By Ray Prouty | December 1, 2017
A continuing concern of rotorcraft engineers is how to maintain the vertical-takeoff capability, but fly faster than conventional helicopters.
One of the most popular concepts involves adding a wing and a forward propulsive device to a “pure” helicopter to produce a “compound helicopter." In a sense, the result is an airplane with a very good low-speed lifting capability. By thus bypassing some of the rotor’s high-speed limitations, it appears possible that speeds of at least 300 kt could be feasible.
In the early 1960s, the U.S. Army gave research contracts to each of four helicopter manufacturers to convert on of their conventional helicopters into a compound by adding a wing and one or two jet engines. Sikorsky, Bell Helicopter, Kaman and Lockheed all flew their versions, with Bell flying the fastest at a speed of 274 kts.
This work was in preparation for the attack aircraft known as the Advanced Aerial Fire Support System (AAFSS) that the Army was then panning. The military branch wanted a speed of at least 220 kt, which was about 50 kt faster than the demonstrated helicopter speed capability at the time.
The jet engines were quick and easy installations for demonstration of the concept. But they were not practical for the new aircraft because of their high fuel consumption. (It was before the days of the high-bypass jet engine.) For instance, the jet engine on the Lockheed XH-51 Compound would suck the fuel tank dry in six minutes!
For that reason, all the engineering teams that were preparing to enter the AAFSS competition decide to use propellers, which at 220 kt could be made quite efficient.
Several teams were planning tilt prop/rotors or tiltwings where torque was balanced by symmetry. Other teams, however, were faced by the torque problem because they chose to develop variations of the single-main-rotor configuration. All these teams investigated the possibility of eliminating the tail rotor by using the propeller to generate the required antitorque moment.
At Lockheed, where I worked at the time, we investigated two possibilities. The first was to use two propellers mounted on the ends of the wing and operating one in reverse pitch during hover and low-speed flight. This scheme was discarded when calculations showed that to produce the required anti-torque moment, the wing would have to be longer than was compatible with ground clearance during slop landings or rotor clearance during maneuvers.
The other proposal was to use a pusher propeller and put turning vanes in its wake, thus producing a sideforce similar to that normally generated by a tilt rotor. This proposal got as far as wind-tunnel tests. Although it worked as required for the hover condition, the flow broke down when the testing simulated left sideward flight before getting to the 35-kt Army requirement.
The scheme was abandoned, and Lockheed chose to incorporate a conventional tail rotor on its design, which won the AAFSS competition as the AH-56 Cheyenne (Figure 90-1).
Perhaps this was giving up too easily, since another team actually got the turning-vanes system into the air. Piasecki Aircraft built and flew the 16H-1 Pathfinder (Figure 90-2) with a “ring tail,” incorporating turning vanes in a duct behind a pusher propeller. I do not know whether its left sideward flight performance was satisfactory.
Sikorsky tried to make one device do everything by using a swiveling tail rotor. It was positioned as a tail rotor in hover and as a pusher propeller at high speed, using a movable rudder to provide the antitorque moment. The system was flown on a modified S-61. It was satisfactory in hover and in high-speed flight. But as might be expected, the transition when the rotor was being swiveled between the two positions was troublesome. I recently saw another proposal for a single-rotor compound helicopter. It uses wings that swivel differentially in the rotor downwash during however and low-speed flight to produce the required antitorque moment and directional control. In forward flight, the antitorque force is provided by a rudder.
The basic concept is not much different from that used on the McDonnell Douglas NOTAR system, which uses aerodynamic forces on the tail boom to do the same job. The NOTAR, however, has a backup system provided by the airflow ejected through a quick-reacting turning nozzle at the end of the tail boom.
The examples above simply illustrate that engineers can usually find several ways to meet the challenges that arise on the “cutting edge of technology.”