Flexible flapping airfoil propulsion at low Reynolds numbers

S Heathcote, I Gursul

Research output: Contribution to conferencePaper

28 Citations (SciVal)


Water tunnel experiments on a flexible airfoil plunging with constant amplitude have been carried out for Reynolds numbers of 0 to 27000. Peaks in thrust coefficient at intermediate values of airfoil stiffness were observed at both zero and non-zero Reynolds numbers, indicating that a degree of flexibility is beneficial at low Reynolds numbers. Time-averaged velocity fields and momentum flux data revealed a broader, higher-velocity jet in cases of optimum airfoil stiffness. Stronger vortices, separated by a larger lateral distance, characterised the corresponding instantaneous velocity fields. The flexibility causes the airfoil to pitch passively; the phase angle of the pitch was found to lead the plunge. Pitch amplitude and trailing-edge amplitude were found to be single-valued functions of pitch phase angle. The shape characteristics of the airfoil could therefore be described by the pitch phase angle only. Thrust coefficient was found to be a function of only two parameters: Strouhal number and pitch phase angle. For each Strouhal number, a peak in thrust coefficient was observed at a particular value of the pitch phase angle. The optimum pitch phase angle was found to tend to a limit of 105±5 degrees at very large Strouhal numbers. A significant thrust benefit was observed over very stiff airfoils when the optimum flexibility is utilized.
Original languageEnglish
Number of pages15
Publication statusPublished - 2005
Event43rd AIAA Aerospace Sciences Meeting and Exhibit - Reno, NV, USA United States
Duration: 1 Jan 2005 → …


Conference43rd AIAA Aerospace Sciences Meeting and Exhibit
Country/TerritoryUSA United States
CityReno, NV
Period1/01/05 → …


  • Airfoils
  • Reynolds number
  • Propulsion
  • Tunnels


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