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.

I have found very little new data on any of the fans I listed ...

Whatever data I found I have added; it seems that the lowest performing data points are (not unexpectedly) associated with outrunner motors, some with rather miserable efficiencies (again some, eg Sk, a bit better than others).

I have not broken down each fan data sets into inrunner (Neu, Het, TGY edf) vs outrunner (Sk, TGY heli motors etc); would have been too much work to plot. But it's discussed in detail earlier for the older data.

It's clear that if you couple a poorly designed fan with a miserable motor you will get a low efficiency score (=good hand warmer) .

The result is a 1.5 min flight with a smoking hot battery and the end of the flight.

I think I'll use the CS-12bl fan as a paperweight

Red dots : JetFan 90mm

Green dots : WeMoTec MidiFan 90mm

Purple dots : Stumax 90mm

Orange dots: Het 90mm fan

Yellow dots : VasaFan 90mm

Cyan (light blue) dots : CS 90mm 12-blade 90mm

Magenta (purple red) dots : Schuebeler DS-51 carbon fan

Black dots : JePe 4-bl carbon fan (Spiderfan)


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A tip with which I agree 100% from years of mounting, installing and running EDF's :

EDF sound has a lot to do with the way the fan is balanced, and mounted

"80% des Sounds eines Impellers liegen in der sauberen Wuchtung und Montage begründet. Deshalb bieten wir das an und sortieren die Antriebe auch paarweise auf Differenzen < 5%."

or :

"And another tip: 80% of the sound of a ducted fan are due to a clean installation and balancing. That is why we offer the sort and the drives in pairs with differences <5%."

http://translate.google.com/translat...peller&act=url

I am just using very basic, well known physics (that I did not invent btw ) , and refrain from speculating on things I don't know, or can't measure.

So the comparison I am doing has been done hundreds of time before for edf's, it is certainly not my invention, and it is based on very simple and transparent physical assumptions that allow one to make an (imperfect and approximate) comparison.

I've added a few more points to the graph, in fact all the data points I could find in a reasonable amount of time / search .

Red dots : JetFan 90mm

Green dots : WeMoTec MidiFan 90mm

Purple dots : Stumax 90mm

Orange dots: Het 90mm fan

Yellow dots : VasaFan 90mm

Cyan (light blue) dots : CS 90mm 12-blade 90mm

Magenta (purple red) dots : Schuebeler DS-51 carbon fan

Black dots : JePe 4-bl carbon fan (Spiderfan)

Past experience with EDF data suggests that the fan efficiency curves tend to be rather flat as a function of power (more power in the same fan does not imply less efficiency), and this fact seems to be borne out, more or less, by the data below.

Note again that some of the lowest efficiencies in the plot seem to be associated with low efficiecncy outrunner motors, i.e. they are not just due to the fan design.

Theoretical considerations (related to critical Reynolds numbers) also suggest that high-blade-count fans will pick up some efficiency at higher power levels (i.e. they will compare less favorably to low blade count fans at lower power).


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... I had lengthy discussion on this topic with Daniel Schuebeler a few years ago.

EDF fan conditions are never static as the air enters the duct (say of a 90mm F-16) already at 70 m/sec, so that the description of it as "static" (whatever that means) is physics nonsense.

That is of course the reason why "static" fan measurements with a good sized intake lip or intake duct, albeit imperfect, correlate very well with "flight data" ( I found that out a few years ago).

The absence of any significant fan unloading in flight (which we measured by video and doppler, thus obtaing the fan rpm in flight vs static), also pointed to the validity of the static data.

Daniel himself described to me all he details of his wind tunnel setup at the Technische Fachhochschule Lippe, and how difficult it was to quantify efficiencies once you started the additional airflow.

In any case the present exercise is, for me, a way of avoiding costly and time-consuming mistakes of putting a junk fan in an otherwise quality airframe (eg AW Me-262). I install my fans, for a number of reasons, in such a way that I want to avoid as much as I can having to take them out if I can ...

Incidentally, compressibility effects can be estimated and are at (v/c)^2, where v is the airflow speed and c is the speed of sound.

For airflow at 70 m/sec that is a (70/330)^2 = 5% effect.

Here are a few more interesting comments related to fan efficiencies and relative tradeoffs :

http://translate.google.com/translat...A4tter%2Fpage5

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More here on the use of simple formulas that allow an estimate of the efflux from given thrust figures :

http://translate.google.com/translat...745552&act=url

Original (in German) :

http://www.rc-network.de/forum/showt...=1#post2745552

The discussion there is of course consistent with basic physics principles, and with what I presented earlier in this thread ...

It is also the basis of number of "fan calculator" programs.

Discussion of closed vs open intakes at JetPower 2012, by the EDF design master himself

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Daniel Schübeler zum Open-Duct-Einbau (5 min 24 sec)

DS presenting the new DS-215 fan

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Daniel Schübeler zum DS-215-DIA HST (5 min 4 sec)

WeMoTec news at JetPower 2012, presented by Mr. WeMoTec himself

More excellent JetPower videos by Peter Kaminski here:

http://www.youtube.com/user/EDFJets

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News auf der JetPower 2012 von WeMoTec (8 min 45 sec)

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CArf EDF news:
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CARF EDF-News - JetPower 2012 (7 min 6 sec)