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Aviation History
1957
1957 - 1771.PDF
8616 December 1957 The Gannet T.5 prototype, VVN.365, the author in front and Roy Morris flying from the middle seat, seen from "Flight's" Gemini. HANDLING THE GANNET T.5 Fairey*s Twin-Turboprop Operational Trainer By C. M. LAMBERT WITHOUT any doubt, the Fairey Gannet T.5 is one of themost interesting aircraft I have flown. Actual weight andperformance figures are still classified, but the machine can be flown at weights approaching that of a loaded Dakota yet is verylight to handle and without any aerodynamic peculiarities. At first acquaintance the controls of the two turboprop engines seemvery complicated but in practice engine handling is perfectly straightforward. Above all, the approach and touch-down areremarkably simple. A carrier-type landing is generally made, flying straight on to the ground without reducing power orrounding out, the touchdown being completely cushioned by the long-stroke undercarriage. But first a word about the engines, because these are the keyto Gannet flying. In accordance with Naval requirements, each engine of the Armstrong-Siddeley Double Mamba is a constant-speeding unit. In all normal flight conditions therefore the r.p.m. of each compressor is held constant and the pitch of the propellerdirectly connected to it is varied to absorb the power selected by the fuel input. But it is self-evident that at low power settingspropeller pitch will be extremely fine if compressor r.p.m. are to be maintained; and if protective devices were not installed thepropellers would form virtually a flat disc and give not only tre- mendous drag but blank off the tail surfaces and reduce controlalmost to nothing in the event of a runaway propeller. A flight fine-pitch stop is therefore included in the propeller controls toprevent them fining off below 21 deg in flight. There conies a point (at about 130 kt) where with power off, full compressorr p.m. can no longer be maintained; but even at the stall the r.p.m. drop is not serious. For carrier landing it is essential to maintain compressor r.p.m.so that full power should be immediately available in case of a missed approach. The 21 deg stop is therefore automaticallywithdrawn when the wheels are lowered to allow the propeller to fine off further and maintain constant-speeding r.p.m. Thiscan be overridden, if required, by a switch on the power console. The basic fine pitch stop is fixed at 6 deg, used for starting on thepound when propeller loads must be at a minimum. High propeller drag at low power and approach speeds is avoided by aflight-idle gate on the throttle quadrant. An acceleration control a;'ows the throttles to be slammed open from the flight-idle gate,aid full power follows in three to four seconds. The high-pressure cock levers need some explaining since theyJw a number of jobs. Each h.p. cock lever carries a button and o«s three positions: "on," "off" and "feather and brake." With the lever on, fuel supply is open and the pressing of the buttoninitiates the lighting cycle if the throttle is closed and starts the unfeathering pump. In the off position, the pressing of the buttonsimply unfeathers the appropriate propeller. In the feather and brake position, propeller feathering begins and the pressing of thebutton completes feathering and applies the propeller brake. In flight the propeller is actually held still by being taken past thefeathering angle and being held by the airstream against a ratchet in the reduction gear. On the ground, casual wind-milling isprevented by the brake which is released by moving the h.p. lever from the off to the on position. Two further systems need description. A control in the pro-peller system senses any reverse torque load, i.e., wind-milling torque tending to drive the turbine. In case of engine failure adoll's eye on the panel flickers and the appropriate propeller will automatically be coarsened by the feathering pump to a near-feathered setting—except during ground starting, of course. This avoids the disastrous eventuality of a propeller discing on take-off.In addition, an automatic j.p.t. control ensures that limiting j.p.t. for maximum power is not exceeded (e.g., in tropical conditions)by reducing fuel input to the engine. A bell and light give warning of this; and fuel restriction can be cut out by a switch.When this was explained to me at White Waltham by Fairey test pilot Roy Morris, with whom I flew, my mind was becomingas fogged as perhaps is that of the uninitiated reader, but we repaired to the cockpit and looked at the hardware related to allthese systems. The fog rapidly cleared. With over 120 different dials and gauges, the front (pupil's) cockpit of the Gannet T.5 iswell filled but not confusing. There is just enough space to hold everything without anything being out of reach. The seat isreasonably comfortable and adjustable for height, and the visibility in all directions except rearwards is remarkably good. In fact itis an excellent compromise between roominess and accessibility, and the main instrument panel is not obscured by the electricallyretractable gun sight. Since a large number of submarine sight- ings are still made visually the visibility factor is most importantand, with two forward-facing crew members and one rearward- facing, the Gannet offers complete 360 deg coverage. The almoststandard blind flying panel and the majority of engine and fuel system indicators, together with several doll's eyes and warninglights, are accommodated on the main panel. Engine instruments include miniature gouges for shaft horsepower, r.p.m., oil tem-perature and pressure, together with a dual j.p.t. gauge, reverse torque doll's eyes, excessive j.p.t lights, total/main tank gauge and
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