Plane going out of controll over 28k height add reaction wheels?

[n0xiety]

Trying to build a really fast and long range plane. Reached the point where i can theoratically make more than 7 kerbin circumnavigations with it but have a problem. I am supposed to get up to 30k with it as the fuel gets used so the next circumnavigation uses less and less fuel as i climb up very slowly. The problem starts at 5th circumnavigation. The plane reaches about 28k and starts loosing yaw controll. it can go 1450m/s at 27k height stable but if i go over that it starts yawing towards sides and goes out of controll eventually. Now before you get started about com sliding because of fuel getting consumed know that is not the reason... The plane can fly really stable as long as i don't pass 27k and even land with the fuel that i am loosing controll with. So this is not a com issue. Maybe the air gets so thin up there that the controll surfaces are not enough to keep the plane stable? I am indeed using a small surface for yaw authority for reduced drag.

PS. Please don't talk about noob mistakes here.

Edit: Added Images

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Edited June 7, 2015 by n0xiety

[MarvinKitFox]

Judging by the picture of the plane you have supplied, the problem is clearly that it does not have enough wings.

Looking at the picture, I cannot see ANY wings, actually.

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nor engine.

..

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nor picture, for that matter.

[rudi1291]

Maybe the air gets so thin up there that the controll surfaces are not enough to keep the plane stable?

Exactly. You will need a bigger control surface for yaw at that altitude and as a stabilizer. Without a stabilizer, the plane will start to drift sideways, and each control input can make it spin out of control...

[n0xiety]

Judging by the picture of the plane you have supplied, the problem is clearly that it does not have enough wings.

Looking at the picture, I cannot see ANY wings, actually.

.

nor engine.

..

.

nor picture, for that matter.

No need for picture i explained the situation as it is. Like i said if the plane can reach 28k height while going 1450m/s you can assume that wing area is enough... If it wasn't i wouldn't have been able to go up to that height and keep that speed. Engines have nothing to do with it thrust is aligned with point of mass perfectly. Point of mass stays stable as the fuel gets consumed so no problem there either. As i said i have small yaw controll compared to the plane but it was enough untill i passed 28k mark. Haven't tried yet but the problem might get solved by adding a bigger yaw controll surface but i don't really want that because of extra drag... I am trying to build a plane that can make 7+ circumnavigation after all. Picture isn't going to show you anything else than these information i gave you.

[Zzabur]

I had about the same issue there, and i found a way to deal with it.

When reaching 28-30km, check your air intakes, then decrease throttle slightly, you'll earn a bit of air intakes. Continue to decrease slighlty, i found that losing control is because :

-either an engine got less air intake (reaching very low intake like 0.05, it seems that it feeds engines from left to right, so right engines flameout a bit before left, and your plane begin to turn on the right)

-either too much thrust for control surfaces (as they become less efficient because of less atmosphere)

You need also to be almost at prograde 0° on the horizon to "build" your speed to 2km/s.

I don't know exactly which is the main reason causing this issue at 30km, but i could resolve it by lowering throttle to keep control. I can reach orbit without using any space engine (just raising a bit periapsis sometimes with a second burn at apoapsis, but i often manage to reach periapsis >70km with only jets). If i keep full throttle at 30km, no matter how much SAS or winglets i have, i lose control.

[MarvinKitFox]

Picture isn't going to show you anything else than these information i gave you.

Picture tells a LOT.

You have Yaw control problems, while flying high supersonic but not hypersonic speeds at 27k?

Is your wing shape appropriate? You do NOT want to build it along the normal plan for low-flying planes.

Nor do you want to use the design for SSTO spaceplanes, as at 28k they are flying much faster and not relying on aero lift any more.

Are you flying west or east?

At 1450m/s, that's the difference between your plane weighing 7/8ths or 4/10ths of its mass.

Single or multiple engine?

If multiple, are you *sure* that they are getting enough air?

Even ultimately airhogged designs cannot supply their engines max at 28k.

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.

