • In parachutes like the Mears and Smith Salvus, where the rigging is not exposed until the parachute is well clear of the machine, it is unaffected by eddy currents, but there is still the same liability to tangle as is always present with every parachute under the most normal conditions. During the War dozens of kite-balloon parachutes came down all or part of the way with varying tangles in the rigging. In my opinion positive-extension is also a sine qua nan. By this I mean that there should be a positive reaction against which the rigging and silk can be pulled until the whole is fully extended. In most parachutes, including the kite-balloon type, this is very simply effected by the apex remaining attached to the case by a small " breaking-cord " until the last moment. Air gives a sufficiently strong elastic reaction, but it is not positive. There are, all the same, a good many practical types which work quite satisfactorily on this principle, amongst them the following : Robert, Leo Stevens, Broadwick, Floyd-Smith, Irving, Jahns, Smith-Salvus and Skyhook, the first five of which have been tested by human drops. The extension of the rigging subsequent to the develop- ment of the silk throws a big peak of pressure on to the latter, unless the kinetic energy of the falling load is partially absorbed by letting the rigging pay out against a friction device or by the introduction of ample shock absorbers. More cases of bursting have occurred with parachutes of this type than with positive-extension parachutes. Positive-extension means that the reaction must be taken from the aircraft, though the actual point of reaction may be indefinitely produced by the simple expedient of attaching the parachute-container to a long cord. " Ha ! that's just what you can't do," say the critics ; '' the kinetic energy developed by a falling parachute-container will snap the cord if you have it more than a few feet long." Now that's just where they are wrong. They seem to think that with all the positive-extension parachutes the container must be so close up under the fuselage at the moment of delivery that the parachute must foul the tail. Not at all. The kinetic energy of the falling parachute- container can be overcome by a suitable shock absorber, a device which has already been adopted with success. Furthermore, the faster the aerolpane is falling the less the relative speed, and hence the less the impact of the parachute- container when it reaches the limit of the connecting cord. In order to get the utmost possible speed, the D.H.4 from which I jumped at 250 ft. at Dayton, Ohio, July, 11, 1919, was at the moment in a fairly steep glide. Except for an unusually sudden yank backwards, I noticed no untoward feature. My companion, Lieut. Caldwell, was killed a few minutes after by an accident in no way attributable to the parachute. That the positive-extension type of parachute is free from all the dangers immunity from which Col. Holt and Mr. E. E. Smith li th il i f hi ti~ . .. Smit claim as the special prerogative of their respective - a arop that uncertain opening is as good as positive inventions, is amply proved by the success of the German ing or that positive-opening is overrated. " pilots who were using parachutes of this type—the Heneicke i observer was reported to have fallen 3,500 ft. and the Fokker—during the last four months of the War. a K..B. parachute before it opened, and I have direct Exact statistics are not available but in the opinion of personal records of over a dozen observers who had free falls Capt. Bishop, when in the 211th Aeroplane Squadron, and who tedat 500 ft and over. had seen a good many Germans descend from burning or Dest test of the psychological value of positive-opening broken machines, quite 90 per cent got away with it safely, it a competent meH,ral inWS+icra+r.r mat» ™™™,-o+;,,0 Whether parachutes will or will not foul the aeroplane in it,in p is to let a competent medical investigator make comparative tests of the nervous reaction of several parachutists before making drops with a positive opening and a problematical opening parachute respectively. It is quite easy to do. I have submitted myself several 'times to the tests, and got a pat on the back once from Col. Flack, C.B.E., M.D. I would rather not publish the incriminating record, for you would think me a bigger coward than I really am, but if I tell you that when about to use the best positive-opener I know of my sluggish pulse rises from 56 to 112, and with a problematical opener it rose to 129 (there was also some- thing about apostolic blood-pressure as far as I remember), it will reveal to you what a considerable nerve strain para- chuting imposes, even on the initiated. There now, I'm afraid I've rather let the cat out of the bag. We alwa 's pretend we don't turn a hair. Of quite as much importance as positive opening is tangle- proof rigging. For aeroplane work, where the eddies of the accidents depends entirely on the nature of the accident. If there are no projections on the machine, it does not matter if it does touch. A falling machine generally has periods of rectitude during which the pilot should seize his lifeline and his opportunity. A burning machine can be controlled until the moment of jumping. The idea that a spinning machine will roll the parachute up is merely evidence that the critic who advances this non- senss in a serious argument has never been in a spinning machine. It takes 8 sees, for an average-sized machine to rotate about its longer axis, and only 2 sees, to get away in a para- chute. What will happen when a machine is falling like a stone I don't know, and am certainly not going to try ; but who- ever heard of a machine falling like a stone ? fwi iiggiug. ror aeroplane work, where the eddies of the I've seen plenty fall. They always dart about, glide and slip-stream whip and flog the rigging, it is positively asking for otherwise evolute.trouble not to use tangle-proof rigging. Now then, let any of the positive-opening-no-use experts 210
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