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Aircraft freaks out at high-alt... but not how I'd expect...


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So, at the suggestion of several posters here, I've taken to flying aircraft for my parts testing missions since I can more reliably produce altitude/speed combinations with an aircraft than I can with a rocket.

I've attached a .craft for my two-engine test-bed. (Two-engine, so I can mount a single part to be tested, and for better climbing/speed performance for envelope pushing parts tests.)

https://drive.google.com/file/d/0B7Rn2EgE0DrDVzhFYjFwcVdnMmc/view?usp=sharing

Below 8,000m, this thing flies like a dream. It's fast. It can fly nigh straight up. It rolls sharply and stops on a dime. It turns pretty well. (It'll turn too well if you're not careful and you can spin it out.) The rudder is useless, but good for trimming landings. I wish I had flaps for the final approach, but I've emergency landed it just fine every time I've taken it up.

But it's not without some sweat-inducing attributes. It flames out around 22,000m - which I guess is normal for a basic jet engine - which can make thin-atmo testing a bit hair raising. I often dive on 0% throttle to hit my speed targets.

What's worse, however, is the way that, at that altitude, the plane wants to tumble. It seems to have no rhyme, nor reason, on all three axes. It tumbles all the way down to 8km, and then suddenly aerodynamics start to take over.

Normally, I'd assume this was the atmo being too thin to interact with the wings and elevons... but it enters the tumble so quickly and easily - and becomes impossible to control so fast - that it's clearly being acted on by /some/ force. This is at zero throttle, too.

More expert analysis than I possess is required to diagnose this problem. As you probably figured out, this is where you come in.:cool:

I'm looking for a craft that has ample thrust and lift for high-altitude, high-speed tests. Long endurance for visual surveys (though this will eventually be replaced by a VTOL craft to get the EVA requirements), and a stable flight profile so I can focus on getting into position, instead of wrestling out of a tumble.

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I downloaded it and went to try it out. The centre engine doesn't function. Were there fuel lines to run fuel from the two outer tanks to the centre engine?

In any case, it doesn't look like it needs three, and it's named twin engine.

Anyway, I'll try flying it and see what happens.

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Stock, NEAR or FAR?

Screenshots from above and the side in the SPH with CoM/CoT/CoL indicators turned on would be helpful, as would some stability analysis screens if you're using FAR.

Stock. I'll get those screenshots presently.

I downloaded it and went to try it out. The centre engine doesn't function. Were there fuel lines to run fuel from the two outer tanks to the centre engine?

In any case, it doesn't look like it needs three, and it's named twin engine.

Anyway, I'll try flying it and see what happens.

Middle engine was the last part for testing. Was a 'basic jet engine' test @ 19,000m - 22,0000m with a speed of 340-440m/sec.

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Well, it can definitely be flown. I found it extremely touchy and crashed it on my first two test flights.

It seems to have way more control surfaces than it needs, including those winglets near the nose. It is very unstable as soon as you move more than about 30 degrees off prograde.

In general there is a tradeoff between maneuverability and stability. This craft is very maneuverable. :)

There's a good tutorial here that goes into a lot of detail about aircraft design.

When you are flying at high altitudes, you fly fast and you definitely want to favour stability over maneuverability at that point.

If I could make a friendly suggestion as to the name... Perhaps 'Death Trap' rather than 'Twin Engine'. :)

Good luck and happy landings!

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- - - Updated - - -

Well, it can definitely be flown. I found it extremely touchy and crashed it on my first two test flights.

It seems to have way more control surfaces than it needs, including those winglets near the nose. It is very unstable as soon as you move more than about 30 degrees off prograde.

In general there is a tradeoff between maneuverability and stability. This craft is very maneuverable. :)

Yeah, the extra control/lifting surfaces are for having SOME control in thin atmo. I fly it with capslock on in low atmo. You are correct that it can spin on a dime if you ask it, though. (Don't ask it.)

I don't grok why at high-altitude, though, where the control surfaces should have /less/ purchase, it flips out like a rabbit on crack. My goal is a stable, responsive craft @ 30km. I've looked at all the tutorials, and this is as close to their advice that I can model...

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I see a few potential problems here.

#1 is your lift centered on the mass when you remove the control surfaces? Having longitudinally unbalanced control surfaces can make a plane tumble at high speeds.

#2 Your intakes are by far the draggiest part of the plane. Having the intakes so far forward (especially the intakes you're using) can easily make your plane want to fly backwards at high speed.

#3 Did you build in sequence so that your engines don't suffer asymmetric flameout?

