From: email@example.com Date: 26 Jan 94 04:12:53 PST References: 1 2 3 4
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>> How was this speed ratio maintained, in the GE design? >> >> -dk In both designs, the speed ratios of the props were controlled by prop pitch. I'm not familiar with the method GE used, but the PW/A design used electric beta (prop angle) actuators (one for each blade if I'm not mistaken) that rotated with the props. I'll have to assume the GE approach was similar. The considerations were not only resonance, but power, prop efficiency (something like 8-10% recovery from a counter-rotating prop versus a single row prop, if memory serves), acoustics (prop tip speed) and, in the case of the GE design, power turbine aerodynamic considerations, since the prop speeds determined the turbine blades and vanes relative speeds. For those that are interested, the PW/A engine is on display in the Allison museum at the Allison Engine Company (new name, used to be called Allison Gas Turbines, division of General Motors before being sold to a New York investment firm), in Indianapolis, Indiana. Carl Nordstrom P.S. For those that aren't familiar with a propfan, the blades are scimitar-shaped. Up until a few years ago all prop aircraft had a practical limit of around .5 mach or so (many have gone much faster, but paying a high efficiency penalty due to compressibility effects). Then Hamilton Standard (and NASA?) developed the propfan design to be able to get prop-like efficiency at turbofan cruise speeds. So an efficient .7 mach then became possible. Many aircraft have had counter-rotating props to recover the efficiency lost by a first prop row (the Russian Bear bomber is a notable example), so counter-rotating two propfan rows became a natural evolution for this technology. The first propfan to fly was the Propfan Demonstrator. It was an Allison 570 engine mated to a modified Allison T-56 gearbox, driving a single propfan row. I don't remember who did the prop, but it was probably Ham Standard. It flew on a Gulfstream aircraft that was heavily modified for the task. It seems that this was a NASA project, if I remember correctly. The propfan idea would require more development to be a viable commercial product. The program was canceled before GE and PW/A were able to fully develop their engines. Areas such as dynamics (vibrations), performance, life, materials, prop strength, acoustics, etc. were identified as needing special attention. These types of issues arise during *every* engine development program, but in this case the issues involved new technology that had never been tried before. I do feel, however, that with time and money none of the issues were particular show-stoppers.