Expert: D. Norkus - 6/30/2005

Question
What is/are the differences between turbojets and turbofans?
Would you happen to know the reason(in some countries) for higher pilot requirements on turboprops rather than jets?

Thanks

Answer
1)What is/are the differences between turbojets and turbofans?

In turbojet engines (like those on the 707) 100% of the air sucked in thru the inlet is directed into the engine core. The engine derives it's thrust by highly accelerating a mass of air through the engines core. Since a high velocity is required to obtain an acceptable of thrust, the turbine of turbojet is designed to extract only enough power from the hot gas stream to drive the compressor and accessories. All of the thrust produced by a jet engine derived from its exhaust gas.


With turbofans (like the 777), only part of the air sucked in thru the inlet is sent to the core and the rest passed thru fan. Turbofan engines have a duct enclosed fan mounted at the front of the engine and driven either mechanically at the same speed as the compressor, or by an independent turbine located to the rear of the compressor drive turbine. The fan air can exit seperately from the primary engine air, or it can be ducted back to mix with the primary's air at the rear. About 75% of thrust comes from fan and less than 25% comes from exhaust gas.

Here are some pages on the subject with diagrams:
http://www.aircraftenginedesign.com/custom.html4.html
http://www.hq.nasa.gov/office/pao/History/SP-468/ch10-3.htm

2) Would you happen to know the reason (in some countries) for higher pilot requirements on turboprops rather than jets?
As far as why some countries would require MORE time to fly turboprops, I have never heard of that. Here in the US, the airlines that fly turboprops (regional carriers) require less experience to get hired than jet operators (major carriers). Maybe as jets are usually more automated, while turboprops generally are not and are a higher workload aircraft? Requirements like that are backwards from what I have experienced.