Spaceplane won't lift off the ground

[Pie Guy]

The spaceplane in the images I've linked below won't lift off the ground. I tried removing the engines on the wings but that doesn't work either. Any ideas?

https://imgur.com/a/fp8YPNI

Note: It flies, and it flies well. Once it has reached the end of the runway I have perfect control in the air, it just doesn't lift off while on the runway, and I don't know why.

Edited August 22, 2020 by Pie Guy

[Gargamel]

Move your real wheels to just barely behind the COM.   The wheels are supposed to work like a fulcrum when you pull back on the stick, and if they are too far back, the downward force of the control surfaces can't create enough force to lift the nose.   If they could, your plane would probably do back flips in flight. 

[jimmymcgoochie]

2 things- wheel positioning and control authority.

When you’re trying to take off, the plane has to rotate, lifting the nose up. The rear wheels are the point where the plane has to rotate around, and in your design the elevators at the back of the plane are your only source of pitch control worth mentioning- the main wing elevon parts won’t provide any real pitch authority as they're too near the middle of the plane.

The elevators have to push down on the tail like a lever in order to get the nose up, but they’re too close to the pivot point and so have a limited effect; with levers, you apply more force with the same amount of effort the further from the pivot point you are.It’s also worth mentioning that the centre of mass is in front of the rear gear, which will make it much harder to rotate the plane- think of trying to pull a see-saw down with someone sitting on the other side.

Moving the main landing gear further forwards will give those elevators a longer lever to work with and will even out the masses either side of the pivot point too, both of which will help your elevators force the nose up. Just be careful that you don’t then start hitting the runway with your engines when you pull up- using slightly larger wheels all round and then adding a single small wheel to the tail, moved upwards a little bit to keep it off the ground, will help to avoid tail strikes without getting in the way.

Alternatively or indeed additionally, you could add a pair of canards near the nose. They’ll provide a second source of pitch authority and help to rotate the nose on the runway, but will also have an effect on the centre of lift. Another trick is to adjust the rotation of the wings slightly- in rotate mode, hold shift to get fine control and angle the wings 5 degrees up; this will make them generate lift when your plane is on the runway as in KSP wings generate lift solely due to their angle of attack through the air. This will also reduce body drag in flight, although this is less important with the Mk1 setup and matters more for Mk2 and Mk3. Adjusting the positions of the landing gear to give the plane a slight nose up attitude on the runway might also help.