The technology demonstrator will ultimately be used to demonstrate mission configurations. The company – working with 40 partners in 13 countries as part of the European Clean Sky 2 project – has submitted key sub systems of the demonstrator to a preliminary design review and will begin manufacturing its first components.
The final assembly of the Racer prototype is expected by the end of 2019.
Aeromechanical engineers have long grappled with the challenge of making a vehicle able to take off vertically, hover and achieve very fast cruise speeds—an almost-mythic combination of lift and forward thrust. But making one that is affordable and multi-mission.
Advances in the field of V/STOL aircraft have given rise to scores of variations. Many models succeeded in experimental stages, some achieved production status—most at a significant cost or for a limited market.
Airbus Helicopters’ RACER demonstrator aims to go further, by showing that its particular combination of rotor, box wings and lateral propellers is the answer to cost-efficient, fast flight.
The only way to fly faster is to add either a wing or a second rotor, and propellers to increase the speed. If you want an aircraft to hover, nothing is better than the large diameter of a rotor.
But in cruise, the rotor is limited by its aerodynamic behaviour: at very high speeds, through a combination of the rotation speed and the vehicle’s forward motion, the tip of the advancing blade reaches the speed of sound (Mach 1)—a critical point that affects the aircraft’s ability to maintain lift.
At the same time, the retreating blade compensates for its lower speed (compared to the advancing blade) with an increased pitch angle, until it reaches the point of stall (loss of lift).
The solution? Reduce the rotor’s speed of rotation. But this reduces its lift. The only way to go beyond this is to develop a compound formula which partially takes the burden of lift and propulsion off of the main rotor.
The open question was, is the RACER architecture the right way to proceed for a compound formula? Airbus’s Paul Eglin said: “Other concepts add a second rotor and have only one propeller, when we have two propellers. For us, Racer is the simplest formula to solve the problem of high speed.”
Racer’s two propellers are mounted on box wings—think of very thin, sleek biplane wings joined at the tips by the propellers’ bullet-shaped casing. The wings attain good cruise efficiency, and have a small surface area above to minimise their interaction with the main rotor wake when the aircraft is in hover.
The propellers are two lateral pusher types. In addition to providing thrust for high-speed cruise, they counteract the main rotor’s torque effect in hover, thus replacing the role of the tail rotor. In hover, the asymmetric tail boom benefits from the interaction from the main rotor wake to provide a significant contribution to the anti-torque function as well.
The underlying philosophy behind Racer’s design was simplicity illustrated in the transition – or lack thereof – when moving from hover to cruise.
When the aircraft is stopped or cruising at slow speeds, the main rotor is responsible for lifting the aircraft. As speed increases, the wings then begin to progressively take over the job of providing lift, as they do in a standard fixed-wing aircraft.
The pilot just has to reduce the angle at which the main rotor blades are positioned to maintain a constant altitude. For propelling the aircraft forward, a similar shifting of the burden takes place: as the main rotor propels the aircraft forward, the lateral propellers take over as speed increases. It all happens in progression, to the end benefit of the pilot at the controls.