Mini-IMP Aircraft Company
The Mini-IMP features retractable landing gear to improve performance. The nosewheel gear of the Mini-IMP is constructed of cut and drilled aluminum extrusion and tube. Aluminum plate is cut on a metal cutting bandsaw to provide bearing mounts and nylon flanged bushings are used for mount bearings. The nosewheel is equipped with a shimmy damper, which is easily constructed as part of the nosewheel fork. The nosewheel leg is a heat treated, and bent spring steel rod which bolts in to an aluminum mount block. An assist spring, not only helps retract the nosewheel, but also provides over center locking of the gear in the DOWN position. A nosewheel UP lock is provided to assure that the gear will not be displaced during aerobatic maneuvering. The nosewheel retracts forward into the fiberglass cone ahead of the nosewheel bulkhead. Retraction and extension on the prototype is mechanical through a single lever on the left side of the cockpit. A grip on the lever provides both up and down locking and unlocking, and gear retraction requires a single motion of about 12 inches travel and is almost “instantaneous”. The main gear is retracted mechanically through cables, which run to suitable chain sprockets, which rotate the main gear outward and up into wheel wells on the bottom of the wing outboard of the fuel tank. The main gear legs lay in recesses on the side of the fuselage and pass in front of the air ducts to the engine in such a way as to provide straightening vanes for the cooling air. The gear system is also equipped with both mechanical and electrical gear position indicators so that the pilot can be assured that the gear is not only where he wants it, but is also locked there.
The prototype was originally fitted with a fully manual, mechanical retraction system. However, the aircraft can be fitted with a pneumatic or hydraulic retraction system to power the main gear. With these systems the nose gear continues to be mechanically actuated, but the main gear in lifted by a power cylinder which in turn is powered by a pump. Either gear retraction system can be fitted and it is entirely a matter of personal preference as to which system is the more desirable. The nosegear can also be fitted with powered retraction if desired and several builders have done this.
The main wheels are fitted with extremely effective disc brakes, which are individually controlled by toe pedals. The system includes a parking brake arrangement, which employs a bypass valve so that inadvertent “brake lock-out” cannot be accomplished. Ground steering is easily accomplished through use of full castering nosewheel and differential brake steering. The full swivel nosewheel permits pushing the aircraft backward with out nosewheel impairment to motion to the rear as is common with most nosewheels.
The Mini-IMP is fitted with nosewheel doors which fair nicely into the nose of the aircraft while the main wheels are fitted with backing plate covers which serves to streamline the main wheels when they are retracted into the bottom of the wing. With the fully manual (unpowered) gear retract system, a portion of the main wheel backing covers is hinged and actuated by the gear retract grip so that the pilot can deflect the hinged portion of the backing plates to obtain a “flap-air-servo” effect to assist in retracting the landing gear. The nose gear is tied-in to these systems so all three wheels are easily actuated. All wheels are mechanically locked in the UP positions and main gear is mechanically LOCKED in the DOWN position. The nosegear is locked down due to an over-center positioning of the actuator. With the pneumatic or hydraulic systems, the nosegear is operated exactly like to manual system but the main gears are moved with the power cylinder and are BOTH locked in the UP and DOWN positions mechanically with the power system only doing the actual gear movement. With the power system, the gear has a “free-fall” emergency provision in the event of pump failure. The electric pumps only operate while the main gear is in motion.
Flight evaluations have shown that nosewheel doors are extremely desirable and the drawings show a suitable nosewheel door installation. The nosewheel itself has been changed in the prototype from the original 5 inch wheel (the same as the main wheels) to a 4 inch wheel to further lighten the nosewheel installation and facilitate retraction and nosewheel door operation.
The special geometry of the Mini-IMP landing gear is such that, despite the use of rather small wheels and tires, the gear moves slightly AFT under loading. This gives the main gear an effective “TRAILING ARM” characteristic, which tends to compensate for the small wheels by giving them an effectively larger rolling radius. The extreme smoothness of the prototype landing gear (which has been obtained without resort to complicated oleos, dampers, etc.) indicates that this extremely simple system is most effective. The nose wheel is equipped with an adjustable shimmy damper, which can be built in a few minutes. Extensive testing has shown that the prototype exhibits absolutely no tendency for nosewheel shimmy.
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