Expert: Paul Soderman - 7/11/2006

Question
Dear Paul,
I am a flying instructor for the RAF. One of my students asked me why the ceiling fan in the office is shaped like an aircraft propeller but the blades on the desk fan are more like the blades on a boat propeller.  I haven't a clue.  Please can you help? Thanks
-James

Answer
Ah James - you didn't like my economic analysis.  OK - let's stick with aerodynamics mostly.  To design a propeller, a designer would start with requirements for a class of aircraft and mission.  That gives him things like cruise speed, altitude (density), required thrust.  The aircraft size limits the propeller diameter.  He also has a range of engines that would fit the aircraft, so he knows available horsepower.  So now he can iterate on propeller rpm, blade number, and surface area to compute blade load and angle of attack distribution along the blade.  To get a fairly uniform spanwise load without going supersonic, he would choose a proper airfoil section, blade twist, and taper (maybe even sweep) and model the propeller aerodynamic performance.  He might also coordinate some wind tunnel tests to generate a map of propeller performance (thrust, absorbed power, torque, efficiency) versus advance ratio (flight speed divided by propeller speed).  Behind all this is the detailed structural analysis required to get the blade loads to the hub in a safe way.   All this results in a pretty sophisticated aerodynamic device for a given mission.

None of that would be appropriate for a ceiling fan, because it only has to push a little air down at low speed.  High speed air would not feel comfortable to people.  There is no flight speed.  So the aerodynamic requirement is so trivial that almost anything rotating in a circle would push the air. Course rectangular thin blades are best because the flow can be directed downward, the fan volume is small, the fan weight is low, and the fabrication cost is low.

The desk fan requirement is similar, but somewhat higher velocity air is required, so the blades become slightly more complicated.  But it would be entirely inappropriate to go through the design process and fabrication complexity needed for aircraft propellers.  Not that it wouldn't work.  Bill Gates might like to have such a thing on his desk.

Paul