Positive spin

Helicopters are bizarre, improbable, and rather wonderful contraptions. The bumblebee of the aviation world, they have nevertheless become an indispensable piece of technology for modern society - from saving stranded beach goers to waging military campaigns.

In 1843, George Cayley, sometime father of the modern helicopter, saw the potential in flying craft capable of ’landing at any place where there is space and of ascending again from that point’.

But it’s the forward flight of helicopters once airborne that has always proved a little more problematic - essentially being a delicate balance of forces and controls working in opposition to each other.

’There is no such thing as a gliding helicopter,’ wrote US journalist Harry Reasoner in 1971, reporting on the Vietnam war: the first real helicopter campaign. ’This is why, in general, aircraft pilots are open, clear-eyed, buoyant extroverts and helicopter pilots are brooding introspective anticipators of trouble. They know that if something bad has not happened, it is about to.’

It comes down to the key defining feature of a helicopter: the main spinning rotor, which has to provide both lift and thrust.

All helicopters have more or less the same physical limits; there is a need to fly faster

Jean-Michel Billig, Eurocopter

There is a central dichotomy in that at any one time there will be a blade (or blades) advancing and one retreating from the direction of travel. They each experience different forces. The speed of the advancing blades combines with the flight velocity of the craft itself to produce overall very high flow velocities. On the retreating blade, it’s the other way round - the rotational speed and the flight speed of the helicopter subtract and you have low flow velocities.

’The tip of the advancing blade should not reach Mach numbers [supersonic speeds] nor should the tip of the treating blade get to such a low speed that it leads the blade to stall,’ said Jean-Michel Billig, chief technology officer at Eurocopter, a Franco-German-Spanish division of EADS. This ’dynamic stall’ is why helicopters are mostly limited to an airspeed of 180 knots (333km/h; 207mph) and have poor agility at such speeds.

’All helicopters have more or less the same physical limits; it is frustrating to have such a limit and there is a need to fly faster,’ added Billig. But helicopters are never going to be used for transcontinental passenger flights, so what’s the issue? Well, there are good commercial and practical reasons why faster helicopters might be needed. ’Oil platforms are getting further and further out to sea, so the flight takes a very long time. And of course it’s non-value-added time, so the idea is to get to the platform faster so you have an effective full day of work on the platform,’ Billig said. ’Also, when a fisherman has fallen in the sea, it’s usually not next to the coast but 200 miles away and he’s probably expecting a rescue team to come quickly. These are just two examples.’

Engineers have long recognised this problem and have tried to come up with design solutions. Tilt-rotor helicopters were all the rage in the 1980s, culminating in the Bell Boeing V-22 Osprey. However, they are necessarily something of a compromise, and the transitions between flight modes are difficult. ’They have extreme technical complexity and poor hover performance, and being so expensive only the US Marine Corps can afford it basically,’ added Billig.

Another solution is the hybrid approach, which includes fixed, permanent features of both gliding aircraft and helicopters. US company Sikorsky has demonstrated hybrid flight with its X2 light tactical aircraft, which claimed the outright helicopter flight speed record with a 260-knot blast in September 2010 (see panel). While hugely impressive, the X2 is a light, sleek, tactical aircraft and might struggle carrying a team of oil engineers and equipment to and from a rig.

Taking a more utilitarian approach, Eurocopter has been developing its own high-speed hybrid rotorcraft since 2008, keeping in mind tight defence budgets. Indeed, the aim of the X3 project is to achieve a 50 per cent speed gain over standard helicopters at an extra price of no more than 25 per cent. In fact, X3 is built around an existing mid-sized helicopter commercially available from Eurocopter. ’We’ve used a Dauphin airframe - just because we had a prototype available - and we’ve selected off-the-shelf components, so it’s platform agnostic,’ Billig said.

Tilt-rotors have extreme technical complexity and poor hover performance, and they’re so expensive that only the US Marine Corps can afford them

Jean-Michel Billig

As well as the Dauphin’s five-blade main rotor (powered by two turbofans), the X3 has a fixed wing spanning approximately 6m with two pod-mounted propellers on the tips to provide thrust and one rear horizontal stabiliser with two vertical fins.

The fixed wing provides additional lift and relieves the load on the main rotor, which prevents stalling on the retreating blades. However, as drag increases on the airframe, the lift-drag ratio of the rotor drops, meaning additional thrust is required. In standard helicopters, the pitch of the rotor is increased to drive the helicopter forward, but with the X3 the propellers take over. The propellers also provide anti-torque and yaw controls, making a tail rotor unnecessary.

’When you are in the air, when the rotor is rotating, if you do not have anti-torque the airframe will turn in the other direction, so you need a tail rotor to compensate this,’ said Billig. ’On the X3, we don’t have this, so you need to play with the pitch angle on the propeller on each side; so basically if you have a low pitch angle on the left and a large pitch angle on the right, you will turn left.’

Just two years after the original concept was signed off, the X3 was assembled and took its maiden flight in September 2010, performing the complete flight envelope with cruising speeds of around 180 knots.

’There are different areas where we learned a lot about the physics, in particular the control loads,’ said Billig. ’At high [craft] speed, we have to reduce the speed of the rotor and transfer the lift function to the wings, and this transfer is heavily dependent on the advancing speed [of the rotor] and we have to understand which control loads allow it to be done automatically, without any crew intervention, which is extremely important.

’We want any function not directly related to classical helicopter flight to be done automatically - so that the X3 is basically behaving exactly like a helicopter from the point of view of the pilot,’ said Billig, adding that aspects of this automated pilot function are actually borrowed from the EC155 craft.

Lessons from initial testing were clearly well learned. In the most recent test flights in May 2011, the X3 reached a true airspeed of 232 knots at only 70 per cent power. The goal for the entire project was just 220 knots, so hopes are high for the next round of testing, which begins in mid-February.

The X3 was never about outright speed and tilting at records, but rather manageable and affordable extra speed. Nevertheless, with just 30 knots to go to get the record and 30 per cent more power in reserve, the team is quietly confident it can take the mantle.

Regardless, Eurocopter plans to build a variant of X3 by 2018 that will have 20 seats and weigh around 13,607kg. It will likely be marketed at the oil industry.

headtohead

hybrid hopes

The X2 and X3 helicopters have both demonstrated higher speeds in flight

Sikorsky X2

Powerplant: 1x LHTEC T800-LHT-801 turboshaft at 1,300shp
Rotor: 2x four-bladed co-axial rotors
Propellers: 1x variable-pitch six-bladed pusher-type propeller
Top recorded speed: 260 knots (481km/h; 299mph)
Successor: S-97 Raider tactical attack helicopter

Eurocopter X3

Powerplant: 2x Rolls-Royce Turbomeca RTM322 at 2,270shp each
Rotor: 1x five-bladed single rotor
Propellers: 2x five-bladed side-mounted, variable-pitch propellers
Top recorded speed: 232 knots (430km/h; 267mph)

Successor: Speculative 20-seat oil platform transporter