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An EDF Fieseler Fi103

The Fieseler Fi103 is much better known as the V-1, the first true 'cruise' missile.

Using Depron as the main structural medium allows planes to be built of a size and weight that would be difficult to achieve in balsa. This in turn enables unusual scale types to fly quite well as I found with by Bachem Ba349 Natter.

The V-1 is actually quite an efficient airframe with a reasonable wing area but was powered by a pulse jet.
The question was could an EDF be placed in a scale pulse jet body fly a V-1 reasonably well?

On the plus side the Argus pulse jet was pretty inefficient so the duct had quite a large diameter for the overall size of the plane, certainly much bigger than a typical turbo jet of the period.
At least with a true circular fuselage section creating the formers is easy!
I chose to size the V-1 round a 55mm EDF that in theory gave 18oz static thrust on a 3s.
So the first task is to build an EDF pulse jet and see just how much thrust was lost to determine if it would be worth going any further.
The first planks go on building round a 55mm diam cardboard tube.
The inner duct complete with formers added to support the outer skin.
The Argus pulse jet complete.
The fan looks an awful long way a way down that duct!
On the plus side with the fan right at the front the inlet should be as efficient as possible.

So far so good.
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Built in Depron the intention is this will e light and powerful enough to hand launch just like all my other planes! :p

The next issue was to find out just how much thrust the "pulse jet" produced.
But the problem was how to actually measure it?
The answer proved quite simple. Just strap metal plates to the outside until it would just 'hover' in my hands.
Maybe not accurate but all I wanted was an idea just to confirm whether it was worth going any further. It ended up just able to hover at a total of 14oz.
With an estimated flying weight of 18oz it looked like it should fly ok - so on with the fuselage.
This was going to be an all Depron structure built in 4 major sections out of 3mm Depron.
The 'ring' formers for the centre section.
Each has an inner flange so that in conjunction with the skin the formers become in effect a circular 'I' beam.
Like the Natter each section is built vertically and planked.
The inside of the completed centre section looks more like part of a submarine!
The 3 rear fuselage sections that carry the wings, tail and pulse jet.
The centre section has a substantial balsa/Depron/balsa box spar across its centre into which the wings will be glued in.

It is not generally known that the USA actually built V-1s called the JB-2 'reverse engineered' from captured bits and first flew in October 1944 just 5 months after the first German V-1 landed on London.
The US geared up for mass production for use in the invasion of Japan and had built just over 1000 by the time the contract was cancelled after their surrender.
From my point of view the JB-2s that were test flown were brightly painted, a good feature in an RC plane, rather than the camouflage of the V-1 which were intended to be be hard to see!
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Next build the tail plane. A simple 2mm Depron skin over a hard balsa spar with a symmetrical section.
The elevator has a scale centre pivot hinges with balsa shrouds and driven by a 5g servo buried in the tail end of the fuselage.
The fin added. The rudder is fixed.
The wing are fairly conventional with ribs and a wide but tapered balsa/Depron/balsa box spar. A symmetrical section like the original.
With the bottom skin added the aileron is cut out and the 3.7g servo added.
Of course the original did not have ailerons but then it only had to fly in a straight line! ;)
The left and right wings are identical except for top hinged aileron.
The cables run in the fuselage. Just in case they are all long enough to reach right up to the nose.
The pulse jet and the fuselage nose section added but without the battery, ESC and radio.
At this point I realised that even with the 1500mAh battery, ESC and radio right in the nose the CofG was going to be much too far back. I would have to at least double the weight of the battery to get it to 30%.
The problem was that elevator servo right in the tail. A quick calculation showed that over half the battery weight in the nose was required to counter balance it.
The only solution would be to move the servo into the nose and use long (48"!) pull/pull cables.

So some rebuilding required and I haven't even completed it! :mad:
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It was always the intention to cut open the nose to install the radio, ESC, and battery box but now the elevator servo as well.
First the elevator servo has to extracted and a double sided horn fitted to the elevator.
The nose cut open.
The Depron battery box set as far forward as possible.
With the elevator servo underneath.
The radio is in its own box on the left with the ESC on the right. The ESC has a finned heat sink protruding slightly into the air stream.
The nose replaced leaving an opening just big enough to slide in the 1800mAh 3s battery.
The battery is secured in place by the battery hatch which has a sliding bolt latch.

It weighs just 20oz a bit over my 18oz target but most of the difference is made up by the bigger battery.

Nevertheless it certainly flies. An edited video of its second flight.
It glides remarkably well and lands ridiculously slowly!
The biggest problem is at the launch.
Its not the speed required but rather maintaining a sufficient grip on the large diameter smooth fuselage without risking crushing the structure. :eek:
In addition the ESC overheats after a couple of minutes at full power and automatically reduces power. No great problem as it flies very well on much reduced throttle!
More cooling (and possibly a bigger ESC) is next on the list.
To improve the its cooling the heat shrink was removed from the ESC and it was brought up flush with the surface.
A duct carries air to the underside of the ESC as well.
Now decorated as a 1948 USAF Republic JB2 Loon a video in rather windy conditions.
I am still amazed at how well it glides.
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