Could Eurocopter's X3 high-speed helicopter lead to a rekindling of interest in intercity vertical take-off and landing transports, an idea that was popular in the 1950s and 60s?
Photo: Mike Hirschberg, AHS
During a demonstration of the 232-knot X3 at Manassas regional airport outside Washington on Monday (pictured above), I asked flight-test engineer Dominique Fournier why previous compound helicopters had not worked where the X3 seems to fly so simply and so well.
A compound helicopter generally has both wings and a means of propulsion, so that the main rotor does not have to do all the work of providing both lift and thrust. This allows the aircraft to fly faster, and makes the compound helicopter suited to missions, like intercity transport, that require higher speeds over longer ranges than conventional helicopters.
After pointing out that the X3 is the 26th compound helicopter to fly - none of which have gone into service - Fournier immediately mentioned one of the most famous, and one of my favorites, the ill-fated Fairey Rotodyne. In many respects, the X3 is a very similar aircraft.
First flown in 1957, the 40-passenger Rotodyne was being developed for British European Airways, to fly city-center-to-city-center routes such as London-Paris. Some pretty daunting technical challenges had largely been overcome when a cost-saving consolidation of the aircraft industry by the British government effectively cancelled the program in 1962.
Comparing the two designs, Fournier says the Rotodyne used a tip-driven rotor for take-off and landing. The tip jets burned a lot of fuel and caused noise problems. Also the rotor autorotated in forward flight, which is an inefficient way to fly a rotor, he says. The Rotodyne's two wing-mounted Napier Eland turboprops were also gas guzzlers, he adds.
Fast-forward to the X3 and what has changed most, Fournier says, is the availability of fuel-efficient engines. In the X3, two Rolls-Royce Turbomeca RTM322 turboshafts drive both a conventional helicopter main rotor and wing-mounted variable-pitch propellers via the same main gearbox.This allows to X3 to hover and maneuver like a helicopter and accelerate and cruise like a fixed-wing aircraft.
Photo: Mike Hirschberg, AHS
The 2,300shp RTM322s are vastly overpowered for the X3 demonstrator, but in a production H3 Eurocopter would be able to instal appropriately sized fuel-efficient engines and meet its goal of a 50% increase in speed over a conventional helicopter for a maximum 25% increase in life-cycle cost. On longer-range missions that would benefit from higher speed, that could reduce mission costs by 25%, says Eurocopter.
Meanwhile, if you remember the Rotodyne, or are intrigued by it, check out this classic British newsreel. And make up your own mind about the aircraft's viability: