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Bachem Natter


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It is realy strange, It was only this week I noticed the same feature, whilst browsing the David Myhra book.
 
I am not sure what you are asking, my own plan is to plank the lower part to increase the thickness.
 
I intend holding my fin to the body using wooden skewers cut down, so that half is in the body and the other halve through the fin and tailplane, with enough to go partly up the top part of the fin and white glue (PVA)..
 
The picture you have posted is excellent, where have you found it?
 
The more I have looked at the concept, the greater has been my admiration of the concept. Today we all take for granted the various rocket type missiles with their sophisticated electronics and sensors in conjunction with logic programmes. The Germans in particular could see the possibilities of the concepts, possibly being the first. Whereas the limitations of the electronics of that era, effectively stymied all the various programmes of missiles that the Germans were undertaking. The Bachem replaced the missing electronic packages with quite a sophisticated human device to undertake the roll of the electronics, saving the most sophisticated part of the missile system, the pilot after the mission was completed. I do believe it could have worked very well. For one, I thank God, it came to late, as I for one probably would not of existed (a little selfish, I know).
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Simon, looking at the defector vanes, which were positioned in the rocket efflux, I noticed that there were only two.
 
I then looked again and thought, were they connected to the elevators, by which they were driven.
 
I guess the idea of vectored thrust comes from rocket technology, rathere than being particularly new idea.
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I have found the reference to the Siemens K-12 servomotors. Hmmm do not what to make of it.
 
So I got out the William Green book, and was somewhat surprised to find that "Large by comparison with the wing, the tailplane contributed an important proportion of the total lift"
 
It is/was my intention to locate CG at 30%, I just wonder if the CG could go further back and maintainn stability. The question is how much further back, 33%, or more? I guess if I calculated the static margin, it may provide a clue, as to what may be possible.
 
Given that the wing was a NACA 0012 section, mounted along the longitudinal axis, it was not intended to glide. Or the glide being at a high speed, where no real lift was sought, just maximum velocity.
 
I was also surprised to read that the original intention was to install an ejector seat. Being omitted when it became apparent that there was insufficient room fro the spring activated device. I guess perhaps was the same unit fitted to the Heinkel 162 and Dornier 355?
 
Although the Myra book has little text, there is a lot of info, once you actually read it.
 
I see that those vanes only had ashort life before being burnt away.
 
Anyway I am back to the WG book to see if it has any clues as to what and how the servo motors were controlled.
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With a tailplane of that area , could you not treat the model as a tandem wing type?
 
The C.G. could perhaps then be back near the T.E. of the front wing.
 
How about making a small scale profile model and test it for gliding, altering the C.G. to get a stable glide, this should indicate where to put it on the " fullsize model ".
 
I believe Chris Golds does this to " prove " his models, before drawing his plans.
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Rex
 
I think you have a good idea there.
 
Simon
 
There are probably two reasons why the lower fin is quite chunky, compared to the upper fin. The first is that the rudder mechanism is in the thickened tail driving a torque rod, I guess because it has to go somewhere and that the jet efflux is out of bounds. The second and perhaps a bonus it helps provide a good base for the tailplane.
 
I note a number of differences in the two sources. WG recons that the "B" had a variable thrust motor Walther 509c, which increased the duration, without increasing the tankage. The new motor required a deeper body to the "A". Which would explain the more curvy nose.
 
WG also reckons that one "B" was flown by a pilot.
 
WG recons that there were 3 further man flights after the unfortunate Lothar. He also recons that there were 10 ready for launch at Kircheim nr Stuttgart, which were destroyed as the Americans came near.
 
WG reckons that max thickness was at 50%, which would not suggest a NACA 0012, although both agree on 12% thickness. Both agree on 11% thick tailplane.
 
I guess we will never know now, as most of those involved are probably no longer available.
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Although progress is slow, it is because I have other things to do. Not that you would think so, listening to my wife. The first major time consumer, is repairing my little glider, which I mentioned (extensively) in the "Weather's improving ...... thread. The other is my house painting and guttering.
 
I repaired the model quite quickly, then managed to dislodge the motor today when landing.
 
Anyway, the bench is once more clear, so it is back to the build.
 
Being an electric flier, who flew extensively thermal gliders, I am aware of weight, as an advantage and a detriment. Weight being the double edged sword. Lack of weight enables a model to fly well with little power, yet lack of weight prevents effective penetration in any sort of wind. Any glider guider will tell yo, there is no substitute for ballast in a wind, if you are to avoid going down like a lift, or worse still, backwards, when down trim is applied to move forward, particularly if down wind.
 
