How can a glider or an airplane do a loop without thrust

[jbriggs444]

I am just getting confused, let's see what I'm doing wrong, I take this picture:
[snip]
Offcurse my plane is not climbing but just free fall on axis Y (my plane have angle of 0 let's say) so my plane isn't moving backward ?

If you have a plane that is pitched upward and is stopped dead in the air then that plane will not be airborne for long.

If you have a plane that is moving forward... Well then that plane is moving forward, not backward.

 

[plaguebreath]

Ok guess I am just looking stupid now, thank you everyone for the help.

 

[Khashishi]

I think you can still work through this without advanced physics training. The Glenn research center slide shows some useful equations.

Here's the deal: if there is no thrust, the plane has to be pointed downward to gain horizontal momentum. Note that when pointed downward, the lift has a component that is pointed forward. It is this lift that propels the plane forward and allows it to glide. If the plane tilts up, the lift will have a component pointing backwards, so the plane is now losing horizontal momentum. If there's enough momentum to begin with, the plane can execute the loop de loop. If not, the plane loses too much momentum and starts to stall. That's because the lift depends on the forward (not necessarily horizontal) velocity squared of the plane. If you lose too much speed, you lose your lift. The instinctual action when you start dropping is to point the plane up more, but the correct action is to point it down more, so you can gain some speed and lift.
$L = \frac{1}{2} \rho v^2 S C_L$
from https://en.wikipedia.org/wiki/Lift_(force)

 

[Khashishi]

the first plane is the step at time 0, my plane is on starting position with velocity 0 and thrust =0

Ah, therein lies your problem. You can't fly if your velocity is zero. Point your nose down so you can gain some forward speed and hope you get enough lift before you hit the ground. (When pointed down, lift will provide horizontal acceleration. Once you've gotten enough horizontal velocity, you can start to point the nose more horizontal again, where lift will point up and start to slow your fall.) Planes have to be moving forward to work.