I know the q400 is one of few airliners that has a turboprop engines. I know that some military airplanes use turboprop engines so why don't many airliners use them?
Turboprop engines are used in ATR aircraft too. The main reason the airlines are not using turboprop engines are,
- They are not as efficient as jet engines for high altitude long distance flights. This is why all the turboprop aircraft are used of short haul flights.
- Their efficiency decreases as the speed increases above Mach 0.6~0.7.In comparison, Boeing 747-8 cruises at around Mach 0.85.(Their efficient (cruise) speed is less compared to jet aircraft(~700-750 kmph versus ~880-840 kmph). This adds quite a bit of travel time to long haul flights.
- The cabin (and outside) is significantly quieter in case of turbofan engines when compared turboprop engines. This is an important issue in case of civil aircraft where the noise (and vibrations) affect passenger comfort.
- The turbofan engines produce a lot more thrust compared to turboprop engines, for a given size(the turboprop engine output is measured in shaft power though). This makes them unsuitable for large aircraft.
- The huge propellers in the large turboprop aircraft require long landing gears which increase the weight and complicate the maintenance.
The largest (and fastest) turboprop powered civil aircraft, flew slower and lower than jet aircraft from comparable era. It wasn't known for its comfort either.