Aircraft/SSTO keeps bouncing/flipping forward on landing

[Diddly Feelerino]

In the title lads. I have an SSTO I've built to land on Kerbin and/or Laythe. It works and performs brilliantly (reasonable rate of turn, good lift and enough fuel for take off into orbit, as tested on Kerbin). 

I have one problem though. No matter what I do (or where I put the landing gear), on landing the aircraft flips forward. It's particularly odd, given that I don't have brakes on, the lift is generated just behind the centre of mass and the weight is concentrated in the centre to rear of the craft (not the front).

I've prepared a small video demonstrating the issue (trying to land again and again on Laythe, bouncing almost every single time). If you're wondering about the terrain it's immaterial - the bouncing problem is apparent on the KSC Runway too. 

Any ideas on how to solve this? =/

PS - The video is below, just click on the spoiler link!
 

 

https://vid.me/e/bPe8

 

Edited May 18, 2016 by Diddly Feelerino

[bewing]

Generally, this is caused by "clipping" in the landing gear. If they are clipped too much, the wheels automatically disable -- and your landing gear become "metal sticks" instead of wheels. Right-click on your gear and check if it says "Gear blocked: yes".

 

[Diddly Feelerino]

No, the gears deploy fine (and it says "gear blocked: no"). When landing the front wheel always "shudders" or rapidly glitches, moving from its original spot.

Could it just be slightly clipping maybe, with the suspension on landing impact causing it to become a stick?

[Diddly Feelerino]

I found this in my search. Maybe it's a bug post 1.1?
 

 

[bewing]

Hmmm. You may want to post a screenshot of the plane, or a craft file. @GoSlash27's bug fix is mostly to do with jittering, AFAIK. You can certainly try it and see if it helps, though. It's possible that your suspension is bottoming out, I suppose. You may want to try increasing the "spring strength" to max to see if that changes things.

Squad is still making large changes to wheels & landing gear & landing legs. So don't make permanent modifications to your design based on current behavior ... save a copy to try on the next KSP version.

[Diddly Feelerino]

I've ended up tweaking the dampeners to 0.6 and the springs to maximum. The craft appears to be much more stable now (albeit still a bit jittery on landings).

If the problem persists I will keep you posted, thanks!

[Diddly Feelerino]

Hi, just replying.

Yeah, the plane still bounces, flipping forward. Moreover, the landing seems to force the plane to totter forwards on the 1st landing gear, which then shudders/rubber bands then the plane breaks up.

It really seems like a bug. Coming in well below 100m/s horizontal speed and good vertical speed....


I should also add that if I take off at speeds above 100m/s the landing gear is "overstressed" and either 1 or 2 of the landing gears will be destroyed (although I still achieve lift). What is going on???

Edited May 18, 2016 by Diddly Feelerino

[KerikBalm]

you can see the pitching forward starts before your nose gear contacts at all. Your main landing gear are too far behind the cg, so when they touch, they lift upthe back of the craft/put a torque on it causing the nos to come down. Your wheel base does seem very short.

You may want to reduce the spring strength, but keep the dampeners to max (so its less bouncy), then move the rear landing geat to just ever so slightly behind the center of mass... but nearly right under neath it.

Also, try landing on the runway.

You say:

"No matter what I do (or where I put the landing gear)"

"If you're wondering about the terrain it's immaterial - the bouncing problem is apparent on the KSC Runway too."

But you should really show us, because its hard to diagnose a problem if you don't show us the details... otherwise its just like you're fishing for people to validate your belief that its not your problem, its a bug.

 

Also... from the looks of it, your craft isn't very pitch stable to begin with, and a little torque from wheel contact could send it tumbling.

Pics of the plane's CoM(full and empty), CoL, and wheel position, or ... well, you know the saying

Edited May 19, 2016 by KerikBalm

[KerikBalm]

Also, for rough terrain landings, I like to use drogue chutes mounted at the very back (even a normal radial mount chute as a drogue chute, not specifically the one with the word drogue in its name)... a braking chute is much more stable than wheel brakes, and tends to keep the plane pointing in the direction that its moving (I also like to set SAS to follow prograde upon touchdown).

If the plane lacks pitch stability... moving the center of drag way behind the CoM (as with braking/"drogue" chutes) will massively increase pitch stability if mounted as far aft as possible.

Such chutes have long been standard equipment on all my planes designed to land on away from the KSC's nice flat grassy field (oh yea, or that runway is ok, if upgraded, I guess ). As such, trained (well, just lvl 1) engineers are needed for all my spaceplane operations on distant worlds.

