Coosing your Weapon

To many pilots, deciding between a jet and a turbo-prop would be a nice problem to have.

Turbo-prop engines harness the rotating turbine inside a jet engine to drive an external propeller through a gearbox, whilst turbofan engines derive thrust from the reaction force created by hot expanding gas that exits the engine. Modern turbofan jet engines create thrust not just from combustion exhaust, but also expanding bypass air that has been capture by the engine’s fan blades and heated by the engine’s core.

While turbo-props also derive some thrust from reaction forces, the majority of a turbo-prop’s thrust is generated by the propeller blades.

Conventional aviation wisdom–if there is such a thing–says that turbo-prop aircraft are more efficient at lower altitudes and airspeeds, while jets work in the inverse, gaining efficiency with high altitude and airspeed. It also says that jets need long runways to be safe, and long hops to make money, but at least some of those conventions are being swept away in a tide of technology and design innovation aimed at making both types of turbines safer and more efficient.

Points to ponder

Factors that weigh the scales in a turbo-prop/jet comparison include:

• mission distance – jets stand out above about 500 nautical miles• speed – jets being typically much faster• fuel efficiency – at• runway lengths – business jets traditionally demand upwards of 1500 m and most require a sealed surface, while many turbo-props can operate comfortably on 1000-metre runways or less• performance – turbo-props generally deliver instant power, whilst a jet can involve some lag in attaining maximum power, a big advantage for turbo-props at take-off and in a go-around situation• ruggedness – many turbo-props are capable of handling unsealed runway surfaces, including some with durable fixed landing gear• altitude and weather – turbo-props typically operate below flight level 300, where turbulence and weather may be found on days when there is none above that altitude• cabin size and comfort – larger business jets provide greater headroom, are less noisy and are frequently more luxuriously furnished than turbo-prop aircraft.