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Aviation History
1947
1947 - 1991.PDF
NOVEMBER 13TH, 1947 FLIGHT 555 Landing Gear Developments have evolved the, basis of a fluid whichappears to meet the full requirements; long- range tests are, however, needed. Inreference to increase in speed of operation, French practice is to use compensators ofvarious forms which, however, do not appre- iiblv reduce inertia loading in the retractiongear due to vertical accelerations. Further, each jack, etc., to be controlled is providedwith a hydraulic relay mounted close to it and. using either a secondary relay hydraulicor an electric system to effect remote control. The lecturer then briefly reviewed the ad-vantages and disadvantages of various methods of control before passing on to thesubject of strength requirements. Captain Lucien questioned how the stress-ing case demands specified by the I.C.A.O. have been arrived at, and suggested thatthe individual faults of particular aircraftr 'r been responsible for the increase inseverity of stressing cases applicable to other types of aircraft not suffering fromthe defect. The fact that each country studies the requirements of others very oftencauses further revision which is, obviously, always in the direction of increased severity.The lecturer then suggested that the future development of stressing requirements shouldbe based on systematic experiments where, by varying a single factor only at a givenmoment, it should be possible to determine laws, or at least to draw envelope curves,which will give some idea of the extreme limit of loads to be applied to the variouscases. As an example, an investigation has beencarried out into the application of the latest French stressing cases to an undercarriageof German origin, manufactured in France since the liberation to meet the immediateneeds for civil communication. These under- carriages are used daily in more than 200aircraft and refuse obstinately to be broken, yet Fig. 3 shows how far short of theofficial requirements they are. The lecturer expressed the belief that it would be in thegeneral interest, (a) for require- ments to be established on aninternational plan based on systematic tests which would allowthe cases at least to approach reality and, (b), that each countryarranged to use the internationally established requirements as a basis,complementing it if it must, and even exceeding it, to meet its par-ticular needs. But it is illogical for a designer to have to check tohis own national requirements and then to have to check again againstthe 17 cases of the I.C.A.O. Referring back to braking, it hadbeen shown that the use of a torque factor of 0.8 could bring about adynamic loading of the nosewheel leading to the use of a much largerwheel and tyre than would be needed to deal with the static load,and thus to an appreciable weight flpsialty on the structure. Further-Sore, as regards the main wheel, the increase in landing speed causeda serious increase in the weight of the mass which is used to store thelauding energy on braking. " One could say without exaggeration," observedthe lecturer, " that wheel weights as they have been known up to recent times maybe increased by as much as 15 to 30 per cent. All these considerations have led usto believe that the conditions will be made less stringent in the future by discountingthe possibility of an eventuality which may never occur." Tricycle Three-point Landings It seems appropriate to investigate meansfor making full use of the nosewheel tyre capacity provided to deal with the dynamicloads. To achieve this, means must be found to allow touch-down on all threewheels (or sets of wheels) simultaneously. To take full advantage of this, the under-carriage shock absorbers must be put in balance, by hydraulic inter-connection, toeliminate pitching loads which occur when one wheel touches the ground first. Theadvantages of this system are as follows: (i) the possibility of transferring part of themain wheel load to the front wheel; (ii) the possibility of having all three wheelsidentical; (iii) a reduction in the height and weight of the undercarriage; (iv) improvedperformance due to the fuselage shape being an optimum aerodynamically, and uncjpm-promised by considerations of take-off angle of attack; (v) improved take-off due to theairscrew thrust line remaining parallel to the direction of motion; (vi) reduction instructure weight; and (vii) improved braking. The lecturer commented that the basis ofstrength calculations of undercarriages in Compo- nent Axle Piston Tube External Tube Casing Toggles Material Steel Casting Steel . Steel Magnesium Casting Steel Allow- able Stress Ib/m' 128,000 156,000 156,000 13,450 128,000 Section a-a b-b c-c d-d I-I e-e f-f g-g (A)g-g (B') h-h i-i k-k lug A lug B Attach. Lug Stress- ing Cass X X X X X X X X X X Y X Y X Actual Stress Ib/in* 238,000 204,000 277,000 235,000 287,000 270,000 282,000 20,200 16,900 45,300 19,000 22,600 18,800 15.350 107,500 RF .54 .63 .56 .66 .54 .58 .55 .66 .79 .30 .71 .60 .71 .87 1.19 X Sidetoad Outboard. Y .