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Pusher Prop dirt clearance

Old 01-04-2006, 03:56 PM
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KidEpoxy
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Default Pusher Prop dirt clearance

What is a decent pitch angle minimum when looking at pusher prop clearing the dirt in Rotation & landing flare? I'm looking at some skid-fins (belly dorsalfin) to limit the aft going down, but was wondering how much clearance I needed for a 049~074 pusher.
Old 01-07-2006, 12:20 PM
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allanflowers
 
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Default RE: Pusher Prop dirt clearance

I'm no expert in this but, since no one else has answered, I will give you my thoughts.
I would design for a minimum of 10 degrees with a small clearance (.25") added. If you run into problems you could go to a three blade prop with a smaller diameter.
I do know that if you move the landing gear rearward to help the prop clearance issues, you may experience problems getting the plane to rotate on takeoff.
Good luck and keep us informed of how it works out.
Allan
Old 01-07-2006, 11:51 PM
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KidEpoxy
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Default RE: Pusher Prop dirt clearance

Thanx, already was looking at using 3 blade to reduce the problem, but was thinking of modeling something military jet like, and it don't have a lot of ground clearance in the landing gear.
Old 01-07-2006, 11:55 PM
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Tall Paul
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Default RE: Pusher Prop dirt clearance

If you put something at the rear to keep the prop away from the ground when the tail thingy whatever you come up with is on the ground, you should be OK.
Old 01-08-2006, 12:01 PM
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Default RE: Pusher Prop dirt clearance

Here's my 'military jet like' foamy design with pusher prop. I have the lasercut parts, foam for skins, AXI motor and pattern printouts, just waiting for the time to do it. It has a 10 percent angle. Done in Alias/Wavefront.
Allan
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Old 01-08-2006, 12:41 PM
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Default RE: Pusher Prop dirt clearance

KId, have you considered skipping the landing gear? I've experimented with .049 models over the years and pretty much concluded that landing gear on this size model just adds complexity, weight, and drag. Your jet style model will look sleeker in the air, too. Hand launching a pusher will be "interesting". Remember to follow through
Old 01-08-2006, 01:10 PM
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Tall Paul
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Default RE: Pusher Prop dirt clearance

I'd leave the gear off also.
Retracts on that small a plane add way too much weight.
Old 01-08-2006, 02:19 PM
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Default RE: Pusher Prop dirt clearance

I think some folks are confusing the pix of my design, shown a few posts back, with Kid Epoxy's plane. He has not said anything about having retracts on his .049-.074 powered model.
Mine is probably somewhat bigger (46"wing span) and, although my CAD images show retracts, it will have conventional LG.
Sorry about confusing things.
Allan
Old 01-08-2006, 03:08 PM
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KidEpoxy
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Default RE: Pusher Prop dirt clearance

The probable 911 call after the toss was the reason I started the whole gear design stage

I was looking at the F14, a nice set of lower dorsal/pectoral fins under it at the back, easy enough to have them be the limit to rotation. All I needed was a number of degrees to be reasonable, and y'all been helpful- thanx. Yup, no retracts for me, maybe on a 074 twin, but not this model. I suffered a Landlord Induced total loss of rc stuff, so gettin back in I'm gonna start with a $7 cox model and start collecting stuff again.

If I have to slide the mains back to get a decent balance & rotation angle, what taxi problems if any would result? Cause I moved some mains on a taildragger forward, and it was a pinwheel on the ground.
Old 01-08-2006, 04:52 PM
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Default RE: Pusher Prop dirt clearance

If you move the main gear backward, the plane may not want to rotate on takeoff, especially with power on. Then you decide you are running out of runway and try to abort the takeoff by throttling back - at which point the plane suddenly takes off because of the lack of thrust "pinning" it to the runway. I went through this on a canard, finally solved the problem by changing the thrust angle of the pusher prop to cause the aircraft to pitch UP under power. That may have just been a fluke with my particular canard design, but it is something to watch for.
Allan
Old 01-08-2006, 05:21 PM
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KidEpoxy
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Default RE: Pusher Prop dirt clearance

I'm drawing it with the propnut about 1" below the wingline, that should give slight up pitch when powered, right?
And I was considering rolling balance when moving the gear back... under wing mains look like they need to be further back on the wing... wont know untill I start balancing it out with the servos up front... Once I get all the equipment on a cardboard layout I'll know more... and I'll start a build thread here or 1/2a with tons of pics- and questions
Old 01-08-2006, 06:57 PM
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kdheath
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Default RE: Pusher Prop dirt clearance

LG placement is related to the location of the center of mass. Not CG or balance point, center of mass. On a trike, if the mains are too far aft, the tail won't have enough leverage, if you will, to rotate the plane. On the taildraggers, if the gear is too far forward, the concentration of mass is way back and causes ground loops because the model rotates like a barbell.

BTW, most airfoils stall around 17 degrees angle of attack. Seems like, unless you burn the landings on, 10 degrees isn't enough to slow the thing down enough for landing.
Old 01-08-2006, 08:30 PM
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Default RE: Pusher Prop dirt clearance

10 degrees may not be enough but to design for much more is very difficult. I don't think one aspires to landing or taking off a F14 like a WW-I bipe though.
By the way, center of mass and center of gravity are the same thing.
Allan
Old 01-08-2006, 09:57 PM
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Default RE: Pusher Prop dirt clearance

With the rather small wings for the size of the ship, even if I enhance the span & taper, I know I'm looking at a ground screaming takeoff & land. Add to that a thin symetric or plank foil... go fast or fall. I'm toying with an idea for a servo for flaps / dragchute / drop anchor

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