So I am actively both seeking a CG balance, whilst achieving a low overall weight, yet I am conscious I need some weight. I have been balancing using CG gauge, which is unfortunately a fraction to narrow, for an ideal fit, but just useable.
 

I have inserted a Lipo in the nose and strapped all the bits onto the model, which includes, an ESC, 2* servos, RX. Also you can see a Matchbox bus, which is the approximate weight of motor, propeller assembly and a corgi car at the nose, to estimate what balance weight may be required to achieve a positive nose down balance. less than 24g.

I have also being experimenting with model tissue paper and tissue paper from the local "Card Shop". Although no good for doping the Card Shop tissue paper is far better for the blue foam sheeving, not taking up any significant glue, yet adhering really well, it peel strength is far better than Modelspan. It should also have the advantage of not taking up much paint. Modelspan is far better for the wings however where some porosity is wanted,
 
I will be endeavoring to loose a little tail weight, which should help CG management. I think the wing loading may be OK, Yet I suspect that such a low aspect ratio wing, will not glide well, or glide at all. With such a large body drag could well be very high, although the "cd" could be good, that cross sectional area, well...????????

Edited By Erfolg on 12/06/2011 13:13:02

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I have noticed that myself and others, within our club have found that in windy weather that if you allow the model to get down wind, with low power, that regaining the field can be an issue. This particularly so with light aircraft or one with a lot of built in drag, such as a DF.
 
As I suspect that Bachen will have lots of drag, even putting the nose down to penetrate or keeping airspeed up, the glide angle will not be 1 in 15, I certainly do not want it to be zilch, just coming down, or as I have seen with a delta, going backwards without power.
 
Unlike a delta, I expect the Bachem to stall, in a conventional way.
 
So for m there is a balance between weight, duration and the weather that the model can realistically manage.
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I have spent a little time hot wiring out part of the back end.
 
I will shortly stick the halves together and then leave foe a day or two. I have a lot of work to do. As I am trying to do the house painting and on Wednesday a plaster is coming to render and paint the bay.
 

I have lost a few grams down to 175, or 6 oz, just a fraction. But back end weight matters a lot.

Edited By Erfolg on 13/06/2011 15:59:44

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I have been making some progress, though I guess I am a long way behind Simon. Although my model may appear to be near completion, in the case of a relatively conventional build, still some way.
 
I was thinking about a comment of Simon with respect to vertical launch. I came to the conclusion unlike a tractor propeller, a pusher model will not prop hang at zero speed. That is because a tractor model has its CG below the propulsive force, and effectively hangs as a pendulum. Whereas in the case of a pusher the pendulum forces will cause the model to topple, unless some stabilising forces are applied to keep the model axis as required.
 
On that basis it would seem that the model will need to be flying at a speed where the control surface are really effective.
 
On that basis I have done a few simple calcs to approximate the minimium pole height that would be necessary. Using what approximate data I have to hand.
 
I used the classic F=ma rearranged to a = F/m
 
using 745 grams thrust from here and estimating my total weight to be 600 grams
then available force is 745-600= 145g
 
a = 145 * 9.81 * 1000/ 1000*600 ( I left the 1000 in to show I had converted to Kg).
 
a = 2.37ms^-2
 
again using those GSE physics mechanics formula (yet again just shows that physics is not wasted)
 
v^2 = U^2 + 2as
 
re-arranging, in terms of distance
 
s = V^2 -U^2/ 2a where U = 0 as it is stationary and 20 miles per hour or 9 ms^-1 is estimated as the minimum speed for effective control surface use.
 
s = 9^2/ 2*2.37
 
s = 17m
 
that is one long pole, about 17 yards. My model will never take of vertically , without a couple of Rapier motors strapped to the side, even if I hit my target weight and thrash the motor.
 
I suspect Simon will have similar issues, although he may only need a 7m pole
 

 
 
 
 

Edited By Erfolg on 16/06/2011 14:31:08

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Coming near to completion I had another browse through the David Myra, book on the Natter. Surprisingly, after having read it many times over years, and it being a pretty small book, I found something very interesting.
 
I had postulated that it might be possible or even desirable to have a rearward CG on the model, as the Tailplane is so large relative to the wing. On Page 62, there is a sketch/drawing of BP 20-07 for the production of a wind tunnel model.
 
