Landing Gear

[AuddieD2015]

I have read dozens of posts on "Why does my plane turn 90 degrees and explode on the lawn?" I have made certain that my wheel are straight and started using universal to line them up. I have made sure CoL is slightly behind CoM and wheels are just at the CoM so you can actually lift off. That hasn't changed. I am making bigger aircraft and they need bigger wheels. Ever since I started using anything bigger than the small retractable or the fixed landing gear, no plane makes if off the runway. They wobble back and forth at around 35 m/s, turn hard left or right at about 45 m/s.  I would not think that you could get away with using small gear on a medium or large airplane or STO. For one, the back end of the plane would get knocked off if you nosed up even slightly.

Is it something with the level 3 runway? With KSC in general for plane takeoffs?

As a side note, I took off in a small supersonic plane and flew to and landed on then took off from the northern ice caps. Planes are working. 

[Streetwind]

I usually don't suggest this, because I'm of the opinion that it's an unnecessary crutch in 99% of all cases... but for planes, it might actually be required:

Have you tried autostrutting every part? Especially the wings, and the parts the wheels are attached to?

[Coyote Foxtrot]

Are you using a tail-dragger or tricycle gear configuration? There is a certain condition with a tricycle gear configuration that causes the aircraft to be unstable in yaw while on the ground. It happens most with landing due to braking action, but I have seen it on some of my crafts where the aerodynamic configuration of the plane pushes the nose into the runway. If force is applied in pitching down whether from engine thrust, aerodynamics, or braking, more force is applied to the nose wheel increasing the nose wheel's normal force and therefore friction. With that increased friction any deviation will cause the plane to turn and with it being in front of your center of mass, it will cause the craft to turn more. The default settings for the steerable gear have their angle limiter decrease with greater speeds (where taxi speeds are essentially below 10 m/s).

You can change it so it doesn't do that, but your nose gear will still have too powerful of a correcting force and can cause you to spin out (and if you don't use a steerable wheel for your nose gear you can't even  steer). The better solution is at a medium speed to start pulling up to get pressure off the nose wheel disempowering its turning power. You will lose the ability to correct yaw from the nose wheel and will have to rely on your vertical stabilizer and rudder, but there should be at this point sufficient speed to use them assuming it is large enough and far enough away from the center of mass.

[AuddieD2015]

11 hours ago, Coyote Foxtrot said:

Are you using a tail-dragger or tricycle gear configuration?

Tricycle arrangement. As much as I knock my tail off taking off, I should do a tail dragger since I am already dragging it! I will try your suggestions along with those of @Streetwind. I generally don't use autostrutting unless my rocket is flying apart on the way up but it's usually too late by then.

[18Watt]

I suggest mounting all landing gear to the center fuselage, then offsetting the main gear if needed.  Avoid mounting landing gear to wings or engine pylons, or any part laterally mounted from the center fuselage.

[jimmymcgoochie]

I have a few suggestions that might help you:

• Put a single small wheel on the back to counteract tail strikes. It won't save you from a bad landing, but for takeoffs it should do the job.
• Activate advanced tweakables (main menu settings) and then override the friction control settings on your front landing gear and reduce it to 0.5 or lower. Excess front wheel friction can make it act like a pivot and since the thrust is (almost) always coming from behind that pivot point, once it's going even slightly sideways it'll tend to keep going until it spins and/or crashes, but reducing friction can make a huge difference to this.
• Tweak the landing gear positions so that you're at a slight nose up pitch on the runway and/or angle your wings so that they have a few degrees of positive angle of attack (leading edge above trailing edge). KSP wings work solely by their angle to oncoming air so angling them like this means you get some/more lift on the runway which makes it easier to take off, though it's a balancing act as too much AoA can produce excessive drag at higher speeds.
• Override the suspension settings and increase spring and damper strengths. Weak suspension is more likely to sag under load which can make a bad situation worse, but strong suspension will resist this and keep your plane stable on the ground.
• Add some pitch control at the front of your plane. These are called canards, but you don't have to use the "canard" parts for this- although they are pretty good for it with fairly fast response times. When trying to take off with just pitch controls at the rear, the elevators actually push the plane down in order to lift the nose up, reducing lift; canards at the front will actively raise the nose and provide more lift when you need it most. You should also disable pitch controls on control surfaces that aren't at the very front or back of your plane as they'll provide minimal benefit but increased drag and could even make it harder to control.

[king of nowhere]

On 2/10/2023 at 3:30 AM, AuddieD2015 said:

I have read dozens of posts on "Why does my plane turn 90 degrees and explode on the lawn?" I have made certain that my wheel are straight and started using universal to line them up. I have made sure CoL is slightly behind CoM and wheels are just at the CoM so you can actually lift off. That hasn't changed. I am making bigger aircraft and they need bigger wheels. Ever since I started using anything bigger than the small retractable or the fixed landing gear, no plane makes if off the runway. They wobble back and forth at around 35 m/s, turn hard left or right at about 45 m/s.

 

you did not mention friction, so i assume it's the problem. the front wheel is making drag on the runway. you can right-click on the wheel and manually lower its friction parameter

[Vanamonde]

Another thing that can help is setting the braking force higher on the rear wheels and lower on the forward wheels. 

[User_1234]

No friction control for the front wheel. High friction control for the rear wheels. No steering for all wheels. Make sure all gear are perfectly vertical (use the snap tool in the SPH).