Saving my Favourite FT Design.

[Hai-Lee]

Recently I had someone tell me that Balsa was too heavy for use as a covering material so I set about discovering for myself if he was correct or just prejudiced against everything he had not experienced. I had been planning to remove the crinkled and de-laminating paper on a few of my favourite FT FB models and coat the foam in 1mm Balsa and then covering film as I do for my Balsa models.

So I though about what to cover and how. As I have a local club and there are almost a dozen flying examples of the same design, ((Das) Little Parkflyer Stick which has plans posted in the plans thread), I rationalized that the resultant stream of spare fuselages and wings could assist in keeping them in the air and so the test plane was selected.

For the test I have already cut out 3 FB fuselages. The first one is standard, the second has the outer paper removed and the third has absolutely no Paper on the FB. A fourth version may be completed eventually completely using Balsa but that would not be required for the exercise and therefore not included in this thread.

The weights of the resultant fuselages prior to assembly are/were:

First fuselage, Taped in clear packing tape and glued together:- 37 grams, (58 Grams with the taped tail feathers attached).
Second Fuselage, Outer paper stripped only at this time, (Not glued and no tail). :- 21Grams (a reduction of 10 Grams).
Third fuselage, All paper removed, (Not glued and no tail). :- 14 Grams (a reduction of 17 Grams, I suspect the paper on each side is of a different or grade because of the different weight savings).

Today I had a requirement for a replacement fuselage and as the first fuselage was effectively finished I passed it on to the one who required it.

With such a significant weight reduction on the third Fuselage I have started assembly and covering the fuselage in 1mm Balsa, (Medium grade), so far the added strength is incredible though I do really need to find an alternate glue as the White glue does not really like the smooth finish of the freshly stripped FB.

Some progress photographs will be supplied next time I am at this thread with perhaps some updated weight figures for the third fuselage coated in 1mm Balsa.

Next time!
Here are the pics mentioned. Sorry about the Quality!

The three fuselages cut out and weighed



Completed Fuselage 1



Balsa Sheeting Fuselage 3



Update on Fuselage 3 the coated weight with Tail came out to 50 Grams With a little lightening of the tail foam with holes. This compares very well with the taped original version, (Fuselage 1), So that leaves me a weight budget of 8 Grams for covering which is a very generous amount and I still have not tried to lighten the fuselage at all and the strength has improved out of sight. The fuselage is very strong and distortion resistant in the direction of the grain , (flight direction).

The wing is next after which I will rummage through my covering film stock and select a colour scheme.

More to come!

Started the wing and so far the weight saving is over 40 grams. Pics included at various stages of the proceedure.

Wing cut out



Stripped of paper



Edge coating



Joined together



Current progress



So far the weight reduction has been remarkable even though I left the internal paper attached. Basic film covering selected , Silver with orange and blue trim. More tomorrow if I get time.

 

[Hai-Lee]

Update!

Covered the tail and the fuselage using "Heavy" covering film and rechecked weights.

Fuselage weight is 1 gram lighter than the version I covered in packing tape alone.

The tail turned out considerably heavier so I am redoing the tail using a false "Built up" arrangement similar to the main wing. Wing still in progress doing Ailerons at this time.

Here is a pic of the finished balsa/foam fuselage with tail fitted.



Current deductions so far. The 1mm balsa weighs just slightly less than the same area of the papers from the 5mm FB i use here.

The fuselage lost 2 areas of paper and had a single area of balsa substituted and so its all up weight was just slightly less. The first tail had 2 areas of Balsa fitted and it of course ended up about 20% heavier as a result.

the wing which is using a false built up arrangement is still significantly lighter.

All structures have been found to be considerably stronger and the structures could easily accept lightening holes to save weight and this could be done without decreasing airframe strength significantly.

Wing completion for the next post if I have the time!

 

[Craftydan]

Interesting process.

I agree: Balsa isn't inherently heavier, particularly when you composite it with foam -- balsa skinned foam allows you to use MUCH thinner balsa for the same strength as only foam or only balsa. Balsa sheeting foam is an old and valuable technique, and quite a few classy planes were built this way.

Cost, maybe not so much. Depends on if you've got a good source. For a Tiny Trainer, the areas are so small scrounging for the thin sheeting isn't that hard, but I recently spec'ed out a set of 3m balsa sheeted foam core glider wings that didn't fair so well. With graded balsa from a few online sources, I found that in weight and cost it was cheaper to cover in a carbon/Kevlar fabric -- including the resin and tooling costs. Again, tiny trainer scale cost is down in the noise after you purchase the electronics, but the square growth of surface with scale can get spendy quick. Admittedly the materials for the glider wing are all still sitting in my shop mocking me, but other projects have priority


I am curious what you're using to laminate the materials in your layup -- wood is porous and soaks in thin glues well, but foam? Depends on the foam . . . but Depron is fairly cooperative.


