What is your primary X-Plane version?

Wednesday, November 16, 2011

Center of gravity

I was in process of tuning the glider's flight model to follow speed polar of the real one. However I noticed that model is quite much pitching up. I had to push hard forward to get the nose down. Definitely was something wrong with the balance. I did open the plane maker to double check the CoG position and wings position. Everything seemed to be OK. Somewhere over the x-plne forums I found that x-plane calculates resulting lift forces at 25% of wing's chord for any airfoil. Actually this can be visualized within the x-plane by pressing "/" key.

Clearly it is at 1/4 of the chord lenght

Gliders usually have laminar airfoils that may have resulting force at about 50% of the chord. That means the CoG (defined as in real DG1001M) is too much off of the resulting lift force in the X-plane. Thus a large moment is produced and the aircraft tends to pitch up.

So how to solve this problem?
A) Solution might be to simply change the CoG position in the plane maker. However that breakes designed balance for ground operation as the CoG moves towards the main wheel.
B) Move the wings so that chord's 25% (1/4) will match desired position of the CoG. Then just hide the wings and replace them by ones imported as misc. object that will not be taken into flight model calculation.
C) Calculate airfoil's moment correction.

I'll go for B). This sulution seems to be most accurate at given circumstances

B) solution applied
Edit:
As I found later, there is no need to move the lift force (wings). I misunderstood some things here. So the solution is C). Explanation is here.

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