HILLER XROE-1 ROTORCYCLE
LATE IN 1954, TWO CONTRACTORS were selected by the U.S. Navy’s Bureau of Aeronautics to build evaluation models of an ultralight one man helicopter for the U.S. Marines. Hiller Helicopters, Palo Alto, California, produced the XROE-l and Gyrodyne Company of America, St. James, Long Island, New York, produced the XRON-l. (The Gyrodyne design eventually evolved into the DSN-l drone helicopter as part of the Navy’s DASH (Destroyer Antisubmarine Helicopter weapon system.)
The design criteria set forth by the Navy called for a small "foldable" helicopter which could be used for observation, liaison, messenger missions, self-rescue, escape (i.e., para dropped behind enemy lines), and small tactical maneuvers.
The Hiller configuration chosen as a result of their preliminary design studies was a two bladed main rotor machine with a conventional tail rotor for torque compensation and directional control. The basic helicopter body consisted of a semi-monocoque pylon which enclosed a 40hp reciprocating engine, a tapered tripod type landing gear, and a simple tubular tail boom. The “foldability” was accomplished by the use of quick release “pip pins” at the landing gear legs, main rotor blades, and tail boom.
A single prototype, by then dubbed the “Rotorcycle,” was completed and first flown in November of 1956. The 290 lb. XROE-l had all the big helicopter speed, range, and altitude performance its exposed and lonely pilot could mentally accept. Hiller test flights demonstrated that the Rotorcycle was capable of an 1,160 feet per minute rate of climb and a 13,200 foot altitude ceiling. It also had a gross to empty weight ratio then unprecedented for conventional helicopters of any size. Numerous flight demonstrations were made where the XROE-l would arrive before the gathered onlookers as a 27 inch diameter package in the back of a Station wagon and assembled by its pilot in less than 10 minutes. Although funded by the U.S. Navy, the one man collapsible Rotorcycle was demonstrated to the U.S. Army (reported in Aviation Week magazine December 30,1957) and numerous military units of NATO countries in Italy, Switzerland, Germany, England, and Holland. Hiller test pilot Richard L. Peck once flew it for the British Royal Marines in gale force winds which had broached ships in the surf and kept even the largest helicopters in the hangar. By early 1958, the single XROE-l prototype had logged 60 hours of flight time, 10 of them in Europe.
After the extensive demonstrations of the Rotorcycle had proved the feasibility of a one-man collapsible helicopter, the U.S. Marine Corps ordered five additional machines to be delivered in December of 1959 for testing at the Naval Air Test Center, Patuxent River, Maryland, and tactical evaluation at Quantico, Virginia. However, the small Hiller manufacturing facilities were preoccupied building H23D Raven helicopters for the U.S. Army and conducting extensive tests on their X-18 tilt-wing VTOL research airplane for the U.S. Air Force. Consequently, 10 Rotorcycles were built for Hiller by Britain’s Saunders Roe aircraft company—five for the Marines and five for foreign speculation. The evaluation Rotorcycles for the Marines were identified as YROE-l’s (Y for prototype aircraft procured in limited quantities to develop the potentialities of the design versus X for experimental).
The engine chosen to power the Rotorcycle was the Model H-59 developed by the Nelson Aircraft Corporation, Irwin, Pennsylvania, and manufactured by Barmotive Products. That engine was originally designed for use in powered sailplanes. The H-59 was a horizontally opposed, four cylinder, two cycle, air cooled engine with a rated power of 40 hp at 4,000 rpm. The engine was designed to run on 80/87aircraft fuel mixed with S.A.E. 30 oil with a ratio of one part oil to eight parts gasoline. The engine could be operated with standard automobile gas but with a slight reduction of power. The XROE-l engine was started with a lawnmower-type pull cord. The later YROE-l’s utilized a battery powered starter. The bare engine weighed only 42 pounds—but the addition of exhaust stacks, carburetor, ignition coils, electrical generator, starter motor, and a 12-volt battery brought the total engine weight to 57 pounds.
The H-59 air cooling system consisted of a 13-inch axial flow fan providing low-pressure, high-flow air through the engine shrouding and which also served as the engine's flywheel.
Engine vibration isolation was accomplished by the use of four rubber mounts at the attach points of the engine crankcase housing to the air-frame.
A 2.5-gallon-capacity fuel tank was mounted above and behind the engine and functioned as a gravity feed system.
