Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Tune for RPM or Thrust ??

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Old 01-20-2003, 01:07 AM
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jamcg
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Default Tune for RPM or Thrust ??

To fine tume the prop selection for the fastest possible plane. Should I select for higher RPM or higher thrust number ???

James
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Old 01-20-2003, 02:15 AM
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ChuckAuger
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Default I'd have to say neither...

of the above. It's a trick question, right??
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Old 01-21-2003, 04:38 AM
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ewstap
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Default Tune for RPM or Thrust ??

It is a good question! I've been fooling around with props on an outlaw with a TT Pro36. One thing I have noticed is that when I turn a 7.25 X 9 prop the same speed I turn an 8 X 8, the plane dosn't seem to go any faster. I havn't proved this with a radar gun but I really can't see any difference in speed.....?
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Old 01-21-2003, 01:36 PM
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jchuston
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Default Tune for RPM or Thrust ??

it seems to me that at some point, most planes will reach their maximum speed (due to wind resistance and such). Maybe a small difference, but not much increase in speed...

John
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Old 01-21-2003, 05:04 PM
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banktoturn
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Default Tune for RPM or Thrust ??

jamcg,

The bottom line is that for maximum speed, you tune for thrust, but there seem to be a lot of misconceptions about this. Usually, when the word 'thrust' is used, people seem to assume that it means thrust at low speed, and implies that a large diameter, low pitch prop is used ( as in 3D ). What you need for speed is maximum thrust at the target speed. This tends to mean a high pitch prop, since that is required for thrust at high speed, and small diameter, since the engine won't have the power to turn a large diameter, high pitch prop at high speed. What you are after is operating the engine as close to its peak horsepower as you can get, with a prop that turns that power into the maximum amount of thrust at the highest speed. This is complex, so it turns into a trial & error process. All the things you do to the prop and engine are done to increase the thrust. The amount of thrust you generate determines the speed at which the airframe's drag equals the thrust. If the plane is very clean ( low drag ) this will be a high speed. If the plane has high drag, this will be a low speed.

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