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Aviation History
1926
1926 - 0418.PDF
SUPPLEMENT TO FLIGHT 58 THE AIRCRAFT ENGINEER JUNE 24, 1926 speed ; and for very stable machines, 3 per cent, on climb to 4 per cent, on top speed. These latter figures are only important as showing variations of degrees since many factors enter into the equation of motion which are really beyond the control of the designer who has to make a com- promise. Probably the best way to keep tail plane drag down is to keep the lift losses due to body interference as low as possible and so keep down the area of the tail plane. TAIL PLANE REACTION DIRECTION OF DOWNWASH Fig. 14. The undercarriage offers per se little prospect of aerodyna- mical improvement. The largest items are always the wheels whose frontal area is determined by the permissible bearing pressure on soft ground. The usual figure for this is W projected area in square feet = The area is measured as tread by diameter : the resistance of wheels faired is between 4 and 5 lbs., per square foot for W small wheels, so that the resistance of the wheel is about ~^zrc\ ooO lbs./sq. ft. at 100 ft. per second. For larger wheels the figureW falls to . So far as military aeroplanes are concerned,OOU ground conditions are hardly likely to alter, since these machines may have to operate from unprepared ground. Civil aircraft, particularly on regular routes, might be made more independent of ground conditions if engine reliability can be maintained at a very high figure. The possibility of specially surfaced aerodromes or launching gears is not very near. Attempts at disappearing undercarriages are unsound mechanically, and probably give rise to more resistance in attempting to provide accommodation of the undercarriage than they save. I do not believe any aeronautical engineer regards such a scheme seriously. Wheels are not only the most important items of undercarriage resistance in them- selves, but they are particularly liable to give rise to inter- ference resistances with the chassis struts. Wind channel tests show that these interferences may be avoided, but as with most other interference phenomena, these tests are largely a matter of trial and error. As an interesting example of what wheels can do I may instance the analysis of the resistance of a racing car carried out in the Boulton & Paul channel, which showed that the front wheels and their interference with the body were the biggest items of resistance. It is difficult to divide the various factors which go to determine the performance of an aeroplane into water-tight compartments—they are all more or less mutually dependent. I propose, however, to treat the propeller and engine together, as the power plant, since so many of the propeller character- istics are really determined by the engine. In this section of these notes I will, therefore, make only a brief mention of slip-stream effects. The flow through a propeller is stream- line, i.e., total head is constant, hence the inflowing stream contracts in front of the propeller and the outflowing stream contracts behind it as the stream velocity increases ; this is shown diagrammatically in Fig. 15. This streamline flow is liable to interference. In multi- engined machines interference in the inflowing stream may obviously take place even where the propeller diameters do not overlap in front elevation. Similar interference may take place between the body and inflowing stream, particularly if the body projects far forward of the propeller disc, has a poor entry and small clearance. These interference phenomena are common knowledge and experience on machines of quite recent design, and it is sufficient to point out that clearance in front elevation is not the true criterion. Experimental work in the wind channel on arrangements designed from carefully plotted streamlines will ensure freedom from inter- ference. Judging from personal experience on these lines I would express as my opinion that all three-engined aeroplanes, of which drawings have been published in technical papers to which I have access, have some measure of propeller stream interference, not always enough to cause vibration, but sufficient to affect performance, particularly at low speeds. It is always possible to be wrong in expressing an opinion on interference, but I give this opinion with fair confidence. The outflowing stream has a rotary component varying with intensity at different radii, and, of course, at various values of V /' linear speed \ ml „ „ , . ,,. . 1 * The effect of this obliquitynD \angular speed X diameter/ of the slip stream is well known as disturbing the symmetry of an aeroplane and causing a yaw when the rudder is left free. This obliquity must also affect the resistance of bodies in the stream, but so complete is the flow that it is difficult to allow for in design. It is probably better to aim for stability of flow, i.e., small change of drag with incidence, than attempt to adjust the attitude of the parts likely to be affected. There is another pitfall for the designer, whose only safeguard can be wind-channel work on a large and representative scale. The general effect of the slip stream as affecting resistance by increase of air velocity I prefer to discuss under power plant and propeller efficiency, since it is in the power plant balance sheet that the drag from this cause should properly appear. The reader will have gathered that I do not see open any way to substantial aerodynamic improvement, over best practice in the aeroplane proper. It is, however, only too easy to fail to achieve what theory and practice tell us can be. and ought to be, possible. If the specification of an aeroplane is simple, e.g., if merely carrying a man or so is its sole require- ment, design is fairly easy ; aerodynamical considerations can be put first, as they were in the aeroplanes of ten years ago, but aeroplanes are now highly specialised machines, particu- larly those intended for military purposes. Numerous cockpit openings, large bodies to accommodate equipment and allow free movement to the crew, the provision of awkward angles for view and gunfire, all decrease not only the aerodynamic efficiency that can be attained, but decrease to a far greater degree the probability that that best will be obtained. OUTFLOW STREAM AIRSCREW y -—-— INFUOW STREAM ^——__ DIRECTION OF ^ 'MOTION OF AIRCRAFT Fig. 15. There is, outside the ranks of professional engineers, a complete misunderstanding of engineering work. Nowhere is this misunderstanding greater than in the aeronautical world. Some imagine that all the data necessary for design is provided by the work of national research establishments, whereas in fact, such is the barest skeleton and scarcely touches the real problems which make for successful design. It is difficult to see how it can be otherwise. Scientific research is a map on a very small scale and endeavours to cover a very large country and to open up fresh territory. Engineering design is cartography on a very large scale, so large in fact as to miss no feature large enough to affect the desired result. A mining prospector, whose sole equipment is a small-scale geological survey of the continent of America, is not likely to discover an El Dorado. Theory is like a map—it helps you to get near your goa but finally you must depend on " local " knowledge. Local knowledge, on the other hand, is of little value away from its own locality. We have heard a great deal about " unorien- tated " and " ad hoc " research. The former term is. I think, unfortunate, being too nearly a Latinised form of " aimless," 3626
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