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CG? Thrust angle? Neither?

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Old 06-11-2003, 01:36 AM
  #1  
Scott Godbee
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Default CG? Thrust angle? Neither?

I am having trouble with a newly built trainer. It is the hanger nine solo sport, powered by a TT .46 pro.

This is really the third solo sport I have worked with. The first was my trainer that tangled with a tree and lost. But it flew great before that. The second was the replacement, which also flies great. The third is my brothers, whom I have introduced into the hobby recently. Having set the third plane up identical to the first two, it flies nothing like either of them. I consider myself of moderate experience, atleast past the trainer stage. Now on to the problem.

At full throttle the plane wants to climb like a homesick angel. Adjusting the trim with a lot of down corrects the problem for straight and level flight. Flies Great! Throttle down and it heads straight down, due ofcourse to the great amount of down trim.

I have put the cg as far forward as possible,i.e. battery and reciever as far forward as will go, and engine all the way forward. When trying a balance test it is deffinately nose heavy. Still no change.

So I tried changing the thrust angle. Increasing it until it was at a noticeably rediculous angle. Still no change.

With the cg in the same place as before, I added a brass propnut for more weight on the nose. Still no change.

With all the weight forward, and the thrust angle at a really unreasonable down angle, what else could it be?

Everything else seems normal. wing incidence and all that.

Any suggestions??

Thanks to all who reply.

Scott
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Old 06-11-2003, 01:46 AM
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DBCherry
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Default CG? Thrust angle? Neither?

When you added down thrust to the engine, did you return the CG to the proper spot? Not that it matters to your problem but I'm curious

By the way, changing CG really won't affect the problem you're describing. Aft CG may make the plane "squirrelly", or unstable, but it won't cause the plane to climb significantly when adding power.

Are you sure the wing incidence is correct? What about the horizontal stabilizer, does it have negative incidence in relation to the plane's thrust line?

I'd guess the latter if adding much downthrust to the engine didn't improve things.
Dennis-
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Old 06-11-2003, 01:55 AM
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Scott Godbee
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Default CG? Thrust angle? Neither?

Dennis, Hi.

Yes the cg has been left alone while attempting the thrust angle change. And yes, the incidence SEEMS to be correct. Giving the fact I having nothing to guage it with other than eyeballing it, and comparing it side by side to the other. But my problem with that is if the hz stab did have some negative in it, wouldnt the down trim compensate for it, even at low throttle? You can even see how much trim is set in the elevator when on the ground, lots of down.
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Old 06-11-2003, 08:07 AM
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HarryC
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Default Re: CG? Thrust angle? Neither?

Originally posted by Scott Godbee
At full throttle the plane wants to climb like a homesick angel. Throttle down and it heads straight down, due ofcourse to the great amount of down trim.

I have put the cg as far forward as possible
When trying a balance test it is deffinately nose heavy.
With the cg in the same place as before, I added a brass propnut for more weight on the nose.
Scott, it is the forward CG that is causing your "problems" The aircraft wants to balance around a particular point, but you have made it nose heavy. To counteract that, the tail plane must push down so it is installed at a particular angle or has up elevator trim. The amount of down force that the nose exerts is fixed, because it is weight. But the amount of balancing down force that the tail exerts is variable because it depends on the speed of air flowing over it. Therefore there is only one airspeed at which the tail balances the nose. Less airspeed and the tail goes up, more airspeed and the tail goes down. The speed of air over the tail is not just the plane's airspeed unless it is gliding. The tail sits in the draught from the prop, so altering throttle alters the airspeed over the tail. More throttle means more airspeed over the tail, so more down force and the nose rises.

The more nose heavy the plane is, the more downforce, hence angle of attack, the tailplane needs to balance the nose. So the more nose heavy the plane gets, the more sensitive it becomes to airspeed over the tail altering its flightpath.

This is desirable in a trainer! Put the plane into a dive, the speed increases, the tail down force increases and lifts the nose out of the dive, getting the beginner out of trouble. Yes, the irony is that a nose heavy model will raise its nose out of a dive, the more nose heavy it is the faster it will lift its nose up! Aerobatic models have the balance point further back so that hopefully the tail is producing no balancing force and then in a dive it does not promptly try to lift itself out of the dive.

