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Old Nov 07, 2012, 03:14 PM
Current project: Electrolyte
pdawg's Avatar
United States, OH, Dayton
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Hoeby,
Aerodynamic advantages are possible in the twin configuration...
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Old Nov 07, 2012, 05:49 PM
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pdawg,

I doubt that efflux speed was a limiting factor in a J-58 in afterburner mode.

Just saying.

Now, back to my cave.
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Old Nov 07, 2012, 06:11 PM
Current project: Electrolyte
pdawg's Avatar
United States, OH, Dayton
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Every design has limiting factors! Its all a compromise and lessons are learned after doing numerous speed airframes. The 25mm twin speed airframe design that Hoeby pasted into this thread was designed with fan efficiency in mind. These tiny fans have limited power so minimizing intakes and thrust tubes has a big advantage to maximize efficiency and top speed. This is a way to maximize efflux...not limit it. Long ducting and fans located in turbulent regions near the back of a fuselage are a great way to prevent a fan from reaching its peak potential. Once again...its all a compromise.
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Old Nov 07, 2012, 06:42 PM
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I'm afraid my response was a bit vague.

I was merely echoing Erik v. Schaik's comment that efflux speed will be a limiting factor in the ultimate speed of an EDF, and with this in mind, gram for gram and watt for watt, a twin power plant will be less efficient than a larger single power plant.

That the fastest 1:1 jet was a twin has no bearing on what the fastest configuration for a small scale EDF would be. Efflux speed was not the limiting factor to the SR71's top speed.
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Old Nov 07, 2012, 08:51 PM
Current project: Electrolyte
pdawg's Avatar
United States, OH, Dayton
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Rclurker,
Your comments suggest you have not taken an airframe in to account? With a draggy airframe the efflux is no longer the limiting factor...drag is. While I would choose a large fan over a small one...given the choice I would happily use two large ones. Once again...it's all a compromise.
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Old Nov 08, 2012, 04:50 AM
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Holland
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It is not only efflux what you need, also thrust.
Look at some of those turbine jets. The efflux of most turbines is almost the same.
When they put a 8 kilo thrust engine on a plane, and want to go faster they put a 16 kilo thrust turbine on it, and it goes faster. The efflux is almost the same.
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Old Nov 08, 2012, 09:25 PM
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Canada, ON, Mississauga
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Okay , so is there any 'rule' or optimal calculation about the length of a thrust tube compared to FSA? What would be considered too long for a 70mm HET with an 80% exit nozzle?
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Old Nov 09, 2012, 06:24 PM
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United States, WA, Shelton
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I am no expert but I do know that when dealing with friction loss in fluid dynamics that the longer the run, or tube in this case the greater the friction loss experienced. By necking down the flow we increase velocity, hence the 85% FSA, however if we lengthen the tube we must increase power (thrust) to compensate for the friction loss to maintain exit velocity. It is very interesting
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Old Nov 10, 2012, 06:34 AM
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taper should be no more than 10deg so 70mm to 60mm you need to draw on paper to see what ever it come's out is what you need
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Old Nov 10, 2012, 07:17 AM
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Holland
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Quote:
Originally Posted by space 1 View Post
taper should be no more than 10deg so 70mm to 60mm you need to draw on paper to see what ever it come's out is what you need
Isn't that only whit the inlet? To prevent air comming loose from the wall.
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Old Nov 10, 2012, 10:18 AM
My project: FAIREY DELTA 1
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Uden Volkel, Netherlands
Joined Dec 2003
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Nope, If the thurst tube is too short it is more likely you'll compress the air instead of increasing efflux. Fans do stall when air gets compressed thus efficiency will get bad.

Have a read in the sm110-52 manual. Great tutorial! My Fd1 has been designed around that manual. Note the vid shown here is made at very close distance from intake and thrusttube.
fearey delta1 testrun#1.MP4 (5 min 58 sec)
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Old Nov 13, 2012, 09:41 AM
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What is that small twin ? is there a link to the thread please ?

ty
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Old Nov 13, 2012, 01:37 PM
Current project: Electrolyte
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Quote:
Originally Posted by scousethief View Post
What is that small twin ? is there a link to the thread please ?
Here you go. The airframe is currently in Australia getting a pair of custom built 25mm fans. The airframe and power system are both custom built strictly for speed. Looking forward to seeing how it performs with this new set of fans...rpms will exceed 100k. We expect this design to destroy the current 25mm edf speed record.

http://www.rcgroups.com/forums/showthread.php?t=1513465
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Old Nov 13, 2012, 02:06 PM
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Nice ty Pdawg
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Old Nov 17, 2012, 09:59 AM
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Holland
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I made the first steps for the twin design.
As whit the other design i first start in 2D. When the drawing is how it needs to be, i will converted it to 3D to see if everything fits where i thought it needs to go.

The design is based on the next setup.
2x wemotec minifan
2x HetRC 2W20
2x 4S1P 3000mAh
2x Brushless controller, robbe roxxy 80A

I also thought about changing the fan to a wemotec HW505. This is a 66mm minifan rotor. So efflux will be a little higher. Also about the motors i am not sure if they will be the 2W20. Maybe i switch to the 2W18. It is for sure, when i am going for the HW505, that the motors switch to 2W18.

The fuse will not be round, but square. This to get a tight fuse around the lipo's.
The lipo's are now projected on there site. But the lipo in front of the wing also can be routated. When doing that, it can move forward a few cm's (the get the CG right).
For the wing design, i took the wing of a pylon racer called zwoadrittl. Why?
I know where the CG is for this wing. And the wingprofile is also designed for speed.

What you guys think about the first design. Need to change something, otherwise i start to redraw it in 3D
( you guys think, why first in 2D and than in 3D. That's because i am not that good in 3d cad)

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