On a clear morning in early summer, John Langford and a test pilot climbed into a twin-engine plane at Manassas (Va.) Airport. The pilot taxied down the runway, lifted off and headed west. As soon as the plane reached cruising altitude, Langford, sitting in the back seat, pushed a button, and a robot pilot took over from the human one.

"It was an amazing feeling," said Langford, the chief executive of Aurora Flight Sciences in Manassas and a pioneer in the development of unmanned aerial vehicles, or UAVs. "It was a lot of fun. You are looking over the robot and thinking, 'I hope all the engineers did their jobs right.' "

The left side of the aircraft's cockpit looks pretty typical, with a bucket seat, joystick and airspeed and engine pressure displays. But on the right side, the seat has been replaced with a collection of bundled wires and mechanical arms connected to the dashboard. This robotic device is operated remotely either from the ground or the back seat.

Langford calls this craft the Centaur, a play on its half-human, half-robot makeup; a team of engineers at Aurora has been building it for the past decade. They hope it will someday fly scientific missions across Greenland, ferry passengers around the United States and perhaps even carry patients to the hospital when no human pilots are available.

"You use either a keyboard or mouse to set the heading, altitude and airspeed," Langford explained.

The Centaur is one of several dozen nonmilitary UAVs, better known as drones, that have been taking to the skies in the past few years. Many have been developed by military aerospace contractors. The Federal Aviation Administration has granted permits to 46 federal, local or state agencies and universities to operate these vehicles, which go by such catchy names as Cobra, Kestrel, Sparrow and Skate.

Civilian drones can fly only under certain conditions. Most can't land or take off from civilian airports or fly over populated areas. Models whose total weight is less than 55 pounds must remain within the operators' sight and go no higher than 400 feet. But FAA officials say they expect the number of commercial and scientific UAVs to rise as the agency develops new rules to integrate drones into civilian airspace by 2015.

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That's good news to environmental researchers such as David Schmale, an associate professor in Virginia Tech's plant pathology department. Schmale and his colleagues are using drones to sample air currents in their study of a fungus that has been devastating crops and fruit orchards.

Flying a drone from the ground is cheaper than hiring a pilot and gassing up a private plane, according to Schmale, and that's important for a plant scientist on a grant-funded budget. The drones that Schmale and his team use are made from balsa wood or fiberglass and are powered by an electric motor. The craft, with wingspans of five to eight feet, are available on the Internet for $1,000 to $10,000. Schmale and his graduate students purchased a simple drone online, learned how to fly it themselves, and then began adding additional scientific sensors.

Schmale has rigged his drone with a set of Petri dishes connected to extendable arms that open and close during the flight. The dishes hold a growth medium that captures the type of fungus spores he is studying.

But Schmale can't fly the plane wherever and whenever he wants. The FAA allows him to use a patch of sky above Virginia Tech's agricultural research farm near Blacksburg and another one above Fort Pickett, an Army National Guard base south of Richmond, Va. He has to file flight plans two days ahead of time.

Like many scientists, Schmale is looking forward to the time when he's able to fly drones with fewer restrictions.

That day may be a few years off. There's still an uncomfortable relationship between the researchers and do-it-yourselfers who want to fly drones on demand, and the civilian aviation industry and its regulators who worry about mixing remote-control and human-piloted aircraft.

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UAV developers also say they face a public relations problem. When people hear the word "drone," most think of the foreboding Predator aircraft that fires missiles on terrorist camps, according to Paul McDuffee, vice president of government relations at Insitu, a Boeing subsidiary that makes UAVs.

Firing weapons "is just one aspect of the capabilities of these systems," said McDuffee.

Insitu's ScanEagle UAV, which is launched by a catapult, can be used for both military surveillance missions and environmental monitoring. The vehicle is about 4 1/2 feet long, with a wingspan of 10 feet. It has been used to study migration of wildlife in Alaska, floods in North Dakota and the spread of invasive weeds in Australia.

"We have a lot of educating to do as an industry to make sure that people know that these things do not pose a threat or any kind of additional imposition relative to privacy," McDuffee said.

Drones are an aviation technology — like metal fuselages, computerized flight controls and jet engines — that was developed by the military and that then migrated to commercial aviation. UAVs date back to the 19th century, and increasingly sophisticated models have been used in many conflicts since — most notably in Iraq and Afghanistan, where they have been used for both intelligence-gathering and airstrikes.

Today, advances in battery technology, GPS navigation and computer software, and lightweight composite materials are making UAVs more affordable and flexible for scientific missions. Still, because of federal restrictions and public unease, it's unlikely that we'll soon see the skies above the United States filled with patrolling drones. For now, experts say, UAV manufacturers will find more opportunities to fly their drones in remote parts of the world.

One company seeking such opportunities is AAI, a Hunt Valley, Md., firm that builds long-duration military surveillance drones as well as a scientific UAV called the Aerosonde.

"It can go anywhere where it's dirty, foul and dangerous," said Mark Hender, general manager of AAI's Aerosonde unit.

The Aerosonde's specialty is long-duration flying on just a little gas — up to 26 hours with a small payload. Aerosonde flew into the eye of Hurricane Noel in 2007 on a NASA-sponsored mission. It's headed to Antarctica this fall with a team from the University of Colorado to help researchers understand how carbon dioxide from the atmosphere is being absorbed into the Southern Ocean.

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For Aurora's Langford, the next step is figuring out how to get drones and planes to recognize each other, something called sense-and-avoid capability. Right now, each airplane carries a transponder that identifies its position in relation to all the others around it. Air traffic controllers manage the flow of these planes.

In fact, when he was testing the Centaur earlier this summer, Langford had to turn the controls back over to the human pilot in order to steer clear of another plane that was crossing their path. The Centaur received a transponder signal from the other airplane, warning of its speed and flight path, but under current rules, unmanned aerial vehicles are not allowed to maneuver autonomously to avoid other air traffic.

Researchers from NASA and the Swiss military are working on ways that drones can interact with planes in the sky and controllers on the ground.

In Afghanistan, remotely piloted U.S. drone helicopters have shuttled food and other cargo to front-line troops, but it may be a long time before drones take human passengers into the sky.

"The real problem comes when things go wrong," said John Hansford, director of the Center for International Air Transportation and a professor of aerospace engineering at the Massachusetts Institute of Technology.

"What happens when an unmanned aircraft is flying along and something like sunspot activity takes out the GPS? What does the airplane do? Part of the reason we are comfortable having humans operate airplanes is we believe humans can help compensate when there is a problem that is unanticipated."

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Niiler writes about science and technology.