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Jan 16, 2016, 08:40 PM
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An EDF with only cheat holes!

I have two scratch build scale Deporn EDFs that use the "fan at the back" principle with no thrust tube what so ever.
The pros and cons of going this are open to question but they work very well in the designs concerned.
But the layout did set me thinking. Would it be possible with this layout to avoid a long inlet and feed the EDF entirely from big(ish) cheat holes?
I had in mind to use this layout on a 'plane' that had no inlet at all because it was a rocket! Known as the A4b but perhaps better known as the German V-2 with wings on.
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The intention was to fire the V-2 conventionally but at its apogee to extend its range by gliding. Range had become an issue as the standard V-2 did not have the range to hit London as its launch sites were driven back by the advancing allied armies in the autumn of 1944.
Several A4b were built and two were actually fired. One failed at launch and the other appeared to break up at its maximum altitude.

With an EDF right at the back you end up with something like this.
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Not intended at this stage to lift off vertically but to be a "hand launch belly land"
To give it best possible chance of working it would be an all Depron super light weight.
However before investing time and effort in building a complete A4b it seemed logical to build a test 'tail end' with just the EDF and the cheat holes.
The test duct.
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The first stage is to build the inner cone.
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The EDF is supported by 4 fins glued on to the inner cone.
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The external skin is then built over the EDF.
The complete test duct.
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On a 3s the 70mm EDF delivers 26 oz (737 g) thrust drawing 44 A. This is quite close to its 'free air' figures so the duct is not costing that much performance.
Even if it does not have the excess thrust for a vertical take off it should have quite sufficient to fly normally.
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Jan 16, 2016, 09:12 PM
Need 4 Speed!
pdawg's Avatar
construction and intake design looks great!
Latest blog entry: original Y/A F-18 Kit
Jan 17, 2016, 06:46 AM
great job
Jan 17, 2016, 10:26 AM
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jumo004's Avatar
Too cool !!!!
Jan 17, 2016, 12:28 PM
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turbonut's Avatar
I like it
Latest blog entry: In flight
Jan 17, 2016, 05:46 PM
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As the test duct seemed to work OK I had a look at what the final A4b might look like.
As it will be a hand launch belly land clearly scale control surfaces would have to be cut back so they did not extend past the fin. Although this only needed to be done on the lower fin I felt an 'unequal' arrangement might look odd.
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It would have a span of 20" (510 mm) and be 47" (1195 mm) long.
The intention is to use 4 servos with the horizontal pair mixed as elevons and the vertical pair mixed as rudderons. In other word all four control surfaces would act as ailerons.
Jan 18, 2016, 08:13 PM
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As this was intended to be a 'super lightweight' it would use ring formers in the fuselage each with an internal flange.
To save Depron each larger ring would be fabricated from four quadrants.
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To get things under way the tail inner cone is built first.
It is built vertically.
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Quite a slow job as each plank has to be shaped to fit.
It is quite a milestone when the last plank goes in!
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As with the earlier test duct the EDF is supported primarily by the tail fins.
Each fin is a built up stream lined two skin structure.
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Quite a delicate job to get it all together with the EDF hub exactly lined up with the inner cone.
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At this point the usual "Am I just wasting my time" worries start to kick in!
Last edited by Quorneng; Jan 19, 2016 at 07:12 AM.
Jan 19, 2016, 07:34 AM
cut build fly
Jan 19, 2016, 05:16 PM
My project: FAIREY DELTA 1
Erik v. Schaik's Avatar
Nice concept!

Have a search at the Miles M52. It has been build with minifan and FSA was 100% with minimal airgap.
Leduc research jets have interesting ducts as well.

good luck!
Jan 19, 2016, 05:36 PM
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turbonut's Avatar
I bet it works great! nice work so far. Maybe scale the wings up a bit for the first try
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Jan 19, 2016, 06:34 PM
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The outer skin added over the EDF.
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Each long intake has an area of just over 1/4 of the FSA.
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The next tricky job is to 'insert' the 3.7g servo into the fin.
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The servo is the same thickness as the fin.
The servo wire is fed down through the fin and into the fuselage.
Now to do the same for the other 3 fins!
Jan 20, 2016, 04:46 AM
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Very cool model. I think CG may be diffucult.
Jan 20, 2016, 05:49 PM
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Now for the middle bit.
Basically a tube but slightly barrel shaped but it does mean each plank can be quite wide.
The 'stack' of 8 formers all slightly different!
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The centre section complete
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It only required 16 planks to complete instead of 26 narrow one used on the tail section.
The wings will be made in one piece and simply slotted through the centre section.
Jan 20, 2016, 08:04 PM
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jumo004's Avatar
Are you using a fixture to hold the formers in place as you add the planks ?

Man you work fast !

Great stuff !
Jan 21, 2016, 05:50 PM
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No fixture.
Simply two planks cut to the right length and glued to the first and last formers. Then the intermediate one are glued into the correct position between them.
Its all done vertically. I use an engineers set square to make sure any taper is equal on both sides.
From another build but it looks something like this.
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Very wobbly to start and to prevent distortion the planks have to be added in opposite pairs. Best to let glue set on each pair before you add the next!
It may seem a bit tedious but when finished you have the complete finished fuselage shape that only requires minimum finishing.

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