Bi-plane C of G

[ericrw]

Hi,

When locating the C of G on a bi-plane , should it be on the top or the bottom wing ??

[FlyinBrian]

You can probably take an educated guess as the distance given will seem a long way back if it is for the top wing and quite forward for the bottom wing (assuming a forward wing stagger - top wing ahead of the bottom)

[The Wright Stuff]

Hi Eric,

Just to be clear, are you asking how to determine where the CoG should be using a rule of thumb (e.g. relative to the wing chords)?

Or are you asking how to find it on a model by balancing (i.e. you know where it should be, but you don't know how to check it)?

[Peter Miller]

Basically you calculaste the CG for each wing and then put it in the middle. This normally works pretty well. I tend to use 25% chorrd on the top wing and 25% chord on the bottom wing and then draw a vertiocal line in the middle of the two points.

If the top wing is a lot bigger than the bottom one then move the CG further forward.

[The Wright Stuff]

Hi Peter, to continue on your answer, how then do you physically balance the model to ensure that the CoG is as per your calculation.

I assume that one option is to mark the underside of the top wing where your vertical line intersects the bottom surface of the top wing, and use fingertips there...

[Jon H]

I do it differently to Peter but the result is pretty similar. I measure from the leading edge of the most forward wing to the trailing edge of the rear most wing (including any wing sweep) and then divide by 3. Whatever that distance is i measure back from the leading edge of the forward wing and that's my start point.

As the top wing is usually the most forward and easiest to balance from i balance it there at the point i just calculated.

'my' method is shown, along with Peters, on this website. https://www.modelairplanenews.com/balancing-biplanes/

Edited By Jon - Laser Engines on 08/05/2018 13:53:48

[ericrw]

I`ve found it, 77mm from leading edge of top wing !!