Things With Wings

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  • Short and Fast - Can STOL Jetliners be Practical?
    Posted by Graham Warwick 8:09 PM on Nov 04, 2010

    Looking ahead at traffic densities projected within the next 20 years or so, there is interest on both sides of the Atlantic in the potential for "metroplex" operations - the ability to use all the runways available around a large city, be they reliver airports or GA airfields.

    For researchers at NASA and the German Aerospace Center (DLR) that means looking at short take-off and landing (STOL) airliners able to operate from under-used runways shorter than 3,000ft. The problem with traditional STOL aircraft is they are slow, and don't mix well with conventional jet traffic in en route or terminal airspace.

    NASA has been working with the California Polytechnic State University (Cal Poly) at San Louis Obispo for some time on the concept of a "cruise efficient" STOL - or CESTOL - aircraft. This would take off within 3,000ft, but cruise at normal jetliner speeds. A 10ft-span model of Cal Poly's latest design is to be tested in NASA's NFAC wind tunnel in 2011.
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    CESTOL (Concept: Cal Poly)

    The 737-sized CESTOL design uses a combination of upper-surface blowing (USB) and a circulation-control wing (CCW) to increase lift and reduce takeoff and landing distances. Boeing YC-14 airlifter used USB and NASA's Quiet Short-Haul Research Aircraft (QSRA) flight-tested both USB and CCW in the early 1980s. Neither aircraft could be described as cruise-efficient, however!

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    QSRA (Photo: NASA)

    CCW works by ejecting high-pressure air from slots over the rounded trailing edge of the wing. USB works by blowing engine exhaust over the upper surface of the wing. Both increase lift at low speeds, but USB has the disadvantage of icreasing drag in the cruise as the engine exhaust flow "scrubs" the wing surface.

    So Cal Poly has recently modified its CESTOL design, which used to have the engines embedded in the upper surface of the wing. Now they are mounted above the wing, on pylons like the VFW 614 airliner and HondaJet bizjet, but ahead of the wing so they still provide upper-surface blowing. The wind-tunnels tests will show how successful this latest design is at reducing cruise drag.
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    QSTOL (Graphic: DLR)

    DLR, meanwhile, has been focusing its research an A320-sized quiet STOL aircraft, or QSTOL, that uses upper-surface blowing. Simulations of airport operations are showing the advantages and disadvantages of STOL. While they would undoubtedly unlock unused runway capacity, DLR finds STOL aircraft could adversely impact overall system performance because their slower arrival and departure jets could interfere with conventional jetliner operations.

    And then there is the environmental impact, as aircraft with propulsive lift systems like USB are inherently noiser and thirstier than conventional airliners. DLR says an advance in engine technology will be needed before a QSTOL can be truly quiet.

    Tags: tw99, NASA, DLR

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