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Aviation History
1941
1941 - 2111.PDF
SEPTEMBER IITH, 1941. FLIGHT 157 Schurter single-piston and opposed-piston compressor aggregates. A double- acting compressor piston is directly attached to each single - acting, two - stroke engine piston. Ai air- cushioning cylinder is pro- vided below each piston. The work expended on the working stroke is returned in raising the piston on the compression stroke. . Another illustration shows two of these units installed in the wings of a high-altitude aircraft. From each of these - a regulated supply of com- pressed air is tapped from chamber, B, to charge and warm the pressure cabin. Plants of similar type are proposed by Aktiebolaget Ljungstroms Angturbin. In addition to a unit construc- tionally resembling the Milo design previously described, the one illustrated is of inter- est, as it employs a pair of twin-rotor, screw-type com- pressors, A, gear-driven from the turbine shaft. Air taken in at the forward end is delivered by the blowers through the receiving chamber, B, to combustion chamber, C, where the fuel is added, and thence to a turbine, D, with a double reversal of flow direction as before. Special consideration, how- ever, is given to means for increasing the propulsive effort with the utmost rapidity. This is of importance for the take-off of the craft and also when landing, should the pilot overshoot his point, or danger arise of collision with an obstacle. Under normal conditions control is effected by regulation of the supply of injected fuel. Should it be necessary rapidly to accelerate the speed of the craft, a sub- stantial increase of the fuel supply would need to be suddenly applied. The initial temperature of the propul- sive gases would then exceed safe values, since the com- pressor could not be speeded-up with sufficient rapidity to deliver a corresponding increase in the quantity of air. t Rapid Acceleration Accordingly, to permit rapid acceleration without ill- effects, that part of the energy available for propulsion is temporarily reduced, and thus the power input to the com- pressor is increased simultaneously. The compressor speed is thus accelerated, the quantity of air delivered increases to an excess over that lequired for the fuel supplied, and the temperature of the propulsive gases is lowered. There- after it is permissible rapidly to increase the supply of fuel to correspond with the increased quantity of air, the normal fuel-air ratio is re-established, and the aircraft is acceler- ated. These operations can be completed in considerably less time than is necessary for the normal gradual increase of the supply of fuel. Control can be effected by either decreasing the back pressure of the turbine exhaust gases or by decreasing the quantity of air admitted to the compressor. The former method is employed in the example illustrated. Normally the gases leaving the turbine are discharged through con- duit, E, of diminishing cross-sectional area and furnished with a valve. From points adjacent to the turbine outlet, two auxiliary conduits, F and G, are fitted, each provided with a valve. Extending to the exterior of the casing, the outlets are directed forward and downward respectively. *or normal operation the three valves are positioned as shown jn full line, but if the valve of either conduit, F, or ^- is opened, the back pressure will be reduced. This results in an increased heat and pressure drop in the tur- Dme which is transformed into additional mechanical and applied to the compressor. Regulation of the uit E valve and, selectively, either conduit F or con- liftin Jalve' wil1 Produce respectively a braking or a is su^r startin§ UP only a relatively small quantity of fuel PPhsd. and the major portion of the energy is absorbed by the compressor whilst the propulsive means operates under no-load conditions. Consequently, an excess of air is provided in the combustion chamber, and the fuel sup- ply may be suddenly increased to full-load quantity without ill-effect. With the plant running at approximately half- load when approaching a landing, a similar procedure will enable a sudden increase of fuel to be given to accelerate the craft in order to correct the direction of approach or to avoid an obstacle. When regulation of the air admission to the compressor is employed for control, a throttle valve is provided in the air intake. With the valve partly closed, the quantity of air delivered is reduced, and the velocity of the discharged gases is lowered. If the fuel supply remains unaltered, the amount of energy by which the propulsive effort has been lowered will be imparted to the compressor. Obviously, both methods can be employed simultaneously on the same plant to produce the desired effect with more rapidity. Swiss Reciprocating Units Turning to Switzerland, we find the system proposed by W. Schurter, of Zurich, makes exclusive use of compressor aggregates of the reciprocating type. Single-piston or opposed-piston, high-compression, two-stroke engines are employed, and each piston is directly connected with a Diagrammatic arrangement of a Schurter plant employing three single-piston compressor aggregates. The propulsive gases feed into a distributing conduit having controlled dis- charge nozzles and supplementary combustion chambers for boost. The detail sketch shows the distributing conduit arranged as a main spar for a wing and also serving to prevent ice formation on the leading edge. coaxial compressor piston. Both types are illustrated in schematic section. Dealing first with the smaller unit, the single-acting engine piston is rigidly combined with a double-acting, annular compressor piston. Groups of admission and dis- charge valves are provided at each end of the compressor cylinder. One side of the compressor delivers scavenging and charging air, at approximately twice atmospheric pressure, to the engine cylinder by way of duct, A, and the scavenge ports. From the other side air is discharged into conduit, B, where it mixes with the combustion gases as they are leaving the engine cylinder by the exhaust ports. The engine piston is of the cross-head type and by means of a stationary abutment with a gland for the piston rod, a cushioning cylinder is arranged below the piston at C. Of similar design the opposed-piston unit is virtually self-balanced, and the engine has the more satisfactory end-to-end scavenging action. The upper side of the lower compressor delivers air to chamber D, whence part is taken for scavenging and charging the engine, and the remainder is passed through the shallow annular passage, E, to tho
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