change_altitude does nothing for a vehicle with zero horizonal speed

[alexdawn]

Sorry I'm not sure if it belongs here or in Bluebird.

I created using create_aircraft a vehicle with a type "Phan4" from https://github.com/project-bluebird/bluesky/blob/master/data/performance/OpenAP/rotor/aircraft.json

The change_altitude command only seems to work in the simulation when the vehicle has some horizontal motion and the altitude remains fixed when the craft has a ground_speed of 0. How do a get a rotor craft behave more like a rotor craft and less like a fixed wing craft?