How do I make a successful VTOL?

[Kuleigori]

Hi! I just finished my first session of (failed) VTOLs, and I still not sure what went wrong. I understand that the CoM should be closely aligned with the CoL, but whenever I test my spaceplane it either roles to the side so much it flips upside down, or it just rotates in a circle without rolling. Both turn into uncontrollable free falls and spirlas before it disappears into the water. Could anybody give me advice? Also, 75% of my plane tests veer off the runway and explode before I get halfway of the runway. Thanks!

[Coyote Foxtrot]

When is comes to CoM and CoL, there is a bit of nuance to it. When it comes to standard forward flight, the purpose of having CoL behind CoM is to have passive stability to the craft. With the CoL infront, the AoA will naturally want to increase; this is considered to be unstable. Having the CoL behind means that the AoA naturally wants to decrease; this considered to be stable. The further back the CoL is, the more stable (the more the craft will want to return to its flight path vector/prograde) it is, but also the more resistant it is to change. Having CoL close behind the CoM is to balance between stability and maneuverability.

Placement of the CoL however is also dependent on the torque produced by thrust based around the CoT's vector and position relative to the CoM.

This sort of balance between stability and instability is on two planes for pitch and yaw.

If the issue is purely on vertical takeoff with little to no horizontal speed, aerodynamics play almost no effect and it now comes to a balance of the CoT relative to CoM. You want the average vector mainly to go through the CoM; it does not necessarily have to be directly underneath the CoM, but it would require a greater angle the further away resulting in horizontal movement. Without RCS thrusters or different engines/gimbling engines for thrust vector effect, you will essentially have no control over the craft in VTOL mode. You can either have engines rotate/gimbal around the CoM so it results in torque in different directions or you can place multiple engines that you can quickly vary the thrust in to allow for uneven thrust and control over the craft.'

You will need to maintain some of this vertical thrust and balance until you achieve enough airspeed for you control surfaces to be effective.

With the issue of veering off the runway, assuming you are mostly using a tricycle gear design with a singular nose wheel, the same idea with the stability and instability with lift applies with the gears. It is best to get pressure off of the nose wheel as soon as you can to reduce its friction with the surface and power to change the crafts direction and then instead rely on your vertical stabilizer and rudder for yaw control. Make sure the main gear is just behind the CoM to allow the needed force to pull your aircraft up is reduced as it's like a seesaw. The further back your main gear is, the smaller your moment arm for your elevators are and the less force you have to rotate your aircraft (assuming you are not using canards).