Surging in from the west through one of Provence's many beautiful valleys, a peculiar-looking aircraft is preceded by an unfamiliar sound.
The deep chugging rumbling of a conventional helicopter rotor is mixed with the loud whining noise of two wing-mounted forward-facing propellers, making it difficult to guess what is coming.
As the aircraft swoops over Montagne Sainte-Victoire, shaking the windows in holiday cottages and farm houses below, it becomes clear that this flying machine resembles nothing else in the skies.
Eurocopter's X3 rotorcraft - pronounced "X cubed" - is basically a chopper with wings, which will be seen for the first time by the public next week as part of the aerial displays at the Paris air show.
The prototype combines the versatility of a helicopter, by way of vertical take-off and landing, with the higher speed of a plane.
"It's exactly like a helicopter," says flight test engineer Dominique Fournier. "But as soon as you've taken off, it's exactly like a fixed-wing aircraft."Game changers
The X3 is one of the fastest rotorcraft in the world, having achieved a cruising speed of 232 knots (430 km/h or 267 mph) during a test flight on 18 May.
Though not quite as fast as US rival Sikorsky's equally futuristic-looking but differently designed X2, which achieved a true air speed of 250 knots last September, the X3 has nevertheless made the prospect of ultra-fast helicopters going on sale within years much more likely.
Consequently, both helicopter companies describe their innovations as "potential game changers".
"The aerospace industry today has a new horizon," according to Sikorsky's president Jeffrey Pino. Eurocopter's chief executive Lutz Bertling says "it will be a totally different way of flying".Mission capability
For the pilot and for passengers, the difference lies in the "very different sensation from flying this when compared with an ordinary helicopter", according to experimental test pilot Herve Jammayroc. "In the X3 we accelerate and decelerate horizontally."
And although the X3 is perhaps a more complex machine to build, "it is easier to fly than a conventional helicopter", Mr Jammayroc says.
For Eurocopter's customers, it is all about balancing costs with how quickly and how far the aircraft can travel.
Hence, although the X3 is at least 50% faster than conventional helicopters, "the key message is not speed", according to chief executive Mr Bertling.
"The key message is productivity," he says, insisting that the X3's greater size makes it a more versatile rotorcraft than Sikorsky's X2.
"We are not selling helicopters, we are selling mission capability," Mr Bertling says.
"If you can do it with a balloon or a fixed wing or a bicycle, you don't buy an expensive helicopter."Productive aircraft
Eurocopter's aim is to deliver an aircraft that increases cruising speeds by 50%, while limiting any resulting increase in costs to 25%.
"The target is a productive aircraft," Mr Bertling says.
"So 210-220-230 knots for us is quite reasonable. And 270-280 knots may be conceivable, but fuel costs get too high."
With the X3, the required technology is pretty much there, according to Eurocopter's chief technology officer Jean-Michel Billig, who is in charge of research and development.
"Today, we believe it should cost in the region of 20% more than a similar size helicopter in terms of cost of ownership," he says.Replacement programme
The X3 forms part of a broader restructuring of Eurocopter, which includes plans to replace its entire current offering of six different helicopter models.
"We have a road map to renew our current product family over the next 10 years," says Mr Billig.
A helicopter programme costs about 1bn euros ($1.4bn; £876m) per year and typically lasts for about six years, so it is a costly exercise.
The company is also working on more fuel-efficient models, such as helicopters powered by diesel-electric hybrid engines, or unmanned or optionally manned helicopters, even full-sized ones that carry passengers.
Improving safety, both in terms of reliable systems and crew awareness, and to reduce operating and maintenance costs, are also central tasks.Buoyant helicopter market
Some replacement models might be similar to X3, says Mr Billig. "We are assessing the performance of X3 and we will apply it to helicopters where it makes sense," he says.
But his boss, Mr Bertling, adds there will still be a buoyant market for conventional helicopters. "For example, one of the great growth areas is servicing wind parks offshore, and here high speed doesn't make sense," he says.
Typically, the faster an aircraft moves horizontally, the less able it is at vertical take-offs and landings, so any aircraft that tries to be both helicopter and plane will be a compromise that is neither fish nor fowl in some situations.
Hence, rather than compete with fixed-wing planes or even with conventional helicopters, which will continue to serve growing markets in Asia, Latin America and the US, as well as here in Europe, the X3 and other helicraft of its ilk are carving out new niches in the aviation market.
Such aircraft could be used on new routes between city centres, such as between London and Brussels, or even within mega-cities, such as Mumbai, where vertical take-off and landing would save time by not having to travel to and from airports.
Other customers, such as the oil and gas industry, could speed up air shuttles to and from the rigs, thus enabling crews and experts to spend more time actually working.
Such customers would be particularly sensitive to the cost of the helicraft, Mr Bertling observes.
Whereas for others, such as search-and-rescue or military customers, it is "less a question of money and more about mission success".
That does not make it a licence to print money, however.
"Operating with high margins in military areas - outside the US, I have to say - is not that easy in the current climate," Mr Bertling observes.
This year's Paris Air Show will take place at Le Bourget exhibition centre on the outskirts of Paris from 20 to 26 June 2011.