Suddenly there’s a red-hot competition under way to develop rotorcraft capable of flying at sustained speeds well above the theoretical limits of conventional helicopters.
To meet the challenge, engineers at some of the top helicopter manufacturers are turning out radical concepts that look downright bizarre. But the big surprise is these odd birds actually fly beautifully, and, more to the point, they are attaining speeds that are attracting notice among influential customers, especially at the Pentagon.
If your pilot certificate doesn’t say the word Rotorcraft on it, you may have wondered at some point why it is that even the fastest helicopters in the world — we’re talking modern military marvels like the British-made Westland Lynx or Boeing’s AH-64 Apache — can’t outrun a lowly Beech Bonanza. Shouldn’t some bright engineering mind have figured out by now how to build a helicopter that can cruise at speeds above 200 knots? Or even 300 knots?
Well, here’s the reality that even early helicopter pioneers like Igor Sikorsky and Arthur Young knew all too well: The rotor blades of a helicopter do just fine when they’re slicing through the air in a steady hover. It’s when the pilot eases the cyclic stick forward and starts to gather forward speed that the physics get complicated. And above 200 knots, they get really complicated.
Without going into the nitty-gritty details of why helicopters are limited by how fast they can fly, what you should understand is that a spinning rotor creates a lot of drag. The drag, in fact, is proportional to the cube of rotor rpm. It stands to reason, then, that the slower a rotor spins, the less drag it creates. Yet for a helicopter to remain in equilibrium, both sides of the spinning rotor must produce about the same amount of lift. In a hover with no wind, this isn’t a problem because the airflow velocity over the advancing blade and the retreating blade is equal. But what happens when a helicopter transitions from a hover to cruise flight?
Let’s say for argument that the tip of the advancing blade of a helicopter you are flying is moving through the air at 300 knots in a no-wind hover. This means that the tip of the retreating blade must also be traveling at 300 knots. Now you enter forward flight and accelerate to a speed of 100 knots. This means that the total effective velocity at the tip of the advancing blade is now 400 knots. But the air velocity over the retreating blade is decreased by the same amount, meaning it’s traveling at an effective velocity of only 200 knots. This causes a phenomenon known as dissymmetry of lift. Rotor blades are designed to compensate for this in many helicopter designs by a combination of flapping and blade feathering in such a way as to reduce lift on the advancing blade and increase lift on the retreating blade.
You can see where this is all headed: If you were to accelerate a helicopter to, say, 300 knots, the advancing blade would have an airflow velocity of 600 knots (approaching the speed of sound) while the airspeed of the retreating blade would effectively be zero. For the blade to produce lift it must have some airflow over it, and in this case the retreating blade would stall (hence the term “retreating blade stall”).
Practically speaking, a pure helicopter is limited to a top sustained cruise speed of around 175 to maybe 225 knots — and most current production helicopters fly slower than that. While it might be possible to fly faster with a helicopter if you had big enough engines for propulsion and the right rotor system, the drag penalties would usually cancel out the benefits. After all, the more drag you encounter, the more fuel you’ll burn, and that means you’ll have to carry more fuel, and the vicious performance-eroding cycle comes full circle.
But what would happen if you mounted not one but two rigid counterrotating rotors on a helicopter to solve the problems associated with dissymmetry of lift? Or, taking a slightly less radical approach, you instead bolted an airplane propeller on each side of your helicopter and connected both to the engines, or you designed a system whereby you tilted the entire rotor system 90 degrees midflight to transform your helicopter into a turboprop?
Here we have three dramatically different approaches to speedy rotorcraft design, and yet each has the same goal: greatly increased cruise speed while maintaining VTOL (vertical takeoff and landing) characteristics without taking the impractical leap of, say, adding a 43,000-pound-thrust, fully afterburning Pratt & Whitney F135 turbofan engine to the equation. (In case you’re wondering, that describes the propulsion system of a $240 million, VTOL-capable Lockheed Martin F-35B Lightning.)
