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What's the correct motor pitch when mounting to a firewall?

Old 08-31-2007, 05:47 AM
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phupper
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Default What's the correct motor pitch when mounting to a firewall?

I converted one of those Ultra Flyer hand-toss foam planes to electric RC. The wings are flat on the underside. It's a tail-dragger, and on take off it would pitch forward and the prop would strike the grond. I put washers under the lower motor mounting screws to counter this nose-dive, but now on take off it likes to climb steeply and stall. I'm hoping that removing the washers, and changing to a smaller prop, will solve the problem, but I'm just guessing.

Should the pitch of the motor be parallel with the flat wing bottoms?

Thanks!

Last edited by phupper; 08-31-2007 at 09:12 PM.
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Old 08-31-2007, 11:45 PM
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phupper;
Most people agree for general flight about 2-3 degrees down and the same to the right (from the cockpit) and then tune from there. I ussually put it at the same incidence as the horizontal stab. 0---0 degrees. Then on a flat bottom airfoil as yours I put the main wing at 2 degrees positive to that line. When the wing is positive to the other two then as you add throttle the plane tends to rise. Thats just me. Some people dont like the rise with throttle and set o\everthing 0----0-----0. The 2 degrees to the right counteracts the prop torque. But you will still have to give a taildragger a little rudder on takeoff for a straight run.
I converted one of those gliders to RC a few years ago used a GWS slow stick 300 geared motor. Flew it so much it just turned to mush. There is a rumor out that a new model is due made of the EPP type of foam that bounces good instead of the beer cooler foam. Havent seen any, they are blue in color I heard........

Good Luck
Bob
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Old 09-01-2007, 12:23 AM
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phupper
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Thanks, that's some good information. I didn't know about the right-tilt to counter the torque. And no wonder it wanted to climb and stall!

I'm not using a rudder as this was intended to be a cheap aileron trainer. But it takes off so quick (6 or 8 feet) it doesn't really have time to turn. However that may change once the correct downward motor pitch is applied.

Thanks again!
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Old 09-01-2007, 12:24 AM
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Originally Posted by phupper View Post
I converted one of those Ultra Flyer hand-toss foam planes to electric RC. The wings are flat on the underside. It's a tail-dragger, and on take off it would pitch forward and the prop would strike the grond. I put washers under the lower motor mounting screws to counter this nose-dive, but now on take off it likes to climb steeply and stall. I'm hoping that removing the washers, and changing to a smaller prop, will solve the problem, but I'm just guessing.

Should the pitch of the motor be parallel with the flat wing bottoms?

Thanks!
phupper, Glad to see you made one of these things.

While mine had some down thrust built in to the firewall I think it would still be manageable without the down thrust.

I have to ask though,
How does it glide with the power off?
Does it glide slightly downhill?
Or does it want to climb?

If it glides normally,(slightly downhill) maybe the problem is in the long nose and landing gear position. Not in the thrust line.

Where did you mount your gear?
On the wing? or on the fuselage?

I would mount the main gear on one of these ahead of the wing.

How about a few pictures?

Paul
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Old 09-01-2007, 12:57 AM
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I haven't actually got it to fly yet. With the upward thrust from my incorrect motor tilt, it just stalled and fell. But on those takeoffs i had to start with up elevator to keep it from leaning forward and striking the prop, so that could have contributed to the steep takeoff. The smaller prop should clear the ground now so I can get a more gradual lift. The CG is about 3/4" behind the leading edge.

I made the landing gear struts form some aluminum sheet I had, the wheels are foam. I found that old cigar boxes make excellent firewalls etc.

At this point I haven't concentrated much on actually flying. I'm still trying to make a plane that can survive more than one crash! This latest (untried) version has the fuse coated with nylon reinforced packing tape, top and bottom. The wings only survived the previous crashes because I glued them together with dowells, then ran a single strip of tape from wing tip to wing tip, top and bottom. That tape is amazing. I think once the whole thing is coated with NR packing tape, I'll be able to get more than one crash out of it...
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Old 09-01-2007, 01:14 AM
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phupper,
It looks good but I think the nose is too long for that gear arrangement.

