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Aviation History
1950
1950 - 0791.PDF
FLIGHT, 27 April 1950 With a strghtly longer fuselage, the VarsityT.I crew-trainerisessentiallyatricycledevel- opment of the Valetta military transport. providing that incidence without incur- ring a high drag penalty for the cruising condition. The landing run should, in general, be somewhat better for nosewheel aircraft as a result of higher braking effi- ciency—except, perhaps, for very light aircraft not equipped with brakes and flaps. There may, however, be a differ- ence in the tail-down incidence between the two undercarriage configurations which would result in lower landing speeds on the tailwheel types. As few people bother to make a three-point land ing nowadays—aircraft are usually '' wheeled-in ''—this eliminates the inci- dence advantage of the tailwheel type. Nevertheless, a nosewheel permits the pilot to apply the brakes hard, whereas similar action on a tailwheel aircraft can incur the danger of nosing-over. On large aircraft equipped with tricycle undercarriages, the wing can be positioned at an angle of incidence reason- ably close to the. incidence required for landing safety- AIRCRAFTINERTIA CASTORINGNOSEWHEEL FRICTION Figs. I and 2. Whereas with a nosewheel under-carriage (above) the sideforce moment arm about the e.g. gives a stabilizing effect, with a tailwheelundercarriage .(right) ft has a de-stabilizing action. CASTORING TAILWHEEL. NON-STEERABLE speed. Furthermore, the elevator can be made powerful enough to cope with the ensuing pitching moment, pro- viding that an appropriate fore and aft location of the undercarriage relative to the e.g. is used (Fig. 3). The fact that braking airscrews can be used on tri- cycle types without danger of tipping back is due to the fact that the resultant <>f the horizontal inertia forces is not far vertically from the thrust and drag line, and the magnitude of these factors is equal during the deceleration. In making a comparison between nose- wheel and tailwheel undercarriages from the structural viewpoints, it has to be admitted that the nosewheel type is the more difficult to deal with. Having a shorter wheelbase, it has to withstand greater loads—more especially in landing cases, when the loads can be as much as twice those that a tailwheel assembly would be called upon to accommodate. The full brake-application load-capacities The immediate post-war Viking makes use ofthe conventional tailwheel configuration of its Wellington and Warwick forebears. specified by the Air Registration Board for tricycle under- carriages are, in fact, worse than the manoeuvring loads on aircraft equipped with tailwheel landing gear. With high- wing aircraft, for example, the taxying loads are 45 per cent higher for a tricycle layout than the manceuvring loads on a tailwheel type. As to landing loads —which are usually the most critical— although the A.R.B. vertical velocities of descent are the same for both types of undercarriage up to a stalling speed of 50 m.p.h., the energy absorption coefficients are approximately 100 per cent higher for the nosewheel type. It is, in fact, true to say that nosewheel loads are, in general, about double the value of tail- wheel loads when the leg lengths are the same. But the very fact that nosewheel struts are, as a rule, considerably longer, means that the loads they are called upon to bear are, at the very least, twice as large as those for the corresponding tail- wheel units. Other structural considerations involve static and dynamic stability on the ground. Here, it must be said that, owing to the main wheels trailing aft of the centre of gravity, the tricycle layout —with, of course, a castoring nosewheel —is inherently statically stable, whereas the tailwheel undercarriage is inherently statically unstable. Consequently, the nosewheel under- carriage is essentially an anti-ground-loop configuration (Fig. 1) as compared with the tailwheel layout (Fig. 2). An examination of the tricycle aircraft in side elevation also
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