Bachem Natter

[Erfolg]

I agree with the undercambered wing section comments.

 

I did try a few models with undercamber, perhaps the Flair Sunrise 100 being the best known. The reality is they are a complete waste of time, unless, just floating is the aim. For a glider where you need to seek lift, or move to known lift, they are outclassed by more commonly found sections.

 

With respect to the Bachem, progress will continue, I have many doubts, with respect to many things. It is intersting that the cross sectional area, is about the same as my Ta 152

 

It is the wing area that is the issue and getting the wing loading to an accepatbel/viable level. As I find that for some reason my Cloud model 163,just completed, will not go to date, and the wing loading is quite low at 16 oz/ft^2.

[Simon Chaddock]

I just couldn't resit this.

 

After the war Erich Bachem teamed up with Edwin Hymer to produce the now very famous Hymer caravans and campers.

Indeed one of the models is called the Eriba Hymer after ERIch BAchem)!

[Simon Chaddock]

Erflog

Any progress?

[Erfolg]

Hi Simon

 

I am 3/4 of the way through the tailplane, I will post a picture later, after I have taken it.

 

I have involved my self in an investigation in the end play with snakes under and without load. That will b a temporary brake, or slowing up whilst i re-undertake the tests, but with a bit more diligence, than my first quick and cheerful test. My trouble is designing a half decent experimental process, without lots of effort and points where arguments may occur.

 

I guess my next Bachem issue is how to drive the elevators, which will also take some thinking in my case.

[Erfolg]

As promised I have just nipped upstairs to take a few snaps.

 

The picture is of the tailplane to-date, it is Naca 0015, I tried Naca 006 and 0010 on Profi, just a little thin to be as strong as I would like it. I have used the same method i used on +120" Open Class Barcs league gliders. The way to do it was pinched, as most, if not all my methods and knowledge for that matter.

 

The following picture is a rough ensemble to get the feel of the thing.

 

I have part made the under fin.

I now need to make the fin and top skin the tailplane

[Simon Chaddock]

Erflog

Very nice!

Glad you are making progress. I didn't want to catch you up, I am still on the wings.

[Erfolg]

I have just finished the Fin.

 

So all the main parts are now completed.

 

A little while ago I made an appeal to those who have used Blue foam in making large assemblies, as how to increase strength (as well as carve/fabricate) of foam. To date I have had no replies. So I am trying again, who has experience, be it good or negative (I can learn from others difficulties).

 

I have covered blue foam tips previously with light weight glass cloth and WPV. I am reluctant to do the same, with a model which I suspect will need a lot of modification and possibly remade parts.

 

Model below has glassed tips, and under belly (with no glass) solar film over skin as it were. It works well.

 

I am thinking of using heavy weight or lightweight tissue paper, PVA pasted on, then WVP top coat, before painting. Has any one any experience, thoughts?

 

 

Edited By Erfolg on 30/05/2011 13:21:42

[Simon Chaddock]

Is not brown paper over blue foam a recognised technique?

 

I am sure tissue would work as well and although it might be lighter its rather porous nature might mean the extra PVA weight negates any advantage.

 

How much power do you think it will need?

[Erfolg]

Simon

 

I have read that to, but I do not think I can stand the weight of brown paper. Like you I am concerned about weight, yet want a robust structure.

 

I know glass cloth works quite well, it is the ease of mods and repairs I am looking for.

 

I was re-reading about the glide tests of the full size, where it was described as highly controllable, but glided like a brick. Some how the two do not seem that compatible, but then again I guess the Shuttle was similar,

 

[Tom Wright 2]

Posted by Erfolg on 30/05/2011 13:20:42:

 

 

 

A little while ago I made an appeal to those who have used Blue foam in making large assemblies, as how to increase strength (as well as carve/fabricate) of foam. To date I have had no replies. So I am trying again, who has experience, be it good or negative (I can learn from others difficulties).

 

The fus i would have fashioned in a similar way .....rough saw cut and sanded to final shape ,and then cut in half and use a simple hot wire tool made from a trigger iron to scoop out the interior leaving a wall thickness of 1/4 ". the two shells are glued back together after consideration is given for hatches ,cable runs servo placement etc .

