A vertical-takeoff-and-landing (VTOL) design with wings that both tilt and fold, providing stable hover and efficient flight, is under development by Boston-area startup PteroDynamics. The company is targeting the military unmanned-aircraft market initially, but has ambitions to develop an air taxi.
In PteroDynamics’ Transwing concept, the wings tilt and fold back along the fuselage, and the aircraft acts like a multicopter in vertical flight. To transition to forward flight, the wings rotate into horizontal position, allowing the Transwing to fly like a conventional aeroplane.
“Our vehicle is completely unique,” says CEO Matthew Graczyk. “It’s 100% fixed wing in horizontal flight and 100% rotary wing in vertical flight, with no compromises.” Unlike a tiltwing, the Transwing can transition between vertical and forward flight with no stall range, he says. The aircraft can have a long wing for cruise efficiency, but folds into a compact size for VTOL.
PteroDynamics has flown a 12-ft.-span model, but has not yet attempted transition between vertical and horizontal flight and does not know how well the design scales. “We did get lucky with the prototype,” says Petrov. “What will happen at 40-ft. or 400-ft. span we don’t know, but as we understand how it works we see no fundamental reason it will not scale.”
As the wing rotates aft it has more dihedral and sweep and the angle of attack never exceeds 20 deg., while propeller wash helps prevent flow separation. “A tiltwing goes past 45 deg. into wing stall and the wing becomes an obstacle to forward flight,” Petrov says. Transwing “can stall, but mildly. We tufted the wing and video shows the airflow does separate, but not badly. Mostly the flow stays attached.”
Petrov describes the tilt/fold articulation as similar to the wing-fold mechanism on Grumman carrier-based aircraft such as the Avenger and Hellcat, except that Transwing folds leading edge up. The hinge mechanism will carry high loads, but reinforcement of the wing at the fold “should not penalize the aircraft too much, perhaps 5% of maximum takeoff weight,” he says.
I can’t wait for a full size test.