Expert: Paul Soderman - 7/9/2003

Question
Hi,
I see many different aircraft propeller designs. Naturally there are advantages with fixed blade props for smaller craft, but what exactly are the trade offs on blade length, breadth and number? In other words, what makes a two, three, four or five blade propeller optimum for any one plane type?
Many thanks.

Answer
Chris
Propeller design is complicated by the many variables involved.  But lets consider some basics.  For a given thrust required, which depends on aircraft drag, cruise speed, climb performance, etc. you can choose an engine that is sized for your airplane and choose a blade area (span, chord, number) to achieve the thrust.  Smaller is better to minimize engine weight, torque, etc.  And since most small aircraft engines turn around 2700 rpm, the propeller diameter must be chosen to give a subsonic tip speed.  Supersonic tip speeds are not desirable.  Ground clearance is a must.  The number of blades is related to the aircraft mission; for high cruise speed, few blades are desired.  For good takeoff thrust and climb rate, more blades are required.  WWII fighters had few blades, transports and bombers had many.  It seems that propeller efficiency goes up as the number of blades goes down.  Why this is, is unclear although I suspect it has something to do with induced swirl.  My answer is somewhat vague because propeller design is becoming a lost art - in the 40's there were people who specialized in that field for obvious reasons.  Most NASA people like myself have never designed a propeller.  
Good question.
Paul