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Aviation History
1936
1936 - 1357.PDF
» 556 FLIGHT. MAY 21, 1936. hill 250 feet high within two miles of Southampton aero drome, flying on the direct course from Ryde, and which produces a most violent bump at 1,oooft., even in 10 m.p.h. winds. Since we cannot go to the expense of carrying out prolonged test flights in order to discover the exact details of wind speeds, directions and heights at which this very sudden bump occurs, we have to fall back on the simple expedient of flying around it. But there does seem to be ample scope for some of our glider enthusiasts to do a very useful and interesting job of work, in making extended tests over such vicinities. They might even prove their value to the materialists who keep POUCULIARITIES Y OUR editorial in the issue of May 7 suggests that there is something very mysterious about the behaviour of the Pou-du-Ciel in flight. Its peculiarities are, however, due to the following factors :— 1. Lack of inherent longitudinal stability. 2. Elevator control (front wing) too powerful and sensitive. 3. Elevator control too close to C.G. to give adequate "feel" to the pilot, and 4. Lift interference with rear wing by the front wing. An elementary force diagram is drawn here, from which it is seen that T is, say, ift. above D, and for equilibrium L must be in front of W. Assuming a L/D ratio of 6 (the Pou wing arrangement is notoriously inefficient), L will be roughly Jan. forward of W. Now L is made up of components from front and rear wing, the greater part of lift being supplied by the former. The first feature to become apparent is that for horizontal flight (at uniform velocity) there can be very little range of speed, for L must remain equal to VV whilst its position is governed by the magnitude of the T/D couple, or by T. For instance, if T is increased L must move forward, which in turn means that the incidence of the rear wing must be decreased more in proportion than that of the front wing, and this is difficult of achievement: for by decreasing the incidence of the front wing (for the higher speed), the downwash on to the rear wing is reduced, giving that wing a greater effective incidence, and hence L moves back, which is opposite to the desired effect. From the little evidence available, it appears that the fatal accidents have taken plarce through sudden dives from a few hundred feet, at a point where the pilots commenced the land ing approach. If we may assume, as seems reasonable, that the engine has been throttled back, there is a diminution of the T/D (nose-down) couple and the nose starts to rise. This tendency is aggravated by the increased effective incidence of the front wing due to loss of height, the effect having been modified by the time the flow (relative) meets the rear wing. The pilot instinctively pushes forward on the stick, and, due to (2) and (3) above, probably brings L behind W, in other words he overshoots the mark (as is not unusual with pendulum-type elevators, which is what the pivoting wing vir tually amounts to) and the nose falls. The correcting back ward movement of the stick probably places the front wing in a stalled attitude, relative of course to the descending flight path, with considerable loss of lift on that wing. Concur rently the rear wing, relieved of the downwash from the front wing, experiences a sudden gain in lift, with the inevitable result that the aircraft dives to earth. Thus it is seen that the system of forces provides a condition asking the question: "What is the use of gliding? " Our problems are very similar to those of the longer airlines. We are hampered in the same way as they by the iniquitous petrol tax, which is used to subsidise road transport. We also bear our share of loss caused by the difficulty and waste of time in getting pilots' licences en dorsed for new types of aeroplanes by the Air Ministry. The greatest difference between us is in the number we carry. In this we have the advantage of being able to spread air-mindedness over a wider field which, if our work is good, will ultimate benefit the longer and more expensive lines. little removed from unstable equilibrum, and only by skilful juggling, or great luck, may disaster be avoided. The take-off is made possible, incidentally, by the high posi tion of T, which compensates for the forward position of L. The safest method of landing would appear to be to fly the aircraft on to the ground with engine on, or alternatively the engine should be throttled back gently, whilst the stick is pushed slightly forward for the glide-in, but on no account should the stick be pulled back suddenly. In conclusion, it is earnestly to be hoped that any drastic steps that may be taken to control the activities of this type aircraft should be limited to the Flying Flea so as not to hamper the progress of those types of light aeroplane that are being developed along more normal lines. C. H. LATIMER-NEEDHAM, M.Sc. (Eng.), F.R.Ae.S. Henlow, Beds. [Mr. Latimer-Needham's ingenious explanatipn probably holds good when applied to Poux in general. The machine does appear to need a rather special flying technique compared with orthodox aircraft. The fact remains, however, that many machines of the type have been flown for very many hours without disclosing any tendency to go into a dive from which recovery is impossible.—ED.] LANDING FEES I AM gratelul to Mr. C. A. Herring for further ventilating this subject, but I am afraid that he must only have scanned my original letter on the subject. Usually private pilots do sp°nd money at the aerodromes which they visit, and they help to support their home aerodrome and frequently pay subscription^ to more than one club and so indirectly con tribute towards the upkeep of a number of aerodromes. If the landing fee is to be small, the total amount raised is not likely to go far toward 9 providing elaborate ground equipment, and if it is large private pilots will not go to those places where such fees are charged I particula.ly complained because, having spent nearly 30s. at Bristol Airport, I was charged a 2s. landing fee, and ob jected to the principle. It may interest Mr. Herring, and private pilots generally, to know that I have received a letter from one well-known airport to the effect that they did not, and never would, charge landing fees tc club members, and that since the date on which I wrote my original letter I have landed at a number of places and have never been asked to pay a landing fee, ?o it would appear that the suggestion to do so, and its practice, has shown itself to be objectionable and to do more harm than good. ERIC W. WALFORD. Coventry. DECLUTCHING THE PUPIL T HE letter appearing in Flight of May 8, on the subject (if "declutching the pupil" reminds me that the old D.H.6 which was used for instructional purposes in the last year of the War was fitted with a trigger gear which permitted the in structor to render the pupil's stick and rudder-bar inoperative in an emergency. Cases of "freezing-on" were by no means rare in those days, and instructors found the D.H. arrangement more effec tive and less barbarous than the earlier method of dealing with a panicky pupil, in which he was bashed on the head with a spanner, or had his crash helmet forced down over his eyes by a violent blow from behind. As far as I remember, the trigger device was quite simple, and I have often wondered why it has not been incorporated in present-day machines. Watford. GRENVILLE G. O. MANTON. CORRESPONDENCE The Editor does not hold himselj rssponsible for the views expressed by correspondents. The names and addresses of the writers, -not necessarily for publication, must in all cases accompany letters intended for publication in these columns.
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