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# C.G. displacement influence

## C.G. displacement influence

(OP)
Hello,

I am trying to develop a MatLab program that mathematically models an aircraft in trim flight. Everything fine so far, except that the reference system used does not coincide with the centre of gravity. I denoted the distances from the origin of the reference system to the centre of gravity with CGX, CGY and CGZ. However, CGY=0.

Now, how does these distances affects my equations of motion and the aircraft's derivatives? Thanks!

### RE: C.G. displacement influence

i don't think it matters where the origin is relative to the CG.  your equations of motion are describing the motion of the CG, right ?  so that the pitch moment balance about the CG is the summ of applied moments about the CG ... expressed that way it doesn't look very helpfull ??

### RE: C.G. displacement influence

(OP)

Well, I have the stability and control derivatives for my aircraft, all calculated with respect to the c.g. assumed at the origin of the body axes (nominal c.g.). Now I found out that the final c.g. is displaced in the Ox and Oz directions by some distances. I believe this will affect the derivatives I obtained so far, I just don't figure out how it affects them mathematically.

### RE: C.G. displacement influence

are you designing your own plane ?

### RE: C.G. displacement influence

(OP)
I am trying to improve a plane by adding a forward fin to it and changing its engine. But this will obviously move the c.g., so it's derivatives will also change. Any suggestions how this is achieved is still welcome.

### RE: C.G. displacement influence

if the CG is still within the range allowed for the plane then you'd be ok.

but this is only part of the problem ... you're changing the power and maybe the line of thrust ... this'll affect the stability of the plane.

a "forward" fin ?

i'd look into a text on airplane stability and control ... something like "etkin".

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