In Detail - Tailwheel unit

 

GENERAL:
The tail wheel unit consists of a main tube E (on which is mounted the retracting and locking mechanism), a pivot bracket assembly P, a pivot bracket assembly P, which houses the shack-absorber unit M and the self-centrinp mechanism, which houses the shack-absorber unit M and the self-centrinp mechanism. and a wheel fork assembly W which is hinged to the bracket by means of a pivot pin V. The unit is retracted hydraulically. The wheel runs on ball bearings on an axle located in the ends of the fork, and is fitted with a self-earthing twin-contact tyre. A ferrule bolted between two lugs on each side of the wheel fork enables a special tail wheel steering am to be used when manceuvring the aircraft on the ground without the aid of the engine. The tail wheel is retracted and lowered by a hydraulic jack attached at the front end of the retracting mechanism on the strut, and, at the rear end to the stempost.

CONSTRUCTION:
The main outer tube E is pivot-mounted in bearings at the junction of the front and rear diagonal diaphragms of the tail end of the fuselage by means of a pin passing through two lugs Jl to which are attached a pair of levers L; a spigot sleeve A is fitted over the end of the tube E and is loaded away from the tube by a spring B. A pair of lugs at the bottom of the spigot sleeve A are coupled by a T-piece G and tie-rod assembly Hl to two bell-cranks K pivoted between the outer ends of levers L. The hydraulic jack eye-bolt is bolted between the other arms of the cranks K, and the pin Fl securing the tie rod assembly to the lugs on the spigot sleeve extends beyond the lugs to engage a slot in each guide bracket J. The lower end of the tube E carries a bearing to support the pivot bracket P, and in the top of the tube a collar C and cam support tube F are secured by pins D. The top and lower self-centring cams (Rand S) and the thrust race T are mounted over the inner tube F. The lower cam S is secured by pins H to the pivot bracket and can rotate about tube F on the thrust race T. The upper cam R is prevented from rotating relative to tube F by a pin Q but can slide axially on F as pin Q passes through an elongated hole in the tube. The two cams are held in contact by a spring 0. The pivot bracket P carries a top shock-absorber pin U, and the shock-absorber is retained in position by two pins Ml, mounted in eccentric bushes Nl.

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RETRACTION:
When the tail wheel is in the DOWN position the hydraulic jack is fully compressed and the pin Fl is at the top of the slots in the guide brackets J. This allows the spigot sleeve A to be extended away from the main tube E by spring B so that the spigot engages in a fitting in the top diaphragm of the tail end structure and locks the tail wheel down. As the jack piston rod extends, the bell cranks K are pivoted about the bolt supporting them between the levers L, and the tie-rod assembly HI draws pin Fl down the slots in the guide brackets J, at the same time withdrawing spigot A from the locking fitting in the top diaphragm. As the guide plates are bolted to the rear diaphragm further extension of the jack acts on levers L causing the tail wheel unit to rotate forward and upward about the main attachment pin. Simultaneously the pin F1 follows round the inside arc of the guide plates, holding spigot A in the unlocked position.

 

LOWERING: 
As the hydraulic jack is compressed it acts on levers L, causing them to rotate the tail wheel unit downward and rearward about the main attachment pin. This operation continues until pin Fl reaches the bottom of the locking slots in the guide brackets J, when the tail wheel is in the correct DOWN position. Final compression then pivots the bell cranks K, allowing pin F1 to move up the locking slots and so permit spring B to force spigot A into the locking lug in the top diaphragm.

 

SELF-CENTRING:
Rotation of the pivot bracket in tube E caused by landing and taxying loads causes the lower cam S to rotate relative to the upper cam R. and the action of this cam forces cam R up the support tube F, so compressing spring 0. When the side load on the wheel is relieved spring 0 acts on cam R, returning cam S and consequently the pivot bracket to the neutral position, thus aligning the wheel and fork with the fore-and-aft datum of the aircraft.

 

SHOCK-ABSORBER: 
The pneumatic shock-absorber consists of an outer oil cylinder C in which slides an air cylinder assembly M. The outer cylinder incorporates a bearing at the top which engages the top attachment pin U. On one side of the bearing is a bleeder screw R and on the other a filler screw A. The air cylinder assembly M is closed at one end by a fitting K, which houses the inflation valve J and is bushed for the lower attach-ment pin in the wheel fork, while the upper end is enclosed by diaphragm D on which a recoil valve P is mounted by means of a bolt and nut B. A floating piston E, sealed by a ring 0 at one end and carrying a piston ring N at the other end, is free to slide inside the air cylinder M. The inner cylinder assembly M is retained in the outer cylinder by a bearing nut G, oil leakage being prevented by rings F and H.

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OPERATION:
Landing and taxying loads cause the wheel fork to rotate about the main pivot pin, thus compressing the shock-absorber. Since the outer cylinder C is filled with oil and the inner cylinder M inflated with air, this compression causes the oil to lift the recoil valve P and flow through holes in diaphragm D into the inner cylinder, thus driving the floating piston E down the cylinder and further compressing the air until the load has been completely absorbed. When the compression is arrested, the high air pressure forces the oil out of the air cylinder thus re-extending the shock-absorber. This return oil flow immediately closes the recoil valve P, however, so that the oil is confined to passing through the small hole in valve P. This retards the recoil of the shock-absorber unit and prevents sharp rebound of the fork and wheel.

 

TAIL WHEEL FAIRINGS DOORS:
Two tail wheel fairing doors are hinged to the lower edge of the skin of the rear monocoque. The doors are shaped to conform with the contours of the rear manocaque and, when in the closed position, completely seal the tail wheel cutaway in the latter, small bulges being incorporated to clear the tyre. A mounting bracket and cap are bolted to the tail wheel oleo-leg and adjustable connecting rods attach the doors to this bracket. The operation of opening or closing the doors is thus governed by the tail wheel; when the iatter is retracted the connecting rods first of all open the doors wider apart to clear the tyre and then close them as the oleo strut moves round its pivot, until, with the wheel fully retracted, the doors are just closed.