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My plane keeps spinning out when I try to turn.


Curebores
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Hi there. I'm having problems with a plane I am trying to build.

 

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This plane can take off (if a little bit squirrley), fly straight, climb to 6k+ (or 30k+ with the rocket), and land.

 

The problem comes when trying to do any kind of sustained turn. You can just about fight it round in small increments, but try and turn hard either by banking or roll/pitch and it will have you doing aerobatics until you hit the ground.

 

A previous version of the plane flew very well, with none of those problems. It had no science or service bay (0.35t less weight and about 1.5m shorter) and different landing gear (the basic ones, which would randomly explode sometimes) but otherwise identical. Obviously the wings etc were in slightly different positions for balance.

 

I have no idea what's wrong with it. According to everything I know about aircraft design in KSP (Which admittedly is only slightly more than nothing) this thing should fly true. It is too long/heavy? Have I just completely messed up everything?

 

Any help would be appreciated.

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Welcome aboard!

The CoL is a bit close to the CoM for good stability, try adjusting your wings rearward until the CoL marker is half in and half out of the CoM marker.

I'd also make sure that pitch is disabled for the control surfaces on the wings, they can reverse sometimes when the wings are attached ahead of the CoM but the surfaces are behind it.

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41 minutes ago, Red Iron Crown said:

Welcome aboard!

The CoL is a bit close to the CoM for good stability, try adjusting your wings rearward until the CoL marker is half in and half out of the CoM marker.

I'd also make sure that pitch is disabled for the control surfaces on the wings, they can reverse sometimes when the wings are attached ahead of the CoM but the surfaces are behind it.

Yep, I think his pitch is just so hard that it causes the plane to stall.

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46 minutes ago, Red Iron Crown said:

Welcome aboard!

The CoL is a bit close to the CoM for good stability, try adjusting your wings rearward until the CoL marker is half in and half out of the CoM marker.

I'd also make sure that pitch is disabled for the control surfaces on the wings, they can reverse sometimes when the wings are attached ahead of the CoM but the surfaces are behind it.

 

That seems to have done the trick. Thank you very much.

 

Another quick question. Using the rudder for more than a second or so at a time will cause the plane to roll rather than yaw, which makes the fine tuning for landing a bit of a challenge. Any idea why that's happening?

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2 hours ago, Curebores said:

Another quick question. Using the rudder for more than a second or so at a time will cause the plane to roll rather than yaw, which makes the fine tuning for landing a bit of a challenge. Any idea why that's happening?

I would think because the rudder is quite large for such a small plane, it overpowers the other surfaces' ability to resist the roll it induces. You could try tweaking down the control authority on the rudder or using a fixed surface with a smaller moving surface at the back.

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You can limit the amount of authority on the rudder if it is overpowered.

I usually don't use the 'Tail Fin' for a tail fin on small (Mk1 or Mk2 under 20t) planes just because it is such a powerful control surface. The Standard Canard is a good tail for those tiny planes.

Also, add a slight amount of dihedral angle (tilt your wings and tail fins upward) which will help with roll stability. It will move the CoL slightly above the CoM which will cause the plane to re-level itself when you're not applying any control inputs. Imagine the plane to be "hanging" from the lift forces like a set of strings, which converge in a single point straight above the CoM if you tilt the wings upwards slightly.

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18 hours ago, GoSlash27 said:

I actually disable yaw response completely on my rudders unless I happen to be building an aerobatics plane. Most planes fly better without it. Elevators respond only to pitch and ailerons respond only to roll.

Best,
-Slashy

 

That leaves your only answer to adverse yaw being pod torque (or RCS, but we're talking planes) and natural stability. I don't think that's a great place to be in. You don't necessarily need a lot of aerodynamic yaw authority -- a delta-deluxe would probably do here, with how small the plane is and how much torque the pod can provide relative to it -- but you should have some.

 

Speaking of which, try disabling the pod torque. It may be pushing your aero surfaces into stall, rendering them useless.

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2 hours ago, foamyesque said:

 

That leaves your only answer to adverse yaw being pod torque (or RCS, but we're talking planes) and natural stability. I don't think that's a great place to be in. You don't necessarily need a lot of aerodynamic yaw authority -- a delta-deluxe would probably do here, with how small the plane is and how much torque the pod can provide relative to it -- but you should have some.

 

Speaking of which, try disabling the pod torque. It may be pushing your aero surfaces into stall, rendering them useless.

foamyesque,

 I definitely agree with your suggestion to disable the pod torque.

As for the subject of adverse yaw, IME the only source of that in KSP for a reasonably designed plane *is* the yaw authority itself. Particularly when SAS is engaged. That is to say; airplanes in KSP with a vertical tail will not sideslip unless commanded to. There is therefore no reason to have active rudders on anything other than aerobatic aircraft.
 If you disable yaw response entirely, you can fly with the SAS engaged without the normal cross- controlling it tries to induce and turns become much smoother.

Best,
-Slashy

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5 minutes ago, foamyesque said:

@GoSlash27: I can't agree about the lack of sideslip. Certainly, large fixed tails will act to minimize it as well, but tailless planes will sideslip very easily indeed. This can be problematic :v

True... but tailless planes don't have rudders, active or otherwise :P

For planes that *do* have vertical tails, they fly better if those vertical tails don't respond to yaw inputs. They are also not likely to ever encounter a situation where active yaw is needed.
 Trust me on this one. Better yet, try it yourself. You'll see what I mean.

Best,
-Slashy

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I have actually done both variations. I tend to build very lightly stabilized planes (space or otherwise), so I notice the presence or lack of yaw authority pretty readily. Especially during reentry, a yaw control surface is a huge help in keeping you nose-to-airstream, in situations where any sideslip at all can be very, very bad.


EDIT:

 

They're also useful for landing approach, of course.

 

 

Edited by foamyesque
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