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Aviation History
1933
1933 - 0660.PDF
SUPPLEMENT TO FLIGHT 64 SEPTEMBER 28,1933 THE AIRCRAFT ENGINEER to be extrapolated somewhat, and it is doubtful if this i6 justified, since the minimum velocity possible for the Autogiro might well be above 32 ft. per sec. Taking the two possible C.G. positions, x = 0.5 and x = 0.25, it will be seen that a great deal is to be gained by having the C.G. at 0.25 in preference to 0.5. The gliding angle is increased from 20.8 deg. to 54 deg. with only an increase of about 7 ft. per sec. in the vertical speed. The vertical component at x = 0.25 should still be low enough to be dealt with by the undercarriage, and the horizontal component is only 20 m.p.h. TABLE 1 ai = ± 20° TABLE 2 C.G. x V F.P.S. Horizontal Comp. of V F.P.S. Yrertiea] Comp. of V F.P.S. Ghding Angle 0-5 0-25 0 —0-25 -0-5 47 35-5 32 42-5 54-5 44 29-4 21-8 38-6 520 16-3 23-5 23 17-2 161 20-8° 34° 47° 24-5° 17-2° Tf the tailplane had an available range of 90 deg. it would make but little difference to the possible gliding angles, since the maximum-lift coefficient of the tail- plane would be reached at a speed very little less than the speed corresponding to 20 deg. setting, and the aircraft can then no longer be in equilibrium. An aircraft rarely lands in absolutely still air; this, no doubt, accounts for the steep angles at which an Autogiro descends, and is in fact the only way in which to account for vertical descent. The horizontal speed of the Autogiro is y/V - "W. If Vw is the wind velocity the speed relative to the ground is then vA" - W - \\. Hence, if is the gliding angle relative to the ground with wind velocity VW) then Vv Tan <j>w = - .... (17) Values are given in Table 2 of wind speed necessary to convert f into different values of ^w. Considering the case of the C.G. at x — 0.25, the possible gliding angle in still air is 3d deg. and a wind speed of 29 ft. per sec, or 20 miles per hr,, is necesary to change this to a gliding angle of 90 deg., or, if we take 70 deg. as constituting an apparent vertical descent, the wind speed necessary is only 23 ft. per sec, or 16 miles Wind Speed (F.P.S.) necessary to alter <f> to 50° 60 70° 20 25 30 35 40 31 23 17 12 8 35 28 22 17 12 39 32 26 22 18 42 34 30 25 22 45 37 33 29 26 80" 90' per hr. The horizontal landing speed under these con ditions is only 6 miles per hr., which means practically zero landing run. It is quite likely that the figures given above might be considerably improved upon in the latest Autogiros, more particularly in the C.30, in which control is obtained by tilting the rotor backwards. This has the effect of putting the C.G. further aft at the higher inci dences, and so securing a steeper gliding angle. Then again the characteristics of the latest Autogiros may somewhat modify the calculations. Cierva records in the Journal of the Royal Aero nautical Society that an Autogiro has for a short time reached an incidence of 87 deg. This is no doubt pos sible at the instant of landing provided the pitching moment of inertia is sufficiently great to balance the forces out of equilibrium. This incidence could, of course, be reached by means of gust velocities. It has been assumed in the above calculations that the incidence of maximum KL is about 40 deg. This again may be increased in the C.30, thus resulting in the con siderable advantage of slow vertical speed at high angles of incidence. There is, of course, no reason why an Autogiro should not fly well beyond the maximum-lift coefficient, since the danger of a sudden stall due to loss of control is entirely absent with the Autogiro. Pro vided there is the control available, there is no reason why any incidence should not be reached, and once this incidence is attained, although control falls off, the air craft will just sink down slightly more rapidly, since it must be laterally stable at all angles of incidence. It is this fact, that the incidence of maximum KL can safely be passed, that gives the Autogiro such an advan tage over the aeroplane, for, after all, safety must be the first criterion of any aircraft. There is little doubt that the Autogiro will play a big part in the future of air transport. •06 •05 04 03 •02 0 •01 02 •03 04 05 •06 H (-0 -25\ IF! \ i i G.6. \ \ \\ i i \ [0 i y v *v s i s I P i «• 2 > 2 I i t> i l i 4 3 > 3 4 0 30 20 10 0 <*T 10 20 40 4o VF.PS. / / 6 0 8 / > , -5 -25 .0 •25 •s •- =&t T CONTROL ASSUMED AVAILABLE F ii G.7 — -- 972 d
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