Improvement of high-speed coaxial rotor performance using redundant controls and camber morphing

Jacobellis, George
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Gandhi, Farhan
Hicken, Jason
Sahni, Onkar
Julius, Anak Agung
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Aeronautical engineering
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Attribution-NonCommercial-NoDerivs 3.0 United States
This electronic version is a licensed copy owned by Rensselaer Polytechnic Institute, Troy, NY. Copyright of original work retained by author.
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CFD simulations were then undertaken for a rotor with reflex camber on the inboard sections of the blades. Two modified blade geometries were derived from the geometry of the X2 blade, one with a larger chord, greater twist, and sharp trailing edge airfoils on the inboard section, as opposed to the X2 which has a reduced chord and twist, and elliptical airfoil sections on the inner part of the blade. This geometry was further modified to include 15 deg reflex camber on the inboard section of the blade. Simulations were run at the same flight condition as previous X2 simulations (230kt, 4 deg aircraft pitch). While the addition of reflex camber did reduce the reverse flow drag compared to the uncambered blades as expected, the reflex camber led to a large amount of negative lift on the advancing side, resulting in reduced rotor efficiency.
Lift-offset coaxial helicopters are of great interest due to their high speed and range capabilities, while retaining excellent hover performance. These types of vehicles can travel at speeds of up to 250kt, which is over 50% greater than conventional configurations. The presence of the two rotors, as well as an auxiliary pusher propeller, offers enough control authority to enable the aircraft to hover or cruise at a range of different pitch attitudes and with different rotor loading distributions. This study aims to identify and understand how these helicopters may fly in the most efficient way possible, through utilizing either the already present redundant controls or by using in-flight variable blade geometry.
For the first part of the study, which focuses on using redundant controls to achieve different trim states, a model of the XH-59 Advancing Blade Concept demonstrator aircraft was developed using Rotorcraft Comprehensive Analysis System (RCAS) and a prescribed vortex wake aerodynamic model. The effects of varying three redundant trim variables: rotor speed, differential lateral pitch, and vehicle angle of attack were examined at a flight speed of 230kt by trimming the aircraft in level flight while the these three redundant controls were parametrically varied. States exhibiting the best performance and the lowest vibration are highlighted. The propulsive power is the dominant component of total power, accounting for more than 85% of total power. The rotor power decreases as vehicle angle of attack is increased, reaching zero power slightly above 3 deg. Operating the rotor in a negative power state, similar to how a wind turbine operates, is theoretically possible, but may not be feasible for all vehicle configurations.
With the aircraft pitch at 3 deg, the rotor power is constrained to be greater than or equal to zero. While also constraining the lift-offset to manageable values, low power and low vibration trim states were identified and the performance and vibratory loads of these trim points were compared. The lowest total power at 3 deg aircraft pitch occurred near 80% rotor speed and 3 deg differential lateral pitch. This state also corresponded the highest rotor lift to drag ratio (L/De) at 3 deg aircraft pitch, which was just above 12. The hub vibrations at this point can be reduced significantly by increasing the rotor speed to 90%Nr and decreasing the differential lateral pitch to 0 deg. This state exhibits a 36% decrease in the 3P hub pitching moment, with only a 3.5% increase in total power.
Assuming that the negative rotor power can be used to decrease total power, those states utilizing negative rotor power are compared to states in which the rotor power is constrained to be non-negative. The optimal trim state for a system with a powered or free spinning rotor lies at about 3 deg pitch attitude, while the optimal trim state for a rotor operating at negative power lies at 6-7 deg pitch attitude. With the aircraft operating at a high pitch attitude, the horizontal tail is able to provide a significant amount of the necessary aircraft lift, while the rotor provides the longitudinal moment necessary for trim. At 7 deg aircraft pitch the total power is decreased by 9.6% compared to 3 deg pitch and the rotor L/De is increased to 17.7. The blade loads and hub vibrations for the negative rotor power state are similar or less than for the marginally powered state.
The second part of this thesis investigates the impact of camber morphing blade sections on the inboard portion of a rotor operating at high advance ratio. Slowing the rotor at high flight speeds results in the rotor operating at a high advance ratio, the ratio of forward flight speed to tip speed. This subjects the blades on the retreating side of the rotor to a large region of reversed flow, where the air flows from the geometric trailing edge to the leading edge. Traditional airfoil sections with large chord, high twist, and a sharp trailing edge on the inboard sections of the blade perform poorly in reverse flow, leading to increased rotor drag (H-force) which contributes to total power. Modern coaxial helicopters, such as the X2 Technology Demonstrator utilize elliptical airfoil sections with shortened chord and reduced twist on the inboard sections, which reduces this reverse flow H-force. This thesis investigates blades with large chord on the inboard sections, which are capable of morphing the trailing edge section in high speed flight as an alternate method of reducing reverse flow drag.
Through computational fluid dynamics (CFD) simulations and wind tunnel tests this study examines a NACA63-218 airfoil in reverse flow at Re = 375,000, and demonstrates reduction in reverse flow drag through the introduction of reflex camber when the airfoil is pitched nose up in reverse flow. Of the three dominant sources of reverse flow drag - ram pressure on the upper surface near the trailing-edge, suction on the lower surface near the trailing-edge, and bluff body separation at the rounded nose, reflex camber influences the first two, while leaving the third mostly unaffected. The change in trailing-edge geometry reduces exposure to ram drag on the upper surface while the suction on the lower surface rotates to result in a force opposite to the direction of the free stream. Although the 2D CFD simulations had difficulty predicting the bluff body separation at the airfoil nose well, the change in flow at the trailing-edge was well captured yielding drag reductions of around 60% for a 10 deg reflex camber (compared to reductions of around 50% in the wind-tunnel test). Even greater percentage reductions in drag (up to 70%) were observed with a larger 15 deg reflex angle for nose-up pitch angles greater than 5 deg in reverse flow. Simulations at a higher Reynolds number (1.5 million) showed very similar drag reductions. These results show promise for the potential of camber morphing on the inboard sections of a rotor at high advance ratio to reduce reverse flow drag.
Before an analysis of camber morphing on a full rotor was undertaken, a helicopter representative of more modern designs, the X2, was analyzed at 230 kt using three rotor analysis methods to assess their applicability. The first and simplest method, dynamic inflow, is used to determine the trimmed flight conditions at speeds up to 250 kt which was 4 deg pitch attitude and a lift-offset of close to 0.2. Two higher order methods, Viscous Vortex Particle Method (VVPM) and Computational Fluid Dynamics (CFD) are then used to evaluate the same trimmed condition at a speed of 230 kt. VVPM is capable of capturing the rotor to rotor interference from first-principles at a significantly lower cost than CFD, however the high resolution CFD solution revealed significant deviations in the airloads from the values obtained with the first two methods.
December 2018
School of Engineering
Dept. of Mechanical, Aerospace, and Nuclear Engineering
Rensselaer Polytechnic Institute, Troy, NY
Rensselaer Theses and Dissertations Online Collection
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