The Germans had a word for it - triebflügel, or "thrust-wing" - but the Austrians have a new spin on the idea. Gmunden-based Aerie is at ILA 2012 displaying long-endurance VTOL unmanned aircraft that switch between rotary- and fixed-wing operation.
S-25 (Photos: Aerie)
On display are Aerie's S-25 Swift prototype (above) and the new I-2 ILE backpack version (below), developed in response to interest received when the company exhibited at the Farnborough airshow in June. Formed last year, Aerie has signed a letter of intent with Austrian sailplane manufacturer HB Flugtechnik to produce a range of VTOL UAVs.
Aerie's concept has propellers mounted at about mid-span on a rotor-cum-wing. For vertical take-off and landing, the propellers pull in opposite directions to spin the rotor. For forward flight, the rotor stops and coverts to a wing, the props providing forward thrust.
The S-25 is a 30kg UAV with two electric motors mounted on the 5m-span rotor/wing. Aerie is also designing the 150kg, 10m-span D-150, planned to be available in 2013 and the 200kg K-200, with greater than 10m span, to follow in 2014. In both of the larger UAVs, the wing-mounted props will be shaft-driven from a fuselage-mounted engine.
Like the S-25, the D-150 and K-200 UAVs (above, with S-25, in scale-model form) are tail-sitter VTOLs. All three are intended to have an endurance exceeding 24hr. The smaller ILE, however, is a nose-sitter, with forward-swept wings. Check out this video:
Aerie founder Johannes Reiter says the S-25 prototype, which weighs 25kg, has lifted off vertically under the power of two electric motors, each producing 4kg of thrust, for a VTOL thrust-to-weight ratio of 0.3. The vehicle lifted off at a rotor speed of 120rpm, below the expected 180rpm. "There is still a control. issue under development, in order to get it completely controlled as a helicopter in flight," he says.
Reiter says advantages of the on-blade propulsion include stability in rotary-wing flight, absence of a tail rotor, "and even use of the effect of the precession force." By careful positioning of the rotation point of the wing relative to the fuselage "you have an automatic rpm regulator...that comes just by nature."
If the wing pitches, its drag increases and rpm reduces. "After that the precession force reduces, so the wing stays in one proper balanced position between the precession force and the resulting momentum on the wing caused by the lift force," he says. The effect is similar to single-lever propeller control, but "all done by nature's laws."