R&D: Beneficial Bird

By Steve Colby, Photos by Shannon Bower | December 1, 2007
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R&W gets an exclusive rundown on Piasecki Aircraft’s latest effort to develop a compound helicopter

THE PIASECKI VECTORED-THRUST DUCTED PROPELLER (VTDP) compound-helicopter demonstrator, dubbed the X-49A in May 2003, was conceived to answer a need for affordable rotary-wing performance and mission-envelope advancements. It proposed to do this by inserting incremental technological enhancements to fill the developmental gap driven by the historical 20-year, rotary-wing acquisition timelines.

Piasecki Aircraft Corp. is proposing the VTDP compound-helicopter technology as a means of upgrading conventional helicopters to conduct missions well beyond their original design capabilities.


The U.S. Navy awarded Piasecki a contract to design, fabricate, and flight test the X-49A demonstrator to explore H-60 speed, range, and survivability enhancements while decreasing life-cycle costs. Another motivator was improving organic H-60 airborne mine-countermeasure mission towing capability. Piasecki envisioned demonstrating with the VTDP towing without the extreme nose-down attitude required for an H-60 to pull a high-drag load in water. Thrust provided by the ducted fan generates a controllable force vector below the rotor, permitting a near-level towing attitude. This approach improves safety and zeros out adverse rotor hub moments, saving wear and tear on the rotor system.

The compound helicopter concept has been around for a long time. Piasecki’s first-generation version, the 16H-1A Pathfinder 2, demonstrated compound-helicopter speeds up to 195 kt in 1967. The fascinating thing about this second-generation technology is that it is 46 percent more efficient than its predecessor, and it is being implemented in a way that achieves high rotorcraft speeds without sacrificing helicopter performance. The reason is the VTDP’s use of the ducted fan with thrust-vectoring devices for anti-torque, yaw control, pitch and thrust — four functions compared to the conventional tail rotor’s two. More on the mechanics of that later.

The maiden flight of the demonstrator, N40VT, was on June 29.

Piasecki Aircraft founder Frank Piasecki has a long history as a pioneer and innovator in the helicopter industry. His long list of innovations includes the first fan-jet tail reactor for anti-torque and yaw control in the PV-1 in 1940, preceding the NOTAR by nearly 50 years. Piasecki built the world’s first tandem-rotor helicopter, the X-HRPX, capable of lifting over three times the payload of any other helicopter flying at the time. The tandem-rotor design led to a number of extremely successful helicopters and, in turn, to the development of today’s H-46 and H-47.

Piasecki sold his original company to Boeing in 1960, after spinning off the Piasecki Aircraft Corp to focus on new and innovative rotorcraft technologies. For his innovation and contributions to vertical flight, Piasecki was awarded the United States’ highest technical award, the National Medal of Technology. His legacy of innovation continues today in the X-49A Speedhawk.

The Naval Air Systems Command initially funded a $26.1 million program in October 2000 to demonstrate the VTDP. In 2004, lead responsibility for the program transitioned to the U.S. Army’s Aviation Applied Technology Directorate. Serious consideration was given to the fledgling program during the Air Force’s Personnel Recovery Vehicle (now called Combat Search and Rescue-X) analysis of alternatives, but the X-49A was eventually eliminated due to development timeline risk. Yearly funding since the program shifted to the Army has been a meager $2-3 million a year. Despite these budget limitations, Piasecki continues to make remarkable strides in the evolutionary process to develop a revolutionary aircraft.

Shannon Bower and I were welcomed by John Piasecki, the company’s vice president of contracts, to witness a flight test and visit with the test team, led by his brother Fred Piasecki, vice president of technology. We arrived at New Castle Airport KILG in Delaware on a foggy Thursday morning, Oct.18, thinking it would be just an interview day due to the weather. Shannon, John, and I were pleasantly surprised as the fog lifted and the test team resumed their pre-flight preparations and briefings.

Steve Schellberg, the chief test pilot and program manger for the X-49A VTDP project, is a former chief flight instructor at the Navy Test Pilot School at Patuxent River, Md. He gave us a very in-depth description of the modifications from nose to tail and the systematic, and carefully-controlled, phased approach to developing this demonstrator. Jimmy "Hammer" Hayes, a former U.S. Marine Corps Marine Aviation Weapons and Tactics Sqdn. 1 graduate and Marine instructor pilot, is Piasecki’s director of military requirements and provided timeline and program development perspectives on the technology. The day’s flight was designed to characterize flight performance at 1,000 ft msl and further define the operating envelope for the control-blending nomograms.