The wings could have been hot wire cut or just sanded to the desired section,at such a short span i doubt if any further strengthening would be needed.

Tail and rudder 6mm depron is light and easy to work ,or blue foam in the same thickness will do with no additional added structure,

This approach is simple cheap and quick ,and should result in a reasonable weight .

I would test fly uncovered as mods and repairs are so simple at this stage.

The wings tail rudder can be covered with solar film ,with or without a 1mm balsa capping .

The fus with all those curves ,tissue and epoxy would probably be the way to go.

Don't know if any of that is helpful ,but i find the methods used very much depends on the type of model .

My 5ft span twin boom design is made entirely of 1" thick blue foam as the strength to weight ratio falls just right ,and no hot wire work is needed.I Have done other designs using the same principles and used solar tex applied directly to the foam just useing a balsa l/e t/e as the anchor and overlap points.

 

I hope you do see this one through as the difficulty is the sort of challenge that keeps us going .

 

Tom.

 

[Erfolg]

I have just done a moments calculation for all the bits, to estimate where the battery should go. Unless my calcs are wrong, I have a problem, the model will be hopelessly tail heavy.

 

I am now trying to think of a way out, without putting the motor in the nose.

 

Initially I was pleased as all the bits, including Lipo came out at 480g, with some opportunity at a little weight loss.

 

Now?

[Simon Chaddock]

Erflog

Sorry to hear about the CofG issue.

 

The engineering solution would be to go 'mid engined' with a long extension shaft!

 

Could the battery go right in the nose if it were glass skinned and hollowed out?

 

 

 

[Tom Wright 2]

Erflog.

A Picture of the converted trigger iron used for hollowing out as suggested in my previous post made in response to your request.

 

The wire shape shown is for recessing u/c wooden rails.

 

Edited By tom wright 2 on 31/05/2011 23:43:56

[Erfolg]

I thought a little about my present predicamant, may interest.

 

 

To help solve the problem I have set up a spreadsheet to enable some of the numbers to be adjusted to see the impact on true CG or what is needed to get to the wanted CG.

[Erfolg]

I thought that my predicament and how I am going about evaluating the issue with an aim of finding an acceptable solution, may interest some.

 

I have set up a spreadsheet with the relationships inserted, so that I can play with the values, to see the impact on the CG.

 

 

Please double click on image to see it, with a little detail.

the basic relationship, is established by taking moments about the CG.

 

126x = (40*383) + (327 *87) + (71*405)

126x = 15,320 + 28,449+28,755

126x = 72,524

x = 72,524/126

x = 576mm

 

As the body to front from the CG is only 325mm, positioning the Lipo is an issue.

 

I guess the calc sent you all back to GCE Physics? Then you thought, what earthly use has this got to do with real life, now we know.

 

I have investigated removing material internally from the back end of the fuz. to get the CG of whole body and model to be coincident, of may be it will be possible to move the CG slightly forward. But the impact is very little on the positioning of the Lipo.

 

I am now considering a smaller different motor as it is this item which is having a big impact on the CG.

 

I am also looking to having a lead nose, but do feel that needs to be covered with impact absorbing material.

 

So many options, and such indecision.

 

Can invigilator remove the previous post, something has gone wrong, sorry

Edited By Erfolg on 01/06/2011 15:02:13

Edited By Erfolg on 01/06/2011 15:02:46

[Myron Beaumont]

Erfolg

Fly it backwards ?

[Erfolg]

This is the motor under consideration the CF2822 which is rated up to 14 amps, providing 700g thrust, not enough for vertical performance perhaps, Though the estimated weight could be 600g, if efforts to contain and reduce weight is successful.

Rather than the motor ordered Turnigy 2217 rated at 18 amps.

[Simon Chaddock]

A good motor (and cheap too).

 

I used 2 on my Hamilcar and got about 500g thrust each with a 9x3.8 at 12amps on a 2200mAh 3s. They were happy with a 10x3.8 but I simply did not need the extra thrust.

 

I have also used the 1600kV version which is rated a bit higher and can turn an 8x6 ok if you want thrust at a higher flight speed.

[Tom Wright 2]

Re c/g chop the nose off and add some on ,no one will notice lol .