With the motor so far out in front, you could be getting a rocking motion pivoting around the wheels.

Put the down thrust back in, I don't think that was your problem.

Maybe a larger elevator will help in holding the tail down.
Or moving the wheels further forward.

Maybe just hand launching until you get the plane trimmed might help.
Once you get the plane flying correctly, then all that is left is the gear.

Keep us informed of your progress, sorry I couldn't help more.

Paul
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Old 09-01-2007, 01:37 AM
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Ya, I think you're right. I guess i have to think about not only the CG in the air, but also the balance point on the wheels when on the ground. I'm gonna try to put the wheels more forward. A little more tail-heavy when on it's wheels, and I wouldn't need the up-elevator to keep it from nosing over frontwards. I'll post the flight results.

Thanks!
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Old 09-01-2007, 02:31 AM
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Originally Posted by phupper View Post
I haven't actually got it to fly yet. With the upward thrust from my incorrect motor tilt, it just stalled and fell. But on those takeoffs i had to start with up elevator to keep it from leaning forward and striking the prop, so that could have contributed to the steep takeoff. The smaller prop should clear the ground now so I can get a more gradual lift. The CG is about 3/4" behind the leading edge.

I made the landing gear struts form some aluminum sheet I had, the wheels are foam. I found that old cigar boxes make excellent firewalls etc.

At this point I haven't concentrated much on actually flying. I'm still trying to make a plane that can survive more than one crash! This latest (untried) version has the fuse coated with nylon reinforced packing tape, top and bottom. The wings only survived the previous crashes because I glued them together with dowells, then ran a single strip of tape from wing tip to wing tip, top and bottom. That tape is amazing. I think once the whole thing is coated with NR packing tape, I'll be able to get more than one crash out of it...

Wow thats a big prop on that plane the prop can be smaller, dont need it that big, check your wing, to see if it has 0 incedence, if you have + incedence i will want to climb and stall, normally 3 degrees of down thrust works well on the motor, and about 1 degree right thrust works well to counter the prop/torque from wanting to make the airplane go left, hope that helps, Chellie



Ps I agree with PD1 the nose is too long, and will make it want to pitch down in front,
move the landing gear forward more, that should help a lot, Better yet, a tricycle landing gear will work great
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Old 09-01-2007, 02:40 AM
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"Wow thats a big prop on that plane"

That prop was big. It was the 11x4.7 recommended for max thrust at the correct current draw with the motor. I have since replaced that with a 9x6 that draws about the same (10A at WOT). So hopefully I won't lose too much thrust.
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Old 09-01-2007, 02:41 AM
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Originally Posted by phupper View Post
Ya, I think you're right. I guess i have to think about not only the CG in the air, but also the balance point on the wheels when on the ground. I'm gonna try to put the wheels more forward. A little more tail-heavy when on it's wheels, and I wouldn't need the up-elevator to keep it from nosing over frontwards. I'll post the flight results.

Thanks!
I have not seen bell cranks on a plane in ages you must be a former/current control line pilot take care & keep up the good work, Chellie
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Old 09-01-2007, 02:48 AM
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Bell cranks? You mean that's not the way everybody does it?:<

I had built an Ace Simple Corsair that used a plastic tube and shaft method. If you bent the tube too far it would bind the shaft. This seemed like a logical work-around for a single servo setup.

Ok, so I did some control line in the past. I admit it, I did it, and I liked it. And I'd do it again if I had to!
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Old 09-01-2007, 03:30 AM
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Originally Posted by phupper View Post
Bell cranks? You mean that's not the way everybody does it?:<

I had built an Ace Simple Corsair that used a plastic tube and shaft method. If you bent the tube too far it would bind the shaft. This seemed like a logical work-around for a single servo setup.

