Extendable Chord Rotors for Helicopter Envelope Expansion and Performance Improvement

This study examines the effects of chord extension over a spanwise section of the rotor blades of the UH-60A Blackhawk helicopter. Chord extension is effectively realized by extending a Trailing-Edge Plate (TEP) through a slit in the trailing-edge between 63–83% span at an angle 2 deg down relative to the airfoil chord line and resulting in a 20% chord increase relative to the baseline. Since TEP extension changes the baseline SC-1094R8 airfoil profile, Navier-Stokes CFD calculations of the two-dimensional aerodynamic coefficients of the modified profile over a range of angles of attack and Mach number variations, were used in the helicopter performance simulations carried out with RCAS. The aerodynamic model also used 8x8 state dynamic inflow and Leishman-Beddoes dynamic stall model, and the structural model of the blades represented them as beams undergoing coupled elastic flap, lag and torsion deformations with nonlinear finite elements. The study showed that at high speed (130 kts), TEP extension resulted in power reductions between 12–15% over the range of altitudes, at gross weights where the baseline aircraft was stalling. At 90 kts cruise condition, corresponding power reductions were in the 10.5–16% range, primarily at higher altitudes. Increases in maximum gross weight of around 1,400 lbs were also observed with TEP extension, relative to the baseline, near stall. Increases in maximum speed of around 20 kts could be achieved with TEP extension when flying high and heavy. TEP extension reduced disk angles of attack on the retreating side, alleviating deep stall experienced on the baseline aircraft. Lift increased over the annulus where TEP extension is present, but reduced over the outer rim of the disk because the nose down pitching moments of the TEP result in larger nose-down elastic tip twist of the blades. The offloading at the blade tips reduced drag, rotor torque and power for the configuration with TEP extension.