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All of these can be answered by 2 pictures.

one of your plane in hanger, aero tools shown.

and one of your plane at altitude, experiencing the problem you face, again with aero display on and resource tab open.

[n0xiety]

All of these can be answered by 2 pictures.

one of your plane in hanger, aero tools shown.

and one of your plane at altitude, experiencing the problem you face, again with aero display on and resource tab open.

Uploaded pics hope you can solve the problem. Maybe i should try angled upwards wings at the bottom of craft but that will reduce the low altitude efficiency.

[Tex_NL]

With the 5 intakes your craft is very 'drag heavy' at the nose. But has no vertical stabilizer to speak of. In the low, thicker atmosphere that tiny fin might be enough to keep the nose pointing forward but not at high altitude. Try giving it a bigger stabilizer. The Big-S Tail Fin perhaps.

[n0xiety]

With the 5 intakes your craft is very 'drag heavy' at the nose. But has no vertical stabilizer to speak of. In the low, thicker atmosphere that tiny fin might be enough to keep the nose pointing forward but not at high altitude. Try giving it a bigger stabilizer. The Big-S Tail Fin perhaps.

Actually i am using Rapierspike for that. Added 1 more pic to the album where you can see the Rapierspike. Clearly the stability effect doesn't work as well when passing 27k altitude. That might be the only reason why i am able to get away with that tiny fin untill i reach 27k altutide.

Edited June 7, 2015 by n0xiety

[n0xiety]

Well added a big-s tailfin which solved the problem. But the bad part is the fuel consumption is 0.25 at 27k while it was 0.24 before. Like i said trying to circumnavigate kerbin atleast 7 times so that 0.1 diffrence is actually alot if you think about the long run.

[KSK]

I'm probably missing something here but why the orange tank body? Unless you're using rocket power (which, at first glance, seems unlikely on something that you're circumnavigating Kerbin with), wouldn't you be better off with a dedicated liquid fuel tank rather than lugging around a load of oxidiser that isn't helping you?

Fuel consumption is less of an issue if you can carry twice as much fuel without penalty.

[n0xiety]

I'm probably missing something here but why the orange tank body? Unless you're using rocket power (which, at first glance, seems unlikely on something that you're circumnavigating Kerbin with), wouldn't you be better off with a dedicated liquid fuel tank rather than lugging around a load of oxidiser that isn't helping you?

Fuel consumption is less of an issue if you can carry twice as much fuel without penalty.

Drag my friend. Mk-3 is too big tried that. When you use mk3 than you have to use mk3 adapter too to smooth the curve. Well when i use the mk3 then i can't use just 4 engines. Everything has to get big as the craft gets bigger. Another thing is that im trying to keep the speed over 1400m/s at 27k altitude or it will take forever to circumnavigate 7 times. If it was up to me i would use droptanks to do this and possibly would be able to circumnavigate more than 10 times easily but challenge doesn't allow that.

Edited June 7, 2015 by n0xiety

[MarvinKitFox]

Your original design has a lot of frontside drag due to those inlets, and *very* little rudder for Yaw control.

You seem to be always flying as high and as fast as it can go.

Still, you say it manages fine until that point. 27.5k, and fuel is about half depleted.

While you are using a single monster tank for fuel, most likely to prevent CoM creep due to uneven fuel use, your CoM will still be moving to the rear as the tank empties.

With drag remaining front, and mass moving aft, your instability is worsening.

With increasing altitude, your rudder is becoming less relevant, also worsening stability.

Frankly, I expect that at any point after about 25k alt that if you disable your autopilot and keep hands off the controls, your plane will enter a flat spin.

I think it is purely the autopilot and the gimbal on your engines that is keeping it balanced at that point.

The small solution is to greatly increase your rudder size. But this will add drag, worsening the fuel efficiency of your endurance flight.

The real solution would be to move those very draggy air intakes as far back as practical, and mounting a very low drag nose on the plane.

Edited June 7, 2015 by MarvinKitFox