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Aviation History
1956
1956 - 1494.PDF
650 FLIGHT, 19 October 1956 Better Braking for Miles Geminis Palmer Disc Installation : "Flight" Staff Aircraft Converted The neat, unobtrusive installation as it appeati on a Gemini mainwheel. ADISADVANTAGE of the otherwise entirely admirableversion of the Miles Gemini with Gipsy Major enginesis that the small Bendix brakes, already rather overtaxed by the weight of the standard aircraft, arc less than adequate forstopping or running-up the heavier Gipsy-Gemini, even on dry tarmac—a deficiency that the higher residual thrust at tick-overspeed of the more powerful engines does nothing to relieve. The fact that only a comparatively small number of convertedGemini aircraft are in use was, it seemed, an obstacle to obtaining the assistance of a brake manufacturer to remedy this situation—until a chance conversation between Air Couriers of Croydon and a Palmer tyre engineer resulted in the problem being brought tothe notice of The Palmer Tyre, Ltd. They were immediately sympathetic and, after a preliminary investigation, enthusiasticthat a solution could be found. Inevitably, whatever design of disc brake—the obvious choice—was adopted, a low-pressureclosed-circuit hydraulic system would be required. Palmer had previous experience of this type from a Provost installation. In practice, a considerable amount of modification was requiredto the various components that Palmer had available to suit the Gemini installation, but this was tackled with the determinationto make good the previous deficiency. The mainwheel is an adaptation of a type designed for use on a helicopter, and may befitted directly on to die Gemini stub axle. The only undercarriage modification required is the replacement of the taper pin whichtransmits the torque from the brake disc into the axle by another pin of a larger size—testimony of the very greatly increased brak-ing power available. An annular-piston disc brake—of exceptionally neat design—has been adopted to make the best use of die fluid pressure that is generated in the master cylinders, and a special anti-fade high-working-temperature lining has been developed and supplied by Ferodo. The annular-disc piston—contained within a housing inthe forged light-alloy main frame of the brake—bears against a pressure plate registered in slots in the brake carrier which formsthe inner part of the assembly and is bolted to the main frame. The brake disc is driven by slots in the main wheel, within whichit is located. The wheel and brake assembly carry six-ply, heavy- tread 6.00 X 6^in tyres of Palmer design. Installation of the new braking system was undertaken in theCroydon workshops of Air Couriers, who successfully modified (from schematic drawings supplied by Palmer) Mr. Ken Mac-Alpine's Gipsy-Gemini before undertaking conversion of the Flight aircraft—a task that was not an entirely simple repeat opera-tion, as some additional modification had to be made to clear die starboard (dual) rudder bar widi which our Gemini is fitted. The brake pedals, which are of the toe-operated type, are Palmer adaptations of die Provost pedal and are connected to the twin hydraulic master cylinders via short push-rods. A vented reser- voir is mounted on die nose bulkhead, where it is accessible for filling and priming the system through the luggage compartment in the extreme nose (an operation that may be performed from the head provided by a gallon can held over die nose of the fuselage). The remaining components of die system are a pair of thermal pressure-compensator units—mounted on die wing spar and accessible through the detachable inner leading edge—and a park- Schematic diagram of the installation. BLEED SCREW ing valve and pressure indicator in the cockpit. Hydraulic linesbetween each unit are of hard Tungum pipe and Palmer Silvoflex high-pressure flexible hose. The parking valve, which is essen-tially a double spring-loaded non-return valve, is tripped by a lever on the valve block; repeated toe pressure on the pedals dien buildsup and sustains hydraulic pressure on the brake pistons. This pressure is reflected in die pressure-compensating units, which aredisc-spring-loaded to crack at 130 lb/sq in and thereafter create a "reservoir" for excess oil. Variations in selected brake pressureresulting from temperature changes are absorbed in this variable capacity. The ends of die operating piston are attached to alength of piano wire which runs in a conduit and terminates at die cockpit indicator—a Perspex block with coloured divisions toindicate die system pressure (die fluid is hydraulic oil D.T.D.585). The indicator (an Air Couriers adaptation of die Palmer pressure-indication method) provides die pilot witii a simple and effective check upon die brake effectiveness, since, widi the parking leverapplied, the pressure generated in each brake can be read off against die indicator scale. In operation, die brakes are effective and reasonably progressive.Differential operation (which may, of course, be assisted with the throtdes) is independent of rudder-bar position and is appliedwith one pedal only or by a greater load on one pedal than the other. The design of the brake ensures that no snatch occurs whenmaximum application of one brake is made while taxying at a PIPELINE FROM MASTER CYLINDER Brake details in the cockpit of the "Flight" Gemini. The installation is confined to the port pedals. brisk walking pace; but heavy braking requires high pedal-loads,which tend to discourage rough handling. Manoeuvring die Gemini on the ground has become the simple and straightforwardoperation that the private pilot welcomes. For running up, die parking lever is set and die pedals pumpedtwice until die indicator rods reach die division below die junction of die blue and red segments of die scale. This mark correspondsto a pressure of about 150 lb/sq in applied to the brake pistons and is sufficient to hold the aircraft at full throttle on one engineor bodi. More pressure may be obtained by some hard pumping on the pedals, but die increasing foot loads and the five- or ten-second delay that occurs while the rather sluggish master cylinders recover will in practice generally limit die pressure applied to alitde more man the minimum necessary for run-up. This pres- sure, in any case, has been selected as a mean, about which diurnaltemperature variations will not reduce the effectiveness of die brakes when the aircraft is parked. Providing the Gemini with brakes has been the result not ofa shopping expedition but an unusual and heart-warming degree of co-operation and enthusiasm by a leading manufacturer andsome keen installation engineers. Brochure preambles and advertising slogans can only infrequentiy be relied upon to givea realistic appraisal of a manufacturer's attitude to a project where commercial gain is unlikely to be guaranteed; but the first twosatisfied customers are inclined to subscribe to die declared aim that "Palmers are interested in any problem whose solution mayadvance aviation, and gladly offer die services of dieir develop- ment organization for work on it."
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