Tuesday, April 1, 2008
R&D: An Art at Risk
Some would argue that rotorcraft engineering is as much an art as a science, and they could muster a compelling defense of that premise..
Take the case of Frank Robinson, the R44 Raven 2 main rotor blade, and the hacksaw.
The engineers at Robinson Helicopter Co. were trying to come up with a design that would get more altitude performance out of the R44. They knew one thing they needed was more main rotor blade area, so they designed one with a chord wider than that on the R44 Raven 1.
When Robinson Chief Pilot Doug Tompkins returned from a flight test of the new blades, however, he reported a nettlesome vibration at high altitudes and higher airspeeds. The engineers took strain-gauge data and crunched numbers. They tried all different kinds of trim tabs. But they couldn’t come up with a good solution for the vibration.
After a couple of days of letting the engineers work, to no avail, the company’s founder and president went out to the flight line to talk to Doug after a flight test. He questioned the pilot about the conditions under which the vibration occurred. He reviewed the strain data.
"It seemed to me that it was a resonance," Robinson said. "So I decided to change the in-plane frequency of the rotor blades as a way to get around it."
The best way to do that in this case, he said, was to lower that frequency by narrowing the inboard chord. That would reduce the chord-wise stiffness and lower its natural frequency. "At the same time, I also wanted to reduce the drag on the retreating blade in high-speed flight."
So, "by guess and by God," he said, "I figured out about how much I wanted to have cut out of the trailing edge without it bothering anything else."
As his son Kurt tells it, Robinson told one of the experimental mechanics to get a hacksaw. He then pointed at the inboard trailing edge of the main rotor blade, told the mechanic to start cutting there, and walked away."
"Everybody kind of stared at him," Kurt said, "and thought, ‘Are you kidding?’"
He wasn’t. When Tompkins returned from the flight test with the modified blades, he reported the vibration problem was solved. And that is why the Raven 2 main rotor blades have a long, inboard notch on the trailing edge.
Kurt said the funniest part was watching the engineers, with the solution in hand, go off to their computers to crunch numbers, "then come back and explain why Frank was right."
This tale turns out to be a cautionary one, at least for the U.S. rotorcraft industry. Frank Robinson honed that instinct and judgment through years of working on a variety of new designs for and studies of helicopters, gyrodynes, and gyroplanes. Today’s rotorcraft engineers simply don’t have such opportunities. As a result, their skills individually and the collective ability of the U.S. industry to advance the state of rotorcraft design are withering.
"A failure to invest in new technologies and systems is leading to a clear and lasting reduction in the nation’s capability to create the next generation of safer and more capable rotary-wing aircraft."
That’s the conclusion of a task force of the Aerospace Industries Assn of America that assessed the state of the U.S. rotorcraft industry and weighed whether it can be revitalized. The task force’s 11 members included executives from Bell Helicopter, Boeing, and Sikorsky Aircraft.
Their report, "Securing the Future of America’s Rotorcraft Industry," calls for establishment of a joint Pentagon office to foster rotorcraft development (including new X-projects), restoration of NASA funding of rotorcraft R&D, and greater corporate investment in R&D. How likely their recommendations are to be enacted, given federal and corporate budget and spending priorities, remains to be seen.
Their group makes three basic arguments: that rotorcraft engineering is very different from the fixed-wing variety; that it is much harder to forge a good rotorcraft engineer than a fixed-wing one, and that U.S. military and corporate strategies and spending priorities have for years been strangling any hope of building and sustaining an adequate cadre of new, skilled rotorcraft engineers.
It seems a strange argument, given the pace of helicopter activities today, which the 11 freely admit.
"Take a picture of America’s rotorcraft factories today, and all would appear well," their report says. "Production is healthy. Five different helicopters and the V-22 tilt-rotor are in production for the U.S. military. Several other models continue rolling off the lines for the civilian market."
However, "all this activity masks a few difficult truths," they warn.
First, consider the unique aspects of rotorcraft design. An engineer designing a rotorcraft faces the same challenges of range/payload, speed, and efficiency as a colleague on a fixed-wing project, the task force writes. But the rotorcraft engineer has "to optimize these elements while facing a second set of parameters that do not exist in other aircraft," they note.
A rotorcraft must be unusually light to achieve vertical takeoff. The aircraft and its dynamic systems all have to bear much higher operating cycles. (Depending on its mission, the group says, a helicopter may face as many cycles in an hour as an airplane does in a day.) Its simplest parts — fasteners, auxiliary lines, electrical connectors — must be designed to withstand a high-vibration environment.
A rotorcraft has many more moving parts than a fixed-wing one, which requires expertise in lubrication, durability, and diagnostics, the group says. Bearings and gears must work throughout the flight envelope, not just occasionally. Also, its every dynamic part is flight-critical.
"Although the basic equations of aerodynamics, stress, and fatigue are the same, the issues, technologies, and priorities are completely different," the 11 note. They say the high degree of coupling involved among rotor, fuselage, engine, structure, and payload requires a multi-disciplinary analysis approach. But there is a smaller base of basic research into rotary-wing aerodynamics. Issues long solved in fixed-wing design "must be solved anew for each rotorcraft." To add to the challenges, "performance predictions lack fidelity," and rotorcraft design and engineering tools, which demand more complex fluid-dynamics modeling but are in much lower demand, "are not nearly as developed as their fixed-wing counterparts."
The bottom line, the group argues: "The people who design and engineer rotorcraft are a breed apart. Their training begins where typical aerospace engineering ends."
It can take a decade to qualify a trained aerospace engineer as a rotorcraft engineer. The group says this has been seen in practice. When a rotorcraft engineer has been brought into a fixed-wing program (and vice-versa) to help accelerate development, it says, "companies commonly find a difficulty because the two communities approach problems so differently." The 11 note that the short-takeoff-and-vertical-landing (STOVL) version of the U.S. F-35 fighter has proven the most difficult to develop. "Rotorcraft engineers were brought in to assist with the STOVL-unique aspects," they write, "underscoring the difference in engineering regimes."
This brings us to the difficult truths the group mentioned.
While U.S. rotorcraft production lines are full, new designs are scarce. The Pentagon and its agencies form the engine of rotorcraft R&D in the United States. But for years they have been focused on refining and expanding the production of existing rotorcraft and extending the service lives of those flying. As the group notes, the military aircraft on today’s churning production lines were designed more than 10 years ago. "The last competition for a new-design helicopter was 16 years ago."
Manufacturers have adjusted their design and production capabilities to those market realities. Some focus on derivatives and modifications of their designs. Others have pared design staffs and outsourced airframe work, concentrating on making specific items, such as dynamic components. Others focus on mission systems and integrating them into aircraft.
U.S. manufacturers have invested substantially in internal R&D to occupy technical staff pending new program starts, the group notes, and some of that work has "led to invention of dramatic new technologies." But the civilian market is driven by aircraft operating costs and "civil demand for these advanced technologies is not sufficient to drive designs," it adds. "This corporate investment cannot be relied on to carry the industry to the next round of military rotorcraft."
The 11 warn that "with no new rotorcraft programs funded and only two under consideration, rotary-wing design and engineering staffs — the true core of America’s rotorcraft advantage — have little prospect of future military work, and are losing the technical skills necessary to develop the next generation of rotorcraft."