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Aviation History
1950
1950 - 0792.PDF
514 FLIGHT, 27 April 1950 NOSEWHEEL or TAILWHEEL? . . . makes it apparent that it is an anti-bounce and anti-nosing- over gear (Fig. 3) by contrast with the tailwheel layout (Fig. 4). On the score of dynamic stability, which involves such high-frequency phenomena as shimmy (lateral oscil- lation) and porpoising (pitching oscillation), the nosewheel undercarriage does not show to advantage against its tail- wheel counterpart. The shorter wheelbase not only reduces the damping moment arms, but also incurs higher aircraft moments of inertia about the wheels. There are, however, various ways of suppressing, if not eliminating these troubles. Porpoising can be mitigated by increasing the wheelbase or the stiffness of the shock-absorbers, or by decreasing the TABLE II: DESIRABLE UNDERCARRIAGE DESIGN DIMENSIONS- Fig. 3. The force vector diagrams shown here makes it clear why a tricycle undercarriage k stable in pitch after touch-down.V- PITCHINC MOMENT OF INERTIA Fig. 4. By contrast with Fig. 3, the force vector'diagram for a . tailwheel type moving on the ground shows instability in pitch. height of the undercarriage. Shimmy can be reduced by the introduction of anti-swivel damping such, for example, as is provided by hydraulic steering, by hard twin-tread tyres, by twin-wheels on a common axle, or by changing from an inclined to a vertical swivel axis on a twin-wheel type. The fact that a nosewheel assembly is almost inevitably required to bear about twice the loads of a tailwheel unit means, almost equally inevitably, that it comes out at least twice as heavy as a tailwheel unit of the same height. There are, however, other weight penalties as well. On a multi-engined aircraft, for example, the front fuselage is required to accommodate the heavy nosewheel loads—if ditching loads are omitted—whereas tailwheel loads can TABLE I : UNDERCARRIAGE PERCENTAGE OF ALL-UP WEIGHT. Tailwheel Aircraft Tricycle Aircraft Type Percentage a.u.w. j u/c. ! Struct, i Percentage a.u.w. Type Struct. Tudor I ... Tudor II Tudor IV Tudor V York Wayfarer Prentice Proctor Viking ... Dakota .. •4.0 4.0 4.0 4.0 4.9 5.2 5.2 4.9 3.8 7.0 27.8 32.1 28.7 31.5 30.1 38.0 42.5 40.4 30.2 33.6 Mean value for tailwheel under- carriage = 4.8 per cent of all-up weight. Mean value of tricycle undercarriage = 4.92 per cent of al!-jp weight. Dove Ambassador Hermes IV ... Marathon ... Prince Convair 240 DC-4 Canadair DC 4M-2 DC-6 DC-7 Stratocruiser Constellation Constellation MB Martin 2-0-2 Rainbcw 4.2 4.8 4.1 S.I 5.2 27.8 32.1 28.5 32.0 32.1 3.7 5.7 5.3 4.8 6.2 5.15.1 4.4 4.5 5.6 27.9 1 26.9 25.3 24.8 29.5 28.0 30.4 17.2 27.8 25.9 Dimension (1) Zero liit incidence with all wheels on ground (Fig. 3 and 4). (2) Angle Beta between vertical and line join- ing e.g. to tyre con- tact centre. (3) Nose- or tail- wheel loads as percen- tage of a.u.w. (Fig. 5). (4) Wheelbase as per- centage of aircraft length. (5) Wheel track as per- centage of wing span. Noiewheel only Tailwheel types Not less than 5 deg. to prevent " jump " take-off. Not less than 15 deg. to prevent tip-back ten- dency. 10 to 20 per cent, for a.u.w. below 10,000 Ib. ; 8 to 16 per cent for a.u.w. less than 20,000 Ib. ; 4 to 13 per cent for a.u.w. over 20,000 Ib. Not less than 33 per cent. 28 per cent. Not less than 2 deg. below stalling incidence, to prevent ground-loop- ing. Not less than 20 deg. to prevent nosing-over, and not more than 25 deg., to prevent ground- looping. 4 to 11 per cent, average. 57 per cent average. Not more than 30 per cent to prevent ground- looping. easily be dissipated in the rear fuselage structure which, in any event, has to be made strong enough to accommodate empennage loads. On this basis alone, therefore, the fuse- lage of an aircraft equipped with a tailwheel undercarriage should be lighter than that of an aircraft with tricycle landing gear. All in all, the weight penalty on the introduction of a nosewheel undercarriage on a given aircraft may vary from 0.5 to 2.0 per cent of the all-up weight, and this in turn incurs a decrease of payload from 2 to 8 per cent—a serious loss. If, on the other hand, an aircraft is djesigned from the outset for a nosewheel undercarriage, it is possible to use a higher wing loading and, therefore, a lower structure weight, by being less demanding in taxying, take-off and landing. This factor may offset or even overcome altogether the weight penalty involved in the use of a tri- cycle undercarriage. In Table I is shown how small, in fact, are the final weight differences for both basic types of undercarriage. The fact that most of the aircraft in- cluded in the table are transport types is ascribable simply to the lack of other data at present available. In order to provide some approximate undercarriage design data for this article, the characteristics of some 36 British, American and German undercarriages on civil and military aircraft have been examined. The main values of these data are presented in Table II and in amplification of some of the various items it is of interest that, regarding item (i), the wing incidence on the Marauder 3 was in- creased by 3 deg to improve the take-off. By the same GLOBEM ASTER STRATOCRUISER CONSOLIDATED B-32 CONSTELLATION DC-6 SKYMASTER LIBERATOR NEPTUNE AMBASSADOR 1 ALBEMAHLE MARAUDER BLACK WIDOW BMMBWMIXINVADER LICHTN1MC MARATHON SHOOTING STAR KORSA2 rj'fmm •>. ~^. PRINCE DOVE AESCWN "2345 1O 2O 30 MOSEWHEEL LOAD ( PERCENTAGE OF LANDING WEIGHT) Fig. 5. This chart is of interest in showing nosewheel loads as a percentage of landing weight plotted against landing weight for a variety of types, most of which are, necessarily, American.
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