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Aviation History
1913
1913 - 0126.PDF
(fijSgjj FEBRUARY I, 1913. STABILITY DEVICES. By MERVYN O'GORMAN. Paper read before the Aeronautical Society on Wednesday, January 29th, 1913, Brig. Gen. D. Henderson, D.S.O., C.B., in the Chair. 1. Devices thus spoken of are for the most part not concerned with stability of aircraft in the strict sense, they mostly pay no heed to the rotational energy stored in the aeroplane's mass, or the oscillation due thereto, or to tbeir decrement, but, still, they strive in a vague way after one of the following :— (as) Its safety in the air. (b) Its staying truly on its path. (V) Its steadiness as a gun-platform or view platform, and as such are worthy of review. 2. These three do not flow from one another, nor does safety follow from stability. Indeed, the three are in some measure mutually exclusive, so we will chiefly dwell on the urgent one—the first—and if needs be, let the others go. The number of such inventions is so large that only a few can be considered in a paper of reasonable length. 3. The Essential Speed Limits.—To be in the air at all, " aeros,"* i.e., aeroplanes, must move through it at a certain mini mum speed, and not to break up, they must not go faster than a certain maximum. The upper limit concerns the strength factor of Aeroplane Competition last August), is so entirely dependent on the engine, that this is not suitable for an automatic device to rely upon as yet. 6. We may conclude :— (i) That in examining the merits of a device, the omission or pro vision of a speed maintainer is of grave importance. (ii) That should the aeroplane give a large range between the slowest gliding speed and the fast flying speed, this adds to safety by moving the danger limits further away from each other. (iii) That automatic schemes must not avail themselves of this full range, but must needs keep the speed well above any of the tricky limits. (iv) That if the extreme limit is approached, powerful warp con trol and light wing loading will be useful. 7. Before going any further with the devices themselves it is use ful to glance at the curve, Fig. 1, which gives for an average calm day an indication of one-half of the problem set to us by the wind in this matter, viz., the horizontal air-speed variations measured parallel to the axis of an aeroplane when in flight. In addition to A/0 S^reo f'C /a-. * K ! > \ , •• / / 7 1 1 I f / r \ V. ^—-v^ '—\_ "\ 100;. ... AL r/ rt/£>£ f~/c /& Fig. 1.—Diagram showing the horizontal wind variations, and a certain fraction of the vertical variations, over an aeroplane wing while in flight- the construction as a rule. As regards the lower limit, no device gives safety to the flyer, unless he is protected from unwittingly reaching such limit. We touch here a cardinal feature at the outset. 4- The Slowest Speed.—The minimum safe speed is the speed below which the rudder, elevator, or warp cease to have quick enough control to outvie the gustiness of the air at the time ; hence this minimum is not a fixed quantity, but depends at any time on the irregularity of the wind. In any device providing against the undue falling off of speed, engine failure must be reckoned with and hence potential energy from the height and the pull of gravity must be drawn upon by the working scheme of the device. The effort derived from the organs of control falls off as the square of the air speeds past them, therefore the lower limit, which is less low in proportion as the air is more unsteady, is again less low when the engine is stopped. Twin Propeller Device,—Accordingly, though some freedom from dependence on gusts is achieved when the propeller stream is made to pass over the wing flaps, as in the " two-propeller " machines of the Wright and Short types, the low limit of slow flying must not be based on the speed with engines turning, but when the engines are stopped. This is the case in all aeroplanes, but is particularly so in these designs, since an appreciable strengthening of the lateral control is probably obtained with them. 5. Slow flying may be rendered more difficult by the inability of particular engines to turn slowly at partial throttle, but with this again we are not now concerned. Moreover, the slow flying which can, with skill, be effected by the process of setting the main wings cabre', keeping them there and forcing the aeoroplane forward against the very high dragt, so caused (as was done at the Military * I have often wished for the liberty to abbreviate the word " aeroplanes " in this Way, and since the meaning is not obscured by the shorter term, I trust it, or some other short term, may come into use. t I adopt " drag," the word suggested by Mr. Archibald Low, in preference to the word "drift," to express the " resistance to forward motion through the these, and partially expressed in the same diagram, there is the other half, the vertical pulsations which we also have to deal with and cure by sensitive pitching or otherwise. As to the latter I am unacquainted with any published diagrams or measurements. The indications of Fig. IA even are approximate only, since they were taken at the end of one wing of the aeroplane, and include winds other than axial up to 15° divergence. They probably give a general idea of the extent of variation of the " head-on " velocity at the various heights above the ground indicated in Fig. IB on the day in question, but we also assume that the size of each air disturbance was large enough to include the whole wing or the whole aeroplane, though we only know of the disturbance over an area the size of a threepenny piece. Experiments are being made as to the size of disturbances at one particular locality, and when these have been made in many places some better generalisation may he possible. 8. Vertical Gusts and the Shape of Air Disturbances.—If the vertical gust is a frequent additional disturber of the peace of flyers, and no one has made any show of a reason against this view, it is certain that we must give it more special consideration than it has yet had, and in doing so, observe that an up-current unfor tunately results in a forward movement of the centre of lift of a wing curve, which is precisely what we do not want at that moment. 9. A rough pictorial notion of the shape of the air movements should, I think, help the consideration of the effectiveness of devices such as this paper is directed to, and the following, based on a note of Mr. Mallock's, though obvious when stated, is a suggestive way of putting the matter. air." The word drift is badly wanted in aeronautics in its own time-honoured significance, so that the actual travel of an aircraft may be compounded of the distance which it travels axially, and its drift, i.e., the amount it drifts with the wind. 126
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