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Aviation History
1910
1910 - 0111.PDF
FEBRUARY 12, 1910. The force P cos 0, representing the amount the plane is capable of supporting, is called the lift = L say. l/SOHTl ">* •\: CL drift cot. 6 Thus L = P cos 6, D = P sin 0. The ratio of lift to L = P cos 6 D P sin 6 From this it appears that by making the angle of incidence of the plane smaller and smaller, we might « V eLsV h P Sin 0 f / % 1 ^ \B get as laige a value of the ratio, lift to drift, as we may desire. It would, indeed, be possible, if the plane were frictionless. Skin friction, however, which is always present, modifies this ratio considerably. The force of skin friction on a surface can be repre sented by F = /AV3 where/is a coefficient determined by experiment, A is area of surface and V is the velocity of the fluid past the surface. The forces on the plane are now as shown in the figure. From this we see that the lift L = P cos 6 — F sin 0 and the drift D = P sin 0 -+- F cos 0 the ratio L bein P COS ^ ~ F S'n ° D mg P sin 6 + F cos if' Obviously this value is less for all angles of incidence than the case where the plane was frictionless. The following table has been prepared showing the manner in which the ratio ,-: varies with the angle of incidence for a frictionless plane and one with friction. This table has been calculated by taking the value of the coefficient c to be equal to '006, and the co-efficient /= '00001. These values represent the means obtained by com paring the results of several experimenters. It will be seen that the ratio •= in the plane with friction first rises to a maximum, as we increase the angle of incidence, and then falls off again. v# ® Army Flying School at Hounslow. A LARGE shed is being erected on Hounslow Heath, which is, we understand, to accommodate the Wright degrees. o'5 I "O rs 2'0 3-0 40 f'O 60 7-0 8-o 90 io-o degrees. 0-19 0-38 o-57 075 1-13 i-«a 1-89 2 "26 2*64 3-02 3"4<3 3-77 degrees ii4-6 57 3 38-2 28-7 191 i<T3 11 • 5 9'7 8-2 7-2 6-4 5-7 Angle of Total lift in lbs. j Ratlo L Rad L incidence of per sq. ft. at D D plane. ; 60 ft. per sec. without friction.; with friction. degrees. 5'c. 8-9 11 • 1 I2'I 11-9 io"6 9'4 8'3 7"4 6-6 6-o 5-5 The angle which gives the maximum ratio -yr can easily be worked out by the aid of elementary calculus. In this case it is 2*35°, giving a ratio jr = *2'25- The idea of increasing the ratio of lift to drift, by diminishing the angle of incidence, has, therefore, its limitations, since, after a certain small angle is reached, this ratio diminishes again. The Wrights observed this when their gliding experiments were being carried out. (See FLIGHT, Vol. I, page 723.) It is seen that as the angle of incidence increases, the L ratios "pr tend to approach the same value for a plane with and without friction. The above co-efficient of friction represents a moderately good surface, such as might obtain in present-day aeroplane practice. It is extremely important to notice that the actual lift of a plane travelling with a certain speed at a given angle of incidence, is not appreciably altered by the presence of friction; it is the drift that is affected by this. The head resistance of the frame of the aeroplane and engine has not been considered in the above. The actual ratio of jy would be under that given in the table. The diminution, however, ought not to exceed "5. In any design we can get the same lifting power for a certain speed with a smaller plane area by increasing the angle of incidence. This in itself means less weight. The ratio ^r however, is diminished, and it is a question of calculation by trial and error which of the two systems requires the smaller thrust to propel the machine. In actual practice the ratio jc varies between 4 and 9. L The Wright machine has a ratio of -jy equal to 8 approximately, but the Bleriot aeroplane cannot be much over 4. A good value is anything between 6 and 9, say 75. This admits of a fairly good lifting power, together with a moderate amount of supporting surface. That is to say, the thrust required should be ?!. of the total weight of the machine. (To be continued.) ® $ flyer to be used by the army officers for their aeroplane training. It is probable that the Hon. C. S. Rolls will be actively identified with the initial training of the first flying pilots of the British Army. . 107
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