Aircraft Troubleshooting

[Kyrt Malthorn]

I'm sure this happens a lot...

I'm in career mode, building (non-space) planes with the basic jet engines. I've built some that have been beautifully stable at 4x speed. I just unlocked some Mk2 parts, and I'm trying to take the next step - a two-seater for heavier payloads. It's been a learning experience for building heavy aircraft, a stepping stone as a player to building spaceplanes of my own design. My new design has plenty of lift and probably too much thrust, but I'm having problems getting it off the runway. It likes to bounce from side to side and eventually flip over.

Here's my current design:

Top down.

Side profile.

I made DARN sure the payload was symmetrical. The only things off the center axis at all are physics-less parts.

Maybe I overdid it with the engines but I haven't gotten it off the ground to care yet.

Since I can't figure out how to embed a whole album in a post at the moment, here's a link to it.

I've tried everything I know/can find on the forum. The landing gear are straight as I'll get out - they're even strutted. The main body is strutted. I've tried stitching the wings together and to the main body in case their wobbling is affecting things. I've added reaction wheels. All to no apparent avail. If I could just pinpoint WHY it's giving me a phantom bounce that escalates to flipping out...

P.S. All parts and aerodynamics involved are stock. I have Kerbal Engineer vessel window up in case that helps anyone diagnose what's going on. The torque it shows should all be the tail fins, so it's vertical, not horizontal. I've double checked everything was added with symmetry.

Edited August 25, 2015 by Kyrt Malthorn
Imgur album embed didn't work as expected.

[sal_vager]

It looks like your nose wheels are far too far back, I find if they are too near the center of mass the craft wants to tilt over them, as your thrust is above the wheel pivot point (the contact with ground), it acts like a seesaw.

Also your rear wheels don't look straight, maybe try fitting them to the bottom edge of the fuselage sections then offsetting them to their current position, as they look just slightly twisted.

[Moonk]

Early in game i had issues on runway. Usualy problem was with wheels. It is best to attach them at the near bottom of lower part, and then "offset" them to needed place.

[Slam_Jones]

Might need an extra pair of side landing gear. At that weight and configuration, there could be a lot of pressure on the back wheels, causing them to tilt slightly and cause the whole "veering-off-the-runway-BOOM!" problem.

Also, you may want to try taking off and replacing the vertical tail fins (even if you replace them with the exact same thing). Sometimes a symmetry bug will come thru and cause issues (it's happened to me, at least).

Another idea is to change up the gear so that the nose is tilted higher on the runway, allowing the wings to create more lift quicker, hopefully getting you into the air before instability occurs. Then, you just need to be careful about tail-strikes (slamming the tail into the ground, usually causing an engine to fall off).

You could also try unlocking steering for the front wheels, as sometimes this allows you to correct any wobble. It can also cause much worse problems if used incorrectly. So that one's kinda a 'maybe.'

My final idea is to add a pair of canards on the nose of the plane, to help get the nose in the air quicker and safer.

Also, this is the part you need for the album: (it's the jumble of letters and numbers at the end of the album's URL, minus the # symbol an anything after it.) (And minus the spaces in there)

[ imgur]fhlCs[ /imgur]

Edited August 25, 2015 by Slam_Jones

[match]

One other thing you can try is trimming up before accelerating. This can relieve some of the load from the front gear and prevent wheelbarrow effect while taking off.

[Kyrt Malthorn]

It looks like your nose wheels are far too far back, I find if they are too near the center of mass the craft wants to tilt over them, as your thrust is above the wheel pivot point (the contact with ground), it acts like a seesaw.

Also your rear wheels don't look straight, maybe try fitting them to the bottom edge of the fuselage sections then offsetting them to their current position, as they look just slightly twisted.

Actually my CoM is so far back they're further forward than they really need to be. In the top down view the four radial mounted parachutes form a box around the average between CoM and dry CoM, while the front gear rest underneath the furthest-forward set of wings.

Twisted landing gear maaay be more of an issue here. I thought I lined them up perfectly in the hanger, but...

Might need an extra pair of side landing gear. At that weight and configuration, there could be a lot of pressure on the back wheels, causing them to tilt slightly and cause the whole "veering-off-the-runway-BOOM!" problem.

But THAT might well be perpetrating the issue.

I will say I've used canards up front on several designs, but I tend not to like them so much for my designs aiming for high stability at 4x time speed. Also the CoM is SO far back that it would make it hard to get my CoL behind it. This is what I get for clustering three Mk2 cross-sections to form the main body...

It's main body is also so much lower in the back that it's already tilted up as far as I can get it without offsetting the nose landing gear way below the body.

Crossing fingers, I'm going to mess with the aft landing gear.

Javascript is disabled. View full album

Also, thanks for the album how-to!

Edited August 25, 2015 by Kyrt Malthorn

[Guest]

If your aircraft is bouncing from side to side, even with those widely spaced back wheels, then your COL is too far behind your COM. That would make you unable to pitch up and rotate. Post screenshots inside the SPH with COT, COL, and COM displayed. Otherwise, if that is not the problem, try using larger vertical stabilizers.

[Red Iron Crown]

Your swept wings are attached ahead of the CoM but the control surfaces on them are behind it, a configuration that sometimes causes problems. See if disabling all axes for those surfaces or removing them fixes the problem.

[Kyrt Malthorn]

Thank you!!!!

I placed them flat and offset them to where they needed to be, and added another pair. No more bendy gear!

And it took off successfully! It kinda had to be helped by the drop-off at the end of the runway but I could probably improve that by letting the engines throttle up while the brakes are on. There will be more tweaks, but at least I can get it into the air to try things out.

But hey, even the ACLS (Advanced Cheapsake Landing System) works!

[ajburges]

Need to see SPH pics with the centers displayed.

3 wheels is plenty. I launch bigger SSTOs on that many.

What happens if you try to launch without SAS. Sometimes any control input can induce fatal oscillations. If that is the case, your fuselage is too flexible.

- - - Updated - - -

Good to see you got it going. On further review, your rear gear had dynamic camber. That will always kill you.

[Kyrt Malthorn]

As requested...

Red ball is the dry CoM from RCS Build Assistant mod - so the CoM doesn't travel very far forward at all.

As for flying without SAS: I flat out forgot to turn it on the first time I lifted off and didn't realize until several seconds into flight. So, as of reworking the rear landing gear, it performs the same with and without SAS.

Edited August 25, 2015 by Kyrt Malthorn

[Vegetal]

Your aircraft isn't going to rotate well with the rear landing gear so far back, the control surfaces don't have much leverage to push the nose up. But with this configuration you can't do much....maybe lowering the front gear so your plane already points skyward.

[Guest]

As requested...

Yeah, that COM and COL is just fine. The trouble is you can't adjust your rear landing gear too much or you'll strike the tail when you takeoff.

If it's lifting off in it's current configuration there's nothing else to be done.

It's a cool looking plane!

[ratchet freak]

You need higher landing gear further forward or a way to add upwards force at the nose (canards/seperatrons) to start your rotation.