COT infront of COM Spaceplane question.

[DreadZombie]

Is making a stable/good spaceplane where the Centre of Thrust is in front of the Centre of Mass possible? If it is, how would you make it?

I ask because I've recently been using the Firespitter mod pack to make a plane, on which the propellers are on the front of the wings, similar to WW2 bombers. This makes the COT in front of the COM. I'd also tried to make the COL slightly behind the COM.

The plane, however, wants to pull up and then refuses to pitch down again. (If it does, it's only slightly.) I've made sure that the COT is in line with the COM but this still didn't seem to work.

If pictures are needed, I'll post them after the next time I get on KSP.

Edited August 26, 2013 by DreadZombie

[M4ck]

It does matter where your COT and COL are located relative to the COM. Basically, the COM works like an axis for any forces affecting the plane. I'll use a shot from a project of mine to describe what I mean.

It's a small craft with a propeller at the front. You can see the COT is a tiny bit below the COM. If you pinned the ship at the COM and grabbed the COT to pull it forward, the vessel nose would rise a bit. This means that in flight the vessel would slowly point the nose up. The same goes for lift(in this case, the lift-generating part is a balloon, not a wing, so it also works while the vessel is stopped); the COL is a tiny bit to the front of the craft, so it also adds to the "point up" tendency, as it'll lift the front part of the vehicle more than the back, until the COM is directly below it(it's a large simplification, but it works in this case). Basically the farther away is the line you would get by extending the COT/COL marker from the COM, the larger the "rotating" forces will be.

In this craft, the differences are small enough that they can be countered by SAS, but if the COT was a lot lower, the ship would begin to rotate faster and faster all the time the propeller was turned on.

I hope this clears everything a bit. If not, maybe the others will be of more help.

[DreadZombie]

Ah, I knew the basics but I'd thought that it was just the fact the centre of thrust was in front of the centre of mass that was causing the problem.

Anyway, thanks for that, you've cleared everything, I'll just have to check the design again and adjust it to fix it.

[M4ck]

Well, in a lot of cases the COT at the front of a ship is actually better than one at the back, like with long interplanetary ships assembled in orbit. The ship "compressed" by an engine pushing it from behind tends to wobble much more than a similar one pulled behind an engine, plus the joints aren't stressed as much by bending when stretched.

Edited August 26, 2013 by M4ck

[arq]

One way to keep the plane stable in this configuration is to have significant wing-area in the rear, so that the plane is 'fletched' like an arrow. In fact, this is the best way to keep an aircraft stable in atmo. One nice thing about the engines in front is that they are heavy so the CoM will be more forward, which amplifies any 'fletching' you may add. I've lost many spaceplanes on re-entry because the CoM shifted too far back as fuel burned.