Help with spaceplane sideslip

[A_name]

I’m having some sideslip issues with one of my new spaceplanes. Basically, the plane insists on yawing to the left.

[URL="http://imgur.com/a/JdKju"]Here is an album with some screenshots. [/URL]

Notes: (1) the plane is not suffering from asymmetrical thrust or air intake, (2) the problem occurs with several types of tail planes, (3) it happens whether I activate or deactivate yaw control on the rudder (in the screenshots where yaw control is deactivated, the yellow line I think just represents the rudder trying to keep the plane straight in its static condition due to its aerodynamic properties), (4) I’ve checked that all parts are symmetrically attached, (5) yaw is disabled on all control surfaces other than the rudder (on some tests as stated above). I can’t for the life of me figure out why this is happening, but needless to say it makes the design practically un-flyable. Please help!

Incidental question: Can wing parts even be used as tail planes? As you can see in one the bonus screenshot, before even taking off, the wing begins providing lateral lift (left in this case), as if it has decided that way is “up” and so its lift should pull in that direction. This came up during my testing of different versions of tail planes and I’m curious if this is normal behavior. Edited November 20, 2015 by A_name

[Hodo]

[quote name='A_name']I’m having some sideslip issues with one of my new spaceplanes. Basically, the plane insists on yawing to the left.

[URL="http://imgur.com/a/JdKju"]Here is an album with some screenshots. [/URL]

Notes: (1) the plane is not suffering from asymmetrical thrust or air intake, (2) the problem occurs with several types of tail planes, (3) it happens whether I activate or deactivate yaw control on the rudder (in the screenshots where yaw control is deactivated, the yellow line I think just represents the rudder trying to keep the plane straight in its static condition due to its aerodynamic properties), (4) I’ve checked that all parts are symmetrically attached, (5) yaw is disabled on all control surfaces other than the rudder (on some tests as stated above). I can’t for the life of me figure out why this is happening, but needless to say it makes the design practically un-flyable. Please help!

Incidental question: Can wing parts even be used as tail planes? As you can see in one the bonus screenshot, before even taking off, the wing begins providing lateral lift (left in this case), as if it has decided that way is “up” and so its lift should pull in that direction. This came up during my testing of different versions of tail planes and I’m curious if this is normal behavior.[/QUOTE]


In a nutshell, you lack enough vertical stabilization for that craft.

I dont have the time to go into more detail right now sorry. But your tail on that size of craft isnt quite big enough. This is leading to some of your issues. Make sure your vertical stabilizer is far enough from your CoM to act as a lever. In addition to that make sure you have enough control surface on it to actually work. It is generally best to error on the side of caution and have a slightly to big tail then not enough.

[numerobis]

That's a tiny tail for such a big plane, no way it'll keep things in check on its own.

That said, I often build planes with literally zero vertical stabilizers (saves mass!) and fly ok -- sure I suffer a bit of slip sometimes, but a small roll will correct it.

[A_name]

[quote name='Hodo']In a nutshell, you lack enough vertical stabilization for that craft.

I dont have the time to go into more detail right now sorry. But your tail on that size of craft isnt quite big enough. This is leading to some of your issues. Make sure your vertical stabilizer is far enough from your CoM to act as a lever. In addition to that make sure you have enough control surface on it to actually work. It is generally best to error on the side of caution and have a slightly to big tail then not enough.[/QUOTE]

[quote name='numerobis']That's a tiny tail for such a big plane, no way it'll keep things in check on its own.

