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Aviation History
1927
1927 - 0898.PDF
SUTFLKMENT TO FLIGHT 72 THE AIRCRAFT ENGINEER NOVEMBER 24, 1927 eases that have been worked out. Using this value we get : Cg whence ' 6902 ./ If now equation (8) be compared with equation (7) it will be seen that by approximately limiting the time of run to 'wo minutes that the " limiting take off " value of P is decreased in the ratio q- to ' 690- q ; The time of " take off " will, therefore, be approximately two minutes when p __ 4X(g< - I) 690-(K2L- 690- q-) If, now. in equation (6?;) t be given the value of 120 seconds the run to " take off " will be t'iven bv :— .(10) For normal flying-boats whose top speed is between 100 and 120 m.p.h., the value of X will be somewhere between 3-5 and 4-5 depending upon the pitch angles of the propeller and its speed of rotation. For a geared engine the propeller of which runs at 1.000 to 1.200 r.p.m.. the value 4-5 is nearly correct; for an ungeared ensrine the lower value should be used. The value of K is 29-2 for a lift coefficient of 0-5 and 25-5 for a lift coefficient of 0-65. The value of /, which is the ratio drag to lift of machine without the hull and no slipstream effect, will, for normal machines, lie between 0-12 and 0*15. The value of q is four times the maximum water resistance of the hull divided by the displacement, if no figures of the water resistance are available q may be assumed to be from 0-65 to 1. 0-65 representing a very good hull and 1 a rather poor one. It is interesting now to assume definite values of X. 1 and K applicable to a good modern machine and then plot values of q against the power loading P in equation (9) for various wing loadings L. This is shown by figure 2. The value of the constants assumed being :•— X = 4-4 Z = 0-126 K = 27 L = 8 lbs. per sq. ft., and L = 20 lbs. per sq. ft. The length of run required has also been calculated by equation (10), and is written on the graph for values of g = 0-95, 0-80. and 0-65. From these curves, it would appear that in the case considered that the wing loading does not seriously affect the total load with which a flying-boat will leave the water, the water resistance being far more important. The wing loading, however, does seriously alter the length of run required, thus, taking q as 0 • 8. the run required to take off in two minutes is approximately 1.780 yards for 8 lbs. per square foot, and 2,790 yards for 20 lbs. per square foot, the length of run being, for all practical purposes, proportional to the square root of the wing loading for a constant time to take off. If the length of run were used as the criterion of take off. then the machine whose wing loading was 8 lbs. per square foot, could obviously take a little greater power loading than shown on the curves, this increase in power loading would, however, be very small, the dotted line on the graph indicating the absolute limit of " take off " independent of wing loading as given by equation (7). It would also appear that a good modern flying-boat will leave the water at a power loading of between 18 and 21 lbs. per horse-power, which gives a very good indication of the possibilities of long-range flying-boats. It will be observed that the thrust is assumed to be 4-4 times the available horse-power which may not be realised on a high-speed boat. In Fig. 3. the time to " take off " is plotted against fractions of normal loading for various values of q, the other constants remaining the same as in the previous case. The normal loading is assumed to be 10 lbs. per square foot, and 10 lbs. per horse - power ; thus at fraction 1-5 normal load, the wing-loading will be 15 lbs. per square foot, and the power loading 15 lbs. per horse-power. In Fig. 4 the run required to " take off " is shown plotted against the time to " take off " for the same case. The assumption made that the water resistance is proportional to the all-up weight will be approximately correct for a 25 per cent, overload on a well-designed hull, but would not be expected to hold for a much greater overload, so that in Figs. 3 and 4 the curves should not be expected to apply to the same hull throughout. To determine how near the formula given in this article would apply to an actual over- loaded hull, a model hull was tested at various overloads, the propeller thrust calculated as accurately as possible, and the time of " take off "' found by step-bv-step integration. The following table gives the comparison :— Time to take off By Step-by-Step Normal Load. by Formula. Integration. Per cent. Sees. Sees. 100 20 20 111-5 29-7 30 127 52-3 56-8 The constants for the machine at normal load are :— P - 13, L ^ 10-77. X 3-9. K . 25-5. q -= O<>75 I = 0 125. From the above, it is seen that the time is practically correct for 11-o per cent, overload, and that the formula under-estimates by about 8 per cent, for a 27 per cent, over- load. Fig. 5 is similar to Fig. 3 except the normal loading is 10 lbs. per square foot, and 14 lbs. per horse-power. WHEEL BRAKES AND THEIR APPLICATON TO AIRCRAFT. By G. H. DOWTY. A.F.K.Ac.s., M.I.Ae.E. Until quite recently wheel brakes have not l>een seriously considered in connection with aircraft, for their fitment has generally been regarded as a menace rather than an advantage. Their use has always l»een associated with a tendency for pitching of the machine on its nose. and. in any cast1, to give no great advantage when compared with the additional weight and complications consequent to their adoption. Compared with the motor vehicle, brakes on an aeroplane have a very restricted use, and are confined to checking the length of run on alighting and subsequent operations on the ground. It must be admitted that the aeroplane exists under a considerable handicap, in that it requires a greater space within which to arrive and depart than any other means of traasportation. The aeroplane is the only vehicle used which does not apply brakes on stopping, and yet it is the one mostly in need of braking, since its speed is the greatest. The advantages to be gained from braking have not been ignored, and in the search for a suitable method many schemes have been suggested and tried. The following have been some of the most popular methods to receive attention :— (1) Increasing the height of the undercarriage to produce a large angle with the ground. (2) Air brakes of various forms such as expanding rudders and flaps. (3) Sprags on tail skid and axle. (4) Wheel brakes. The first method, while satisfactory, is necessarily limited, and has the further objection that it is not positive. Air brakes have been repeatedly tried, but have always been discarded because of their almost negligible effect at low speeds. Provision of Rprags on the tail skid has the disadvantage of setting up heavy loads in the fuselage and, furthermore, 8l0d
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