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Old Aug 26, 2014, 08:28 AM
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Larry Dudeck's Avatar
Williamsville, NY, USA
Joined Sep 2000
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10 ft WS TBJ

I'm toying with the idea of building a 10 ft wing span Twin Biz. jet. I have built this particular plane three times but with a 57" WS. Kinko's can enlarge the plans for me.

The proportional impeller diameter would be about 6" but not too many EDF's available in that size. So I'm thinking about using a "ducted" prop design.

Looking through my inventory of parts I have an Eflight 15 BL motor, a pair of 7x6 3 blade props and a pair of 35 amp ESC's. Using an online calculator, this motor with the 7x6 prop develops 90 ounces of thrust on 6S. With 4000 mAhr packs, flight durations of 7 to 10 minutes are very doable.

My first iteration of the design has an AUW of 21 lbs, or 336 ounces. The two motors develop 180 ounces of thrust so the thrust/weight ratio is about 0.5. This is more than adequate for my kind of flying. If I under estimate the weight by 10% and the motor calculator is overly optimistic! I would still be ok with a t/w ratio of 0.3

The resulting wing load is about 27 oz/sq.ft. No problem there.

My choice of packs are two and are based on the chargers I currently have. I can use 3S x 2200's in a 2S2P configuration or 4S x 4000 in series with a 2S x 4000. Either way I have to fabricate wiring harnesses so the final selection will be based on price.

The first order of business is to fabricate a duct with an ID of 7.125" and run a power test to see what kind of thrust my existing motor, prop, ESC and battery develops. How does a ducted prop compare to an "open" prop?

So have any of you ever did anything with ducted props? What separates a ducted prop from a ducted fan? It can't be blade count as my WM400 fans are 3 blade impellers.

Your thoughts and comments would be welcome.
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Old Aug 26, 2014, 11:23 AM
EDF Jet Jam 2015, May 28-31st
eatond's Avatar
United States, MO, St Charles
Joined Nov 2002
1,798 Posts
Hi Larry,
interesting idea, I'll be watching the build. Take a look at the Josh Harel F-16 for a better idea on the performance with a ducted prop. His F-16 was 91.38" long with a 60.61" wingspan. I can't find what size prop he used on the plan but the plane weighed 17 pounds. While I think that proves the ducked props should provide more than enough power I also think you could use a smaller sized EDF and have more than enough power. When Bob Ponte and I put together his George Miller A-10 we used 120mm E-Turbax fans that were good for around 15 lbs of thrust each. Plenty of power.

Dan Eaton
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Old Aug 28, 2014, 03:02 PM
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Larry Dudeck's Avatar
Williamsville, NY, USA
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Dan,

What batteries were used with the 120 mm fans?

One of my intentions here is to build this plane "on the cheap", spending as little as possible. Large diameter EDF's need good batts and speed controls.
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Old Aug 28, 2014, 04:01 PM
EDF Jet Jam 2015, May 28-31st
eatond's Avatar
United States, MO, St Charles
Joined Nov 2002
1,798 Posts
Hi Larry,
Bob's A-10 used two E-Turbax fans equipped with NEU 1524 motors and powered by Castle Phoenix 110 ESCs hooked to 10S 6000mAh packs on each side. The plane weighed 31 pounds and we probably had a 1:1 thrust to weight ratio with that setup. Definitely over powered and over priced for what you had in mind but my point was you don't need 8 inch fans and can get by with much smaller fans.

You can probably do what you want with a pair on 90's. Of course like anything else the more performance you want the more you're going to spend whatever size fan you choose.

I think the ducted prop is worth looking into. As a gas/glow solution I can see where it would have had drawbacks but using electric might breath some new life into an old idea.

Whatever you come up with should be interesting to watch.

Dan Eaton
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Old Aug 28, 2014, 11:47 PM
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Tucson, AZ, USA
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Way back in the last century when my son and I started playing with EDF, we made our own fans. Made a 4" fan using 2 Rev-up 9-7 narrow blade props cut down to the 4" dia. Took out the front hub half on one prop and the back hub half of the other and glued them together. Drove the fan with a 05 car motor with a 35A esc and standard 7 cell car battery. Flew the plane pretty well. Latter replaced our homebuilt fan with a Wemotec 90mm. A few differences - homebuilt got off the ground in a shorter distance and accelerated faster. The Wemotec was faster in the air. We didn't have the choices back then in motors/esc/batteries that we have today.

I see no problems building a "prop" EDF. Just balance the prop/fan and play with motors/cell count to get the desired power.
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Old Sep 07, 2014, 01:38 PM
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Larry Dudeck's Avatar
Williamsville, NY, USA
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http://www.rcgroups.com/forums/showthread.php?t=2228776

No sense in posting to two threads on the same subject.
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