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Nose drops when throttle is cut.

Old 06-21-2007, 06:23 PM
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xray328
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Default Nose drops when throttle is cut.

I was flying my electric MM Magpie today. All went well, but when I cut the throttle, the nose drops to about 25 degrees down. I set the CG per the instructions on a Great Planes CG machine. What would cause this? The battery is being used to adjust balance, and even moving the battery back two inches didn't help.

Thanks in advance!
Old 06-21-2007, 07:31 PM
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iron eagel
 
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Default RE: Nose drops when throttle is cut.

For a high wing that is common although that much of a change 25 degrees is a bit much. I would add a bit of down thrust, that should help tame the beast a bit.
Old 06-22-2007, 06:08 AM
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Default RE: Nose drops when throttle is cut.

Would moving the CG back slightly, meaning a very small amount, like a quarter of an inch.. help?
Old 06-22-2007, 09:04 AM
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Rodney
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Default RE: Nose drops when throttle is cut.

As iron eagle says, add some downthrust; most will require about 5 degrees and also usually need about 3 degrees right thrust.
Old 06-22-2007, 09:35 AM
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LuftwaffeOberst
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Default RE: Nose drops when throttle is cut.

Double Post.
Old 06-22-2007, 09:41 AM
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Default RE: Nose drops when throttle is cut.

ORIGINAL: CGRetired

Would moving the CG back slightly, meaning a very small amount, like a quarter of an inch.. help?


I'm with you on this one... I'd re-check your CG. When I hit my flaps on my Mustang my planes nose pitches up slightly because of lift, reduction of speed... and being slightly tail heavy. ( And I mean very slightly tail heavy) I wanted it that way so it would behave more like the real P-51.


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Old 06-22-2007, 11:19 AM
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Default RE: Nose drops when throttle is cut.

... But adding downthrust is only going to help when the propeller is turning. He said that when he cuts the throttle, it pitches down. Sounds like a stall ... like you're going too slow. The extreme dihedral wing will keep the plane straight, but it sounds like the plane is falling out of the sky. After it dives a bit, does it straighten out for a minute then dive again (porpoising) ?
Old 06-22-2007, 11:49 AM
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Default RE: Nose drops when throttle is cut.

On full size airplanes (like cessnas) it's very normal. It's called downwash and it's when the spinning draft coming from the prop hits the stabilizer on the top and keeps it leveled. When you cut the throttle, this force is removed and the nose pitches down. T-tail airplanes don't have this issue.
Old 06-22-2007, 11:52 AM
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Kaos Rulz
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Default RE: Nose drops when throttle is cut.


ORIGINAL: broke_n_bummin

... But adding downthrust is only going to help when the propeller is turning. He said that when he cuts the throttle, it pitches down. Sounds like a stall ... like you're going too slow. The extreme dihedral wing will keep the plane straight, but it sounds like the plane is falling out of the sky. After it dives a bit, does it straighten out for a minute then dive again (porpoising) ?

If there is too much up-thrust, he's having to dial in down elevator for straight flight under power. Reduce the power and the down trim causes the plane to nose downward. To correct this, you shim some down-thrust in the engine to hopefully take out the down trimmed elevator. Once the correct engine thrust is dialed in, the plane should initially continue flying straight when the power is cut. After air speed is reduced, then the plane will lose altitude.
Old 06-23-2007, 04:24 PM
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Default RE: Nose drops when throttle is cut.


ORIGINAL: ED STEFAN


ORIGINAL: broke_n_bummin

... But adding downthrust is only going to help when the propeller is turning. He said that when he cuts the throttle, it pitches down. Sounds like a stall ... like you're going too slow. The extreme dihedral wing will keep the plane straight, but it sounds like the plane is falling out of the sky. After it dives a bit, does it straighten out for a minute then dive again (porpoising) ?

If there is too much up-thrust, he's having to dial in down elevator for straight flight under power. Reduce the power and the down trim causes the plane to nose downward. To correct this, you shim some down-thrust in the engine to hopefully take out the down trimmed elevator. Once the correct engine thrust is dialed in, the plane should initially continue flying straight when the power is cut. After air speed is reduced, then the plane will lose altitude.
I agree, the pattern planes use this method to trim the model for different power settings to stay in level flight.
Shim the thrust angle down a little more and try the throttle test again.
Old 06-23-2007, 07:59 PM
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xray328
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Default RE: Nose drops when throttle is cut.

The motors angle is preset from the manufacturer. It flys fine under power, but as soon as the powers off, its just wants to nose dive. Its not porpoising though, its just nose down, and it would just nose in like that if I let it. CG is dead on.
Old 06-23-2007, 08:14 PM
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dcook07
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Default RE: Nose drops when throttle is cut.

Sounds like there is an alignment problem and you were able to trim it out under power! I'd set it up on your bench and " bench trim it!!

DDCook
Old 06-23-2007, 10:04 PM
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Default RE: Nose drops when throttle is cut.

I'm with DCook and Lufftwaffeon Oberst this one. I'd check the alignment and and CG. Did you do anything like change to a larger (i.e., heavier) engine, or something else that could have changed trim?

I'd recheck the basics first... Let us know what you find.

Bob
Old 06-23-2007, 11:14 PM
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xray328
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Default RE: Nose drops when throttle is cut.

It looks like the motors angle did change after all. The mount is in foam and after awhile the foam gave a bit. It was only a few degrees but it made a difference. Thanks guys!
Old 06-24-2007, 07:07 AM
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Default RE: Nose drops when throttle is cut.

Keep us posted on your fix...

Bob
Old 06-24-2007, 08:13 PM
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Default RE: Nose drops when throttle is cut.

A change of a few degrees will make a big change in how the airplane will fly. If your cg is right it comes down to things like AOI and thrust angle. Typically the thrust angles in models are a max of about 5 degrees, change this by a only a few and there will be a major change in how it flies.

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