### Propeller drag coefficient from thrust coefficient? (EPP1045)

Is it possible to calculate propeller drag coefficient from thrust coefficient? Are they related in any way?

I have calculated propeller thrust coefficient by simple approximation using DriveCalculator. I then made some curves in Excel and would like to repeat the same for drag (prop: EPP1045, engine: EMP2836/09 880KV). #### Replies

• hello

as i know n is rev per sec

and D is diameter in unit of meter

rho is in kg per cubic  meter

• Coefficient of drag (Cd) is an attribute of an airfoil section (or any particular shape) at a particular angle of attack (AoA) and Re.  We can use calculus to integrate those values over the entire radius of the blade, but it appears to me your line of questioning is more along the lines of efficiency rather than Cd.  A holistic look at "drag" in this context really means "energy expended that is not added to thrust".  There are generally two ways of analyzing propeller efficiency: static (most useful for lifting propellers) and at some advance ratio (i.e. moving laterally - best for fixed wings).

I believe this prop has already been tested in a wind tunnel at the University of Illinois.  Perhaps you should start there.  They've already graphed propeller efficiencies vs. V/nD (advance ratio).  For static conditions, figure-of-merit or FM can be calculated by using their measured coefficients of power and thrust.  Here's the formula (in spreadsheet notation) which you'll need to use to calculate it from the "static" table data:  =((CT)^1.5)/(CP)*0.707))

University of Illinois Wind Tunnel Small Prop Test Database

• Hi,
Can you give us some details about the computation of Cd, I've been searching for days and the better definition I had is a polynomial expression which depends on the angle of attack (trend line of data with bigger propeller, eq. 20).