RE: HELPP Airplane 180size on my Car roof.. what is the Max speed???
As long as it is at zero thrust, meaning the tail is in line as if it were flying and it is tied on good, it will be fine. Many do it. There will be no more force on it than if it were flying, other than the tie downs. That you want to do very carefully. The guy that taught me did it with a 1/3 bipe. The only problem was when the tie down broke and he lost a wing. Luckilly the truck behind him missed it and he just put some tape over the holes in it and flew it.
Not something I would do personally, but if it your only choice, make sure it is secure and straight and go for it.
Don't tie it backwards. You are asking for all kinds of trouble doing that.