This concept is intriguing. Any observations or comments about it would be most welcome.
Because the idea of gearing the fan came up many years ago in a seminar held by some people from Rolls-Royce plc. The question arose from the floor and so the answer given, it was stressed, was not R-R official policy but more the opinion of the engineer representing Rolls-Royce.
The seminar was not on the subject of single stage fans - nor of high by-pass fans in general, but it was something that the person posing the question, on behalf of an operator, clearly thought strongly about.
The pros and cons stated were as follows:
Pros were that the current situation where the turbine and fan have to be a compromise in design, in order to maximise the power produced by the turbine and the power consumed effectively by the fan with minimum losses, could be avoided by having both the fan and the turbine rotating at their individual optimum rate. This is bound to give an increase, even if only a small one, in the overall efficiency of the engine. This leads to a decrease in overall sfc that is going to please the operator.
The cons are that the introduction of a gearbox does two things:
1. It increases the overall weight of the engine unless savings can be made elsewhere in the construction. On a twin spool (co-axial) engine, or a triple spool, the length of the LP spool must be increased to accommodate the gearbox. This increase needs a corresponding increase in the case, carcass and fairing (cowl) which will add to the overall weight and this does not yet include the weight of the gearbox itself. The alternative might be to snug up the gearbox into the nose cowl which will prevent the additional weight of the external parts but will not avoid the mass generated by the gearbox itself.
2. No matter how this is done, and, one suspects, a single stage or two stage spur epicyclic will be used, the gearbox will need to be rotated. Rotating any mass, however small, will result in power being used to do it. Is it possible that the optimisation of the turbine and fan speed will produce sufficient benefit to both to increase the efficiency to the point that the residue from driving the gearbox will still give increased performance and improved sfc?
Intriguing. We await figures from Pratt & Whitney and your opinions.