Another Bachem Natter!

[Simon Chaddock]

First I must apologise to Erflog for stealing his idea but he did say the Natter was a challenging design to model and I can't resist a challenge.

 

The Ba349 Natter had a seriously small wing compared to the rest of the airframe.

A quick check of my 'easy to hand launch' planes shows they have wing loadings between 5 and 9 oz/sqft.

So the task I set myself was to build a scale Natter with a loading of no more than 10 oz/sqft or in other words it would have to be very, very light.

Flying relatively slowly should mean the drag of that big fuselage would still be significant but hopefully not catastrophic.

The cruciform tail lends it self to a pusher prop.

 

I settled on a 26" span. Sounds small but its nearly 1/5th scale!

The wing area is exactly 1 sqft so the target weight is 10oz.

 

My Photosuite graphics package allows scaled and tiled printing so with a bit of trial and error I printed out the above 3 view to a 26" span.

As expected the fuselage is vast but it still comes as a bit of shock.

That's a 24" rule.

The fuselage is no less than 6" wide and 8" deep. There will be plenty of room inside that.

 

The wing on the other hand is small & simple. The original had a symmetrical section, a single spar and just 5 ribs per side.

Sticking to what I know this is going to be another Depron build.

To save weight my wing spar is tapered width-wise towards the tip and is 0.7" deep. The flanges are 1/32 (0.8mm) balsa over a Depron core.

The wing will be built in two halves and slotted into the completed fuselage unlike the original where the fuselage was actually built up around a completed wing.

The ultimate would be to install a thrust of over twice its weight and launch vertically up a ramp!

 

 

[Biggles' Elder Brother - Moderator]

OMG - They're breeeding!

 

Seriously, looks interesting Simon.

 

BEB

[Tom Wright 2]

Simon.

Nice approach to this unusual project, the low wing loading and thoughts about roll control sensitivity should ensure a successful maiden ,hope Erflog doesn't mind me saying that his well researched build may come out with a higher wing loading,so both projects will be followed with great interest.

Tom.

[Erfolg]

Simon

 

I did contemplate using Depron, which I think you are using. There were thtree reasons why I have not gone your way.

 

The first is lack of experience of using Depron.

The second I had concerns on durability and ability to fly in wind speeds of about 10 mph, the UK norm.

The third reason, is I have no Depron, but do have "Blue Foam".

 

I will be watching your model with great interest. I could well build a replica, if my own model does disappoint, and even if a Depron model is limited to wind speed?

 

Have you considered a Douglas X3? I have been seriously considering this in Depron, But is it do able?

 

 

Edited By Erfolg on 27/05/2011 17:04:46

[Simon Chaddock]

Erflog

Beside the X3 the Natter looks positively chunky!

 

In Depron? The wings not really. Most of the fuselage possibly, but the long pointy nose much beyond the nose wheel would have to be in something rather stiffer.

 

I am sure it would fail the BFMA "nose radius" requirements!

 

As it had two small J34 turbojets the fuselage shape is not ideal for a single EDF fan.

I understand the X3 was rather a disappointment for despite its racy looks (and a take off speed of nearly 300mph!) it was only able to exceed Mach 1 in a dive but did nevertheless provide quite a bit of data put to good use in the design of the "century" series of jet fighters.

 

[Simon Chaddock]

The LH wing.

Initially the 3mm Depron ribs have no shape at all and the underside is completely flat, with the leading edge strip glued in place at the mid point.

When dry the wing is removed and the rib undersides very carefully sanded to shape by eye.

The 2mm Depron under surface is added. When dry this provides sufficient support to to sand the rib upper surface to shape giving a symmetrical section and the top surface is added.

The hollow Depron leading edge section is hand formed around a length of small diameter tube.

The hollow wing tip is built up from 6mm Depron sheet and sanded to shape.

The complete wing half.

It weighs just under 1/2oz yet it will easily support a 10oz weight (the expected weight of the complete plane) at its tip.

 

I had to order the 2mm Depron through the post which limits the maximum size but the Natter wings are so small it was not a problem!

 

The fuselage however will be built from thicker 3mm Depron to limit the number of big formers needed to support it.

 

My guesstimate of the weights.

