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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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#7
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Differences between automotive & airplane engines
In article ,
"Kyle Boatright" wrote: "Alan Baker" wrote in message ... In article .com, wrote: 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. Absolutely and utterly wrong. It is *torque* which must always be associated with the rotational speed at which it is being produced. -- Alan Baker Alan, You do understand that if you know the HP at a given rpm, you can easily calculate torque. KB Yes. But if you know torque without knowing RPM, you can't tell what performance you can get out of an engine, but if you know horsepower, you do know what performance you can get. -- Alan Baker Vancouver, British Columbia "If you raise the ceiling 4 feet, move the fireplace from that wall to that wall, you'll still only get the full stereophonic effect if you sit in the bottom of that cupboard." |
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Differences between automotive & airplane engines
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. Absolutely and utterly wrong. It is *torque* which must always be associated with the rotational speed at which it is being produced. Read that first sentence again. He's not wrong; he just didn't specify "torque" for those who don't know the relationship between it and RPM and HP. When you say "absolutely and utterly" it should be used only where it applies. Clearly, that's not here. |
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Differences between automotive & airplane engines
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#10
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Differences between automotive & airplane engines
On Fri, 10 Feb 2006 16:28:10 GMT, Alan Baker
wrote: In article .com, wrote: 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. Absolutely and utterly wrong. It is *torque* which must always be associated with the rotational speed at which it is being produced. Read that first sentence again. He's not wrong; he just didn't specify "torque" for those who don't know the relationship between it and RPM and HP. When you say "absolutely and utterly" it should be used only where it applies. Clearly, that's not here. But that's my point. He is absolutely and utterly wrong, when he says that you need to know the rotational speed before you know all you need to know when you know the horsepower. With horsepower, you can use gearing to get any rotational speed you want; the horsepower remains constant. Torque changes with gearing. Yes, you CAN use gearing, at the expense of complexity.And efficiency. Much better to design the engine to produce the power you need at the speed you need it. However, sometimes you trade efficiency and durability for weight - and a geared 1.2 liter 80 hp engine running at 6000 RPM can weigh significantly less than a direct drive 2.7 liter engine providing the same power at 2800 rpm. (well, about 40 lbs less, anyway) |
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