Paper 2019-01 Starr Whittle Thesis.Pdf
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Journal of Aeronautical History Paper No. 2019/01 A Commentary by Dr F Starr FIMMM, MI.MechE, C Eng. on FUTURE DEVELOPMENTS IN AIRCRAFT DESIGN A Thesis by Officer Cadet Frank Whittle, Cranwell 1928 1. Introduction This paper reviews the thesis that Officer Cadet Frank Whittle wrote when he was a student at RAF Cranwell in 1928, during his fourth and final term. With this thesis, Whittle began to consider what type of engine would be best suited for high speed flight at high altitude (1, 2). This put him on the road to jet propulsion, as we now know it, and he was able to file a patent in 1930. As will be seen, although Whittle makes use of the concept of the gas turbine, at this stage he was using it to drive a high speed propeller. An invention does not suddenly appear out of thin air; rather, it almost always involves false starts. So it was with Whittle’s invention of the jet engine. The thesis illustrates his first tentative steps towards eventual success, but, just as important, it shows how Whittle was beginning to develop as a professional engineer. It culminated with him being able to build and run his first engine, the WU, in 1937. Whittle was indeed a unique individual. To paraphrase what James St Peter says of him:- Sir Frank Whittle was perhaps the last of the individual innovator-inventors, who combined the hands-on skill of a mechanic and pilot, with the acquired knowledge of the trained engineer and aerodynamicist (3). At this point it is worth addressing the claim that the German, Hans von Ohain should be given equal status with Whittle. This is largely done on basis that the first jet aircraft that flew, the Heinkel He 178, was powered by an engine based on Ohain’s approach. A more objective analysis would be that this engine, the HeS 3B was something that simply (4) demonstrated the principle of jet propulsion . It was still suffering from difficulties with the burners, a persistent problem during all through the bench testing, from the 1937 HeS 2 prototype onwards. Accordingly, start up was with hydrogen, before the burners got hot (5) enough to vaporise the liquid fuel . The configuration that Ohain used, and those that followed, had no development potential or commercial future. The underlying weakness was the use of a radial inflow turbine, which is fine for small engines of low thrust and output. But as engine size increases, a high temperature radial turbine is beset with ever increasing thermal stress and fatigue (6). By 1942 the Ohain team had switched all work onto a conventional axial flow compressor/axial flow turbine, the HeS 011, of which there was only a prototype. He was being bypassed by what was happening elsewhere in Germany. 1 Journal of Aeronautical History Paper No. 2019/01 One must compare this to Whittle’s contribution. His design, which utilised a double sided centrifugal impeller and an axial turbine, was the basis of his experimental and flight trials’ engines. This approach became the basis of the Rolls Royce Welland, Derwent, Nene and Tay. The Nene, although rather neglected in Britain, was taken up and built under licence in the USA, France, Canada and Australia. The design was pirated and improved by the Russian as (7) the VK-1. Around 50,000 were built by the Russians and their allies, the Chinese . The Nene, and its clones, served as a teaching aid to companies wishing to get into the gas turbine field. This makes Frank Whittle the real person behind the “Jet Engine Revolution”. This paper is presented in four parts. This commentary is non-technical and outlines the background to the writing of the thesis. It also summarises the content of the thesis without detailing the mathematics and thermodynamics in the document. The second part, Appendix 1, is a facsimile of the handwritten thesis itself. The third, Appendix 2, is a transcript of the thesis, including the original hand-drawn diagrams as necessary. Finally, for those wishing to explore the technical content of the thesis, Appendix 3 is an annotated version of the thesis with the mathematics and re-drawn diagrams in full. 2. The thesis and long range flight Cranwell students were required to produce a thesis each term during their course. The 1928 thesis, hand written by Cadet Whittle, needs to be considered as a piece of extended homework that had been set by an instructor on the Cranwell course, Professor Sinnatt, who had recognised his student-officer’s potential (8). For many years the exercise book in which Sir Frank wrote the thesis was on display in the Science Museum. It now resides at Wroughton, near Swindon, among the Science Museum archives. There is a copy in the National Archives at Kew, in London. The copy used here derives from a photocopy of the original, which belongs to Sir Frank’s son, Ian Whittle. This was made for Sir Frank when he bequeathed the exercise book to the Science Museum. The author is indebted to Ian for allowing the use and reproduction of his copy. Why has it taken so long to turn the contents of the exercise book into a typescript? Perhaps the most important reason is that it does not meet the expectation that it explains the concept of jet propulsion, and possibly contains a drawing of a jet engine in schematic form. Even a casual inspection of the thesis shows that it contains nothing of this nature. Furthermore, it becomes clear, early on in the thesis, that although Sir Frank’s concept is to use a gas turbine to drive a propeller, it is not a turboprop as we understand it. Only towards the end are we told that the system involves a piston engine driving a medium pressure reciprocating compressor, as well as an ‘air turbine’ as it is called. The exhaust gases from the piston engine are mixed with the air from the compressor, which are then fed to the turbine. This drives a high revving propeller, designed for an aircraft that would fly at 600 mph and 120,000 ft. 2 Journal of Aeronautical History Paper No. 2019/01 The early part of the thesis explains why, for a long range aircraft, flying at high speed and altitude is essential. Sir Frank argues that a new type of power unit will be needed, capable of high rotational speed and excellent altitude capability. The remaining parts of the thesis are essentially a basic thermodynamic assessment of the power unit. Although not difficult in themselves, the equations that Sir Frank deploys do imply the need to have a good grasp of elementary calculus. Sir Frank, unfortunately, has made life difficult for those of us who do not have his insight in moving from one equation to another. There are some pages which have been lost, which does not help, although it is possible to surmise what they might have contained. Here one needs to keep in mind the likely circumstances under which the thesis was written. Sir Frank was an Officer Cadet undergoing academic training, as well as learning to fly and become an officer (9). Furthermore, he was not writing the thesis for publication. It was from him to Professor Sinnatt. Hence a major object was to show that he understood what had been formally taught in the classroom. Accordingly, the early part contains some rather strange material on 'why does an aircraft fly’. One also gets the impression that Whittle had to write the thesis in his spare time, in which equal, if not more important, priorities would be to show that he could look after his kit, and dress and act an officer. As a result, the thesis is somewhat disjointed, reading more like a first draft than a coherent effort. Graphs and sketches are drawn in a rather hurried freehand, and it is not always clear how the figures are referred to in the text. Nevertheless, one can see why Whittle’s tutor was impressed. Whittle’s thinking about what was needed from an aircraft of the future was quite beyond what anyone was then envisaging. In that period, a high speed for a commercial transport was 150 mph. Range was around 500 miles (10, 11). And the idea of driving a propeller directly from a turbine was quite novel. Whittle’s proposed power unit was patentable and could have been built, even with the technology of the 1920s. The reason Whittle suggested flight at an altitude of 120,000 feet was that current aircraft used short grass airfields which required a low stalling speed. This in turn limited the maximum Indicated Air Speed (IAS) that was achievable. To attain a high True Air Speed (TAS) the aircraft had to fly at an altitude where the density of the air was a small fraction of that at sea level, so that the high TAS was associated with a low IAS. Whittle chose a density of 1% of sea level, to give a TAS ten times the IAS. In 1928 there were no tables of atmospheric density at great heights, and he estimated he would need a height of 120,000 feet. A modern table of atmospheric density shows that a height of about 106,000 feet is required. In addition, in 1928 variable pitch propellers were not generally available. With a fixed pitch propeller, as the TAS of the aircraft increased, the rate of rotation of the propeller would have to increase to maintain the correct incidence of the blades.