Oct 162010
 

A view inside the X-21 laminar flow control research aircraft. The system used on the X-21 was ingenious and held great promise, and was utterly impractical. The wings of a Douglas B-66 were removed; the engines were moved to two pods near the tail. A new wing was added… a wing with a difference. The upper surface of the wing was riddled with spanwise slots (around 800,000 of ’em) which were manifolded together. Beneath the trailing edges of the wings near the roots were small turbine units that served a role something like vacuum cleaners. These devices were powered by bleed air ducted from the main engines, and sucked air through the slots in the wings. The result was that turbulent air over the wings could be sucked into the wing, with the result being a notable drop in drag and an increase in lift. In the early 1960’s, Northrop and others believed that this could be used to create aircraft, subsonic and supersonic, with far greater range and fuel economy. The problem was that the slots were *really* narrow, and were a performance and maintenance nightmare. Bugs, dust, ice and rain would be sucked in and would plug up the works, not only negatively affecting performance, but requiring considerable manpower and down time to keep clean. Painting the wings was of course impossible, negating the idea of camouflage. The basic idea turned out to be one of those concepts that’s really great on paper but begins to fall apart when it runs headlong into reality.

 

 Posted by at 11:40 pm

  7 Responses to “Northrop X-21 Inboard Profile”

  1. Avation Week’s 11 October 2010 issue describes an experimental UAV
    with a similar control system.

    “In pursuit of lower manufacturing and maintenance costs, a U.K. team
    has flown an unmanned aircraft in which conventional flight control
    surfaces can be replaced by air jets blown from the trailing edge of
    the wing.

    Developed by Cranfield University, BAE Systems and nine other
    universities, the Demon UAV results from an unusual partnership
    between U.K. industry and academia under the five-year, £6.2-million
    ($9.8-million) Flapless Aerial Vehicle Integrated Interdisciplinary
    Research (Flaviir) project.”

    http://www.flaviir.com/home.htm
    http://www.aviationweek.com/aw/generic/story.jsp?id=news/awst/2010/10/11/AW_10_11_2010_p20-259707-01.xml&channel=misc

  2. Always one of my favorite obscure test aircraft.

    Sure, it’s impractical, but on the other hand, it also tests the math.

    I recall a B-47 test pilot saying that early jet aircraft were faster than they had expected, apparently because they had spent 40 years under-estimating the drag produced by a propeller.

  3. > a similar control system.

    Actually, no similarity whatsoever. The X-21 sucked in air through a vast number of tiny slots atop the wing; the UAV blows air through a small number of large slots at the trailing edge. The X-21 did it for increased L/D; the UAV does it for aerocontrol. Different methods to achieve different ends.

  4. You would have thought Northrop would have figured out the plugging problem long before they built the actual aircraft.
    It seems like all Northrop aircraft were required to have something odd about them, even if it was something like the super-skinny tires like on the F-89 Scorpion, or the guys sliding around in seats with gunsights on them in the P-61 Black Widow.

  5. As with the Rockwell XFV-12, sometimes it turns out the only way to find out that the really neato concept is fatally flawed is to build it full scale. And like the XFV-12, apparently it seemed the cheapest way to go full-scale was to build an actual full-scale flying aircraft.

  6. I’ve wondered if modern composite fabrics, with carefully tailored porousness, might be practical in some circumstances where the slots weren’t. An to clean out the inevitable dirt clogging the pores, run the system in reverse, over-pressuring the wing during the preflight check.

  7. The problems with that are two (at least):
    1) A system designed to both suck and blow (hmmm, we seem to have come back to “government”) must be designed for both positive and negative pressure. Which means stronger, heavier, more expensive.
    2) No matter how hard you try, you’ll never blow it all out. And the contamination will accumulate. So every now and then you’ll have to go in and do the deep cleaning anyway.

    The only way to make a system like this practical is to make the slots themselves practical. in other words… large and easily cleaned and difficult to clog. While that’s certainly doable, it’ll reduce the efficiency of the system compared to ittybitty slots.

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