Originally Posted by
helispotter
I hadn't seen anything about the X3 for a while, so looked it up on Wikipedia (
https://en.wikipedia.org/wiki/Eurocopter_X%C2%B3). It apparently
achieved 255 knots in level flight in 2013, indicated to be an unofficial helicopter speed record. Yet it has already been in a museum since 2014!
I hadn't come across Racer (
Rapid and Cost-Effective Rotorcraft), so looked it up too (
https://en.wikipedia.org/wiki/Airbus_RACER ) and now understand it is an evolution from the X3. Wiki reports "
first flight anticipated at the beginning of Q2 in 2022" but that has clearly passed so progress presumably still slowed due to COVID?
The Racer article in turn reminded me of the Piasecki X-49 and Sikorsky X2 as compound helicopters of 'comparable' configuration. The X-49 (and its predecessors) seemed like a mechanically straightforward alternative to any swivelling tail rotor concepts. Is there any future in such an arrangement as opposed to the X3 / Racer configuration?
Finally, is there any future in revisiting the DTNSRDC X-wing concept that had been built and fitted to the S-72 (
https://en.wikipedia.org/wiki/Sikorsky_S-72), but was never flight tested? It seemed like a radical idea to achieve high speed flight of a rotorcraft at the time. Rotor is only used for take-off then becomes stationary for high-speed forward flight. Not sure how they managed to solve the problem of the aerodynamic loading on the forward swept pair of stationary blades (wings)!
X-49 as a research platform has been going on for a while, but not a ton of public data. The configuration adds a wing (which is key to high speed performance with a flapping rotor) and the ducted prop. Yaw control is via rotating segments of a turning duct structure into the prop thrust. The entire tail assy looks really heavy and I suspect yaw control is still somewhat marginal. A production configuration was slated to add a third engine to handle the required prop power (similar to the discussion above) and a nearly 4 foot fuselage extension which I imagine goes forward of the wing to help move the center of gravity close to the main rotor mast axis. So... can the configuration work? Yes. In fact, with the new ITE engine and a new drive train, a third engine probably isn't needed. Is it better than X3? Not sure. The Airbus X3 concept splits the anti torque and forward thrust into two, though smaller, diameter propellers. Two props is more drive system and rotor components, but the turning duct isn't needed, shaft torques are lower, and the hardware is much closer to the aircraft center which reduces yaw inertia. The ducted prop has protection for people on the ground which is something the X3 notably lacks.
The Sikorsky X-2 is a different beast. Remember my comment about X-49, X3, Invictus, and other high speed helo having wings to achieve high speed? The wing is a solution for overcoming the drop in lift/thrust performance of a flapping main rotor as the aircraft speed increases. In textbooks, this is the "Ct/sigma vs mu" plot, a way of plotting lots of rotor data in a non-dimensionalized form. Adding a wing offloads the rotor so that as aircraft speed increases the lift demand from the rotor drops to stay within its capabilities. On an aircraft without a propeller, this also means the lift available to pull the aircraft through the air via rotor flapping also drops, but if you can flap the rotor a lot and/or have a low drag fuselage in a nose down attitude, you can still achieve higher speed than usual for a helo. The propellers on X3, X-49, and AH-56, etc. replace the missing forward thrust from the main rotor with pure horizontal thrust... at the cost of adding more rotors. In the extreme, a large enough wing and prop can completely unload the main rotor and then you just need to slow the main rotor down to keep the advancing tip below Mach 1 and add tip masses to keep the rotor stable. This is the Carter Copter (and Jaunt Journey) solution. Larger and larger wings do add download, though and harm hover performance, but going fast does require more installed power on the aircraft, so that can be overcome with a larger main rotor (and more mass). There's no free lunch in any of this.
Back to X-2... it does not have a flapping main rotor, so it's Ct/sigma vs mu plot doesn't show rotor lift decreasing rapidly with increasing airspeed. The rotor itself really can move to a different level of performance at speed than conventional flapping rotors. Eliminating the flapping compliance largely eliminates the natural flap response of a rotor that keeps the center of lift over the rotor mast and rotor controls can trim the retreating blade to produce less lift and shift the rotor lift heavily onto the advancing blades. The rotor now behaves a bit more like an airplane wing which is why X-2 aircraft don't have the wing. You also eliminate the download from the wing, so the aircraft is more efficient at hover (coaxial rotors also have some efficiencies here). All this sounds great... on paper... but the Iron Law of No Free Lunch applies. The X-2 eliminates the most important invention in the history of rotary winged flight, the flapping hinge, and the consequences are severe. Rotor loads are roughly an order of magnitude higher than on a flapping rotor. Vibrations in the aircraft are severe. Hub drag is high. Part count is high. Yaw agility is low and sometimes nearly non-existent. The configuration has gone head to head with tilt rotors twice now (XH-59A vs XV-15 and SB>1 vs V-280) and lost both times. It might even lose to a simple winged helicopter in a few years. The prop isn't the key technology, its the rigid rotor that eliminates the wing, but the rigid rotor is also the source of all the aircraft problems.
X-Wing IS even more radical. It is a rigid rotor (bad) that uses a hugely complex pneumatic "swashplate" to direct variable amount of air to leading edge and trailing edge vents along the elliptical cross section blade airfoils to both control lift and simulate cyclic blade pitch (bad). Had it flown, the vibrations in horizontal flight would have been severe. The rotor design was an innovative stiff in/out of plane "bearingless" hub design that used composites to achieve the stiffnesses required to make forward swept wings work. Integrating the mechanical, pneumatic, software, and other systems together for such an aircraft led to the development of the "System Integration Lab" approach that is now common for advanced helicopter programs. The volume and power inefficiencies of the pneumatic systems and rigid rotor probably doom this concept from further development. Remember... No Free Lunch.