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        Question Clipped wing - change of CoG?

#1 Kahnx Apr 11, 2007 07:23 AM

Clipped wing - change of CoG?
 
I have a quick question:

I have a Windrider Bat with the Predator style Superfreak mod, which gives it a straight trailing edge .. so effectively it is a plank - come - delta. I have been learning to DS with this wing, and have crashed numerous times by clipping the wingtip on the ground. The wingtips had become soft, so I have trimmed a couple of inches off each tip, reducing the wingspan from 54" to 46".

My question is: how will this affect the location of the CoG? will it move forward or rearward? I want to shift the CoG in the right direction to match the new centre of pressure before going back to the slope.

Anyone done this who can offer some advice?

Cheers

Kahn

#2 The Predator Apr 11, 2007 09:11 AM

I just shredded a wingtip on my DS Bee yesterday as well, actualy both tips, in a cartwheeling digger through the rocks at SCARY speeds while DSing. So, I'm right there with ya. I'm no expert on this but, I'm pretty sure that if you cut off the wing tips, the most aft section of the wing that generates lift, your aerodynamic center would have to move forward since there is less lift generation on the aft section of the wing, the part we're cutting off.
If I am indeed correct, the optimal CG will move forward BUT, so will the actual CG, since we cut off that aft section of wing. They will somewhat compensat for each other BUT I'll bet your best bet will still be starting out with an extra 1 OZ of nose weight and then ease it back as flight tests reveal the true optimal CG.

#3 surfimp Apr 11, 2007 10:01 AM

Can you post a picture of the plane? What is going to determine this is whether or not the quarter chord line of the wing is straight or swept. The CG is located relative to the Mean Aerodynamic Chord (MAC), which means that if you have a swept quarter chord line, then changing the span will affect the CG location. The greater the sweep, the greater the change in CG position, and vice versa.

On a rearward swept quarter chord, shortening the tips will move the CG forward from its original location. I suggest you re-calculate using a CG Calculator based on your plane's current specs.

Steve

#4 raptor22 Apr 11, 2007 11:57 AM

Quote:

Originally Posted by Kahnx
I have a quick question:

I have a Windrider Bat with the Predator style Superfreak mod, which gives it a straight trailing edge .. so effectively it is a plank - come - delta. I have been learning to DS with this wing, and have crashed numerous times by clipping the wingtip on the ground. The wingtips had become soft, so I have trimmed a couple of inches off each tip, reducing the wingspan from 54" to 46".

My question is: how will this affect the location of the CoG? will it move forward or rearward? I want to shift the CoG in the right direction to match the new centre of pressure before going back to the slope.

Anyone done this who can offer some advice?

Cheers

Kahn

Like surfimp said, it depends on the planform.

It will move forward a bit, so I would personally add a little sticky weight to the nose and move it progressively back until you find the new CG>


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