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Aviation History
1941
1941 - 1903.PDF
AUGUST 2IST, 1941. FLIGHT READY FOR THE FUTURE range from fine pitch to feathered position. As in the single airscrew, the pilot selects his engine speed, and the governoi maintains this under all conditions by in- creasing or decreasing the pitch, thus keeping the load on the airscrew constant. Mention has been made of the fact that the contra- prop, is not " worth while ' on all types of aircraft. This requires some little explana- tion. In transforming en- gine horse-power into thrust there are two limiting fac- tors : engine speed and the speed of the tips of the air- screw blades. The first of these two is fairly obvious— the engine is designed to run at a certain maximum speed for long periods; if that speed is exceeded, the life of the engine will be shortened, and the engine may even be wrecked. The second is not quite so obvious. The speed of sound in air is just over 1,100ft. /sec. at ground level. If the air- screw tips reach this speed, or even if they approach it, a change in airllow takes place, and the airscrew thrust suffers a serious drop. By interposing gearing between the engine and the airscrew, the tip speed can be kept down, but a limit to this is set by the fact that maximum airscrew efficiency depends, among other things, on the ratio of pitch to diameter, and as the slow-running air- screw has to have a coarser pitch than a fast-running, for a given aircraft speed, one soon reaches this limit. In addition to these considerations, the unfortunate airscrew designer is faced with many other problems. For instance, he is being asked to convert into useful thrust, with a maximum of efficiency, greater and greater horse- powers. To absorb these high powers he can increase the size, i.e., diameter, of the airscrew, but he then runs into the problem of tip speeds. He can increase the blade width, but here again he cannot go beyond certain limits without incurring a loss of efficiency. Our old friend aspect ratio, well known from the aerodynamic design of aircraft wings, comes into the picture and may not be reduced beyond certain proportions. Helix Speed vie While aeroplanes were slow, and the airscrews were ungeared (i.e., were running at engine speeds), these problems were comparatively simple. Now. however, with aircraft reaching or exceeding 400 m.p.h., they are in- creasing by leaps and bounds. It is not always remem- bered that the tip of an airscrew blade travels through space in a spiral path, compounded of the forward speed of the aircraft and the rotational speed of the airscrew. It will be obvious, when this fact is recalled, that the airscrew tip travels a greater distance in a revolution than does the aircraft, so that the local speed of the tip is much greater, and the speed of sound is reached locally much earlier than at other parts. It might have been thought that this list of limitations would exhaust the subject. Unfortunately this is far from being the case. The poor airscrew designer has yet an- other aspect to take into account. Hitherto we have con- sidered low-altitude flying only. The tendency of modern military aircraft is to fly higher and ever higher. At 20 oooft., which is a moderate height nowadays, the air In fine pitch : This picture of the Rotol contra-prop. shouldbe compared with that on the preceding page. The pitch angle range is 75 deg. , density is less than half of the density at ground level. It is an unfortunate fact, from the airscrew designer1 s point of view, that the speed of sound decreases in proportion as the density decreases. Thus, a tip speed which might be per- missible at low levels would be excessive at 30,000ft., for example, and once again a compromise has to be sought. Reference has been made to the subterfuge of in- creasing the blade width, and the limited extent to which the pov\ ur can be efficiently absorbed in that way. An obvious alterna- tive is to increase the number of blades. But al- though four is a wotkable compromise, a greater number introduces inter- ference effects. The easiest way to picture these is. perhaps, to think of bi- plane, triplane and multi- •plane wing arrangements. The monoplane is the most efficient, then the biplane, and the triplane begins to be somewhat inefficient, although it does give extra lift. So with the number of airscrew blades. Single-bladed airscrews are in existence, and are efficient. The two-bladed airscrew so common a few years ago represented the biplane, the three-bladers now in common use the triplane, and it is fairly efficient. But a greater number than three begins to be inefficient, although four-bladers are used effectively on certain types in which airscrew diameter is limited. Straightening the Slipstream w . " ~ ~ -'•: Airscrew efficiency pure and simple is but one aspect. A single airscrew sends a spiral slipstream back over the aircraft. This causes unsymmetrical airflow and thus a tendency for the aircraft to "swing," as it is termed; that is to say, to swerve to one side. This tendency is always present, but is worst when the amount of slip is greatest, such as, for instance, during take-off. Now, obviously, an aircraft designed to fly fast at 30,ocoft. or even higher will have an airscrew of fairly coarse pilch, and this will give a high degree of slip at low forward speeds. Consequently, the tendency of such an aircraft to swing will be more pronounced than that of one which is designed to operate at low heights and moderate speeds. Over and above the troubles caused by the twist in the slipstream there is the torque reaction. This has to be opposed by "rigging down " the ailerons on one side, or in some similar way. Evidently such rigging cannot exactly balance torque except at one engine speed : at others there is a residual torque which is not corrected. As engine powers go up so does torque reaction increase. On some aircraft types this is not very important, but on others it can cause much trouble to the pilot. Deck- landing aircraft, for example, have very little room for swinging. Quite obviously, therefore, there is much to be said for an airscrew system which does away at one and the same time with the twisted slipstream and with torque reaction, ir addition to absorbing with good efficiency at great heights the power of the latest high-power engines in fast- flying aircraft. It is for such that the contra-prop is specifically intended, although its application to more "modest" types for certain work is also worth while.
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