Intermeshing-rotor helicopter

An intermeshing-rotor helicopter (or synchropter) is a helicopter with a set of two main rotors turning in opposite directions, with each rotor mast mounted with a slight angle to the other, in a transversely symmetrical manner, so that the blades intermesh without colliding.

Yaw is accomplished through varying torque, which is done by increasing collective pitch on one of the blade sets.

[1] During the Cold War the American Kaman Aircraft company produced the HH-43 Huskie, for USAF firefighting purposes.

One example of the Kaman K-225 experimental synchropter was fitted with a small turboshaft engine in late 1951, becoming the world's first gas turbine-powered helicopter.

The latest Kaman K-MAX model is a dedicated sky-crane design used for construction work, and has been modified for trials by the USMC as an optionally-unmanned cargo transporter.

Kaman K-Max, Missoula MT / 2010
HH-43 Huskie with intermeshing rotors
Kaman K-MAX is optimized for external cargo load operations. Unmanned , remote control or optionally piloted variant