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Old Sep 21, 2011, 03:41 PM
DX5e fatal flaw- PM me!!!!
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Effect of prop slightly above centerline

Had a thought, what would be the effect of placing the prop and propshaft about 1/4 to 1/2 inch above the 'correct' placement on a 5lbs high-wing trainer?

All I come up with is you'd get a slight desire to nose-down, which might not be all bad, if even noticeable.

Just thinking about a slight cheat that might simplify my life a bit on my flightstar 40 build...

Dave
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Old Sep 21, 2011, 03:46 PM
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Cromer,Norfolk, UK
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You won't notice much difference, as the thrust line will still be below the wing, the model won't particularly care where the thrust line is unless its in an unusually high or low position.
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Old Sep 21, 2011, 04:29 PM
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It is the relationship between the center of drag and thrust line that effects pitch and this may or may not be on the centerline of the fuse.
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Old Sep 21, 2011, 04:37 PM
DX5e fatal flaw- PM me!!!!
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Hmmm, yes, the motor/prop would still be below the wing which would *probably* mean the center of thrust would still be below the center of drag.

I should have said 'mount the motor a bit up from the factory location' to better state what I may do.

Dave
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Old Sep 21, 2011, 07:02 PM
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Joined Jan 2008
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You could hang it from a string tied to where the center of the new motor mount will be. If it hangs vertical it should be good.
Not sure what that will tell you.
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Old Sep 22, 2011, 08:27 AM
DX5e fatal flaw- PM me!!!!
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I was thinking of doing this as a way to re-use the gas mount, which I've decided not to do. However, seeing how close to the ground a 12-inch prop comes, I'm thinking maybe this is an easy-cheesy way to gain 1/2-inch ground clearance.

Need some tundra-tires too, or maybe just one for the front.

Dave
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Old Sep 22, 2011, 08:58 AM
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United States, VA, Arlington
Joined Dec 1996
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Hi DAve
What you're trying to achieve by way of higher ground clearance is good here. The difference you'll make to how the model flies will be minimal compared to the savings in prop tips!

Aerodynamic changes - you'll get a smoother ride between power on and power off. The thrustline being closer to the wing CL will cut down on the leverage of the motor, power on, to the wing, which is the largest source of drag around.

Physical changes - more prop clearance, which you can use. If this is a glow powered model under conversion, likely it has clearance for a smaller prop than we'd use.

If you really want to solve the issue, fit a taildragger UC and get a main gear that gives you all the clearance you need. Taildraggers aren't hard to handle on the ground, they just take a little concentration and a different mindset to a trike. Big plus - you only have to buy a pair of wheels

At least we don't have to worry about fuel tank height vs needle valve level, like the oily bunch - electrons are quite happy to go uphill

Hope that helps

D
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Old Sep 22, 2011, 09:41 AM
DX5e fatal flaw- PM me!!!!
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If it weren't for the fact this is an ARF and I have no idea how to build a steered tailwheel (and I'd have to unskin part of the plane) I'd probably just make it a taildragger right now.

My UM Champ has a steered tailwheel, I like it!

My UM T28 has tricycle gear, not as much fun to land and takeoff, and the prop is a bit beat up.

How much prop clearance is 'typical'?

Dave
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