A low power EDF R4D Skyray.

[quorneng]

My simple low power "DuctFan" seemd to work well enough but could the same low power principles be applied to a conventional EDF jet layout.
A low power EDF has to be considered as the equivalent of a high bypass turbo fan, moving a lot of air relatively slowly.
For a plane travelling at say 30 mph the best use of the available energy is made if the nozzle exit is no more than 40mph. Higher exit velocities will produce more thrust but require disproportionally more power to generate it.
The Douglas R4D Skyray has some advantage as an EDF as it has a relatively short fuselage of almost constant cross section combined with a generous wing area.
Scaling up 4 times from this rather battered 40 year plastic kit gives an 18" wing span.
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This 55mm diameter fan will just fit inside.
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At 18" span the Skyray has a wing area of 1.1 sqft. Keeping the same 6.8 oz/sqft wing loading as the DuctFan would give an AUW of 7.5 oz however on a Skyray almost 50% of the projected area is 'fuselage' and of questionable lifting capability so my target will be 6oz.
As it will be such a light airframe I built a wing to tryout the construction technique using all 3mm Depron.
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It is a symmetrical 8% section. The wing is so thin that even a 3.7g micro servo will only fit when flush with the under surface.
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The thin leading edge is protected by a 1mm balsa strip.
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The complete wing including servo weighs 14g (0.49oz).

[Richard]

Your a master in depron

[quorneng]

With the wings in their correct relative position shows just how wide the fuselage/centre section is.
After much thought I am going for a conventional EDF layout with a duct tube, formers and skin but all in Depron.
I found a stout cardboard tube of exactly the right diameter for the duct. The radius was to small to wrap the depron so it was planked.
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The fuselage formers.
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The radii here are gentle enough to cover the underside with a single piece.
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The centre section is now just rigid enough to remove the cardboard tube.
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The inside of the duct is lined with paper for a smooth finish and to add some stiffness.
The forward part of the fuselage is next as it determines the profile of the bifurcated inlet duct.

[Tony Smith]

This is a top notch build...
Looking forward to the next update...

[quorneng]

The cockpit section has 5 formers.
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Although the formers were cut as matched L&R pairs they were glued together and the fuselage built 'free standing' off the both keel strip.
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Very delicate to start with but it allowed equal planking to make sure the very light structure remained straight.
The cockpit section will be grafted onto the front of the duct once the planking is complete.

[quorneng]

The cockpit fully planked.
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A bit of a pain to do but it is remarkably strong and light(0.275oz).
The former in the cockpit will be cut away to suit the battery location.

[quorneng]

The cockpit section grafted onto the duct with the LH duct planked in.
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As my original intention had been to build a completely hollow fuselage adding the inlet duct 'in situ' like this is not easy. Next time I will build the complete duct first and add the fuselage around it in the conventional manner.
My only deviation from scale has been to enlarge the actual inlets so that together they have the same cross section as the main duct.
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With the wings laid out it is at last starting to look like a Skyray.


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[Tony Smith]

What does it weigh so far..?

[YOUNG GUN]

That is looking very very good - Like a computer enlarged depron version of the model!

[quorneng]

Tony
What in this picture (with both elevon servos) weighs 58g (2oz).
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Just a tiny bit more than I hoped for but it shouldn't end up much over my all up target of 6oz.

As the motor, battery, radio amd ESC will weigh twice as much as the rest of the plane their positioning is critical. There is not much space to move things around once the fuselage is completed.

Indeed I might have to build a same size simple sheet Depron Skyray glider to get the position of the CofG.