Landing Gear problems

[Gauss H2K]

Here is my SSTO

Here's my query, during takeoff this vessel likes to yaw left and right randomly and violently, what about my landing gear placement is causing this?

Edit: since it's not apparent in the pictures, the back wheels aren't attached to the wings. They are attached to a structural pylon attached to the distal fuel tanks.

Further Edit: After increasing the size, and control authority of the rudder, I was able to rule out the rudder as an effect, as the problems persisted. Furthermore I saw that at 4 wheels, the back wheels tended to "break" under the weight. Adding more wheels prevented the breaking, but didn't solve the yawing problem. At this point I believe it has something to do with using the small hardpoints themselves, since there were no wheels that were inverted or at an angle other than 90.

Final Edit: Look at these pictures:

The only thing changed about the landing gear, was switching from a small hardpoint to 2 cubic octagonal struts. Landing Gear problem solved. Don't use small hardpoints.

Edited March 12, 2014 by Gauss H2K
Clarification purposes

[Claw]

It looks like your landing gear is attached to the wings. If your wings are flexing during takeoff roll (which they probably are as you go faster), then it will cause your landing gear to flex.

Try either attaching them to a more structural member or strengthen your wings with struts.

Also, make sure your landing gear aren't too far behind the CoM. Otherwise when you try to rotate the downforce from your control surfaces will cause additional force on the wings and gear, making them warp further.

[Hodo]

They aren't straight.

That is the simplest answer without actually testing your aircraft.

[*Aqua*]

Often it helps to have two front wheels. Have only one usually make my planes flipping over.

[Behemoose]

I'll have to agree with Claw here. If your wings are flexing at all, it's going to cause the gear to bounce, which will induce the yaw you're talking about. Make sure you've used plenty of space duct tape on those wings.

[Gauss H2K]

It looks like your landing gear is attached to the wings. If your wings are flexing during takeoff roll (which they probably are as you go faster), then it will cause your landing gear to flex.

Try either attaching them to a more structural member or strengthen your wings with struts.

Also, make sure your landing gear aren't too far behind the CoM. Otherwise when you try to rotate the downforce from your control surfaces will cause additional force on the wings and gear, making them warp further.

I know about this, which is why I placed them on structural pylons, attached to the radial fuel tanks. I've also already "over" strutted the wings to everything on the ship.

Often it helps to have two front wheels. Have only one usually make my planes flipping over.

On the earlier model I had, I had 2 front wheels attached on pylons, I removed them because with 4 wheels, it was still yawing.

They aren't straight.

That is the simplest answer without actually testing your aircraft.

I used snap-to to place all of them making sure they would be straight and only rotated the front gear by 180 degrees. If it were the case that the snap to didn't cause your gear to be straight by default, then this issue should've plagued the other 5 spaceplanes I made, but alas this isn't the case.

[DeMatt]

A few ideas:

• MOAR STRUTS. Yes, put more struts between each wheel as well as the main body of your spaceplane. Structural pylons are only more resistant to bending because they're so short.
  
• MOAR TAILFINS. I look at that and wonder where, exactly, the rudders and tailfins are.
  
• LESS ELEVONS. I'd want to see a dorsal and ventral planform before definitively saying this, but maybe reducing the rear control surfaces to "Small" from "Standard" might actually help.
  
• I have to wonder if you aren't just a tad overengined, running (apparently) four RAPIERs and two LV-Ns. Just how far away is this spaceplane expected to fly?
  
• What happens if you take it to the runway, turn on SAS and the engines, and let it drive straight ahead? Does it still wobble? How about if you don't even turn SAS on?
  
• What happens if you turn it on its tail, add launch clamps, and try launching it vertically? What happens if you (mostly) empty the fuel tanks, and act like it's returning?
  

[Othuyeg]

Your front landing gear is backwards. If you use SAS it will not understand this and it will keep turning one way, trying to fix the course but the steering in that gear steers it the wrong way, merely putting it more off course.

[Snooze]

Your front landing gear is backwards. If you use SAS it will not understand this and it will keep turning one way, trying to fix the course but the steering in that gear steers it the wrong way, merely putting it more off course.

You can "Invert Steering" to correct this.

[Claw]

On the earlier model I had, I had 2 front wheels attached on pylons, I removed them because with 4 wheels, it was still yawing.

I used snap-to to place all of them making sure they would be straight and only rotated the front gear by 180 degrees. If it were the case that the snap to didn't cause your gear to be straight by default, then this issue should've plagued the other 5 spaceplanes I made, but alas this isn't the case.

I doubt you need 2 front gear unless you're hiding some massive weight in that plane somewhere. I'm sure people will disagree with me, but the stock plane landing gear are actually pretty stout. I also believe that you're using symmetry and angle snap, so the parts should be sitting correctly on the plane.

I know about this, which is why I placed them on structural pylons, attached to the radial fuel tanks. I've also already "over" strutted the wings to everything on the ship.

