Hi "Ron"

I think it is probably a combination of material flexing and fitting...........if you think of the door,no matter if it is flat or "profiled",in an air stream it will act like a wing.........producing lift in one direction or another + plus drag !!! a curved profile probably more so like a wing,flat profile would depend more on the relative air flow over it.........bit like driving you car and holding you hand flat into the wind...change the angle of you hand and it goes up or down.

I think it is a question of material strength against speed of airflow.........as Doug says,ali 100% perfect........it has the strength for its shape / weight of the door.

( Full-size limit the speed range with gear and or flaps deployed,the ones that I used to fly...all marked in a white "arc" on the ASI )

For the "stock" set up,I guess it's just making the material stronger in the areas that have caused the "flexing"..........plus don't fly at high speeds with ill fitting or weak doors !!!

Good luck with your maiden...........and doors !!!

Let us know how she goes..........

Cheers.................Nick (UK).