help with spaceplane

[FusionNexus]

Hello! I've been building and testing a spaceplane (or rather, just a plane, it cant really go much higher than 13k altitude atm), and need some assistance. I'm playing with a good few mods, namely ones like kspie, near future stuff, mechjeb, and a whole bunch of other recommended stuff. This is technically my first fully functioning plane (can take off and land alright). But I'm currently having issues with stability mid flight, the plane likes to roll one direction (mainly to the left) or the other even without input, and does not like to stay pitched up, without input it eventually nosedives. I'm not entirely sure what to do to fix it. Every part on the sides of the fuselage was built using mirror mode.

here's some images I took after my last flight attempt:

[18Watt]

15 minutes ago, FusionNexus said:

does not like to stay pitched up, without input it eventually nosedives

That one is fairly easy- your COL is too far behind your COM.

15 minutes ago, FusionNexus said:

the plane likes to roll one direction (mainly to the left) or the other even without input

That is a little harder, could be a lot of things.

• Is the plane built symmetrically?  (I'm assuming it is, but just asking..)
• Have you accidentally introduced rudder or aileron trim?  If so, I think Alt-X will cancel.  If you're using SAS, the trim will be zeroed as well.
• Looks like you have two intakes, but three engines.  Is one engine getting starved of air?  KSP models resource flow to engines in unexpected ways.  If you are running out of air, it will take air away from one engine first, instead of reducing air to all three engines.
• The rudders, or large vertical fins, are placed too close to the COM in my opinion.  They become much less useful in that location- they should be placed much farther back on the plane to be effective.
• Double check that your control surfaces are tied to the correct axes.  In other words, you don't want an elevator or canard to be trying to control yaw.  That will introduce odd flight characteristics.

Edit:  I'd start by moving the large rudders as far aft as you can get them.  Also, after moving them aft you probably don't need those huge rudders.  Smaller rudders mounted as far back as you can get them would be better.

[FusionNexus]

4 minutes ago, 18Watt said:

That one is fairly easy- your COL is too far behind your COM.

That is a little harder, could be a lot of things.

• Is the plane built symmetrically?  (I'm assuming it is, but just asking..)
• Have you accidentally introduced rudder or aileron trim?  If so, I think Alt-X will cancel.  If you're using SAS, the trim will be zeroed as well.
• Looks like you have two intakes, but three engines.  Is one engine getting starved of air?  KSP models resource flow to engines in unexpected ways.  If you are running out of air, it will take air away from one engine first, instead of reducing air to all three engines.
• The rudders, or large vertical fins, are placed too close to the COM in my opinion.  They become much less useful in that location- they should be placed much farther back on the plane to be effective.
• Double check that your control surfaces are tied to the correct axes.  In other words, you don't want an elevator or canard to be trying to control yaw.  That will introduce odd flight characteristics.

Symmetrically, yes, all the parts besides the fuselage itself was built via mirror mode, so every part on each side of the vessel is on the same plane and axis as their counterpart on the other.

Ive never actually heard of this type of issue, just tried to test it, alt X doesnt do anything that i notice, but ill check controls if it can be found in there to see what the right keys are.

Im not 100% sure how air intake works, the intakes take in quite a lot of air, way more than the engines attached to them even need, I assumed that they require air intakes to actually work properly, but in flight the middle engine does fire and seems to run just fine

thanks for the rudder tip

I solved this issue earlier today after reading the basic mk2 spaceplane guide i found, all the control surfaces have been adjusted accordingly, rudders are the only thing controlling yaw, and the elevons on the outsides of the engines control roll, everything else controls pitch

[18Watt]

3 minutes ago, FusionNexus said:

I solved this issue earlier today

Awesome!  Welcome to the KSP forums!

 

[AeroGav]

22 hours ago, FusionNexus said:

 This is technically my first fully functioning plane (can take off and land alright). But I'm currently having issues with stability mid flight, the plane likes to roll one direction (mainly to the left) or the other even without input, and does not like to stay pitched up, without input it eventually nosedives. I'm not entirely sure what to do to fix it. Every part on the sides of the fuselage was built using mirror mode.

here's some images I took after my last flight attempt:

Grats on your first foray to the SPH.

I Like to zoom in a bit on that Stability analysis graph by setting a smaller scale.      A quick check to make sure the line keeps sloping down at up to 30 degrees is good to make sure there's no nasty deep stall tendency,   but outside of aerobatics you're going to be using no more than 10degree AoA.   Lift to drag ratio goes to the toilet after that.    So I usually fine tune with an AoA range of 10 and marks every 1.

