Would turbofan aircraft benefit from adjustable fan blades as do turboprops? Propeller aircraft adjust their propeller size, i.e. shorter propeller for cruising at maximum design altitudes where the air is thinner for efficiency, and a longer propeller where the air denser for increased thrust used for increasing altitude at early stages of flight. Would turbofans benefit from this when flying at maximum altitude for efficiency? If not why not?
Both turboprops and turbofans want their blades to be as long as they can be, in all circumstances: that way the largest amount of air is accelerated, and a given amount of thrust created with the highest efficiency. It never pays to reduce the bypass ratio of a subsonic turbofan.
Turbofans with adjustable blade pitch would make sense though, like the constant speed propellers that turboprop engines already have. This article from Flight Magazine 1973 gives the advantages of a high bypass variable pitch turbofan:
- Inherently low noise levels because of high bypass ratio and low fan tip speeds.
- Rapid thrust response, allowing thrust modulation for accurate rate of descent control on the approach and maximum use of reverse thrust during the landing run.
- Reverse thrust available without the weight or maintenance penalty associated with reversers and spoilers.
- High ratio of take-off to cruise thrust well matched to the needs of short-range, Stol aircraft.
- Good cycle efficiency and specific fuel consumption.
- Wide engine operating margins.
- Provides many of the advantages of the turboprop while avoiding vibration, ground and fuselage clearance problems and allowing better matching with higher cruise speeds.
Assuming, that engineers would have solved the design problems of such a system and making it light and easy to maintain, this would greatly enhance that flexibility of a fan engine and possibly also increase efficiency.
It would give the designers one additional degree of freedom when designing an engine.