Some unmanned aircraft designs attempt to combine the vertical takeoff and landing (VTOL) and hover capabilities of a helicopter with the increased speed and range capabilities of fixed-wing airplanes. Stop-rotor “nose-sitter” configurations — so named because the aircraft takes off and lands from a nose-down orientation — may offer good hover efficiency and aerodynamic design, but can require complex mechanical systems. These designs can also suffer a significant loss in altitude during transition from helicopter to airplane mode, and involve uneven weight distributions, rendering the aircraft “top heavy” and unwieldy during takeoff and landing. Further, the counter-rotating fuselage and tail of some nose-sitter designs are less practical than aircraft designs with a conventional fuselage orientation and tail rotor. Tiltrotor configurations with tiltable rotating propellers also involve mechanically complex systems and decreased hover efficiency due to higher disk loading. “Tail-sitter” designs — so named because the aircraft takes off and lands from a tail-down orientation — are associated with poor hover efficiency due to high disk loading and an awkward 90-degree attitude change between hover and forward flight modes.

The stop-rotor rotary wing aircraft transitions between a helicopter mode (top) and a fixed-wing (bottom) flight mode.

The compound helicopter has a rotor system driven by an engine for takeoff, hovering, and landing, and an additional propulsion system and supplemental wing independent of the rotor system. At higher speeds, the rotor system does not drive the aircraft, and is substantially unloaded by the lift of the wing. Compound designs also have disadvantages: they are heavy due to additional systems, and can suffer a significant download penalty when hovering due to the presence of the wing in the rotor downwash. Similarly, tilt duct designs, whose propellers are shrouded in ducts and rotate between flight modes, suffer from poor hover efficiency and high drag in forward flight mode.

A patented system and method of transitioning an aircraft between helicopter and fixed-wing flight modes was developed. The stop-rotor aircraft is capable of both a helicopter mode VTOL and efficient, high-speed, fixed-wing flight by flipping the left wing/ rotor blade 180 degrees between flight modes.

Each wing has a spar and a flap movable with respect to the spar, a flap actuator configured to move the flap, and a center section rotatably coupled to each spar and including at least one spar actuator. The spar actuator is configured to rotate at least one of the wings about a rotational axis of the spar when the aircraft transitions between helicopter and fixed-wing flight modes.

Conversion between flight modes will take about 1-2 seconds, and simulations indicate altitude deviations of less than 50 feet. A prototype battery electric aircraft is being developed that is capable of more than 30 minutes of flight duration and a cruise speed of 100 knots. Hybrid power systems could provide much greater duration and range. The 38"-long removable payload bay can carry up to 25 pounds.

For more information, contact Amanda Horansky-McKinney at This email address is being protected from spambots. You need JavaScript enabled to view it. ; 202-767-5815.


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This article first appeared in the July, 2017 issue of Tech Briefs Magazine.

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