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Aviation History
1912
1912 - 0304.PDF
IAS Mr. Mervyn O'Qorman, Supt. of the Royal Aircraft Factory :— I. I cannot consider Bleriot's suggested explanation ot top pressure on aeroplanes correct technically. It belongs to the order of speech which may be called " chatty." For all that, he deserves credit for pointing out a great risk. *2. What is wrong with it is that it ignores the existence of the air for one premise, and makes it fundamental for the other, while omitting to mention the true essential "centrifugal force" (though that, too, is only a " chatty " expression). 3. An aeroplane at flight-speed has not naturally a parabolic downward path even when the throttle is closed, as he suggests, because whatever one may do in omitting air resistance for elementary diagram of projectile movements, air effects may not be omitted from aeroplane reactions. Such a path is given by com pounding a uniform velocity with a uniform acceleration continually at right angles to it (downwards in the case of a projectile). 4. Centripetal force on the constant mass of the aeroplane is greater in proportion to its velocity squared and in proportion to the acuteness of curvature of its path. In Bleriot's diagram, at B (and I think it is his own hasty diagram that has led him to the form of statement which he uses), the radius of curvature is so small (it is nil) that all aeroplanes must collapse under top pressure if they could ever take such a path. They do not; the air yields, and they move outwards towards the top of the plane. That is a centrifugal motion, which is resisted by a centripetal force called up from the air into which an attempt is being made to move a large plane broadways. This is the cause of the top pressure. 5. Any aeroplane at some such speed as 65 m.p.h. turning on a circle whose diameter is about 100 to 120 yards, is subject to a centripetal force which exceeds the weight of the aeroplane. The resolved component of the weight force down the line of action of this force at any moment and opposite to it in direction may easily be far less than it, and then the top pressure on the plane exceeds the lift pressure, and puts an important effect on the " cabanne " wires. 6. How do movements of sufficiently acute curvature arise ? It is unusual, save when a landing must urgently be made on some spot almost below the flyer, to turn on a sharp circular path downwards, save when it is particularly desired to increase the aeroplane's velocity. The absence of a proper velocity indication on most aeroplanes leaves the matter to the flyer's judgment, and as wind gusts affect the question in a most erratic manner, it would be well to consider such as a fundamental part of every equipment. 7. There is another more remote, but not impossible cause for rapid changes of path curvature, particularly on some of the best aeroplanes, wherein the masses are carefully concentrated on the turning centre, and the tail control ample, and that is simply the elevator movements for longitudinal equilibrium which continue, of course, during the movement of descent. 8. I have had photographic " graphs " of the instantaneous changes of inclination of BE I and BE 2 taken by a recording apparatus made by Mr, F. Short, one of the assistants at the Aircraft Factory, and these show several Hiinutes on a scale representing three or four inches per minute, the normal variations when the aeroplane is in flight. The number of changes is very considerable and rapid. I will have a copy made and sent to you at a later date if you consider the matter of interest. Mr. R. Blackburn's (Blackburn Aeroplane Co.) views:- g I have read with^ profound interest the translation in your paper of M. Bleriot's report to the French Minister of War, and I feel sure that his commujiijeation must have come as a revelation to most aeroplane constructors. That any one've the prevailing conditions of flight should absolutely reverse the loadingiof the planes has hitherto not been entertained, and yet this ve^sfmple phenomenon disclosed by M. Bl6riot can leave no doubt thijt there is one pre vailing condition which causes this reversal of s&jpss. It is, however, only possible under the one condition disclosed "by M. Bleriot, and is absolutely momentary. As to what this effect must have on the designing of the top stays will naturally depend on the speed of the machine. For normal velocities of 55 miles per hour, the factor of safety need not be as great for the upper bracing as for the lower, because the momentum of the machine, due to a sudden descent from the horizontal, can never exceed the value given by the flying speed of the machine, whereas the strains on the under bracing can reach an enormous figure by a very steep vol pique' depending on the distance between the heights from which it is made and the point where the machine is flattened out. It is quite possible for this