Also, a SSTO that small shouldn't be too hard to land with just parachutes on Kerbin/ laythe. You could even consider a VTOL tailsitter (although I know, I know, you want to fix your current design, not make a completely different one):

Ok, that uses no airbreathers, and is for duna, but you get the point... In the case of duna, a horizontal landing is even harder because of the really thin air, and the low gravity making it really bouncy. Its still a spaceplane... it glides to get a landing close to the surface base. Its wings supplied the majority of the lift (I think) for the SSTO from kerbin with pods containing airbreathing jet engines (that decoupled and landed back at kerbin)

 

 

[Diddly Feelerino]

Here you go;

[bewing]

Maybe try using the "move" gizmo in the editor to move the rear wheels backwards as far as they will go. I'm amazed you can actually fly that thing, with the CoL and CoM on top of each other like that -- and no canards for stabilization.

[KerikBalm]

I think moving the rear wheels back farther will only exacerbate the way it will pitch forward when the rear wheels contact. If the rear wheels were almost directly under the CoM, then there wouldn't be much torque exerted when it touches down.

But yea... that plane seems to be pitch unstable, and my guess would be that the torque exerted by the rear wheels contacting first is enough to send it into a tumble.

Not a bug, but a plane with neutral pitch stability and a large force "lifting" the rear of the plane when it contacts the ground.

If the plane was more level when it lands, the torque would be less, but then the wings would need some incidence to be able to generate enough lift (can't generate lift in KSP with 0 AoA, and pitching the whole craft up means that the rear wheels exert a force long before a balancing force from the nosewheel is there)

In theory, it should land ok with the rear wheels exerting arbitrarily low torque (assuming the whels have no rolling friction) as your descent rate (relative to terrain.. 0 m/s vertical speed going landing uphill is not 0 m/s relative to the "ground") gets arbitrarily low.

My guess is it would do fine (intitially at least) if you landed on the runway with a 0.25 m/s vertical speed.

Braking would still be a problem... if you aren't perfectly level, one of the rear wheels will grip more than the other, causing a turn and osciallation as the other wheel starts ot grip more.... etc. Also disabling the nose gear brakes is a very good idea... as I said, I vastly prefer parachutes for braking.

[Val]

Position of landing gear works much like the position of CoL.

The main load bearing landing gear needs to be right behind CoM on craft with nose gear.

If you move the load bearing gear back, it effectively transfers load to your nose gear, which for landing gear would be the same as having CoL in front of CoM.

It becomes unstable at speed.

[Diddly Feelerino]

So I should move the wings back a bit behind the COM?

As for the rear wheels - do I move them forward or back?

[Diddly Feelerino]

Moved the wheels closer to the COM and added canards to try and control up and down easier (not that I ever had a problem with aerodynamic stability, but following all advice). Increased dampeners to 2 and springs to 1, took brakes off the nose wheel.

Moved the wings back a bit and slightly tweaked their angle to allow AoA.

End result? Exact same. Bounces on landing, flipping forward. Here's the new configuration;

[KerikBalm]

craft file so i can test?

[Diddly Feelerino]

Sure! - just one issue; how do I send you that exactly...

[KerikBalm]

host it on drop box? its what I normally do

[Diddly Feelerino]

https://www.dropbox.com/s/p2gvmo0iuh2l78g/SSTO%20Mk%20II.craft?dl=0

Here you go; hope you have better luck with it =/

The craft has modded parts; I think it's parts from TAC:LS and Surface Mounted Stock-Alike Mod.

Edited May 21, 2016 by Diddly Feelerino

[KerikBalm]

hmmm, I don't have that mod... question though.... is there any reason you are making the physicaly dimensions so compact?

[Diddly Feelerino]

4 hours ago, KerikBalm said:

hmmm, I don't have that mod... question though.... is there any reason you are making the physicaly dimensions so compact?

Yeah - it has to be carried by an orbiter to Laythe, and has a heatshield on the rear for a Jool aerobrake. So the smaller and lighter = the better.

[KerikBalm]

is that also the reason that its got a normal docking port instead of a more aerodynamic shielded one? tyou cant connect anything to the shileded one in the VAB?

It just has to fit behind a 10m infaltable heatshield then?

[Diddly Feelerino]

Pretty much. The shielded port wasn't researched when I made the craft - also connecting the two ships is a pain with shielded ports it seems......

[Val]

On 20/5/2016 at 2:31 PM, Diddly Feelerino said:

So I should move the wings back a bit behind the COM?

As for the rear wheels - do I move them forward or back?

If it flies well, then there's no need to move the wings.

Move the gear forward. Nose gear should be as forward as it needs to stop it from flipping.

 

On your craft I'd also rotate the nosewheel so it faces opposite direction. Just to get the wheel further forward.

 

Main gear should have it's wheel roughly below CoL. Just far enough back from CoM that the nosewheel stays on the ground.

Not my craft in picture, but shows main gear position.

[Diddly Feelerino]

Thanks for the feedback. I did what you advised, and the craft still flips. In fact it flips forward, always to the left. Impossible to control unless I touch down at the lowest possible vertical speeds.

Here's the current version;