Okay, so if that's the case, the same bending phenomenon applies to fuel tanks and pylons as well. I'm going to guess you're not actually using the "structural pylon" piece because it would show up in the staging stack. Even if you are using the small hardpoint, the added lever arm can cause the FL-T800 tanks to flex. I'm not saying this IS the problem, I'm just saying it often causes what you're describing.

Your pictures are helpful, but it's hard for me to guesstimate how much it weighs because I can't see your center stack. With the FL-T800s mounted the way I see in picture, you're getting two bending connections. Plus, if you're using hardpoints, they will cause more sideways torque than can amplify this. The actual "structural pylon" part is worse for this than the hardpoint.

You can check if this is indeed the problem by launching your craft. Roll your camera angle around to look at the front of your plane, but slightly off to the side (so you can see down one of the gear). Launch your plane and snake it back and forth in yaw. It may look like your main gear is "skating" on the runway. Watch as the speed builds up and you keep snaking. If the gear bends under, then the hardpoint/pylon isn't helping.

There are a few ways to fix that, but I won't bore you if it's not the problem.

[Othuyeg]

You can "Invert Steering" to correct this.

Yes, indeed he can. Somehow for me it has caused problems however. The steering just doesn't seem to work properly when inverted.

[Gauss H2K]

• 
  

A few ideas:

• MOAR STRUTS. Yes, put more struts between each wheel as well as the main body of your spaceplane. Structural pylons are only more resistant to bending because they're so short.
  
• MOAR TAILFINS. I look at that and wonder where, exactly, the rudders and tailfins are.
  
• LESS ELEVONS. I'd want to see a dorsal and ventral planform before definitively saying this, but maybe reducing the rear control surfaces to "Small" from "Standard" might actually help.
  
• I have to wonder if you aren't just a tad overengined, running (apparently) four RAPIERs and two LV-Ns. Just how far away is this spaceplane expected to fly?
  
• What happens if you take it to the runway, turn on SAS and the engines, and let it drive straight ahead? Does it still wobble? How about if you don't even turn SAS on?
  
• What happens if you turn it on its tail, add launch clamps, and try launching it vertically? What happens if you (mostly) empty the fuel tanks, and act like it's returning?
  

• I have 2 large Tail fins at the back, which are two delta wings with two small standard control surfaces, Edit: In an attempt to improve flight characteristics even better I added a larger Tail Fine with 4 standard surfaces for better yaw control. It flies better now, or at least FAR tells me it does... since the same problem persists.
  
• The large standard control surfaces are being used as Ailerons, not elevons. The forward canard and rear elevators are there to help at mach speeds
  
• It weighs in at 80t, 4 rapiers is just enough for TWR, and the LV-Ns are to suit it's purpose of being a shuttle between moon bases in Jool's SOI
  
• It wobbles in both scenarios, I do believe it's something to do with the hardpoint wheels.
  
• It flies just fine, I've built many an SSTO so I know how to design them to either remain at the same level of stability as the fuel drains, or become more stable. I won't try a vertical ascent because it has 1-1.1 TWR on rapiers.
  

I doubt you need 2 front gear unless you're hiding some massive weight in that plane somewhere. I'm sure people will disagree with me, but the stock plane landing gear are actually pretty stout. I also believe that you're using symmetry and angle snap, so the parts should be sitting correctly on the plane.

Okay, so if that's the case, the same bending phenomenon applies to fuel tanks and pylons as well. I'm going to guess you're not actually using the "structural pylon" piece because it would show up in the staging stack. Even if you are using the small hardpoint, the added lever arm can cause the FL-T800 tanks to flex. I'm not saying this IS the problem, I'm just saying it often causes what you're describing.

Your pictures are helpful, but it's hard for me to guesstimate how much it weighs because I can't see your center stack. With the FL-T800s mounted the way I see in picture, you're getting two bending connections. Plus, if you're using hardpoints, they will cause more sideways torque than can amplify this. The actual "structural pylon" part is worse for this than the hardpoint.

You can check if this is indeed the problem by launching your craft. Roll your camera angle around to look at the front of your plane, but slightly off to the side (so you can see down one of the gear). Launch your plane and snake it back and forth in yaw. It may look like your main gear is "skating" on the runway. Watch as the speed builds up and you keep snaking. If the gear bends under, then the hardpoint/pylon isn't helping.

There are a few ways to fix that, but I won't bore you if it's not the problem.

It does weigh a lot, as the SSTO's I build are usually <40t. This one is 80t, but I am quite proud, as aside from the landing gear, the FAR data tells me it flies very well.

Edited March 12, 2014 by Gauss H2K

[Behemoose]

It wobbles in both scenarios, I do believe it's something to do with the hardpoint wheels.

I'm inclined to agree. I was having the same trouble with one earlier tonight. I strutted the hardpoints together and it helped. Another thing that might help is anything that induces shaky SAS. Make sure you're not using too many control surfaces, or that they're too far out to the edges of the wings. That can cause the SAS to lose its mind.