Best,

-Slashy

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The control surfaces do have less purchase in thin atmo, so if the craft becomes unstable it is harder to recover. Also, because the air is thinner, there is less of the medium that gives you both stability and maneuverability to work with.

The biggest problem I can see is the front winglets (canards). I tried removing them and took it up to 20 km.

With two engines, you definitely have to worry about asymmetric flameout at those altitudes. I found that without the front winglets is was still quite controllable, although responding slowly in the thin air. Also, if the engines do flame out, you lose thrust and, therefore, control.

A couple of other specifics. The wings seemed to flex quite a bit, making the control somewhat sloppy. Some struts (even placed invisibly inside the wings) could help a lot.

It's probably best to place intakes as far to the rear as possible. If your drag is further back, the craft will be more stable.

Hope this helps.

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I see a few potential problems here.

#1 is your lift centered on the mass when you remove the control surfaces? Having longitudinally unbalanced control surfaces can make a plane tumble at high speeds.

This may be the culprit. I'll check it out. I suspect not, the mass is probably just slightly behind the lift if I pull the elevons and tail, and very ahead if I pull the canards. Starhawk also doesn't like the canards it seems, I feel like I had to put them on for mass or lift reasons...

#2 Your intakes are by far the draggiest part of the plane. Having the intakes so far forward (especially the intakes you're using) can easily make your plane want to fly backwards at high speed.

Hmm. I assumed, perhaps incorrectly, that the intake fuselage wasn't anywhere near as draggy as the big chrome ramscoop deal (Which is between the tail verticals). I'm still having trouble keeping enough intake air in this plane, however, even at lower altitudes. But flying backwards at high speed is one of the modes of dead-fall so you're probably right.

#3 Did you build in sequence so that your engines don't suffer asymmetric flameout?

I don't know what you mean, exactly, so I highly doubt it. I built the body, using intake fuselage because, at the time, I didn't have structural, and knew I needed intake air. Then I stuck the engines aft of their fuel tanks. I'm guessing this is incorrect?

The control surfaces do have less purchase in thin atmo, so if the craft becomes unstable it is harder to recover. Also, because the air is thinner, there is less of the medium that gives you both stability and maneuverability to work with.

The biggest problem I can see is the front winglets (canards). I tried removing them and took it up to 20 km.

With two engines, you definitely have to worry about asymmetric flameout at those altitudes. I found that without the front winglets is was still quite controllable, although responding slowly in the thin air. Also, if the engines do flame out, you lose thrust and, therefore, control.

A couple of other specifics. The wings seemed to flex quite a bit, making the control somewhat sloppy. Some struts (even placed invisibly inside the wings) could help a lot.

It's probably best to place intakes as far to the rear as possible. If your drag is further back, the craft will be more stable.

Hope this helps.

Two votes against the canards and to move intakes to the rear. Copy that. I'll do a rebuild with those principles in mind. I'm confused about the advice in re: quantity of control surface though:

If I want less control surface for more stability, how do I steer at 22km?

EDIT: Oh, and how do I do invisible struts? I can't seem to figure that one out.

Edited by qoonpooka
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This may be the culprit. I'll check it out. I suspect not, the mass is probably just slightly behind the lift if I pull the elevons and tail, and very ahead if I pull the canards. Starhawk also doesn't like the canards it seems, I feel like I had to put them on for mass or lift reasons...

Hmm. I assumed, perhaps incorrectly, that the intake fuselage wasn't anywhere near as draggy as the big chrome ramscoop deal (Which is between the tail verticals). I'm still having trouble keeping enough intake air in this plane, however, even at lower altitudes. But flying backwards at high speed is one of the modes of dead-fall so you're probably right.

I don't know what you mean, exactly, so I highly doubt it. I built the body, using intake fuselage because, at the time, I didn't have structural, and knew I needed intake air. Then I stuck the engines aft of their fuel tanks. I'm guessing this is incorrect?

Two votes against the canards and to move intakes to the rear. Copy that. I'll do a rebuild with those principles in mind. I'm confused about the advice in re: quantity of control surface though:

If I want less control surface for more stability, how do I steer at 22km?

EDIT: Oh, and how do I do invisible struts? I can't seem to figure that one out.

Well... it's not necessarily that you want "less" control surfaces, just that they be balanced front to back so that you're not generating funky drag when they're moving. Having too much control surface forward or aft can make the plane very unstable at high speeds. If it looks balanced without the control surfaces attached, you're good to go.

The intakes use weird math for calculating drag. The mass of the entire part is figured into the drag coefficient, so the fuselage type intakes generate horrendous drag. The XM-G50 is a pretty good intake for this duty. About 4 total should do fine for this job (2 per engine). Just mount them as far back as you can get them. They act like tiny parachutes at high speed, so having them in the back will aid stability.