stress to reach a value equal to 10 to 15 times the live load, the velocity at the end of the dive being much in excess of the normal flying speed. Under these conditions, therefore, I should -deem it necessary to have a factor of safety of 15 for the under APRIL 6, 1912. bracing, and 5 for that of the top. For normal flying this is toler ably excessive, but then how often is it a machine is not flown normally, and this possibility must be provided for. When the velocity of the machine, however, reaches a much higher figure— which of course will be the case in the near future—then, because of momentum being proportional to the square of the velocity, the staying above may have to be made equally as strong as the under stay wires to provide for the reversal of stress caused by a sudden change in the direction from the horizontal to a steep dive. Mr. T. W. K. Clarke's Analysis :— M. Bleriot's report on the possibility of top pressure existing on aeroplane wings opens up a field of view of the very highest im portance. While in the main agreeing with his conclusions, a quantitative study of the question seems to raise some difficulty in a complete acquiescence in his conclusions. The finding of the extra load on the wings, due to motion in a vertical curve, presents no difficulty. If any machine is made to travel in a vertical circle of radius R feet, with velocity V feet per second, it must be acted upon by a centripetal force, — lbs., per lb. of machine. In the case of a machine whose normal wing-load is W lbs. ( — the whole weight of the machine), and which, after flying horizontally, V2 starts to turn down, we should have W —L = W.s~ where L is the R-£" . . new loading considered as positive when in the same direction as ,he normal load. So, for a machine on a rising curve, we should have L-W = W.^-. From these we can construct the following table :— Table Connecting the Radius of the Vertical Path, the Speed, and the Whole Load. Falling curve—Negative load Rising ,, —Positive ,, : O :2W w 3W 2W 4W 3W 5W Speed 40 m.p 1 > t1 11 SO 00 70 80 J) )7 1 3 h. 00 110 ft. 55 ft. 36 ft. 27 ft. 170 „ 85 „ 56 „ 42 „ 240 ,, 120 ,, 8l „ 60 „ 330 „ 165 „ no „ 82 „ 430 » 215 .. 143 .. i°7 ». 540 ,, 270 „ 180 „ 136 „ Note.—Table shows why Bleriot gives a factor of safety of 3 for the top guys to correspond with a factor of safety of 5 for the bottom. Now the question arises, do or can machines turn in such small vertical circles ? For example, a machine travelling at 70 m.p.h., in order to produce a negative load equal to the normal positive load, would have to travel in a vertical curve of radius 165 ft. ; this can best be appreciated by saying that it would have to be turned from the horizontal path to a downward path of say I in 5 in 33 ft., or one third of a second, i.e., in less time than it takes to count two. That this should be so, seems to me extremely improbable ; but on this point there are others much more qualified to speak than myself. Without such evidence the above cause by itself does not seem to me sufficient, but we do not have to seek far for other factors. In normal flight, the surfaces have an angle of incidence of say 5° to the relative wind, if there should be a negative pressure on the surfaces as above, the surfaces must have a negative angle of incidence of say 9°, and in passing from one incidence to the other the surfaces must have passed through all intermediate incidences. Now, although at all ordinary incidences the centre of pressure does not vary much, and what motion it has is on the safe side, that is to say as the angle of incidence decreases the C.P. moves forward and so produces a moment tending to increase the angle again. But as the angle of incidence is still futher decreased, there comes a limit after which the C.P. moves back very fast, almost suddenly, thereby soon producing a very large moment, and a very dangerous one in that its direction tends to still further decrease the incidence or to make it negative. In the very early days of flight the Wright brothers pointed this out as one of the chief dangers. Now such a state of affairs having come about, it is my opinion that the pilot would in any case have the time and power to counteract it, it would be " all up " with his control over his machine, no matter how strong the upper guys might be. If it was possible to recover after having experienced a negative pressure on the main surfaces, then we might have expected that in a fair proportion of the cases cited by M. Bleriot, recovery would have been effected, even though a guy or two on the top had been broken ; but there is only one such case, viz., that of Lieut. Ducorneau, particulars of which I have not seen. What then should be the preventive ? The length of the fuselage, and the size of the fixed portion of the tail would both be determining factors in the radius of the vertical 3°4
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