 

As for the continual rolling problem,  I think there is a bug in the game engine that can cause a continual roll in one direction or other for no apparent reason. Its normally about 3 degrees per second or similar.   It may have something to do with attachment order or joint flexure.  Some of my craft do it , some dont.  It makes long range atmospheric flight a real PITA without some kind of mod FBW/Autopilot.  

 

As for the lawn dart tendency...     Yeah when fully fuelled your Center of lift is too far behind your centre of mass.   Blue arrow touching the back of the yellow ball is how i go.   It looks like your Centre of mass shifts rearward as fuel burns off,  and you had to set the wings real far back to stop the plane flipping out when the tanks are low.   You can see that  from the yellow (low fuel state) line on that graph being a much shallower slope than the full fuelled one.

To fix that CoM shift,  you've either got to :

 

• leave forward fuel tanks empty (but pay the drag penalty for these empty fuselage sections regardless),
•    find a way to shift engines futher forward,
• or find a way to add more fuel in the rear. (  Perhaps some big S strakes orientated vertically instead of the tail fin you chose, since that will add fuel tankage to the back end of your plane.  Likewise Big S strakes with elevons attached to the back instead of tailplanes. Not enough to fix the problem on its own though)

Yeah this is a perennial problem with KSP..

Just wait till you try balancing a plane with NERVAs...

RE: rolling

for some reason this plane doesn't do it and is quite steady.   Which proves that size and part count are nothing to do with it then..

 

Notice again, how i'm trying to get my dry CoM forward without making it look too wierd.  3  ton NERVs are mounted as far fwd as possible.    Next go 2 ton rapiers.  The 1.2 ton Panther engines are hung off the back....

[FusionNexus]

27 minutes ago, AeroGav said:

Grats on your first foray to the SPH.

I Like to zoom in a bit on that Stability analysis graph by setting a smaller scale.      A quick check to make sure the line keeps sloping down at up to 30 degrees is good to make sure there's no nasty deep stall tendency,   but outside of aerobatics you're going to be using no more than 10degree AoA.   Lift to drag ratio goes to the toilet after that.    So I usually fine tune with an AoA range of 10 and marks every 1.

 

As for the continual rolling problem,  I think there is a bug in the game engine that can cause a continual roll in one direction or other for no apparent reason. Its normally about 3 degrees per second or similar.   It may have something to do with attachment order or joint flexure.  Some of my craft do it , some dont.  It makes long range atmospheric flight a real PITA without some kind of mod FBW/Autopilot.  

 

As for the lawn dart tendency...     Yeah when fully fuelled your Center of lift is too far behind your centre of mass.   Blue arrow touching the back of the yellow ball is how i go.   It looks like your Centre of mass shifts rearward as fuel burns off,  and you had to set the wings real far back to stop the plane flipping out when the tanks are low.   You can see that  from the yellow (low fuel state) line on that graph being a much shallower slope than the full fuelled one.

To fix that CoM shift,  you've either got to :

 

• leave forward fuel tanks empty (but pay the drag penalty for these empty fuselage sections regardless),
•    find a way to shift engines futher forward,
• or find a way to add more fuel in the rear. (  Perhaps some big S strakes orientated vertically instead of the tail fin you chose, since that will add fuel tankage to the back end of your plane.  Likewise Big S strakes with elevons attached to the back instead of tailplanes. Not enough to fix the problem on its own though)

Yeah this is a perennial problem with KSP..

Just wait till you try balancing a plane with NERVAs...

RE: rolling

for some reason this plane doesn't do it and is quite steady.   Which proves that size and part count are nothing to do with it then..

 

Notice again, how i'm trying to get my dry CoM forward without making it look too wierd.  3  ton NERVs are mounted as far fwd as possible.    Next go 2 ton rapiers.  The 1.2 ton Panther engines are hung off the back....

Thx for the tips and explanations Aero, Using your mk2 guide as a reference helped with designing the plane overall, after this post I made a bunch of massive adjustments, and the use of AtmosphereAutopilot kinda trivializes flight (or well, makes it feel more game like if that makes sense). my current version now runs Direct Cycle nuclear engines from KSPIE, and is well capable of getting to orbit via fuel switching. thx for the help!

Edited August 27, 2022 by FusionNexus