The asymmetric flameout deal... I have a write-up on that in my old spaceplane tutorial. Gimme a sec and I'll post a link.

Best,

-Slashy

- - - Updated - - -

http://forum.kerbalspaceprogram.com/threads/102182-So-you-want-to-build-a-space-plane-%28-25-stock%29

Check the section on "multiengine spaceplanes".

Best,

-Slashy

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I suspect you're getting asymmetric flame out. What happens is that when jet engines start getting starved for air (e.g. at high altitudes), they can "flame out". If you have more than 1 jet engine, the jets may not both shut down at the same time. This is due to the crappy programming the 'idiosyncrasies' of the fuel flow logic in the game. When one engine shuts down, it gives all the air to the other engine.

Obviously, the asymmetric thrust that results from this situation causes serious flight control issues.

Some solutions the problem are:

  • Don't fly in situations that lead to "flame outs"
  • design vessels with single jet engines to avoid asymmetric thrust
  • use action groups to shut down engines simultaneously

The last one is may not be feasible for you right now since you have to unlock action groups by upgrading your facilities.

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Yeah, the craft did suffer asymmetric flameout when I tested it.

@qoonpooka - You can run struts from one wing section to another, and if you place them right, you won't see them. Also, you can mitigate asymmetric flameout if you check out the link Slashy posted.

Edited by Starhawk
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I have had flameouts, but I'm pretty sure I'm not flaming out when I experience this particular scenario. Both engines are clearly still thrusting as I tumble, if I raise the throttle. (It becomes much more rapid in the tumble, too, but I clearly get two contrails throughout.) I believe that I can hear as the engines are nearing flameout, and I tend to throttle them nearly to zero when I hear that. Most of the time I can prevent it. When I do get a flameout, I tend to see it as a flat spin, rather than the tumbling on all three axes I've been seeing. But I'll certainly re-wire the intakes and see if that helps. :)

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Sounds like you have a handle on that part of it, then. Is it possible you're stalling out or otherwise losing lift?

Oh I definitely stall. I'm holding the thing up by engine thrust alone at this point and then suddenly it'll run away from me - almost all the time it'll pitch /up/ no matter the input, and then once it flips over the pitch once it's just total pandemonium for a 20,000 meter fall.

Any one of these symptoms would make sense, but it's the combination of no control input, and the aircraft acting like it's suddenly got ZOMG ALL THE INPUT ON ALL THE AXES HOLY CRAP EHRMAGERD DOGEWOW.

As the atmo density increases around 4-5k It'll stabilize from the tumble and I can pull out of the stall just fine.

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I'm at work (ahem) so I can't fly it, but is your CoT going through your CoM? If that's not the case as soon as your control surfaces stop working (like, say, when you reach thin air), your plane will start to tumble as you can no longer offset the torque.

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Part of the issue is just a limitation of the jets. They lose thrust in that high, thin atmo and, without thrust, you don't have the basis for control. Once you unlock turbojets, the high altitude flying changes completely. They have great thrust at 20 km.

I'm no expert, but it seems to me that any craft based on jets will contend with difficulties at that altitude.

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I'm at work (ahem) so I can't fly it, but is your CoT going through your CoM? If that's not the case as soon as your control surfaces stop working (like, say, when you reach thin air), your plane will start to tumble as you can no longer offset the torque.

CoT is inline, yeah. But you're right that it behaves like it's not.

Part of the issue is just a limitation of the jets. They lose thrust in that high, thin atmo and, without thrust, you don't have the basis for control. Once you unlock turbojets, the high altitude flying changes completely. They have great thrust at 20 km.

I'm no expert, but it seems to me that any craft based on jets will contend with difficulties at that altitude.

Yeah, I was expecting there to be issues. It's just this specific failure mode, almost like the physics model can't figure out what to do, so it has a seisure until atmo density overwhelms whatever glitch it gets into. I guess it's possible that it's simply a combination of factors. I'm going to try tightening the design today and see how it handles, then.

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I was just looking through some of my old designs. One of the things I always used to do was add a reaction wheel (ASAS module back then) to my designs, right at the back of the fuselage, and attach the jet engine to that. It adds significantly to the control, especially if the aircraft design is still in need of tweaking. Basically, it just adds torque to the back of the craft which makes it easier to steer.

Here's a pic of one of my old designs - it actually has reaction wheels both at the back and right behind the cockpit. Talk about overkill! Anyway it flies pretty well even at 20 km.

Z3Mhzbl.png

I'm actually a bit ashamed to show this craft. I would never build it like that now. But it flew.

Live and learn!

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