A two-stage transmission reduced the engine output shaft rpm to drive the main and tail rotors. Between the engine and transmission was a drive tube whose length allowed the engine to be mounted low enough to provide a proper center of gravity for the machine. The engine connected to the transmission drive tube through a centrifugal clutch that automatically engaged at 1,800 rpm after starting the engine. At the transmission end of the drive tube was a free-wheeling clutch which provided rapid (and automatic) disengagement whenever engine rpm fell below that required to maintain main rotor speed (i.e., upon engine failure or intentional practice auto-rotation landings).
The transmission, a Hiller design, was of conventional construction using spiral bevel gears to reduce the engine output of 4,000 rpm to 1,625 rpm in the first stage to drive the tail rotor (plus oil pressure and tachometer) and 542 rpm from the second stage to drive the main transmission, lubrication was accomplished by a self-contained pressure jet system and used the same type S.A.E. 30 oil as used in the engine fuel mix.
A splined joint, for easy assembly/disassembly, connected the tail rotor drive shaft from the transmission to the tail rotor gearbox. The gearbox, in addition to providing a 90-degree change of drive direction, had a step-up ratio of 22:1 which converted the transmissions’ first stage output of 1,625 rpm to the tail rotor at 3,250 rpm.
The main rotor and tail rotor were of conventional all-metal design. Formed steel served as the main blade leading edge/spar with aluminum skin providing the remainder of the blade. Tail rotor blades were aluminum sheets.
Main blade collective feathering cyclic feathering was accomplished on one set of angular contact bearings on each blade which contributed general simplicity of the rotor system. The rotor hub contained oil internally which provided lubrication for the feathering bearings.
Mounted above the main rotor blades was a Hiller Rotormatic control rotor. That control rotor system, which consisted of two small airfoil shapes that functioned as an aerodynamic servo-rotor to control the cyclic pitch of the main rotor blades, was of the same design used on Hiller production models of UH-12 and H-23 helicopters. The main advantage of that type system was that very low cyclic feedback loads were transmitted back to the pilot’s cyclic control stick. The cyclic Stick itself was located in an overhead position to avoid complicated linkages and facilitate folding of the Rotorcycle. A reversing link in the cyclic control system was necessary to maintain conventional helicopter control movements.
The collective control stick was in the conventional location. i.e., in the pilot’s left hand, and incorporated a twist grip throttle control.
Changing the tail rotor pitch for directional (rudder) control was by conventional pedal and cable system. The pilot’s instrument panel on the XROE-l prototype contained an air-speed indicator, rotor tachometer, engine cylinder head temperature gauge, and a voltmeter. These were required during testing of the Rotorcycle. Production models would probably have had only a single combination instrument with a tachometer and a meter for reading minutes of fuel remaining.
The landing gear consisted of three double-tapered aluminum alloy spring tubes. That arrangement seemed best qualified to satisfy the Rotorcycle’s foldability feature and the tapered tubes proved an efficient and light-weight method of absorbing landing energy. The front landing gear pad also served as a bucket for holding ballast if required to adjust the center of gravity in the event the pilot weighed less than 180 pounds.
Hiller marketing types touted that “To qualify as a Rotorcycle pilot, it is anticipated that a layman without previous flying experience would require about eight hours of dual time and two hours of solo time in a standard training helicopter. This would be followed by one or two hours of tethered flight in the Rotorcycle. Then the student would be prepared to undertake free hovers and transition to normal maneuvers. Such a training schedule would suffice for normal flight conditions; however, to prepare the student for all emergencies which might be encountered during any helicopter operations, a course extension would be recommended.”
It is not known what the Marine Corps evaluation report from Quantico said about the Rotorcycle, but Dick Peck, who had an enormous amount of Hiller helicopter time in his log book—once confided that he did not really enjoy flying the XROE-l. Being so small, the machine was quite sensitive on the controls, and with the pilot being so out in the open with no real attitude reference, each flight was really very exciting.
For whatever reason, by 1960 the Rotorcycle’s technical success had lost its importance; and “. . . the operational need for a one man helicopter had vanished. Therefore, the average G.I. never had an opportunity to flit over the battlefields in his own personal flying machine—and should probably be very thankful.
Kent A. Mitchell
Excerpted from the Journal, American Aviation Historical Society/Fall 1994