Aerodynamic and structural investigation of an active back-flow flap for dynamic stall control


The design and experimental investigation of a back-flow flap for helicopter dynamic stall control is described. A spoiler-type flap is designed, and shown by CFD to reduce the pitching moment peak during dynamic stall by 34 %. Initial experiments with a passively actuated flap in a low-speed wind tunnel showed that the opening and closing times for the flap due to the aerodynamic forces are sufficiently short and that the flap is not affected by the inertial forces of the model pitching. The experiments showed the need for a flap restraint, and that an active actuation is needed so that the flap angle is sufficient at the time of stall to have a control effect. Initial demonstrators for the structural concept of the active back-flow flap using glass fiber reinforced polymer and a solid state hinge are presented, showing the possibility of fabrication as an after-market add-on.

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\(\alpha\) :

Angle of attack (\(^\circ\))

\(c\) :

Airfoil chord (=0.300 m)

\(C_{\mathrm{D}}\) :

Drag coefficient

\(C_{\mathrm{L}}\) :

Lift coefficient; mean; peak

\(C_{\mathrm{M}}\) :

Pitching moment coefficient; peak

\(C_{\mathrm{P}}\) :

Pressure coefficient

\(f\) :

Frequency (Hz)

\(M\) :

Mach number


Reynolds number based on the model chord

\(\rho _{\infty }\) :

Freestream flow density (kg/m\(^3\))

\(t\) :

Time (s)

\(v_{\infty }\) :

Freestream flow velocity (m/s)

\(\omega ^*\) :

Reduced frequency: \(\omega ^*\): \(\omega ^*=2\pi fc/v_{\infty }\)

\(x\), \(y\), \(z\) :

Coordinates in flow direction, breadth and upward (m)

\(y^+\) :

Dimensionless wall distance


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Opitz, S., Gardner, A.D. & Kaufmann, K. Aerodynamic and structural investigation of an active back-flow flap for dynamic stall control. CEAS Aeronaut J 5, 279–291 (2014).

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  • Back-flow flap
  • Active flap
  • Flow control
  • Solid state hinge
  • Helicopter
  • Rotor blade