(space)plane once more

[KerbMav]

As usual my plane starts to spin around every axis at 10km / ~400m/s after a rather uneventful takeoff.

All reaction wheels are off, control surfaces at strength 10, flying with soft controls

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"Fun Fact": Its predecessor flew straight, reached space (apart from not enough fuel it might have attained orbit), but its tail swam like a fish when pulling a bit harder. So I removed the wobbly parts (an RCS tank and a inline battery) and strutted the tail a bit.

Are my planes to rigid??

Using: FAR, KIDS, DRE, HotRockets

Edited April 24, 2014 by KerbMav

[MKI]

What happens when you remove the fuel on the jet engines using tweakables Does the center of mass move closer to the center of lift?

I can't see to well, but another possibility is the engine exhaust is hitting the tail, thus creating unusual effects.

Finally your tail isn't very significant, it might not be providing enough stability.

[breakthrough]

I don't use FAR, but the tail really looks like its blocking the thrust from the TurboJets. I assume it must not when its sitting on the runway, so I think the tail flexes such that one engine or the other is blocked when flying at speed. The large asymmetry of thrust would easily cause a spin when that happens.

Presuming that the trust is being blocked, I recommend you move the tail upward so it clears the engines by a good margin or redesign so that the engines are closer to the centerline with the tail surfaces outboard.

[KerbMav]

What happens when you remove the fuel on the jet engines using tweakables Does the center of mass move closer to the center of lift?

The CoM moves further to the front - and it happens quite early after takeoff doesnt take long to reach the "critical zone" described above.

I can't see to well, but another possibility is the engine exhaust is hitting the tail, thus creating unusual effects.

Was not a problem in the first design - jet engines also do not heat up parts behind them.

Finally your tail isn't very significant, it might not be providing enough stability.

As I said, all I changed was removing a inline tank and battery, so the tail was the same, only maybe half a meter further back.

Presuming that the trust is being blocked

Interesting idea.

1. Will try the old design once more.

2. Will clear up behind the jets.

Edited April 23, 2014 by KerbMav

[SpenSpaceCorp]

Make sure Jeb is the pilot. That may improve things

[KerbMav]

If it does not whirl, it refuses to be steered at all ...

[O-Doc]

There's two basic problems. The first is that your fuel tanks are not in and around you center of mass. That means your aircraft is getting increasingly unstable as the flight wears on. Does it move to an instability point when fuel tanks are empty?

The second problem is all your control surfaces are in a very problematic location. There are all behind you center of lift but, perversely, equally behind and in front of your center of thrust. Those outside ailerons no doubt are being told to do pitch control and will vector upwards when you pull up. In thick air you will get upwards lift from that. In thin air you get downwards lift from that because your CoT becomes the pivot point of your aircraft. That with increasing aerodynamic instability is going to end in the results you described.

The simple fix is to tweak out the pitch vector from those ailerons. If you are not getting enough pitch authority then add some canards. If you don't like that style then hide small winglets in the front of the nose, attached to the back of the cockpit.

EDIT: Also, move your ASAS to between your CoM and CoT,

Edited April 24, 2014 by O-Doc

[KerbMav]

There's two basic problems. The first is that your fuel tanks are not in and around you center of mass. That means your aircraft is getting increasingly unstable as the flight wears on. Does it move to an instability point when fuel tanks are empty?

While flying with jets the CoM of this aircraft moves forward, but only very slowly and not very far - the problems already set in when the tanks are all almost still full.

(CoL behind CoM is a good thing?)

The second problem is all your control surfaces are in a very problematic location. There are all behind you center of lift but, perversely, equally behind and in front of your center of thrust. Those outside ailerons no doubt are being told to do pitch control and will vector upwards when you pull up. In thick air you will get upwards lift from that. In thin air you get downwards lift from that because your CoT becomes the pivot point of your aircraft. That with increasing aerodynamic instability is going to end in the results you described.

The simple fix is to tweak out the pitch vector from those ailerons. If you are not getting enough pitch authority then add some canards.

If I tweak out the pitch vector from my tail, only the rudder for yaw will remain, the two ailerons will then only move my CoL back (which is not a bad thing, but that will be all they do) - but wont they also work against any pitch control by stabilizing the aircraft as static fins?

Will try adding canards, although I am still below 10km in still quite thick air when it starts dancing. Can it be the supersonic speed that is causing this?

EDIT: Also, move your ASAS to between your CoM and CoT,

I want to fly without reaction wheels while in atmosphere - and use RCS in thinner air/space mostly. (I rarely say this: Scott Manley showed it works. )

Where is the ASAS located if all torque has been deactivated in all cockpits/passenger cabins and A/SAS units?

Edited April 24, 2014 by KerbMav

[O-Doc]

While flying with jets the CoM of this aircraft moves forward, but only very slowly and not very far - the problems already set in when the tanks are all almost still full.

(CoL behind CoM is a good thing?)

Yeah, sorry. That's a bit of a brain explosion from me. You don't have an issue with the mass moving. CoL should be behind CoM

If I tweak out the pitch vector from my tail, only the rudder for yaw will remain, the two ailerons will then only move my CoL back (which is not a bad thing, but that will be all they do) - but wont they also work against any pitch control by stabilizing the aircraft as static fins?

