The Challenge of High Speed

The helicopter’s current cruise speed capability in the range of 150 to 175 knots has been achieved through a great many incremental refinements in helicopter and engine design during the past 65 years. At its current stage of development, it is generally accepted that increasing cruise speeds much above 175 knots will require far more than incremental changes or design refinements. What will be required are either major helicopter configuration changes or a major improvement to the way that rotors produce lift during high-speed flight. New aerodynamic approaches to rotor design have long been sought after to elevate helicopters to a significantly higher speed plateau. Such a new approach has been demonstrated and it is the subject of this essay.

Non-helicopter configurations such as the tilt wing/rotor and direct lift types have successfully achieved substantially higher speeds than the helicopter. But they have done so at the expense of the helicopter’s most important operational attributes. Sacrificing those attributes that have earned the helicopter such wide acceptance in order to achieve higher speeds has not been an acceptable trade-off for most missions that require vertical flight.

The search for ways to enable helicopters to fly at speeds above 250 knots and approaching speeds typical of propeller-driven conventional aircraft has long challenged both industry and government R&D communities. The dominant technical challenge has centered on the need to overcome the inherent loss of lift capability that occurs on the retreating side of the rotor disk as the helicopter flies faster. Because of the loss of dynamic pressure on the blades when they move in the direction opposite to the flight path, the retreating blades gradually lose their ability to produce lift as flight speed increases. Conventional rotors cannot make up for this loss by increasing lift on the advancing side even though the advancing side has surplus lift capability. It cannot do so because large rolling moments would be created and no means have been available with conventional rotors to balance these moments in order to control roll attitude of the helicopter.

During earlier decades, the solution to extending the helicopter’s inherent speed limitation has tended to focus on unloading the rotor at higher speeds to minimize the impact of the retreating side’s loss of lift capability. Adding wings to the helicopter to supplement rotor lift, while also adding propulsive devices to supplement forward thrust, have been tried with some success. Compound helicopters, with wings and propellers, have been flown at dash speeds approaching 250 knots. However they have never succeeded in reaching operational status because of the weight penalties of the wings coupled with the loss in hover efficiency due to rotor downwash impinging on the wings. Low forward propulsive efficiencies plus the immaturities of rotor system and blade design in early periods of experimentation, discouraged pursuit of compound helicopters.
Added to the challenge of high-speed helicopter flight has been the reluctance of designers and operators to sacrifice the accepted strengths of the basic helicopter. Those strengths, which have been the foundation of the helicopter’s success, center on the benefits of its low disk loading. The low disk loading of typical helicopter main rotors produces low downwash velocity resulting in efficient generation of rotor lift during hover and low speed flight. Low downwash velocity has permitted safe operation in unprepared landing sites and has permitted in-flight rescue over any terrain or surface including over water. Large diameter rotors bring another safety related benefit to the helicopter, which is its ability to autorotate to a safe landing in the event that all engine power is lost.

Until the Advancing Blade Concept was created several decades ago, no innovative designs have been advanced that retain all the accepted operational benefits of the helicopter while providing a substantial increase in cruise speed. This aerodynamic concept directly addresses the most important issue that has limited helicopter speed.

A Concept with Promise

Sikorsky Aircraft created the Advancing Blade Concept during the 1970s with the goal of significantly increasing helicopter flight speed. It took advantage of the rotor’s ability to develop lift on the side that advances into the line of flight in such a way as to relieve its retreating side from the need to produce any lift. Reducing dependence of the retreating side to generate lift is the first necessary step in extending the helicopter’s speed potential. By employing two counter-rotating rotors whose blades were extremely stiff in the upward bending direction, the blades were able to produce the total lift on only the advancing side of each rotor with no lift required from the retreating sides. Since the retreating side was no longer called upon to generate lift, the rotational speed of the rotors could be gradually reduced with increasing flight speed. This reduction in blade tip speed delayed the drag increase caused by blade tip Mach number so as permit higher flight speeds.

This new aerodynamic approach was demonstrated in rotors of several sizes from model size up to full scale in the NASA Ames 40 by 80-foot wind tunnel. The concept was found to work as planned leading NASA and the US Army to fund construction of a demonstrator aircraft together with Sikorsky Aircraft, designed around the advancing blade concept. This design, identified as the XH-59A, achieved a level flight speed of 240 knots, increasing to 263 knots in a shallow dive. These speeds were demonstrated in 1974 with the XH-59A flying in the compound mode with two off-the-shelf J-60 jet engines strapped on for auxiliary propulsion. The reader is urged to read Sikorsky’s S-69 model description in the Rotary Wing Legacy section of Sikorsky Product History on this site for a comprehensive review of both the advancing blade concept and of the XH-59 demonstrator aircraft.

