Aircraft instability at 12-16K, and other small issues

[Turd]

I've been working on this aircraft for about a solid week, just trying in general to get it to fly. Its on the heavy side with a takeoff weight of 40T but I had no problem getting it to 60K+ and 1600m/s two days ago, but now every time I hit about 12K it noses down. I've tried changing the CoM and CoL relationship, as well as shifting the fuel locations to better balance the craft as it gets used up. Unfortunately the solution eludes me... Any help or tips would be appreciated, I've torn it down about 50 times just adjusting mass-lift relationships but I just can't figure out where I went wrong. Normally the problems start at 11K, and 450-500m/s. Until that point I can climb at 45 degrees and still accelerate on just the jet engines. If I kick on the rocket engines early instead of at 20-25K when the jets flame out I can then pull the aircraft at any vertical inclination but it soon spirals out of control until it drops to about 8K at which point I can kick the jets back on and fly it back for a landing... Generally ASAS will have this thing fly at whatever elevation I tell it to, but nose down at about 11K. Once the rocket motors are lit ASAS will go nose up until its doing back flips all over the sky... It actually lifts off at about 80m/s a third of the way down the runway. I appreciate any tips or help you can give me, I know I need to keep lightening this thing and remove some extra struts.

Is my CoM shifting and throwing off stability? Why didn't I have this issue before and why can't I fix it by altering tank positions to shift CoM and CoL?

Is it in some way aerodynamic? Do I not have enough flap to pull up even though I have them all trimmed?

Is my fat, overweight aircraft just to big? Why did if fly when it was even heavier?

I do have a few mods currently installed on this plane, but it ran fine on both the pulse det engines and stock turbo jets until recently.

TT Mk3 expansion

TT Landing gear set

Pulse detonation jet engine

One other minor issue I've been having is the plane violently pulling to the right side after I retract the gear. When I put them back down sometimes the aircraft will straighten back out. Other times, it explodes as seen above in one picture...

Edited June 11, 2013 by Turd

[Sephta]

what pitch did you have, when your plane went out of control?

[WhiteWeasel]

Shouldn't your center of lift be in the same place as your center of mass?

@ 0:35

Edited June 11, 2013 by WhiteWeasel

[Turd]

It drops to negative pitch when under jet power, about 5 degrees downward. When I turned on the rockets it would buck further downwards, but then I would be able to wrest control. Unfortunately any upwards pitch after that point at around 500+ m/s and it will violently kick upwards going past vertical and start going end over end, only becoming stable again down at 8K meters or so.

[Quorthon]

Are you still climbing at 45° when you are at 11km altitude?

Try a more shallow climb smaller than 30°. By doing so you should pick up more speed thus creating more lift.

You might also consider extending your wingspan a bit to provide for more lift up high.

Canards in the front that have more lift might also be a good idea.

And don't forget to trim your elevator!

The B9 Aerospace pack has some nice wings for heavy planes.

Shouldn't your center of lift be in the same place as your center of mass?

This would make it only react more agile but could make the plane unflyable once the center of mass has shifted too far back due to fuel consumption.

Center of lift behind center of mass is the most stable solution and can easily be adjusted with elevator trim.

[Ruedii]

Try turning off the jet engines before they flame out.

This is a known issue, the jet engines turn off one by one when they flame out, instead of turning off all at once. This will throw your plane into a flatspin. You need to kill them before hand. You can choose to turn them off a pair at a time using an action, if you like. This will reduce air consumption and allow you to stretch their use a little longer. Just don't push it too far.

Furthermore, you can get to a higher altitude before you have to shut down the jet engines if you increase your number of air intakes.

Another option is to use Mechjeb2 to auto-kill the engines right before they flame out. This feature is experimental, so I can't be 100% sure it will work, but you can try it.

Edited June 11, 2013 by Ruedii
corrections

[Sephta]

Up/Downward spin is most likely the result of Drag and Thrust producing to much torque in one direction. Would actually explain why you can hold it with a negative pitch.

You can counter this by adding some angle to your wing-position, angle your thrust or add a "counter-spin"-segment. There are other methods to avoid such a thing completely, but for that you'd had to redesign your whole ship...

[Turd]

It was indeed due to thrust torquing the aircraft, the jet engines were too far below the center of mass causing it to be unstable at high altitude where the air was thinner. I changed the location of the wing root, and just by chance the CoM and CoL line up right on top of each other now. The reason I had them apart at first was due to fuel consumption, it would go unstable after a bit before but now I have the fuel location ironed out.

I got up to a very shallow orbit and released a small satellite ate 101K, then returned back and landed on a random continent... I need to work on establishing and breaking orbit so I can land at KSC and move the cargo bay back to the CoM so I can carry larger payloads!

Here is the flight path I took, theres probably a more efficient way to do it...

Take off at 3 seconds, 80 m/s

Fly at 20-30 degrees pitch until 10K or 500 m/s

lean back to 45-55 degrees until air intake reaches .20 at around 20K

Engage rocket engines and climb to 80 degrees

Turn on RCS and SCS as the aircraft still wants to slightly nose over the top

Cut jet engines at .03 air intake level, airspeed should be 700-800+

Climb until orbital altitude id reached, then perform orbital burn

Cut main engines, stabilize craft at 100K+

Kill RCS

There should be a little rocket fuel/oxidizer left in the main tanks and about a third in the jet engines at this point, just enough to fire retrograde, de-orbit and land.

The toughest part seems to be the point where I engage RCS on the main climb. I've lost control due to aerodynamic forces a few times there and tumbled to 50K on momentum before falling back to 8K/ 300 m/s and regaining control.

Edited June 12, 2013 by Turd