Early Work
In 1920 W.J. Stern of the Air Ministry Laboratory in South Kensington wrote a report in response to an Aeronautical Research Committee (ARC) request about the possibilities of developing a gas turbine engine to drive a propeller. His report was extremely negative. Given the performance of existing turbocompressors, such an engine appeared to be mechanically inefficient. In addition to high weight and poor fuel efficiency, Stern was skeptical that there were materials available that would be suitable for use in the high-heat areas of the turbine.
Griffith, who was at this point the senior scientific officer at the RAE at Farnborough, read Stern's report and responded with a request that the National Physical Laboratory should study the materials problem. Griffiths, meanwhile, started studying the issues with compressor design. In 1926 he published An Aerodynamic Theory of Turbine Design, which noted that existing compressor designs used flat blades that were essentially "flying stalled" and that efficiency could be dramatically improved by shaping them aerodynamically.
In October, Griffith presented the paper to a small group from the Air Ministry and the RAE. They unanimously supported starting a development project to study Griffiths' compressor designs. Initial work started in 1927, and by 1929 this project had progressed to the point of building an extremely simple 4-inch-diameter (100 mm) "engine" consisting of a single-stage compressor and turbine with a single row of stators in front of each. Designed solely to test the basic concept, the rig nevertheless demonstrated superb aerodynamic efficiencies as high as 91%.
At the same the RAE team introduced the "cascade", consisting of multiple rows of compressor blades attached to flat plates. Unconvinced that the aerodynamics of a single blade in a wind tunnel would match the real world performance of a multi-stage compressor, the cascade allowed various compressor layouts to be tested simply by moving the plates on a mounting plate inside the wind tunnel. This also allowed the angle of attack to be easily varied by rotating the plates with respect to the airflow. According to NASA, one of the reasons UK engine design remained ahead of the US into the 1950s was that the cascade tests and theory were widely used in the UK, while generally ignored in the US.
During this period Griffith was promoted to principal scientific officer at the Air Ministry's South Kensington Laboratory. Here he returned to theoretical work and published a report in November 1929 that outlined the design and theoretical performance of a 500 hp turbine engine driving a propeller. Contrary to Stern's earlier report, Griffith demonstrated that if the existing testbed design could be scaled up successfully, it would have performance far superior to existing piston engines.
The engine outlined in the report was quite complex, consisting primarily of a fourteen-stage "contra-flow" gas generator. Each stage combined a compressor disk, on the inside, with a turbine, on the outside, and were free to rotate relative to each other on a central shaft. Air would enter at the front of the engine in the center where the compressor half was located, passing through the contra-rotating stages until it reached the other end of the engine where it was ignited. The airflow then reversed direction at the back of engine and flowed back through the outer half of the disks, powering the turbines, eventually flowing out of a front-mounted exhaust. Each turbine stage powered only its associated compressor, and as they were free to rotate on the central shaft, no mechanical power was produced. Power would have to be extracted externally through a free turbine.
In April 1930 Griffith proposed building a testbed version of his design, but the ARC concluded that it was simply too far beyond the current state of the art. In 1931 Griffith returned to the RAE. At some point during this period he was given Frank Whittle's engine design using centrifugal compressors and returned a negative response; after pointing out minor errors in the calculations he stated that the centrifugal design was inefficient and its large frontal size would make it unsuitable for aircraft use. He also stated that Whittle's idea of using the hot exhaust directly for thrust was inefficient and would not match the performance of existing engines, in spite of Whittle concentrating on high-speed use where it would be more effective (propellers suffer a dramatic drop in efficiency below the speed of sound (M.1)).
Read more about this topic: Turbojet Development At The RAE
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