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Old Nov 16, 2012, 05:24 PM
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Build Log
Bachem Ba349 Natter in Depron

As other have pointed out a very light scale airframe can be built out of Depron.
A scale Bachem Natter would test this to the extreme as the full size only had a wing span of 12 feet!
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I expanded the 3 view to just 26" span but in comparison the fuselage was huge, 48" long, 6" wide and 8" deep. Clearly this was going to have to be extraordinarily light to stand any chance of flying without the benefit of a scale Walter rocket engine and 4 Schmidding boosters!!!
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For the fuselage its generous proportions meant that it would be quite strong and stiff enough as a true monocoque with all the loads taken by the 3mm Depron skin.
The first job was to cut out 16 fuselage formers in 3mm Depron. To save weight only the nose and tail formers were solid all the rest are 'rings'. The original fuselage was in 3 sections. I did the same but built each vertically using carefully shaped Depron planks.
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The planks have to added symmetrically each side one at a time. It is a very delicate structure until a substantial area in covered.
The completed centre section.
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Remarkably strong and rigid yet it weighs less than 1/2oz!
Like the original the wings are very simple with a single spar and just 5 ribs of a symmetrical section.
The full depth wing spar has 0.8mm hard balsa flanges over a 6mm Depron web.
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With 3mm Depron ribs.
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It has 2mm Depron skins.
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The tips are sanded to shape from 2 layers of 6mm Depron.
The wings mounted on the fuselage centre section,
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It weighs 1.25oz.
So far so good but this is likely to be the easy bit.
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Old Nov 16, 2012, 05:41 PM
The Hun in the Sun
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Now this is something different and very cool! What is your planned power system? I'm guessing a prop in the nose?
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Old Nov 16, 2012, 06:06 PM
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Not in the nose but at the back, protected by the lower fin but I expect the battery will have to go in the extreme nose to compensate!
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Old Nov 16, 2012, 09:48 PM
Übung macht den Meister..
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Very spiffy!

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Old Nov 17, 2012, 05:16 AM
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The rest of the fuselage sections were built in the same way but the more complex shape made it an even more delicate job.
The nose
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The tail
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The 3 pieces just resting on top of each other gave the first impression of its considerable bulk!
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The tail plane and fin(s!) added to the tail section. Tiny 3.7g servos are mounted in the tailplane to work the scale size elevons.
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This whole project was really an exercise in emptyness!
With the only servos in the tailplane and motor out the back there was obviously not going to be a shortage of space to fit the rest.
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This photo does show just how little internal structure there is.
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The completed Natter.
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With a 1500mAh 3s right at the nose and a 1500kV Blue Wonder at the rear driving a 7x4 the CofG comes out at 25% chord. It weighs just a touch over 12oz ready to go.

Just a point of scale the lack of glass in the cockpit is correct. Most of the unmanned prototypes were built this way.

But will it fly? and even if it does will it be controllable? Those elevons have a very short lever arm, which may have been ok with a torque free rocket motor but with a prop?
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Old Nov 17, 2012, 08:29 AM
Slip the surly bonds...
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Congratulations on an impressive build combining simplicity with boldness. I do hope it flies, perhaps on the same day that someone is flying a B17?

(That would keep Frank happy!)
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Last edited by Sopwith Mike; Nov 17, 2012 at 08:30 AM. Reason: Winding up the von machine
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Old Nov 17, 2012, 09:15 AM
Übung macht den Meister..
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Do I see correctly that you planked the tapered sections of the fuse? What adhesive did you use that allowed for even sanding?

That looks great! Waiting impatiently for the flight video!

James
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Old Nov 17, 2012, 09:24 AM
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Hooray for you, trying something really different. I suppose that an EDF power system would be too heavy at least in this size. You could cheat on wing area. I imagine that the real Natter was not required to turn tightly and mostly flew a trajectory.
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Old Nov 17, 2012, 10:33 AM
The Hun in the Sun
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Quote:
Originally Posted by Sopwith Mike View Post
Congratulations on an impressive build combining simplicity with boldness. I do hope it flies, perhaps on the same day that someone is flying a B17?
(That would keep Frank happy!)
C'mon Mike, I'm over that now.

That Natter looks positively fantastisch! No matter what happens, I suspect you will personally fair better than the world's first Natter pilot!
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Old Nov 17, 2012, 12:14 PM
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Quote:
Originally Posted by vonJaerschky View Post
That Natter looks positively fantastisch! No matter what happens, I suspect you will personally fair better than the world's first Natter pilot!
That much is certainly true.

This project is awesome! I'm all about the rare, uncommon and few built (if any) designs. I've always loved the Natter and am stoked to see you going after it with such finesse and gusto. I can't wait for the flight report.

Cheers,

Sperling
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Old Nov 18, 2012, 01:25 AM
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Very cool, how will it be launched?
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Old Nov 18, 2012, 02:41 AM
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Funny little plane. Will probably fly like a F-104.

Took a picture of one while visiting Deutsches Museum, München in 2011. Didn't get all of the nose. Look at the simple aiming device in front of the canopy !

It is partly replica and partly made of spares from the Ba-349 project. On the stabilizer the text says: Report founds to the "Kommandantur", adress, and then a promise of a reward ! Some of us write the same thing on our model airplanes.
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Old Nov 18, 2012, 07:47 AM
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It has a wing area of almost exactly 1sqft. At 12 oz all up that's also its wing loading!
Quite a modest figure.
The prop delivers close to 12oz thrust so the intent is to simply hand launch.
Early low power glide flights seemed to go well although roll control was poor.
The first attempt at a full power flight did not go too well!
A Bachem Natter crashes (0 min 37 sec)

The damage was severe but repairable.
What puzzled me was the complete loss of pitch control as it stalled. It was almost as if the tail was being dragged down when under power yet the thrust line is neutral, on the line of the CofG and centre of drag which of course the rocket motor on the full size would have to be.
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After much though it occurred to me that the airflow into a prop takes an inward radial path particularly when static or at slow speed. The prop runs very close to the tailplane trailing edge and when mounted at the fuselage centre line the majority of the prop disc is below the tail plane. The inward radial flow at slow speed would thus generate a significant tail plane down force directly proportional to the power.
Thus applying full power to recover from an approaching stall would simply make the situation worse, which seemed to agree with the video evidence.
The solution was to bring the prop centre line as close to the tail plane as possible.
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It looks odd with so much of the prop disc 'shaded' by the end of the fuselage but at this point stability rather than thrust was the primary concern.
The other worry was the lack of aileron power. At launch even with full right elevon it still rolled left from the motor torque.
Rather than add non scale ailerons a neat alternative was to use the rudders as additional ailerons - rudderons?
So another 3.7g servo was placed in the nose with pull/pull cables to the rudders but with one set crossed over and added to the aileron channel.
Nattter combined elevon/rudderon test (0 min 20 sec)

Only a small increase in weight it also moved the CofG a touch further forward as well.
This is the result. Not the easiest thing to fly (I very nearly crashed it) but got bolder as I gained confidence.
Bachem Ba349 Natter (5 min 14 sec)

It actually flies and even glides quite well.
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Old Nov 18, 2012, 09:57 AM
The Hun in the Sun
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That's great! I love the first "Touch & Go" in the last video.Just like you planned, right? Seriously, nice work getting a design like that to fly!
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Old Nov 18, 2012, 10:14 AM
Übung macht den Meister..
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United States, OR, Fairview
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Ausgezeichnet!

Now you need to find a paint scheme that makes it stand out from all the other Natters on the flight line!

While I tend to prefer balsa as a building medium, I see all the great work in Depron, and will have to try at least one subject with that...

James
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