Glider Tips

[CalculusWarrior]

So I've been attempting to make spaceplanes which return unpowered (like the Space Shuttle), and I've found it surprisingly difficult. A plane which flies normally while the engines are on can be very dodgy when they're cut off. I've also discovered that some ships are very unstable at supersonic speeds, while others' noses drop like a brick when approaching the runway.

My question to the master aircraft builders out there (or anyone who cares to throw in their two cents) is: what are basic rules to follow when creating a glider, and how do they differ from conventional planes? The standard trick of placing the CoL behind the CoM doesn't seem to work very well, as it results in the aforementioned nose dropping behavior when approaching the runway.

Many thanks, my kerbal test pilots will hopefully lead much longer lives due to your responses!

[metl]

Try angling the wings upward so that the weight sits below and in front the lift. Also, add some small control surfaces to the front (but keep the CoM in front of he CoL, so it might require moving the main wings back some.) Works for me, but I don't do a whole lot with space planes.

[Hammer Tech]

The way I do it is to have the wings as high as I can get them, and not too far behind the CoM, angle them straight out, and always have canards for keeping the nose up. Other than that just try to keep it as light as possible and have long straight wings.

[Murph]

I'm not certain, not an expert on it, but it's probably a good idea to try to place the CoM and CoL in more or less exactly the same spot for a glider, or at least very close together. That should minimise the automatic pitch effect caused by the distance between them. If there's a big distance between them, the CoM pushing down and CoL pushing up is always going to give a net pitch force. With a powered plane, I believe that the thrust and drag forces significantly counter that pitch force. If there are any fuel tanks included in the ship during its gliding phase, remember to use the dry CoM, not the fully fuelled CoM.

The real world gliders that I've flown have always had the wings pretty much aligned with what should roughly be the CoM (by visual inspection/guesswork), but I've only flown the things, didn't study the precise dynamics of them.

Edit: Oh, and I believe that "flying brick" has been used to describe the real world Shuttle when it was in the glide phase of its flights, so perhaps you're close to accurate already.

[numerobis]

Keep in mind that the CoM marker is off when you put the small gear bay on your spacecraft: the marker assumes they are 0.5t, whereas they're massless in flight.

[arq]

I find that conventional planes (with the main wings in front) are more stable than canards (with the main wings at the back). It can help to get your controls far from the CoM to increase their leverage, and a conventional tail can help to accomplish this. Having control surfaces far back can also help to increase the stability of the plane (think of it as being fletched like an arrow). Most planes with a centered CoT will glide alright, in my experience.

Most of my SSTOs go through all of reentry and landing without engines. It is important to stay pointed very near prograde, especially at high speeds, to prevent flipping. Stay high to fly further, or fly low to slow down (thanks, drag) and land sooner.

[numerobis]

Another thing; if your airframe can handle it, you can tumble wildly through the atmosphere. When you enter the lower atmosphere, you'll more easily be able to regain control, particular if your airframe is dynamically stable (CoM ahead of CoL no matter which way you're pointed).

[bsalis]

Do you mean hands free and SAS free?. Or simply to just do controlled unpowered landings (that can even be done with kOS).

If it's just unpowered landings then you just need plenty of lift for the mass of the craft. For hands free it simply needs to be well designed, with CoL and CoM together, then add some trim to pitch up.

Edited December 17, 2013 by bsalis