CoM and CoL Rule and Gliders

[firerider521]

So everyone knows the fundamental Center of Mass in front of Center of Lift rule, right?

I'm having trouble with designing a shuttle-like craft that glides to the surface from orbit. Right now, a craft following this rule will not glide efficiently and will eventually nose down to death. Is this my problem, or does this rule not apply to gliders?

[MrTaxman]

I couldn't say for sure - I've always found that the CoM needs to be a touch *behind* the CoL for planes. Maybe my experience is flawed, but that's what I've seen.

[stupid_chris]

Atmospheric planes and orbital planes behave differently, I'm not expert, but IIRC, a space plan needs it'S CoM behind it's CoL when re-entering the atmosphere, else the nose will plunge down and result into a spin of death, like you are experiencing :l

[firerider521]

Ah, that makes more sense.

[Raptor831]

The further the CoL is behind the CoM, the more stable your craft is. It also means your maneuverability goes way down. If you make a craft that reenters with the CoL in front of the CoM you do death-flips into the cold, hard ground. I've done that many times. Ideally, they should be close to each other for normal flight. If you want a quite stable craft then you really want the CoL about one "ball" behind the CoM.

Also, if you can't pull up, is it because you simply don't have enough lift? Or not enough control surfaces? Both of those will hurt you in glide.

[firerider521]

Well, I once tried infinite control surfaces...

It... well, it didn't work so well.

[bsalis]

Ok, well first up... be mindful that landing gear does not have any mass when flown, but the SPH includes it in the CoM marker.

Which is why I removed the landing gear to take this pic of a working glider...

So yes, I have found that for a stable glider with no trim, you should have the CoL slightly in front of the CoM. However, if you have something you want to bring down from orbit, you probably still want to have the CoL slightly behind, then use trim to pitch up.

You also want some of this for full hands-free flight...

http://en.wikipedia.org/wiki/Dihedral_(aircraft)

Then you can have some fun chasing your glider in another plane...

[firerider521]

Wow, I didn't know that about the landing gear! I didn't want an unmanned glider, but that's really cool.

Also, I found out that I simply needed more lift.

Edited July 10, 2013 by firerider521

[Cranium]

I've actually found that for unpowered flights like gliders, a perfectly centered COM/COL is really good for just detatch and forget, as it's going to continue in the original direction you point it in with minimal course correction.

I've done tests like this by making a small, probe powered glider, perfectly balanced, and then detaching it from a decaying orbit. I've found that I can make it fly perfectly without much control, and can even use a centered probe core as the only means of control if the plane is small enough.

[timpossible]

Also account for the change in mass as the fuel is burnt up, if you burn up all your fuel on launch, the CoM could've moved a lot

[Kosmo-not]

My shuttle has its lift vector slightly in front of the CoM (draining of RCS fuel can move the CoM slightly more aft). I stuck two small control surfaces on the rear of the shuttle to give it more control authority and prevent it from spinning out. It works beautifully.

My spaceplane used to have issues with flipping out (center of lift forward of CoM). Putting a few of those small control surfaces on the back fixed the whole problem while keeping the wanted maneuverability.

Edited July 13, 2013 by Kosmo-not