Both the gyrodyne and the compound helicopter are typically rotorcraft with driven rotor, additional stub wings and thrust provision. Are they two terms for the same thing or do they differ in some way?
I have now found a couple of pretty authoritative definitions for a gyroplane:
"The 'gyrodyne' is a hybrid rotorcraft that is capable of hovering and yet cruises in autorotation. The first successful example of this type of aircraft was the British Fairy Rotodyne, certificated to the Transport Category in 1958. During the 1960s and 1970s, the popularity of gyroplanes increased with the certification of the McCulloch J-2 and Umbaugh. The latter becoming the Air & Space 18A."
- Electronic Code of Federal Regulations (e-CFR) Title 14. Aeronautics and Space Chapter I. FEDERAL AVIATION ADMINISTRATION, DEPARTMENT OF TRANSPORTATION Subchapter A. DEFINITIONS AND GENERAL REQUIREMENTS Part 1. DEFINITIONS AND ABBREVIATIONS, Section 1.1. General Legal Information Institute, Cornell Law School.
"Gyrodyne means a rotorcraft whose rotors are normally engine-driven for takeoff, hovering, and landing, and for forward flight through part of its speed range, and whose means of propulsion, consisting usually of conventional propellers, is independent of the rotor system."
So it may be that the gyrodyne fully offloads the rotor and relies on autorotation during cruise, while the compound helicopter drives its rotor throughout. But I can find no authoritative definition of a compound helicopter.
Technically, under autorotation the rotor disc tilts backwards and air flows up and over the blades from below, while under powered drive the rotor disc tilts forwards and the blades drive air downwards from above. It would be nice to think that this was reflected in the two definitions, but there are obviously borderline cases where the disc angle and loading are not quite textbook.
A gyrodyne is a type of VTOL aircraft with a helicopter rotor-like system that is driven by its engine for takeoff and landing and also includes one or more conventional propellers to provide forward thrust during cruising flight. Lift during forward flight is provided by a combination of the rotor, like an autogyro, and conventional wings. Due to a number of issues, there is some confusion over the term "gyrodyne", and the terms compound helicopter and compound gyroplane are frequently used to describe the same design. The gyrodyne is one of a number of similar concepts which attempt to provide helicopter-like low-speed performance and conventional fixed-wing high-speeds