Spaceplane Flight Balance

[Varyon]

So I used to be able to build airworthy spaceplanes. I placed the CoL near the CoM, as is the norm, and they flew fine. Now when I make spaceplanes, placing the CoL near the CoM, they refuse to leave the runway until my velocity is 190 m/s or greater, and then they faceplant. I know it's not as simple as lining up the CoL and the CoT with the CoM, but there must be something fundamental I'm missing. After looking at the Kerbalife spaceplane line and Zokesia Skunkworks (and after extensive independent testing) I've determined there's little to no correlation with where the CoL is in relation to the CoM in terms of getting the spaceplane off the ground and stable. So what's the secret?

Edited May 29, 2013 by Varyon

[lobsterbark]

Having a higher aoa on the runway. Wings don't produce lift at zero aoa, because ksps drag model is not complete yet.

[ferram4]

Put your CoL slightly behind the CoM in order to make sure your plane is stable. The further behind the CoM the CoL is, the more stable your plane will be; conversely, the further in front of the CoM the CoL is, the more unstable the plane will be; putting the CoL on the CoM will make the plane neutrally stable. The more stable your plane is the more control surfaces you need to change its direction, which is why the general rule is to put the CoL just behind the CoM: the plane is stable, but only a few control surfaces are needed to control it.

You also want to make sure to put the landing gear directly underneath the CoM; if you don't do that you can have a lot of trouble rotating the plane on the runway to create lift and take off.

[Varyon]

Well, that's the basics, straight from the expert. I'll try to keep those points in mind, but is there any reason you can think of why space planes would flip forward rapidly with a CoL in what has been stated as an optimal position?

[RoboRay]

Asymmetrical thrust? If the thrust vector of your engines is above the CoM, it will push the nose down. It can be a little off for jet engines, as long as you have sufficient aero control to hold the desired attitude, but putting it in line with the CoM simplifies things for you.

Or, you may just need a little adjustment to your elevator neutral position. Angle the rear-most horizontal control surfaces a notch or two upward and see if that helps.

Edited May 29, 2013 by RoboRay

[AnalogAddict]

Well, that's the basics, straight from the expert. I'll try to keep those points in mind, but is there any reason you can think of why space planes would flip forward rapidly with a CoL in what has been stated as an optimal position?

I've noticed at high altitudes, symmetrical placement of intakes is a must, especially if you're using the radial mount intakes. If you have a single radial intake on top of your fuselage without a matching one below then the drag from that intake will cause you to pitch upwards uncontrollably. Naturally, the opposite is true, one on the bottom means nose down at high altitudes. Disabling the intake will momentarily correct the issue; however for the end all solution remove them all together and use the ram air intakes. I'm not sure if the description is accurate on the item, but Ram Air intakes are for use at high altitudes so I deploy them as such.

[Mystique]

Well, that's the basics, straight from the expert. I'll try to keep those points in mind, but is there any reason you can think of why space planes would flip forward rapidly with a CoL in what has been stated as an optimal position?

I had same issue with my first plane made of whatever was there on store. I was planning to do some data collection around the globe (bad idea taking into account low time warp values) so I placed all scientific parts I had just behind cockpit. And also added 3 fuel fuselages. No matter what I tried with wings, control surfaces, etc, problem was solved only when I made plane shorter (-1 fuel fuselage) and moved most of scientific stuff backwards. That way CoM moved slightly backwards and I adjusted CoL to be right behind it. And it suddenly started to fly greatly

[Kosmo-not]

Put some small control surfaces on the back of your spaceplane. This has made my spaceplanes go from fliptastic pieces of **** to mostly unflipable.