High altitude operations with piston engines powerplant design optimization part III: The diffuser critical design

Low BSFC (Brake Specific Fuel Consumption) and flat-altitude-rating make piston engines ideal choice for subsonic flight at altitudes up to 20, 000m-65, 000ft. These propulsion systems are more complex than traditional applications that are normally limited to 5, 000-7, 000m (16, 000-23, 000ft). In fact, the air propulsion (propeller or fan), the air intake and the cooling system take part have huge volumes. Therefore, their design influences vehicle aerodynamics as a whole. The cooling system is an integral part of aircraft design. As assessed from WWII design heritage, the cooling duct can be a static subsonic ramjet: the Meredith cooling duct. At high altitudes, the Meredith duct air is taken from highpressure areas into an alternate, extremely optimized, path. This path should end with a nozzle in a low pressure, high turbulence area of the aerial vehicle. In subsonic ramjet cooling ducts, the "static compressor" or diffuser is the most critical part. In fact the maximum compression ratio is below 1.5. Its efficiency highly influences the total thrust and the cooling efficacy of the duct. The Meredith duct should be embedded in the fuselage or in the wing to avoid excessive external drag. Only the air intake is positioned outside. In propeller systems, the intake is positioned in the lower part of the aircraft at about 2/3 of the wing chord, where the pressure reaches its maximum. In propeller systems, the high altitude engine intake can be positioned at the end of diffuser to increase the engine boost. In this way the turbomachinery mass and volume is reduced and the power to mass ratio of the propulsion system is increased. In fan systems, higher pressure is present inside the fan duct. In this paper, the preliminary design of the cooling duct is introduced. However, a CFD/wind tunnel optimization is strictly necessary to achieve a fully effective system. In any case, the requirements of low weight, high reliability and long endurance HALE (High Altitude Long Endurance) UAVs (Unmanned Aerial Vehicle) requires further work on this specific subject.