Will try adding canards, although I am still below 10km in still quite thick air when it starts dancing. Can it be the supersonic speed that is causing this?

Tweak out pitch from those control surfaces which are in front of the CoT. If you have control surfaces on both sides(front and back) of the CoT then you get diminished effect the higher you go because CoT becomes the central point around which your craft turns, not the center of lift which is more dominant in the lower atmosphere. In your design it look like they are cancelling each other out.

I want to fly without reaction wheels while in atmosphere - and use RCS in thinner air/space mostly. (I rarely say this: Scott Manley showed it works. )

Where is the ASAS located if all torque has been deactivated in all cockpits/passenger cabins and A/SAS units?

It's mostly about the way the physics engine figures out the movement from CoM and the corrections to get CoM heading in the right direction. I'm pretty sure the SAS unit is used as the point of reference to correct direction. The further away from the CoM your SAS is the more movement it goes through and the more force it applies. That's why you get violent shaking from SAS as it over corrects itself. It doesn't matter if your reaction wheels are on or off, SAS takes control of the control surfaces too. That's where the infini-glide bug comes from. Don't take this as gospel, just my working theory from countless hours of experiments.

If you do the tweaking then the shaking should go away without having to move your ASAS unit. But, this is still a good design principle to follow.

[KerbMav]

For as long as FAR and later Stock KSP made tweaking possible pitch and yaw were only enabled on the tails and roll on the wings of my planes - special care is taken to place the control surfaces at a reasonable distance from the CoM to get good leverage, hence the use of the long structural tail part.

What if I remove the SAS unit - command pods have controle authority (since .22 ?), while A/SAS units only adds torque (as far as I know, from memory).

The more I try to fly this craft the more I think that speed is more a problem than thinning air.

(Btw., it is not just shaking, it starts yawing left to right until it suddenly starts spinning and tumbling around itself.)

[O-Doc]

It's probably best to post the craft file. That way would be easier to know more about the issue.

[KerbMav]

ETA: ~6h

[SnappingTurtle]

I can't see a vertical stabilizer in the image. You need one or the plane will sideslip, which will cause a difference in the sweep angles of the wings, which will cause different amounts of lift, which will cause roll. Check that the thrust from the engines isn't impacting the wings by bringing up the log in flight (F3 IIRC) and look for collision events.

[KerbMav]

I can't see a vertical stabilizer in the image.

The second image?

There is no "damaged by heat from engines" or similar entry in the flight log.

[SnappingTurtle]

You could try a larger vertical stabilizer or twin stabilizers, that might help. Also the center of lift moves forward when you begin to reach sonic speed so try moving it back. See what the FAR analysis window shows for various mach numbers, particularly mach 1 and thereabouts.

[KerbMav]

You could try a larger vertical stabilizer or twin stabilizers, that might help. Also the center of lift moves forward when you begin to reach sonic speed so try moving it back. See what the FAR analysis window shows for various mach numbers, particularly mach 1 and thereabouts.

"What the image shows is that an increase in control surface deflection increases the camber and the effective angle of attack of the wing, both which increase lift. However, camber only produces lift at subsonic speeds; it is entirely dependent on the airflow at the trailing edge of the wing being able to affect the airflow at the leading edge of the wing. Once in supersonic flow the only way changes in lift can be achieved is through increases in the angle of attack of the wing; camber will only increase drag. This would be the result of any roll controls feeling sluggish at high speeds, since they're less effective at Mach > 1 than at Mach < 1. This is also why supersonic planes fare better with all-moving tailplanes / canards than tails with separate elevators."

So it really looks like my flexy unstrutted tail in the first design was the reason for my retained control of the aircraft - as it not just flapped with the aileron but actually move the whole tail fin by flexing the aircraft's structure?

[Hodo]

The problem I see is your vertical stabilizer is woefully insufficient for that craft.

[KerbMav]

The craft files of three versions - should not contain any modded parts.

https://www.dropbox.com/sh/wyj1zv4dxgp8hry/u_pK6sS8rU

[KerbMav]

Guess who went to space today!

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But I still have no clue about that big one from the first post ...

[Hodo]

and you did it with an outstanding looking craft!

Great job.

[O-Doc]

Congrats on getting to space. I can't reproduce the problems you described on the crafts you uploaded. All the turbos are blocked by the tail. Which means they don't get off the runway.

[KerbMav]

That is really strange ...

I suppose you are using FAR, too?

It is the only mod I can think of that might change the obstruction of engines - I also use DRE, KIDS and HotRockets, but I never read anything about them changing airflow.

[O-Doc]

Ah, sorry. I'm not using FAR, I thought you were using stock. It's hard to problem solve as I haven't used FAR yet.

[KerbMav]

I built some ordinary jet planes to further my knowledge in this field - was not very succesful unfortunately.

If someone wants to take a look and has any advice, here are the craftsfiles.

Edit: But! My spaceplane seems to still work at least!

Edited April 29, 2014 by KerbMav