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Aviation History
1957
1957 - 1717.PDF
FLIGHT, 22 November 1957 807 GNAT M fc I deflection angles generally used at high subsonic speeds withlittle increase in force at the larger deflections used at speeds above Mach 1 and on the approach. The Hobson unit can take a maxi-mum load of 750 1b, and can achieve a rate of 1.5 to 2 in/sec with a 500 1b end-load. The electric trim motor, at 24 V, can exert amaximum of 400 lb per jack. In case of hydraulic supply failure, a warning light indicatesloss of pressure. Though loss of tailplane response follows, the hydraulic accumulator will still provide sufficient pressure for a12 deg movement of the tailplane, this being sufficient to restore the tailplane to a suitable angle for effective manual control. Inthe meantime, the electric trim-motor remains available to change tailplane angle at one-tenth of the speed provided by the hydraulicmotor. To continue in manual control the pilot can pull a lever on the left side of the cockpit in order to disengage the elevatorlocks through a Teleflex linkage. This also locks the telescopic reds to close the mechanical control-loop. From this momentthe pilot has direct mechanical control of the elevators with normal aerodynamic feel. Stick loads are then fairly heavy. Compared with those of other aircraft in the same opera-tional category, the hydraulic system is delightfully simple. As already described, there are only six hydraulic jacks to be oper-ated; and three of these are controlled by p single selector-valve. Four of the jacks are close to the engine-driven pump and manyof the system components. The whole system holds nine pints of D.T.D. 585 fluid, some 159 cu in of which are in the headertank when the single pressure-accumulator is uncharged. In this condition, the accumulator is pressurized with air at 1,500 Ib/sqin; and this air can be used for emergency undercarriage lower- ing. Accumulator pressure is also sensed on the air side and dis-played on a gauge in the cockpit reading from 0 to 3,000 lb/sq in. The accumulator, made by Lockheed, holds air in an annulus sothat the piston travels the full length of the body, the overall size of the unit thus being kept to a minimum. Mounted onthe lower port side of the forward engine bay, the accumulator holds a maximum of 40 cu in of fluid. Hydraulic pressure is built up by a standard Lockheed Mk 9engine-driven pump. Operating pressure is 3,000 lb/sq in and maximum delivery is 900 cu in/min. A relief valve opens at3,500 lb/sq in; and a cut-out valve returns pump delivery to the header tank when the accumulator reaches 3,000 lb/sq in. Itcuts in again at 2,500 lb/sq in. Main delivery pressure is branched three ways, to the under-carriage, ailerons and Hobson unit, but a pressure-maintaining valve is interposed upstream of the aileron and undercarriagesupplies so that, if line pressure drops below 2,000 lb/sq in, these two circuits are cut out and manual reversion and emergencyextension respectively have to be used. In this way the remainder of a failing pressure supply is reserved for the tail control exclu-sively. A pressure switch downstream of the maintaining valve lights a warning lamp in the cockpit when pressure drops below1,600 lb/sq in, which occurs as soon as the maintaining valve closes. Normal working is re-established if the pressure onceagain rises to 2,400 lb/sq in. The undercarriage is controlled by a single selector valve,located under the tank floor just forward of the engine bay and operated by cable from a single cockpit-lever marked "up", "air-brake", "down" and "emergency". In the up and down positions fluid flow is passed directly to the appropriate side of the under-carriage jacks. In the airbrake position the flow is passed from the jacks through sequence valves operated by cams en the main-wheel legs and nosewheel jack so thar flow is cut off when the air- brake position is reached. From the up position, the nosewheelreaches the airbrake setting in one second while the mainwheels take 0.4 sec. The full extension cycle takes about nine seconds. The Gnat carries a complete air-conditioning system, includ-ing pressurization with temperature-controlled air, separate hot air demisting for windscreen and canopy, provision for g-suit andventilated suit and radar cooling. Charge air