The sketch has marked the "Momenkenhezugspunkt" which I take to be the CG, we probably need Vecchio to translate properly. I just recognise punkt, as spot, kenhe, which I take to be kenne, as "to know". I guess it is either technical or dialect, maybe. Anyway this point is 180 from the leading edge, with a chord of 400. Which is 45% point. I guess it was a 1:10 model.
 
It is also interesting that the tailplane was set high on the body. So the reference to the tailplane beibg raised is almost certainly from this wind-tunnel model.
 
I have had some difficulties with covering the tailplane, which has caused me to cover the wing with glass cloth. I have used WBV, this time Ronseal Diamond Hard. To date it seems no different from Wickes WBV, just about 60% more expensive. Although Poly "C" is 300% more expensive. Both Wickes and Ronseal lok very similar, being milky white in colour, similar viscosity, seem to dry in similar times, seem equally as hard, flexible etc., certainly in modelling observable terms. I will attempt to evaluate further, in terms of modelling. Even the trade WBV seem pretty much the same as each other, although I would guess that they must have something different at the scientific level and application as a floor finish. As original varnish I was given, made by Ronseal was something like £60, and the Aski, not a clue, but a large container (only a small amount left). All were the same milky white.
 

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According to the on-line translators "centre of gravity" is "Schwerpunkt" or "Massenschwerpunkt".
 
However
Google gives "zugspunkt" as "Datum" and as you say "kenne" is "know".
"Momente" is "moments"
 
So "Momenkenhezugspunkt" equates to "moments know datum" which is a fair description of the centre of gravity!

The Natter description in Wikipedia says the CofG with the Schmidding boosters in place was at 65% returning to 34% when they were released.

 
I presume that with its symmetrical wing and tail sections and the incidence and thrust lines all at zero the Natter retained "arrow" stability (which is all you need for vertical flight) even with the CofG that far back.
 
And your Natter its looking good by the way.
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Some of the word did throw me a little, although I felt I knew the meaning, but could not explain it. If you see what I mean. Such as Zug, which I take at gut feeling level as , zug, as to pull, I did take momente, as in taking moments.
 
I guess in many ways that aerodynamically it is a very good design. The wings being at 90 degrees to fuselage, removing the need for fairings, keeping the thrust of the motor in line with the centre line. The fly in the ointment being the raised tailplane. I guess that the Miles M52, took the idea to the absolute perfection.
 
I think I will be starting with 30%, for the CG. it seems, more cautious, the first flight can be traumatic enough.
 
I think I will be delayed, as the motor has not been shipped yet. Still there is enough to do,but would like to be better placed to access the as built CG.
 
 
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Hi Simon
 
It is on hold at present.
 
I have been busy changing an aerial on the roof, as trees have grown near to the house, in wet, windy or both, the signal breaks up now, that is since we went to digital. I used to be able to see Winter Hill from the garden, now I cannot see it from the roof because of the trees. So the aerial is pointing some 30 to 40 degrees away from the transmitter to get a mostly solid picture.
 
Whilst doing that, I discovered that the cement flashing on one of the chimneys needed major repair, plus both the chimneys needed a lot of pointing.
 
Not planned, I dislike heights such as that,
 
I was originally replacing the old plastic gutters with new plastic gutters, and painting the fascia boards. I would have liked to put capping boards over the old fascias, but the tile overhang is insufficient, without danger of rain getting at the back, rotting out the wood fascia. Although it would have been easier and quicker than painting.
 
Hope to finish the guttering tomorrow, Saturday
 
I also have had a disaster with a newly complete model, the nose ripped of, whilst running it up, as part of commissioning, ready for a Sunday test flight.
 
So I am hoping that I will be back to modelling come Monday.
 
All though my wife now wants the drive and parking area, relaying. The problem is there is a +6" concrete base, total about 10", under it, Built by Farther and myself, who was in the Royal Engineers and believed in mass,
 
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Errfolg
With such small aileron power motor torque is a serious problem.
After 4 short flights I have temporarily grounded mine to prevent further damage until I have a torque free motor installation.
 
With a higher wing loading yours may be better but the fundamental lack of aileron power will remain.
 
If you can mix in aileron with the throttle you might be ok but I had to set up my linkage so that it needed full left trim for neutral elevons and then use full right trim power on, but it uses no less than 20% of the available movement. Of course any compensation is speed sensitive so too slow and it rolls to the left no matter what.

