Expert: Paul Soderman - 7/13/2007

Question
On a propeller if you were to add a blade but leave the pitch the same what effect would you see, Greater thrust
but the same speed, greater speed but the same thrust, or a combination at what ratio?

Answer
Ken
This is a difficult question to answer without more information about the propeller and mission.  And I am not sure what you mean by speed - propeller speed or flight speed. ( I will assume flight speed.)

First, the number of blades on a propeller are chosen after considering many factors such as takeoff thrust required, cruise speed required, engine horsepower available, engine torque, noise, weight, range, etc.  For example, WWII bombers needed lots of takeoff thrust and had many blades on the propellers for high static thrust.  Fighters needed high cruise or attack airspeed and had few propeller blades to keep drag down.  

From a first level analysis, adding a blade with fixed pitch and rpm would generate more thrust and aircraft airspeed.  However, it would require more engine horsepower and torque.  A higher level analysis might show that once a propeller is optimized for a given aircraft, engine and mission adding a blade would degrade it's performance by creating drag to the engine and aircraft which would reduce airspeed.  And to really hold rpm, you would likely have to install a larger engine with more weight, which also affects aircraft performance.  So, you are really asking what will happen to a system when you change one thing, and the answer requires a system analysis or tradeoff study with knowledge of the system component interactions.  This is why aero engineers spend a long time in school and a lot of time in wind tunnel testing.
Paul