During this initial phase, the X-49A demonstrator’s VTDP flight-control modifications are purely mechanical to reduce development risk, control cost, and prove the concept. As such, it’s important to quantify stresses, power, speed, rates, authority, attitude, and response to properly build the fly-by-wire rules, scheduling, and control formats.

Compound-helicopter technology increases helicopter performance by unloading the rotor with an auxiliary thrust device and a lifting wing. This delays the onset of retreating blade stall, allowing the helicopter to operate at much higher speeds, gross weights, and altitudes, significantly expanding the operational reach beyond the limits of the conventional helicopter.

Emerging performance requirements, like hovering at 10,000 ft msl, are driving manufacturers toward higher-output power plants. Rotor aerodynamic limits prevent efficient employment of this additional power across the operational spectrum, especially at high speed. The VTDP compound technology rectifies this imbalance by enabling the helicopter to use this additional installed power to fly nearly twice as fast, particularly at high gross weights and altitudes.

N40VT started life as a Navy pre-production Sikorsky Aircraft YSH-60F Seahawk. NavAir contracted with Piasecki to modify and test the demonstrator with a very conservative, phased approach to expanding the envelope. To reduce cost and development risk, the aircraft was modified where possible with parts that were already flight-qualified.

The basic airframe is the most obvious flight-qualified piece. The tail-wheel lineage is from an Kaman SH-2 Seasprite, and the hydraulic actuator servo for the sectors (moving clamshell) and the rudder is from an LTV A-7. The wing is from an Aerostar FJ-100, but was clipped 3.5 ft.

The ailerons were replaced with full-span flaperons whose control scheme varies by speed. In a hover, the flaperons deploy full down to minimize the flat plate area exposed to rotor downwash. In forward flight, they retract and become ailerons that work in a mechanically-constant, blended roll solution with the cyclic.

The wing panels are joined through the fuselage with a wing box designed to collect wing stresses through five, strain-gauge instrumented mount points.

Over 200 instrumentation data collection sensors are installed to collect and build necessary data for usage spectrum, performance, and control blending analysis. In flight, these parameters are monitored by the onboard instrumentation pallet and simultaneously transmitted back to the flight test center at New Castle, where they are analyzed in real time by Piasecki engineers.

The most significant modification is the ducted fan, which is made in large part of composite material and weighs only 220 lb more than the entire Seahawk tail it replaces. The H-60 tail boom design is so robust that no strengthening was required for the fan. Retracted, the duct sectors are stowed within the starboard wall of the duct. It’s designed to absorb as much as 80 percent of the installed horsepower to provide thrust.

For the anti-torque application, when left pedal control demand increases, the sectors are deployed, the rudder is displaced to port, and prop pitch is increased. A walking beam assembly suspended in the cabin takes the stock H-60 mixing-unit yaw control output and schedules the control authority continuum based upon propeller beep setting.

The propeller beep switch varies how much a given pedal movement is actually manifested at the rudder and sector. At low thrust, full sector and rudder deployment is allowed, but at higher thrust, less rudder movement is required to facilitate equivalent rudder yaw-control effectiveness. The ongoing flight test program is examining within the constraints of the current mechanical control system a variety of control schemes that include variation of prop thrust and rudder and sector deflection. The experiments will be used to lay the ground work for development of the fly-by-wire control system. There are currently no mixing inputs for rudder and aileron at speed as the adverse yaw was described as "in the noise" by Schellberg.

The mechanical linkage will all be removed and replaced with a flight control computer with VTDP-unique control scheduling programs derived from mechanical flight test results.

Flight testing to date has included the envelope from sea level to 10,000 ft, 15 kt backwards up to 170 kt straight and level. Testing is designed to optimize control-blending efficiencies. For this demonstrator, NavAir has flight-restricted forward speed to 180 kt. Phase 1 test data in combination with the supplemental power unit (SPU) and drag-reduction mods will then enable expanding the X-49A flight test envelope in Phase 2 out over 200 kt.

Tracing back up from the tail, the astute observer notes that the tail-rotor drive shaft cover is significantly wider to accommodate a new, beefy, off-axis drive line. This line couples with a gear-reduction unit called a combining gearbox that reduces the standard Black Hawk tail-rotor output speed required to drive the propeller at an appropriate speed, keeping the tips well below transonic.

