Spaceplane loops then changing climb rate.

[magnemoe]

I made an spaceplane with the new J parts, it works very well, only gripe is that the huge engines should either be pure jets or behave like rapier.

However I have a weird issue, plane is unstable along the climb axis. Typically I take off and climb until Ap is 20 km, then level out however on this plane once I start level out the plane will continue into decent and typically ending up facing straight down or even backward.

This at 300 to 600 m/s.

I have an second set of radial intakes at bottom so drag should be similar. the intakes should also give drag at the back of the plane.

Center off mass don't change much

Stock aerodynamic and no extra reaction wheels. For other planes with the MK2 parts the problem tend to be that its hard to change the climb rate.

[worir4]

Make sure that your centre of mass is on or infront of your center of lift. If it is behind then your plane will flip.

[LordFjord]

Your fuel tanks are all in the rear part of your plane. When you use up fuel, your center of mass shifts to the back -> center of lift gets in front of your center of mass, then it becomes unstable. RCS Build Aid can help you find the right balance of fueled/dry craft.

Edited November 26, 2014 by LordFjord
corrected stupid Kerbalish advise :)

[Scarecrow88]

Make sure that your centre of mass is on or infront of your center of lift. If it is behind then your plane will flip.

Your fuel tanks are all in the rear part of your plane. When you use up fuel, your center of mass shifts to the front -> it gets in front of your center of lift, then it becomes unstable. RCS Build Aid can help you find the right balance of fueled/dry craft.

Excellent. 2 replies, and each giving conflicting advice. How typically Kerbal.

[LordFjord]

Thanks for pointing that out, I of course meant: "shifts to the back" - and I mixed up mass and lift.

Two wrongs makes one right?

I've corrected it now

Cheers

[Red Iron Crown]

CoL/CoM relationship looks good, it sounds like a control authority issue to me. There's not a lot of reaction wheel torque on that craft, and the control surfaces used are rather small for such a massive craft. I would try replacing the control surfaces at the rear of the wing with larger ones, making sure they're all enabled for the pitch axis.

[Hodo]

The CoM is shifting back as the fuel is draining from the craft, till it is behind your CoL, this is causing the front to generate more "lift" then the rear thus making it do flips.

Welcom to Kerbal Soupo'sphere.

[magnemoe]

The CoM is shifting back as the fuel is draining from the craft, till it is behind your CoL, this is causing the front to generate more "lift" then the rear thus making it do flips.

Welcom to Kerbal Soupo'sphere.

Should not move back much as the CoM is close to center of tanks and they drain even, however it might be enough.

Also i guess using the controll surfaces change CoM, also then rotating the plane get an momentum and then movement end i'm in thin atmosphere too.

Will do some more testing.

[Hodo]

The CoM will move back because the engines are at the rear of the craft, and they account for a large chunk of dead weight.

Check your CoM by right clicking and empting each one of your fuel tanks and see where your CoM is in relation to the CoL.

[MarvinKitFox]

300m/s, at 20km alt?

You are not flying, you are standing on your jet thrust.

You need a lot more speed, or a lot more wing, to stay airborne at 20km.

[Hodo]

300m/s, at 20km alt?

You are not flying, you are standing on your jet thrust.

You need a lot more speed, or a lot more wing, to stay airborne at 20km.

You are aware that 300m/s is pretty damned close to mach 1 right?

And 20km 300m/s isn't really that slow. It isn't anywhere near orbital velocity but it isn't standing still either.

[GoSlash27]

I have no idea what the "J parts" are, so the following might not be terribly helpful...

300-600 m/sec at 20KM up *is* a bit pokey. I can see how your controls would be railed nose- up trying to hold the attitude, then overwhelmed when you suddenly change the setting. You might try a shallower profile above 15KM and see if the extra airflow over your control surfaces and wings helps add stability.

A good generic climbout profile would be as follows: 45* pitch to 10KM, 30* pitch to 15KM, 20*pitch to 20KM, then 10* pitch thereafter.

You state that your fuel tanks are reasonably centered and from what I can see I have no reason to doubt you. Everything looks pretty well balanced to me. I think if you just fly it a little faster your problem will resolve itself.

Good luck!

-Slashy

[Hodo]

I have no idea what the "J parts" are, so the following might not be terribly helpful...

300-600 m/sec at 20KM up *is* a bit pokey. I can see how your controls would be railed nose- up trying to hold the attitude, then overwhelmed when you suddenly change the setting. You might try a shallower profile above 15KM and see if the extra airflow over your control surfaces and wings helps add stability.

