I had this question: Could this helicopter's body be a flying wing? and this picture came up.
What kind of helicopter is this? What is the extra wing for?
It‘s a BV-347 technology demonstrator:
In 1969, the Army and then-Boeing Vertol entered into a partnership to improve upon their cargo helicopter fleet, and thus the BV-347 was borne from a CH-47A Chinook that was provided by the Army as a technology demonstrator, according to retired CW5 Jim Kale, who now serves as a tour guide for the U.S. Army Aviation Museum.
Primary objectives as found in above article:
The BV-347 was tested for more than two years and flew about 350 hours with the intention to meet five primary goals: improve flying qualities with external loads and instrument flight; im-prove stability, control and maneuverability in hover flight and forward flight with high gross weight; reduce rotor noise; reduce vibration stress; and create a modern cockpit environment with improved instrument and navigation displays.
The wing was intended to provide extra lift (works in forward flight only, of course), but apparently didn’t add net benefit:
The wings were added as a means to improve lift, and although the addition served its purpose, the added weight canceled out any positive effects it provided [...].