Flexible flapping airfoil propulsion at low reynolds numbers

S Heathcote, I Gursul

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310 Citations (SciVal)


Water tunnel experiments on chord wise-flexible airfoils heaving with constant amplitude have been carried out for Reynolds numbers of 9,000 to 27,000. A degree of flexibility was found to increase both thrust coefficient and propulsive efficiency. Measurements of the flow field revealed stronger trailing-edge vortices corresponding to higher thrust coefficients, and weaker leading-edge vortices corresponding to higher efficiencies. By analogy with a rigid airfoil in coupled heave and pitch, thrust coefficient and propulsive efficiency were found to be functions of the Strouhal number and pitch phase angle. Propulsive efficiency peaks at a pitch phase angle of 95-100 deg (consistent with experimental and computational simulations of rigid airfoils in coupled heave and pitch), and a Strouhal number of 0.29, which lies in the middle of the range observed in nature. Thrust peaks at pitch phase angles in the region of 110-120 deg, but at higher Strouhal numbers. The results suggest the effect of chordwise flexibility is beneficial for purely heaving airfoils at low Reynolds numbers.
Original languageEnglish
Pages (from-to)1066-1079
Number of pages14
JournalAIAA Journal
Issue number5
Publication statusPublished - 2007


  • Aircraft propulsion
  • Flow fields
  • Airfoils
  • Strouhal number
  • Vortex flow
  • Reynolds number


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