The problem is compounded by the Natters minimum lateral stability and rapid roll rate. If you don't get the roll/power trim change more or less right things happen pretty quick.
 
In hindsight to improve aileron power (and keep to scale) I could have used the rudders as additional ailerons or cheated a bit and put on wing ailerons as well.
 
In aerodynamic terms it would be better to add a wing tip weight equal to half the motor torque but the same overall power trim change would still exist.
 
The best solution however must be to adopt what the original had - no motor torque!
 
 
 
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Hi Simon
 
Bit of a bummer, the control issue.
 
I cannot see counter rotating propeller being viable on my model, as the tail weight issue, would require a significant additional nose weight, resulting in even higher wing loading.
 
It seems probable that my initial guess that success could be difficult to achieve.
 
Having crashed another model has me thinking about effective repairs, and what I think is the control issue. Is the wing stalling because of design, and should it end in a crash when at least 30 feet up, or is the tail design incapable of regaining control after the stall?
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  • 4 weeks later...
Whilst waiting for motor for the Natter,I started to question some of the decisions with respect the wing area.
 
The bigger wing had rather compromised the look. I decided that the body needed lengthening to regain feel, although this would further compromise the scale.
 
It seemed real easy, mark around the body a line for cutting, cut, and slip in a new section.
 
After doing the simple task, I discovered a whole raft of issues. It had to be taken apart. The tail plane removed and realign.
 
The start of the operation involves my build jig, which I had made for conventional fuselages. It has come in for this blue fuselage, rebuild.
 
I am learning a bit, little, by little how to use Blue Foam beyond wing tips etc.
 

I guess i am making some small progress again.
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Hi Erfolg
Nice to see your Natter making progress again.

By conventional standards the Natter tailplane is actually quite large in comparison to the wing so elevator authority should not be too much of a problem.
 
Where there might be an issue is what happens when the tailplane enters the wing down wash and possibly the wing tip vortices as well.
 
It is possible a model Natter could have a steep stall with a dramatic nose drop that will require some height to recover from.
 
We shall see!
 

 

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I have had plenty of food for thought, Simons experiences have caused me to conclude that as a RC Model, the Natter is far more challenging than i had thought. I was not optimistic to start with.
 
One of the fundamental issues is that if a motor is placed at the tail, there are issues in achieving an acceptable CG. Simon had suggested a motor at the front driving a rear propeller via a drive shaft. I do agree conceptually this is probably the best solution. I am now thinking, would this be practical using a model boat drive shaft?
 
For the moment I have dug out two old Futaba S128 servos, as they are heavy. The idea instead of lead in the nose, the servos would provide ballast, located about the cockpit area. This is a total contrast to what I consider the norm, using 9g servos. For this model I initially was thinking of 5g to drive the tailplanes. With electrics, it is my opinion that weight above a certain wing loading is a major enemy
 
If Simon's issues do prove to be the result of the principal masses being placed in the extreme nose and the extreme rear, I think it will have to be the drive shaft solution. But how to engineer it?
 
Now this is when I could do with BEB help, would a 3mm shaft reach the first critical whirling speed. Old age has dimmed my memory, to the extent that I have forgot about the principals, other than there will be a Sine(omega?) as there always is with a wave form approximation. Will I need to go into the loft, am I able to actually climb up there, never mind find what I want, and then make sense of the subject? Oh, problems, problems and yet more problems
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Definatly need BEB, I have done a calc, using a formula found on the internet, which indicates the first critical speed for a 3mm dia shaft 750mm long is 38 revs min-1. So I guess i cannot even use a calculator now, as the speed of rotation is so low.
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I have received my two motors today.
 
I will make a physical trial to establish If I can get a remotely acceptable CG, with a viable wing loading. In some respects, I have a view, that even if I do not have much confidence, I should throw it together, and give it a go, in hope.
 
I have re-examined the whirling speed issue, it seems I really do need a much larger dia shaft than 3mm if whirling is to be avoided, so in principal I need a tube if I were to go this way. I guess it would need to be CF., with metal fittings (aluminum), as a solution, very messy.
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Erfolg
That whirling equation does look rather complicated but your result sounds about right to me.
 
To prevent whirl on such a long shaft you would, as you say, have to adopt the principles of an auto motive prop shaft with a large diameter tube over most of its length and of course a tail bearing.
Messy? maybe but interesting to engineer.
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