Spaceplane weirdness

[Wanderfound]

Okay; I'm trying to test the limits of my heavy lift spaceplane. It can comfortably lift 30 tons to orbit, but I'm now pushing 70 ton. It can fly just fine with that amount of weight aboard; what it can't do is take off. No matter how much bracing I apply, the front gear wobble and then fail.

The weird thing is that they don't start wobbling until I begin to lift the nose. This doesn't make sense; if they're going to buckle, it should be when the weight is on them, not when I take it off.

Any ideas?

[Col_Jessep]

Gear bays can be a pain with heavy planes. Here is a nice vid from Cruzan AK:

Or you could add some boosters for a rocket-assisted takeoff for increased Awesome.

[Laie]

Are the wheels straight? I had the very same issue just yesterday and eventually found that my gear was pointing inward, like this / \.

No problem as long as the full weight of the plane was on them, but during liftoff, it showed. Alternatingly, one or the other wheel would have more load, making the entire plane wobble left and right and tipping over.

They weren't off by much, either. I totally strutted them down in a thousand ways without noticing that they weren't pointing the right way.

[Hodo]

I have one simple answer.

Your landing gear are to far behind your CoM to rotate to take off.

This is why your gear are wobbling as soon as you try and lift the nose. You are lifting the nose and pushing the rear gear into the tarmac.

[Wanderfound]

I have one simple answer.

Your landing gear are to far behind your CoM to rotate to take off.

This is why your gear are wobbling as soon as you try and lift the nose. You are lifting the nose and pushing the rear gear into the tarmac.

It takes off just fine with a 30 ton payload, and even with 70 it will take off on the one in three attempts that I get it up before the gear fail completely. The rearmost gear are mounted high, as tailstrike guards. They don't contact the ground until the nose is in the air.

The rear gear are fine. It's the front gear that are failing. CoM is bang in the middle of the plane.

[Sean Mirrsen]

It takes off just fine with a 30 ton payload, and even with 70 it will take off on the one in three attempts that I get it up before the gear fail completely. The rearmost gear are mounted high, as tailstrike guards. They don't contact the ground until the nose is in the air.

The rear gear are fine. It's the front gear that are failing. CoM is bang in the middle of the plane.

The front gear no longer contact the tarmac when you lift the nose. If you really have a problem with the gear wiggling under pressure, maybe you could try running off the edge of the runway?

Or better yet, redesign the landing gear so that you don't use tailstrike guards, but rather the whole aircraft is angled upwards while parked. This way you will take off at a certain speed without applying pitch, and won't have landing gear wiggling issues because you'll lift off evenly.

[Hodo]

It takes off just fine with a 30 ton payload, and even with 70 it will take off on the one in three attempts that I get it up before the gear fail completely. The rearmost gear are mounted high, as tailstrike guards. They don't contact the ground until the nose is in the air.

The rear gear are fine. It's the front gear that are failing. CoM is bang in the middle of the plane.

Sean Mirrsen said it best below.

Or better yet, redesign the landing gear so that you don't use tailstrike guards, but rather the whole aircraft is angled upwards while parked. This way you will take off at a certain speed without applying pitch, and won't have landing gear wiggling issues because you'll lift off evenly.

Couldn't agree more with this post.

[RoboRay]

It looks like the front gear is tilted at an angle, leaning forward. That can cause problems.

Or is that the problem? They're straight when you start, but tilt when the load shifts as you try to get the nose up?

[Wanderfound]

Just got it into the air with 65 ton again. This time, the gear started to fail just before I started to lift the nose; it looks like it may be nothing to do with the initial lift, but instead just be failing when it hits a speed that coincidentally is around the same as the takeoff speed (110m/s in this case). I suspect the suspension may be doing some sort of pogoing thing which is sending the steering haywire, which in turn overstresses the gear. I'll post a closeup screenshot of all the gear when I can.

Regarding the tailstrike guards: the plane is designed to carry very heavy loads. In order to avoid having to redesign the aerodynamics every time I change the cargo, it is necessary that the cargo bay be on the centre of mass. This meant that I had the centre of mass further forward than I would normally prefer, which meant that my rearmost conventional gear needed to be brought forwards far enough that tailstrike became an issue.

It can safely take off without them, but you need to be careful, and when the cargo load gets truly ridiculous it's helpful to be able to just crank the nose up as hard as you can before you run out of runway. In extreme cases, you pivot off the tailstrike wheels in the last moment before takeoff. They're also handy for cushioning the shock on the rear gear when landing with a heavy load. The extra weight is trivial when dealing with a plane this size.

[Wanderfound]

Re: angling up. The rear gear is as low as it can be without risking bulky cargoes striking the tarmac during takeoff (it's an open-air cargo bay; no roof or floor, and the cargo is often big enough to protrude substantially from top and bottom). The front gear is as high as it can reasonably be; it's already mounted on structural pylons.

