Propeller Pitch Calculations

[DaveSawers]

I have put a Javascript calculator on our web site to calculate prop pitch from blade angle and vice versa. Go to: http://www.cmpra.com/Tips/flying.htm and click on Propeller Pitch Calculations.

Hope you find it useful. :cool:

[Dick Huang]

Dave,
I looked at your Java script prop calculator and used it. Being the engineering type, could you provide the formula that relates pitch, distance from center and blade angle?
Dick Huang

[DaveSawers]

Sure. It's just simple trignometry.

Blade Angle = ATan(Pitch / (Distance from hub * 2 * Pi) ) * 180 / Pi

Pitch = Tan (Blade Angle / 180 * Pi) * Distance from Hub * 2 * Pi

Simple, eh? :D

[Ollie]

The hard part is to do the actual blade angle measurement based on the chord line of the prop's airfoil so that the true geometric pitch can be calculated. All the pitch angle measuring devices I have seen measure to a line that is tangent to the bottom of the airfoil rather than the airfoil chord line. Then there is the problem of determining the airfoil so that the zero lift angle of attack can be estimated to determine the aerodynamic pitch of the prop. Without proper angle measurement and airfoil evaluation, pitch errors in the range of 20 to 35% can be expected.

[DaveSawers]

That's certainly true, but all you really want to know is whether propeller A has a larger or smaller pitch than propeller B and by how much. As long as you measure consistently, you will get consistent results.

[Ollie]

The error angle tends to stay fairly constant along the diameter. The measured pitch angle gets smaller from hub to tip. Therefore the percentage of pitch error increases considerably along the diameter from hub to tip. An increment of diameter near the tip produces a lot more thrust than a similar increment closer to the hub because the tip is traveling faster. Therefore, the bigger percentage pitch error near the tip is even more significant.

When comparing a high pitch prop to a low pitch prop, other things being equal, the high pitch prop will have a smaller percent pitch error than the low pitch prop.

Unless more acurate instrumentation is developed and used for measuring pitch angle and airfoil zero lift angle of attack accounted for, pitch comparisons will have serious errors.

[Dick Huang]

Ollie and Dave
Thanks for the formula and other info on prop pitch.
Dick Huang:)