A geared turbofan engine is a variant of a conventional turbofan engine in which a reduction gearbox is installed between the fan and the low pressure (low speed) turbine.
In a conventional turbojet engine, the fan, low pressure compressor and low pressure (low speed) turbine are all rigidly joined by a single shaft. This results in the rotational speed of the entire N1 spool being limited by the maximum tip speed of the fan blades. High bypass ratio airflow demands a relatively low fan speed which can necessitate a lower than optimum compressor and turbine speed. This lower speed means that additional compressor and turbine stages are required to maintain acceptable component efficiencies.
The addition of a reduction gearbox between the fan and the low pressure shaft in the geared turbofan design allows the fan to turn at a different rate than the turbine and compressor. The gearbox is designed to optimise the speed of all components resulting in the need for fewer compressor and turbine stages and a smaller and lighter engine core.