889,024. Vertical take-off aircraft power plants. BRISTOL AIRCRAFT Ltd. Sept. 25, 1958 [Sept. 25, 1957], No. 30125/57. Class 4. [Also in Group XXVI] A vertical take-off and landing aircraft has power plant comprising engines such that with any one engine failed, the remainder have sufficient power to sustain the aircraft in hovering flight, the engines being connected to a common thrust producing system unable to endure the maximum power that all the engines can develop, and comprising a control system including means preventing the total power delivered by the engines from exceeding the maximum that the thrust producing system can endure. The engines may be gas turbines, driving ducted fans or sustaining rotors. In the helicopter system shown, two gas turbines 1a, 1b, drive two rotors 2a, 2b. The two engine systems are similar and in general one only, having reference suffix a will be described, together with common parts having no reference suffix. The engine 1a drives bevel gearing 7a through epicyclic gearing 3a and a one-way clutch 4a, and the rotor 2a is driven by the bevel gearing through further gearing 5a. The two bevel gears 7a, 7b, are connected by a shaft 6, so that either engine may drive both rotors. The engine power output is controlled and limited through the fuel supply fed through ducts 12, 13a, controller 10a with control lever 11a, and duct 14a to fuel sprays 9a. The controller 10a involves fuel pumping and metering devices, and has pressure relief valves 19a, 20a, 21a in by-pass ducts 16a, 17a, 18a. Duct 16a is permanently open, and valve 19a opens at the highest pressure difference corresponding to maximum engine power. Valve 21a opens at a lower pressure difference corrsponding to half of the power normally needed in flight and valve 20a opens at a still lower pressure difference corresponding to idling of the engine. Ducts 17a, 18a are closable by valves 22a, 23a and a further valve 24a can shut off the fuel supply altogether. vavles 22a, 23a and 24a are controlled by a cam 25a, the position of which thus determines the limit set to the engine power output, viz. the maximum engine output, half that needed for flight, or idling, or shuts down the engine. Cam 25a is positioned by a link 26a connected to a lever 27a pivoted at 29, having a handle 30a and working in a quadrant 31 with detents 32, 33. The handles 30a, 30b are normally connected by a clutch, to move together, and in the neutral position of the composite lever 28, cams 25a, 25b are in similar positions allowing each engine to develop half the power needed for flight. Movement of lever 28 to one or other detent 32, 33 sets the idling limit for one engine, and the maximum limit for the other. By exerting a higher force, the lever can be moved past the detent, the corresponding cam then shutting down the corresponding engine. The handles 30a, 30b can be disconnected to allow both engines to be set to the idling limit or be shut down, but levers 27a, 27b cannot be moved apart in the sense to allow each engine to develop full power. The pilot's collective pitch and throttle control lever 34 is connected by linkage 46, 47 to collective pitch controls 8a, 8b on the rotors, and to a floating lever 36 connected by link 37 to one end of an arcuate slide 38 rockable on a pivot 39. Link 40a has one end sliding in slide 38, is connected to the fuel control lever 11a by linkage 42a, 43a, and is connected by rod 44a to a lever 45 connected to lever 28. Movement of the collective pitch and throttle control lever 34 rocks the slide 38, and increases rode creases the fuel supply to both engines. Normally the fuel supply is equal for both engines, but if lever 28 is moved to decrease the power limit of one engine and increase that of the other, lever 45 moves the ends 41a, 41b of rods 40a, 40b to be asymmetric about pivot 39, so that the engine allowed to produce more power has a bigger fuel supply by virtue of the alterations in the velocity ratios of the linkages 37, 38, 40a, and 37, 38, 40b. The connections between the collective pitch lever 34 and the fuel control levers 11a, 11b makes approximate adjustments to the total power output to maintain constant rotor speed irrespective of pitch setting, and a correction is made by a governor 49 driven by shaft 6 and connected to lever 36 by link 48. In addition to manual idling or shut down of an engine, automatic idling devices are provided. The reaction member of the epicyclic gear 3a is connected to a piston 55a in a cylinder 50a supplied with fluid by a pump 52a and reservoir 54a through line 51a. The cylinder 50a also has a return line 57a. In known manner, the pressure in cylinder 50a represents the torque supplied by engine 1a. The pressures in cylinders 50a, 50b, are applied to a shuttle valve 60 centred by springs 67a, 67b, so that if the torque of one engine falls below that of the other by more than a predetermined amount, the valve 60 admits pressure fluid to one of jacks 65a, 65b to operate cams 25a, 25b and lever 45 to idle the failing engine and increase the power developed by the other. Faults not themselves immediately resulting in loss of power are arranged to cause loss of torque, e.g. overheating is detected by an element 71a connected to the fuel controller 10a so that overheating results in reduction of fuel, loss of torque, operation of valve 60, and idling of the overheating engine. If the fault is remedied, the idling engine can be re-powered by centralising lever 28 displaced by jack 65a or 65b. To enable this to be done, valves 70a, 70b can be turned to connect the shuttle valve and therefore jacks 65a, 65b, to reservoir pressure. This may be done automatically by ganging the valves 70a, 70b to be operated whenever lever 28 reaches detent 32 or 33. In a numerical example, the power normally needed in flight may be 1800 BHP, and that needed to sustain hovering 1600 BHP. Each engine would then have a maximum rating of 1600 BHP, but be normally limited by valve 21a or 21b to 900 BHP, which may be the maximum continuous rating, 1600 BHP being developable only for a limited time. The gearing 3a and clutch 4a would then be designed for 1600 BHP., gearing 5a for 900 BHP and gearing 7a, 7b and shaft 6 for 800 BHP. A modified form suitable for three engines is described, Fig. 2 (not shown). In this form shafts corresponding to shafts 26a 26b, one for each engine, are connected by a system of ropes and pulleys so that movement of one shaft, corresponding to increase or decrease of fuel flow rate of one engine, is automatically accompanied by opposite movement of the other shafts, to keep the total power constant. Also, the plurality of valves 19a, 20a, 21a are replaced by a single spring biased valve, the spring bias being continuously varied by a cam replacing cam 26a. The shuttle valve 60 is replaced by a valve for each engine, each valve comparing the torque of one engine with the average torque of the others, the average being determined by a hydraulic circuit. In a further modification suitable for four engines, four shafts corresponding to shafts 26a, 26b are interconnected by differential gears so that reduction or increase in the fuel flow rate of one engine is accompanied by compensating alterations in the fuel flow rates of the other three engines.