As for your tails . . . tails are almost always overbuilt -- if they'd been built right they'd disintegrate with the rest of the plane in a crash

You might consider reducing the material in the tail boom first (there's more material there, and it doesn't need much more than four stringers headed back with a little bit of reinforcement around the mid-point) and perhaps a spar in the tail feathers and no more. 6mm depron has a good bit of stiffness to it -- the spar may be more than enough for the feathers . . . but you're right -- caps and stringers allow you to cut a few more lightening holes. If you had it, I'd recommend a piece of carbon tow (6k carbon tow ~0.5g/m and ~3-4mm wide) and an equal weight of resin with just the Depron alone, but too few folks keep carbon tow in their shops. In this case, the balsa will have to do

Looking forward to seeing this progress!

 

[CrossedUpChris]

This is really interesting. The FB I use here in the UK is comparitavely heavy compared to dtfb. I have a full box of 6mm depron to experiment with. I might have to try something like this.

 

[Hai-Lee]

This exercise is the result of a few converging requirements and problems I have observed over the past few years using FB for building FT designs and then designing my own aircraft. Please be aware I do LOVE the simplicity of FB designs and my work in this particular area is not intended to denigrate FB designs but to perhaps enhance them and possibly even improve their lifespan. Sadly I do not see currently where people will next century find an old FB design and then refurbish it and fly it.

Foam board is a very good entry level medium to build RC model aircraft but the step up to the next building material is a huge one and often either requires some form of tooling or a totally different set of skills and materials.

Some observed areas of research on FB and the designs rendered in FB.

1. The FB has very little compression resistance and can Wrinkle easily even in the slightest impact or even rough landing. The paper has a great resistance to being stretched but in all other force directions it is weak. There are numerous stories in the forum of how to add reinforcement to the FB where it seems that weight is a secondary consideration. Personally there have seen some planes which ended up with so much reinforcement that they no longer flew as they did originally.

2. Here, (In AUS), we have been cursed with heavy FB which has rendered some FT designs as difficult to get to balance properly if at all. Also the FB I use is notorious for having paper which refuses to be separated from the foam. Whilst it took some trial and error, (More Error I am afraid ), but eventually a technique for removing the paper and replacing it at will was developed which is chemical free.

3. The paper on the FB requires special attention to resist water and de-lamination. When the paper absorbs water it can swell, gain a little weight and separate from the foam causing a loss of strength and lead to premature FB failure or even serious issues like wing spar separation and the like.

4. On large FB designs the weight of non-structural areas of FB can become so great that structural strength starts to suffer greatly and sagging of the FB can generate a requirement for extra structure to support unnecessary weight and suddenly weight balloons as does the cost and complexity of the design.

5. The FB planes have a functional look about them and yes there are curved decks and the like most are formed using paper or card which do not stand up to hangar rash very well in the long term. Card can be heavier than some other materials, i.e. Balsa, CF, or the like.

6. Covering of FB is an often debated topic with as many different approaches and ideas as there are forum members though the waterproof covering films used on balsa models are never used or discussed though I have had some experimental success in applying iron on film directly to the FB, (Temperature control is the key). Remember that paper is actually wood that has been processed extremely. Currently the movement towards to "Wraps" is great and they could equally apply to other modelling materials as well.

7. The divide in RC modelling used to be between foam and Balsa and now it is between foam, FB, Composite, and Balsa. Each side of the divide seem to view the materials used by the others as rubbish or somehow inferior. There should be no divide as any engineer will tell you that materials should be chosen for their properties including ease of use, cost, and physical properties. All construction materials and combinations are VALID. As a side thought the greater demand for a material the lower the cost will be eventually.

This list will be updated in the future as other properties or concerns become prominent.

My work on this experimental combination of FB and Balsa is to help bridge the gap between FB, (Disposable), designs and the classic Balsa designs. To see a RC aircraft rendered in either FB, Balsa or whatever and not be able to tell its construction material without close examination is my dream. To see the gentle streamlined curves of a scale Spitfire or P51 as it screams overhead is still a serious thrill even at my age.

 

[Hai-Lee]

Update 2:

Finished the wing and ailerons. Applied the covering and weighed the wing.

A standard taped wing weighs in at 100 grams and the balsa covered wing came out at 82 grams.

The weight result was a saving of 1 gram on the bare fuselage, a weight gain of 10 gram on the Horizontal stab, a weight gain of 3 grams on the Vertical Stab and a weight saving of 18 grams on the wing. Total outcome a weight saving of 6 grams.

Keeping in mind that I double coated the stabilisers in Balsa rather than a false rib structure and that the fuselage and wing had NO lightening holes I estimate that I could save an additional 20 to 30 grams.

