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Aviation History
1935
1935 - 0337.PDF
FEBRUARY 14. I935- FLIGHT. it* [TRACTILE UNDERCARRIAGES A German Heinkel He. jo retracting its wheels. Note the covers for the wheel wells attached to the undercarriage struc ture ; these covers fold up clear of the ground when the wheels are extended. It is well over a year ago that the first British retracting gear was fitted to a production-type aircraft. This was the Airspeed " Retractor " undercarriage in the " Courier " monoplane. Subsequently the " Envoy " and " Viceroy " were equipped with an identical under carriage. The " Retractor '' gear has been found depend able and has the advantages of being light and simple According to its designer, it adds not more than 301b. to the weight of the machine over and above that of a con ventional fixed-type undercarriage. It is in two halves, each half comprising a Vickers oleo pneumatic shock-absorbing unit working at a pressure of 3001b. /sq. in., having a gin. travel, and being hinged to the front spar, as is the bent axle. The radius-rod runs to the lower edge of the rear spar and is divided, the shorter of the links being hinged to the telescopic leg. An oil cylinder containing a piston is joined at one end to the top corner of the rear spar and at its other end to the radius rod. Oil is forced into the lower end of the cylinder, raising the piston and pulling the radius rod upwards with it. When the piston reaches the top of its stroke, the radius rod, telescopic leg and bent axle are all housed within the wing, with about three quarters of the wheel, which is a Dunlop "semi-balloon." In an emergency it would be possible to land with the wheels in the retracted position at the expense of a damaged airscrew. In fact, Sir Alan Cobham landed a " Courier " in Malta in this manner, when he had to abandon his non-stop refuelling flight to Australia. When the wheel is in the fully-extended position, the link in the radius rod is slightly past " dead centre," so that a wheel-load inclined backwards will merely serve to jam the piston harder against the end of the cylinder. An electrical indicator system tells the pilot the position ^wex"OS?d Thiswasthefirst British retractile un- fcrriage y e tW0 draw'n§s above show the method of '8 *) Another British gear which retracts longitudinally is mat on the Monospar S.T.11. of the wheels, and a high-frequency horn may also be fitted to give warning when the wheels are up and the throttle is more than half closed. The pump which forces oil into one side or the other of the piston is actuated by a long handle within easy reach of the pilot. The retractile undercarriage on the De Havilland " Comet " monoplane contributed in no small measure to the outstanding performance of that aeroplane in the England-Australia race. In this design the wheels draw up into the engine nacelles, and when in a raised position their mudguards form part of the bottom fairing, leaving an opening just large enough to let the air escape from inside the engine cowling. The medium-pressure wheels are carried on steei forks with telescopic legs, and they are raised and lowered by a worm gear. When the drums are rotated by cables from the cockpit, they draw the worm upwards, and shorten one member, which forms one side of a triangle, thereby raising the triangle and, with it, the wheel. As the worm gear is self-locking no danger arises if the machine should land with its wheels not quite fully extended. The re tracting mechanism does not prohibit the use of brakes. A Very Simple Design An extremely simple type of retracting gear has been produced by General Aircraft, Ltd., makers of the Monospar. As the sketch below shows, each wheel is carried on a fork, the fixed telescopic members of which are built up of sheet metal and are of rectangular section. The ends of the wheel axle are carried on tubes passing inside the upper members, springing being by compression rubbers. The built-up fork is hinged at its upper end, where there is a backwardly projecting pyramid from which a link tube passes to a collar which slides on a longitudinal member. The fore-and-aft position of this collar is adjusted by the pilot by means of cables passing over pulleys. When the link tube is at its rearmost position it rests
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