negative drag in flex gliding gull?

Discussion in 'Boat Design' started by lunatic, Jun 6, 2008.

  1. lunatic
    Joined: Jan 2008
    Posts: 111
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    Location: nyc,ny

    lunatic Senior Member

    I have often observed non-flapping flight of gulls trailing a ferry boat, positioned to feed on stunned marine life in wake. Assumed ferry's turbulant air wake was a contributing factor, but in calm or light wind gulls must flap to stay in position. In 15-20 kts wind they can hang for extended peroids but have problem of moving upwind past the stern and need to repeatedly turn downwind to be over wake. It is back to flapping if boat is steaming downwind, but if wind is abeam, gulls hang heading 45 degrees into the wind with their glide path following ferry. Other venues: over beach in stiff onshore breeze, out over water in offshore wind, high in the sky overhead, seems anywhere with enough wind, and often with forward motion dead upwind and no loss of altitude. An updraft might explain the latter, wing rotation the former though hard to judge small angles even with bird at eye level. In the very turbulant conditions behind the ferry I expected some revealing action of what seems to be negative drag.
    Flexgliding sweepback increases with wind velocity and especially in gusty conditions, the hand wing is anhedral, being rapidly flicked up for control. Other small control movements are seen but in average conditions, when bird glides by at eye level, it seems to be in a state of relaxed equilibrium, good enough to distort its body to extend its foot for a good head scratch. Flexed wing plan form reduces area, arm wing has good upwind leading edge section, but flat, sharp edged hand wing would produce high drag leading edge vortices if generating lift and seems proportionately large if only a control surface, could it reduce tip vortices?
    Observation conditions not ideal, and with no measurements, I could be missing the obvious. Glide diagrams have a sink velocity and resultant apparent flow rotating lift and drag vectors forward, but seem too simplistic for observations. How can such chaotic input result in such steady performance and if done by very subtle trim on a relatively fixed wing, are there lessons for the sailor here?
     
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