Expert: Paul Soderman - 2/3/2010

Question
Dear Sir,

I just started my study on aerodynamics/fluid mechanics and I found many important references with your name on it, making me a big fan of you now.

I encoutered some confusions regarding propeller fans:

1.Is the mechanism of moving air in propeller fans much the same with the mechanism of generating lift/thrust in propulsion system?(say ship's propeller for example) Is it because the pressure difference between the suction and discharge face of the blade?

2.What static pressure coefficient really is? And is it related to  pressure difference I mentioned above?

Thank you very much Sir

Answer
Akbar
Yes, a propeller moves air by generating lift on each blade much like the lift on a wing is generated.  The pressure on the blade upper surface is lower than the pressure on the lower surface.  Integrating the pressures around the surface results in a net lift force or thrust.  The air is deflected downward or aft because of the thrust.  In other words, the thrust is equal to the increased momentum imparted to the air.  This is true for ship propellers as well as aircraft propellers or office fans.

The static pressure coefficient is a non-dimensional quantity equal to:
(p-pinf)/(rho * Vinf^2/2) = 1 - (V/Vinf)^2   where
p = local static pressure on the blade surface
pinf = pressure at infinity
rho = density
V = local velocity on the surface
Vinf = flight speed

This is a common coefficient used to describe the pressure field around an airfoil because it only changes with geometry and angle of attack.  Two airfoils of the same shape and flight angle will have the same pressure coefficient distribution regardless of velocity or density as long as the flow is subsonic. The pressure coefficient is defined for a pressure at a point on the surface, not the difference in pressures at two points.  However, that calculation can be easily made using the distribution of coefficients.
Paul