Spaceplane COM/COL question

[Spyritdragon]

Allright, here goes

I've seen alot of questions about spaceplances here, and the governing tutorial seems to be this one.

Now, when making spaceplanes, i've always been advised (and so far it works pretty well) that the Centre of Lift goes behind the Centre of Mass, so our plane will tilt slightly downward when not controlled. Put the COL in front of the COM and your plane becomes flip happy, put them too close to each other and your plane becomes excessively maneuverable.

I don't get it though. Why is this? Say my COM is right in the middle of my plane, what is the fundamental difference between the front of my plane and the back of my plane that makes the plane flip-happy if my COL is in the front, but stable when its in the back? Okay, sure, i'd rather be plummeting down towards the earth rather than get in a stall and lose use of my control surfaces, but i dont see how that affects stability in normal flight.

I can also see that a plane that naturally wants to point downward makes pulling the noise up less twitchy, but in return, it'll make pulling it back down more twitchy, and the general maneuvrability of the plane isn't much affected. The plane can now be controlled by letting it naturally pitch down and pulling it back up, which ends up being controllable, but you can do it the other way too.

In general, why not have a plane that just keeps pointing in the way you put it?

Edited July 25, 2014 by Spyritdragon

[Wanderfound]

You'd probably benefit from reading this post: http://forum.kerbalspaceprogram.com/threads/87753-Airplane-doesnt-fly-well?p=1302513&viewfull=1#post1302513

Short version: lift is drag. Drag wants to go backwards. Rear centre of lift = back of plane gets pulled backwards, keeping the front of the plane pointed to the front. Front centre of lift = front of plane gets pulled backwards, entertaining Kerbal screenshots happen.

[peadar1987]

The centre of lift also corresponds roughly to the centre of drag, which KSP doesn't show you, but you really want to be behind your centre of mass. The plane wants to fly with the COD behind the COM, (if you have a dartboard in your house, try throwing a dart sideways or backwards at it, if you're far enough away, it'll realign). If the COD is in front of the COM, you'll have to constantly be making corrections to stop the plane from flipping to its preferred orientation.

Edit: Ninjad!

[RoboRay]

Allright, here goes

I've seen alot of questions about spaceplances here, and the governing tutorial seems to be this one.

Now, when making spaceplanes, i've always been advised (and so far it works pretty well) that the Centre of Lift goes behind the Centre of Mass, so our plane will tilt slightly downward when not controlled.

That is not why you want the CoL behind the CoM, though I do see a lot of people trying to "teach" that here. The reason you want the CoL behind the CoM is for positive aerodynamic stability. Lift is a form of drag. When a solid object is moving through a fluid, it naturally rotates so that the drag is toward the back, behind the CoM.

Put the COL in front of the COM and your plane becomes flip happy, put them too close to each other and your plane becomes excessively maneuverable.

Right. That's negative stability and neutral stability, respectively.

I don't get it though. Why is this? Say my COM is right in the middle of my plane, what is the fundamental difference between the front of my plane and the back of my plane that makes the plane flip-happy if my COL is in the front, but stable when its in the back? Okay, sure, i'd rather be plummeting down towards the earth rather than get in a stall and lose use of my control surfaces, but i dont see how that affects stability in normal flight.

The effect I described above. The CoM is the pivot-point about which the plane rotates. Look at a weathervane... one end has high drag, the other end has low drag. The wind makes it rotate around the pivot-point (the pole) so that the high-drag end is downwind, behind the pole.

It's the air flowing past your plane that makes it "flip-happy" or not based on the CoL/CoM relationship, not gravity pulling down on the plane.

I can also see that a plane that naturally wants to point downward makes pulling the noise up less twitchy, but in return, it'll make pulling it back down more twitchy, and the general maneuvrability of the plane isn't much affected. The plane can now be controlled by letting it naturally pitch down and pulling it back up, which ends up being controllable, but you can do it the other way too.

All of this is just a misunderstanding of the underlying principles. It's not about "balancing" your plane, it's about fluid dynamics pushing the high-drag end harder than the low-drag end, and the CoM is the point about which it forces rotation.

In general, why not have a plane that just keeps pointing in the way you put it?

If you make a plane too positively stable, by putting the CoL too far back, it can actually become impossible to turn no matter how much you pull on the controls... they simply cannot apply enough force to overcome the airflow trying to keep the CoL directly behind the CoM. That makes it pretty difficult to fly, since you can't steer.

If you design a plane for neutral stability, it also becomes very difficult to fly... by being so easy to point it in any direction, it becomes very difficult to make it point in the direction you want it to actually go. The slightest disturbance and you're facing a different direction. That's great for fighters with flight-control computers, where the pilot is really just voting for what the controls should be doing and he's in the minority party, but not so great for anyone trying to just fly in a straight line by hand.

Edited July 25, 2014 by RoboRay

[NoClass]

RoboRay has the best answer above, but I'd like to expand on one thing...

...the Centre of Lift goes behind the Centre of Mass, so our plane will tilt slightly downward when not controlled.

Speaking precisely, a rear CoL will not by itself pitch the craft downward. All it will do is pull the tail out of the windline, away from the direction of flight.

If there was no gravity then the aircraft would continue to point in the same direction. However, if you're not producing lift, gravity will begin to pull the direction of flight downward, the angle of the airflow will point downward, and your stable craft will follow.

Aircraft that are nothing but tail (like an arrow or a fin-stabilized rocket) will follow a nice ballistic curve, with the nose pointed in the direction of flight (which curves) the whole time.

But, if you have wings that produce lift, and hold the nose slightly above the direction of flight, you'll keep flying forward.

[Hodo]

Well I am glad I see others have given a great answer before i could even get here. RoboRay is dead on with his answer.

The only thing to factor into for KSP is if you are playing stock KSP or with FAR/NEAR KSP. Stock KSP you can get away with less realistic CoM vs CoL placement then you can with FAR/NEAR.

[Spyritdragon]

Ah yes. Very nice explanations, thanks ^-^. I'd totally forgotten about the drag factor in keeping your plane straight, for some reason.

[Pecan]

Just to add that the Centre of Drag thing becomes really important when you start trying to add more intakes for better performance. Typically, for looks, they get added in front of engine nacelles/fuel tanks. Looks fine, adds a lot of drag in front of the CoM, plane stops flying!