I Suck at Airplanes: Help, Please

[daawgees]

I started a new career and really want to get better at building airplanes. They can be pretty helpful grabbing all those contracts in the vicinity (kind of) of KSC. I read all the tutorials here and been working on this one plane over the course of a couple days. Here's what happens:

I deploy it to the runway and it bounces...on it's own.

I start the engines and it starts to veer off of the runway. 

I've aligned the wheels as best as I can see, auto strutted them and turned off yaw on any control surfaces. I've attached a link to my craft file. Just need some advice, thanks everyone.

https://www.dropbox.com/s/wt7knpai60n8oeg/Eagle.craft?dl=0

[HvP]

Hello @daawgees. Let's see if we can figure this out then.

There a a couple of things I notice right away about your plane. The engines are sitting much higher than the center of mass of the craft.

If you click on the Center of Mass Overlay button on the bottom left side of the SPH interface (it looks like a weight icon) you'll see a yellow round marker show up in front of your plane. This shows you where the average center of mass is for the plane in its current configuration. If you drain the tanks you'll see it move around a bit as the weight of the tanks get lighter. Now there's another button that looks like a rocket engine right next to the COM button. It shows you a purple ball that indicates where your direction of thrust lines up.

See how your direction of thrust is so much higher up than your center of mass? This means your engines will push on the top part of your plane more than the bottom part and cause it to pivot forward around its middle like a pinwheel firework. This is also forcing your nose down into the runway when you are trying to take off, making it difficult to pitch upwards. You want your engines to be in line with your center of mass as close as possible from the top to the bottom. Moving them down where your horizontal tail fins are would be a better place for them, and you can just stick your elevator fins on the outside of the MK0 fuel tanks.

Your steering problems can be fixed pretty well by adjusting the height of your landing gear. You want the front wheel to lift the nose up a little bit taller than the back, so raise the back pair of gear a tiny bit higher on the fuselage. Not too far or you'll drag the rear end when taking off. Also, the small front LY-05 wheel at the nose is still not quite straight up and down. If you chose the rotate gizmo and click on the wheel it brings up that multcolored set of interlocking circles. Press "F" on the keyboard and it switches from "absolute" to "local" rotation. You'll find that in the local rotation, which shows you how the part is rotated relative to itself rather than the plane's grid, the gear is still rotated a little too far clockwise. This is because the underside of the cockpit is sloped. Give it a couple clicks of rotation in the counterclockwise direction. Hold shift down for more precision if you have angle snap turned on, or turn the angle snap off if you have to.

I think that your plane will still bounce a little at first, partly due to the weight of all that fuel being a little more than those wheels were intended for. Turning up the damping strength and spring strength on those back wheels should calm them down for now.

You probably DO want yaw turned on for the vertical stabilizer fin on the back. You can safely turn the authority limiter down on that tail fin to about 25 percent or so and still get plenty of yaw control. Roll control works best for control surfaces on the outer ends of the wings, so you don't need those control surfaces close in to the body of the plane. Pitch control surfaces work best near the far back (and sometimes rarely on the front) of the plane. Putting pitch elevators on wings near the center of mass won't really give you any extra control. It's best to keep pitch controls on the rear horizontal elevator like the one you have on the tail of your plane already.

One more tip about the wheels. Right click the back wheels in the SPH and turn the brake level up and then click "Friction Control". A new slider will appear at the top of the menu (this might need Advanced Tweakables enabled in the main settings menu, I can't remember.) Set this friction control slider to be about twice what the front wheel is set at. I usually leave the front wheel at 1 and the back wheels at about 2.5. This makes your back wheels more "sticky" than the front wheels and tends to keep you straighter when rolling along the ground.

I hope this helps and isn't too wordy. Planes are harder to build and fly than rockets, but they're so much fun to fly when you get the hang of it. Don't be afraid to ask if there's anything I didn't make clear.

Edited May 26, 2017 by HvP

[daawgees]

Thank you, I appreciate the time you took in writing that explanation. I will try all of those things and report back. That rotation gizmo thing was really throwing me off, I couldn't figure out if it was trying to tell me if my wheels were aligned or not and thank you for clarifying the difference between local and absolute.

One question though, I thought having the center of thrust was 'ok' as long as you adjusted the angle of the engines (which I clearly neglected). Is this accurate?

[HvP]

6 minutes ago, daawgees said:

One question though, I thought having the center of thrust was 'ok' as long as you adjusted the angle of the engines (which I clearly neglected). Is this accurate?

Yes, if you angle the engines to point through the center of mass then you can adjust for the offset weight. I've not done enough testing to see how efficient that technique is for small planes like this though. You may find you like that solution better, so don't be afraid to experiment.

I've also edited my first response to add a bit about increasing the spring strength and damper on your back wheels. It should help to keep the initial bouncing under control.

[daawgees]

That worked, I even left the engines as they were and gave them just 1 'tick' at a downward angle. 

The rotation gizmo is rather finicky with wheels, even while holding the shift key to make fine adjustments they would sometimes spin all around. Either way, once I got her on the runway it took off rather smoothly. Thanks for the help!

[HvP]

You're welcome, and have fun!