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.

By Don Van Dyke
ATP/Helo/CFII, F28, Bell 222

Traffic congestion and indirect routing lengthen typical intercity commutes to 2–3 hrs. At projected speeds up to 275 kts, the AgustaWestland AW609 would cover these trips in just 10–40 min.

Urban aviation refers to aircraft operations both within and between cities or towns. The central built-up hub around which outlying satellite areas are arrayed comprise challenging terrain with numerous flight hazards from physical, environmental, natural, air traffic and other sources. Maneuverability is commonly limited by high-rise structures, urban canyons and obstacles.

The US Congress's Office of Technology Assessment (now defunct) identified the target urban aviation market as time-sensitive travel over a range of 100–500 miles, with NASA opting for a 200-nm mission baseline for comparative purposes. To put this in perspective, the distance between New York and Washington DC is 206 nm. London to Paris is 213 nm.

Society in general expects urban aviation to operate safely, reliably and with respect for the environment, with a business model offering the 4 Cs (choice, convenience, comfort and cost), intermodality and service range.

Given these challenges, let us examine air transport needs in, around and between urban areas. Which aircraft designs are viable, which only show promise and which strategies may no longer be worth pursuing? What are the timelines?

Urban area aviation needs

Urban aviation is part of the communications network which facilitates society's efficient use of urban areas, addressing market needs:
• within and between urban areas
• between urban and satellite areas
• within and between urban satellite areas as well as:
• by linking with other transport modes serving urban areas.

Cities rely on intra-urban aviation for public services such as administration, search and rescue, firefighting, peacekeeping, surveillance and security. In these roles, aviation's most sought feature is endurance, not time-saving. Noise concerns are minimal in view of the life-saving nature of these missions.

All societies need to link distant points physically, but key differences exist between travel in the US and elsewhere. In Europe and Japan, for example, trains carry the majority of commercial passengers, whereas in the US almost all common carrier carriage is by air.
The feature most demanded of interurban aviation is time-saving.

Especially when aviation is in competition with rail, this hinges on convenient access, probably using multiple sites throughout the urban area. In this context, aviation tenders only convenience—and mitigation of environmental impact becomes a greater societal need.

For obvious reasons, airports are sited as closely as possible to urban cores. Most of the world's 25 busiest airports serve capital cities or major commercial centers, each handling roughly 10–20 million passengers annually. However, major airports tend to become ever more crowded and their appetite for increased land use is usually resisted by a public aware that airports often make poor neighbors.

Areas surrounding urban cores are usually less restrictive, allowing operations by vertical or short takeoff and landing (V/STOL) aircraft as well as VTOL. YTZ (City Centre, Toronto ON, Canada) is one of the few North American STOLports, but even its detractors are vocal.

It is clear that, while urban aviation's life-saving features are crucial, equally essential is its ability to offer time-saving and convenience as elements of commercial attractiveness.

Meeting urban needs

Moller M400 VTOL Skycar takes to the highway in the sky. This prototype roadable vehicle bypasses the need for STOLports.

By air, unless they are immediately adjacent to a coastline or similarly open feature, most urban cores are directly accessible only by aircraft with VTOL capability such as helicopters. However, in horizontal flight the helicopter is inefficient, with speed and range only 30–50% that of a typical light airplane. Also, partly due to their relative complexity, helicopters suffer a higher event rate than do light conventional airplanes.

Interest in vertical flight likely stems from man's earliest attempts to mimic the behavior of birds. Enhanced helicopter performance and utility ensure the class's future—but the lure of designs offering the benefits of both rotary and fixed-wing flight persists, especially to access urban areas whose characters and needs are wide-ranging. Acronyms like STOL, V/STOL, ESTOL, STOVL and PLTC hint at the many alternatives tested over the past 60–70 years, but commercial success remains elusive.

To put it simplistically, only 2 dimensions of V/STOL performance are important to urban aviation—hover performance and forward speed. The former addresses the need for access to urban areas and the second regards the need to link various points within urban areas.

Classic helicopters exhibit superior hover performance but retreating blade stall and blade tip speed effects limit their speed performance. Coaxial rotor and hybrid configurations promise to overcome these constraints.

With their speed and agility, especially over medium-length sectors, tiltrotors bypass busy airports and their inherent congestion.

At the other end of the hover/ speed performance envelope are deflected-jet aircraft. They possess unmatched speed, but their (limited) hover capability comes at the expense of large engines and high fuel consumption.

The ability to morph between rotorcraft and fixed-wing configurations is a design approach optimized to satisfy both intra and interurban aviation requirements, and is epitomized by tiltrotor aircraft.


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