Aircraft Engine–Specific Thrust and Visibility from 2D and Circular Nozzle Exits

In modern warfare, military control of the airspace determines aircraft survivability against the most widespread missile threats. The aero-engine exhaust system is an important source of infrared (IR) signatures from the rear aspect, particularly in the 2–3 μm and 3–5 μm IR bands. Two-dimensional (2D; non-axisymmetric) nozzle exits with high aspect ratio (AR > 5) are widely used in stealth aircraft engines due to their low IR signature, ease in thrust vectoring, and high maneuverability and agility. This analytical study compares the specific thrust (for choked and unchoked flow regimes) and the visible planar areas of a 2D nozzle exit with different ARs with those of a circular nozzle, as seen from the direct rear view. The nozzle’s isentropic efficiency (η_is,noz) is obtained in terms of the total pressure ratio, and the effect of AR on η_is,noz is examined for 1 ≤ AR ≤ 15. It is found that η_is,noz decreases with increasing AR, but this decrease is more rapid in unchoked flow than in choked flow. For the same value of nozzle exit cross-sectional area, the corresponding visible planar areas of aero-engine hot parts are compared for a 2D nozzle exit with different AR with that of the baseline case (circular nozzle), from bore-sight. This study shows that the optical blocking of aero-engine hot parts by a round-to-rectangular nozzle of AR ≤ 4 is almost the same as that of a circular nozzle, from bore-sight.