An Aerodynamic Puzzle Solved
AgustaWestland has been focused for several years on perhaps the most conservative of fast-cruise-speed VTOL designs (as if that adjective even fits here) with the AW609 civil tiltrotor. The approach is conservative if only because it’s proven. Based on engineering work pioneered by Boeing and Bell Helicopter, the AW609 is a commercial tiltrotor aimed at government and corporate VIP markets. Sikorsky, meanwhile, has created a wildly advanced machine called the X2 that relies on rigid, twin-coaxial rotors and a rear facing five-blade propeller that are designed to propel the craft beyond the 250-knot barrier. Not to be outdone, Eurocopter has created the X3 (pronounced “X Cubed”) technology demonstrator, which combines the fuselage, engines and main rotor of an EC 155 with two high-efficiency propellers mounted on short wings astride the fuselage. The strange look of the design has given rise to the nickname “Flying Cuisinart.” A funny moniker, yes, but nobody was laughing when the machine last year attained a sustained forward speed in level flight of 232 knots.
In Eurocopter’s case, the X3 concept is about much more than merely building a faster helicopter. It’s about unraveling the engineering mysteries that will enable the commercialization of a helicopter that can fly 50 percent faster than a traditional helicopter at a cost of just 20 percent more.
The biggest challenge will be in the transformation needed to turn the X3 idea from a technology demonstrator to an honest-to-goodness product. Obviously, a hybrid helicopter based on X3 technology will need bigger engines to drive the main rotor and the propellers, which are linked through combining gear boxes. It’s very likely that a helicopter based on the X3 demonstrator will be tailored to military needs given the obvious benefits of an aircraft that is capable of flying at high speed and inserting troops at precise locations on the battlefield. The Bell/Boeing V-22 Osprey tiltrotor is slowly proving its usefulness as a troop transport — and now Eurocopter and also Sikorsky believe the time is ripe for a more agile VTOL craft capable of serving in both transport and light attack roles. It should come as no surprise, then, that a U.S. tour of the X3 this summer included visits to several military bases, including the Redstone Army Arsenal Airfield in Alabama, Fort Bragg in North Carolina and multiple demonstration stops for Pentagon brass at sites around Washington, D.C.
So what makes the X3 different from other compound rotorcraft under development? The most obvious disparity is the use of two five-blade propellers mounted on short-span fixed wings and driven by the same turboshaft engines that turn the craft’s five-blade main rotor. An obvious difference from conventional helicopters is the absence of a tail rotor, the need for which is obviated by the propellers, which turn at slightly different speeds to offset the torque created by the main rotor. What the X3 concept doesn’t completely solve, of course, is the issue of retreating rotor blade stall (to partially overcome it, the X3’s main rotor turns more slowly than an EC 155’s), meaning that 232 knots is likely about as fast as this helicopter will be capable of flying, even in its final configuration.
Now on to the question almost everybody asks the first time they see a photo or a video of the X3: Won’t those large propellers mounted on each side of the passenger compartment pose a clear and present danger during entry and exit? Even some Eurocopter execs admit they felt the configuration bordered on ludicrous when it was first presented — but the designers are thinking of ways to prevent passengers from being torn to ribbons the moment they step out the X3’s cabin door. Two main ideas are currently under consideration. The first would require that the propellers be stopped during passenger loading. The other would move the cabin door from the side of the helicopter to the rear, which would allow for quick entry and egress, an enticing compromise that would be especially useful for carrying troops into or out of a hot landing zone.
Considering that Eurocopter aims to enter production with a high-speed compound helicopter based on the X3 by the end of the decade, the clock is ticking on figuring it all out.
X2: The Need for Speed
Sikorsky’s approach to the rotorcraft max speed conundrum is, by almost any measure, complicated; yet it’s also arguably a more elegant solution and, the company hopes, a more effective one too. As we’ve already discussed, VTOL rotorcraft flight requires a compromise between hover performance and forward speed. The efficiency versus speed equation especially penalizes low-disk-loading aircraft, such as conventional helicopters, which hover efficiently and have good low-speed controllability and relatively low downwash. With the X2, Sikorsky says it is focused on increasing cruise speed without compromising the attributes that make helicopters so valuable.
At the heart of the X2 design is an integrated fly-by-wire system that allows the engine, rotor and “propulsor” (the rear-facing propeller mounted at the tail) to operate efficiently together, with full control of rotor rpm throughout the flight envelope. The key to making the concept work is the X2’s twin-coaxial, high-lift-to-drag rigid main rotor blades, which spin opposite of each other and thereby ensure that an advancing or retreating blade never has the upper hand in the lift equation and that the whole system is about as efficient as it can be.
