A simple FAI class model

[JNUK]

I have design a model with an aim to try a technique, which is new to me. I’m sure someone somewhere has invented something similar already, but I haven’t come across just exactly the same approach. My aim was to design a model compatible with FAI S3/S6 class dimension requirement and with weight below 10 grams excluding recovery.

In the beginning of last season I built such model using a kind of tracing paper. The overall design was simple enough, dimensions were correct and model flew very well.
However, I did not managed to get its weight below 10 grams. Also, despite the paper good quality and a coat of light varnish the body sucks moisture from the air limiting conditions the model can be flown.

A member of our club who also flies in FAI competitions showed me a body made of laminating pouches. I understood that the building technique is quite elaborate and requires a proper mandrel and long thermal treatment. Both were not available to me, so I devised a simpler method.

The body and, in absence of better alternative, the nose cove are made of two layer laminate. The outer layer is a half of 150 micron laminating pouch (so, it is 75 micron thick). The inner layer is aluminised mylar. I’ve tried 15 and 5 micron mylar. Both types work very well, although laminate with 5 micron film is a bit too soft in my opinion. 75 micron laminating pouch film & 15 micron mylar have area density of 102g/m^2.

Both sheets are wrapped around BT-60 body tube with a 3mm seam. The laminate is temporally fixed along the seam with masking type. After that I used an ordinary domestic iron to iron gradually the pouch to the mylar layer starting from the seam. The same technique is used to form the nose cone and the tail transition. I do not have access to 10mm motors used by FAI chaps , so the model is design for 13mm A-class motors. Larger motor tube may add a little bit of weight, but not much.

Length of the model – 510mm
Max body diameter – 41.6 mm (BT-60)
The total empty model weight without a recovery baffle and a parachute/strummer - 9 grams.

This is a very brief description to give you an idea. I would appreciate your comments and questions.

 

[JPVegh]

Very nicely done. How do they perform?

 

[kjohnson]

It's an interesting technique. I may have to try it using one of my mandrels. When I was seriously doing tube events, I had gotten my fiberglass and tissue models down to around that mass, if not slightly lower.

I can find 3 mil and 5 mil (76 and 127 micron according to the goog) laminating pouches, and have plenty of 1/4 mil mylar. I like that the adhesive is on the substrate already. Is there much shrinkage with the thermo setting?

kj

 

[JNUK]

Very nicely done. How do they perform?

Thank you.
I haven't flown these particular models yet. All dimensions are tested and proven on a different model and I do not expect any surprises there. If you mean the flight performance, all dimensions are pretty standard for FAI models, apart from a different motor weight/diameter of course. A heather motor brings CG down a little bit, but the margin is still sufficient. I flew a model with the same dimensions in 8...10 m/s wind without any significant weathercocking. There is a video on YouTube of the above model's older sister flight in approx 4m/s wind.

As for the structure itself, all seams are very very strong. If correct temperature is used for "soldering", it's practically impossible to tear them apart. The pouch film is originally very soft and flexible , but once exposed to a sufficient temperature its glue hardens and becomes more rigid.

 

[JNUK]

It's an interesting technique. I may have to try it using one of my mandrels. When I was seriously doing tube events, I had gotten my fiberglass and tissue models down to around that mass, if not slightly lower.

I can find 3 mil and 5 mil (76 and 127 micron according to the goog) laminating pouches, and have plenty of 1/4 mil mylar. I like that the adhesive is on the substrate already. Is there much shrinkage with the thermo setting?

kj

If you use a metal mandrel, you may have a problem of heating the laminate up with an iron due to significant mass of the mandrel. You have to devise a technique of heating up the mandrel with the laminate wrapped around it.
With an iron and not-metal mandrel I gradually heat the laminate strip by strip trying to squeeze air bubbles out from between the layers. However, even if some bubbles form or mylar layer wrinkles a bit it's not a problem as the outer layer remains practically unaffected by these imperfections.
There is no any thermo setting at all if temperature/expose time remain below the lamination film shrinkage conditions. This is a function of the film thickness and it may depend upon the film type, although I'm not sure about the later. I tested different temperature setting on my iron and achieved no thermal settings on "Cotton" setting, but you have to be quick with it. I use that temperature for soldering seams. The actual tube laminating is done on 'Silk' setting without any issues and significant exposer time limitation.

The only issue I discovered so far is that you cannot really glue anything to the film that well. That is why I had to use a bit different approach to attach the tail transition to the body tube. I can see that seam on the second picture.
After some experimentation I discover that the best adhesive for the film is EVO-STIK IMPACT.