[Erfolg]

I had noticed that the "B" version had 300mm length added to the body, from reading the story of the development , it is not clear where. The extra duration, suggests bigger tanks, which probably go into lengthening the front area of the body. Though looking at the drawings it is not obvious where, as they all look the same in length, perhaps the draughtsman used a bit of license when drawing the three versions.

 

On that basis you are probably correct, Tom

[Simon Chaddock]

Erflog

The low tailplane appeared to be used on the early glider versions and was carried by one of the fuselage formers as shown in the Bachem "Jordanoff" cutaways.

 

I would include pictures but they are copyright marked.

 

I wonder if it was moved up to give more clearance for the rocket jet pipe.

 

I suspect you are correct that by March/April 1945 accurate documentation of modifications was not a priority.

Edited By Simon Chaddock on 03/06/2011 21:05:39

[Erfolg]

According to the blurb, the tailplane was raised as wing wake caused problems at speed. If I remember correctly verified in the wind tunnel.

 

The aircraft was cleared up to Mach 0.95, this number has reoccurred with other aircraft, so I wonder if it was a wind tunnel limitation. Although again if I remember correctly the 163 was also verified up to this speed, yet actual high speed tests, indicated that the aircraft was at its limits at this speed.

[Erfolg]

It probably seems I have not been doing anything, I have done something however, besides thinking about problems, those mainly of weight against strength.

 

But the real problem has been repairing my cow stomped model, removing the existing gutters on the house, painting the fascia boards and fixing up the new gutters. It seems strange that it easier to take of the old, and replace, than rubbing down the existing gutters and then painting. Which made me think, that I was a slave to convention, that I painted the original gutters, because that what was convention on cast iron etc. I am so disappointied in myself.

 

I know I need to run cables from the very back of the model to the front for the motor cables. I also know I need a recess to take the lipo, which needs to be removable and ideally taking a number of different size of Lipo envelope.

 

To cater for these things I set about grinding out the interior with my Dremel clone and an abrasive drum sanding wheel. I have done this previously on other small items, but not on this scale. When the wheel wears, it more melts than sands the material. On the upside it comes out quick. I used a pair of outside calipers to measure the shell thickness and to maintain a reasonable constant wall of no more than 1".

 

The present weight of the shells is 175g (approx, 6oz) combined, which is more I am sure than Simons. So the debate still goes on in my head, should I reduce the weight a little more. Or is a higher structural integrity better?

 

It seems that the original target weight is still doable of 600g or about 20 oz. Which if my memory serves me well, is somewhere between 13 -15 oz/ft^2. Which if achieved is not to bad, other than the model wing area is small. So probably will need to fly quick.

 

My debate is if I reduce the weight a bit, will it actually make much difference to the wing loading? May be ruggedness is more important?

 

I have also cut out one wing root, so that the wing can be slid across the fuselage, This is the same process as both Bachem and Simon.

 

Before joining and when I have finished this process I will take a few photos

Edited By Erfolg on 08/06/2011 20:21:33

[Erfolg]

Having made a bit of a hash of the first shell root, I tried a different method on the second. Ah well, you learn by your mistakes, just seem a bit old to be still be learning, some simple things.

 

Anyway, the first pictute is of the two shells.

 

The second is with the wing inserted and the other bits lying around.

 

The fins have been covered with tissue, which does seem to stiffen up the materials, at virtually no additional weight.

My next task is to try and evaluate where the CG will be by taping all the bits onto anticipated locations, before joining the shell halves, in case I need or can loose some weight to avoid ballasting.

I may need to wait for the motor, propeller etc. before more progress can be safely made.

 

[Simon Chaddock]

Erfolg

Perhaps you might be able to confirm this.

I am in the process of building the upper fin. The bit above the tailplane is no problem, I have both its plan and section.

The small bit between the tailplane and the fuselage is different. I know its plan but it appears to of a much thicker section (about twice) than the rest of the fin.

 

This picture of the elevon controlled jet pipe vanes is the only one I can find that seems to show it.

If this is the case I presume it was done to ensure adequate support to the tailplane when it was raised above the fuselage.

Edited By Simon Chaddock on 09/06/2011 21:10:18