Big big big spaceplane trouble (plane aligns to retrograde marker on runway)

[Stop_signGEORGE]

I'm trying to make a high altitude supersonic plane for a contract in Realism Overhaul. So far I am failing horribly. On the runway, once it picks up speed, the plane turns around goes butt-first along the runway until the engine stops it, as if the center of lift is in front of the center of mass. I am using FAR.

Screenshots:

As you can see in the last picture the plane lines up with the retrograde marker automatically, even with SAS on. Any ideas?

[bewing]

This kinda sounds like a wheel issue? Try lowering the friction of the front wheel to .2 or less. Maybe increase the rear wheels to 1.4 or more.

(And maybe try your takeoff roll a little more gently? )

Edited July 11, 2016 by bewing

[Stop_signGEORGE]

31 minutes ago, bewing said:

This kinda sounds like a wheel issue? Try lowering the friction of the front wheel to .2 or less. Maybe increase the rear wheels to 1.4 or more.

(And maybe try your takeoff roll a little more gently? )

Okay, that solved my wheel problem. My next problem is that I topple in the air too, and my nose bounces randomly on the runway so hard that it flips my craft all the way onto its back and destroys the cockpit.  Ugh, jet planes are hard. I don't know why I am having such a hard time, because my first propeller plane flew like it wasn't a feat.

[bewing]

go, Stop_signGEORGE said:

My next problem is that I topple in the air too, and my nose bounces randomly on the runway so hard that it flips my craft all the way onto its back and destroys the cockpit.

Heh. Yeah, some planes just want to be quirky. For the nose bouncing problem, try reducing the spring and damper on the front wheel to .5 each and see what happens.

For aerodynamic control problems, my answer is always canard wings on the nose. Maybe that'll work this time, too.

Edited July 11, 2016 by bewing

[Stop_signGEORGE]

I don't think it's the wheels, but the runway. There are certain points on the runway that launch my plane into the air no matter what.

[bewing]

2 hours ago, Stop_signGEORGE said:

Well, the grass next to the runway is not perfect, but you still may get a better takeoff run from the grass. It also has a few little dips and things that may toss you into the air. But maybe your nose won't bounce.

[Snark]

A few things.

Your vertical stabilizer really sticks up high above the body of the plane.  That can cause stability problems by inducing too much roll when it tries to yaw the plane. If you can move it downwards so it doesn't stick up so high, that will help.

Make sure you have turned off roll authority on your vertical stabilizer. This is really, really important . If you don't do that, it will try to help you roll the plane, which will induce yaw, so it ends up fighting itself and therefore loses much of its effectiveness in providing yaw stability.

As shown, I expect that your plane is going to have real trouble getting its nose up when taking off. You've put your rear landing gear almost directly under the only pitch controls on the plane (i.e. the ailerons on the back of the wings). Since the only way your plane can pitch up is by pushing the tail down, and since you've put the rear gear way back there, that means it'll be hard to rotate off the ground. This, in turn, means that you'll have to get up to a really high speed on the runway before you can take off, while simultaneously putting LOTS of load on those rear gear, which is going to make you very vulnerable to bad instability while zipping down the runway.

There are a couple of ways to solve that. First, move the rear landing gear forward, closer to the CoM. This will make it easier to rotate the nose up during takeoff. Second, consider putting canards on the front of the plane. That helps by giving you a pitch control that works by lifting the nose rather than pushing the tail down. Just make sure to disable roll authority on the canards.

Finally, one last comment. From the wording of your post, it appears that you're under the impression that having your CoL behind your CoM will make you prograde-stable. That's a not the case. It's a very widespread KSP myth that a lot of players believe, but it's wrong. So don't rely on the CoL marker alone when you're trying to figure out stability issues.

[Van Disaster]

I would imagine it's wheelbarrowing & digging the front gear in, because the rear wheels are too far aft. Add to that that I suspect ( going on the ball positions ) that most of your pitch is going to go towards countering inherent nose-down I'm not holding out much promise that you're going to have a trouble-free takeoff.

You're using FAR, so the CoL ball is misleading & not useful other than for gross positioning of flight surfaces ( Ferram would tell you to turn it off - I use it as a quick check that I've not done something stupid ). Learn to use the static derivative panel at a variety of heights & airspeeds, it's less scary than it looks.

Last thing to do is to have it pitched up slightly on it's wheels, that way it'll want to start taking off by itself.