Cleared for (Vertical) Takeoff
Will one of the first new propulsion systems since the Wright Brothers lead to a safer alternative to the helicopter? Jennifer Welsh revs up the story. Illustrated by Branden Melendez.
Illustration: Branden Melendez
A high-pitched whirr shatters the serenity of the damp Monterey morning. “We’re all going deaf!” Garth Hobson shouts over the incessant whine, after removing an earplug, grimacing, and leaning away from the screeching machinery. His windbreaker protects him from the sprinkling winter rain as he stands outside of the testing bay. He’s in the middle of a golf course, the unlikely home of the Naval Postgraduate School’s Turbopropulsion Laboratory.
Hobson, associate director of the lab, is developing one of the first completely new aircraft propulsion systems since the Wright brothers flew at Kitty Hawk: the cross-flow fan. He believes this technology, which dates back to an 1892 patent, could one day compete with helicopters. He isn’t the only one; a company called Propulsive Wing in Elbridge, New York, is also developing aerial vehicles using a similar design.
Hobson’s team seeks to generate maximum thrust by perfecting the fan, with the goal to propel a craft that could take off vertically. Hobson’s research originally was funded to develop the technology into personal vehicles—fueled by America's desire for a flying car. More realistically, the cross-flow fan could power safer vehicles to perform many jobs now carried out by helicopters, such as hauling supplies on battlefields or during storms, like the East Coast's “snowmageddon” in February 2010.
“There are certain applications where vertical take-off is almost crucial,” Hobson explains. “Helicopters do amazing things, but they are scary devices.”
Indeed, between 2001 and 2005, there were 8.0 accidents for every 100,000 flight hours, according to the International Helicopter Safety report. In the same period, there were 0.229 airplane accidents per 100,000 flight hours, according to the National Transportation Safety Board. Too many jobs rely on dangerous, expensive, inefficient helicopters.
One of the worst helicopter disasters killed 45 people in November 1986. The passengers were returning from an oil field in the North Sea between Scotland and Norway. When the craft’s transmission failed, the rotors desynchronized and the blades collided. The helicopter crashed into the ocean and sank, leaving only two survivors. Twenty-nine more people died when a military helicopter crashed during foggy weather in 1994. The crash killed members of Northern Ireland’s Intelligence community and a special forces crew.
Despite their hazards, helicopters fill a niche that other aircraft can’t touch. Because they take off vertically, they’ve become a mainstay of executive travel and military operations. Both Hobson and his competitor Joseph Kummer, who directs Propulsive Wing, think a cross-flow-fan-mobile could be a cheap, easy to use, and maneuverable vehicle that could do much of what helicopters do—without the blur of blades slicing the air.
Today, cross-flow fans cool computer towers, ventilate heating and cooling systems, and blow curtains of air to dry the snow-slicked entryways of stores during Christmas shopping season. They propel air efficiently, albeit loudly.
They aren’t much different from the fan first patented by Paul Mortier in 1892. They look like elongated hamster wheels, with blades that resemble toenail clippings with rounded edges. Air passes over the parallel blades of the fan’s elongated cylinder twice as it is pulled in from above the fan, moves through the center and is expelled in front.
People have tried to use cross-flow fans for flight since the 1970s. Engineers at Vought Systems Division of the aircraft conglomerate Ling-Temco-Vought conducted a series of experiments to use the fans for low subsonic transport aircraft. They created 46 design configurations of 12-inch-diameter fans. Vought’s experiments successfully tested the fans, determining that it was possible they could propel a vehicle. However, the company never built a flying prototype. The project lost steam when Vought suffered major layoffs.
“They used what I call the ‘bent-metal’ theory of experimentation—you make something and put it in the wind tunnel, and if it doesn’t work you bend the metal a bit and see if it works now,” says Kummer.
Recently, computer simulations sped up design and testing, resurrecting the project. To date, Kummer has built five successful flying prototypes. Using models, both teams have accurately predicted the efficiency of the system and stresses on the fan, and how much lift the wings can generate.
A traditional airplane wing generates lift as air flows faster over the top than the bottom, due to the wing’s asymmetrical shape. The faster-moving air becomes less dense as the gas molecules move further apart. That air exerts less pressure above the wing, allowing the air below to push the wing up.
