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Aviation History
1944
1944 - 0092.PDF
48 FLIGHT JANUARY 13TH, 1944 Tricycle Landing Gear is A Review of Design and Performance Criteria •GROUP CLASSIFICATION: B.3 THE susceptibility of the conventional tail wheel type of landing gear to nosing over under a strong braking effort, its inherent tendency to ground-loop, particu larly in a side-wind, and the necessity of making a full- stall landing to prevent bouncing have made it less and less satisfactory as landing speeds have increased. If properly designed, the tricycle landing gear will eliminate these undesirable characteristics, but it may introduce other objectionable features peculiar to its type. Among these are porpoising, longitudinal instability with the nose-wheel off the ground, shimmy of the nose-wheel and failure of the nose-wheel strut from execessive dynamic loads. In addition, it should be remembered that the elimination of the faults in the conven tional gear will result in increased loads on the tri cycle gear and airframe structure. The removal of the possibility of nosing over will lead to faster taxying and manoeuvring on the ground ; the elimina tion of the tendency to ground-loop will result in more cross-wind landings, and the lack of any ten dency to bounce will result in more landings at high sinking speeds. A paper read before the U.S. Institute of the Aero nautical Sciences by Jenkins and Donovan, dealt with the design and perform ance aspects of tricycle undercarriages in a most comprehensive manner. In the authors' opinion many of the investigations so fai carried out, both theoretic and experimental, are erroneous, since they are only an adapta tion of conventional landing gear criteria to the geometry of tricycle gears, or have not been reviewed in the light of practical experience. For example, the effect of the wheel- base, i.e., tie distance between the nose-wheel and main wheels' axles, on the characteristics of the landing gear cannot be determined without considerations of its location relative to the centre of gravity and the distance of the main wheels aft of the aerodynamic centre. Mode-of-Landing Effect The distance of the nosewheel from the aircraft's e.g. principally affects the nosewheel load and also has an influ ence on the resistance to porpoising and the stability against overturning. The size of the loads placed on the nosewheel by the landing impact, in turn, depend on the type of landing made, e.g., (i) three-wheel landing, (ii) main wheels first, or (iii) nose-wheel first. In the three-wheel landing, the load distribution between the front and rear wheels is usually assumed to be the same as the static distribu tion ; an assumption which can be considerably modified by the effect of the shock absorber on the nose-wheel assembly. In a main-wheels-first landing, the proximity of the rear wheels to the e.g. will result in most of the energy being absorbed there. The loads on the nose-wheel produced by the subsequent dropping of the aircraft's nose may there- tore be expected to be less than in a three-wheel landing. In a nose-wheel first landing, the nose wheel striking the ground tends to rotate the aircraft about its lateral axis, so that the load on the nose-wheel depends on the pitching moment of inertia of the particular aircraft. The accelera tion at the nose-wheel may be found, in turn, from the shock absorber travel and the aircraft sinking speed; unlike the three-wheel landing case, the total wheel base has no critical effect on the nose-wheel reaction. Load Distribution The general load distribution of the aircraft naturally plays a decisive rdle in the determination of landing gear loads. In order to illustrate these conditions, it is possible to represent the aircraft by an arrangement of two concen trated masses, one* of which is located directly over the nose-wheel and corresponds to the load produced by the landing impact in a nose-wheel first landing, whereas the second is located at a dis tance aft of the e.g. which may be determined by the distance of the nose-wheel from the e.g., and the air--. _ craft's radius of gyration. A . calculation carried out with a single-engined, single- seater tricycle machine, driven by an engine and tractor airscrew mounted in the nose has revealed a radius of gyration of nearly 18.4 per cent, of the aircraft length, whereas the distance of the nose-wheel from the e.g. is between 20 and 35 per cent, of the aircraft . length. This means that .between 46 and 22 per cent, of the aircraft's weight acts on the nose-wheel; since the static percentage is usually only about 15 per cent., the error in designing from the load in the three-wheel attitude ma^, exceed 200 per cent., assuming the same sinking speed in both conditions. These figures apply only to the type of aircraft outlined above, and they vary-widely in other types, depending upon loading conditions, size, etc.; the ratios between nose-wheel first, and three-wheel percent ages of weight acting on the nose-wheel have been found to vary from 4: 1 to I|:I. In order to reduce the weight actually on the nose-wheel, it is useful to select the largest possible distance between the nose-wheel and the e.g.; it is wise, furthermore, to make the travel of the nose-wheel considerably greater than that of the main wheels, by which means the acceleration in a nose-wheel first landing and the loads on the nose wheel and structure will be reduced. Of much greater importance than the location of the nose-wheel is the distance of the main wheels aft of the e.g., because this affects more of the properties than any other single characteristic. It influences not only the directional and longitudinal stability, the length of take off run and. the static reaction on the nose-wheel, but also the ability to land with excess air speed and high sinking speed without bouncing. This distance is determined partly by the requirement that the aircraft's e.g. must be ahead of the main wheels even when the tail buffer is against the ground. Thus the vertical forces acting at the main wheels will tend to decrease the angle of attack of the aircraft, reducing the lift and preventing it from rising into the air again. As it is normally desirable to make the take-off with the nose-wheel off the ground, the aircraft must be stable longitudinally when running only on the rear wheels, other- *THIS is the fourth in the series of articles based on abstracts from the world's scientific and technical Press as compiled by R.T.P.3 Section of the M.A.P. The subjects dealt with in the series are divided into the following groups :— A. Aerodynamics and Hydronamics. b. Aircraft and Airscrews. C. Engines and Accessories. D. Materials and Methods. E. Instruments and Devices. F. Production. G. Physiology and War Medicine. Each article appearing will carry its classification group letter followed by a number indication that it is the 2nd, 4th, 7th, etc., in that group to be published. The articles will not necessarily appear in the alphabetical order of their groups.
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