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A low powered Douglas Skyray EDF

#1
Having built several planes in Depron I wondered if it would be possible to go 'up' the conventional weight/power spiral by building a big, light weight, low power scale 'jet' using a ducted prop rather than a commercial EDF.
In the Skyray Douglas squeezed the biggest jet they could into a small compact airframe so it looked a good starting point.
I began with this 40 year old 1/72 plastic kit!
Pastickit.JPG
A 4.5x4.5 prop driven by a 2600kV out runner gave over 16oz static thrust at 180W.
With a bit if juggling I could get a 4.5" prop inside a 36" span Skyray and stood a chance of keeping it below 16oz all up. Putting the prop in a duct would loose some thrust but it still looked feasible and just as important it would be cheap to do!
The basic centre section of the duct was made from 3mm Depron planks formed around a piece of plastic sewer pipe. Depron 'hoops' added to give it enough strength to be handled.
Ducthoops.JPG
Fuselage formers were than added.
fusecentre.JPG
The underside was then skinned in Depron.
BtmskinF.JPG
The top skin would only be added when all the RC gear was installed.
The wings had no ribs. the wing skins were simply glued over the wing spar and 2 spacer spars. This resulted in a perfectly symmetrical section and with a scale t/c ratio of just 6%.
Wingspars.JPG
The next step to build the fuselage nose section and create the bifurcated inlet. That was going to be much more complex operation.
 
#3
The fuselage nose section is a former and plank job but as before in 3mm Depron. Not for the faint hearted as it is very delicate until the planking is complete.
Fusebuild1.JPG
Next the cockpit lining was added.
Cockpitskin.JPG
This added sufficient rigidity to safely skin the outside.
Fusecmplt2.JPG
With every plank (there are a lot!) carefully cut to shape it is more akin to boat building.
The completed nose section was then literally inserted into the duct and the fuselage formers cut away to create the inlet ducts.
Inletcut1.JPG
The ducts were than skinned on the inside (more planks!). As is common for an EDF the inlets are somewhat over size to match the fan (or in this case prop) swept area.
Inletduct3.JPG
Finally the exterior inlet skin was added.
Inletskin.JPG
Now just the other side had to done in the same way!

At about this point I realised with the hours spent on construction this was rapidly becoming a very expensive 'cheap' plane!
 
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#4
The tail cone built around the same sewer pipe as the main duct.
TailconeA.JPG
It is a double skinned structure.
TailconeR.JPG
The motor is mounted as a pusher and with the prop at the extreme end of the duct.
MtrinductR.JPG
With the tail cone glued in place the ESC & radio radio can be installed.
ESCradio.JPG
The battery (1500mAh 3s) is mounted in the cockpit.
Newbatmnt.JPG
Painted in the standard US Navy colours of light sea grey top side and white under.
Painted.JPG
This meant the underside could be left natural white Depron and saved some weight.
It weighs 16oz ready to go with 12oz static thrust. The wing loading is 5oz/sqft.
To my amazement it not only flies but did so straight out of the blocks. Indeed it has had no modification or any CofG adjustment.
[video=vimeo;37767466]https://vimeo.com/37767466[/video]
About the only problem is there is nowhere to hold it on the CofG so it has to held by the nose for launch.

The ducted prop is remarkably efficient at reduced power so despite its small battery the Skyray has close to a 20 minute 'cruise' endurance.
It also glides pretty well. How many EDFs can do a power off loop?
 
#7
Foam Addict
Well its span is 36" and the rest is to scale, even the wing section, so if you scale up a 3 view to 36" that's it!
It comes out at 46" nose to tail.
In common with most EDFs the inlets and tail pipe are over scale although the fuselage itself is not.
The duct is 4.5" diameter (the 4.5x4.5 prop is very slightly trimmed to fit) and the inlets have the same area as the duct.

Everything is sacrificed to reduce weight. Tiny 3.7g servos (there are only 2) with Sellotape hinges. The elevons have the scale aerodynamic balance which reduce the servo load.
A tiny 6g Corona 4ch 35mHz radio. Even the servo extensions are soldered and insulated with heat shrink to save the weight of the connectors.

The result is a pretty extreme design, certainly with little crash resistance and one that took an inordinate amount of time to build for what is - a lightweight foamie!

But it does fly well.
In this video it soon gets out of sight but if you listen carefully to the sound track after the full power launch it is throttled back in 7 stages and it was still climbing - albeit with some thermal assistance! :)
[video=vimeo;47180912]https://vimeo.com/47180912[/video]
 

Foam Addict

Squirrel member
#9
Thanks a Bunch!
45"? That's a lot bigger than I thought it was.
I might not go the plank route, but I have a method of forming light, strong and flexible foam that might just work.
This will have to be an indoor endeavor, as the winds here rarely drop below 10 or 12 mph.
I have to build pigs to get any stability, and the over power them to keep from hovering in one spot.:black_eyed:
Thanks,
Foam Addict.
PS, I flew a mini version of your endurance plane.
30" span, 1600 2 cell, and a motor of unknown heritage that I partially rewound.
2.5 g resoldered to standard connector.
15 Min flight with 1300 left on the batt. Wing folded though, so not a great ending.:(