Why can't I keep this airplane straight on the runway?

[Zeiss Ikon]

(1.6.1, RSS/RO/RP-1 -- lots of mods, of course)

I'll confess.  I hate KSP airplanes.

And this one demonstrates why.  Simple design, looks like a cross between a Mig 15 and a really early German jet (HE-111 cockpit and engine on top will do that).  If this were an R/C airplane, I could throttle up, wait a few seconds to build speed, rotate, and then trim as I climb out before testing with rolls, loops, and so forth.  In Realism Overhaul, however, I can't get it far enough down the runway to get anywhere close to flight speed (should be around 40 m/s with flaps down, maybe less).

 

I've built and even designed model airplanes -- my center of mass is ahead of my center of lift (maybe too far ahead; in an R/C bird I'd probably find it to be nose heavy), ahead of the main gear.  Steering is disabled on the main gear.  I've got the wing set at a small incidence angle, and the horizontal tail a couple degrees less (I expect to need nose up trim on early test flights).  But every time I start my takeoff roll, the slightest deviation either direction compounds.

I've tried setting the gear parallel to centerline, one degree toe out, and up to three degrees toe in.  Same result.

Here's the craft file.  Any help?

Edited December 31, 2019 by Zeiss Ikon

[bewing]

Have you looked in the FAQ on this forum? Runway veering is a very common airplane issue, but it has many possible causes. Please note that this is not a bug -- all of these issues also happen in real life airplane engineering.

As far as I'm concerned, the most common problem that causes "ground instability" is that the "ground drag" on your front end is bigger than the drag on your back end (when calculated in terms of torques).

And the simplest and easiest way to fix that is to go into your landing gear context menu, switch your front wheel from "auto" to manual friction (not traction control!) and then reduce the friction on your front wheel down to .5 or less. If that doesn't do it, reduce it further.

 

 

[Zeiss Ikon]

I'll go look at the FAQ again -- been a while since I read through it.

For an R/C model, the solution to this would be a few degrees of toe in on the main gear (acts as a brake on the outside of a veer or the "heavy" wing), but I've never flow tricycle (the three powered R/C models I've owned and flown were all conventional gear, which steers a bit differently).

I'll try reducing the nose wheel friction to half or less and see if that helps.

[Guest]

Try turning on advanced tweakables and set spring and dampen to 1.5 both also (more importantly) try moving the rear wheels up closer to your center of mass so that the plane can tilt on the back wheels easier. 

[Zeiss Ikon]

@Dale Christopher Thanks for the suggestions.   @bewing mostly had me covered.  Lowering nose wheel friction and following the FAQ suggestion (and method) of removing all toe (in or out) on the main gear, combined with a couple minor tweaks (moved the engine down and back a little, which moved the CG aft, moved the main gear forward, as you suggest, which lightens the load on the nose gear and eases rotation, shortened the nose gear to reduce wheelbarrowing) and it now tracks pretty straight and corrects predictably.  I also made some further adjustments to wing incidence and decalage based on trim requirements.  As with any longitudinally stable aircraft, it requires trim changes for different speeds, though the top mounted engine stands in for the down thrust commonly used in models.  Also as expected, once off the ground, it flies very much like an R/C advanced trainer -- and makes me wish for a joystick.

I had underestimated the wing loading; it actually needs almost 70 m/s to rotate, and with near-full tank, dirty stall (gear down, 30 flaps) is close to 65 m/s.  IOW, it takes off more like a fighter than a transport or trainer, probably not surprising considering the resemblance to an F-86.  I may mount parachutes; there is no way I'll ever be able to land this, while flying with the keyboard.

I'm on my laptop now, but I'll come back later and show a photo of the corrected version, and upload the working craft file.

[Guest]

I used to make designs that had the rear wheels way back near the end of my craft and never could figure out why my takeoffs were so crappy... as soon as I tried to use control surfaces the plane would wobble all over the ground as it went down the runway and swerve all over the place. But soon as I figured out that the rear wheels were supposed to be a fulcrum point which the plane tilted on I started setting the rear wheels way up near the center of mass. Now all my planes take off like they were light as a feather.

[Zeiss Ikon]

@Dale Christopher If you look at the top of this thread, you'll see my wheels weren't far behind the CoM -- the nose actually bounced a bit when the plane loaded onto the runway.  I moved the CoM back a smidge, and moved the gear forward a bit, and now the nose bounces even more when the plane loads.

Doesn't look like much changed -- just lowered the engine and moved it aft a short distance (20-30 cm at most).  The other changes are hard to see or just plain invisible.

