In 1964, under Army contract, PiAC modified the Pathfinder (16H-1) to attain speeds over 200 mph. A G.E. T-58 shaft-turbine rated at 1,050 shp, a new drive system, a new propeller to absorb the increased power and a 44 foot diameter rotor (H-21) were added and the fuselage was lengthened, accommodating eight people. A three-bladed Hartzell propeller hub was modified so that it would have a direct control from the 16H servo control system.
| Type | High Speed Compound Helicopter |
| Crew | 1 |
| Passengers | 7 |
| Engine | GE T-58-8 |
| Horsepower | 1250 |
| Rotor Diameter | 44 ft |
| Ducted Prop Diameter | 5.5 ft |
| Fuselage Length | 37.25 ft |
| Weight Empty | 4,450 ft |
| Useful Load | 3,450 lb |
| Maximum Speed | 225 mph |
| Cruising Speed | 207 mph |
| Range | 518 mph |
The 16H-1A made its initial flight in November 1965, and logged more than 150 hours under the joint Army/Navy test program, including flight at forward speeds of up to 225 mph. It was highly maneuverable in forward speeds, flew sideways up to 35 mph, was flown backwards at 32 mph and numerous autorotative tests were made.
The “Ring-Tail” anti-torque, forward propulsion and integrated control sub-assembly provides many advantages in compounding the helicopter. The 16H-1 is normally flown in forward speed with the main rotor pitch reduced, the aircraft level, and with the cyclic pitch stick slightly forward. This gives the pilot the opportunity to enter into autorotation while decreasing the propeller pitch. This procedure is not time-critical as it is in a conventional helicopter which requires conversion from power pitch to autorotative pitch in less than two seconds. In the 16H-1, the propeller absorbs the energy of the air flowing by and drives it back into the rotor, assisting in maintaining rotor rpm while the pilot arranges the collective pitch of the rotor and the pitch of the propeller.
The success of the compound-helicopter flight testing program sparked a large Army competition for a full-scale development and production program for 375 aircraft. The result was the “Cheyenne” helicopter. Serious technical deficiencies (not related to the compound configuration) caused several crashes and resulted in the program’s cancellation, and the Army’s downgrading their speed requirements. The operation and cost advantages of the compound helicopter over conventional helicopters, remain valid and represent a formula for the future.
