View Single Post
Old Jan 12, 2012, 06:12 PM
Herb is offline
Find More Posts by Herb
Lithium Member
Herb's Avatar
Irvine, Calif USA
Joined Feb 1999
12,244 Posts
Hi Eddie,

Yes you can compute efflux from measured thrust and vice versa.

Even more simply, if you compare fan A to fan B, then the ratio of effluxes is simply

V_A / V_B = Sqrt (T_A / T_B )

at a given (shaft) power level.

In many ways ducted fans behave like props, so similar equations apply. One of the main differences is in an EDF the air is already accelerated at the intake.

The page below is a very clear yet simple description of some of the basic physics, by a guy (Martin Hepperle) who knows what he's talking about :

http://www.mh-aerotools.de/airfoils/propuls4.htm

You can see that besides thrust and velocity the only other ingredients are the diameter D (more or less fixed in out 90mm fan case) and the density of air rho.

It's funny that the basic (Newtonian) equations go back to steamship prop design, late 19-th cent.
Herb is offline Find More Posts by Herb