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Differences between automotive & airplane engines
How are "normal" airplane engines tuned to run at a lower rpm? What changes would have to be made to an automotive engine to shift the power band down accordingly?
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Differences between automotive & airplane engines
Chris Wells wrote:
How are "normal" airplane engines tuned to run at a lower rpm? What changes would have to be made to an automotive engine to shift the power band down accordingly? short answer: give it cubic inches in a ratio of 2 to 3 times its rated horsepower. |
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Differences between automotive & airplane engines
Chris Wells wrote:
How are "normal" airplane engines tuned to run at a lower rpm? What changes would have to be made to an automotive engine to shift the power band down accordingly? The most common way to move the "power band" into a useable prop rpm range is to use a gearbox or PSRU. Most of the auto conversions I've heard of utilize this approach. Dave |
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Differences between automotive & airplane engines
Chris Wells wrote: How are "normal" airplane engines tuned to run at a lower rpm? What changes would have to be made to an automotive engine to shift the power band down accordingly? ----------------------------------------------------------------------------- Dear Chris, They are not 'tuned' to run at lower rpms, they are DESIGNED to do so. Indeed, although both are engines, in purely engineering terms they have surprisingly little in common. In mechanical terms the two main differences between variable-speed engines, such as found in cars, and 'normal' aircraft engines are in the cam timing and the sizing of the bearings. But you're really looking at an entirely different engineering philosophy, in that with an aircraft engine reliability is given a higher priority than any other factor. Another major difference is the ratio between nominal and peak power. The service life of a car's engine is based on a nominal output equal to about 25% (or less) of the engine's peak output. That is the level of output the engine is expected to produce for approximately 98% of its service-life. The only time it will be asked to produce more is when accellerating or climbing a grade. By comparison, the nominal output of an aircraft engine is about 70% of its peak (or take-off) rating, dropping to about 55% if the objective is to achieve maximum time between overhauls. A major problem in the on-going contraversy about converting car engines for use in airplanes is that most Americans are not well versed in automotive engineering and the best example of that may be seen in the comments produced any time an engineer uses the term. 'Automotive,' of course, means ANYTHING that moves under its own power and 'automotive engineering' covers everything from the space shuttle to motorbikes... unless you happen to be one of the millions of superbly ill-educated Americans who use 'automotive' when they mean 'automobile.' This is far more than grammatical nit-picking, in that it is impossible to carry on a meaningful dialogue without properly defined terms. A by-product of that lack of education is how Americans view 'horsepower,' typically insisting that 50hp (at 5000rpm) is EXACTLY THE SAME as 50hp (at 1000rpm). Indeed, most will whip out their calculator and 'prove' they are identical :-) But as the Wright brothers discovered more than a hundred years ago, horsepower is not a factor in the equation of flight. With powered flight, the factor we must concern ourselves with most is thrust. Working back through the equation, for a given propeller efficiency & rpm we will eventually arrive at a given quanta of torque which then may be converted into units of 'horsepower,' should we wish to do so, although it serves no useful purpose. But when we DO use 'horsepower' we must be careful to never use it in isolation, always identifing the rotational speed at which that 'horsepower' is being produced. And along about here someone will discover the simple solution of putting a gear-train between the thing producing all that lovely 'horsepower' and the propeller producing all that necessary thrust. Indeed, the more astute will point out that the Wright brothers did exactly that, using an arrangement of bicycle chains as a a torque-multiplier :-) -R.S.Hoover |
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Differences between automotive & airplane engines
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Differences between automotive & airplane engines
Chris Wells wrote:
How are "normal" airplane engines tuned to run at a lower rpm? What changes would have to be made to an automotive engine to shift the power band down accordingly? the airplane enemy is weight. Any engine may fit, the lighter is the better. An automotive engine, with PSRU is always heavier than an airplane one. An automotive engine burn the same amount of gas than an airplane one Is automotive engines cheaper than a 2000h core of airplane engine? (with the PSRU). By -- Pub: http://www.slowfood.fr/france Philippe Vessaire Ò¿Ó¬ |
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Differences between automotive & airplane engines
Philippe Vessaire wrote: Chris Wells wrote: How are "normal" airplane engines tuned to run at a lower rpm? What changes would have to be made to an automotive engine to shift the power band down accordingly? the airplane enemy is weight. Ya got that one right.... Any engine may fit, the lighter is the better. That makes two in a row for correctness. An automotive engine, with PSRU is always heavier than an airplane one Ya wanna bet????? .. An automotive engine burn the same amount of gas than an airplane one Bull****.... Is automotive engines cheaper than a 2000h core of airplane engine? (with the PSRU). This answer doesn't make sense.... By -- Pub: http://www.slowfood.fr/france Philippe Vessaire Ò¿Ó¬ |
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Differences between automotive & airplane engines
I'll agree with the automotive engine with PSRU being heavier, but are
you sure about your other statement "the lighter the better"? I'm currently looking at an engine that is 100lbs lighter than the one recommended for my plane. Although cutting 100lbs from the total weight is a dream come true, it brings up the question of weight and balance. I can move the engine forward to make up the difference in balance, but I don't know how far or how to find out. Lou |
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Differences between automotive & airplane engines
Lou wrote: I'll agree with the automotive engine with PSRU being heavier, but are you sure about your other statement "the lighter the better"? I'm currently looking at an engine that is 100lbs lighter than the one recommended for my plane. Although cutting 100lbs from the total weight is a dream come true, it brings up the question of weight and balance. I can move the engine forward to make up the difference in balance, but I don't know how far or how to find out. Lou You weigh the airplane without the engine installed and calculate a balance point for it. Knowing the weight of the engine, you then figure the arm at which it needs to be located to bring the airplane's empty CG to the point the designer calls for it. Not a big deal at all. Pages 134 and 135 of William Kerschner's Advanced Pilot's Flight Manual shows how. Dan |
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