The wing of Kummer’s prototype is much thicker than an airplane wing. In cross-section, it has a teardrop shape. The fan sits at the back of the wing, where the wing comes to a point. There, the fan pulls air from above the wing and expels it below, moving the air faster and creating even more lift than the wing would without the fan. The amount of lift created by this system allows the Propulsive Wing to take off on a runway about one-third the length of a typical runway.
Because of the wing’s large size, the Propulsive Wing’s design does away with many features of a traditional aircraft—including much of the body and tail. The miniature prototype Kummer has built is radio controlled from the ground with a wingspan of around four feet. When it flies, the prototype is very maneuverable, because the fan is much less likely to stall than traditional propellers. It also flies slowly and steadily, almost hovering above the ground.
While Kummer has a working unmanned aerial vehicle (UAV) prototype, Hobson is adapting the fan to allow a vehicle to take off vertically. The goal—still several years away, Hobson believes—would give such a craft many advantages over current UAVs.
Today’s small UAVs conduct missions considered by the military to be too dull, dirty, or dangerous for manned aircraft. They perform military reconnaissance and attack runs. UAVs are also useful for some civil applications, such as firefighting vehicles, security work and aerial photography. One danger of the UAVs now used for military applications is that a standing soldier launches them. Hobson thinks designing a UAV that takes off vertically would help protect soldiers in the field.
UAVs come in all sizes and shapes, but for now they all fly like traditional planes. Introducing a new propulsion design could radically change the way how we use them. However, investments in the technology haven't yet taken off. Hobson hopes visitors to his lab will change that.
To reach the Turbopropulsion Laboratory, drive through the Naval Postgraduate School’s golf course. Take the first right after the clubhouse at the faded blue sign, after the dumpster. Listen for the whine of the fan among the squat cement buildings. Be prepared: most of the equipment in the lab looks so ancient, you might feel like you've walked onto the set of a cheap 1960s science fiction movie (probably involving flying cars).
The lab’s star, the cross-flow fan, is encased in an aluminum box with large plastic pipes sticking out of each end. Probes stick into the case at specific points like meat thermometers, testing the pressure and temperature inside the rig. Hobson and his crew use this information to calculate how efficiently the fan moves air, and how much thrust it is producing.
Today’s test is a three-man operation. John Gibson, the lab’s aerospace technician, is manning the throttle at the fan-testing rig by incrementally opening and closing the fan’s exhaust pipe. He takes instructions from Anthony Gannon, associate research professor, who keeps the power to the turbine steady. Gannon controls the fan’s rotation speed and watches the rig’s vibrations. In turn, he’s instructed by graduate student Vlassios Antonia, who monitors the digital readings from the probes. Antonia decides when to start each throttle sequence again, testing the fan’s efficiency at ever-higher rotation speeds.
Gannon’s head pops into a small, dusty viewing window a story above. Because of the noise, spoken directions are impossible; Gannon communicates his instructions to Gibson by holding up a white sheet of paper with a big zero scrawled in permanent marker. Gibson nods and throws the throttle wide open. The tattered red yarn trailing from the exhaust tube flies out horizontally, and the whine soars in pitch as Gannon boosts power to the fan.
The fan they’re testing is the smallest one they’ve tried. It has fewer blades (22 instead of 30) than previous tests. It’s a miniature version of those created by Vought’s original experiments in the 1970s, just 3.5 inches long and 6 inches in diameter. The fan is one section of the cylinder that would run the length of the wing in a prototype. Antonia’s computer simulations have predicted that fewer blades, spaced further apart, should increase the fan’s efficiency. Such small changes to the size and shape of the wheel and blades, Hobson has found, can create more thrust and make vertical takeoff possible.
Still, computer simulations sometimes aren’t enough to predict the outcome of physical experiments at such high speeds. The lab has had its share of setbacks; sometimes a fan disintegrates in the middle of a run.
“Literally the blades just all go ppptttttthhh” and fly out of the fan’s exhaust, says Gannon. “When they fail, suddenly your turbine just goes vvrrrrmmmm, because it has no load on it any more.” He must shut down the power to the fan before the loose parts wreak havoc on the rest of the equipment. “When you are spinning anything above 5000 rpm [revolutions per minute], it’s kind of scary!” he says.