That said, I often build planes with literally zero vertical stabilizers (saves mass!) and fly ok -- sure I suffer a bit of slip sometimes, but a small roll will correct it.[/QUOTE]


Hi, thanks for replying. I took your advice and added two more Big-S Spaceplane Tail Fins. It's now flyable but it's still doing it. Enough that it's thrown me a couple degrees off heading two minutes into the flight. [URL="http://i.imgur.com/A8Yn6IE.png"]Here's a screenshot[/URL]. These tail fins have a lift rating of 3.5, 25° of deflection. There's three of them total on a 65 ton craft and they're as far back as they could possibly be. Surely this should be flying dead on like an arrow. Yet something is causing it to veer to one side. Any other ideas? Edited November 19, 2015 by A_name

[fireblade274]

You said you have tried restricting the tail fins to be the only "yaw" part. Have you extended this so the wing elevators only "pitch", and maybe one elevator on each wing to only "roll"?

I think the main contributor to this issue is that your craft is very wide and thin. Could you post the craft file? If I find time maybe I could take a look at it and do some tinkering. Edited November 19, 2015 by fireblade274

[Val]

Judging by the pictures, it seems to me that the problem is that your wings are not level. Every picture shows you banking slightly, even the one on the runway.

Normally, that would cause the craft to turn slowly, but because SAS is on, it'll try to hold it's heading and start sideslipping instead.

I usually disable Yaw on all control surfaces so there's only passive yaw stability. That will stop the sideslipping when you bank and will cause the craft to turn instead (unless you have strong reaction wheels or the vertical stabilizer is too small, in which case they will cause it to sideslip, same as rudders) Edited November 19, 2015 by Val
Small vertical stabilizer added

[A_name]

[quote name='fireblade274']You said you have tried restricting the tail fins to be the only "yaw" part. Have you extended this so the wing elevators only "pitch", and maybe one elevator on each wing to only "roll"?

I think the main contributor to this issue is that your craft is very wide and thin. Could you post the craft file? If I find time maybe I could take a look at it and do some tinkering.[/QUOTE]

Thanks for the comments! Yes, all my elevators are restricted to only pitch and the small ailerons on the edge of the wings are restricted to roll.

It is indeed a wide and thin craft, do you think that was a bad design choice?

I'll try to post the craft file tonight as I'm at work right now :)

[COLOR=silver][SIZE=1]- - - Updated - - -[/SIZE][/COLOR]

[quote name='Val']Judging by the pictures, it seems to me that the problem is that your wings are not level. Every picture shows you banking slightly, even the one on the runway.

Normally, that would cause the craft to turn slowly, but because SAS is on, it'll try to hold it's heading and start sideslipping instead.

I usually disable Yaw on all control surfaces so there's only passive yaw stability. That will stop the sideslipping when you bank and will cause the craft to turn instead (unless you have strong reaction wheels or the vertical stabilizer is too small, in which case they will cause it to sideslip, same as rudders)[/QUOTE]

Thanks for replying! I get what you're saying, but I don't think that's the issue. I think the plane is banking in the pics [I]as a result[/I] of the sideslip (except for that last screenshot from the first album, on the runway which is when I tried using a wing part as a tail fin and it decided right was up and pulled my tail in that direction; you can see the blue arrow coming out of the tail fin). My reasoning is that the bank is induced by the fact that the vertical stabilizer is so tall. The direction of the bank seems consistent with this theory. Also, the wings are attached using angle snap and mirror symmetry, so I don't see how they would become un-level. They do have some positive incidence on them, to help me keep a low angle of attack during ascent, but that shouldn't induce a sideways rotation.

I did try disabling all yaw at some point but it was slightly worse than with yaw enabled on the rudder.

I really appreciate you guys helping me on this wild goose hunt! :) Edited November 19, 2015 by A_name

[Val]

[quote name='A_name']Thanks for replying! I get what you're saying, but I don't think that's the issue. I think the plane is banking in the pics [I]as a result[/I] of the sideslip (except for that last screenshot from the first album, on the runway which is when I tried using a wing part as a tail fin and it decided right was up and pulled my tail in that direction; you can see the blue arrow coming out of the tail fin).[/QUOTE]That is a possibility. If your CoL is above your CoM, then sideslip can cause the craft to bank.