But now, I can take off, and it's a nice flier once in the air and trimmed.

Here's the updated craft file.

Thanks for the help, @bewing, I'd forgotten the FAQ had information specific to runway problems.

 

[bewing]

As far as landing goes, once you burn off most of the fuel your craft will either become unstable and crash, or it will land much easier than it takes off.

Additionally, landing and takeoff depend a lot on wing lift. And the wings you stuck on that thing are the very worst wings in the game. They are really only in the game as a cruel trick that the devs play on airplane newbies. They mass twice as much as they should for the lift they give -- relative to all the other wings in the game. Your plane would fly/take off/land better with any other set of wings than these.

 

Edited January 1, 2020 by bewing

[Zeiss Ikon]

Sigh...

My CG does shift from a full tank to empty.  Down, a few centimeters.  No discernible fore or aft movement at all.  I designed it with the CG in the center of the fuel tank.  If the airplane is stable with a full tank, it'll be stable when empty.  And it's heavy enough at just under 4 T that the weight of a single Kerbal more or less (two seat cockpit, may or may not carry a scientist on the science mission it's built for) won't change the CG location enough to notice (test flights were done with only Jeb aboard, so if anything it'll be more stable with the second seat filled).

I may try building another version with the other swept wings, but as you note, this should land better with depleted tanks, because after burning most of the 1200 liters of kerosene in that tank under the engine, it'll be 600-800 kg lighter (roundly 20% of its weight) .  I'll also give another test flight with, say, 10% fuel and see how much more I can slow it down when it's lighter.

The main reason I don't have confidence in my ability to land it is because I can't seem to land anything in game.  I blame keyboard controls.  Thirty-some years ago, I was a pretty good pilot with an R/C transmitter...

[bewing]

I'm not sure if those non-stock cockpits are different, but in stock there is no mass change for empty/full cockpit seats. Only for external command seats.

[linuxgurugamer]

10 minutes ago, bewing said:

I'm not sure if those non-stock cockpits are different, but in stock there is no mass change for empty/full cockpit seats. Only for external command seats.

 I thought that was fixed

[bewing]

Oh, maybe it was. Maybe I failed to pay attention to that one.

[Guest]

Hmmm, when I was building a vtol ages ago with the stock cockpit I did notice a change in handling when there was a Kerbal onboard it would be slightly more front heavy o_o

[nepphhh]

Be very careful of placing gear on wings. Much safer to place it on a known-perfectly-straight fuselage part and shift it out. Even very slight angle on the gear makes KSP crap the bed.

[UncleManuel]

This is an age old question: 

 

But basically everything was covered here already. 

Addition by me: Turn on CoM and CoL indicators and watch how they behave with full tanks and empty tanks. Sometimes weird characteristics on landing are due to a CoM shift... 

[RoninFrog]

Try turning off steering for the rear landing gear (if you haven't already). For all of my planes, the rear wheels tend to overcompensate when steering, leading to a fishtail/crash.

[Zeiss Ikon]

No discernible CoM shift from full to empty tank (CoM marker is inside the tank), steering on the main gear was off from the beginning.

After fixing the gear, I was able to complete the design mission without problems, despite using the "worst" swept wings in the game.  And I slightly understated performance -- while it's only good for about 200 m/s at sea level, it'll do that on 70% cruise throttle at 7000 m.  I was able to visit six biomes (For Science!) near the Brownsville, TX launch site (in Realism Overhaul) and return to base on the 1200 liters of fuel I provided, though I should add at least another couple hundred in the stored version -- I landed (on the grass near the launch center) intact, with literally about a minute of full power fuel remaining.  As expected, the airplane slows down much better when bingo fuel (that's close to 1 T of fuel weight burned off); I landed at around 50 m/s compared to a full-tank dirty stall speed around 65 m/s.

This is actually a nice handling airplane now that I can get it off the runway.  Trimmed for current speed, there's no significant change between MechJeb autopilot and "free" flight without MechJeb or SAS.  Long term no-SAS flight is marred only by a slow tip-off into a bank one direction or the other; likely another degree of dihedral would fix that.  I do need to reduce the built-in wing incidence by a couple degrees; at 200 m/s at altitude, I'm flying with almost five degrees nose down to hold altitude.  Glides well, too, when clean; zero pitch coincides with trim for about 100 m/s as currently set up, which ought to be close to minimum sink or maximum L/D.  Drag brakes would be welcome; the twin rudders are set up for air brake function, which changes pitch trim, but I don't have that tech unlocked yet (all parts in this aircraft are in the starting tech node for 1.6.1 Realism Overhaul).