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The US Army is developing a list of enhancements for the next generation Apache attack helicopter. Among them this was mentioned:
Topping the list of major changes is re-engining with the 3,000-shp Improved Turbine Engine, but the array of potential upgrades includes adding a propulsor and compounding wing to increase the helicopter’s performance. Aviation Week
What is a compounding wing and how does it increase a helicopter's performance?
This paper discusses wing compounding and appears to be a free PDF download. Apparently the added wing provides lift to offload the rotor, allowing them to work more on providing forward propulsion and relieving some of their need to also create lift. Figure at the top of page 6 shows the helicopter with added wing with outboard props on the wing.
The compound helicopter configuration, known as the hybrid compound helicopter, features both wing and thrust compounding. The wing offloads the main rotor at high speeds whereas two propellers provide additional axial thrust as well as yaw control. This study investigates the maneuverability of these two helicopter configurations using inverse simulation. The results predict that a hybrid compound helicopter configuration is capable of attaining greater load factors than its conventional counterpart, when flying a Pullup-Pushover maneuver. In terms of the Accel-Decel maneuver, the two helicopter configurations are capable of completing the maneuver in comparable time-scales. However, the addition of thrust compounding to the compound helicopter design reduces the pitch attitude required throughout the acceleration stage of the maneuver.
The Sikorsky X2 is (was) a compound helicopter, with an engine driven rotor and a pusher type propeller to offload the main rotor.
The Fairey Rotodyne was a compound autogyro: an engine driven rotor, a pusher propeller, and a fixed wing to generate most of the lift at speed. The fixed wing is efficient at generating lift, reduces the interference drag from rotor lift creation.
