Propeller Wake Theory

[Aimes]

I was wondering if anyone out there had any practical experience with propeller or fan testing. My question is regarding the momentum theory of wakes. Using this theory it can be found that the air velocity in the wake region is double that at the propeller. Unfortunately, the tests that I am running on a ducted Multi-Wing propeller indicate zero increase in speed in the wake region.
I am curious to know whether this is a common divergence from theory.

Any thoughts?

 

[insideman]

You have misinterpreted the theory.
Half the CHANGE in velocity happens ahead of the prop, the other half CHANGE occurs aft of the prop. Tests confirm the theory rather well.

 

[Aimes]

Yes, half of the velocity change occurs ahead of the propeller, this is correct. My free stream velocity far ahead of the propeller is, however, zero. This means that the wake velocity will be twice the propeller velocity.
Perhaps this is where the difference lies. My propeller is tested at static cnditions. There is no forward velocity.

 

[Sparweb]

Static test conditions make the simplified form of the theory hard to apply. Look for info specifically on "Static thrust" (carefully omitting all the hits related to jets). Try the NACA/NASA reports

http://naca.larc.nasa.gov

. They seem to have a report on every aeronautical subject under the sun.


STF