What's the general rule for location of CoL vs CoM for things that fly (or at least for aeral maneuv

[smjjames]

(wishes there was more room in the subject)

Anyway, as you can probably surmise from the subject and screenshot, I'm wondering what's the guideline for location of CoL vs CoM for planes and also for aeral maneuvering for those that aren't quite capable of flight on their own.

Edited August 4, 2015 by smjjames

[cantab]

For planes you usually want the CoL a little behind the CoM. Too far behind and you'll tend to nosedive, significantly in front and you'll tend to flip.

For a repulsorlift craft like that I've no idea. I'm not even sure what horizontal wings would do, except help maintain stability in jumps, in which case you want them arranged much as for an aircraft - your design above will want to do a backflip in jumps. A vertical stabiliser and rudder would seem more useful to help you stay flying straight and turn more easily, mounted at the back of course.

[smjjames]

For planes you usually want the CoL a little behind the CoM. Too far behind and you'll tend to nosedive, significantly in front and you'll tend to flip.

For a repulsorlift craft like that I've no idea. I'm not even sure what horizontal wings would do, except help maintain stability in jumps, in which case you want them arranged much as for an aircraft - your design above will want to do a backflip in jumps. A vertical stabiliser and rudder would seem more useful to help you stay flying straight and turn more easily, mounted at the back of course.

That... would explain it I guess. Thanks for the tips.

(more tips welcome though)

[Mic_n]

Erm... not sure about this "repulsorlift" thing, I'm presuming that's the weird non-standard things you've got there.

I'm going to sound like a jerk here, but if you don't understand something as fundamental as that, stop mucking about with mods and figure out the basics first.

Stop and think about what they're representing. Think of the CoL as a fulcrum, and the CoM as the point mass hanging from it (and the CoT the force you're applying to the CoM). If the mass is directly underneath the fulcrum, it'll happily hang there. A thrust 'forward' on that mass from directly behind it will push it forward and rotate it a little around the fulcrum, meaning you'll nose-up a little. Cut the thrust and it'll settle back to its normal orientation. Move the fulcrum down and the 'turn' will be less and less pronounced until it vanishes altogether when they're directly aligned. Move that fulcrum forward, and the mass will drop away and attempt to point the whole craft straight up. That means your craft is quite likely going to stall and you'll lose control of it. Move the fulcrum backward and the nose will want to drop. No stalling there, you'll speed up instead, so as long as you can raise the nose enough to lift off the ground, you can overcome that tendency.

It's not rocket science. Aeronautical engineering, sure. Maybe a little rocket science.

[GoSlash27]

smjjames,

You do not want the CoL in front of the CoM under any circumstances. Having them in the same place will result in a highly unstable (and thus highly maneuverable) aircraft. The farther back you move the CoL, the more stable it becomes and the less maneuverable. Eventually, it will be so far back that it's impossible to keep it from acting like a lawn dart.

You want to position the fuel in such a way that it doesn't move the CoM as it drains.

Having the CoL above or below the CoM will make the aircraft pitch with changes in speed.

Best,

-Slashy

[ajburges]

smjjames,

You do not want the CoL in front of the CoM under any circumstances.

if you want a dynamically unstable craft you want CoL ahead of CoM. Problem is you can't control such a craft without a fly by wire system (and significant control authority). Stock SAS is woefully insufficient as a flight control system for such craft.

Control surfaces use a different AoA from the wing to create torque. You get control authority when that torque overcomes the torque of the CoL (whose strength is based upon the moment of inertia, the lift coefficient and the distance between the centers). In an aerodynamically stable craft (CoL behind CoM), that torque resists deviation from prograde with negative feedback. In a dynamically unstable craft, that force deviates from prograde with positive feedback.

Need some info to help with repulsors. Are they modeled as a thruster or are they negative mass in operation. Still you want those to do most the lifting, so put control surfaces in the back for stability and forget about it. (or skip aero surfaces and use reaction wheels alone for control authority)

Edited August 3, 2015 by ajburges

[smjjames]

I'm going to sound like a jerk here, but if you don't understand something as fundamental as that, stop mucking about with mods and figure out the basics first.

Which is exactly why I'm asking about it in here

I normally make rockets, so I'm not very experienced in making good aircraft. Though the craft above isn't supposed to be an aircraft per se.

Need some info to help with repulsors. Are they modeled as a thruster or are they negative mass in operation. Still you want those to do most the lifting, so put control surfaces in the back for stability and forget about it. (or skip aero surfaces and use reaction wheels alone for control authority)

You'd have to ask Lo-Fi (author of Kerbal Foundries, which is where the mod is from) about that, they definetly aren't thrusters, they apply anti-gravity. Yes I know that's a total non-answer, but I don't even know the answer about how they work in the first place.

Once off the ground, they really don't do anything, my question here is asking about maneuverability while airborne.

Edited August 3, 2015 by smjjames

[Esme]

Which is exactly why I'm asking about it in here

I normally make rockets, so I'm not very experienced in making good aircraft. Though the craft above isn't supposed to be an aircraft per se.

You'd have to ask Lo-Fi (author of Kerbal Foundries, which is where the mod is from) about that, they definetly aren't thrusters, they apply anti-gravity. Yes I know that's a total non-answer, but I don't even know the answer about how they work in the first place.

Once off the ground, they really don't do anything, my question here is asking about maneuverability while airborne.

If your CoM and CoL were the other way around they'd be reasonable for a conventional aeroplane, although personally I'd probably have them a smidge closer again. An easy way to remember teh CoL and CoM's relative positions is to think of gliders, which fly slightly nose down and thus tail up. So the pointing-up one (CoL) should sit behind the pointing down one (CoM).

With the craft you have there though... I have no idea what'd work best, it's a case of try different things and see what works best!

[ajburges]

If your CoM and CoL were the other way around they'd be reasonable for a conventional aeroplane, although personally I'd probably have them a smidge closer again. An easy way to remember teh CoL and CoM's relative positions is to think of gliders, which fly slightly nose down and thus tail up. So the pointing-up one (CoL) should sit behind the pointing down one (CoM).

Pedantic: gliders want CoL and CoM to be the same point. Using control surfaces creates drag. You want to maintain a glide slope without control surface use to maximise range. In fact, if you can't align, you want CoL ahead of CoM so your tail plane is creating upwards force to maintain positive AoA.

After I finish reentry, I redistribute fuel so my pitch plane needs no trim to maintain course.

[smjjames]

Thanks for the info and tips.