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Aviation History
1946
1946 - 1258.PDF
654 FLIGHT JUNE 27TH, 1946 MESSIER HYDRAULICS ane side of the jack piston, whilst the other valve permits the escape back to reservoir of the dis- placed fluid on the other side of the piston. For the converse selection the jack piston moves in the opposite direction since the fluid flow is reversed. In an emergency, should the hydraulic power supply for any reason fail, the undercarriage and flaps may be operated by compressed air stored in bottles, although the emergency air systems for the two services are separate entities. The undercarriage system comprises a bottle supply of air compressed at 1,450 lb/sq in, which is delivered to the jacks through separate pipe lines and, in addi- tion, actuates the jacks without imping- ing on the normal oil space. This is effected by the use of jacks of the type shown in Fig. i, in which fiie hydraulic operation is brought about by means of a third jack tube inside the piston rod, and the pressure theniore operates on the cross-sectional area of this tube. The emergency air system operates by intro- ducing the compressed air into the annular sp^ce between this rod and the cylinder, whilst the hydraulic contraction of the jack is effected by oil pressure acting on the annular underside of the piston. The system is diagrammatical!y shown in Fig. 2, and, as may be seen, is con- trolled by a small selec- tor which, in its normal position, vents the air side of the main and tail undercarriage jacks to atmosphere and seals the air in the emergency bottle. In the emergency down position the vent to atmosphere is closed, and the compressed air is admitted to 4he jacks, thus lowering the undercarriages. To allow fluid to escape from the jacks should the main hydraulic selector not be in the "down" position, a release valve is in- stalled. This consists of a piston, moved by the compressed air used for emergency actuation, which opens a valve and allows fluid in the jacks to be displaced back to reservoir, by-passing the selector. When the aircraft has landed, the emergency selector is moved back to normal, the air in the jacks escapes to atmosphere, the release - valve, being spring-loaded, re-seats itself and, assuming that the fault which led to the emergency system being used has been rectified, the system is ready for use again once the; air bottle has been reinflated; it may thus be appreciated that the bleeding normally required in similar circumstances by the usual run of hydraulic systems is, in this case, not required. As an added precaution, in the extremely rare event of Fig. 3. Layoutof main under- carriage show-ing retraction motion geo-metry. Fig. 4. Diagram of tailwheel retractionmotion geometry. both hydraulic and pneumatic failure, the main and tail under- carriages can be lowered by the hand pump. The " up" and '' down '' locks on the main undercarriage jacks, as shown in Fig. 1, are of special interest. On each side of the rod piston are radially sprung skirt with projecting cham- fered rims which, a1 the appropriate stroke extremity, spring over a butting face and are prevented from disen- gaging by a radial lock- bolt spanning the skirt bore. When a selec- tion is made and pres- sure applied to the appropriate side of the jack piston, the initial movement is made by the lock-bolt being lifted against its spring pre-loading, so freeing the spring skirts and enabling the piston rod to move. The tail undercarriage is similar, but an in- ternal mechanical lock is provided only for the '' down position, as the unit is maintained in the "up" position by hydraulic pressure. Retraction geometry for the main undercarriage is shown in Fig. 3. When the wheel is locked down, the breaker- strut radius rods are slightly over-centre,an d landing loads are transmitted through these members and not through the jacks themselves. Tailwheel geo- metry is shown in Fig. 4 ; as the jack closes, the fork swivel-housing pivots about its hinge axis, thus raising the wheel. Times for undercarriage raising and lowering are as follows: Normal up, 13 sec ; normal down, 7.5 sec ; emergency down, 14 sec. Shock absorption for both main and tail undercarriages is accommodated by straightforward oleo-pneumatic members which differ only in minor detail. Main- leg bridge-castings each accommodate two shock absorbers, one in each leg of the bridge, as shown in Fig. 5. The telescopic members of each leg proper are supported by bearing lands inside each bridge leg, the shock absorber unit being carried as a separate component co-axially within the leg so that it is subject only to end-loads, any bending loads being taken by the lower leg member relative to the bridge casting. In essence, the shock absorber consists of a telescopic Fig. 5. Detail section through typicalhalf bridge showing one leg with shock absorber co-axially housed inside tele-scopic lower sliding member.
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