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Grob 103 Twin Astir, Scratch build, Vacuum forming


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Nearly 4 years ago I started an attempt at scratch building a Short Sperrin (scale 1/72) using CAD modelling, my CNC-machine, 3D printing and Vacuum forming. I got as far as a nearly finished CAD model and a few moulds made in the CNC-machine, but life got in the way, so the project ground to a halt.


I recently felt the urge to take it up again but I quickly realised what a big project the Sperrin is regarding the number of moulds required (at least 18), so I decided to pause it again.

 

Instead, I am doing something much smaller and more simple but using the same techniques.

 

I have always had the ambition to model the most important planes of my real world aviation career, so I thought why not start at the beginning.
I started to fly gliders when I was 14, and much of my training and my first solo was in a Grob 103 Twin Astir registered SE-TZL. Thus, SE-TZL will be the subject of this build.

 

Here is my plan, process and progress so far:

 

I started off by collecting as many images and references of the Twin Astir as I could. These were used in trying to create accurate drawings/profiles.

Twin%20Astir%20new%20sections_zpsadlwnmc

 

The profiles were then imported into the 3D modelling program Blender where I created an accurate (hopefully) 3D model of the Twin Astir.
Here is a simple render of the 3D model:

 

perspective_zps6dm6huch.jpg

 

 

The 3D model was then chopped up and turned into moulds (still just on the computer) and scaled to 1/72. I decided to create female moulds in order to get the dimensions accurate when vacuum-forming.

The 3D moulds are then sent to my kit-CNC machine which will mill the moulds for me. And here it is milling the left-hand side fuselage mould.

 

2018-10-10%2021.04.03_zpsdyj1i8hy.jpg

 

I have managed to mill the moulds for the stabiliser and the two fuselage halves, so that leaves the wings and canopy moulds to be milled. I will hopefully get that done over the weekend.

Here is a picture of the right-hand side fuselage mould and the stabiliser mould.

 

2018-10-10%2020.38.44_zps2vqqnvgu.jpg

 

I have also vacuum-formed the right side of the fuselage and the stab. It really is a small model, and I think it is going to be a little tricky to build, especially since this is my first vacuum formed model!

 

This is my very simple homemade vacuum-forming machine:

 

2018-10-10%2020.40.38_zpscjidzgdo.jpg

And here is the right-hand side of the fuselage vacuum formed together with the stabiliser:

2018-10-10%2020.36.04_zps89aennic.jpg

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Amazingly professional work. Will be watching with interest. I use all of these tools as part of my job but not my hobby, Although I may be close to scoring an old Unimat 1 for hobby purposes, (would be good for cowlings spinners and Wheels), but nothing as fancy as 3D CNC :) 

Edited by Marklo
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45 minutes ago, Marklo said:

Amazingly professional work. Will be watching with interest. I use all of these tools as part of my job but not my hobby, Although I may be close to scoring an old Unimat 1 for hobby purposes, (would be good for cowlings spinners and Wheels), but nothing as fancy as 3D CNC :) 

Thank you very much, but I'm not sure I would call it professional though!😂 It really is a process of trial and error for me. You should see the pile of failed parts that have from of my CNC mill.

The Unimat looks like a very useful piece of kit! 

4 minutes ago, Bandsaw Steve said:

Am very interested in those initial drawings you made prior to importing to blender. Which software package did you use for that work might I enquire?

 

Great subject by the way! 👍👍😀

I used adobe illustrator for the drawings. I'm not very proficient at it but making the drawings doesn't involve much more than using the simple pen tool.

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I remember Astirs from Detmold about 35 years ago now :(. The canopy hinges had a tendency to break due to rough handling cracking the fiberglass.

British sense of humour led to one of them being called Fred...

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On 10/12/2018 at 4:15 PM, John_W said:

I remember Astirs from Detmold about 35 years ago now :(. The canopy hinges had a tendency to break due to rough handling cracking the fiberglass.

British sense of humour led to one of them being called Fred...

Yes, It is now about 20 years ago since I flew a glider but I remember the club having a lot of focus on being careful with the canopies. We were for example strictly forbidden to move the aircrafts with a canopy open.