Wing 1oz

Motor 1oz

Battery(3S 1000mAh) 3oz

Radio, servos, ESC 1oz

Fuselage and tail 4oz

 

[Erfolg]

I have just noted that the tailplane of my model is 15" span, and weights in at 68g which is about 2.4 oz, so the weight saving is about a factor of 5. This pretty much confirms that the weight saving of 500% is worth having.

 

I am convinced that Depron on EPP is the way to go. Such a great pity that it is not more freely available, at sensible prices.

 

I take it the big packs of Depron like material that some were buying from B&Q are no longer available.

 

I do note that your progress is much faster. Your model is bang on scale. Which has the added advantage of not having the interminable debates of what should be stretches, by how much, will it be obvious, will the changes achieve what is desired.

 

Keep it up, best wishes.

 

Erfolg

 

[Simon Chaddock]

Erflog

I am fortunate that I do not live too far away from Steve Webb Models who stock 6mm and 3mm Depron in big sheets and UHU POR to go with it.

 

The last time I was there however I felt a bit cheap buying just 1 sheet and a tube of POR for £5.40 when another customer was buying a Flair Tigermoth kit and engine for over £500.

 

My assistant enquired "Is that all?"

His assistant offered to carry the kit to his car.

Such is life!

 

With a 15" tailplane your Natter is almost exactly the same scale as mine although it is somewhat lighter at only 13.5g.

 

Have you come across this by David Myhra. Its a big pdf file with 130 pages but it must have just about every picture ever taken of the Natter.

It is well worth a read even if you are not actually building one!

[Erfolg]

Simon

 

It is one of books I have

 

The other book, with a section on the Natter, of significant content is War Planes of the Third Reich, which at $25 is incredible value, as it it a very good reference works.

 

Perhaps surprisingly, when the two books are read with Wikipedia entry and the contributions from I think it is ? Bonds, in my thread, you get a very good feel, with respect the RLM policy, the politics and who were the players in the story.

 

With respect to scale, I have pretty much lost track now, with respect to my own, I have compromised so much.

[Simon Chaddock]

The tailplane under construction.

A similar tapered spar to the wing but with thinner 2mm ribs.

Note the oversize elevons as the glider test flying had identified poor lateral control.

Not a good characteristic for a slow flying model!

I would like to use this colour scheme on this full size replica of one of the prototypes.

Good for visibility!

The lettering on the tailplane is also genuine and gives the contact details if found!

Edited By Simon Chaddock on 29/05/2011 18:22:20

[Erfolg]

I guess the idea of the address was pinched from free flight model aircraft.

 

I was also thinking of the same colour scheme, which is OK for me as I am semi-scale, if you use it you would need the square under fin

[Simon Chaddock]

Erflog

Yes I am building an "A" version with the smaller under fin and the oversize control surfaces rather than one of the M protoypes as above.

 

As it is intended only to be a 'demonstration of priciple' model rather than 'competition scale' I am happy that the colour scheme would be 'typical' (and practical) rather than absolutely accurate.

[Simon Chaddock]

Made a start on the fuselage formers.

There will be 12 formers in total, the above are the five that make up the nose section from the cockpit bulkhead.

With a bit of thought I managed to cut them all from the 2 biggest ones.

My intention is to build the fuselage up from a full length balsa reinforced keel beam.

 

[Simon Chaddock]

 

The full fuselage former stack.

I have increased the number slightly to 16.

All have their centres cut out except the first and last.

 

I fear actually building this fuselage is not going to be that easy!

Edited By Tim Mackey - Administrator on 31/05/2011 23:19:42

[Erfolg]

Simon

 

I did not notice the "r". Shows you often see what you expect to see. When i read the post, I thought it must have been me, as I often catch the wrong key, or hit the wrong key and never notice, just seeing the letter I expect to see.

 

Have you considered the natural CG position for your own model?

[Simon Chaddock]

Tim

Thanks for removing the r

 

One advantage of the super light construction is that the airframe itself only has a modest impact on the moments diagram.

In my case the calculation of the heavy bits looks like this.

 

80g LiPo 28cm ahead of the spar. (2240 gcm)

25g motor, 10g prop & 2x3.7g servos 42cm behind the spar. (2247 gcm)

 

The bare Depron airframe will be a bit tail heavy (possibly 1500gcm) but this can be countered by mounting the radio and ESC up to 20cm ahead of the spar.

