Sikorsky X2 helicopter (Copyright: Sikorsky)

Back to the drawing board for the future helicopter

Helicopters were once billed as an alternative to fixed-wing aircraft, especially as a short-haul airliner – but noise, vibration and fuel-efficiency got in the way. Can a radical re-design turn things around?

When helicopters first appeared in our skies in the 1950s, they were touted as the transport of the future; an aircraft which could take off and land in a car park or the roof of a building and fly us high above our traffic-clogged streets.

Congested roads and airways would be a thing of the past, the thinking went. Fleets of helicopters could whisk us safely and efficiently to our destinations. But helicopters proved to have their drawbacks. They are much less fuel-efficient than planes. They are noisy, and vibrations make them uncomfortable to travel in.

In the meantime, fixed-wing aircraft won out. Conventional planes can carry larger loads faster and further than helicopters, and in more comfort for passengers. But they require long runways and therefore, bigger airports.

At London’s Heathrow, for instance, a plane uses the runway every 30 seconds or so. Airports like these are bad neighbours; noisy and polluting, and have to be situated some way outside city centres, adding to travel time. And we need more and more of them.

If helicopters can be re-designed, then they might provide an alternative, cutting congestion and opening the skies to us all.

“They could operate at heliports constructed at other transport centres, like train stations or on top of freeways” says Carl Russell, an aerospace engineer at Nasa Ames Research Center.

Full-tilt

Nasa may be best known for space exploration, but its aeronautics division is doing some cutting-edge research into a new generation of air travel. Their mission is to “solve the challenges in the air transportation system: air traffic congestion, safety, and environmental impacts.”

‘Compound aircraft’ have been investigated in the past, for direct city-centre-to-city-centre travel, but with little success. In the 1950s, the British-built Fairey Rotodyne was developed as a kind of flying bus. It had a conventional-rotor for vertical flight, and two propellers on stubby wings for horizontal flight.

It showed promise, but development was cut mainly due to politics and cost-cutting. Test flights highlighted a number of engineering challenges, in particular the noise from a jet air system used to power the rotors.

Now Nasa designers are using tilt-rotor technology to design a machine that will carry around 90 passengers and travel 1,000 miles (1,600km).

The Large Civil Tilt Rotor (LCTR) looks like a plane, but with two huge rotors at the end of each wing instead of small propellers. For take-off and landing those rotors are parallel to the ground just as in a helicopter. Then during flight, they swivel forwards to act like huge propellers. “The entire nacelle [the streamlined engine housing] at the wingtip will actually tip forward with the rotors,” says Russell.

Right now it’s an engineering concept, and Nasa are doing wind tunnel tests of various components.

The LCTR is designed to work using existing infrastructure. That means it could use airports, but not clog up runways. Short and medium-length trips could be taken on a tilt-rotor, leaving just the long-haul flights using large fixed-wing aircraft.

Need for speed

Another major drawback of helicopters has been their speed, or rather lack of it. Compared to fixed wing aircraft, helicopters are the snails of the sky.

The limit for a conventional helicopter – big rotor on top, small rotor at the back - is somewhere in the region of 170-190 knots (315-350km/h or 185 to 220 mph). The LCTR is designed to fly at 300 knots (555 km/h), which is a significant increase in speed.

“That’s why we have to go to these different type of configurations because you’re just not going to get it with a conventional design,” says Russell.

But to build an even faster helicopter is going to take another huge leap in engineering.

The way conventional helicopters are built makes it almost impossible for them to fly very fast. Their spinning blades slice through the air and work like the wings of a plane, generating lift. Unlike wings, however, the blades don’t provide equal lift on both sides. As a blade on one side moves forward, a blade on the other side is moving backwards. When a helicopter starts to speed up, this difference becomes more serious. Air passing over the blade moving in the same direction as the helicopter travels faster than it does over the opposite blade. It is a problem known as retreating blade stall.

But with radical redesign, helicopters are capable of being speed demons. Sikorsky, a helicopter maker based in Florida, holds the record for the fastest helicopter flight. Its X2 concept flew at over 250 knots (460 km/h) in 2010. The X2 uses two counter-rotating rotors, meaning two rotors stacked on top of each other, spinning in opposite directions. That means there is an equal amount of lift being generated on each side.

Another advantage of the design is that engineers can remove the tail rotor, which is usually needed to stop the helicopter spinning around. That gave them extra room for a propeller at the back.

Steve Weiner, Sikorsky’s Director of Engineering Sciences, and was chief engineer on the X2 programme. “The concept has been around since the late 1960s,” he says. It was originally tested on an aircraft called the XH59A. “That aircraft went pretty fast (238 knots/440 km/h) but it vibrated very badly, it was very noisy and it used a lot of fuel. It also needed two pilots to operate it,” he says.  “There were enough controls that they had both hands, both feet, and anything else available operating controls!”

Advances in computing and ‘fly-by-wire’ flight systems have made the craft much easier to fly, and the proof of the design is in the world speed record. The double rotors mean no retreating blade stall, and a much smoother and quieter ride. Now the team is working on a next generation helicopter incorporating the technology.

It will take 15 years or more before we see these ideas in everyday operation, but the people working on them believe they can give us what we, the passengers, demand; safety, efficiency and comfort. “If you’re trying to fly around passengers, they are certainly not going to fly on something that is shaking like crazy and rattling their teeth,” Russell says.

 

 

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