The two Twin Astirs that we had had a retractable landing wheel. It would normally be covered with doors but the club had removed the doorsI and believe it was because the hinges easily broke.

Edited by martenare
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13 hours ago, Bedders said:

Recalls delightful memories of my Air Cadets gliding course. Loved flying these, and if you find yourself with a spare set of moulds, you have a customer!

 

Justin

Yes, I loved flying them as well (not that I had much else to compare with at that point). Some of my strongest flying memories are from the Twin Astir.

I could probably send you over a “kit” once I have all the parts done.

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18 hours ago, Bedders said:

Recalls delightful memories of my Air Cadets gliding course. Loved flying these, and if you find yourself with a spare set of moulds, you have a customer!

 

Justin

Hi Justin.  Just be aware that there are several differences between the Twin Astir and the Viking T1 as used by the ATC.. 👍

Chris.

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2 hours ago, stringbag said:

Hi Justin.  Just be aware that there are several differences between the Twin Astir and the Viking T1 as used by the ATC.. 👍

Chris.

Yes definitely! I think the two most obvious differences are the nose wheel and the “drooped winglets” on the Viking. 

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7 hours ago, martenare said:

Yes definitely! I think the two most obvious differences are the nose wheel and the “drooped winglets” on the Viking. 

There is also a different wing plan to be aware of in addition to the wing tip mods. I believe there was also a change to the airfoil section but that would hardly matter in modelling terms.

I'm really impressed by the work you have done up to now, and really looking forward to more progress. 

If the moulds were to 1/48 scale then I would be knocking on your door too.  😈.

 

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9 hours ago, stringbag said:

There is also a different wing plan to be aware of in addition to the wing tip mods. I believe there was also a change to the airfoil section but that would hardly matter in modelling terms.

I'm really impressed by the work you have done up to now, and really looking forward to more progress. 

If the moulds were to 1/48 scale then I would be knocking on your door too.  😈.

 

I actually think it is a bit misleading that both the original Twin Astir and the Viking T1 are called Grob 103. Wikipedia calls the Twin II (grob 103a) an unrelated design but from a modeling standpoint I would say they are very similar. It would not take much to convert my Twin Astir parts into a Viking.

 

I chose to do it in 1/72 as I want to model all the aircraft of my career at the same scale. That will include the 737 which will be a little bit too big in 1/48, especially for my wife’s liking! 

Edited by martenare
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I have now managed to get the wing moulds milled in the CNC. I still need to remove the extra material and clean them up a bit but I can hopefully have the wings plus the right-hand side of the fuselage vacuum formed by tomorrow. That means that I am very close to starting to build this little glider.

2018-10-14%2019.55.19_zpsj9cme5to.jpg

 

I have also started to think about how to actually build it once I have all the parts vacuum formed. I have nearly no experience with vacuum formed models so I am not sure what the best way to attach the wings would be? The Twin Astir wings have about 4 degrees dihedral - do I make a spar and attach that to a bulkhead in the fuselage?

Some advice from would be greatly appreciated!

Edited by martenare
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53 minutes ago, martenare said:

 

 

 

The Twin Astir wings have about 4 degrees dihedral - do I make a spar and attach that to a bulkhead in the fuselage?

 

In a word.... yes. But as I have recently struggled with dihedral (If you want the details have a look at my Avro 504 build.   The dihedral dramas start around about page 5) I would suggest a couple of slight amendments.

 

Firstly - I would aim to have at least two spars one forward and one aft. This will help make the wing a bit more rigid and resistant to twisting. 

 

Secondly, I would consider making the spars an integral part of the relevant bulkheads. If the bulkheads are generally ovals cut from sheet plastic I would suggest cutting the - let’s say two -relevant bulkheads, as ovals with spars sticking out either side. I think this will give a simpler and stronger layout. 

 

Finally, as your wings are long and might be prone to flexing or breaking I would suggest investigating the possibility of reinforcing them with one or two fine lengths of carbon fibre - available at any hobby store.

 

I hope that’s some food for thought. 🤔 

 

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Alternatively you could epoxy or CA some brass tube into the fuselage then use brass rod  through the wings mated to the tube and set the dihedral with a jig. This is what I did on my Pup

29296803657_6343a9e2a2.jpg

 

The 'Jig' is just heavy card CA's together and is used to set the diehedral, Using Brass gives you a bit of scope to bend it into shape if the adhesive sets a a little off.

