In an age when fixed-wing aircraft routinely operate at better than Mach 2, helicopters have yet to break the 300-knot mark.
By John Likakis
Helicopters are amazing pieces of technology. The ability to take off and land vertically enables a helicopter to operate out of nearly any space large enough to accommodate its rotor blades. And that ability has made helicopters an integral part of most countries’ armed forces for the last half century. Whether the mission is bringing in troops or much-needed disaster relief supplies, the helicopter’s unique ability to land almost anywhere helps it get the job done.
But the very design feature that enables vertical flight also holds helicopters back. The whirling rotor blades that provide the lift to take off and land vertically succumb to the laws of physics and aerodynamics as the helicopter’s speed increases. In an age when fixed-wing aircraft routinely operate at better than Mach 2, helicopters have yet to break the 300-knot mark.
In A Whirl
Helicopter rotor blades look quite simple. They are basically very long, very narrow, and very thin wings (what engineers would refer to as “high aspect ratio” wings). Instead of flying in a straight line, they whirl around in a circle to produce lift — hence the nickname “whirlybirds” for early helicopters.
But such apparent simplicity belies the overwhelming complexity of the system. Long, thin airfoils do not allow for large internal stiffening structures. Yet the rotor blades must be strong enough to carry more than the total weight of the helicopter and its cargo (to account for the extra loading imposed by maneuvering).
To an extent, a rotor blade gains stiffness as the rotor spins due to centrifugal force. Otherwise, achievable strength and stiffness is limited by the innate design properties of the blade, including the materials from which the rotor is made. Most modern rotor blades are made with a combinations of composite materials, such as Kevlar, carbon fiber, and fiberglass, with some type of foam or honeycomb material used for the blade core and a metal spar for added strength to support the flight loads.
Long, thin airfoils also tend to flutter — a type of aerodynamic instability that can rob the blade of much or even all of its lifting ability. Severe flutter can lead to the rotor blades self-destructing. While stiffness can prevent flutter, the long, thin aspect of rotor blades means that there is only so much structural stiffness available from today’s materials.
Beating the Limits
Over the years, a number of different schemes have been tried to overcome retreating blade stall and the supersonic challenges of the advancing blade. One idea that seemed to hold great promise was turning the rotor into a wing for high-speed flight.
The concept is simple enough: Make a four-blade rotor that can be stopped in flight with two blades pointing forward and two pointing aft to form an “X” atop the helicopter — and make the blades wide enough so that they can act as wings. Back in the 1970s, U.S. Navy engineers began exploring the concept. By the late 1970s, Lockheed was contracted to perform feasibility studies and then build a working wind-tunnel model.
In 1982, Connecticut-based Sikorsky was selected to develop a flying aircraft. Sikorsky’s S-72 X-wing rotorcraft was to be the first aircraft to try this concept, and Sikorsky’s engineers worked wonders. They developed hollow rotor blades with slits molded into the trailing edges of each blade. Instead of varying the actual angles of the blades, a sophisticated valving system allowed compressed air to blow out the slit of any given blade. Taking advantage of something called the Coanda effect, the air forced out of the slit entrained the air flowing around the blade, causing much more air to follow the contour of the blade’s trailing edge.
By causing such a large mass of air to flow around and down at the trailing edge, the rotor blade could produce substantially increased amounts of lift. And by varying the amount of air being blown out the slit, the amount of lift produced by each blade could be precisely controlled.
If all of this sounds hideously complex, that is only because it is. Sikorsky engineers actually managed to design the entire system, as well as design a special aircraft, the Rotor Systems Research Aircraft ( RSRA), that could test the system in flight. By 1987, the RSRA was ready for flight testing. Unfortunately, government funding was cut before the X-wing concept could make its first flight.
Two Rotors, One ’Copter
Another way to beat the problem of retreating blade stall is to use two main rotors and have each turn in opposite directions. This puts an advancing blade on each side of the helicopter, thus balancing the forces. While this seems like a straightforward solution, the engineering needed to make it work is daunting.
An early attempt that involved putting a main rotor at each end of the aerial vehicle was pioneered by Frank Piasecki with the HRP Rescuer tandem-rotor helicopter of 1945. Nicknamed the Flying Banana (due to both its shape and the fact that the U.S. Coast Guard painted their versions bright yellow), it proved to be a very efficient lifter. Piasecki kept the two rotors from interfering with each other by making the fuselage very long and mounting the rear rotor higher than the front one.
Piasecki’s concept was (and is) sound. After Boeing acquired his Pennsylvania-based company, the Piasecki Helicopter Corporation (renamed Boeing Vertol), the best-known incarnation of the tandem-rotor helicopter, the Boeing CH-47 Chinook, was developed and first flew in the early 1960s. Capable of a top speed of 170 knots, the Chinook is still one of the fastest production helicopters in service today. (It is worth noting that in 1955, Piasecki and members of his team started the offshoot Piasecki Aircraft Corporation, which continues to work with vertical takeoff and lift, or VTOL, aircraft.)
While tandem-rotor helicopters can be fast, they also tend to be rather large. So another way to get two rotors onto one fuselage is to put both of them up front, but angle them slightly off to the sides so that the blades intermesh. Kaman Aircraft, of Bloomfield, Connecticut, has been building such designs for decades. The company’s latest in the line of so-called synchro-copters (for the intermeshed and synchronized rotors) is the Kaman K-Max. These designs are powerful lifters, but not terribly fast.
