MD unducted fan [Archive]

enicalyth

11th Jul 2004, 22:50

The question of the unducted fan relates to the differences between propeller and jet/turbofan engines. Propeller efficiency is the ratio of useful work to mechanical energy produced in the system; Jet/turbofan efficiency is the ratio of useful work to heat energy input. With a prop the efficiency term has velocity in numerator and denominator; with a jet/turbofan there is only a velocity term in the numerator. And therein lies the rub, a prop aircraft varies little in efficiency with velocity whereas a jet/turboprop does so dramatically. The jet/turboprop must fly as fast as it can without penalising the Lift/Drag ratio due to compressibility effects. If we are talking about short-haul where aircraft are lower and slower there comes a point when the prop aircraft can be more attractive especially if fuel is expensive. But the travelling public expects short travel times and classical prop aircraft become inefficient above M0.65 as the propeller tips near the speed of sound. So why not sweep the propeller blades to delay the onset of compressibility losses just as in swept-wings? No problem except such a propfan has a large, heavy and expensive gearbox. Instead you could mount the propeller on the same shaft as the turbine and dispense with the gearbox accepting the penalty that turbine and fan have different optimal speeds. Such a propfan might comprise two contra-rotating props of eight swept blades each as opposed to a turbofan which is a multitude of broader blades situated on a separate shaft. The turbofan sits up front and its airflow is ducted to contribute to the overall jet propulsion. Compression is what provides jet propulsion and the up front fan is a compressor. The unducted fan makes no bones about being a propeller, a bulk mover of air and not a compressor as such, and being directly driven from the turbine is happy to sit at the rear leaving the air intake nice and clean. It takes a large volume of air and imparts a modest change of velocity to it. What energy is not robbed by the turbine is imparted into the exhaust jet and the unducted fan is no more than a fancy turboprop that can fly faster thanks to swept propeller blades and is lighter thanks to the absence of a gearbox. Hopefully it is almost as fast as a turbofan thanks to its fancy blades, not as noisy when mounted at the rear of an MD-80 and mechanically less prone to vibration. And it has the prop saving grace as we’ve said of roughly the same efficiency at any speed thanks to there being no velocity term in the prop efficiency equation. Fashion and fashionable ideas had more to do with the death of the UDF than fuel prices, and last but not least the public got fed up with the look of rear-engined jets.