Flying Straight for Long Distances

[Quadkiller]

Hey everyone, aircraft flying noob here.

So I recently I tried flying a simple aircraft halfway around Kerbin while leaving it on time warp. The plane mostly flew straight but it kept pitching up slowly so I had to keep making adjustments. I'm almost sure this is because my COL is slightly above my COM.

From what I read online, adjusting the trim should do the job. So i tried adjusting it using Alt+W but nothing changed. I'm wondering if it has something to do with the fact that SAS was on.

Any ideas?

[Val]

The reason the crafts pitches up is because SAS keeps you flying in a straight line, literally, while the planet surface is curved. (behold my awesome paint skills)

 

Trim is not taken into account while SAS is active. To use trim you'll have to fly without SAS.

 

To do what you want, you need to get a mod that can keep your pitch relative to the surface. Here is one: 

 

 

Edited January 2, 2017 by Val
Split a sentence for clarification.

[AeroGav]

2 hours ago, Quadkiller said:

Hey everyone, aircraft flying noob here.

So I recently I tried flying a simple aircraft halfway around Kerbin while leaving it on time warp. The plane mostly flew straight but it kept pitching up slowly so I had to keep making adjustments. I'm almost sure this is because my COL is slightly above my COM.

From what I read online, adjusting the trim should do the job. So i tried adjusting it using Alt+W but nothing changed. I'm wondering if it has something to do with the fact that SAS was on.

Any ideas?

Yes, it is because of SAS.

SAS maintains a constant nose angle relative to Kerbol (their sun), not taking into account the curvature of the planet you are flying over. If you add trim it will simply fight it, trying to keep the nose pointing where it thinks you should be pointed.

You can try flying with SAS off, but that requires a directionally stable aircraft, and that's something I struggle with despite having thousands  of hours in game, building almost nothing other than aircraft and spaceplanes.  Some say this is due to a bug in the physics engine causing phantom rolling moment.

Conventional aerodynamic wisdom says the following things help a plane fly straight

• Wing Dihedral
• Lots of yaw stability, ie. generously sized vertical stab placed as far behind CG as practical
• High wing design, ie. wing above CG

Whilst these guidelines help, building a stable aircraft is still very hit-or-miss.

Despite having all three of the above qualities and a low part count, it slowly rolls left with SAS off.

OTOH , this thing can be trimmed to fly hands off 40km up, despite smaller stabilizer area relative to weight, lower mounted wing, less pronounced dihedral and more doohickeys hanging off it than a Christmas tree.

I'd say you've got three choices

1.   Download a better autopilot mod
2.   Just don't accept these survey contracts unless they within 5 minutes flying time of KSC.    I play airplane/spaceplane-only career games and i still won't touch them.   They are just not time efficient.  Spend an hour flying to the other side of Kerbin, an hour back, correcting the plane's flight the whole way, for a very small amount of cash?   Or do it with rockets, ok that's a minimum of three rockets (which must have v. close to orbital delta v) assuming your re-entry trajectory is perfect every time,  and get basically zero recovery value on them due to landing on the wrong side of the planet.
3. build a plane that can fly on prograde assist.

Because prograde assist tries to hold zero AoA you need to angle the wings upward when attaching them to the fuselage in order to get any lift.   _{If your plane is a canard,  you must angle the canards upward by at least the same amount so that the canards stall before the main wing.   All of this will radically reposition your CoL indicators, so you'll have to slide the wings aft a bit to get the blue ball behind yellow again.  Finally, you'll want to make further small adjustments to the angle of the canard or tailplane until the plane flies at small positive aoa hands off and untrimmed.}

Prograde assist does not attempt to correct roll but will try to dial out any yaw, so provided you got roll stability with dihedral it should want to keep the blue side up.

At  low altitudes, you prograde plane may drive you crazy with loop-the-loop antics since it'll generate too much lift when you bang the throttle open.   However,  as they get near their max altitude they calm right down and cruise pretty nice.

https://kerbalx.com/AeroGav/Bute-Ranger

This career mode plane can supercruise economically at high altitude with only occasional roll corrections in time warp.  Almost makes survey contracts worthwhile - but not quite.