The latest scheme for two counter-rotating rotors is to use co-axial rotor shafts with one rotor mounted directly above the other. Such schemes have long been used in conventional aircraft (the Russian TU-95 Bear bomber is a prime example), but developing a practical version for helicopters has proved a bit more challenging.
Sikorsky, a part of Lockheed Martin since 2015, has been leading the charge in this area. Its first co-axial design was the S-69, which first flew back in 1973. The model successfully proved the co-axial concept and Sikorsky’s engineering. The company has used much of the knowledge gained from that program in developing the X2 co-axial helicopter.
In test flights, which began in 2008, the X2 hit level-flight speeds of 250 knots, and 260 knots (almost 300 mph) in a shallow dive. The helicopter is driven forward by a pusher propeller mounted on the tip of the tail. The tail rotor conventional helicopters require to counter torque is not needed with co-axial rotors, as the torque generated by each rotor cancels out the torque from its counter-rotating counterpart.
While co-axial rotors overcome the problem of retreating blade stall, the problem remains of the advancing blade tips traveling too fast. Sikorsky engineers overcame this in the X2 by slowing the rotational speed of the rotors to 360 RPM when the helicopter exceeded 200 knots of airspeed.
The lessons learned on the X2 have since been incorporated in Sikorsky’s S-97 Raider. Intended as a contender for the Future Vertical Lift (FVL) light scout/attack helicopter, the S-97 first flew in early 2015. Flight testing is still underway, and a second S-97 prototype was built late in 2015. The S-97 has a projected cruise speed of 220 knots with a full weapons load.
The flexibility and utility of rotorcraft makes their continued development a national priority. The U.S. Army, one of the largest users of helicopters, has been working with the National Aeronautics and Space Administration (NASA) in an effort known as “Vertical Flight 2025.” According to NASA, the program participants are examining every conceivable area of design to identify ways to further improve helicopter performance.
One rather surprising area of inquiry is the design of rotor blade airfoils. According to NASA, “Unlike fixed-wing aircraft, helicopter rotors have traditionally relied upon relatively simple airfoils because of the conflicting aerodynamic requirements, aeroelastic constraints, and the need for structural simplicity and operational reliability. As a consequence, the significant performance benefits of high-lift airfoils . . . have not been exploited for rotorcraft.” So NASA is looking into new rotor blade designs that incorporate variable-geometry airfoils (think ailerons, trailing-edge flaps, and/or leading-edge slats) to both improve lift and create dynamically controlled rotor systems.
Other areas under study include electrostatically controlled boundary layer control (see the previous issue of Aviation Aftermarket Defense for a discussion of advanced boundary layer control technologies), micro-size active control units embedded in rotor blades to drive tiny control surfaces, rotor-tip sweep-back and airfoil optimization to allow rotor tip speeds in excess of Mach .9, changes in rotor tip geometry to increase a helicopter’s maximum payload without increasing the aircraft’s available power, and potential advances in materials science to produce rotor blades that are stiffer, lighter, and stronger. There is even research into using so-called “smart materials” to create rotor blades that can employ active control along their entire length.
In the not too distant future, helicopters may have normal cruise speeds more like those of fixed-wing aircraft. Research by NASA, the U.S. Army, and industry leaders, such as Sikorsky, holds great promise for both the evolutionary development of existing designs and the revolutionary employment of entirely new rotorcraft technologies.
Image #1 - Helicopters are great at getting into and out of tight operating areas, but the dynamics of the rotor blades keeps these versatile machines from going as fast as fixed-wing aircraft. The Apache pictured here has a top speed of less than 160 knots. (Photo by Peter Davies, U.K. Ministry of Defence)
Image #2 - Engineers have been battling the inherent dynamics of rotor systems since the first autogyros flew back in the 1920s. This McDonnell XV-1 was a composite design with wings for lift at higher speeds. The rotor used compressed-air jets at the tips for power, while a small radial engine drove the pusher propeller. Like the British Fairey RotorDyne, the XV-1 proved to be extremely noisey. While the design was capable of a relatively vast 200 knots, it was abandoned after two prototypes were built. (Photo courtesy of U.S. Navy)
Image #3 - Sikorsky’s S-72 X-Wing was an attempt to overcome the limitations of rotors by stopping the rotors in flight and having them act as wings. This design is shown here on the Sikorsky RSRA rotor research aircraft. However, the X-wing rotor project was cancelled before its first flight when funding was cut. (Photo courtesy of Sikorsky Aircraft)
Image #4 - Several designs have used two counter-rotating rotors to overcome retreating blade stall. The Boeing CH-47 Chinook mounts the rear rotor higher than the front to avoid interference between the two. (Photo by Staff Sergeant William Tremblay, courtesy of the U.S. Army)
Image #5 - The S-97 Raider by Sikorsky uses two co-axial rotors and a pusher propeller to achieve high top and cruise speeds. Reducing rotor rotational speed as airspeed increases helps this advanced helicopter avoid aerodynamic issues that can occur when the rotor tips reach supersonic speeds. (Photo courtesy of Sikorsky Aircraft)