The project will develop new engine concepts based on open contra-rotating rotors.

It had no tail rotor but instead had co-axial contra-rotating rotors.

Sikorsky X2: concept demonstrator with twin, contra-rotating rotors and a pusher prop.

His Model No3, built in 1924, employed collective and cyclic pitch control on the tandem contra-rotating rotors.

It achieved lift by use of two contra-rotating rotors powered by an electric motor that was supplied with power from the ground.

This shaft carried two coaxial contra-rotating rotors, and atop it a fin for increased lateral stability.

Contra-rotating rotors on a vertical axis in the tail provided fore-and-aft (pitch) control during hovering and transitional flight.

The original Hoppi-Copter consisted of two contra-rotating rotors on a pole attached to a motorized backpack.

Both crashes occurred under similar conditions - the contra-rotating rotors intermeshed and collided, the blades shattering, causing an immediate loss of control resulting in a crash.

Contra-rotating rotors like on Sikorsky X2 solve the problem of lift dissymmetry by having both left and right sides provide near equal lift with less flapping.

An edition of Popular Science magazine in 1969 featured a backpack helicopter that used small jet engines in a tip jet configuration instead of contra-rotating rotors.

The Advancing Blade Concept system consisted of two rigid, contra-rotating rotors (30 inches apart) which made use of the aerodynamic lift of the advancing blades.

Interestingly, the SLADF did not appear to use a contra-rotating rotor design, using a single rotor with aerodynamic deflection surfaces inside the duct to cancel torque.

In 1784 Launoy and Bienvenu demonstrated a flying model with coaxial, contra-rotating rotors powered by a simple spring similar to a bow saw, now accepted as the first powered helicopter.

Fitted with a de Havilland Gipsy aero engine, it had twin concentric contra-rotating rotors of stainless steel tubing with fabric covering and with full cyclic and collective pitch control.

Crush, tear, curl is a production method developed by William McKercher in 1930 which uses machines with contra-rotating rotors with surfaces patterning that cut and tear the leaves producing a product popular for use in tea bags.