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Old Jun 14, 2012, 08:13 PM
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Central WI
Joined May 2005
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Help!
Trimming question

Test flew a Slip Stream S-Bach 342 today and have one issue I need help with.

Trimmed level at 3/4 to full power but when throttling back to land airplane starts to decend gradually but as airspeed bleeds off and aircraft slows it starts to raise its nose and climb slightly.. fearing a stall, slight forward elevator pressure resulted in an ok landing but it is slightly unnerving! Balance slightly forward of recommended point.

My guess is there may be too much down thrust and shimming the motor up a bit would help.

Am I on the right track? Thanks in advance.
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Old Jun 15, 2012, 12:00 AM
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United States, MI, Macomb
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Removed post to not confuse the issue. Misunderstood OPs post.
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Last edited by tacx; Jun 15, 2012 at 08:15 AM. Reason: incorrect info
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Old Jun 15, 2012, 01:32 AM
Grumpy old git.. Who me?
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Quote:
Originally Posted by tacx View Post
I'm assuming you balanced the plane at a point forward of the recommended point. Which means you are making the tail heavier.
If it balances at a point further forward than recommended then that's nose heavy, not tail heavy


As for trimming.. Trim for lever flight at 3/4 throttle and roll inverted. Hands off it should fly level or perhaps dive very slightly. If it climbs it's tail heavy and the CG needs to move further forward, if it dives strongly that's nose heavy (move CG back)

Another good CG test is to put it in a vertical dive with no throttle, if it carries on vertically down that's ok, a slight pull to canopy is ok too, but if it pulls hard to canopy that's nose heavy (move CG back), if it pulls to the belly that's tail heavy (move CG forward).

If the CG checks out by these methods then adjust thrust like as you suggested.
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Old Jun 15, 2012, 01:36 AM
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United States, OR, Canby
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Funny, if it is balanced forward I would think that means it is a bit nose heavy by the recommended measurement? If the recommended CG is at 100mm and it balanced there then you re-balance it at 90mm that makes the tail heavy so you add weight to the nose to balance which would make it nose heavy at the recommended position, right?

The question to the OP would be which way did you trim the elevator for level flight?
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Old Jun 15, 2012, 04:22 AM
Grumpy old git.. Who me?
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Quote:
Originally Posted by 600Bob View Post
Funny, if it is balanced forward I would think that means it is a bit nose heavy by the recommended measurement? If the recommended CG is at 100mm and it balanced there then you re-balance it at 90mm that makes the tail heavy ................ right?
Wrong.... If the instructions say 100mm (from leading edge) and the model actually balances at 90mm (from leading edge) then it's nose heavy.

Think about it.. if you add weight to the nose (making it more nose heavy) then the CG of the plane moves toward the nose, not away from it.

Steve.

Just goes to show how easy it is for people to get confused when the definition of CG and balance is poorly understood!
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Last edited by JetPlaneFlyer; Jun 15, 2012 at 04:30 AM.
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Old Jun 15, 2012, 12:32 PM
Don't worry be happy :)
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Joined Sep 2001
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Quote:
Originally Posted by Bad Dog View Post
...My guess is there may be too much down thrust and shimming the motor up a bit would help.

Am I on the right track? ...
Yes, you are right!
To much down thrust will make the model to raise its nose and climb when the throttle is cut.
An aft CG will give the same symptoms, but you wrote that your CG was slightly forward so it should not be the problem.
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Last edited by Stark; Jun 15, 2012 at 01:34 PM.
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Old Jun 15, 2012, 01:26 PM
aka: A.Roger Wilfong
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Novi, Michigan, United States
Joined Jan 2001
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I have to disagree about the nose/tail heavy assessment.

The stab is what causes the nose to rise and fall. On a nose heavy plane, the stab is generating downward force to raise the nose. As the airspeed drops, the stab is less effective and the nose falls. OTOH, on a tail heavy plane, the stab is generating a foce to lift the tail, as the airspeed drops, it generates less lift and the nose rises.

A simple way to tell if the plane is either nose or tail heavy of to do the "dive test".

- Take the plane up a coupld hundred feet and trim ti to fly straight and level at 1/2-2/3 throttle.
- As flying past yourself, push the nose down into a 30-45 degree dive - don't change the throttle.
- Let the plane accellerate (going down hill) for 50-100 feet and then let go of the stick.
- If the plane contunies in the dive, it is neutrally balanced.
- If it pulls out, it is nose heavy
- If it does a loop, it's really nose heavy
- If the dive steepens, the plane is tail heavy.

This works, because a plane in trim, flying level at a constant speed is like a balanced teeter-totter with the weights and forces cancelling each other. Assume the plne is effectivey nose heavy by 1 pound at the prop and the the distance from the prop to the center of lift is half the distance from the certer of lift to the stab. In this case, the stab has to generate a half pound of downward force. (alternately, if the pound were half way between the center of lift and the stab, the stab would have to provide a half pound of lift to keep the tail up).

But, the trim on a plane is really only effective at one speed. If the plane speeds up or slows down, the wing and stab generate different lift relative to each other.at different speeds which unbalances the teeter-totter. In the case of the nose heay plane the stab will generate more down force, unbalancing the teeter-totter and rasing the nose.

If the plane only flew at on speed, it wouldn't matter if it were nose heavy or tail heavy - once trimmed, it would fly straight and level. In reality, the reason a tail heay plane is hard to fly is that it is unstable in pitch. Any changes that affect the pitch also affect the speed and that speed change has the effect of pushing the change even further - as the nose drops, the speed increases, causing he stab to generaes more lift, causing the nose to drop more, causing the speed to increase, and so on. Just the opposite happenes with a nose heavy plane - as the nose drops, the speed increases, causing the stab to generate more downward force, causing the nose to come up, reducing the speed, ...

Try the dive test to find out the actual fore/aft balance.

BTW, this works on gliders as well. A long time ago, I had a glider that was just slightly tail heavy. In a thermal it was ok, but if I put it into a dive, it wasn't lnog before it was difficult to get it to pull out - finally it didn't.

- Roger
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Old Jun 15, 2012, 01:37 PM
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United States, OR, Canby
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Quote:
Originally Posted by JetPlaneFlyer View Post
Wrong.... If the instructions say 100mm (from leading edge) and the model actually balances at 90mm (from leading edge) then it's nose heavy.
That is what I was trying to say. If it balances at 100mm then you move your fingers to 90mm the tail will drop. THEN if you add weight to the nose to make it balance at 90mm then move your fingers back to 100mm the nose will drop.

I was replying to Tacx who has now removed his post to avoid confusion since he misread the OP.
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Last edited by 600Bob; Jun 15, 2012 at 01:45 PM.
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Old Jun 15, 2012, 05:17 PM
I fly 3-C Crash,Crunch,Crumble
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USA, NM, Clovis
Joined Feb 2010
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I has been a long time since I have seen a post so screwed up with bad information, and misunderstanding
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