Original taped version bare airframe total weight :- 158 Grams
Balsa composite with covering film bare airframe total weight:- 152 grams
Estimate of build weight with lightening holes and careful build, (without loss of strength :- 122 to 132 Grams)

New progress pics.

Started covering



Underside



Ready for fitout!



With the above airframe I believe anyone could achieve a similar build which could result in a weight reduction of around 20% or a better weight distribution to reduce the need for extra nose or tail weights.

Also discovered an additional bonus characteristic of this methodology and that is that warps can be more easily removed with a little hot water soaked into the balsa and the plane clamped until totally dry.

Additional Information and GOTCHAs:

The wing Leading Edge is a 5 x 5 mm piece of balsa
The outer trailing edge was too flimsy for proper aileron operation and so a 1 x 20 mm half spar was fitted to each wing trailing edge.
If the iron is too hot the covering film can bond with the foam. If the iron is too cold then the film does not shrink properly.
Between each false rib a tiny hole should be made to allow the air trapped under the covering film to escape when applying the film.
Holt Melt Glue is incompatible with the heat required to shrink the film: DO NOT USE!!!!
The internal areas of the wing were left paper covered and so was the main wing spar.
The wing was not totally sealed so that the servos could be fitted in the original designated location.
The edges of the Foam board were not covered in balsa and the covering film was fitted over the bare foam and heated gently to ensure adhesion.

Next month I hope to take a REAL FT design and redo the entire procedure to use a feature of the FB which I am determined to find a way to exploit. It will be a new post entirely. I hope that this exercise has been as informative for you as it has been for me.

 

[Hai-Lee]

Dan, I reread your post for the umpteenth time and in a way I envy the ease you and your countrymen are able to access materials and the prices you pay but sadly living in AUS envy and dream is all I can do. Freight costs inflate the cost and reduce the availability of almost everything.

As for the glue I used to bond the Balsa to the Foam I use a special, Very low cost, clear foam safe craft glue I discovered a year ago and buy in large quantities now. I found the glue when looking for a cost effect alternative to the POR-UHU which was costing $15 a small tube, I later discovered it glued wood and ALL types of foam with equal tenacity. For those who have a few bonding issues with the bare foam I learned, when I was making FB rotor blades for my Autogyro, that a light sanding of the bare foam seems to give the glue something extra to bond with.

Your statements about the scaling of this technique in relation to larger models is well understood and whilst the Balsa would work the cost quickly becomes prohibitive. As with the old style balsa kits as they got larger alternative materials were used and now most LARGE built up wooden models use a lot of Plywood as well as other composites.

This entire procedure was undertaken to discover some of the properties of adding the balsa in place of the paper so that I would know what to expect when I decide to remove the wrinkled forward portions of my fuselages and wings on well loved and often flown FT designs. What comes next may be a rehash of an FT design with a view to lighten it and try out a new, (to FB), control method.

Sadly i have so many projects on the go as well as repairs and building for other flyers that I have little time for doing these exercises even though I do like the challenges. Fly high, Fly safe, and have a beautiful landing!

 

[stvrob_63]

This thread has got me thinking about balsa laminates.
As for laminating the balsa to foam, I am thinking of a aliphatic resin glue, watered down substantially so it could be sprayed as a light mist over the (bare) foam. And then lay the balsa on and work it on firmly with a warm hobby iron. Experiment with getting the temperature to be just below the point it would damage the foam. The glue should set up quite quickly and the balsa is thin enough that the excess water would evaporate thru the balsa easily.

Im also thinking about a covering for the balsa that further strengthens and toughens the balsa. What about the old timey tissue paper, but wet out with one of the very thin laminating epoxies? The ones that are mixed like 4:1 or 5:1 resin: hardener (e.g. http://www.westsystem.com/105-epoxy-resin/ )

After laminating the outside, it seems to me the foam itself has fulfilled its purpose and would no longer be needed. To save weight we could remove it entirely by flooding the inside of the part with Acetone and pouring away the dissolved foam.

Such a structure, where the entire load is carried by the skin is called a monocoque structure, and is among the highest strength:weight load bearing structures.

https://en.wikipedia.org/wiki/Monocoque

 

[Hai-Lee]

Possible but I dispute the foam serving no purpose as it still stops the balsa bending cross grain and provides some volume in the corners to resist the seams splitting.

A light glass or CF coating would supply sufficient reinforcing to allow the foam to be removed.

When thinking Monocoque just remember that such designs normally did not have corners as they are a point of weakness if unsupported.

Have fun!

 

[stvrob_63]

Likely you are correct about the foam. Lots of room for experimentation with various composite structures.

 

[rockyboy]

Hmm.... I happen to have lucked into a large quantity of balsa from an estate sale, and I do love that Das Little Stick design.


Darn it Hai-Lee - now you've given me a new project to add to my list