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All Forums >> RC Airplanes >> Aerodynamics >> Biplane piches down at full power
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Biplane piches down at full power - 8/25/2008 3:56:33 PM   
3dsky



Posts: 235
Joined: 1/1/2003
From: New Berlin, WI, USA
Status: offline 		Looking for a place to start. I have a 120 size biplane that flies true and level at ½ to full power upright and inverted (very little down needed) with a constant throttle setting. The plane pitches down (hard) when throttling up and once you correct the dive it will fly straight. What causing the down pitch at throttle up?

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RE: Biplane piches down at full power - 8/25/2008 4:08:40 PM   
Darryl Usher


 

Posts: 385
Joined: 5/11/2002
From: Cornelius, OR, USA
Status: offline 		I would look at too small stab and too much down thrust.
Darryl


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RE: Biplane piches down at full power - 8/25/2008 5:33:26 PM   
buzzard bait


 

Posts: 1026
Joined: 1/1/2003
From: Ithaca, NY, USA
Status: online 		You say, "once you correct the dive it will fly straight". Does that mean that you are correcting it just with the stick and NOT with the trim? In other words, is it only the transition from low to high throttle that it the issue?

If so, I'm not sure what it going on here, but it sounds like the CG is fairly far back. If you move it forward some, then when you re-trim it will be carrying more 'up' elevator, which might solve the problem.

Excessive downthrust, as above could be contributing. With excessive downthrust and a rear CG, a blast from the engine would tip the nose down, and the plane would continue in a downward flight path because of the rearward CG. Jim

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RE: Biplane piches down at full power - 8/25/2008 7:19:42 PM   
3dsky



Posts: 235
Joined: 1/1/2003
From: New Berlin, WI, USA
Status: offline 		The pitch down is only on throttle up and with the stick and once I compensate for the pitch down it will fly hands off level. The CG is at recommended point and the 2 wings are at the recommended incidence. The plane is not over powered. I think the CG is OK because it will fly level upright and inverted with out a trim change. Thanks for your help, I was thinking of trying to add up thrust, but wanted I second opinion.

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RE: Biplane piches down at full power - 8/25/2008 8:19:32 PM   
Rodney



Posts: 4917
Joined: 12/8/2001
From: FL
Status: offline 		What are the recommended incidence of the wings? On all my bipes (at least 11 with 6 being quarter scale) flew best when the upper wing had minus 1.5 degrees with respect to the lower wing. On most of mine the elevator and lower wing were at the same incidence and thrust line was always right about 3 degrees and downthrust varied on the planes from minus 3 to minus 5 degrees. You may find that minor changes in the incidence of the upper wing will greatly improve the performance.

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RE: Biplane piches down at full power - 8/25/2008 8:35:32 PM   
Villa



Posts: 944
Joined: 8/12/2003
From: Wilson, NC,
Status: online 		I would add a little nose weight. If then still the same, I would add a little more nose weight. I have had only 2 biplanes and am now scratch building my third.

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RE: Biplane piches down at full power - 8/26/2008 7:02:11 AM   
nitro wing


 

Posts: 2439
Joined: 9/29/2003
From: surrey, BC, CANADA
Status: offline 		Check incidence,perhaps take some down thrust out (easy to do) retest
Level flight,power on and off,no pitch should be notable if seeking precise and extreme aerobatics.

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RE: Biplane piches down at full power - 8/27/2008 4:25:22 AM   
BMatthews



Posts: 9015
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline
quote:

ORIGINAL: 3dsky

....I think the CG is OK because it will fly level upright and inverted with out a trim change.......


That may be normal for a hot aerobatic model or for the 3D flat foamie type models but it definetly is NOT normal for a regular sport model and is not at all OK for a scale model.

Not requiring any push to hold it level in the inverted means your CG is too far back and you're dab on the neutral point or VERY close to it. This is likely making your thrust offset highly critical and with a model set up for what basically amounts to 0% stability ANY downthrust is too much.

Now if this is on a 3D style hot aerobatic model then fine. If not then I'd suggest moving the CG about 3 or 4% of the wing chord ahead and retrim the elevator. I'll bet the odd behaviour on power up goes away.


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