CG Question

[jersterT58]

Lets say I am designing a "foamy" (Whether Dollar Tree foam or EPP). How does one determine where CG is? I am assuming somewhere in the wing width. But we all know where "assuming" puts me.

Can someone help me with this, or find someone who can?

JersterT58

 

[wilmracer]

I typically start by looking at similar models and setting the CG slightly forward from there and adjusting as needed. A good method for testing is to make a small scale chuck glider with the same shape to check your estimate. It won't be exactly right, but it will give you another good basis to work from. Just remember that it is always safer to err on the side of nose-heavy.

There are several CG calculators online if you really want to get into the weeds. I wouldn't do that unless I had a huge amount of time and money invested or a REALLY unusual model.

 

[saf99]

Take a look at this... http://adamone.rchomepage.com/cg_calc.htm

 

[jersterT58]

Thanks "Wilmracer" and "saf99" for taking time to answer my question. Much appreciated.

 

[TEAJR66]

Here is an excellent article for designing and finding the CG on a scratch built plane. http://flitetest.com/articles/easy-aircraft-design. most times, someone else's designs have a recommended CG. If you design your own, this will help you establish yours and ensure you have a good tail arm length and tail volume.

 

[Team_Monkey]

Don't forget those locate the "design CG", you still have to verify you actually built in the correct CG location.
Mr. Clean taught me about the CG sling a few years ago and it has worked perfectly on everything from foamies to a 105" span gasser. Although the materials for the wing loops were different.

Good luck!

 

[pinchflat]

As a general rule of thumb. For a straight non tapered wing the CG will be 1/3 of the span back from the leading edge.
For a tapered or swept wing you need to find the mean aerodynamic center and it's 1/3 of that.
For deltas and other tailless aircraft that number can be as little as 10-15% of MAC.
I hope this helps. These are base lines and you can alter them to suit your taste.

 

[wilmracer]

Don't forget those locate the "design CG", you still have to verify you actually built in the correct CG location.
Mr. Clean taught me about the CG sling a few years ago and it has worked perfectly on everything from foamies to a 105" span gasser. Although the materials for the wing loops were different.

Good luck!

I've never used a Vanessa rig before but they sound very cool. Seems like a lot of effort to put one together but I might give it a go for larger aircraft.
http://www.youtube.com/watch?v=afvfShdOuck

 

[RAM]

Haven't done it yet but I'm pretty sure a 1/4 nut or just a dimple in the bottom of the plane where you want it to balance is all you need.

Plus one of these

 

[Team_Monkey]

In an ideal world a tri-pod might work but not only would you have to get the fore-aft CG perfect, you would also have to have the lateral CG perfect. Also, don't forget the weight versus the material of the fuse. You have to support the aircraft in such a way you don't damage it (pole a hole) by locating the CG.

Wow, I've never actually looked up a Vanessa rig, what a PITA.
A simple CG sling is just two loops of string/rope to support the plane wings and a plumb bob (weight) on a single string hung from the same point.
Here's my big Pawnee in a sling made from 1" webbing straps.


Here's a 1.20 size ARF in the sling to determing battery location. You can see the screwdriver bit I used for a bob.

 

[ghost civic]

when I design a plane from scratch, I aim to have the CG be 25-30% back from the leading edge of the wing. IE, if my wing is 6'' wide then I try to have the cg be around 1.75'' from the edge.
typically, this can be achieved by finishing the airframe and attaching the motor/prop and then placing the battery and servos in places to get that desired CG. putting a elevator servo on the tail, or in the fuse with a long pushrod can adjust CG. so if I check CG with just the motor installed and it is tail heavy, ill put the servo in the fuse with a long pushrod. if it seems nose heavy, ill put it on the tail. Tho, most of the time I can make the needed adjustment with the battery's placement. I don't make the battery tray until I know where it needs to sit to get the CG I want.
If you place all electronics and battery first, then the only way to get the CG you want is to add weight (to the nose or tail, depending on what adjustment is needed). There is nothing wrong with that, as long as you don't have to add a ton of weight.

Also, I found some pics via a google search that showed a basic high wing plane design and it gave the ratio's that generally make a good plane. here they are, they will help. I could have avoided a few failed designs had I found the pics earlier.

 

[RAM]

This should help
http://www.instructables.com/id/Introduction-65/?ALLSTEPS