During the first tests of the new fan two days earlier, Gannon noticed some unsettling vibrations. Because of his concerns, the team is only revving up to 6000 rpm today. The test succeeds; even though the vibrations are back, the fan stays in one piece.
The engineers are fortunate they still have enough hardware to keep their experiments running. Their project was funded through a grant from NASA’s Glenn Research Center in Cleveland, under a program to develop propulsion systems for personal air vehicles. In their final year of funding, when Hobson was scheduled to build a prototype, NASA’s decision to plan a manned mission to Mars erased funding of all external aerospace grants. “We were really going pretty hot and heavy on the project,” says Hobson. “Had we been able to sustain the funding, I would say that by now we would have had a flying vehicle.”
Since then, a constant flow of Naval Postgraduate students through the lab have allowed the team to continue its computer simulations of alternate fan designs and small-scale testing. Hobson is now working to secure funding through the Office of Naval Research to develop vertical takeoff UAVs for the military.
“I’m not here to make money out of developing concepts, but I’m developing technology that somebody else can go and market,” he says.
Miniature flying machines
As Hobson and Gannon move toward adapting the cross-flow fan for vertical takeoff, Kummer has taken the technology in a different direction. He is refining a flying wing design for use as a UAV.
Kummer’s Ph.D. work at Syracuse University, which integrated the cross-flow fan into a flying wing, led him to found Propulsive Wing in 2006. The idea started as a chalkboard sketch with his advisor, Thong Dang. Dang was working with Carrier Corporation to use cross-flow fans for heating, ventilation, and air conditioning systems, and he came across the work done by Vought on using the fans for flight. “Up front we had absolutely no idea if we would have any success with it, because no one else had in the past,” Kummer recalls.
Also funded out of NASA Glenn, Kummer used one of Hobson’s early fans to test the accuracy of his computer models. Kummer’s NASA funding also was cut off by the Mars initiative, but his work continued with money from the Astronaut Scholarship Foundation. The support let him build a flying prototype.
Several years later, Propulsive Wing’s two-man operation is still going strong. Now working on the company's fifth prototype, Kummer has high hopes. The only thing holding them back is persuading investors to take that leap of faith on something completely new. “Typically a lot of the issues you have on the miniature side start going away when you start scaling things up, but it costs a lot more,” says Kummer.
Kummer hopes to convince would-be financiers by finding novel applications for his flying wing. “We’re not really competing with people. We are finding completely new missions that this plane can fit into, that nothing else can even do right now,” he says. Propulsive Wing’s large internal wing has 10 times the volume of a traditional airplane wing. Because its integrated fan creates more lift, it can carry three times the weight of a typical airplane with a similar wingspan.
If he conquers the challenges of UAV flight, Kummer wants to market larger unmanned flying wings for civilian uses, including fighting forest fires and hauling cargo over hills and to remote areas. He even believes the same design could fly underwater. Fans would propel the water toward the surface to dive down, then rotate 180 degrees to return the craft to the surface.
When it does reach the skies, the cross-flow-fan-mobile could replace the helicopter in many ways. But like any new technology, this breed of propulsion will carry risks of its own. “I want to prove it out as an unmanned airplane first,” says Kummer. “I wouldn’t want to be the first test pilot.”
Story © 2010 by Jennifer Welsh. For reproduction requests, contact the Science Communication Program office.
B.S. (biological sciences) University of Notre Dame
Internships: The Scientist (Philadelphia); Discover online (New York)
If I won the lottery, I would go to medical school—not to be a doctor, just to learn. Then, I would head to space camp.
I wanted to be an astrophysicist in high school. While in college, I studied cancer in the lab and discovered a passion for infectious disease.
With my degree in hand, I shied away from committing to a graduate program. Instead, I tested my luck at a startup company in the biotech world. I spent three years developing antiviral drugs.
I realized I wanted to be a science writer when I spent more time perusing science websites than doing my job. I wanted to talk about more than viruses and toxicity. Having the freedom to learn and write about any science field feels almost as good as winning the lottery.
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B.A. (anthropology) University of California, Santa Cruz
Since I can remember I have always loved to draw. As the years went by I also developed a deep appreciation for cultures, past and present. But I was curious how I would ever combine my love of drawing and cultures. A professor encouraged me to apply to the Scientific Illustration Program. The program has equipped me with the ability to illustrate cultural subjects. I look forward to continue illustrating things from the archaeological world.