Cobbled together 90 mm fan with 250 Watt Turnigy L2210-1650 Bell Style Motor (250w) seeing if I can get decent efficiency from a small fan turning slower.
On 14.8 Volts its pushing almost 500 Grams, not so bad thrust mapping is as follows.
100 Grams 1.9 A 27.6 Watts
156 Grams 3.0 A 42.9 Watts
197 Grams 3.9 A 55.1 Watts
257 Grams 5.5 A 76.0 Watts
296 Grams 6.7 A 90.9 Watts
340 Grams 7.9 A 105.5 Watts
432 Grams 10.7 A 138.1 Watts
492 Grams 12.5 A 155.7 Watts
The data in a spread sheet
The tests were run on two 7.4V 1300mAh Turnigy 20C Lipo's in series, I figure the motor will survive full throttle bursts on 4C and its 10 Amps short of the max, 100 Watts below its spec.
Its surprising as the same motor with a 8 X 6 prop pulled 23.3 Amps on a 3C yielding 520 Grams of thrust at 232.3 Watts.
Weighs around 150 grams.
Need a plane for it should be pretty ugly with such a big fan
Foam and Coroplast SPAD for the EDF Jet
A mass of 500 or 750 grams give a wing loading's of 13 oz/ft² and 19 oz/ft² respectively
Wing area 1.3855 ft²
Dihedral 5º
Kf2 A symmetric trapezoidal wings and tail plane
My Xbee based controller with appropriate RX software for the Jet
The wings are braced with bamboo skewers cheap as. I will chamfer the top leading edges as that's what most people seem to do.
I am advised that the elevators will need lots of authority so I will fashion quite large ones :)
Comments
Tried to fly the wee jet at Akaroa today and it was dismal and under powered :(
So taking some advise from DIY Drones I did some more testing and have found a sweet spot with some props I have laying around.
Turnigy L2210-1650 Bell Style Motor
Spec.
Kv: 1650 rpm/vVoltage:7.4~11.1v (2~3s)No load Current: 1.3AMax Current: 22AMax Power: 250W
Shaft Size : 3mm
Shaft Length: 28mm
Dimension: 28mm x 17mm
Thrust: 520g Weight: 50g
My results with a 7 X 5 three blade are as follows on a 3C
23.6 Amps
11020 RPM
880 Grams Thrust !
240 Watts
That's on a 2200mAh 3C and a Turnigy AE-30 ESC
I have read that the following formula calculates approximately the max speed of the model
Plane_KMH = Prop_Pitch_Inches X RPM / 800
Given this is correct?
5 X 11020 / 800 = 68.875Kmh
Should give a minimum duration of 5.6 minutes
Using a 4C on a 5 X 5 Three Blade Prop
Next test will be with a 4C and smaller prop, optimized for 250 Watts should be around 16 amps
Tested a 5 X 5 three blade
4C 14.8V
Thrust: 770 Grams
Watts: 238 Watts
Amps 16.4 Amps
RPM: 14,200 RPM
Plane speed 5 X 14200 / 800 = 88.75 Kmh
Should give a minimum duration of 5.8 minutes
I think either of these options will do the job, the plane weighs 685 grams with a 2200mAh 3C on board, wing loading 18 oz/ft²
Conclusion
Given the 4C is no more efficient and the duration advantage in negligible, and the 4C battery is only 7 Grams lighter, I favor the 3C on the bigger prop. There is a theoretical 20Kmh speed advantage for the 4C but the 3C is plenty fast enough.
The Plane
A mass of 500 or 750 grams give a wing loading's of 13 oz/ft² and 19 oz/ft² respectively
Wing area 1.3855 ft²
Dihedral 5º
Where should I set the CG on a design like this?
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
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
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
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.
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
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
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.
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.
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