Stability problems with high altitude aircrafts

[frankieorabona]

I need a plane to do surveys above 17000mts.
I have designed this aircraft with a rocket, but on the runway when I exceed 100m/s the aircraft starts to drift left and right until it crashes off the runway....
Only rarely have I been able to get it off the ground. Do you have any advice?

I post the file of the aircraft and some photos.

My File - High Altitude Jet MKI

[18Watt]

• Your main wheels do not look straight- they look angled slightly down.  Use the rotate tool to correct that.
• That airplane should be lifting off below 100 m/s.  Perhaps move your elevators farther aft, or offset the nose wheel so that it sits slightly nose high on the runway.
• Moving the vertical stabilizer/rudder farther aft might help too.
• Try turning Rigid Attachment on for the landing gear in the PAW.  Autostruts too.
• On Kerbin, you are near the limit of the weight those landing gear can handle without problems.  You should be ok, but might get better results with beefier landing gear.

Edited January 2, 2022 by 18Watt

[frankieorabona]

45 minutes ago, 18Watt said:

• Your main wheels do not look straight- they look angled slightly down.  Use the rotate tool to correct that.
• That airplane should be lifting off below 100 m/s.  Perhaps move your elevators farther aft, or offset the nose wheel so that it sits slightly nose high on the runway.
• Moving the vertical stabilizer/rudder farther aft might help too.
• r in the PAW.  Autostruts too.
• On Kerbin, you are near the limit of the weight those landing gear can handle without problems.  You should be ok, but might get better results with beefier landing gear.

Thanks for the advice...I have followed some of your advice but the situation seems to have gotten worse.
In particular, I straightened the two rear wheels and moved the front wheel aft so that the plane is slightly tilted up.
I have turning Rigid Attachment on for the landing gear
I moved back the three upper flaps, but it did not improve at all, on the contrary now it is not only heeling but also veering to the left.

 

[18Watt]

38 minutes ago, frankieorabona said:

on the contrary now it is not only heeling but also veering to the left.

Just for the heck of it, try removing the vertical stabilizer/rudder completely, and see what the takeoff looks like.  Yes, you’ll need a vertical stabilizer/rudder, but I’m wondering if the one you have is causing issues.  Try removing it completely and just see what happens with your takeoff.

Are you taking off with SAS on?  I do recommend taking off with SAS on, locked Pro-Grade.

Did you check that your flight control surfaces are assigned to the correct axes?  The elevators should only be assigned to pitch and roll, but not yaw.  Likewise, the vertical stabilizer/rudder should only be assigned yaw, but not pitch or roll.

Finally, your nose gear appears to be mounted straight, but sometimes when mounting to cockpits they can be angled subtly forward or back, which you can check with the Rotate tool.

[frankieorabona]

38 minutes ago, 18Watt said:

Just for the heck of it, try removing the vertical stabilizer/rudder completely, and see what the takeoff looks like.  Yes, you’ll need a vertical stabilizer/rudder, but I’m wondering if the one you have is causing issues.  Try removing it completely and just see what happens with your takeoff.

Removing the rudder/stabilizer it no longer turns left but right (LOL), in addition to that after 100 m/s it continues to drift.

39 minutes ago, 18Watt said:

Are you taking off with SAS on?  I do recommend taking off with SAS on, locked Pro-Grade.

With SAS on, things get worse...I can't even reach 100 m/s.

40 minutes ago, 18Watt said:

Did you check that your flight control surfaces are assigned to the correct axes?  The elevators should only be assigned to pitch and roll, but not yaw.  Likewise, the vertical stabilizer/rudder should only be assigned yaw, but not pitch or roll.

Doing so makes the aircraft unmanageable.

42 minutes ago, 18Watt said:

Finally, your nose gear appears to be mounted straight, but sometimes when mounting to cockpits they can be angled subtly forward or back, which you can check with the Rotate tool.

I had already tried to tilt it, but straight remains the best solution with some takeoffs

[Vanamonde]

This is a classic problem with KSP aircraft going back many years. What happens is that as lift starts to reduce the effective weight of the plane, it begins to wobble around on the landing gear. One of the easiest fixes is to adjust the height of the landing gear so that the plane sits with a slight nose-up attitude. This tends to shift the weight to the pair of rear wheels, which is much more stable than trying to unicycle on the nose wheel. Also, replace those wheels with more robust models as soon as you have the science points. They are weak, tend to flex, and the one you're using as a nosewheel is a caster (pivots freely) and does not help with steering or stability. 

[OHara]

I was also unable to get that aircraft to take off. 

Before taking off, an aircraft needs to rotate the nose up a few degrees (unless it already is raised by the landing gear) but the I couldn't do that with the  elevators trimmed (alt-X) fully pitch-up.  There just isn't enough lever arm to the main landing gear for the elevators lever up the mass of the craft.  At 100 m/s (223.604 miles per hour) the wheels start to shimmy.

If I move the landing gear forward, closer to the centre of mass, then I have enough lever arm (barely).

So probably try more of these adjustments, going further than I did.

Sometimes it is nice to rotating the wings up a little (giving them angle of incidence) or rotate the rear stabilisers down, until the centre of lift is right above the centre of mass.  CoL behind CoM is good advice for beginners, to make the aircraft stable, but once you know it is stable you can adjust to make it easier to fly.

1 minute ago, Vanamonde said:

This is a classic problem with KSP aircraft going back many years.

and also real aircraft (wheel-barrowing) .  

[frankieorabona]

The main problem was the wheels...I unlocked the LY-10 and now it doesn't drift or veer any more

[18Watt]

1 hour ago, Vanamonde said:

Also, replace those wheels with more robust models as soon as you have the science points. They are weak

As a general rule, whatever landing gear looks right, use the next size larger.

 

1 hour ago, OHara said:

There just isn't enough lever arm to the main landing gear for the elevators lever up the mass of the craft.

Two suggestions here:

• Offset the elevator surfaces aft, and/or
• Add ‘canard’ control surfaces, or elevators mounted on the nose.

1 hour ago, OHara said:

and also real aircraft (wheel-barrowing)

Yup.  An under-inflated or worn-out nose strut can amplify the problem, and also induce violent shimmies in real aircraft.

[Snark]

A couple of other points:

• If you haven't done so already, turn off everything but yaw on the vertical stabilizer.  You want it to be helping with yaw only; you need to turn off roll control.  Doing this will help with stability, for reasons that I can explain if you're interested in, but it's a long paragraph so I won't bother unless you're interested. 
• Your CoM is pretty far back, which is likely to make the craft want to flip.  If you can move the CoM forward-- for example, by sliding the engines forward-- that may help.