Ok, so I did some control line in the past. I admit it, I did it, and I liked it. And I'd do it again if I had to!
Hi Phupper l love control line too thats how i got started in Model Aviation, Most people use 2 servos and a y connection on the aileron servos, as opposed to a single servo, its very helpful using two servos, when using large aileron control surfaces, either way, as long as it works good, thats all that matters, Take care, Chellie

Last edited by CHELLIE; 09-01-2007 at 11:04 PM.
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Old 09-01-2007, 04:24 AM
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phupper, Chellie is correct, that is a large prop.
I'm using an 8x6 on mine. Plenty of thrust.
Doesn't need much speed.

Chellie, I would never disagree with you, but I have more planes with bell cranks than those with 2 servos. Old school I guess.


Paul
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Old 09-01-2007, 04:36 AM
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guys, no one caught the 3/4" CG? you need that CG back further. should be about 1/4 to 1/3 of the wing chord. for your first flights the cg would be better closer to the 1/3 mark. what is the chord on the chuck glider, like 4 1/2, 5 inches?
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Old 09-01-2007, 04:40 AM
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Originally Posted by xuzme720 View Post
guys, no one caught the 3/4" CG? you need that CG back further. should be about 1/4 to 1/3 of the wing chord. for your first flights the cg would be better closer to the 1/3 mark. what is the chord on the chuck glider, like 4 1/2, 5 inches?

That's right. Mine has the CG at 2 inches from the leading edge.

Paul



I was distracted by the dancing Zoidberg.
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Old 09-01-2007, 04:44 AM
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Ok, CG is an easy fix, I need more tape on the tail section anyway
Thanks!
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Old 09-01-2007, 04:48 AM
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Originally Posted by phupper View Post
Ok, CG is an easy fix, I need more tape on the tail section anyway
Thanks!
Or shorten the nose, move battery rearward, or remove heavy landing gear.


Paul
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Old 09-01-2007, 06:21 AM
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wait! lol the thing is tail heavy! don't make it more tail heavy...


more weight on the nose will move the CG rearward...


hehe....the zoidberg is pretty funny! crazy monkey chasing character.
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Old 09-01-2007, 06:47 AM
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"more weight on the nose will move the CG rearward..."

How can that be? Isn't the CG the balance point? Wouldn't more weight in the nose move the CG froward? And I don't think it's tail-heavy. I'm confused!
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Old 09-01-2007, 06:52 AM
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Originally Posted by phupper View Post
"more weight on the nose will move the CG rearward..."

How can that be? Isn't the CG the balance point? Wouldn't more weight in the nose move the CG froward? And I don't think it's tail-heavy. I'm confused!
you just said it, balance point...you need to shift the balance more to the rear, or the CG back further, so, you need more weight on the nose...or you can lighten the tail, same effect.
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Old 09-01-2007, 06:59 AM
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Wait, I have to think about this. It now balances at 3/4" behind the leading edge. If I slide my fulcrum back to say, 2", the planes nose will fall. I would need more tail weight to balance it again. No?
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Old 09-01-2007, 07:00 AM
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Paul is right the weight needs to go back.
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Old 09-01-2007, 07:02 AM
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If you add more weight to the nose then you would have to move your fingers even further forward to get it to balnce. If you take the weight off the nose your fingers will go back to get it to balance. Do this until you are at a third of your chord.
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Old 09-01-2007, 07:06 AM
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Originally Posted by phupper View Post
Wait, I have to think about this. It now balances at 3/4" behind the leading edge. If I slide my fulcrum back to say, 2", the planes nose will fall. I would need more tail weight to balance it again. No?
Correct. But don't add weight, take it off so it will fly better .
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Old 09-01-2007, 07:06 AM
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So the answer is, to move the CG back, add weight to the tail?
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