Aerodynamic Analysis of a Propeller-Powered Strut-Braced Wing

Abstract

The aerodynamic performance of a regional, propeller-powered strut-braced wing is investigated. The main wing planform and operating conditions are based on an ATR72, to which a strut and jury strut are added. The research focuses on the different drag components in cruise. The first research objective is to quantify the effect the different wing components have on the change in induced drag caused by the propeller. The second objective deals with viscous effects, such as separation and friction. At the strut-wing and strut-jury strut junctions, it is tested whether the propeller affects the size and location of any separated regions. In addition, the influence of the propeller on the skin friction drag is quantified. The final objective deals with interference and compressibility. The close proximity of the strut, jury strut and wing, combined with the increased slipstream velocity of the propeller, and the change in angle of attack caused by swirl recovery might lead to high supervelocities. The regions with the highest supervelocities are localised, and it is verified whether any supersonic flow exists.

Induced drag is calculated with a panel method (Flightstream), which allows simulating different configurations in a relatively short time. By simulating different combinations of wing elements (with and without strut, jury strut and propeller) it is possible to isolate their contributions. The research objectives about viscous effects and interference were investigated using unsteady RANS. The CFD simulation were also used to validate the panel method. In general, it agreed well with CFD. Some discrepancies were caused by the absence of vortex dissipation and an offset in the pressure distribution inside the propeller slipstream.

The propeller reduced induced drag significantly, around 58% for all configurations. The main strut had the largest effect. Both the strut itself and interference of it with the wing lead to an additional reduction of 1% in induced drag. The jury strut had limited effect. Under the influence of a propeller, it had a small induced thrust component. Interference with the main wing cancelled out this benefit. These results were obtained using unoptimised loading distributions, optimising these would increase the gains for both the conventional and strut-braced wings.

The strut-wing junction only showed separation at the strut leading edge. The local flow behaviour was not influenced by the propeller. The strut-jury strut junction also exhibited leading edge separation, in addition to corner separation at the trailing edge of the jury strut, and separation at the trailing edge of the main strut. The size of the corner separation reduced under the influence of the propeller, by favorably changing the pressure gradient on the jury strut. The net effect of the propeller on the separated region at the trailing edge of the strut was to move it inboard, by increasing the pressure gradient there, and moving the location of the horse-shoe vortex system inward. Friction drag increased by roughly 3%, insignificant compared to the reduction in induced drag.

Finally, the region most sensitive to high supervelocities was the strut-jury strut junction. The closely spaced elements, combined with the higher slipstream velocity and increased angle of attack lead to a small supersonic pocket. Due to its limited size, it is expected that using a slightly different airfoil for the jury strut can already eliminate it.

While some attention needs to be payed to junction flows and interference effects, this work has shown the advantage of a propeller-powered strut-braced wing for regional aviation, compared to conventional aircraft.