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Why does my spaceplane not go straight on the runway?


fulgur
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Essentially, the plane wiggles about controllably for a while and then veers off, either into the sea or into the SPH.

3Hc5w3m.jpg

I often have problems with Mk3 planes. I never have this problem with Mk2 planes, or, oddly, the MkIV spaceplanes mod. Does anyone have an idea how I can stop my plane from doing this, and get it to launch safely?

Incidentally, there is a Mun station module in this SSTO. It is autostrutted into the cargo bay.

ZPPzzlH.jpg

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Besides the very common issue @bewing explains and the others he mentions, I see other potential causes. But you haven't shared the craft file, so it's guessing.

  • Are your engine pods and main gear actually attached to the wings, or are they attached to the body and offset to the sides? Wings have very flexible connections by default, and those heavy pods with powerful vectored thrust can cause the wings to independently flex higher and lower, making it veer off course. It's better to attach engines and gear to the body and offset them, so the wing flexibility doesn't affect them.
  • It looks like your CoL is very far aft. As your plane picks up speed and lift increases, it will tend to want to lift its butt before the nose, at which point the main gear have less and less contact with the tarmac - leading to the dreaded wheelbarrowing. Consider designing the plane with a CoM/CoL a bit closer to the body center, or add flaps ahead of the CoM that move the CoL forward during take off.
  • You have placed your main gear in series instead of parallel, and with a good bit of distance between them. Additionally, they are very far aft as well. What this does is create considerable difficulty for the plane to rotate, adding additional stress to the aft gear, which also causes them to 'buckle' slightly in random directions. This too will make the plane veer off.
  • Finally, as you accelerate and get near to take off speeds, aerodynamic forces start to overwhelm gear friction/steering, and it's actually your rudders taking over. But these too look to have very little leverage in your design... if I'm not mistaken, the CoM is quite far aft and very close to them, leaving very little arm to work with. Mk3 parts have quite high drag when even slightly off perfect prograde, so the whole body in front of the CoM probably create strong yaw forces than the rudders, with so little leverage, may not be able to correct sufficiently.
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This is actually a fairly common problem. Common enough that it's in the FAQ on this forum.

There are many possible causes, from overloading the front gear, to wheelbarrowing from having your rear gear lift off first, to "automotive drag". And sadly, each of those problems has a different fix.

However, the easiest thing to try fixing first is the automotive drag issue. And I suspect it is also the most common problem. So, the basic point of it is that to achieve passive directional stability what you need is to have more drag in the back end than at the front end. The "friction" of the wheel system does not take care of this for you automatically. So what you do is, in the SPH, select the front gear, open the context menu, click the button to switch to Manual Friction, and set the friction to .5 or below. Then you can also select the rear gear, set it to Manual Friction, and raise it to 1.3 or above. 

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1 hour ago, bewing said:

So what you do is, in the SPH, select the front gear, open the context menu, click the button to switch to Manual Friction, and set the friction to .5 or below. Then you can also select the rear gear, set it to Manual Friction, and raise it to 1.3 or above. 

I also like to raise the braking power on the rear wheels and lower it on the front wheels. This is more of a concern on landing than takeoff, but it helps with stability when braking.

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All of the above is no doubt good and something in it solved your problem.

The most common cause for steering out of the true for me is a simple thing.,  When you place wheels, without snap and/or possibly with it, they may not be exactly aligned on the absolute direction.  So I have a ritual of turning on Snap Lock after placing them and then wresting them in Absolute Rotation mode to line up with the airplane's longitudinal axes and, notably also the vertical axis laterally.  If your craft tends to track to one side and then reverse and steer opposite, this kind of instability is most likely due to the main gear being splayed either inward or outward, causing one side to track better than the other as weight (centrifugal force, actually) shifts to one side and then to the other causing a nasty oscillation.

Just ensure it's all aligned.

Edited by Hotel26
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It's more unusual to have a plane of any size that doesn't lurch back and forth down the runway like a drunken sailor.

What helps is more wheels on the runway, so one try using bigger landing gear, and two the best method I've found for fixing this is to give them twin side by side nose gear. Yes it's extra mass, but it's better than crashing on takeoff.

Also remember to disable/lock steering on the main gear, and it helps to set the nose gear steering limiter down for both low and high speeds.

Edited by vossiewulf
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I'm not certain, but it looks like the fuel tanks attached to the wing tips might be slightly misaligned.

If they're not pointing perfectly forward it might be contributing to your problems.

You can use absolute rotation, rather than relative, to make sure they're aligned correctly.

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Thank you! I wasn't able to attach the engine pods to the main body as I couldn't pull them out very far and so the plane fell over, but I tried your other suggestions and it now lifts off with only the minimum of wobbling.

AliTKBH.png

I also moved the wings closer to the CoM, which due to cargo is actualy quite far forwards.

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