Rotor-craft X-2 Dragonfly / XR-11 / XH-11
In 1945 the Rotor-Craft Corporation began developmental work on a helicopter with tandem rotor configuration, designed by its President, Gilbert W. Magill, and embodying various novel features, especially the use of a rigid rotor system. The only designer in the United States to have preceded Magill in this line was Landgraf, who employed side-by-side rotor blades fixed rigidly to the hubs.
In 1953 Rotor-Craft acquired the patents of the Landgraf Helicopter Company, including those related to 'rigid rotors'. At the time Magill stated: 'Rigid rotor helicopters will be less costly to produce, and the safety of rigid blades that cannot droop and endanger passengers or others approaching the machine is expected to be attractive to both armed services and commercial helicopter operators.'
The absence of reliable technical data about its own particular version of the rigid rotor concept decided Rotor-Craft to design a test stand which would enable a quarter-scale rotor assembly to be thoroughly tested during the developmental stage.
This helicopter had two three-bladed rotors set in tandem and overlapping though separated by a distance of 3.76m; the level of the rear rotor was above that of the forward rotor.
The Rotor-Craft XR-11, designed by Mr. Gilbert Magill, is so far the only tandem rotor helicopter to have flown with rigid blades, and it is claimed that inherent stability has been built into the design without resorting to the use of auxiliary aerofoils or gyroscopic weights. Soon the designation of the helicopter was changed to XH-11. It was under development for the U.S.A.F. for three years, then tested at the Cal-Aero Technical Institute at Glendale, California.
A feature of the XR-11 is the rigid mounting of the rotor blades, without hinges or flapping joints, the rotor discs constantly remaining perpendicular to the shafts and following broadly the principle of the controllable-pitch airscrew. The blades were connected in such a way that their span axis was at the rear of the driving shaft. Pitch control rods are enclosed in hollow drive shafts, resulting in a very clean assembly. The blade roots embody levers, which enter the hollow drive shafts via slots, connecting with their respective pitch control rods; the two three-bladed oppositely rotating rotors are mounted in tandem and overlap, the forward rotar turning anti-clockwise and the aft rotor turning clockwise. It is of interest to note that the c of g location is some 40% to the rear of the forward rotor shaft, which is therefore carrying some 50% greater disc loading than the rear shaft.
Swashplates for cyclic and collective pitch changes are positioned at the bases of the rotor drive shafts, eliminating a number of components normally used for transferring control movements. For forward or backward flight the swash-plate was tilted fore and aft by a push-pull rod, while collective pitch change was obtained by raising or lowering another push-pull rod. Mr. Magill claims that the control system is extremely sensitive and the lag in control response is almost imperceptible. A number of standard automotive parts are incorporated in the structure, reducing initial costs and maintenance time. The clutch and gears in the rotor assembly are reworked Ford units, a Studebaker free-wheeling unit is employed and all the universal joints are standard car accessories.
Welded steel-tube construction is used for the fuselage and the rotor blades are of laminated birch construction.
The project was ultimately cancelled.
Randall Franklin, 13.01.2007
In 1948 I was 18 going to college and as a hobby designing a helicopter. By chance I met Gilbert Magill, and he was kind enough to advise me on my plans. He gave me a job at Rotot Craft working in the shop while developing the Dragon Fly. I did minor jobs of fileing tubing for welds etc. I witnessed it's disastrous test flight. It got on the ground cushion and fell about 3 feet. The tube framing was hinged and an adjustable link was above the pilot head. That adjustment sheared and the fuselage collapsed in half. The rotors seperated in all directions and one missed my head by a foot or two. The pilot was uninjured but looked stunned. This happened at the Grand Central Air Terminal in Glendale Ca. The swash plates were at the bottom of the masts. Very much like the Enstrom Helicopter of today. Magill was a true genious, and years ahead of his time. One of the big investors in this helicopter was Lucille Ball. This early helicopter experience qualified me to be in the US Army's first helicopter class at Ft. Sill OK. I flew in the Korean War, and later many years with World Wide Helicopters all over the world. I owe many thanks to Gilbert Magill, for a long and exciting career in helicopters. I regret that I have lost contact with him.
Engine: Continental A 100, 110hp
Rotor diameter: 5.49m
Overall length: 4.57m
Weight empty: 408kg
Weight fully loaded: 612kg
Engine: 110hp Continental C-100
Gross weight: 615kg