The XH-59A demonstrated that the advancing blade concept with coaxial rotors could achieve flight speeds as much as 100 knots faster than the helicopter. It also demonstrated flight control characteristics superior to the helicopter as well far better altitude performance. It did so without a wing or any other lifting device. Most importantly, it achieved these benefits without compromise to hover and low speed flight efficiency. It likewise retained the benefits of low rotor disk loading and the ability to make safe autorotative landings. All the key aerodynamic goals set for high-speed helicopter flight were met by the advancing blade concept during its demonstrations in 1974.

The XH-59A also showed where improvements were needed in order to successfully employ this new concept on a mission capable aircraft. It showed that the weight of its rotor, made predominantly of titanium at the time, was appreciably heavier than that of a conventional rotor of similar size and would need to be made lighter. In the compound mode, the XH-59A showed that the fuel consumed by strap-on jet engines was excessive and a far more efficient thrust producer would be needed. It also showed that rotor drag was greater than planned and would need to be reduced through innovative design solutions. Rotor induced vibrations during initial flight-testing were also higher than expected reflecting the complete absence of vibration suppression equipment on the prototype aircraft. In summary, the result of this initial evaluation of the ABC showed that rotor lift boundaries could be extended to significantly higher speeds and altitudes but that more work was needed to reduce rotor weight, drag and vibration while at the same time significantly improving propulsive efficiency.

Solutions for the weight and drag characteristics of the XH-59A would have to wait for materials and blade aerodynamic technologies to advance beyond those available during the 1970s. Using a propeller or ducted fan to achieve efficient auxiliary propulsion was acknowledged to be the clear solution to achieving acceptable fuel consumption in conjunction with drag reduction of the rotor system. Equally important, it was found that rotor vibration control would have to be addressed during the design stage with technologies not available during that earlier period. But the XH-59A did successfully prove the concept’s high speed and high altitude capability as well as outstanding stability and control.

During the 30 years since the original ABC demonstrator first flew, technology advances have indeed been made in those areas of particular benefit to this concept. These advances warranted a second look at this concept because of the operational benefits that it offers. Significant improvements in non-metallic composite materials, blade airfoil and planform shape, adaptive vibration control technology and in integrated propulsion systems offered substantial promise of success. These proven technology improvements encouraged Sikorsky Aircraft to undertake a new flight demonstrator program to confirm the promise of the rigid coaxial rotor helicopter configuration. This new demonstrator was designated as the X2.

Operational Benefits of the ABC Configuration

Rotor Downwash Velocity

The helicopter’s downwash velocities produced by low disk loadings of 12 pounds per square foot or less, have historically been low enough to permit safe operation in unprepared landings sites including sand and snow covered sites and in the presence of nearby personnel. Many of the successful combat operations performed in the desert and sandy terrains of Iraq and Afghanistan was possible because of the helicopter’s low downwash velocity. Such desert environments cover a substantial portion of the earth’s surface and nearly all of potential conflict areas around the world. Planned and sustained combat operations are not possible with the much more intense downwash velocities characteristic of high disc loadings concepts including direct-lift and tilt wing/tilt rotor aircraft. The ability of these concepts to achieve higher speeds than the ABC configuration is acknowledged. However their higher speeds serve little purpose if take-offs and landings require prepared sites in a fluid battlefield where such prepared sites would become primary targets for destruction.

Low downwash velocity also means that in-flight rescue of people in distress over land and more importantly over water can be accomplished in a routine manner. In addition to preserving extended hover capability and minimizing downwash velocities, the ABC retains one of the helicopter’s most cherished features. That feature is its ability to autorotate to a safe landing if all engine power were lost. The rigid coaxial rotor in fact improves autorotative capability by virtue of its heavier, higher inertia rotor system. The helicopter’s unique autorotation capability has saved thousands of lives of helicopter crewmen and passengers as well as preventing loss of aircraft. For this reason, retaining the ability to autorotate to a safe power-off landing is one of the most noteworthy attributes of the ABC configuration. No other high speed V/STOL concept offers this capability.