 

There is space to move the LiPo forward by further 10cm if need be (and/or put in a bigger LiPo!).

 

To make life easier I intend to build the fuselage in 3 pieces and only glue them together when every thing but the battery is installed.

Edited By Simon Chaddock on 01/06/2011 16:37:52

[Erfolg]

Do not dispute the benefits of light construction.

 

In my case it seems the motor and propeller system is the issue. Although an extension shaft would help the motor issue, the propeller, collet and drive shaft remain. I also suspect if you are not careful, the drive shaft system introduces weight and a complication with its own issues.

 

I am pleased that you will have no problem with the CG, he says through gritted teeth (I am joking).

 

I will certainly have to consider another motor.

[Simon Chaddock]

The fuselage centre section under construction.

The middle two formers have thickened lower sections to reinforce the area for hand launching.

To keep things straight it is held vertical for planking.

I must thank Erflog for raising the CofG issue as it showed that the battery, ESC and radio will have to be a bit further forward than my initial guess.

The result is this huge centre section is likely to end up completely empty apart from the motor and servo cables running through it!

 

[Erfolg]

Simon, you mention the servos and the drive to them, it seems we are having similar thoughts.

 

I initially thought that the servos could be placed just under the elevators. I then came to believe that there was insufficient space for the push rods. When the CG issue raised its head, I started to think that snakes to the servos, mounted well forward could help with CG management.

 

It is always reassuring when others come to a similar solution.

[Simon Chaddock]

After reading Erflog's blog again I have just realised I am building a Ba349B, the MkII version that had a deeper more curved fuselage profile.

The mid section complete.

There is not a lot inside!

It feels massive at 12" long, 6.2" wide and 7.9" deep. Amazingly rigid yet it only weighs 0.9oz. The wing will be slotted in next which will not be too difficult as you can easily get your hand inside.

 

The nose and tail sections should be about the same weight so the 10oz all up target still looks possible.

[Erfolg]

Simon

 

I am worrying less and less about scale, particular the detail.

 

The more I read and more pictures I have seen, there seems a lot of differences, from the drawings I have. I guess that a lot of detail changes were made to improve production simplicity during the manufacture of a so called model sub type. Only where some significant changes occurred was a new sub type recorded.

 

I am thinking particularly in that from "a" to "b" it grew supposedly by 300mm, the drawings do not seem to show it. Also the elevator supposedly was raised during development, to me not apparent in the drawings. I have seen a picture where the rudders are steeply raked around the outlet for the rocket motor, yet not all "b"'s have this feature.

 

I suspect the deteriorating position of Germany and the desperate rush to change the situation, that full detail records with all developments and thinking was not undertaken. Although much of the story is from interviews of the people involved, I think we all know our memories are not always as good as we would like.

 

Yours is looking good.

[Simon Chaddock]

The wings fitted - if you can call them that!

The spar joined at the centre with balsa fish plates top and bottom.

It now weighs 1.8oz.

At this stage I usually 'test' the wings lifting capability by swinging it round at arms length. I appreciate that in this case the open centre section is likely to create more drag than the completed fuselage but I doubt the glide angle will be any better 2:1 or 30 degrees!

Edited By Simon Chaddock on 03/06/2011 20:32:14

[Erfolg]

Ohhh Noooo, you are ahead of me.

 

I have started covering the fins with tissue.

 

I am thinking of slotting my wing through the centre as you have, but bolting it onto a brace.

 

I am agonising over where my accessories will go.

 

Have you a set method of where and how to locate them?

[Simon Chaddock]

Ahead?

Not really, I still have the fuselage nose and tail sections to make (lots of planking). And the fins. And finish the tailplane. And I haven't even got the battery, radio or ESC yet!

[Simon Chaddock]

A start on the fuselage tail section.

The bottom former is a dummy used only for construction. It will be almost completely cut away prior to glueing the two sections together.

Unlike the centre section, which was substantially parallel, this one is rather slow going as each plank has to be accurately shaped to fit against it neighbour.

 

Initially I will arrange the accessories so the CofG is at the spar but will make the battery compartment big enough to allow further adjustment after flight testing.