All flight models are raw simulation models for JSBSim flight dynamics simulator, that try to be realistic and don't always come with any stabilizers to help the control. That means, for example, the Cessna is affected by the counterforce from the propeller, getting you into spin. Helicopters are more complex and harder to control without helpers.
To control them they necessarily have to be trimmed, i.e. applied counterforces that balance it so you can fly them without having to constantly apply the forces manually. Unfortunately, the balance changes with speed and altitude, and it has to be occasionally re-trimmed.
There will be also a simpler physics mode that basically turns off the simulated realistic forces from the propellers, allowing you to disregard them, but that needs to be tweaked into the existing simulation models.
