Gyroscopic torques: fans, pods and other moment wheels

Propulsion systems for aircraft, airships and boats incorporate rotating propellers or steerable turbines, either because of the trajectory of the ship or aircraft itself, or because they are part of mobile systems, pods — in which the propulsion motors are integrated —, the two events being linked.

These systems can be likened to gyroscopes, which generate gyroscopic torques as a matter of principle. These torques modify the attitude of the ship or aircraft, and must be controlled. To do this, it is necessary to know their intensity and direction, and even to anticipate them before they occur.

Pod-type thrusters are increasingly used in ship architecture, and have necessarily been present from the earliest stages of airship design. Compared to propulsion by fixed propeller(s), the use of pods offers multiple advantages: they are more efficient due to favorable aerodynamics/hydrodynamics, they reduce fuel consumption, noise and vibrations, and they increase vehicle maneuverability by increasing braking capacity.

Even so, building a pod is more expensive than building a fixed system, the torque generated by each propeller is lower than that of a fixed shaft and, perhaps most importantly, it uses an electric motor which, for equivalent installed thermal power, de facto reduces propulsive power by driving a generator. This last disadvantage is largely offset by the reduction in the size of the installed machinery, and therefore its weight, which in turn increases the transport capacity of the ship or airship.

These factors need to be taken into account when choosing the type of motorization to be used, depending on the vessel's navigation schedule. Airships powered by pods are certainly the best solution, because of the maneuverability they allow. Indeed, moving parts, such as aircraft rudders and — rudder surfaces on ships —, are only active if they are oriented in a fluid flow, which they deflect or use through the Venturi effect to generate forces useful for steering. The ship or aircraft must be in motion relative to the fluid in which it is moving for their mobile but passive control surfaces to be effective. Pods act as both propulsion and control surfaces, rotating along the ship's yaw axis (even when stationary...) and along the aircraft's two perpendicular axes (even when stationary...) of yaw and pitch respectively.

If a propeller is likened to the disc of a gyroscope, it produces a gyroscopic torque if the orientation of its axis of rotation is altered by a controlled torque. The axes of these two torques are perpendicular to each other and to the axis of rotation of the propeller. In this way, the gyroscopic effect of the pod's rotation in one of its two degrees of freedom is supported...