[quote name='A_name']My reasoning is that the bank is induced by the fact that the vertical stabilizer is so tall. The direction of the bank seems consistent with this theory.[/QUOTE]A rudder is also an aileron mounted vertically. If it is mounted above CoM (as they normally are), then using the rudder will cause banking in the opposite direction of the yaw direction. Usually, when the rudder is far enough behind the CoM, the yaw force is much stronger than the roll force, so it is easily and unnoticeably compensated for, especially if you actively bank in to the turn, which is what you normally do.

With a craft where the distance from CoM to the rudder is very short, as it looks to be on your craft, the banking force, from the rudder, becomes much more noticeable.

[quote name='A_name']Also, the wings are attached using angle snap and mirror symmetry, so I don't see how they would become un-level. They do have some positive incidence on them, to help me keep a low angle of attack during ascent, but that shouldn't induce a sideways rotation.[/QUOTE]I meant the whole craft is not level, as in, you're banking slightly.

[quote name='A_name']I did try disabling all yaw at some point but it was slightly worse than with yaw enabled on the rudder.[/QUOTE]This is a sign that the tailfin is not big enough/far enough back.

[quote name='A_name']I really appreciate you guys helping me on this wild goose hunt! :)[/QUOTE]Trying the best I can. :D

Could you post a picture from SPH with CoM and CoL, so we can better judge how much leverage your tailfin(s) has? Edited November 19, 2015 by Val
Removed incorrect stuff. Rephrased. Grammar.

[A_name]

Well you guys, I'm going to mark this as answered now. I finally settled on a design that uses 2 of the large spaceplane tail fins. There's still some sideslip, but it's flyable enough.

Val, since you asked, [URL="http://i.imgur.com/9gUhzpi.jpg"]here is a picture of the plane in the SPH[/URL]. In retrospect, my CoM does sit rather back on it and thus closer to the rudders.

Final conclusions:

-You might need more rudder than you think especially if your CoM is more to the rear of the plane.
-At least for this particular design, active yaw control is better.
-Bad piloting skills can enhance the problem, i.e. allowing the plane to enter a bank and not correcting often enough.

Thanks again for all the help!

[*Aqua*]

Because it wasn't mentioned here:

There's also the possibility that your plane [I]bends[/I] from lift, thrust, weight, etc. This bending isn't always symmetric because of rounding problems. You can minimize this problem by adding struts.


Especially in the early KSP versions this was a huge problem because part connections weren't as rigid as they are today.

[Hodo]

[quote name='A_name']Well you guys, I'm going to mark this as answered now. I finally settled on a design that uses 2 of the large spaceplane tail fins. There's still some sideslip, but it's flyable enough.

Val, since you asked, [URL="http://i.imgur.com/9gUhzpi.jpg"]here is a picture of the plane in the SPH[/URL]. In retrospect, my CoM does sit rather back on it and thus closer to the rudders.

Final conclusions:

-You might need more rudder than you think especially if your CoM is more to the rear of the plane.
-At least for this particular design, active yaw control is better.
-Bad piloting skills can enhance the problem, i.e. allowing the plane to enter a bank and not correcting often enough.

Thanks again for all the help![/QUOTE]


You dont have to go huge.

[img]http://i.imgur.com/CdzGUwD.jpg[/img]
This is one of my UAV SSTOs and it has very small vertical stabilizers but they are more than adequate for the job.

Then there is this... my old and trusty F-119C/A Akula
[img]http://i.imgur.com/xcbSHN7.jpg[/img]

And its replacement the F-134
[img]http://i.imgur.com/7Qhiulx.jpg[/img]

Then the SP-365 which is my current medium lifter SSTO.
[img]http://i.imgur.com/iVIUKr0.jpg[/img]
[img]http://i.imgur.com/XVIfmcH.jpg[/img]

Notice the tail can be big, or small, but its position plays a huge factor in its effectiveness.