Expert: Paul Soderman - 12/4/2008

Question
QUESTION: If a propeller is ground run at cruise flight pitch and RPM, with the aircraft not moving, are the primary loads acting on the blades the same as when the aircraft is flying at cruise speed.

ANSWER: Steve
No.  A propeller with pitch angle designed for cruise will have a small angle of attack and high onset velocity created by the aircraft flight speed and propeller rotational speed.  As a fixed pitch propeller aircraft slows to the static case, the propeller blade angle of attack goes up and the flight speed goes down so the velocity over the blades is due to the rotational speed and induced flow.  Because the static thrusting propeller has high angle of attack, it can have high thrust, perhaps not much less than cruise thrust depending on design, but it will be compromised by flow separation and partial stall in part of the disc.  Thus, the static propeller will have a different load and load distribution spanwise than it would have in cruise.

See:  http://www.mh-aerotools.de/airfoils/prpstati.htm for a discussion of these effects.
Paul

---------- FOLLOW-UP ----------

QUESTION: Paul,

Thanks for the fast, informative reply. Let me follow -up, by asking it this
way. Were the RPM and angle of attack to remain the same for both cases
(i.e., static and flight cruise), would their be a difference in blade loading? If
there is a difference, if a prop is run on the ground without representation of
onset velocity, can pitch adjustment be used to simulate/create the cruise
load case, or would it be necessary to include airflow (e.g. in the static
ground test case, from an source (ducted fan) ahead of the turning prop)?

Answer
Steve
My previous answer was for a fixed pitch fixed rpm propeller.  So, the static and cruise loads are not the same.  The best way to simulate cruise is to put the propeller in a wind tunnel operating at cruise speed.  It is unlikely that a ducted fan could achieve anything like a wind tunnel, but it may be better than nothing.  Of course it also induces swirl into the propeller.  It is also unlikely that changing the propeller pitch could achieve the cruise loading.  Increasing the pitch might cause partial blade stall.  Decreasing the pitch would decrease the thrust.

I am unsure what your objective is.  Each propeller has propeller charts that should show performance as a function of blade angle and advance ratio.  The best performance parameter is efficiency.  From that you can estimate loading as a function of operating condition.  See:

http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm

for details.
Paul