Expert: Paul Soderman - 6/17/2010

Question
Is an electric ducted fan in general more efficient or powerful than an electric propeller for propelling an ultralight aircraft? I see that all-electric ultralights using propellers, but small RC "jets" use a ducted fan.

Modern jets use a ducted fan combined with a gas turbine (turbofan). If gasoline was no longer an option, would ducted fans be a better option that standard propellers?

Thank you,

Jim (Daydreaming again!)

Answer
Jim
Your question is difficult to answer cleanly because a propulsion system must be designed for an aircraft and a mission. In many cases, a ducted fan is superior, in many cases a propeller is superior.  Let's examine some general concepts.

A ducted fan will produce more thrust than a propeller of the same diameter, or conversely, for the same thrust, a ducted fan will be smaller than a propeller.  This is because a ducted fan gains thrust from pressures on the duct and better tip loading because of wall proximity. However, at high speeds the duct will be detrimental because the drag goes up as velocity squared.

Ducts also add weight to the aircraft.  On the other hand, ducts can be integrated into the structural design to carry loads.  Ducts help keep hands and heads out of the rotor, though one can still be sucked into the inlet as happened to me on a small wind tunnel once (the honeycomb grid saved my bacon).

Ducts deliver a compact clean airflow into the fan (assuming no lip separation), but at higher speeds a propeller does that as well.

A turbofan duct is designed for unique requirements.  The duct slows airflow so the fan and compressor stages can operate in a subsonic flow even if the aircraft is flying transonic.  The duct also protects the fan and very importantly reduces noise.  So a turbofan cannot be compared to a ducted or shrouded fan directly.

So to answer your question.  In general, a ducted fan is best for low speed operation and specialty jobs such as vertical lift and transition to forward flight.  They are also best for crafts where the duct is an integral part of the structure.  A propeller is best for high speed clean aircraft.  That said, there are so many factors that can drive a flying machine design that the optimum propulsion system cannot be chosen in isolation.  A system analysis is required to tradeoff thrust, torque, drag, weight, efficiency, structure, fuel consumption, range, ceiling, takeoff versus cruise, mission requirements, etc.
Paul