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Aviation History
1950
1950 - 0790.PDF
512 FLIGHT, 27 April 1950 An Examination of Landing Gear Design Characteristics By H. K. MILLICER*. Dpi. Ing., M.Sc, A.F.R.Ae.S \ The Theseus-powered Hermes V follows the tricycle landing-gear fashion, set by its immediate predecessor, the Hermes Mk IV. NOSEWHEEL or TAILWHEEL? MUCH has been said in the past on the rathercontroversial subject of a choice of undercarriagetype for a given aircraft. Both the nosewheel and tailwheel varieties of undercarriage layout have their own intrinsic characteristics which make them individually suited for different types of aircraft. This being so, it is felt that the best purpose may be served by examining the merits and disadvantages of both types of under- carriage from four distinct viewpoints, namely, (i) stability and performance; (ii) structural problems; (iii) weight comparison; (iv) design criteria. Any examination of the relative characteristics makes it clear that the most pronounced differences between the nosewheel and tailwheel configurations occur in relation to stability. The tendency to bounce or ground-loop after landing is practically eliminated when a tricycle under- carriage is used; furthermore, such a layout also greatly increases the safety of blind- and cross-wind landings and, in this connection, the field of view from the cockpit is greatly enhanced (for ground manoeuvring) on a tricycle aircraft, especially if it is a single-engined type. When a steerable nosewheel is used, the take-off safety for multi-engined aircraft is improved. At the 1948 S.B.A.C. display at Farnborough, the Airspeed Ambassador demonstrated a single-engined take-off, the '' dead '' air- screw being feathered throughout. This performance was achieved through the nosewheel taking approximately two- thirds of the asymmetric thrust yawing moment, the re- * The author is project aerodynamicist to Percival Aircraft,Ltd., and is an A.R.B.-approved design consultant. maining compensation being provided by the rudders in the slipstream. Whilst on the subject of safety, it is apposite to point out that there is a tendency for an aircraft equipped with a non-steerable nosewheel to run down the gradient of an undulating airfield. This could be distinctly unpleasant if the gradient happened to be across the runway, as the aircraft would tend, so to speak, to run off the course; it could, conceivably, become dangerous if brake pressure happened to be low. The reason for such behaviour can be understood from Fig. i: with gravity pulling downhill and the main wheels trailing, the tendency of the castoring nosewheel is to follow the downhill lead rather than to offer opposition. A steerable or lockable nosewheel would prevent such a tendency and it is a fact that most personal aircraft in the U.S.A. have steerable nosewheels despite the complication and weight penalty incurred. A similar instability which, however, acts in the opposite direction, is displayed by the tailwheel-equipped aircraft (Fig. 2): in effect, the e.g. slips downhill with the castoring tailwheel following, and the aircraft tends to ground-loop. If the pilot is not sufficiently quick in checking this tendency, it can have quite dangerous results; the writer has, in fact, seen one fatal crash in similar circumstances. There is little to choose between the two types of under- carriage configuration when their performances for take-off and landing are compared. It has been established that the best take-off run, irrespective of undercarriage layout, is obtained when the lift coefficient in the static attitude, equivalent to ^xixA, where n is the coefficient of friction and A is the aspect ratio of the wing. When this formula is applied, it can be seen that, for small values of /* (of the order of 0.02 and 0.03) such as would apply on good runways, the re quired take-off lift coefficients are reason- ably small (of the order of 1.0) and, consequently, fuselage incidence is small when flaps are used for take-off. On runways, therefore, the tricycle aircraft should have a shorter take-off run than that of its tailwheel counter- part, owing to fuselage incidence and drag beiag smaller and initial airscrew efficiency and thrust greater. On soft ground, these advantages are usually lost owing to the high incidence required (viz., formula) and the impossibility of The Hastings is a successful military transportaircraft which still retains the tailwheel under- carriage of the common Hermes/Hastingsprototype.
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