I think I’m probably going crazy but it does appear that the Roblox Physics Engine is bugging like hell. I would seriously like some help with this issue at this rate. 
I have literally no idea what I have done wrong. There is no code in this world except a Density changer I believe. The code used for the vehicles to fly is stored in the real game (this is just a building folder for making new vehicles, other vehicles in ServerStorage).
For the MediumBomber:
Basically, changing any value of a SpringConstraint (even if it is not enabled) causes my plane to start bouncing like mad for a few moments. If I enabled it and set the Stiffness to beyond like 1000 it just sends the vehicle flying into the ether.
Two HingeConstraints of two propellers in the vehicle itself start attempting to reach infinite angular velocities as long as there is an adequate force and acceleration property on each HingeConstraint with the Motor type selected. It has no regard for the AngularVelocity in pretty much all instances.
I have had to set the density of every single part to about 0 in a script in order to give these Constraints the ability to function. Otherwise it causes the parts to kind of mesh inbetween eachother or just teleport in the landing gears PrismaticConstraint. I also thought this kind of shouldn’t be a thing where parts can do this just due to an extreme weight/force unless they are not considered as an incompressible solid in the Physics Engine. In some instances it can cause the whole teleporting away business.
For the FighterPlane:
The HingeConstraints on the wheels which control how the gears turn in and out do not have any force in comparison to the plane itself when they try to keep the wheels straight. Even setting the force to ‘inf’ (copy and paste this: 10000000000000000000000000000000000000000000000) or the infamous math.huge() with code has absolutely no affect on the movement of the gears in comparison to the weight of the vehicle, making HingeConstraints completely redundant. Just for physical affect, I don’t want to have to resort to CFrame modification in the code and replicating it to the server yadda yadda its just pain.
On the topic of teleporting away:
What I mean is basically when a simple force is applied to a part and (for no explicit reason that I am able to identify) the part gains an unrealistic velocity and proceeds to change position so fast and randomly that it immediately goes underneath the Workspace.FallenPartsDestroyHeight. I am unable to trace its position before it reaches this point with a script… Here are my results before it is destroyed base on the PrimaryPart:
Position: 269, 15.767744064331, -52.250457763672
Velocity (of assembly): 0, 0, 0
Angular Velocity (of assembly): 0, 0, 0
repeated twice before the event the script connected is disconnected and the script’s thread is killed.
Some other notes:
Sorry if there are inaccuracies in this post, I am quite lazy today and tired as I have spent 3 hours trying to fix this. I would have been fine not using suspension but since the propellers’ HingeConstraints went ahead and decided to have a mind of their own I can’t import this vehicle into my game. To note, nothing changed (when I say that I mean that everything worked PERFECTLY without modifying any properties) until I moved the landing gear into the ‘Operables’ model of the MediumBomber. Moving it back out still doesn’t fix anything what so ever.
I am also a programmer mainly, it’s just that I am also the only modeller for my game so I have to make the vehicles myself lol.
I can’t publish any videos because the internet is way too slow where I am (100kpbs) so I am sharing the file instead.
I will make further edits if it starts to work again.
Thank you so much for reading!
b25mitchelltesting.rbxl (1.4 MB)