URBAN AV OPS


Variety of V/STOL design concepts could herald age of urban aviation

As world population continues to rise, long runways and space to accommodate them are increasingly unavailable.


Interurban transportation

NASA concept for a 100,000-lb civil transport capable of carrying 80–100 passengers at 300 kts or more.

Earlier planning theories suggested that airport congestion might be relieved to some extent by more direct interurban air transportation over distances up to 500 nm between airports or—preferably—between urban areas themselves.

A detailed study of civil tiltrotor potential, updated by Boeing in 1991, concluded that a pressurized, 39-passenger derivative of the V22 Osprey had "significant market potential." Notably, New York Air­ways at its peak carried only 267,000 passengers, variously operating 25-seat Boeing Vertol 107 and 28-seat Sikorsky S61L helicopters.

If urban aviation is to profitably relieve airport congestion, it will need the 80 to 100-seat models and higher speeds envisaged by NASA and others. Such aircraft are unlikely to appear before 2030—consequently, the prospects for interurban airline service using large civil rotorcraft will languish until then.

While relief of airport congestion may be a remote goal, a 2007 NASA study notes that "the current hub-and-spoke system does not serve rural, regional and intra-urban travel well for travel distances less than 500 miles, and consumers still choose to use automobiles 20 times more often for trip distances of 100–500 miles.

Automobiles capture 76% of all trips greater than 100 miles, and airlines only 19% with an average auto speed of only 35 mph—an indicator that there is opportunity for greater mobility in the future."

The introduction of 9 to 19-passenger V/STOL aircraft operating on-demand corporate services under Part 91/91K to bypass airports would reinvigorate interurban business aviation over medium stage lengths. To support these missions, modest investment is required to establish suitable vertiports and STOLports with surface links to both target urban areas and associated airports. The motivation for the community is to enhance the utility of the urban area, thereby increasing its appeal, commercial vitality and tax base.

V/STOL designs with potential for urban aviation

Stoppable rotor
Rotors may be stopped in flight for stowage or conversion to wings. Highest speed potential of any rotorcraft configuration.
A stowed or stopped-rotor aircraft has never flown.

Coaxial rotor with pusher prop
Coaxial rotor overcomes retreating blade stall limitation. Holds unofficial speed record set in early 2011.

Deflected jet
In V/STOL flight, thrust is used to achieve hovering flight. In VTOL flight, good lift is achieved with relatively little or no forward motion.

Ducted dual-fan
FBW technology, composite materials and a patented vane control system enable lateral maneuvering without the need to roll. This is a V/STOL version of the flying car.

FanWing
Uses a bladed rotor on a horizontal axis, effectively acting as a thick wing. These blades, rotated by an engine at up to 2000 rpm, suck in air and push it up and over the wing, providing lift.

Gyrodyne
Uses engine and propeller only for thrust while free-spinning rotor provides lift. Fully scalable. Highest payload potential of any rotary-wing aircraft.

Slowed rotor/compound
Rotor speed is slowed to avoid drag from advancing blade tip and to avoid retreating blade stall while small wing increases lift.

Tiltrotor
Pivots only the rotors rather than the entire wing. This trades off efficiency in vertical flight for efficiency in STOL/STOVL operations.

Tiltwing
Pivots entire wing including rotors to retain vertical efficiency in V/STOL operations. Main drawback is control during hover in windy conditions.

Vectored thrust, ducted propeller (VTDP)
Vectoring is achieved by means of rudder and elevator elements within the duct and wing flaperons. Objective is to evaluate a method to increase the speed of existing helicopters.

Intra-urban

There remains the need serve points within the urban area. Other than for public and emergency service operations, noise will continue to be the major source of objection to intra-urban aviation. It is unlikely that aviation will serve multiple points within the urban area until new technologies, like electric propulsion, become available.

The electric helicopter shows great promise. In 2010, Sikorsky unveiled the Project Firefly electric helicopter demonstrator based on an S300C (formerly Schweizer 300C) airframe. Initial endurance was only 15 min at 79 KTAS, but this will improve as lithium air battery technology matures.

New designs will run 2 electric motors—one in the main rotor and another in the tail rotor to reduce weight and eliminate the need for a drivetrain.
Pending the introduction of such technology, intra-urban air transport must—with few exceptions—continue to rely on intermodal links with existing surface transportation services.

Current types

The accompanying table summarizes major V/STOL designs currently available to urban aviation. Range is comparatively limited because the main characteristic distinguishing V/STOL aircraft is the high required thrust-to-weight ratio.

The dominant strategy adopted uses a power system common to both VTOL and propulsion and further development is largely reliant on the evolution of motive power—in the main, turbine engines.

The aircraft at the forefront of urban aviation are the AW609, the Eurocopter X3, the Sikorsky X2 and the GBA Heliplane, each representing very different design approaches.

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