You want to move the balance point back to at least where the manufacturer says to have it. When you can fly it competently at that, try moving it back an 1/8th inch at a time and feel how the plane becomes more responsive to elevator but also more neutrally stable - that is when it is put in a dive it is slower to pull up. Don't go too far back or it becomes unstable and unflyable.

H
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Old 06-11-2003, 09:58 AM
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Default CG? Thrust angle? Neither?

If you have one that flies well, and one that doesn't. Try this:

Get an incidence meter (buy or borrow). Set the CG the same as on the "good" one.

Then, using the incidence meter, set the plane so the wing is at ZERO incidence, then check the thrust angle, and the tail incidence.

Then check the same on the "bad" one, and make corrections as needed.
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Old 06-12-2003, 02:48 AM
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Scott Godbee
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Default CG? Thrust angle? Neither?

Harry, your mom must be proud. I was not expecting such a profound answer! It makes sense when you explain it like that. I had thought about moving the cg back, just because I was running out of ideas, but I was scared to, thinking that it was already tail heavy, and moving it back more would be trouble. But I will certainly try it. A little each time until its right.

Minn, thanks to you also for your reply. A guy at the LHS suggested that very same thing, and knew of someone with a meter I could borrow. I just wanted to get a little schooling on it before I tried it, so as to not make it worse.

I won't have a chance to try the corrections until friday, but I will let you know how it goes.

Thanks a million to you both!

Scott
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Old 06-12-2003, 05:22 PM
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Default CG? Thrust angle? Neither?

Scott Godbee,

You said that this plane was set up exactly like the one that flies good so from your description I would have to say it is likely to be a wing or stab incidence problem. A little too much positive wing incidence will cause the plane to gain altitude as the throttle is increased even with different CG settings within the specified range. I'm not familiar with this particular plane, does the wing attach with rubber bands? If so, be sure to stretch them front to back as this will keep tension on the leading edge of the wing so it won't have a tendency to lift from the fuse in flight when full power is applied. Also be sure to use plenty of rubber bands. If all else fails, you can try shimming the trailing edge of the wing up a little but be sure to neutralize the elevator trim before taking off.
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Old 06-12-2003, 06:19 PM
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Default CG? Thrust angle? Neither?

Check out this thread. The biggest breakthrough for me in my "trimming airplanes" experience, and it still continues to pay off on a weekly basis while trimming my planes or helping others.

http://www.rcuniverse.com/showthread...utral+position]
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Old 06-12-2003, 08:15 PM
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Default CG? Thrust angle? Neither?

Tattoo,
Now that's an interesting post.

I had a small Cub that just would NOT get off the ground. On a smooth field, I could get it going well over the speed it should have needed to fly, and still roll along 200 or more feet. Nothing.

Then I noticed both ailerons were deflected down. Not sure how I could have missed it, but a couple clevis turns on each side and the plane was off the ground in 30 feet.

I have a couple of planes I need to look at.

Thanks,
Dennis
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Old 06-13-2003, 02:59 AM
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Scott Godbee
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Default CG? Thrust angle? Neither?

That seems like an easy thing to try, and I will! Of course I will first reset the cg and thrust angle. One question though. I have a low wing that I feel really doesn't perform rolls as graceful as I had hoped. I recently read a review of a Bipe where the author noted changing the aileron diff to make the rolls more axial. What actually dose the most work. The downward aileron, or the upward? Tatoos advice leaves me with taking the surfaces a couple of degrees up. But as far as setting the differential, would you want more down on the left, or up on the right for a right roll? Being a one servo controller, I will have to adjust this via the control arm.

Interesting stuff guys!! I can't thank you all enough!!

Scott

P.S. Stick Jammer, it does indeed have rubber bands that hold the wing on. I learned that lesson the hard way about having enough bands on when flying. Performing a loop once, lacking the right amount of bands on the wing, caused the plane to make me look like a really daring pilot. About 45 degrees into the loop, it suddenly went into these continuous, loop on a dime maneuvers. Hands off the controls and it kept getting tighter and tighter. Finally when I got my heart started again, I applied a little down and it recovered. Then to a quick landing for a stiff drink. From then on, I kept a full box of rubber bands handy.
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Old 06-13-2003, 11:01 AM
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Default CG? Thrust angle? Neither?

For differential on ailerons you want slightly more up than down.
The "down" aileron has more drag than the "up".
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