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

If drag is backward, that means that initial velocity was forward. Accordingly, the plane will move forward. It will not move as far forward as it would have done without drag, but it will move forward.

If your step size is large enough that the plane stops and then moves backward due to drag, then either your drag formula is incorrect or your step size is too large. Drag scales with velocity. The slower you go, the less the drag. You will never stop and then start moving backward under drag alone.
isn't drag at certain point be equal to thrust ? If drag is based on the speed, and because there is no speed increase deriving from thrust (my immaginary airplane has no starting speed) we will get that thrust = 0 so what's left is only the negative force of drag that will increase as my plane falling because it will increase the speed too right ?

Your plane will have velocity changes continuously as result of acceleration due to gravity.

The air resistance and drag including all the factors as result of flaps and the shape of the glider as well as this gravity will all affect how component parts are accelerated relative to that of the centre of gravity of the object overall. The glider will maintain some speed through inertia.

The initial speed due to inertia from being pulled by the plane will grant an amount of kinetic energy in addition to the gravitational potential due to height.
This means that there is sufficient kinetic energy to move horizontally as well.

jbriggs444
Homework Helper
2019 Award
isn't drag at certain point be equal to thrust
Yes. If drag exceeds thrust, the plane slows down.
If drag is based on the speed, and because there is no speed increase deriving from thrust (my immaginary airplane has no starting speed) we will get that thrust = 0 so what's left is only the negative force of drag that will increase as my plane falling because it will increase the speed too right ?
If the plane is falling and gaining speed, that's drag plus gravity. If it is going to be looping the loop, that's lift as well.

Yes. If drag exceeds thrust, the plane slows down.

If the plane is falling and gaining speed, that's drag plus gravity. If it is going to be looping the loop, that's lift as well.
I am just getting confused, let's see what I'm doing wrong, I take this picture: 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 ?

A.T.
I have to admit I am not good on physics...
Then you should learn the basics first, like Newton's Laws. After that you can look for some tutorials about "physics for games".

jbriggs444
Homework Helper
2019 Award
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.

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