— 5tdeload Inboard. Fig. 3. Comparison of the NC. 701 undercarriage as flying, and as stressed to latest French requirements. almost all countries is the specification of an arbitrary drop height or vertical velocity supposed constant or "airborne." All dynamic tests in the U.S.A., as well as in Britain, make use of free and, therefore, accelerating, drops of the undercarriage, which covers the mass above it. In France, on the other hand, truly representative con- stant-velocity drops are made by balancing the mass when it has reached the required velocity.With the British and American system the mass dropped must be reduced to adjustthe energy to the appropriate amount, bear- ing in mind that it will be that due to tlwtotal height (dropped height plus deflection) With a long-travel undercarriage the massdropped may be only- half to two-thirds of the correct mass, and the lecturer confessedhis belief that the shock-absorber orifices determined by this method will not be cor-rect. He expressed a desire to receive expert views of his hearers on this subject and sug-gested that a test programme might be drawn up to enable comparative tests to bemade to settle this interesting- problem. Some interesting comments were made onthe subject of light-alloy castings for under- carriage structures, France having been,during the last twelve years, the champion of this technique. It was stated that thebelief is now current that castings should, however, not be used for aircraft of morethan 80,000 to 100,000 1b weight because the sections necessary- become excessively thickfor proper casting. Regarding the use of steel, Captain Lucienrecalled that there have been two methods of development evident in recent years, theone originating in Germany using steel cast- ings welded to steel pressings and tubes, andthe other exploited mainly in the U.S.A., making use of large stampings with or with-out the use of flash welding, but he thought that from a weight standpoint the Germanmethod could not compete with the American solution. The difficulties of retracting bulky landing-gear units become increasingly apparent with the advent of laminar-flow wings and,in addition, fuselages which are so often full of jet pipes, engine components, etc.,that there is scarcely room there either. Furthermore, the introduction of swept-backwings increases the severity of the problem, due not only to the obliquity of the spars,but also the position of the centre of gravity in relation to them. The lecturer could notsuggest a general solution to a problem with so many particular aspects, but gave illus-trations of some recent designs, including the Morane hinged-wheel system where, priarto retraction, the wheel is swung about a longitudinal axis until it lies at right anglesto the leg axis, then retracts into a vertical trough in the fuselage, the leg onlyremaining in the wing. Reference was also made to contraction ofshock-absorber travel during retrac- tion, this system being used forMeteor aircraft, although the lec- turer thought it was probably donefirst on a Caudron aircraft in 1938 Tandem Undercarriages Captain Lucien also referredbriefly to the use of tandem under- carriages in the United States and,in this connection, recalled the original Messier aircraft of 1931.which had two wheels mounted in tandem in the fuselage with out-rigger skids for slow-speed stability. This is largely akin to the modernform, except that the skids are re- placed by small wheels. Apartfrom the interest of such a layout for removing the undercarriagesfrom the wings, it has the particu- lar feature of making possible muchheavier braking than is now feasible. "There is no doubt," observedthe lecturer, " that the skid under- carriage is fcoing to come in some form andfor some operational types of aircraft, par- ticularly naval machines. In this connec-tion, the German Me 163 and AK 234 arc- interesting pointers." Referring to runway equipment, it wasstated that maximum tyre pressures of 150 lb/sq in have been the limit of experienceto date, but tyre pressures up to 300 lb/sq in have been tested experimentally in theUnited States which seemed to point to the ultimate possibility of all-metal runways. On the subject of assisted take-ofl runwaydevices, the lecturer made reference to the VVestinghouse Electropult, and to theauxiliary wheel bogie which has been used successfully in Germany in the shape of alight and detachable pair of wheels which (Continued on next page)
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