A good generic climbout profile would be as follows: 45* pitch to 10KM, 30* pitch to 15KM, 20*pitch to 20KM, then 10* pitch thereafter.

You state that your fuel tanks are reasonably centered and from what I can see I have no reason to doubt you. Everything looks pretty well balanced to me. I think if you just fly it a little faster your problem will resolve itself.

Good luck!

-Slashy

That is an odd climbout profile for FAR. If you have a craft that can maintain a 45deg climb to 10km is great. But after that your air intakes will start to lose airflow, and it is generally best to bring your climb to 10-15deg at 12-15km all the way to 25km. You should accelerate to over mach 5 by the time you hit 25km.

[GoSlash27]

That is an odd climbout profile for FAR.

Unless I'm mistaken, the OP said he was using stock aerodynamics. If that's not the case, please disregard everything I've said here. My advice only applies to stock aero.

Best,

-Slashy

[Hodo]

Unless I'm mistaken, the OP said he was using stock aerodynamics. If that's not the case, please disregard everything I've said here. My advice only applies to stock aero.

Best,

-Slashy

Nope your right, I was thinking of another thread.

[magnemoe]

I have no idea what the "J parts" are, so the following might not be terribly helpful...

300-600 m/sec at 20KM up *is* a bit pokey. I can see how your controls would be railed nose- up trying to hold the attitude, then overwhelmed when you suddenly change the setting. You might try a shallower profile above 15KM and see if the extra airflow over your control surfaces and wings helps add stability.

A good generic climbout profile would be as follows: 45* pitch to 10KM, 30* pitch to 15KM, 20*pitch to 20KM, then 10* pitch thereafter.

You state that your fuel tanks are reasonably centered and from what I can see I have no reason to doubt you. Everything looks pretty well balanced to me. I think if you just fly it a little faster your problem will resolve itself.

Good luck!

-Slashy

J- part is the moded fuselage and the huge jets, they are around 4 times larger than the MK2 but pretty light for their size.

Speed is probably higher after I end the climb but i climb fast out of the dense atmosphere, then level out to gain speed, no reason to stay there.

However this plane is pretty heavy as in 75 ton, i wonder if the wing surface is to small to support horizontal flight at 20 km with the low speed.

I'm used to the MK2 part and planes half the weight.

[GoSlash27]

That looks like plenty of wings to me. Roughly 1.0 lift coefficient per ton is adequate. Probably just maintaining more speed will solve the problem. I'm usually doing about 900 when I cross 20km.

-Slashy

Edited November 27, 2014 by GoSlash27

[Kryxal]

I find myself wondering where the CoT is, and more on the vertical than anything else. Given the CoM-CoL placements, it should have some nose-down tendency, but not THAT much. More speed would help a great deal with that, of course...

[sniped1928]

Actually they are both right I ran tests were if the center of lift is to far forward or back it will flip, I find that as long as the center of lift is in the center of mass bubble or touching it you'll be ok.

[magnemoe]

Did some more testing on an second design, more wings COL a bit more back from COM with slower accent from 15 to 20 km, worked far better however at 22 km and 900 m/s any change in climb resulted in nose going straight up and then backward, for some reason it want to fly with engines forward, plane is very light in front compared with the back. but so is also the MK2 planes. The air intakes is far back so drag should also be far back.

[Secuas]

This may be an very out-of-the-topic question.... But uhhh, what is that plane mod?

[Red Iron Crown]

This may be an very out-of-the-topic question.... But uhhh, what is that plane mod?

Large Diameter Space Plane Parts.

[GoSlash27]

Pretty much what Sniped just said. Wings are extremely low drag at low angles and high drag at high angles, which can make for wonky behavior. Don't get your col too far away from your com and don't treat control surfaces like wings.

Your plane should be balanced before you add control surfaces.

Best,

-Slashy

[GoSlash27]

The more I look at this, the more I'm convinced that the real problem is an imbalance between wings and control surfaces.

I think the only thing that makes your CoL appear correct is that pair of delta deluxe winglets up near the nose. If you remove all the control surfaces, your CoL will be well- behind your CoM.

Using control surfaces (especially canards) to correct balance often leads to airplanes that want to fly backwards.

The proper way to do it is to balance the wings to the CoM, then add your elevons/ rudders, then add canards to restore the original balance.

Best,

-Slashy

[BillyBlazeIRL]

You are aware that 300m/s is pretty damned close to mach 1 right?

And 20km 300m/s isn't really that slow. It isn't anywhere near orbital velocity but it isn't standing still either.

Mach 1 would be way different that high up, but that's neither here nor there. That really is pretty slow. Its easy to get up to 1000m/s with nearly any plane.