No, I do not know the formula.
Before, I watched an interesting effect, if you put the linear damping 1, it turned out that the glider is accelerated to nearly the same speed, which I have in the variable thrust.
I thought I may have what some formula that is able to calculate the acceleration on the basis of traction.
Here is my glider
He keeps in the air thanks to the PID controller, the friction of the ground is not available, there is only air resistance, the weight of 1 ton.