Space planes and wheels, narrow is better?

[run1235]

i'm relativly adept at building the bodies of space planes in one go, (both in stock and far) and really only have to test the airframe itself once or twice but the part that takes me the longest and most testing is the landing gear, what i've noticed is that my planes seem to take off and land better with a narrower gear width, 1 wheel up front and two wheels in the back. what i'm curious about is why do i get better stability for having a narrower base? it makes almost no sense to me.

here's a pic of about how wide i have my wheels, any wide apart for the outside ones and i have uncontrolable wobbling on take off and landing is impossible, with this width i have nearly perfect take off but my landings require a parachute assist to slow me down fast enough to avoid crashing.

http://prntscr.com/ankk2r

[HebaruSan]

If you're putting the gear on the wings, it might be an artifact of the wing joint flexing under the weight of the fuselage. Each wing effectively forms a lever with the wheel as the fulcrum and the rest of the plane pushing down on one side, so the further apart those things are, the more the joint will bend under that stress, which can bring the wheels out of alignment just enough to cause problems.

Does it get better if you add struts between the body and the wings? You wouldn't want to fly that way because of drag, but it would test whether this is the issue.

[run1235]

4 minutes ago, HebaruSan said:

If you're putting the gear on the wings, it might be an artifact of the wing joint flexing under the weight of the fuselage. Each wing effectively forms a lever with the wheel as the fulcrum and the rest of the plane pushing down on one side, so the further apart those things are, the more the joint will bend under that stress, which can bring the wheels out of alignment just enough to cause problems.

Does it get better if you add struts between the body and the wings? You wouldn't want to fly that way because of drag, but it would test whether this is the issue.

i actually try my best to not have to use struts. (except when i'm connecting wing segments) but that does make a bit of sense since most of my designs have the gear on the wings. but what about my designs that dont have the wheels on the wings? they usually have the same problem and it's fixed by narrowing the rear wheel distance.

[HebaruSan]

49 minutes ago, run1235 said:

i actually try my best to not have to use struts. (except when i'm connecting wing segments) but that does make a bit of sense since most of my designs have the gear on the wings. but what about my designs that dont have the wheels on the wings? they usually have the same problem and it's fixed by narrowing the rear wheel distance.

In that case it might be something else, and it would help if we could see the problematic craft while it's having the problem.

[Wanderfound]

As Hebaru says: it's probably flex.

The most important thing in landing gear stability is to make sure that it's mounted dead vertical and dead straight (use absolute angle snap if uncertain). But it also needs to be mounted to parts that don't flex, so that it stays vertical; you can only get away with wing mounting if your wings are very rigid and/or your ship is very light.

Within those constraints, however, the wider and longer the wheelbase the better. You can sneak in some extra width while maintaining stiffness if you first mount the landing gear to the fuselage, then translate them outwards so that they appear to be wing mounted.

For lengthwise placement with a tricycle setup, you want the rear gear just behind CoM and the nose gear as far forwards as aerodynamics and aesthetics can tolerate. Really chunky Mk3 monsters may also benefit from some extra gear directly under CoM, and possibly doubling up the rear gear (to prevent them from buckling during takeoff rotation).

Edited April 4, 2016 by Wanderfound

[Ser]

On 03.04.2016 at 3:39 PM, run1235 said:

i'm relativly adept at building the bodies of space planes in one go, (both in stock and far) and really only have to test the airframe itself once or twice but the part that takes me the longest and most testing is the landing gear, what i've noticed is that my planes seem to take off and land better with a narrower gear width, 1 wheel up front and two wheels in the back. what i'm curious about is why do i get better stability for having a narrower base? it makes almost no sense to me.

here's a pic of about how wide i have my wheels, any wide apart for the outside ones and i have uncontrolable wobbling on take off and landing is impossible, with this width i have nearly perfect take off but my landings require a parachute assist to slow me down fast enough to avoid crashing.

http://prntscr.com/ankk2r

By my own experience I find the way you placed landing gear too narrow as they form a tall triangle with the front gear on the top. This design IMHO may tend to flip towards the forward-right or forward-left direction on every little nose wheel turn or side-slip while applying brakes, so I always try to stay around the Golden ratio: the distance from the nose wheel to any of the rear wheels should be approx. 2 times the distance between rear wheels.

But as I've personally found, the landing gear are often not the root of the takeoff/landing issues. The most of my troubles were from center of lift stability issues: having CoL too far back from CoM causes your plane not to be able to hold the nose up while landing, forcing you to approach on higher speed and thus have wobble/jumps/flips on the runway. The same is true for takeoffs. Having your CoL in the correct position providing you the sufficient maneuverability allows you to takeoff/land without having too much excessive velocity on the ground that renders your aircraft uncontrollable on the run. So you should be able to slightly pitch up while descending on final thus your plane won't tend to jump back into space after touchdown. If your plane tends to pitch down while decelerating at low speed and you cannot hold the nose up then your CoL is wrong.

P.S. I'm playing with the FAR mod

 

Edited April 4, 2016 by Ser