Plane's Fairing Fuselage Creates Unexplainable Lift

[Diamond]

Hello everyone,

I've recently hopped back into KSP and I started making the biggest plane I've ever made.  It uses 5m fairings as the fuselage since they are easy to make nosecones and tail cones out of without using mods.  The plane features a folding rear door and fold-out ramp so that I can load cargo in and out of the back of the plane for transport.  I've gotten the plane up in the air however I am having some issues with the fairing creating some lift that causes the plane to yaw out of control.  Since this is my first time making a plane out of fairings, I have never run into this issue before and I was hoping to get some help with this.  The lift freaks out the most when I fly the plane in a straight line and then roll it slightly right or left while still flying in a straight line.  I've attached a video of the failure.  Any help would be appreciated, thanks!

Door and Ramp for Reference: 

 

Failure video:

 

[fleventeen]

It never occurred to me to build an entire plane out of fairing body, but this looks like a cool idea!

At first glance, it looks like when you go into the roll, the craft starts picking up a large amount of body lift as it starts slipping sideways (the cyan arrows), causing a yaw moment, which kicks the nose to the left. This is likely due to inadequate yaw stability.

I would start by building a larger vertical stabilizer.

And now that I look at the craft as a whole, the vertical stabilizer looks quite small given the girth of this beast.

Here is an image of the spruce goose as a reference for proportions, since your craft looks quite similar .

Edit: Someone correct me if I am wrong, but I believe the aero forces for fairings are applied solely at the fairing base, and proportional to the size of the fairing? If this is the case, you may get more stability by putting the fairing base for the main fuselage section at the rear of the plane. You would then obviously need to put the cargo door at the front. This may however have the consequence of working too well, creating a plane that is "overly stable", so I would consider it a plan B if the above doesn't work.

Edited April 11, 2022 by fleventeen

[Diamond]

On 4/11/2022 at 2:19 AM, fleventeen said:

It never occurred to me to build an entire plane out of fairing body, but this looks like a cool idea!

At first glance, it looks like when you go into the roll, the craft starts picking up a large amount of body lift as it starts slipping sideways (the cyan arrows), causing a yaw moment, which kicks the nose to the left. This is likely due to inadequate yaw stability.

I would start by building a larger vertical stabilizer.

And now that I look at the craft as a whole, the vertical stabilizer looks quite small given the girth of this beast.

Here is an image of the spruce goose as a reference for proportions, since your craft looks quite similar .

Edit: Someone correct me if I am wrong, but I believe the aero forces for fairings are applied solely at the fairing base, and proportional to the size of the fairing? If this is the case, you may get more stability by putting the fairing base for the main fuselage section at the rear of the plane. You would then obviously need to put the cargo door at the front. This may however have the consequence of working too well, creating a plane that is "overly stable", so I would consider it a plan B if the above doesn't work.

I believe I have fixed my issue.  Thank you very much for the help.  The issue was a combination of two things I believe.  As you suggested, I think one of the issues was that the plane did not have enough yaw moment to deal with that yaw moment.  The second issue was that since the vertical and horizontal stabs are attached to the door, all the forces generated by it are transmitted to the door instead of the fuselage.  Turns out my method of latching the door was not sufficient enough to withstand the forces of the stabs while in flight so when that yaw moment happened and the vertical stab tried to correct with yaw input, the door would snap off. I reinforced the door with 4 docking ports instead of the two claws and while it still wiggles around when giving pitch and yaw inputs, it doesn't snap off anymore.  Thanks again for the help!