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

Speed Limit Explained

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Old 06-09-2002, 01:42 PM
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Txskydver
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Default Speed Limit Explained

I have seen several post either , wondering why the low 200 barrier can not be exceeded ,or reporting speeds of 250 to 300 MPH. I have also seen several trying larger & larger engines , trying to go faster. Most of this is in vain , unless you go much larger , bordering on full size , w/ horse power in or near the thousands w/ props measured in feet , turned at low RPM. Our small engines & props are designed for very high RPM's,thus we run into tip speeds in the transonic to supersonic range . A 7 inch prop turning 25000 RPM at 200 MPH has a tip speed of Mach .96, which is transonic , shock waves are beginning to form . Propeller effiecency begins to suffer tremedously the faster we go . The formation of the shock waves causes drag on the tips , thus more HP is required to turn any faster , but most important , the shock waves ( depending on prop design ) can actually reverse thrust where the prop is supersonic. Giant scale unlimited racers turning 30 props well under 10000 RPM have been a little faster , but the hit the same limit , You can hear it , tip speeds are well into Mach numbers , but the are running 20 HP or more and can power thru some of the tip drag , but can not escape the effiecency issues .

Our off the shelf model props a designed for subsonic flow at the tips therefore 220 or so is about as fast as we can go w/ available resources . Larger engines (.90) require larger props , thus is not the answer . Smaller props does not work well either , they can pospone the formation of the shock waves , but we have to take off ( static thrust suffers & we need thrust to maintain a high speed ). The only solutions are to get rid of the prop , Ducted fans ,turbines , and gliders have all exceeded the 220MPH barrier! There are a few tricks to stretch the barrier but I am keeping that under my hat .



Brooks Brinson

I can do 200 mph standing on my head!!!! SKYDIVE!!!!

above quote really pisses off NASCAR fans on my T-shirt
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Old 06-09-2002, 08:55 PM
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Rocket
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Default Speed Limit Explained

A 7 inch prop turning 25000 RPM at 200 MPH has a tip speed of Mach .96, which is transonic ,

what formula did you use to figure that mach speed. and is that pitch or diameter.
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Old 06-10-2002, 11:35 AM
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Cactus.
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Default Speed Limit Explained

easy answer is less drag on prop. from what i found out in previos post thats why a ducted fan can go faster, the shroud acts like wingletts, but if supersonic planes have the wings swept to do with the compression wave ( i forget the teci burb right now ) why cant props be the same?
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Old 06-10-2002, 07:31 PM
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Antique
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Default Speed

Pretty good for SMALL planes..Giant scale Miss Ashley, 34 hp Herbrandson twin on methanol, 239 mph on radar..23 diameter prop, 27 pitch, 9400 rpm....
Looks like you guys need bigger planes..LOL....RC...
There's a picture of the plane on my website..
www.rcignition.com
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