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Aviation History
1920
1920 - 0530.PDF
MAY 13, 1920 CORRESPONDENCE [The Editor does not hold himself responsible for opinions expressed by correspondents. The names and addresses of the writers,not necessarily for publication, must in all cases accompany letters intended for insertion in these columns.] WING LIFT [2019] It is evident that the reason an inclined plane tends to rise when facing a horizontal current of air is the result of the reaction on the plane due to the disturbance it creates in the air current, but this has been expressed in various ways. For instance, it has been said to be due to the air entering horizontally and leaving in a downward direction, such direction being the resultant of the angles of the upper and lower surfaces at the trailing edge. This method of expression, however, appears incorrect when it is known that a plane will lift when the rear portion is flattened out to the horizontal, or even turned up. It is also explained as the pressure on the under surface and a tendency to vacuum on the upper surface. This, however, would give a centre of pressure somewhere between the centre line of the plane and the trailing edge, but the centre of pressure is actually in front of the centre line of the plane. Therefore this statement of the case appears faulty. More- over, it does not satisfactorily explain the movement forward of the centre of pressure with decrease of angle, and then rearward again for very small angles down to zero, nor does it explain the reason a lift is experienced when the chord is horizontal. Further, it is stated that the air current follows the curve of the upper and lower surfaces, but, if such is the case, how does a cambered plane whose chord is horizontal experience a lift, since there must be a tendency to create a vacuum and therefore a downward pressure or stress on the under surface, and a downward pressure on the forward portion of the upper surface, the only upward pressure being due to a tendency to create a vacuum on the rear portion of the upper surface ? Since the foregoing explanations do not appear to cover the facts, may I put forward the suggestion that when a cambered plane is placed at a small negative angle of in- cidence to a current of air, or at such a positive angle of in- cidence that there is a tendency to create a vacuum on a portion or the whole of the under surface, the air current does not properly follow the curve of the under surface, but cuts across that portion in which it tends to create the vacuum, and in doing so sets up a revolving motion of the air enclosed therein, thus causing the plane to ride upon an air roller the length of the plane and which is kept revolving at high speed by the friction of the air current in contact with the under surface of the roller, and the greater the velocity of the air current, the more swiftly the air roller revolves. Since this roller, however, will generally be more or less elliptical in shape, and will consequently exert con- siderable force in attempting to become circular, and it cannot accomplish this without either pressing the air current downwards or raising the plane, it thereby gives a lifting force to the under side of theWplane. This explanation I think covers rather satisfactorily the known results. For instance, in Fig. 1, " a " shows a cambered plane at a large angle of incidence, " b " at a small angle, and " c " at no angle to the air current meeting it. 7erpdtncy to Slight /taller ^forward Heller againstma/'o* portion of undert ' except that due to the production of the slight vacuum toward the rear of the upper surface and the energy absorbed in compressing the roller to an elliptical shape ; the. latter however, will give back this energy in the form of lift, and a machine with a specified horse-power would accordingly be capable of very much higher speed when fitted with these wings than with the usual .type of wing, and therefore of greater lifting power. In order to reduce the speed for rising and landing, it would be preferable to have the facility of varying the angle of incidence of the wings. By this means heavy loads could be carried, for a machine having risen slowly with the wings or whole machine at a comparatively steep angle, the angle would be reduced to bring the roller into being, when considerable acceleration would take place due to the functioning of the roller, which increased speed together with the centrifugal force in the roller would con- tinue to support the load. If this theory is correct, it would appear that the utility of the roller would be twofold—namely, reduction of friction between the air current and the under side of the wing, and the fact that the upper surface of the roller will tend to roll the wing forward at the same speed as that at which the periphery of the roller rotates, and it would therefore appear that a wing specially constructed to produce the most efficient roller on being drawn through the air at an efficient angle (probably in the neighbourhood of zero °) by means of an engine would accordingly offer no resistance at any velocity itacfing r&ft theped to•"--•- air currant bahr* - under ' " F/G.2. according toexperimental results Fig. 2 shows my idea of the wing section for specially producing the roller action, the wing preferably being of high aspect ratio in order to bring as much of the under surface as possible within the action of the roller. Crewe. C. W. SLINGO. THE SIDE-SLIP LANDING [2020] Having noticed that" L. T. P." wishes to hear more on the above subject, I must ask you to allow me to reply. In the first place, my letter did not imply that side-slipping is unnecessary. The side-slip landing is extremely useful, and I should like to know where I implied that it is not. The point I endeavoured to make clear was that side-slipping as a means of losing superfluous height without appreciable forward movement is very useful, but to glide in and use your keel surface to check speed every now and then, as Lieut. Courtney implied, is not correct practice. To judge by the extravagance of " L. T. P.'s" praise for one who can glide in from any height and do a spot landing, I take it that he cannot do it himself ; probably through missing the opportunity of being taught while such schools as Gosport and Lilbourne were showing how it could be done. I must ask him to be good enough to read the last sentence of " R.A.F.'s" letter which immediately followed mine. May I suggest that " L. T. P." endeavoured to become a " very clever pilot indeed " by mastering the elementary principles of flying on the glide—a form of flying to which too little attention was paid by the average instructor. I was denied the pleasure of flying a " 9A." An oppor- tunity never came my way. Provided that a three-point landing is made, however, I should judge from what I have seen that the distance run on the ground would be under 60 yards. It is the length of the hold-off usually seen with this type which makes it appear to require an immense amount of space. Adding 10 m.p.h. to one's speed may be difficult to some without looking at the clock. It is a matter of judgment. In para. 4 " L. T. P." again objects to my views, and advocates gliding so fast that an airbrake is necessary from time to time. Why put on speed and then take it off ? Wili " L. T. P." please refer to my last sentence, para..4 ? In para. 5 he says he does not think that correct side- slipping needs practice. Admittedly plain side-slipping does not. Most of us during our first hour's efforts nearly blew our heads off, and wondered why the wind persisted in coming sideways 1 To carry it out correctly is another matter altogether. One cannot do anything correctly without practice, and what " instinct" tells you is right is often entirely wrong. Here again " R.A.F.'s " remark applies. May 8. E. J. D. German Aerodromes to be Destroyed ? FROM a semi-official statement made in Berlin on May 6 it would appear that, in accordance with the Peace Treaty, the German military air service will be disbanded. From an article published in the Tagliche Rundschau it would appear that the Entente Commission intends to destroy all aerodromes and flying establishments, the cost of which is given as 60 million marks, leaving only one hangar and one aeroplane factory, which will be used for an international air service. It is complained that this means a most serious economic blow at Germany at a time when building for industrial purposes is impossible. 53O
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