The combining gearbox couples to an upgraded tail-rotor output pinion on the H-60 main-rotor gear box. Piasecki modified the H-60 main-transmission tail-rotor spur and pinion gears from the old style, designed to handle 600 hp, to a wider gear-face version stressed to transmit well over three times that to the propeller. Modifying the drive train is sensible, since the concept of a compound helicopter is to use power to develop thrust from an auxiliary thrust device which, in concert with a wing, offloads the lift and propulsion duties of the main rotor. In spite of these changes, 90 percent of this modified transmission remains common with the H-60 transmission, enabling incorporation of the modifications during depot overhaul.

The primary objective of Phase 1 flight test is to verify the structural ability of expanding the X-49A’s flight test envelope beyond SH-60 Naval Air Training and Operations Procedures and Standards limits in Phase 2. Phase 2 would involve installing the SPU, a Rolls-Royce C-250-30, which is both a full-function auxiliary power unit and an additional engine. This 600-hp unit would put power into the drive train aft of the main-rotor transmission to assist in powering the propeller. This would allow all 3,400 shp from the two main engines to be delivered to the main rotor.

Phase 2 would also involve streamlining the airframe to clean up drag. About 30 percent of SH-60F flat-plate drag comes from the rotor hub. The VTDP will fly in a flat pitch, so a disc-shaped rotor head fairing could clean up that drag. Retractable gear will further reduce parasite drag. Piasecki is investigating accelerating implementation of fly-by-wire in Phase 2.

Piasecki has designs of operational concepts for existing helicopters, such as the H-60, Boeing AH-64, Bell Helicopter H-1, and others. The Speedhawk operational design is different from the demonstrator in several respects. A 45-in fuselage plug would be added in front of the main-gear sponson to aid in bringing forward the c.g. and provide 30 percent more cabin space.

This fly-by-wire version would employ a new, forward-swept, anhedral wing mounted above the cabin and behind the mast. Its mean aerodynamic chord would be under the center of the mast to keep lift-generated pitching moments to a minimum. The wing would be designed for quick removal for missions, like external lift, where the wing does not contribute to performance.

Of course, all of these future concepts depend on the ability to effectively redistribute lift, propulsion, and control responsibilities from the rotor to lifting wing and thrusting device, which makes the demonstration of this concept on the X-49A the first step in an evolutionary path to revolutionary capability.

The potential for increased survivability is yet to be proven in live-fire testing, but is intuitively enhanced in this version due to directional-control redundancy and robust tail and drive system design. Lastly, operators should enjoy significant enhancements in long-term reliability due to reduced vibration levels and decreased rotor flapping angles. Even small reductions in vibration and fatigue loads have been shown to markedly increase component life.

As prime contractor, Piasecki is leading a team of companies in this technology development and flight test effort.

Purdy and Kaman have made significant contributions in the drive system and airframe efforts, respectively.

Boeing conducted an independent evaluation of the technology’s performance benefits and the feasibility of its practical application to current platforms. That assessment’s positive results prompted Boeing to enter into an agreement with Piasecki; Boeing is providing the flight test facility, instrumentation, and telemetry equipment at no cost. Boeing, could, no doubt, play an important role in transitioning the technology.

BAE is expected to support development of the fly-by-wire system for the production variant. General Dynamics is supplying airframe components and aerodynamic fairings. By teaming with leading industrial partners, Piasecki and its associates have the combined vision and experience to develop and transition this compound technology into vastly more capable rotorcraft production and inject a renewed level of competitiveness into the industry.

From my parochial viewpoint as a former combat rescue pilot, I see huge potential for VTDP compound military applications. I see a "FAST-DAP" platform capable of running to a fight and popping up into forward-fire delivery profile, employing the prop as a speed brake during the delivery-dive run with remarkably stable pitch, roll, and yaw. I see 200 kt + dash speeds combined with extraordinary maneuverability reducing susceptibility to ground fire.

The observant military helicopter expert sees a plethora of benefits like self deployment, and level-attitude air-refueling "plugs". The utility or offshore operator may see huge benefits in terms of rapid personnel movement without having to refuel at forward points. The medevac/EMS operator should see benefits of recovering patients within the "golden hour." Offshore oil exploration may reap the benefit of rigs farther out to sea by capitalizing on the compound’s range and speed to rapidly get there and back and the versatility to stage at distant oil platforms. The beauty of the technology is that it is scalable to help all helicopter sizes and missions. Let’s hope the Defense Dept. decision makers controlling the purse strings share similar visions of excellence, innovation, and revolutionary capability. Fully funding this program could easily lead to military and commercial spin-off versions to meet the needs of nearly all aspects of the helicopter market.

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