Hover Efficiency

Several factors inherent in the coaxial rotor configuration contribute to its high hovering efficiency beyond simply the benefits of low disk loading. The coaxial arrangement balances rotor torque as well as provides for yaw control through differential thrust, so that the need for a tail rotor is eliminated. The power loss, noise and safety concerns of a tail rotor are eliminated. In addition to balancing torque, coaxial rotors greatly diminish slipstream rotation, which eliminates another source of power loss. And finally, no wing is required on the high speed ABC thereby eliminating power losses associated with the vertical drag created by a wing. These factors are additive and inherently make this configuration the most efficient hovering machine of any high speed V/STOL concept yet proposed.

With its helicopter-like rotor size, the ABC is therefore not only able to generate more lift per horsepower supplied to the rotors than conventional helicopters but it generates far more lift per horsepower than higher disk loading configurations like the tilt wing or tilt rotor designs. This means significantly lower fuel consumption during hover and low speed flight enabling ABC configured helicopters to perform many missions as well or better than conventional helicopters. Missions that require prolonged hover time combined with precision control like search and rescue, anti-submarine warfare, shipboard resupply and others can easily be performed by this concept.

Transition to High Speed 

One of the most important benefits of the ABC is that its transition from hover to high speed flight involves no configuration changes to the aircraft. The transition is not only seamless but the process is neither visible to external observers nor to crew and passengers. Its transition is fundamentally an aerodynamic process that transfers lift to the advancing side of each rotor while transferring propulsive thrust from the rotors to the propeller system. Unlike the tilt-wing or tilt-rotor concepts, no pivot axis, no tilt actuators, no dual position lubrication systems for engines or transmissions are needed. The ABC’s transition between low and high-speed flight does not change visual, infrared, acoustic or radar signatures. The absence of mechanisms, sensors, and actuators typical of tilt wing/rotor configurations means that inherent system reliability is increased and ballistic vulnerable area is reduced dramatically. The acquisition, training and logistics support costs are decreased for the same reasons.

Durability and Survivability

It is well accepted that a system weight penalty will be incurred for any concept that provides a speed capability beyond that of the pure helicopter. For the tilt rotor and tilt wing concepts, weight penalties are associated with wings and with airframe structure to provide the axis for tilting. Also contributing to weight penalties are the need for tilt actuators, complex flight controls, provisions for engines and transmission systems to operate in both vertical and horizontal positions and for redundancy as well as added ballistic protection to flight-essential mechanisms and components.

The ABC also pays a weight penalty for its high-speed capability. However its weight penalty is not for added mechanisms, actuators or for a wing as in the case of the tilt rotor. The weight added to the ABC is needed to strengthen and stiffen the rotor blades and rotor hubs to withstand the increased bending moments applied as a result of the concentration of lift on the advancing blades. The lower rotor cancels the bending moments applied by the upper rotor so that only the rotors themselves need to be strengthened. What distinguishes the ABC’s weight penalty is that it is incurred simply by adding wall thickness to the blade spars and to the rotor hub structure and not by adding actuators, mechanisms or wings. This “beef-up” of the rotors makes them far less vulnerable to ballistic damage or to bird strikes and only adds raw material to the rotors rather than adding new parts. Inherent system reliability of the ABC configuration will surpass any tilt type V/STOL configuration because it does not add subsystems or increase complexity.

Operational Versatility

The ABC design can be configured to serve the entire spectrum of traditional helicopter missions with the added benefit of offering cruise speed 100 knots faster. One mission stands out as being especially well matched to the configuration and capabilities of this concept and that is the attack helicopter mission.  The ABC’s outstanding controllability together with level body attitude from hover to high-speed flight achieved by its auxiliary propulsion system makes it a natural candidate for such missions. Every pilot who flew the XH-59A was impressed with its crisp flying qualities and its absence of controls cross coupling. Its very rigid rotors with their equivalent hinge offset of over 25% enabled the demonstrator to exhibit pitch and roll response similar to fighter aircraft. In addition to flight speeds well beyond the helicopter and its precise responsiveness to pilot commands, the ABC provides great flexibility in types of ordinance that can be carried. Weapons mounting locations on the airframe are unconstrained since the rotors’ plane of rotation is constant and cannot compromise weapons management.

Outlook

Sikorsky Aircraft launched a program to build and flight test a new ABC demonstrator in 2005 taking full advantage of the significant advances in technologies that have materialized since the earlier S-69 demonstrator was designed in the 1970 decade. The primary goal of this program, called the X2, was to confirm the viability of this coaxial rotor concept for operational speeds approaching 250 knots. When this goal is achieved, this new helicopter configuration will dramatically change the future of rotary-wing operations in both military and commercial aviation fields and mission capable versions will make their appearances.