ROTATION
Counterrotating props ( also called dual rotation propellers ) can be used to avoid the tan 2 φ term in the energy loss of (9.1), although due to tip effects, the cancellation will never be perfect.
Counterrotating propellers were used on late WWII fighters. They were probably introduced more to eliminate the swerve on take-off and climb due the massive yaw torque from the swirl on the fin, than for reasons of efficiency. If a tail sits in the wake of a propeller, it is often offset by a few degrees to avoid yawing the airplane when thrust is applied. From (8.8) the offset needed depends on the thrust, so this can never be more than an average setting. Counterrotation has also been used in high Mach, high pitch propellers and “unducted” fans.
Besides being mechanically very complex, counterrotating propellers are incredibly noisy. A slapping noise is heard each time an aft blade passes through the wake of a front blade, and a quick succession of these slaps creates a very loud siren-like, highly unpleasant tonal noise. The same distinctive tonal noise occurs when pusher propellers cut through the trailing edge wake of a wing, or through an engine exhaust flow.
We briefly discussed the swirl in chapter 8. A mechanically simpler solution than counterrotation is to recover this swirl by stator vanes behind the propeller.
The vanes will act like sails, generating thrust as they bend the sideways componenet of the induction velocity to the rear. Due to the small angle of the swirl, a symmetrical vane can usually point straight into the direction of flight without risk of stalling the vane leading edge.
The vanes will only be useful if their section glide ratio is better than the swirl angle, which makes them a difficult proposition. Their use is mainly limited to pre-existing fixtures, like fairings on struts holding a propeller hub inside a duct, or a ship’s rudder which is “already there” in the wake of the propeller.
Pre-swirl vanes ahead of the propeller are not self correcting like aft swirl vanes. Their alignment can only be set for an average thrust condition. In all other conditions they will do more harm than good.
On wing mounted propellers some swirl is recovered by the wing itself. Since alignment of aft vanes is not critical, there is no need to curve the shape of a rounded wing leading edge into the swirl. Optimal efficiency is typically achieved when the propeller is mounted ahead of the wing on the same level, despite some small increase in wing viscous drag.