GUY NORRIS / LOS ANGELES
Research study also reveals unexpected drag benefit but not overall performance effect
Studies show that placing engines over the wing of potential supersonic designs could significantly reduce the sonic boom, helping achieve one of the main targets of the US Defense Advanced Research Project Agency's (DARPA) Quiet Supersonic Platform (QSP) project.
Gulfstream Aerospace, which conducted the research study, also discovered an unexpected drag benefit to the overwing location. Gulfstream QSP technical specialist Donald Howe says: "Contradictory to traditional thinking, the over-wing engines do not incur higher drag levels than the underwing placements. In most cases, aero-dynamic drag is equal to or better than the underwing position."
Howe cautions, however, that studies to date have focused simply on the low boom potential and have not looked at the effect of the upper-wing surface location on engine performance or, in the case of higher nose-up attitudes, such as take-off, the overall performance of the aircraft.
The Gulfstream studies analysed three axial engine positions in four basic configurations. These included external and mixed compression inlets under and over the wing, resulting in a total of 12 computational models. The foremost inlet position was mid-chord on a 63º swept wing, while the mid-axial position was at the 75% point of the local chord. The aft position placed the inlet centrebody just aft of the wing trailing edge.
The results showed that, in all cases, the underwing engines had a stronger tendency to coalesce the shockwaves in the far-field towards a classic "N-wave" resulting in higher peak overpressures and louder sonic booms. Gulfstream says the benefit of the overwing location is "due to shielding of the inlet shock from the flow field below the aircraft. However, even with the engines far enough aft so that the inlet shock does not impinge on the wing, the overwing placements still have improved boom characteristics."
The research team says that this is due to the higher onset Mach number at the inlet face in this aft position. Studies show the higher Mach number causes a more highly-swept inlet shock which then has less of a tendency to move forward and coalesce with the other shocks. Crucially for low boom control, says Gulfstream, this allows the shaped signature to be propagated all the way to the ground.
Source: Flight International
