Numerical and experimental investigation on the effect of turboprop engine exhaust gas impingement on propeller and aircraft surfaces

Abstract

Typically, commuter/transport category aircraft are powered by turboprop engines due to the benefit of fuel efficiency and high power-to-weight ratio. These aircraft are normally configured in either pusher or tractor versions based on the position of the powerplant (engine-propeller). Generally, in turbofan or turbojet engines managing hot engine exhaust is not critical as the exhaust plume is totally overboard of the aircraft. However, in turboprops, exhaust plume management gains importance as the plume gases exit at an angle or perpendicular to the engine axis, crossing the propeller (pusher) plane and airframe surfaces. This becomes especially critical in reverse thrust operations, with a composite propeller. This requires an exhaust nozzle or stub to duct the engine exhaust hot gases out. The design of these exhaust stub is dictated primarily by the structural and aero-thermal requirements, such as aircraft's configuration, drag contribution, exhaust jet thrust, and engine performance. Exhaust stubs are designed to meet pusher configuration aircraft requirements. The stubs are designed to minimize the impingement of the hot plume gases on the propeller and airframe surface without significantly affecting the benefit of jet thrust. The critical geometrical parameters of stubs are exit area, turning angle, plume trajectory etc. A fluid-thermal-structure coupling analysis is performed to understand the thermal effects of the engine exhaust jet flow on the thermo-mechanical behaviour of pusher-configured light transport aircraft propellers and affected structural components. The steady-state thermal flow field of the aircraft with both forward and reverse thrust in which propeller blade angle vary, were analysed for various forward velocities. Further, investigated a three-dimensional analysis of flow around the nacelle-airframe and the effect of exhaust flue gas impingement on the propeller blade surface. Based on the results from this analysis, the designed exhaust duct was integrated into the aircraft. Experimental evaluations were carried out on the prototype aircraft for ground static and flight trials to validate the numerical findings. During the experiment, propeller blade and fuselage surface temperature were measured. The experiments show that the numerical analysis carried out is in good agreement with the measured results. Further, it is observed that the estimated temperatures on the propeller blade surface are within the allowable limit as prescribed by the original equipment manufacturer. The confidence gained from this numerical study and its experimental validation, facilitates a numerical approach that can be adopted for new developmental activities such as configuration change studies and new aircraft designs where the experimental data are unavailable. Subsequently, the numerical studies were extended to design and develop new exhaust stubs for tractor-configuration aircraft fitted with the same engines having twin exhaust ports per engine. The new tractor aircraft chosen is a twin-engine airplane with its powerplant mounted in the high wing configuration. Even though the tractor configuration propeller is not in the critical path, the finding from the studies were extended for the design of a light weight composite airframe. The airframe design team used such numerical results to select skin material for the fuselage, wing, nacelle, plexiglass passenger window, flap design, and geometry. The use of thermal barrier coating on critically exposed surfaces was also assessed. Airframe developmental activities are in progress. Special attention was given to the nacelle for thermal flow field mapping as it houses hot components like the engine and its subsystems. This also facilitates design of thermal ventilation to keep the zone and bay temperature inside the nacelle within the applicable limits prescribed by the engine manufacturer. In addition to mapping of the thermal flow field, another study carried out was to estimate an effect of exhaust stub angle and its orientation w.r.t engine axis on the jet thrust with symmetric and asymmetric inboard-outboard stub angles. This study was carried out for critical flight operating conditions such as maximum continuous and cruise rating covering Minimum Off-Route Altitude (MORA) on the overall aircraft performance. To summarise, the aim of this research was to estimate the temperature levels caused due to the impingement of hot exhaust gases on the propeller and aircraft surfaces such as fuselage, wing, nacelle, and flaps using numerical analysis. In a few cases, the numerical study was validated by experimentation. The effect of stub geometry on the engine’s performance was also studied. Detailed findings are discussed in the respective sections. This study is very much useful to powerplant and aircraft designers.