This study starts with the design of a reversible airfoil rib for stopped-rotor applications, where the sharp trailing-edge morphs into the rounded leading-edge, and vice-versa. A NACA0012 airfoil is approximated in a piecewise linear manner and straight, rigid outer profile links used to define the airfoil contour. The end points of the profile links connect to control links, each set on a central actuation rod via an offset. Chordwise motion of the actuation rod moves the control and the profile links and reverses the airfoil. The paper describes the design methodology and evolution of the final design, based on which two reversible airfoil ribs were fabricated and used to assemble a finite span reversible rotor/wing demonstrator. The profile links were connected by Aluminum strips running in the spanwise direction which provided stiffness as well as support for a pre-tensioned elastomeric skin. An inter-rib connector with a curved-front nose piece supports the leading-edge. The model functioned well and was able to reverse smoothly back-and-forth, on application and reversal of a voltage to the motor. Navier–Stokes CFD simulations (using the TURNS code) show that the drag coefficient of the reversible airfoil (which had a 13% maximum thickness due to the thickness of the profile links) was comparable to that of the NACA0013 airfoil. The drag of a 16% thick elliptical airfoil was, on average, about twice as large, while that of a NACA0012 in reverse flow was 4–5 times as large, even prior to stall. The maximum lift coefficient of the reversible airfoil was lower than the elliptical airfoil, but higher than the NACA0012 in reverse flow operation.
For helicopters in very high-speed flight there is interest in stopping the rotor and having it operate as a fixed-wing. However, with conventional airfoils, one half of the stopped rotor/wing would be in reverse flow. To overcome this challenge, this paper focuses on airfoil reversal, where the sharp trailing-edge morphs into the rounded leading-edge, and vice-versa. Extending on a previous study that reversed symmetric airfoils, the current paper presents a design methodology and solution for reversal of a cambered airfoil. Navier Stokes CFD simulation results show that rather than using straight links to approximate a reference airfoil profile, curved links that exactly represent the nose to mid-chord section reduce aerodynamic penalties considerably despite the "bumps" they produce over the trailing-edge region. The aerodynamic performance of a curved-link reversible NACA 23012 was seen to be close to that of the reference NACA 23012. A curved-link reversible NACA 4-digit airfoil, with the same thickness and camber as the Fairchild Reverse Velocity Rotor (RVR) airfoil performed significantly better, aerodynamically, than the RVR airfoil in reverse flow. The study examined actuator force requirement, and showed that both the magnitude and direction of the actuator force required to resist the aerodynamic loads vary depending on the airfoil angle of attack and the internal mechanism design.
This study uses the Rotorcraft Comprehensive Analysis System (RCAS) to examine the effect of control redundancy on power and vibratory hub loads of a lift-offset coaxial rotor helicopter operating at 230 kt cruise speed. An aircraft nose-up pitch attitude of 3° resulted in very
low main rotor power (less than 10% of the total power), with the majority of the power consumption attributed to an efficient axial propeller. At this 3° pitch attitude, the rotor speed and differential lateral pitch, which are redundant controls, were parametrically varied, and low power
(LP) and low vibration (LV) states identified. The LP state (80% Nr and 3° differential lateral) required 3.5% lower power than the LV state (90% Nr and 0° differential lateral), but the latter had substantially lower 3/rev vibratory hub loads. The lower power in the LP state
is primarily due to reduced main rotor power on account of smaller drag on the advancing blade tip at lower rotor speeds. The rotor drag is comparable for the two states, but the LV state has larger drag contributions from the advancing side, whereas the LP state has larger contributions from
the reverse flow region (accounting for 14% of the total rotor drag) due to higher pitch on the retreating side and larger reverse flow velocities. Even so, the rotor drag accounts for under 30% of the total propulsor thrust requirement, with the fuselage (and hub) drag being the dominant
component. Rotor L/De values for the LP and LV states were 12.3 and 11.3, respectively.
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