is taken from two engine bleeds just aft of the Orpheus compressor section. At fullpower at sea level, bleed temperature and pressure are about 450 deg C and 94 lb/sq in; and a cold air unit and ram-air heatexchanger can reduce this to about 0 deg C. Pressurization is, of course, controlled by throttling the outflow of conditioning airfrom the sealed cockpit. The cabin remains unpressurized from sea level to 15,000ft, after which the pressure builds up to adifferential of 3j lb/sq in at 40,000ft and above. Separate warn- ing devices come into operation if cabin pressure drops more than0.5 lb/sq in below the required level and if the dangerous cabin altitude of 42,000ft is reached. Of the two compressor-bleeds which serve the cabin, one isled direct, through appropriate reducing and non-return valves, to the windscreen demisting pipe, the flow being controlledmanually by an on/off valve. A branch from this also supplies the Dunlop g-suit valve; and a further branch is taken througha differential-pressure control valve to the canopy seal, keeping this at 3 lb/sq in above cabin pressure whenever the canopy isshut. Either normal or emergency canopy unlocking devices operate a sequence valve to deflate the seaL The other compressor bleed is led straight to a Normalairdouble-butterfly valve controlled by Teleflex cable from the cockpit. Through this unit all the hot charge-air can be directedstraight into the spray ring round the base of the canopy and into a spray pipe over the pilot's feet. Alternatively all the chargeair can be directed through the Marston ram-air heat exchanger mounted in the intake which passes ram air into the cooling shroudround the hot part of the Orpheus. At full speed, this exchanger reduces charge-air temperature from 240 deg C to 168 deg C.The air then flows into a small Normalair brake turbine cold-air unit mounted just aft of the heat exchanger. It is a 12.4 Ib/minunit and can reduce charge air temperature to —14.7 deg C. In-between temperatures can be obtained by setting the double-butterfly valve to appropriate intermediate positions. A special tropical cold-air unit may be fitted. The air space between the thin outer and thick inner layersof the Triplex glass windscreen is maintained at ambient pressure by a special bleed to atmosphere fitted with a silica-gel desiccantcartridge. A muff round the pressurization valve on the front cabin bulkhead collects dumped cool air and passes it to theradar boxes in the nose bay at slightly above ambient pressure. The Gnat carries no fewer than seven internal bag tanks forfuel within the fuselage giving a total capacity of 174 gal. They are numbered 1 to 6, an additional 20-gal unit, called 1A, havingbeen added in the bottom of the equipment bay behind the cock- pit. Their layout is shown on page 805. Nos. 1, 2, 3, 4, 5 and 6tanks hold respectively 54, 39, 18, 14, 14i and 14i gal. With the two 66-gal wing tanks total fuel capacity is 306 gal. No. 2 tankis the master unit and holds the main refuelling vent with its float valve and the S.P.E. booster pump with its negative-g provisions.From here fuel is led via the low-pressure cock to the engine. The two tanks forward of this and the four tanks aft of it formtwo fuel blocks, total contents being measured by Smiths Way- mouth capacitance units in Nos. 1, 2 and 6 tanks and totalized ona single gauge, reading in pounds, in the cockpit. Tank pressurization and fuel transfer are by engine-bled air(from a third bleed) which is passed through a Hymatic pressure- reducing and vent valve direct to the wing tank pylons. Here it iseither led into the wing tanks or straight back through the wing- tank feed lines to the fuselage tanks, according to whether droptanks are in place or not. The starboard wing tank feeds into No. 1 tank while the port fuel is passed to No. 3 tank thus pre-serving the e.g. Special Exactor couplings at the wing pylons ensure that air pressure is passed directly back to the fuselage Horizontal tail control, showing feel spring and manual reversion layout I, Hobson power unit; 2, rod to actuator valve operated by special cam on quadrant; 3, double, pre-loaded feel-springs; 4, telescoping push-rod; 5, manual- reversion control cable; 6, elevator-locking bolt; 7, elevator push-rod; 8, twin- pawl lock tor 4.
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