how to know where the CG should be on rc plane?

Merv

Moderator
Moderator

M4RT1N1

Member
The location of the CG is a matter of personal choice. For most planes the CG will be 25-30% of the MAC. The CG at 25% will be a stable flight. I prefer a bit of instability, I prefer a 30% CG.

Here are a few FT articles that go into more detail
https://www.flitetest.com/articles/where-should-an-rc-airplane-center-of-gravity-be
https://www.flitetest.com/articles/aerodynamics-simplified-mean-aerodynamic-chord
https://www.flitetest.com/articles/easy-aircraft-design
I have question about the 3rd link. How can I draw this in full scale?
 

Merv

Moderator
Moderator
I have question about the 3rd link. How can I draw this in full scale?
I’m guessing you want to draw the X on the wing, to find the MAC.

I do it in SketchUp, but any similar drawing program will work. Before PC’s, I would get a large paper or hunk of cardboard and draw it by hand.

The whole ideas is to have 25% of the wing area ahead of the CG & 75% behind. Any way you have to calculate area will work.
 

M4RT1N1

Member
I’m guessing you want to draw the X on the wing, to find the MAC.

I do it in SketchUp, but any similar drawing program will work. Before PC’s, I would get a large paper or hunk of cardboard and draw it by hand.

The whole ideas is to have 25% of the wing area ahead of the CG & 75% behind. Any way you have to calculate area will work.
Please can You drop alink how to use it?
 

M4RT1N1

Member
The location of the CG is a matter of personal choice. For most planes the CG will be 25-30% of the MAC. The CG at 25% will be a stable flight. I prefer a bit of instability, I prefer a 30% CG.

Here are a few FT articles that go into more detail
https://www.flitetest.com/articles/where-should-an-rc-airplane-center-of-gravity-be
https://www.flitetest.com/articles/aerodynamics-simplified-mean-aerodynamic-chord
https://www.flitetest.com/articles/easy-aircraft-design
Is it normal to have CG in the middle of the wing if it is rectangle?
 

Merv

Moderator
Moderator
Is it normal to have CG in the middle of the wing if it is rectangle?
With the X you find the average cord, which is parallel to the root. Take the length of the average cord times 25%. Set the CG on this point.

With a straight rectangular wing, there is no need to do the X, just use 25% of the cord.
 

M4RT1N1

Member
With the X you find the average cord, which is parallel to the root. Take the length of the average cord times 25%. Set the CG on this point.

With a straight rectangular wing, there is no need to do the X, just use 25% of the cord.
What do I need to do different if I have rectangle wings and stabilazers?
 

M4RT1N1

Member
Like he said, instead of finding average chord, you just need the actual wing chord times 25%. You set the cg 25% back from the leading edge of the wing.
How do I know how big should tail and H stab bee and how far from wings should be H stab?
 

Ketchup

4s mini mustang
How do I know how big should tail and H stab bee and how far from wings should be H stab?
I'm not sure if there is any standard or formula to find the size of the tail surfaces since it depends on the type of plane and what you want to do with it. I usually just go by feel but only because I have some experience and know how big is big enough for tail surfaces. For the distance from the H stab to the wing I'm actually not sure. I know that if they are too close that could cause issues but having the tail of a plane too long could make the plane tail heavy. I think that this really depends on the size of the plane.
 

M4RT1N1

Member
With the X you find the average cord, which is parallel to the root. Take the length of the average cord times 25%. Set the CG on this point.

With a straight rectangular wing, there is no need to do the X, just use 25% of the cord.
Bu if it does not work with those settings?
 

M4RT1N1

Member
With the X you find the average cord, which is parallel to the root. Take the length of the average cord times 25%. Set the CG on this point.

With a straight rectangular wing, there is no need to do the X, just use 25% of the cord.
But if I want to know the points of the plane can I still use this method or is it wrong?
 

leaded50

Legendary member
some reference advices given in an article (somewhere..) on sizes vs other surfaces. Perhapsok on a "standard" plane.. but changes on eg. jets, delta .....
 

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