Building a Drone Whirly Bird: Copter Mechanics

Drone planes have gained significant traction in recent years, especially in the military. Once you step outside the military, the unmanned vehicle that gets the most attention is the drone helicopter. Though some think it's far-fetched, Amazon is testing a drone copter for package delivery. Mostly, these tiny helicopters are used for video and still photography.

Drone helicopters have the advantages of drone airplanes, but they also come with a set of disadvantages. "When you have a helicopter you don't have to have a place to take off and land, and that's a big deal," Stephen Rayleigh, instructor at the Unmanned Vehicle University, told Design News. "Say you want to take a picture of a house for sale, you won't have a place to land." As for the downside, the helicopter has to work harder to stay airborne. "The disadvantage is you won't get as much flight time," said Rayleigh. "Airplanes are not far from a glide at any point, so it doesn't take a lot to keep them flying. With a helicopter, everything is hanging on the blades."

Getting the frame right
One of the most important requirements for a successful drone is a frame that's stiff enough to withstand the forces of the wind and the blades. "You want the frame to be lightweight, and you want it to be inflexible," Rayleigh told us. "A lot of times they're made out of carbon fiber. If I were to make one myself, I'd use round or square carbon fiber tubes. They're stiff and lightweight."

Rayleigh noted that a less expensive alternative to carbon fiber is aluminum. "Lightweight aluminum square tubes are inexpensive. I've also seen frames made out of injection plastic," he said. Sometimes, he added, the center of the drone is constructed from printed circuit board. "This puts the circuitry in the frame itself," said Rayleigh. "That conserves weight. The arms can be injection-molded plastic with a lattice work to keep it light."

One of the mechanical goals for a drone helicopter is to keep the design simple enough so the frame is strong while not interfering with the camera's range of sight. "You want to have a lot of motors and not as many arms, so you can keep the design clean and give the camera an unobstructed view," said Rayleigh. "The bottom propeller gets starved for air, so you make the bottom propeller larger to make up for that."

Build it for the camera
Most helicopter drones carry a camera, so they're designed for that primary function. "Carrying cameras is their purpose, so the camera is a big design consideration," said Rayleigh. Designing for the camera has an impact on the materials used to dampen vibration. "Usually you need to have some vibration-isolating mount," said Rayleigh. "You need to keep the vibration out of the camera mount. Plus, the camera is slung underneath, so you also need to have good legs to protect the camera when it lands."

The motor that moves the camera into position or scans a landscape has to move without vibrating. It also has to be able to hold a fixed position. "There are a couple different ways to stabilize the camera," Rayleigh told us. "We've used servo motors, but the newer solution is to use a brushless motor. The camera motor is like the motor on the propeller, but it has more windings and more toque, since it has to hold a position. With the brushless motors, you don't get the motion effect on the video."

One of the toughest challenges in using a drone for videos is to keep the camera steady. "People who are just getting into this spend a lot of money on a mount and frame for the cameras, and what you see on the video ends up [looking] like Jell-O," said Rayleigh. You look at the camera and swear it's steady, but the video tells the story." He suggests using a rubber mount to keep the video steady.

Running on auto pilot
The brains and navigation of the drone sit up on top. Rayleigh noted that builders use double-sided tape to take care of the vibration and keep the auto pilot in place. "These have digital compasses and it's necessary to keep it away from the wires and the motors," he said. "The wires create a magnetic field, and the GPS is susceptible to interference, so you have to keep it clear of the motors and wires."

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