Developer Comment by R_Lefebvre on November 13, 2012 at 9:07am

So... just to summarize, you got 492 grams thrust at 155 watts with the fan on 4S, and the same motor achieved 520 grams at 232 watts on 3S with a prop?  Well that's looking not so bad for the fan, but I wonder if the prop test wasn't optimized?

Comment by Nick Goodey on November 13, 2012 at 11:47am

That's correct

Seems odd to me the scales are working correctly btw. The 8X6 prop did develop a flutter at high rpm but i'm still surprised myself. I retested the fan after final assembly same result, 476 with out the inlet lip and 507 with it with a freshly charged pair of 2 cell 1300mAh in series.

Developer Comment by R_Lefebvre on November 13, 2012 at 11:57am

You should try running the prop again with just a 2-cell.  Obviously this will run much slower, but I think it will be more efficient.  The fact that you were getting flutter tells me you were overdriving that prop.  I think this is the case often.  It would be interesting to test that motor again on 3S with a 10" prop.  Something like that.

Comment by Nick Goodey on November 13, 2012 at 1:38pm

You are quite correct a 10 X 3.8 was pretty good nearly 700 Grams at 22A 202 Watts.

The 8 X 6 was recommended by HK for the L2210-1650 

That said it is a pitchy wee fan a bit of a torque monster I suspect

698 / 202.5 = 3.446 for the 10 X 3.8

492 / 155.7 = 3.1599 for the 90 mm 5 Blade fan

So 8 % less efficient turning electrical power into thrust for the fan much less than I expected. I will do an RPM measurement for the fan also

Please see the test data here

Comment by Nick Goodey on November 13, 2012 at 1:46pm

What do you think about a thrust tube aft for the jet?

My feeling is it will drag on the high speed from from the fan as a boundary layer builds up inside the tube and causes blockage

Developer Comment by R_Lefebvre on November 13, 2012 at 2:00pm

Ah, well, that makes sense.  So a prop is still more efficient, all is right with the world.  ;)  Also, I just notice the kV on that motor is also fairly high.  You might find that if there is a lower kV in the same size motor, it would be even more efficient with a prop.  

I agree with you, I can't see how a thrust tube would make the fan any better.  

If this is an airplane application, you might find that at high speeds the fan gains efficiency while the prop loses efficiency.  There's probably a cross-over point somewhere, just don't know how high it would be.

Comment by Nick Goodey on November 13, 2012 at 2:13pm

That's right the DT850 (850Kv) is very good on an 11 X 4.7 

235 W for 1400 Grams

1404 / 235.6 = 5.59 for the 11 X 4.7

A big SF prop turning slowly is Über efficient in static trust terms it seems, almost twice as efficient

Comment by Nick Goodey on November 13, 2012 at 2:16pm

I wonder what happens with no close running fan housing it must interact quite significantly with the blade tips, I have heard that the blade tip clearance with the housing is vital to EDF performance.

May be worth and experiment see how a naked fan performs

Comment by Nick Goodey on November 15, 2012 at 12:08pm

Space 1 Jet builder On RCgroups

Yellow Jet 758 Grams with remaining Coroplast 2 X 7.4 V 1300 mAh batteries Arduino Nano RX Xbee with antenna, servos and misc hardware. Chamfered the top on the leading edges. All control surfaces cut and hinged. ESC mounted

Intake lip attached to fan intake.

 Keel mounted 

 

Comment by Nick Goodey on November 15, 2012 at 12:09pm

Where should I set the CG on a design like this?

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