PID control loop gains are tuned for a specific response for a specific type aircraft but can accommodate some variance while still proving good control and stability. It is very possible that if you change the plant parameters, then you'll have to re-tune the PID gains(Kp,Ki,Kd). If you model your the plant(Heli/Plane, and possibly servos) behavior then you can investigate PID gains that provide sufficient control response for a given input(step,ramp) or for a variety of inputs. Aerodynamic Models of Helicopters are hard to develop, easier for a Plane, but you have to be very knowledgeable in Controls, Dynamic Modeling and Simulation, Aerodynamics and of course Matlab/Simulink. Without a sophisticated computer model of your RC, you are essentially doing this manually by using your brain, vision, and sensitivity on the controls to measure the plants response to varying inputs(flying) with certain gains..This is called experimental flight testing!..Every newly designed plane or helicopter goes through it. Engineers want to understand the relationships with models, parameters and flight performance.
Moment of Inertia, Mass, Stability Derivatives, Control Derivatives, Lift, Drag, Thrust, Weight are just a few parameters in these Aerodynamic models.