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Old Jan 07, 2002, 04:02 PM
Dude, where's My Plane?
JasonJ's Avatar
Green Lane PA USA
Joined Mar 2001
2,167 Posts
How do I pick the right size prop?

Hey all, is there a genral formula for figuring out what size prop to use for a given wing area ?
I am making a direct drive speed 400 P-51 with a 20" wingspan. About 100 sq inches or so.
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Old Jan 07, 2002, 04:26 PM
LJH
Moths do not fly inverted
LJH's Avatar
Greenwich CT.
Joined Sep 2000
4,191 Posts
Jason,
I would think that with that little wing area, though you did not state a weight, but the thing should go pretty fast. Iwould think that you could start out with a CAM or APC 4.75X4.75,5X5, or a 5.5X4.5. Just a guess since those are the props that the pylon guys seem to use.

Cheers,
Jim
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Old Jan 08, 2002, 11:53 AM
Dude, where's My Plane?
JasonJ's Avatar
Green Lane PA USA
Joined Mar 2001
2,167 Posts
Ok, thanks, Ill start there. The wing span is more like 24 ". Its the P-51 I donwloaded form this site a while ago. Weight will be in the 20 oz range I think. I will use 8 NiMh 1600s with a cheap speed 400 can motor, esc, ele and ali controls. Just playing around with this blue core depron but have no idea about props. I dont want it crazy fast though, dont want to rip the wings off .
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Old Jan 08, 2002, 04:06 PM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
6,716 Posts
Umm Jason

I don't want to rain on your parade, but you might want to look at the specs on Jim Ryan's speed 400 planes before you keep going. You seem to have diverged fairly radically from his formula and his planes aren't exactly slow. In fact, there seems to be a certain similarity with what you're coming up with and a pylon racer.

Sam
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Old Jan 08, 2002, 04:24 PM
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Kodiak AK USA
Joined Jun 2000
609 Posts
If it's the Python it should have a 29" wing span. Why so small?
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Old Jan 08, 2002, 04:25 PM
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San Diego, CA, USA
Joined Mar 2001
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Jason,
Sam's right... you're building a pylon racer. For a scale job, you'll probably want at least 150sq", and you definitely want to keep the weight below 20oz. To give you an idea, the pylon ship I'm working on right now is 100sq" and will weigh about 11oz RTF. You'll never get a S400 scale bird that light, but if you're at more than 18oz I'd strip the gear and try again. 100sq" is very small for S400... it's not enough wing for sport flying... it's really only acceptable for racers.

Your prop should be a 5x5 to start with, or maybe a Master Airscrew 5.5x4. You can try a smaller prop later, but I'd start with a bigger prop to get a little more "pull" on the launch.

Sam (Megow): Have you seen the new Buckaroo DVD? I just picked it up last week... it's sweet!

-David
(Yoyodyne Propulsion Systems)
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Old Jan 08, 2002, 05:02 PM
Dude, where's My Plane?
JasonJ's Avatar
Green Lane PA USA
Joined Mar 2001
2,167 Posts
Sorry guys, I dont have the plans or dimentions here to look at so the wing area was a guese, Ill get a better calc on it.
Its not ment to be a pylon racer, but I understand its got a simalar setup and all. I just printed the plans out, cut the templates out and layed em on the foam to get a sorta scaleish look of a P-51. It dose look smallish but we shall see, no big loss if it smashes into the ground, Ill just scale it up and try again. I may make a scaled up fuse to slap on a depron wing I already have made for another jet.
Its going to be a hand or snow launch, so no gear. A set of foam skies maybe .
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