EXHAUST SECTION

The hot gases are exhausted overboard through the exhaust diffuser section. Internally, this section supports the power turbine and aft portion of the powershaft. The exhaust diffuser is composed of an inner and outer housing, separated by hollow struts across the exhaust passage. The inner housing is capped by either a tailcone or a cover plate which provides a chamber for cooling the powershaft bearing. A typical exhaust diffuser section is shown in figure 1.29.

Figure 1.29. Exhaust Diffuser Section.

Turboshaft engines used in helicopters do not develop thrust by use of the exhaust duct. If thrust were developed by the engine exhaust gas, it would be impossible to maintain a stationary hover; therefore, helicopters use divergent ducts. These ducts reduce gas velocity and dissipate any thrust remaining in the exhaust gases. On fixed wing aircraft, the exhaust duct may be the convergent type, which accelerates the remaining gases to produce thrust which adds additional shaft horsepower to the engine rating. The combined thrust and shaft horsepower is called equivalent shaft horsepower (ESHP).

Figure 1.30. Divergent Exhaust Duct.