 

25974430167_c4e8c90280.jpg  

 

]39036616220_4648216cd0.jpg

Even simpler on my  Depredussin I just ueed 2mm brass rod CA'd in place but the wings have a solid plastic core

Edited by Marklo
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I got the two wing moulds tidied up and had some time to put them on the vacuum forming machine tonight. The detail has come out nicely so I think I am ready to start with the build.

2018-10-15%2020.52.22_zpsm0hlbxki.jpg

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The first step will be to separate the parts from the styrene sheet. This is likely to be tricky as the parts need to be separated at the top of the styrene sheet in order to get the correct dimensions and shapes. I believe that the best method to achieve this is to sand away the styrene until there is just a thin film of plastic left - but how do I sand it evenly?

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It's been years since I attempted a Vac form but most people on the forum put a pencil line around the part and sand till they reach that. Alternatively I've seen modelers cut around the form with a very sharp blade (keeping the blade vertical) then sanding off the resultant lip, this is quicker but trickier.

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Today was sanding day! I read through Steves super helpful topic where he thoroughly describes how he separates the vacuum formed parts for his Percival Prentice. If I end up with a model looking half as good as Steve's then I'll be more than happy.

19 hours ago, BritJet said:

Looking good so far. You might find this useful for how to deal with the vacform parts.

 

Steve

I initially tried to be clever and decided to sand the whole box around the parts as I thought this would give me a good grip. I turned out to be right about the grip but it took forever to sand such a large surface area.

2018-10-16%2015.03.57_zpsvxfsu23h.jpg

So I tried Steve's technique with a lump of masking tape attached to the part as a grip - which worked really well.

2018-10-16%2015.37.16_zpstd0ds8bp.jpg

I struggled quite a bit with getting the sanded surface even and I over sanded one wing half. It should be salvageable but the good thing about making your own vacuum formed model is that I can very easily produce a new part. The screwed up wing will probably be used as a training and experimentation wing.

2018-10-16%2018.36.45_zpsr3hycoxb.jpg

 

Here is a picture of the separated parts:

2018-10-16%2018.35.39_zps7xr9t9p8.jpg

And I did, of course, have to hold the fuselage halves together in order to get an idea of what the finished model will look like.

2018-10-16%2014.43.46_zpsxm6kcepb.jpg

 

 

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3 hours ago, martenare said:

Today was sanding day! I read through Steves super helpful topic where he thoroughly describes how he separates the vacuum formed parts for his Percival Prentice. If I end up with a model looking half as good as Steve's then I'll be more than happy.

I initially tried to be clever and decided to sand the whole box around the parts as I thought this would give me a good grip. I turned out to be right about the grip but it took forever to sand such a large surface area.

So I tried Steve's technique with a lump of masking tape attached to the part as a grip - which worked really well.

 

Glad you found it useful, I thought it would make more sense to link to that with the photos rather than me just try to explain it.  The masking tape does help but the water get to it after a while. Small parts are always difficult to hold while sanding

 

3 hours ago, martenare said:

 

I struggled quite a bit with getting the sanded surface even and I over sanded one wing half. It should be salvageable but the good thing about making your own vacuum formed model is that I can very easily produce a new part. The screwed up wing will probably be used as a training and experimentation wing.

I've been there a few times! As you can vacform another one easily I wouldn't bother trying to repair it unless you want to do it for practice. The trick is to count how many rotations you do and keep looking at the part, reverse direction of the rotations and look again. Reversing the rotations helps to keep the sanding even. If there are only a couple of areas still needing plastic to be removed then you can always use a sanding stick or files, but be careful to keep the mating surfaces correct. Vacforms always need filler so a bit more due to 'operator error' won't hurt!

 

3 hours ago, martenare said:

And I did, of course, have to hold the fuselage halves together in order to get an idea of what the finished model will look like.

It helps to identify where you need to remove more plastic and to check how the parts will fit together but it's also very satisfying to see it start to come together. Are you using PET for moulding the canopy?

 

Steve

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