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Old Mar 12, 2014, 07:12 AM
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Build Log
Concorde - a Depron micro EDF?

If using 40mm EDFs qualifies as being a 'micro' then my Depron Concorde is one but it will be rather big!

This project follows on from my other relatively big but very light Depron EDFs that allows true scale size inlets and exhausts to be used.
The principle is to size the plane so that the EDF itself matches the scale exhaust nozzle diameter. By placing the fan right at the back it can result in the inlet duct being significantly larger than the fan swept area.

For example placing 2x40mm EDFs at the back of a scale Concorde duct actually gives an inlet area of nearly 1.7 times their combined FSA.
As there is no exhaust duct at all with the fan right at the back the generous inlet should allow the fan to produce a static thrust quite close to its 'free air' figure.
Yes, the relatively big inlet will produce more drag at speed but being a very light plane it is never going to fly that fast anyway.
So does this theory actually work?

A 3 view.
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Although all Depron this will be a built up as an exact scale plane even down to the the wing with all its twists and curves.
First a simple test rig to measure the thrust from a cheap 40mm AEO fan.
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This showed just over 4oz on a 2s and nearly double that on a 3s but at this power level the motor burnt out after just a few seconds so a 2s it has to be!
With the fan as the jet nozzle the bell mouth has to be cut off.
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A rather delicate job!
Clearly I had to establish what level of thrust could be expected so a 'test' duct was built.
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With the two 40mm fan units attached to the rear.
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A view looking down the duct showing the internal fairings to smooth the airflow from a rectangle to the fans.
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With a generous capacity 2s battery and short leads it did indeed produce close to 8oz thrust.
So far so good but could a complete Concorde, 90" long with a span of 36" and four EDFs be built and still maintain a realistic thrust to weight ratio?

The target would be to get as close as possible to 20oz all up.
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Old Mar 13, 2014, 06:50 AM
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My Depron Fairey Delta 2 has wings of nearly the same size as the proposed Concorde and proved adequately strong but it did not have a fuselage 7' 6" long!
The 27" long Concorde nose section would be a 'proof of concept' for the fuselage construction.
For minimum weight it would be built entirely in 2mm Depron with closely spaced formers.
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Constructed as a half shell over the plan using individual planks.
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The completed half shell with the RH side formers added.
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The planking built up in the same way to complete the full shell.
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Although this method produces a light and efficient structure it certainly is not a quick construction method!
Two views of the completed nose section with a solid Depron nose block added.
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To save weight there is no glazing. It will be painted on.
Amazingly rigid and eerily light at 1.4oz (40g) it does suggest it might just be possible to build the rest and keep to the target of 20oz all up.
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Old Mar 13, 2014, 03:10 PM
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If the all up weight is too low it will fly like a fluttering feather. I think you could fly it at a much higher weight. So I would not worry about it, and just press on.
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Old Mar 13, 2014, 05:12 PM
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Hampton, VA
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Subscribed!!! There's a 35mm EDF unit used in the Sky Angel series of jets, that might be a bit better than the units your using. Rated for 3S out the box and can handle 4S.

Ksqm
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Old Mar 13, 2014, 09:01 PM
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Roto rob
Not sure that light weight equals a fluttering feather.
My Depron Fairey Delta 2 has a wing of similar size to my intended Concorde and is actually shade lighter at 19oz.
No speed merchant but I don't think it could be described as a fluttering feather either.
Depron Fairey Delta 2 EDF (3 min 19 sec)
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Old Mar 14, 2014, 05:52 AM
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Awesome depron work!

Do you always plank rather the heat and form the depron?
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Old Mar 14, 2014, 06:29 AM
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meerkat13
The benefits of planking in a stressed skin structure are that the plank side joints are in shear (which glue resists well) rather than direct tension and the planks can be long so eliminating (or at least limiting) the number of plank end joints which with care can be well staggered.
By appropriately tapering the planks complex and quite severe double curves can easily be incorporated with only a minimum of final sanding required but it does put a lot of emphasis on getting the formers correct shape in the first place.

To be fair it is sometimes necessary to 'form' a plank as well to avoid the load required to hold it in place deforming an incomplete structure.

Building a shell over formers is of course a similar technique to carvel boat building where smooth lines and double curves are essential.

This form of construction gives the lightest possible structure for its weight and is not that difficult to do but it is not quick, however for electric any weight saved means more battery capacity giving more duration whilst maintaining the same level of performance.
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Old Mar 14, 2014, 08:11 AM
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Thanks for the details explanation.

Don't you just love the engineering and thought that goes into building these micro models.

Cheers, Steve
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Old Mar 16, 2014, 03:42 PM
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The wing is a major part of Concorde so that is next.
Like my Fairey Delta the Concorde wing is just Depron, top and bottom skin in 2mm Depron with closely spaced Depron shear webs.
Although basically of a simple biconvex section it has all sorts of camber changes over its area.
I doubt if I can match the shape exactly but I would like the wing to have the right appearance.
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The RH lower skin with the 19 shear webs glued on. The wing skin has been cold formed to something like the right shape before the webs were glued on.
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The LH wing added with a 6mm thick central 'rib' to provide support for the wing skin joints.
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Having got this far I am concerned that when the fuselage is built on (and under) the wing there will be a lot of redundant structure.
The fuselage will have considerable strength in its own right and thus the wing structure should be integrated into it rather than have the fuselage just sit on it.
With at best only 16oz of thrust I have to be almost paranoid about weight!

Some rework of the wing joint will be required.
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Old Mar 17, 2014, 07:31 PM
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The first step was to add formers to create the fuselage profile under the wing.
It is quite a complex shape.
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The formers corresponded to the position of the wing webs.
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The complex double curves required quite a number of planks.
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With the additional stiffness provided by the underside the centre rib and the lower wing skin could be cut out..
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Quite a long process slowly working down the fuselage.
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The end result saved a significant amount of weight and with no strength penalty.

It may be Depron but this is not going to be a quick build.
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Old Mar 19, 2014, 09:55 AM
Current project: Electrolyte
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Tremendous build so far!
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Old Mar 19, 2014, 12:09 PM
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The C20 motors in My 40 mm aeo Gizmos, rewound to ~7000 kv run 'repeatably' close to 300 watts on 3s, 26/8 amps.
Big battery required , especially so with a twin .
Whether that tradeoff suits is up to you.
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Old Mar 22, 2014, 06:50 AM
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With this sort of stressed skin structure everything is very 'wobbly' until the skinning is complete so it was essential that the top wing skin was added but this required the elevon servos to go in.
As usual for me tiny 3.7g jobs!
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The top skin could then go on.
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It required a huge glue line to be applied which took some time so it had to be positioned very accurately as there was no adjustment possible. The UHU POR did have just enough flexibility left to allow the wing to be carefully shaped to give the required droops and twists. The real test would be to achieve exactly the same on the other side!
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The structure was now quite rigid enough to be handled safely so the main fuselage formers could be added.
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Beginning to look like a Concorde but there is still an awful lot more to do and with so little thrust available the chances of it actually flying are not that great.
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Old Mar 22, 2014, 07:39 AM
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Awesome work !!!

Would it be possible to have a like to the 3.7g servos your are using.

Looking forward to your next post

Cheers, Steve
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Old Mar 22, 2014, 10:59 AM
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meerkat13
These are the servo I am using.
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Nothing special and very cheap 6.25 delivered for four
I have current 60+ in use (and all permanently built in!) and had no trouble at all.
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