Sikorsky announced the development of the X2 technology demonstrator in 2005 and flew the prototype for the first time in August 2008. The 23rd and final flight was conducted in July 2011 after Sikorsky explored the X2’s full envelope from hover to a maximum cruise speed of 253 knots in level flight — an unofficial record for a conventional helicopter. Having achieved its goal of flying the X2 demonstrator helicopter at more than twice the average top speed of a conventional helicopter, Sikorsky will next design, build and fly two more X2 technology demonstrators, one of which it will hand over to the U.S. military for flight test and evaluation. Both will become the baseline prototypes of an all-new light tactical helicopter called the Sikorsky S-97 Raider.
While military customers are clearly Sikorsky’s aim with the X2 and S-97 programs, the company says the experience gained in their development will lay the groundwork for a commercial variant of the design, which could hit the market in five to seven years after the military version enters production. The big unanswered question is how much more a helicopter based on X2 technologies would cost than, say, a stock S-76. Sikorsky isn’t saying, probably because it doesn’t yet know. What’s clear is the leap won’t come cheap. But such a product will make up for the difference in price, it is hoped, by providing better high and hot hover capability, improved maneuverability and agility, less noise and, of course, much faster cruise speeds.
A final point about Sikorsky’s entry in the high-speed-helo sweepstakes: The X2 demonstrator is a small, two-person machine, and that has led some observers to question whether helicopters based on the design will be limited in size. Sikorsky is adamant that the technology will be scalable through a “full range” of weight- and passenger-carrying capabilities. Insiders say Sikorsky has drawn up plans for several X2 technology aircraft with passenger capabilities ranging from one to (if you can believe it) 100 passengers.
As is the case with Eurocopter’s X3, the X2 probably won’t require additional licensing to fly. In fact, during development work on the X2 technology demonstrator, test pilots who were previously familiar only with conventional helicopters were able to transition to flying the X2 with about 15 minutes of practice in the simulator, Sikorsky claims.
A 100-Passenger Rotorcraft?
AgustaWestland CEO Giuseppe Orsi has said he foresees a day when very large tiltrotor-based aircraft could fly as many as 50 to 100 passengers from a downtown city vertiport to another city a few hundred miles away at ticket costs comparable to what airlines charge for the same routes, but without the extra time and hassle of driving to an airport outside a city center. In fact, Orsi said he sees a day when congestion at major airports would make such a travel concept a necessity.
AgustaWestland is currently working on the development of the AW609. No doubt you’ve heard and read a lot about this product, which drew on experience gained from the experimental Bell XV-15 tiltrotor designed by Bell. After partner Boeing pulled out of the project in 1998, Agusta came on board and the machine was rechristened the Bell/Agusta BA609. Development continued through the merger of Agusta and Westland Helicopters in 2000 and beyond, until Bell sought to exit the program last year citing high costs.
Solely in charge of the program, AgustaWestland announced a name change for the aircraft to the AW609 at the Paris Air Show last year and said it would buy out Bell Helicopter’s investment in the program, which it finally did last November. AgustaWestland has since established a subsidiary at a site in Arlington, Texas, to manage FAA certification and flight-testing of the first AW609 prototype.
A second AW609 prototype is continuing testing in Cascina Costa di Samarate, Italy, while a third is being assembled and will be devoted to icing certification testing. A planned fourth prototype will be used for the development and integration of new avionics and mission avionics.
AgustaWestland says the first two prototypes have surpassed 650 flight hours and have proved the aircraft’s ability to fly at altitudes of up to 25,000 feet and cruise at speeds of up to 275 knots at the aircraft’s maximum takeoff weight of 16,800 pounds, figures made possible by the fact that the AW609 in forward flight doesn’t abide by the rules that limit helicopter cruise speed.
AgustaWestland is aiming to gain FAA certification for the AW609 later this decade. The company says it has received orders for around 70 aircraft from buyers seeking to fulfill a range of commercial and government roles. AgustaWestland says the AW609’s capabilities make it ideal for applications such as emergency medical services, search and rescue, transport to deepwater oil and gas facilities, and transporting VIPs and heads of state to and from congested urban areas inaccessible to fixed wing aircraft.
Of course, we have to temper our enthusiasm for future high-speed rotorcraft by noting that nobody has yet advanced a civil design beyond the testing phase. The joke is that the V-22 was the first aircraft in history to enter a museum before it went into production. What’s exciting is that designers appear to have unlocked the aerodynamic puzzles that will finally herald an era of rotorcraft that can excel in a variety of flight regimes, and especially at speeds above 200 knots.
We’re looking forward to the day we get to fly them all.
View our photo gallery featuring the helicopters of the future.
