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Aviation History
1955
1955 - 1482.PDF
592 FLIGHT, 7 October 1955 PRINCIPLES OF GAS TURBINE CONTROL . MAXIMUM THRUST 250- 2OO- ISO- KX> SO TURBINE TEMPERATURE LIMIT AFTERBURNERFUEL SURGE LIMIT IDLE 20 40 60 80 100 PERCENT MAXIMUM THRUST Fig. 6. Typical requirements of fuel flow for various conditions. higher value which cannot be exceeded without causing over-heating, or compressor surging (Fig. 6). Also, in many engines, there is a lower limit to the correctedfuel flow, below which the flame in the combustion zone may be blown out. The most common way of approaching the upper limit so asto obtain the fastest possible acceleration is to set up a schedule of fuel flow versus shaft speed, which may not be exceeded. Thisschedule must, of course, be corrected for both ram temperature and pressure. As the exact shape of the limit line cannot, withpresent design techniques, be predicted with sufficient certainty, the schedule must be determined during engine tests.Another solution of the problem is to schedule the limiting fuel flow only roughly, and to use a feedback control to trim down theflow to a more accurate schedule. This method can be simpli- fied by two design features of the fuel system: first, as the pumpsare driven by the engine (or by compressed air from the engine), the greatest now they can deliver increases with engine shaft speed.Next: the metering valve that varies the fuel flow with engine pressure level can be made to depend on compressor delivery— rather than entry—pressure, making the maximum available fuelflow even more strongly proportional to engine speed. For trimming the fuel flow to an accurate acceleration schedule,a signal must be fed in, which is a measure of the difference between the actual value of some engine parameter, and the valuecorresponding to surge of the compressor. Several parameters may be used: compression ratio, compressor temperature rise, ormass flow, the choice depending largely on the application. Yet another parameter which may bfi limited is the angular accelera-tion of die shaft, and this type of control is, for some engines, the most attractive of all. The chief disadvantage of all these scheduling methods is thateven when the schedule has been set up experimentally, large variations are liable to appear between individual engine builds.An ideal acceleration control would therefore sense incipient surge in the compressor, as well as over-temperature in theturbine, and control the acceleration via a closed loop. Limitations on Materials. It is today quite widely appreciatedthat the flight speeds now within reach produce very high ram air temperatures. Even at a flight Mach number of 2 in the strato-sphere, the ram air temperature exceeds the boiling point of water, and any higher temperatures approach the limiting regionwhere organic materials break down. This means that, unless extensive cooling is applied to engine accessories, no electronic orelectrical controls can be used on very high-speed aircraft. Neither can hydraulic positioning actuators, using lubricating oilsupply, be retained. Indeed, one is forced to attempt the whole control job with few materials beyond stainless steel and air. For this reason, much work is being devoted to producing all-pneumatic systems, using the engine compressor as a source of air (5 to 15 atmospheres) to actuate the nozzle, and to trimming thepneumatic circuit with mercury probes to limit engine temperature. Possible Extensions of Control Systems. The control prin-ciples described above treat the engine as a self-contained system, and still leave to the pilot the job of calculating the bestaltitude and speed for each flight, and of continually adjusting the thrust selector to obtain these. The only way in which flightconditions are taken into account is that an engine control intended for supersonic aircraft generally includes a pressure-ratio device which measures flight Mach number, and limits it to a preset safe value by over-riding either the main or theafterburner fuel flow schedule. For many aircraft duties, it would be attractive to extend thecontrol system so that it could accept commands, not of engine shaft speed, or even of thrust, but of the Components of aircraftvelocity—true air-speed vector, and rate of climb. The final step is then to store a pre-computed flight plan into a memory device,which issues commands to both the engine control and the auto- pilot in proper sequence. Such an arrangement is already used,in elementary form, in missiles. ARMY HANDMAID (continued from page 589) air only, has a rather lower optimum tip speed. Were the Faireydesigners to choose a larger rotor or a design with a lower tip speed, it would probably be possible to fly on less power, butthe present arrangement has certain singular advantages. It is obviously compact and readily transportable and blade sailinghas been found non-existent in winds up tC 25 knots. Another advantage of the small size of the rotor is that it hasmade possible considerable simplification in mechanical design. No drag-hinges are employed and the two blades form a see-sawcombination without individual flapping hinges (each blade can be removed with the aid of a ring spanner). It has also been foundpossible to employ a direct tilting-head type of control, the system adopted being carefully designed to obtain both rotor stability andthe correct feel. Small bob-weights are fitted at 90 deg to the axes of the blades to improve stability at the expense of a slightincrease in the loads on the cyclic-pitch lever. Flight trials have so far indicated that die machine is easy to fly, there being noundue loads transmitted to the pilot and the general level of vibration being low. It is unfortunate that mechanical details of the most interestingparts of the aircraft may not yet be published. It is apparent, however, that the leading portion of each blade is a drawnsteel tube which not only carries all the principal rotor loads but also forms the air duct. The remainder of die blade is a simplestructure formed from thin light-alloy sheet. At present the prototypes are flying with directional andyawing control provided by a simple steel-skinned rudder mounted behind the efflux from the Palouste. Tunnel resultssuggested that it would be advisable to employ a tailplane and additional fins and some flying has in fact been carriedout with a tailplane in place. The structure has in any case been designed to take a fixed tailplane and fixed fins if longitudinal and directional stability should require such surfaces. The basic structure could hardly be simpler. The basis of thehelicopter is a large light-alloy box containing a bag-type fuel tank. From the centre of diis box rises a curved box-sectionpylon to which is attached the rotor and from which die remainder of the helicopter is hung. To die rear is attached the simplelight-alloy box-girder boom carrying the rudder. This boom is split in die centre by a bolted transport joint and the Palouste,powerplant is slung from the forward fixed portion. The landing gear consists of a pair of simple skids formedfrom light-alloy sheet with a wood filling. These are attached to a pair of light-alloy tubes which run across the underside of thenacelle, to which they are attached rigidly on the aircraft centre- line. Vertical "springing" is provided simply by die upwarddeflection of the cross-tubes. The cockpit, while not being in any sense cramped, is probably the minimum envelope for a crewof two. Provision is fitted for dual controls, although in die A.O.P. role the observer would face aft. Comprehensive instru-mentation is provided and racking is fitted for an Army radio set, although a V.H.F. transmitter/receiver is at present employedfor prototype flying. Faireys are very proud of die fact diat die first prototype wasairborne in August of diis year, barely thirteen months after die stan of design. Much of die constructional work of die prototypeshas been done at the company's main factory at Hayes, but assembly and test-flying is being carried out at White Wakham, Berks. The pressure-jet concept was formulated by Capt. A. G. Forsyth,chief helicopter engineer; the design of the Ultra-light has been directed by Dr. G. S. Hislop, chief designer (helicopters), andMr. R. L. Lickley, Fairey chief engineer. The test flying has been in die hands of S/L. W. R. Gellady, A.F.C., senior helicoptertest pilot.
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