 

[Quadkiller]

@AeroGav Thanks for the detailed answer. Since a stable enough aircraft is way beyond me I'll just stick to nearby contracts for now.

[bewing]

Are you familiar with a TiltProbeCore? This is one of the problems it was meant to solve. 

(Especially if you combine it with 2 or 3x physwarp, as I understand it.)

Edited January 2, 2017 by bewing

[ForScience6686]

I would rather adjust trim, your never at a constant when flying, speed, altitude and mass constantly change.  Find a good term seeing to get parabolic flight, then make small adjustments as needed to compensate.  A lot of pilots fly with trim only, and land and take off with the stick.

[bphft]

I'm aware this post is a few years old but I've found a fairly easy way to do this (thanks to knowing how to fly real aircraft). In real life what trim does is adjust a flap on the Airlerons (they face UP with more trim, down with less trim; up points the nose up, down points the nose down) to maintain the angle required to maintain a specific altitude. Speed needs to be adjusted too. The game's mechanics and available parts design (at least the parts I have access too, I only have maybe 8 hours in and this is my first time playing) don't include everything needed to maintain a specific speed and "trim" to accomplish this. At least that I've found, maybe further along in the progression of career mode the parts exist that are needed, and gadgets to control the throttle better are included, but they're not at the progression I'm currently at. I found a way around it to almost perfectly maintain a specific heading, altitude, and speed: 

1. And most importantly the plane has to be constructed so that it's very stable in the air so it stays at least fairly stable without SAS on. This is done by using multiple wings, and multiple "fins" (they're called winglets in the game). 

2. On the wings put 2 sets of Airlerons (the game calls them something like Elivators or something like that) on the back of the main wings, and one set on each of the other horizontal (not any of the vertical) wings and "winglets". On a real aircraft the outside control surfaces are the Airlerons, the inside (usually longer) control surfaces are the "Flaps" (I'll get in to flaps below). The airlerons are what has the Trim control surfaces on them. So I set up an action to force the outer control surfaces on the main wings and furthest rear wings to "Extend" (face UP). This replicates / simulates the Trim surfaces on a real plane's Ailerons. I adjusted the deployment angle to 10 on all of the ones I used as trim and set the action to toggle deployment. 

- This helps pilot a super smooth take-off. The plane lifts it self in the air after it gets to the proper speed. The runway is plenty long enough so start off with half throttle until it winds up and then full throttle. As I said, if you do it right and toggle the "trim" up (flaps are used for this in a real plane) there's no need to pull the nose up. The plane lifts off the ground by it self and continues to climb. 

- With the trim still toggled so they're facing up put the nose just slightly above the horizon line in the "NavBall" (of course keep it level too), back the throtle off until it's at 1/4, the first line, and turn SAS on = Level flight if the plane is designed with the wings 'right' so the wings do most of the work / lift. 

FLAPS. I set up an action so the inner "Elvators" as the game calls them on the main wings and secondary wings retracted (face down). If you line up with the runway and point the nose slightly up (at the first line above the horizon line on the navball) and back off the throttle to halve way between 1/4 and idle the plane will decend nice and smooth for a very nice, smooth landing.  

I dont have any parachutes on the plane. I have "shutdown engines" on a custom action so I cut the engines and turn the brakes on and it comes to a smooth stop. 

Hope this helps someone. I may perfect this more if I'm right about the parts needed being available later in the game when I get further alone in the tech tree. It can be done with just enough tech researched to have landing gear wheels. 

Edited March 29, 2020 by bphft
Saw a spelling mistake. I probably missed other grammar mistakes....oh well.

[UncleManuel]

On 1/2/2017 at 10:32 AM, Val said:

The reason the crafts pitches up is because SAS keeps you flying in a straight line, literally, while the planet surface is curved. (behold my awesome paint skills)

 

Trim is not taken into account while SAS is active. To use trim you'll have to fly without SAS.

Holy crap! Now I know why my planes are constantly slowly pitching up... 

With the rockets in orbit this behavior is not noticed because SAS is in "orbit" mode and follows the trajectory...