Abstracts Category : Other

The effect of turboprop propulsion on the aerodynamic benefits of formation flight

by Cameron Cedric McKenzie

In order to determine the effect of turboprop propulsion on the aerodynamic benefits of formation flight, a 3D Vortex Filament Method (VFM) programme, which made use of a Burnham-Hallock viscous core model, was formulated and employed to model the progression and interaction of the wing and turboprop trailing wakes. Within this programme, an initial prescribed wake for the turboprop engines was discretised by shed helicoidal vortex filaments generated by the use of an amalgamation of the propeller theory of Goldstein and Theodorsen. The downwash velocity field of a B747 during cruise, obtained via the use of the VFM Burnham-Hallock (VFM B-H) model programme, was verified against both the simulation conducted by Ehret and Oertel, in which an integrated Biot-Savart law VFM was utilised, as well as against experimental results obtained by Burnham et al. in their ground-based measurements of the wake vortex characteristics of a B747 aircraft. The VFM B-H model produced peak upwash and downwash velocities which matched those obtained by Ehret and Oertel to within 95% accuracy. Furthermore, a distance of 47.48 m between the rolled up vortex centres was achieved utilising the programmed VFM B-H model, which differed from the Ehret and Oertel model by only 0.48 m. Qualitatively, the 3D VFM B-H plot displayed similar degrees of roll-up and descent when compared to their Biot-Savart VFM plot. As a result of this positive validation process, the programmed VFM B-H model was utilised to simulate turboprop aircraft wakes of a higher complexity. In order to compare the effects of the number of turboprop engines on the aerodynamic benefits of formation flight, the three-bladed single turboprop engine Lancair Propjet, the six-bladed twin turboprop engine ATR 72 and the four-bladed four engine Lockheed Martin P-3 Orion were selected for comparative simulations. As extended formation flight makes use of aircraft downstream separation distances of more than ten wingspans, a wake length of 330 m (which equates to 10.9 span lengths for the P-3 Orion, 12.4 span lengths for the ATR 72 and 36.3 span lengths for the Lancair Propjet) was selected. All aircraft were simulated via the use of the VFM B-H model programme for a range of flight states from cruise conditions to zero g wing loading with full propeller thrust, such as in vertical ascents. From said simulations a novel viscous core radius to simulation convergence relationship equation was developed. The induced velocity fields at 330 m downstream in the wake were then generated in order to investigate the effects of the inclusion of turboprop engines on the aerodynamic benefits of formation flight. From said downwash plots, it was found that the helicoidal vortices affected that region of the wake within an average value of 35% of the wingspan, measured from the fuselage symmetry plane, for all simulated aircraft. In aircraft design, wing mounted engines are placed in a more inboard position in order to reduce rudder strength requirements as well as to minimize the yawing moment dueAdvisors/Committee Members: Redelinghuys, Christiaan.