Wingtip-mounted propellers installed at a nonzero yaw angle

Abstract

Propellers were the first means of powered propulsion for aircraft. However the development of propellers stalled due to the growing interest in jet propulsion. Due to the growing interest in more fuel efficient and environmentally responsible aviation, propeller research is currently seeing a resurgence.

Wingtip-mounted propellers are seen as an attractive option for increasing the efficiency of aircraft. Indeed, due to the favorable interaction between the propeller slipstream and wingtip vortex, the performance of the integrated propeller-wing system is improved compared to the same elements in isolation.

Propellers installed at the wingtip have a considerable moment arm. When a thrust imbalance occurs, a fairly large moment about the aircraft yaw axis can negatively affect the aircraft trim. As a result, the aircraft directional static stability requirements are more stringent compared to the conventional layout. It is known, that a propeller at an angle with regards to the flow direction develops a considerable side force. The change in thrust and the side force that develops in this case could potentially be used to aid in the aircraft stability, instead of adversely affecting it, if the tip-mounted propeller were installed at a so-called "toe-out" angle. This configuration would take advantage of the benefits of the Wingtip-mounted propeller configuration, while also potentially enhancing the aircraft directional stability as well, thus reducing the adverse effects on aircraft trim of the wingtip propeller configuration.

With the aim of understanding the possible impacts of modifying the propeller installation angle, RANS CFD simulations were used to compute the flowfields of various configurations. First an isolated NACA-series wing and small diameter propeller were simulated at steady-state, in order to validate the methods used in this investigation. Following this, an isolated wing based on the experimental NASA aircraft X-57 was simulated, as this aircraft uses wingtip-mounted propellers. Then, the previously simulated propeller was scaled up to the operational size of the X-57, and simulated in transient conditions at zero and a non-zero sideslip angle of 10 degrees, since the inflow angle is non-uniform for this case, leading to unsteady blade loading. Finally the propeller was installed on the wing and power-off and power-on transient simulations were performed at installation angles of -10, 0 and +10 degrees. For each of the computational domains, grid convergence studies were performed to assess the numerical error associated with the chosen element sizes. The domain used for the isolated wing was calculated to have an uncertainty of 2% and 6% respectively when it comes to C_L and C_D. The domain used for the propeller simulations was calculated to have an uncertainty in C_T and C_P of almost 4% and 6%, respectively. These domains were combined for the investigation of the installed propeller cases and, to reduce computational costs, no grid convergence study was performed.

The results obtained from the simulation of the wing installed with the nacelle and spinner only, showed that there is a significant blockage effect due to the presence of the nacelle, which reduced lift over the wing, and increased drag. The presence of the propeller in power-on cases were observed to reduce the negative effects of the nacelle.

It was observed for the -10 degrees installed propeller, that the lift and drag performance of the wing was increased in this configuration compared to the 0 degree case, while the +10 degrees installed propeller showed decreased wing lift and drag performance. This was due to the orientation of the propellers. At $\epsilon_{TMP} = -10^\circ$, a larger portion of the wing is washed by the propeller slipstream, and thus affected by the interaction. This effect reduces with increasing toe-out angle as less of the wing is washed by the propeller slipstream. Additionally, due to the toe-out angle, the loading over the propeller disc is not distributed uniformly. Because of this, also the propeller slipstream is not uniform and thus the trailing wing is affected differently by the interaction for varying toe-out angles. The +10 degrees installed propeller was estimated to have an efficiency $.2%$ lower than its counterpart and $1.3%$ lower than the 0 degree installed case.

The generated side force of the installed propellers varied in sign, which depended on the sign of the installation angle. The side force for non-zero installation angle was estimated to be at least 13% of the zero installation angle thrust, for a representative cruise thrust setting and flight conditions. The side force imbalance for an aircraft experiencing sideslip, with non-zero angle wingtip-installed propellers with an aft center of gravity (CG), was investigated with regards to the directional stability. The thrust imbalance was estimated to be a slightly stabilizing factor when the propellers are installed at +10 degrees and, when combined with the destabilizing nature of the resulting side force imbalance, a slight improvement to the aircraft directional stability was found, though not sufficient to greatly affect the aircraft. At 10 degrees of sideslip, the resulting stabilizing moment due to the force imbalance in this case was found to constitute 1% of the magnitude of the aircraft restoring yawing moment caused by the vertical tailplane. This effect was a destabilizing factor when installed at -10 degrees, with the resulting destabilizing moment due to the force imbalance at 10 degrees of sideslip found to constitute 13% of the magnitude of the aircraft restoring yawing moment caused by the vertical tailplane. Since these calculations were done for a configuration where the propeller forces act ahead of the aircraft CG, it was noted, that the side force imbalance would be stabilizing in both cases for a forward CG. The contribution to the moment is largely determined by the location of the propellers, and the moment contribution of the thrust imbalance of the non-zero installed wingtip mounted propellers is estimated to be 20% larger than that of the side force imbalance, on the X-57.

The present study has contributed towards a better understanding of the interaction effects and performance of wingtip-mounted propellers installed at a nonzero toe-out/toe-in angle. The results indicate that both inboard- and outboard-directed propellers have benefits and drawbacks with respect to the zero installation angle configuration. Nevertheless, future studies should still be performed on this topic. At higher Mach numbers, actively controlled propeller installation angles or tailplane-mounted propellers at an incidence angle could take advantage of the unique characteristics of this configuration.