This is probably a large part of the problem, which I've attempted to deal with by a spiderweb of struts which obviously aren't doing the job.

[Wanderfound]

Some clearer pictures:

[Hodo]

Quick question.

Which version of FAR are you running?

The latest version with the nerfed engines or the previous version 14.02?

The only thing that I can guess is that you have some kind of flex somewhere causing the gear to buckle due to the mass.

[Wanderfound]

Quick question.

Which version of FAR are you running?

The latest version with the nerfed engines or the previous version 14.02?

I thought I had the latest version of FAR, v0.14; I re-downloaded all my mods after First Contract came out. Looking at their site right now, though, it looks like there's a v0.14.1, and the changelog mentions an engine nerf.

Updating now. How heavily did they cripple the turbojets and RAPIERs? May have to swap the outer pair for aerospikes...

[Wanderfound]

The only thing that I can guess is that you have some kind of flex somewhere causing the gear to buckle due to the mass.

Yeah, it looks to be largely a mass issue. Adding another pair of gear to the front of the plane (also on pylons) delayed the wobble until about 130m/s (with a 65 ton payload; they're fine with a sensible load), which may be enough to get off the ground so long as I don't dawdle.

The main problem is that I'm trying to use fighter-plane landing gear on a C-17.

[MarvinKitFox]

Very simple solution.

Mount a couple (or six or so) seperatrons around your nose, oriented vertically.

Don't try to lift the nose by leverage on the gear, just *lift* the nose by rocket power!

Empty seperatrons weigh virtually nil, yet for a brief moment they will provide the energy needed to accomplish your runway rotation without stressing your wheels.

[Wanderfound]

Very simple solution.

Mount a couple (or six or so) seperatrons around your nose, oriented vertically.

Don't try to lift the nose by leverage on the gear, just *lift* the nose by rocket power!

Empty seperatrons weigh virtually nil, yet for a brief moment they will provide the energy needed to accomplish your runway rotation without stressing your wheels.

Rotation is not a problem. Lifting the nose is not a problem. The problem is the front gear (not the rear) collapsing when they're carrying ~100 ton of spaceplane down the runway at 110m/s.

The connection to lifting the nose was just a timing coincidence; I can get the same behaviour to occur without touching any controls except throttle.

[O-Doc]

They are failing because you are putting your craft's weight onto two gears. Make a line with the rear gears so you can have more weight bearing points of contact with the ground.

[Wanderfound]

They are failing because you are putting your craft's weight onto two gears. Make a line with the rear gears so you can have more weight bearing points of contact with the ground.

Again: front gear, not rear.

At the time that they're failing, there are seven sets of gear in contact with the ground, and the weight of the plane is shifting away from the gear that fail, not towards them.

[Bloodlance]

i found out that in the new patch 0.24.1 and 0.24.2 there is a problem with parts flying off our crafts more easier then before.

basically i mean that in example, earlier builds of fighter craft i have made, now loose 100% off all external wings and parts if i simply try to turn @150++ speeds. Even a slight turn makes my wings be torn off.

i assume part of your gear stress+pressure from the force against them when you move faster makes them get loose.

Meaning you need some VTOL in your craft for landing and takeoff.

my 2c.

[O-Doc]

Again: front gear, not rear.

At the time that they're failing, there are seven sets of gear in contact with the ground, and the weight of the plane is shifting away from the gear that fail, not towards them.

Thanks, I got that the first time. With your strut work, the gears are the only soft part of your plane and they just happen to be the primary load bearing point. When the weight comes off the front wheels they have more freedom to roll. Because they are still bearing so much weight, they are going to roll sideways. Splat. Those strange middle gears are alwyas going to create a condition whereby the front wheels come off the ground under different loads to each other. You need to remove that see-sawing effect by moving those gears to the back and in line with the rear gears.

Another pro tip is not to use single gears on heavy planes. Thy are slightly off center which is fine for light craft. Not so fine for heavy planes. Always use gears n symmetrical pairs.

[Wanderfound]

Thanks, I got that the first time. With your strut work, the gears are the only soft part of your plane and they just happen to be the primary load bearing point. When the weight comes off the front wheels they have more freedom to roll. Because they are still bearing so much weight, they are going to roll sideways. Splat. Those strange middle gears are alwyas going to create a condition whereby the front wheels come off the ground under different loads to each other. You need to remove that see-sawing effect by moving those gears to the back and in line with the rear gears.

Another pro tip is not to use single gears on heavy planes. Thy are slightly off center which is fine for light craft. Not so fine for heavy planes. Always use gears n symmetrical pairs.

Cool, thanks. I'll try replacing the front centre gear with another lateral pair up front.