Landing planes in KSP2

[azgar]

I have being building some airplanes in KSP2 recently.
And I have noticed that I do not quite understand how to design them to land properly.
Some of my planes land quite well and reliably. Especially the slower ones, with a low wing load and low landing speed.
Bit some of them are bouncing off the runway and just won't stay on the ground. Or they turn over the wing on a ground run while braking.
I must mention I prefer my planes to be flyable all the way without SAS turned on.
My thoughts are I am probably locating my landing gear not quite correctly. Usually it's classical trike scheme, but the relative hight of main and nose wheels, placement of main gear relative to center of mass, something else I am doing wrong?
If someone could formulate a design principles and methods of landing that make a well behaved plane, that would be beneficial for lots of people, I think.

[Periple]

Hi azgar! Yeah I think it's your landing gear. Here's some things to look at – this applies to tricycle landing gear like you have on your plane, it's a bit different for tail-draggers.

1. The main gear should only be juuust aft of the center of mass – your plane rotates on it when taking off and you want it to be as delicately balanced as possible without tail-planting when it's parked.
2. All landing gear should be vertical relative to the ground on both axes. Not tilted.
3. If your nose gear is taller than your main gear, the plane will take off easily but may bounce on landing, as your wings are still generating lift when you touch down.
4. If your nose gear is shorter than your main gear, it'll be harder to rotate for take-off, but your plane will want to stay on the ground on landing because your wings will be generating less lift with all the gear on the ground.

So if your problem is that your plane wants to bounce on landing, then move the main gear aft or make the nose gear shorter/main gear taller. If you have the opposite problem – plane doesn't want to take off – then make the opposite corrections.

I hope this helped and happy landings!

[K^2]

The big difference between KSP1 and KSP2 is that KSP1 had very strong reaction wheels from cockpit alone, giving you a lot of authority even at low speeds. With KSP2, you have to be more mindful of the aerodynamics. You also can't rely on stability assist as much, especially at low speeds and low engine thrust. So again, you have to make the plane more controllable with aerodynamics alone.

I can see too potential problems with this particular plane - though it's hard to be sure without looking at the overlays available in VAB.

1. Pay attention to your CoM in relation to your fuel tanks. If your CoM is not centered on fuel, as the fuel drains, the CoM shifts. On this plane, I would guess the CoM shifts back, closer to the engine. If you start with CoP very close to CoM, it might end up forward of CoM, which will make the plane want to pitch up at low speeds, making landing hard. A good test is to enable CoM overlay in VAB, note where it is, then set all the fuel tanks to hold zero fuel. If your CoM shifted, that means it will shift in flight. If the CoM is shifting back, you need your fuel further back or move other parts to shift CoM forward. Once you have CoM fixed on the plane regardless of how much fuel is being used, place CoP just behind or just above CoM, and that will give you stable flight.

2. Pay attention to the control surfaces. This plane has a very shallow v-tail with almost no control surfaces, so your wings have to provide a lot of the pitch control. The control surfaces themselves are very close the center of mass due to the plane being fairly short. This means that at low speeds, you'll get very low authority. You can rectify it in two main ways. First, in the procedural wings setup you can increase both the width and the length of the control surface. That will give you more authority. Second option is to give it more leverage. You can replace your wings/stabilizers with control surfaces for the v-tail. These have more authority, generally. You can go with a more conventional tail, with a dedicated horizontal stabilizer that's taken out further back. Or you can also add a canard in the front that will help you with pitch control.

Between these two, you can usually get something that's fairly good about gliding onto the runway. Once on the ground, another thing that's worth checking is making sure that your rear landing gear has no steering and front one has no braking. If you set the parking brake before touchdown with a configuration like that, you generally stay stable on the runway and have maximum braking immediately. You can play with adding a bit of braking power to the front to stop faster, but that might not be worth the sacrifice in stability.

[Sequence]

46 minutes ago, K^2 said:

another thing that's worth checking is making sure that your rear landing gear has no steering and front one has no braking

This makes a big difference to the space planes I've built. 

Something else that helped my planes be less bouncy on landing is to adjust the suspension strength on the front gear, make it really stiff. I noticed it was the front gear being so springy that was making me have to make multiple touch downs before coming to a stop. 

[Vl3d]

There's a bug in which the control surfaces are deployed inverted left to right, correct? This makes flaps impossible and rolls the aircraft.

[azgar]

There is not much shift of CG in this plane. The fuel tanks is sitting pretty much where CG is. And usually I am trying to land it right after I took off anyway, with maybe 10% of fuel spent max. But yes, that is one thing to consider.
I think the front landing gear being too high might be the problem indeed. Though It seems I have tried all the possible configurations moving them around already
Disabling the steering on main wheels and brakes on the front is what I actually have to try. Will see how it goes.
Checking the suspension on front gear is a great advice, will definitely try, thx.
The control authority does not seem to be a problem. It steers quite well in on every axis. Though I did increase an area of elevons a bit on later version of the plane.
One thing that I am suspect I am doing wrong might be coming in too fast. But again, when it is 70 m/s it is bounce off and recover. When it is 35 m/s it is bounce off, stall and crash.
The problem with the flaps is definitely there. But somehow I managed to fix it on some of my planes. I suspect, that it makes a difference, if you save/launch your plane with the property dialog still open or close it first.

Edited March 7, 2023 by azgar

[Razor235]

The default landing gear settings really suck and are bouncy as all hell.  It was like this in 1 as well until a fix later in development.   You can try adjusting spring stiffness and damper.  Higher stiffness will mean it will ultimately compress and bounce back less, while a higher damper will (or should anyway) absorb energy, letting it compress without bouncing as fast or at all.  Lowering friction on the nose will stop it veering too (why did that have to come back).

I personally don't think your main gear height is the problem.  I tend to make most of my planes sit slightly nose-up on the ground because I frankly don't care enough to perfectly align the things if they don't start on the same plane to begin with, and I don't have many issues at all.  You can try making your mains sit further out if you'd like.

 The way SAS is now may also cause havoc on landing, if possible disable it at or just prior to touchdown, or fly without it entirely if you can (I see you already do that, but as a general tip).  This is probably compounded by wobbly joints in some cases.

A "perfect" landing is stalling just at the moment of touchdown for a nice smooth and slow rollout. In practice especially with no physical feedback, third person view, keyboard controls, and different planes every flight a carrier landing near-bellyflop or a long float and mushing into the pavement is what's going to happen 9 times out of 10 and will serve just fine.  We just fly the things, some other poor shmuck has to service the gear!

[JoeSchmuckatelli]

I just wish the wheels did not bounce me quite so high.

[regex]

Flaps help immensely, as does adding in an action for reversing thrust on engines that allow it. Naturally you'll also want to keep your fingers on the brake button but make sure to reduce or eliminate braking force on the front wheels, you'll tend to flip otherwise.

8 hours ago, Vl3d said:

There's a bug in which the control surfaces are deployed inverted left to right, correct? This makes flaps impossible and rolls the aircraft.

Flaps are absolutely possible.

Spoiler

 

[Vl3d]