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Neutral Point Help - 1/8/2013 2:34 PM   
WWIIbirds


 

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Status: offline 		I'm building a canard glider with offset wings to minimize the effect of any downwash that the canard wing may have on my main wing. I'm able to calculate the neutral point of the glider if both wings were in the same geometric plane (IE laying on the x axis). However because my wings are offset wouldnt there be a variation of the neutral point in the z axis? and If so how can I calculate this?

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RE: Neutral Point Help - 1/8/2013 2:46 PM   
da Rock



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Status: offline 		If you're figuring for pitch stability, the 2D planform view info works perfectly. The simplest formulas that only use planform measurements and ignore everything else provide far more reliable results than close to what is needed.

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RE: Neutral Point Help - 1/8/2013 2:50 PM   
da Rock



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Status: offline 		BTW, if the downwash angle is part of your present calculations, the displacement should already be taken care of. If it isn't, then it would follow that your calculations don't need further modifiers.

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RE: Neutral Point Help - 1/8/2013 3:04 PM   
WWIIbirds


 

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Status: offline 		I assume the effect of downwash to be negligible due to my offset wings so I'm not including it my calculation. My concern is that to calculate neutral point it's all about calculating moments, with in line wings I have 2 moments created by the lift of both wings but with offset wings not only do I have lifting moments but I also have moments that are created by the drag of each wing.

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RE: Neutral Point Help - 1/8/2013 9:49 PM   
da Rock



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Status: offline 		Got a planform view and side view to share?

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RE: Neutral Point Help - 1/9/2013 2:52 PM   
da Rock



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Status: offline 		Perhaps the reticence this is experiencing stems from the fact that the available formulas and online applications that figure NPs and CGs don't consider what you're worried about at all.

If your available formulas don't consider what you want to, that suggest there is no real importance.

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RE: Neutral Point Help - 1/12/2013 1:34 PM   
Lnewqban



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quote:

ORIGINAL: WWIIbirds

I assume the effect of downwash to be negligible due to my offset wings so I'm not including it my calculation. My concern is that to calculate neutral point it's all about calculating moments, with in line wings I have 2 moments created by the lift of both wings but with offset wings not only do I have lifting moments but I also have moments that are created by the drag of each wing.


I believe that your reasoning is correct.

Please read this old thread:

http://www.rcuniverse.com/forum/m_10456424/tm.htm

Consider however, that for flight stability purposes, the Z axis is irrelevant and the stagger of the wings will move the CG along that axis as well.

We want the center of lift and drag to be aft the CG in line with the airstream, which is what created the aerodynamic forces that stabilize the plane in pitch and yaw.

The drag introduced by the wings is also directly proportional to the lift produced by each wing at each different speed: it is a dynamic force.

If your canard is much lower than the main wing, for example, a nose down pitch will be generated by drag; however, that moment is to be compensated by trim (because is dynamic or dependent on speed) rather than by manipulation of the CG.

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RE: Neutral Point Help - 1/29/2013 7:28 AM   
BFoote


 

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Status: offline 		Unless your front canard is at least 1, more realistically is 1.5 chord length(s) "above" or "below" your main wing you WILL be adding incidence to your main wing. Better off just putting them in the same plane or close and doing a rule of thumb for incidence. Gets back to where are you going to balance your plane. In other words what will your static margin be? Large SM = larger incidence, smaller = unstable! No canard lift = no incidence if you align the downwash angle of the airfoil at 0 lift to the main wing AoA.

I assume you are perfectly aware that your AoA v lift slopes need to lower on your canard than main wing right? Otherwise it really will not matter one damn where your NP is... If this does not make sense, ask.

PS. Have put flaps on a RC Cozy before with perfect stability and identical stall characteristics as the original configuration. A cozy is a 4 seat canard based on same design as the Burt Rutan Long EZ.

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