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Wednesday, June 1, 2011

Hurdles of High Speed Flight

By Frank Lombardi

By the time helicopters routinely fly beyond the 250-knot speed goal, which is quite literally buzzing around the industry today, designers will have had to overcome some hurdles. What kind of hurdles? Grab your pocket protector, and let’s brainstorm. We know the rotor of a “pure” helicopter does double duty, providing both vertical lift and forward thrust. To go faster, we must get more of the thrust pointing forward by increasing the tilt of the disk in that direction. This, of course, has limits. The combination of higher forward speed, increased cyclic feathering, and increased collective pitch eventually brings our retreating blade to stall. High drag and twisting moments on the stalled blade cause vibrations felt in the controls. Continued acceleration can bring uncommanded pitch and roll. Not good. To delay the dreaded “retreating blade stall” and continue increasing our forward speed, we’ll need to spin the rotor faster.

Now on the other side of the coin—er—rotor, it seems we create different issues. By increasing rotor RPM, the advancing blade tip speed begins to approach the speed of sound as it meets the oncoming air faster and faster. Again drag rises, shock waves form and vibrations return along with an increase in noise, this time because the startled air ahead of the advancing blade is confused as to what to do, a condition we know as “compressibility.”

With typical rotor systems, the limits of retreating blade stall and compressibility seem to converge just below the 200-knot mark. How do we keep rotor speeds and blade angles in their optimum range, yet fly faster? We have to find ways to relieve the rotor of some of its responsibility. It’s difficult to design helicopters to be as efficient airplanes, since the rotor disks can’t be designed to cruise near optimum angle of attack on all sides. One idea to address the imbalance of aerodynamic forces that limit speed is to spin a set of rigid coaxial rotors in opposite directions so there is an advancing blade on each side, and retreating blade stall becomes a non-issue. A bonus with this system is that torque effects are eliminated, and so then, is the need for a power-robbing tail rotor.

Sikorsky has embraced this idea with its X2 technology demonstrator. Eurocopter’s alternate solution is to partially unload the rotor at high speed by outfitting their X3 (X-cube) demonstrator with a small lift-producing wing, allowing reduced rotor angle of attack and avoidance of retreating blade stall. With retreating blade stall remedied, Sikorsky’s X2 addresses compressibility by adding a pusher prop, or “propulsor,” to provide the majority of the forward thrust at cruise. The coaxial rotor can then be slowed down, keeping tip speeds safely below the speed of sound. Similarly, the Eurocopter X3 slows rotor RPM and supplements forward thrust with two wingtip propellers that work as anti-torque in hover.

Still, other issues must be tamed. Rotor blade design is critical, since in all cases, at high speed the majority of the retreating blade will see increased reversed flow, and therefore a large increase in drag. Increased complexity of transmission and drive systems will require redundancies to limit the number of “single-point failures,” an engineer’s worst nightmare. Higher speeds seen by multiple rotating systems will require active vibration suppression to minimize noise, preserve component life, and maintain passenger comfort. Fly-by-wire will be a must, to provide control laws that can seamlessly transfer engine power from main rotor to the forward propulsion system and back again, while always maintaining desirable handling qualities.

Speaking of handling, thought must be given to the type of ratings one might need to operate such a versatile machine. What kind of cockpit information should designers provide to the pilot during each phase of flight, and how would emergencies be handled? Depending on final design outcomes, will it be more akin to an airplane that can hover, or “simply” a fast helicopter? Your existing rating may or may not cover all the necessary skills required. Operationally, hybrid helicopters may challenge the creators of instrument approaches, the designers of helipads, and the rule-makers of aviation law.

An enduring dilemma of helicopter design has always been that whatever helps hover, hurts forward flight, and vice versa. For the high-speed helicopter to be successful, it will have to do both exceedingly well. Both Sikorsky and Eurocopter demonstrators strive to achieve this. Perhaps some will argue that the complexity of such a craft will outweigh its virtue, that it is a niche product or only has military value.

Finally, of course, there is the “cool factor.” The undeniable human condition that we all are familiar with; the feeling that if it looks good, it will fly well. Any craft that seems to fly effortlessly, that seems to enjoy itself while flying, will invite us to enjoy flying in it. This will be the final hurdle to clear if high-speed helicopter flight is to go from fantasy to reality.

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