Stability at low speeds and on the runway!

[Blue]

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For the life of me I can't get this thing to fly reliably. This small SSTO was inspired and developed from Narcosis's Jian SSTO, using a Sabre engine instead of the Turbojet. While Narcosis's flies in a very stable and easy manner, my version has been nothing but trouble as long as it's near the ground.

The primary issue at hand is the fact that it wobbles uncontrollably once its speed exceeds 20 meters per second. If I pull up hard and cross my fingers, perhaps I can get it off the ground, in which case the only safe way to fly it is straight upwards until it's 10km high and going faster than 250m/s (and even before them, it's still pretty wobbly).

I'm completely out of ideas on how to fix it. I've tried many different landing gear configurations and shifting the center of mass around as much as I could. There are a couple of things which I remained invariant about, like trying to keep the landing gear within the fuselage when retracted, and trying to conceal the RCS tank by more than half, but I can't get this thing to drive at all.

Edited April 1, 2014 by Blue

[GDJ]

Double check to see if the wheels are not toed in or out, and do the SHIFT + orientation keys to get them as close to straight up and down as possible. KSP doesn't seem to like toe in/out and caster/camber on wheels.

[Claw]

When you say "wobbly in flight," is it wobbly in pitch or yaw?

- Yes, your landing gear definitely looks angled out to the sides. You'll want them straight up and down or else your craft will fishtail wildly. The rapier engine has a bit more vectoring which makes the problem a bit worse. Use SHIFT+WASDQE as GDJ pointed out above.

- It looks like you might have angled your wings down in the front (hard for me to tell in the picture). I'm not talking about the dihedral, but you have negative AoA on your wings. That will cause downforce as you accelerate, which will make the bending on the angled landing gear worse.

- Your CoL is also below your CoM. And I suspect it's even worse than it looks because of the landing gear causing the CoM to be lower. This will cause your plane to be a bit unstable.

- There is a bug with stock aerodynamics and flight control surfaces (not wings). As AoA increases, lift from the flight control surfaces continues to increase. For your design, this will cause your aircraft to pitch down which is okay, but might be contributing to your wobbly nature because you're using an angled twin-V tail design. Try disabling the pitch and roll axes for the V-tail.

- Also, the landing gear causes the CoM marker in the SPH to lie. Although landing gear has mass in the SPH, it is considered massless in flight by the physics engine. Remove your landing gear and tweak your CoM/CoL markers. It's probably okay, but it's hard to tell from the pictures. If it's too far forward then it will cause problems when you try to pitch up.

- It also looks like you could remove the I-beams. The only thing I see attached to them is the monopro tank, but you could attach that to the underside of your probe core. Your landing gear is probably better attached to the body. That's mostly personal opinion so if you want the I-beam there then have at it.

Edited April 1, 2014 by Claw

[generalguy26]

[deleted]

Edited March 21, 2023 by generalguy26

[Claw]

The I-beams were there to show the plane expanded piece-by-piece. I don't think they are there in the final design.

Duh, makes sense. I suppose you can disregard that. haha

[lammatt]

dont angle your yaw control.

it's better (easier) to fly with orthogonally placed control surfaces instead of angled ones.

and dont angle the landing gears too (it will make you tilt to one side and crash when you try to take off)

[O-Doc]

Have you tried tweaking the pitch out of your tailfins?

[Blue]

Thanks for your suggestions, guys.

Straightening out the landing gear immediately worked to eliminate taxiing and takeoff wobble, despite now in the new revision its wheelbase is narrower than ever.

I'll continue working to improve the gear configuration so as to keep it wide and stable, far enough back so I don't get engine-destroying tailstrikes, and still remain compact.

I am fairly certain the edit to the rudder will help as well. Though for clarity, the wings never had a modified angle of attack. They were always just dihedral low mountings.

[mellojoe]

Put the rear landing gear on the wings. Turn OFF angle snap, as that makes weird interaction between gear and wings. I find that this keeps the front gear longer but allows you to put the rear gear straight and still get that wide base. Give it a shot. GOOD LUCK