The designers of aeronautical engines for commercial aircraft are constantly seeking to reduce the mass, the specific consumption of their projects, that is to say the consumption expressed in kg of fuel consumed per kg of created thrust, and the level of noise which they generate in use. To that end, the engine most used at the current time is the turbofan which consists of a twin-spool bypass engine with a high bypass ratio, with a fan of large diameter which is driven directly by the low-pressure turbine.
This configuration has the disadvantage of rotating the fan at the same speed of rotation as the low-pressure turbine. Now, in order to obtain good specific consumptions and a reduced noise level, this fan should be rotated as slowly as possible while adapting the profile of the blades in order to maintain a high air flow rate and speed and hence a desired level of thrust. However, slowing down the speed of rotation of the low-pressure shaft results in a degradation of the operation of the turbine which, itself, by contrast, needs to rotate quickly in order to have a good efficiency and to help achieve a low specific consumption.
A first solution for combining a low speed of rotation of the fan and a high speed of rotation of the LP turbine consists in placing a reduction gear on the shaft which connects these two elements.
The advantages of this architecture are offset by the weight of this additional equipment and the complexity of its development taking account of the power which passes through it.
Also known are patent applications FR 2641333 and GB 2226599 which describe aeronautical engines of which the fan and the turbine can rotate at different speeds without a reduction unit with a high reduction ratio between them. These engines comprise two LP turbine shafts which are driven by contrarotating turbine wheels. The two sets of turbine blades can rotate and are imbricated in one another. A gear device serves to reverse the direction of rotation of one of the shafts in order to drive the fan of the turbomachine. The reversing module consists of two radially staged pinions which do not have the same diameter and therefore which are not driven by shafts rotating at the same speed. One of the two shafts is then mechanically more loaded than the other, something which is unfavorable with respect to the overall mass of the engine.