Turboprop engines are the most common engines in small propeller driven aircraft. The propellers are fitted directly to the front of the engine, and when you think of a small aeroplane with a propeller on the front the engine is directly behind it.
A turboprop engine directly drives the shaft of the propeller via gearing in the engine; in a turboshaft engine the engine and gearing are separate, but this essentially the only difference between them.
A turboprop engine is a simple gas turbine engine. A gas turbine engine can be divided into three sections, a compressor, a combustion chamber and a turbine. In the aircraft turboprop the air enters through the front of the engine and is compressed before being mixed with fuel and ignited. The energy released from the ignition drives the turbines behind the compression chamber. These turbines power the propeller by means of a shaft and gearing.
The gearing is necessary because without it the propellers would move too quickly, the ends of the propellers reaching supersonic speeds. The only aircraft to make use of this potential is the long serving Tupolev TU-95, a highly successful aircraft first produced during the cold war and expected to remain in service until 2040.
Some of the turbine power is used to drive the compressor. In modern engines this is usually achieved with a separate shaft from that driving the propeller so that propeller speed is independent of compressor speed. The exhaust gases leave through the rear of the engine, and this produces additional thrust, but a small amount compared to the propellers. The exhaust is responsible for about ten percent of the thrust produced by the engine; the rest is due to the propeller.
Turboprops are not suitable for high-speed aircraft because at high speeds the propellers lose efficiency. The usual maximum speed for a turboprop powered aircraft is about 450mph
The Second World War played a part in the development of the turboprop with the need for more efficient, faster and longer-range aircraft fueling research in the area in different countries. Frank Whittle had the basic idea for a turboprop and for jet engines as early as the 1920’s but could not convince anyone to fund his research, the Second World War changed that.
The invention of the turboprop engine is credited to Gyorgy Jendrassik, working in the Ganz factory in Budapest between 1939 and 1942. He had a working smaller model in 1937 that he developed into the Jendrassik CS-1, but this engine was not used on a working aeroplane.
The first aircraft to feature a reliable turboprop engine was a single Gloster-Meteor with two Rolls Royce Trent engines fitted. This first flew in 1945. The Trent or RB5.0 was one of Frank Whittle’s designs. The experience of building and designing these engines led to the design of the Rolls Royce Dart, an engine that was still in commercial production until 1987.
Today the most popular turboprop engine in production is the Pratt and Whitney PT6 (known as the T74 for military use). It was first used in flight in 1961 and is still in production today. Part of the reason for its success is the ease of maintenance. The engine is separated into two sections. The front has the intake, compressor and combustion chamber sections as well as a small turbine. This turbine drives the compressor. The hot gases then enter a separate section where a turbine powers the propeller.
Today turboprop engines are most commonly used on small commuter aircraft. They do not have speed necessary for most modern large capacity or high performance aeroplanes but are reliable and have the efficiency required for these shorter flights.