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Old Jul 11, 2001, 02:30 AM
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Tiny test fan

I made a tiny Ducted fan with a depron hub and balsa blades (4). It has thin balsa stators sanded to a wing like shape. I didn't calculate angles, or put any twist into the blades. The blades take up the outer 1/3 of the fan diameter,with the hub making up about 2/3 of the diameter. This was setup for a little freeflight motor about the size of a n20 but rated for 3 cells. On 3 50 mAh cells it draws .52 A fresh and 3.68 V. Assuming a fairly low effeciency for this first test, what AUW should I be aiming at for flight (FF)? Is there a rule of thumb for a minimum power level for a ducted fan?
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Old Jul 11, 2001, 02:29 PM
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Try to measure static thrust.

I just hang two loops of cotton thread from the ceiling to act as a cradle and measure the thrust on electronic kitchen scales.

Then a trust to weight of 1:2 or 1:3 should do it.

You may have to use more cells than recommended if the thrust is lacking? If you can get say 1oz. of static thrust, then the sums should work out and you could build a 50-80 gm. model?

Steve

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Old Jul 11, 2001, 02:54 PM
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Sounds like a great idea. Now if I can only find our scale... This fan was/is more of a proof of concept. If the numbers look remotely close, then I will try to make a better version.
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Old Jul 11, 2001, 10:39 PM
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With no blade twist and random stator angles I would guess the fan is about %40-50 efficient. Youw on't get much more than %60 from a fan this small. Without proper formulas it will be hard to design an efficient fan.

Greg
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Old Jul 12, 2001, 02:05 AM
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I'd be happy to hit 50-60% . I redid the fan blades using thiner sanded balsa with a bit of twist (10 deg or so) in them. The blades are each less than 1 cm long and I only have 4. I might need more.
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Old Jul 12, 2001, 07:32 AM
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I'm saying that 50-60% is the best you'll get with a very careful design. Random guessing won't get you close.

Greg
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Old Jul 12, 2001, 11:27 PM
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Am I correct in thinking of the rotor blades in terms of prop pitch where the aim is to provide the same forward velocity through the air from the inner to the outer end of the blade (less pitch at the outside, as it travels faster)? I'm not sure about the curves for the stators, how does one compute the best curves for these? They are basically flow straighteners in our little fans? I figure that working with a fan, and observing it as well as reading what I can about them should help my understanding a great deal. For freeflight, I think there is a wider tolerance of low effeciency as there is less mass to be hauling around (generally) so a lower effeciency fan should be ok (as long as it has enough thrust to fly the plane, I'm not really concerned about long flights with ff either)
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Old Jul 13, 2001, 04:45 AM
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Mike

Just as an example. Using simple trig. and a calculator.


Dia of hub = 1". Distance travelled by blade in 1 turn of the motor = 1 x phi = 3.143"

Say you decided on a 3" pitch.

Angle of blade at hub = Tan^-1 3/ 3.142 = 40 degrees.

Dia of tips is say 1.75". Distance travelled by tip in 1 turn of motor = 1.75 x phi = 5.5"

Angle of blade at tip = Tan^-1 3/ 5.5 = 28 degrees.

So twist the blade to have 40 degrees at the hub and 28 degrees at the tip.

The figures for your fan will be different, depending on the dia. of the hub and fan and the pitch you finally opt for.

I agree. For free-flight, max efficiency is not required. A short flight with moderate/adequate power is all thats needed.

Good luck


Steve



[This message has been edited by steve glass (edited 07-13-2001).]
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Old Jul 13, 2001, 08:03 AM
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For a very simple design designing the rototr to have a constant pitch is OK, but by no means optimized. The chords are very important also. To calulated the stator angles you need to calculate how much swirl is being generated by the rotor. It's not the easiest thing to do. There are a few text books that give proper design methods.

A more efficient fan would allow a smaller battery.

Greg
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Old Jul 13, 2001, 06:52 PM
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Steve is bang on with his reply. aim for 1:2 thrust ratio to allow for the servos and reciever

Is the fan in a duct? if so make it short and put it way back in the body.

What airframe? a Sabre would be nice...or a Mig or a Lightning how about a simple stick and 1/16 balsa wing to test the fan?

You have many options but even a 1000 mile walk starts with the first step.

Hear come the micro EDF's..........

Doc

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Old Jul 13, 2001, 07:57 PM
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At such small sizes, how much better would an airfoil section perform than a flate plate? I suppose speed would be more the issue, as I think the airfoil will come into it's own above a certain speed. Hopefully I'll have some more time this weekend to work on the fan a bit. Would it be ideal to have the stators start at the same angle as the airflow and then work back to straight? Would rounded or sharp edged stators be better? I imagine rounded might be better for varying fan speeds to compensate for the differing flow angles better. I suppose it should be possible to "stall" stators as well, if the angle was off too far? Perhaps cassete tape mylar streamers would give a bit of an indication of flow direction after the fan (and stators for testing). I guess I should make some method of easily moving the fan and motor from shroud to shroud as well.
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Old Jul 13, 2001, 08:59 PM
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At this size a curved plate airfoil is fine. Rounded leading and trailing edges are better than square.

The actual angles, curves, and number of the blades can be varied but all interact.

Greg
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Old Jul 14, 2001, 05:00 AM
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I made a rubber powered fan unit 2" dia. It gave an initial burst of 14gm. static and a cruise of 8gm.
It flew a 1oz. delta-shaped thing for two laps of the sports-hall....great fun and in my eyes a success.

For micro-electric-fans, could there be a benefit with a single-blader fan? Would this solve some of the design problems without needing the brains of a rocket-scientist?
How does the pitch of the fan-blade vary, if it is not constant?...or are we back into rocket-science.

Steve
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Old Jul 14, 2001, 05:19 AM
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Mike,
Try to read the article on Pete Shepherds EDF300 unit in the March1998 EFI. Or buy an EDF300 unit.
This will show you how to work to the neccessay accuracy using very basic workshop practice.

Steve
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Old Jul 14, 2001, 08:29 AM
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You'll never have a singe blade fan. I seriously doubt the numbers would ever end up telling you a single blade is best. Ducted fans can't be compared to props in design. There are several factors and constants used to determineoptimum blade shapes. Then you have to decide if you can build the optimum and make a decision were to compromise.

For a prop single blade is "best" because you have to deal with tip vortices and such. Even this can be argued when you take into account weight and balance.

Greg
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