MacLarry
How is this done? I know how to find the COG on a finished model, but how do you determine where it is in designing the model.
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dtribby
It's pretty much an art. Lots of variables to consider. Just remember that the further back you plan to put heavy components (motor, ESC, servos), you'll need to allow for your battery to go that much further forward to balance them. Canard planes carry their CG further forward. If its an established type of plane, then others' work can be a starting point. If it's something completely new, then guesswork turns into art.
Yes, I've known that I'm "different" for some time now...
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squishy
Do you mean find the COG (center of gravity)? Or find the proper COG? If you want to find the COG you just balance the airplane on your fingers and when it balances longitudinally, BAM! That’s the current COG of your airplane. But, if you mean find the proper COG for longitudinal static stability during flight then you came to the right place.

The longitudinal stability is basically your stability in the pitch axis. If your airplane is longitudinally stable, a small increase in nose up will cause the airplane to slightly go down the same amount, and vice versa. Translation: it will fly flat.

Static stability is to do this in all axis, see thrust angle, torque and battery placement discussion for that. I will just cover longitudinally because it’s the most important.

You will get a few different answers to the question what is the best COG because small adjustments to the location of the COG affect flight which is advantageous for different styles. You will ultimately have to decide for yourself where the proper COG is for you and your airplane. Most plans/kits are marked (rarely incorrectly) and you just adjust the components in the airplane till the airplane balances at the marked location. Most likely you will move the battery because it’s like the heaviest thing dude.


Obviously, I am saying the longitudinal static stability of your airplane is very affected by the location of the COG.

Here’s the best method for finding the ideal COG location, of any airplane. There is no substitution for the glide test, if you want to find the proper COG of every airplane in the room, accept no substitute, the glide test. A video is better than reading more words, here:



With the exception of extreme 3D flight and aerobatics, a proper COG based on the aerodynamic center of the wing or the COP (center of pressure). Dun dun dun, stop, that’s the key word of this lesson. The COP is a great concept to understand/visualize/feel about your airplanes and will help you tune them. When your airplane flew flat in the glide test there was balance between the aerodynamic center of the airplane and the point at which gravity centers it's pull, the COG.



Mental health moment



    • The COP of your airplane is the point where all of the aerodynamic pressure may be represented by a single point.
    • The aerodynamic center of an airfoil is like the pitch axle of the wing based on the air pressure and lift it produces in flight.


Now that you have it gliding perfectly and longitudinally stable, you need to think about the control and style of flight the airplane will be asked to perform. After All it isn't just gonna glide all day right? As your COG moves forward the longitudinal static stability of your airplane will increase. Basically the distance between the stabilizers/control surfaces and the aerodynamic center/COP increases and the stabilizers have more influence over the airplane, like duh right? Of course the opposite is true, if you want a less stable airplane (aerobatic), you shorten that length and move the COG to the rear of its proper point. If it’s too far to the rear the stabilizing forces will be unable to keep the airplane stable. If it’s too far to the rear the control surface positions needed to keep the nose up will cause inefficient drag which will slow you down and cut flight time considerably.

So I guess my answer to the question "how do you find the COG of an airplane?" is: I don't know, how do you?

Some people start at 1/3 of the wing chord, others just literally guess and pray (me). There are calculators for flying wings and other tools online, but I think when it comes down to it, you just need to do your homework, understand all the forces involved and engineer your own method that works for you.

Cheers and happy crashing..
"Education is not about filling buckets; it is lighting fires." W.B. Yeats

http://www.youtube.com/user/squishy654
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squishy
This sentence: If it’s too far to the rear the control surface positions needed to keep the nose up will cause inefficient drag which will slow you down and cut flight time considerably.

Should read: If it’s too far to the front the control surface positions needed to keep the nose up will cause inefficient drag which will slow you down and cut flight time considerably.

An important difference, sorry for the typo
"Education is not about filling buckets; it is lighting fires." W.B. Yeats

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LukeWarm
If you need more information on that subject, you may enjoy reading in these two threads.
Useful-Center of Gravity-Information
The controls we use.
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