Tangling with Aussie Foamboard

Have you ever observed that the devil is always in the detail? In diagnosing the cause of one problem I made an assumption. Generally this assumption turned out to be true. However the way I interpreted the assumption was not true and therein lies a story that I hope others can benefit from. Along the way I encountered two other simpler problems and so I hope my solutions help people there as well.

Living in Australia I can only dream of using Adams foam board. The only foam board that I can get my hands on comes from Fix-a-Frame Pty Ltd at foamboard.com.au (hereafter refered to as AFB – Aussie Foam Board). APNewton posted an article (http://flitetest.com/articles/foamboard-in-australia) where he compared AFB with various other foamboards (including Adams) as well as Depron. He established that AFB is about 30% heavier than Adams Foamboard. Red20RC posted an article (http://flitetest.com/articles/ft-spitfire-australian-foam-board) where he used AFB to build an FT-Sprifire and showed it could be very successfully used to construct FT models.

Basically the differences between Adams foam board and AFB is that AFB is about 30% heavier and has a much stronger bonding between the foam and the paper. The stronger bonding between the foam and paper is good in that it results in a model which can take quite a bit of wear and tear and last quite a while. I have used it to construct a number of models and have not noticed any deterioration on models that I have flown lots. However it does present problems during construction. The paper being well bonded to the foam means that two Flitetest construction techniques need a different technique to get the required result.

Firstly removing paper from a section of foamboard requires more than just a gentle peeling of the paper. Secondly removing the little strips of foam as part of creating an A, B or C fold is more difficult and also requires a rethink.

I read on various forums that rubbing alcohol could be used to remove paper from an entire side of foamboard. I doubt its possible to remove the paper from an entire side of AFB but rubbing alcohol ( methylated spirits wroks as well) can be used to remove a small section of paper from AFB. The technique is straight forward. Rub the area concerned with rubbing alcohol until you can see it soaking through the paper. Give it another few seconds and then start to peel the paper away. If you are really lucky the paper will peel away in one piece. However it sometimes tears and leaves a lot of paper behind. When this happens you need to apply more rubbing alcohol and gently but firmly rub the paper of with your thumbs. You need to press firmly but not so much that you end up squashing the foam. Practice with a bit of scrap first and you will have no problems.

Similarly peeling the foam strips away from the areas where there are A, B or C folds also requires more work than simply bending and peeling the foam away. My solution is to do the score cut as per normal and then fold one of the sides involved 180 degrees so that it is flat against the other side. I then run a knife along the edge cutting into the foam the width of the fold trench as close to the paper as possible. I do not try to get the knife exactly between the foam and the paper as this will most likely result in cutting through the paper. When doing this I proceed slowly leaving a small amount of foam behind. The cut foam is then removed and the remaining foam removed using the technique demonstrated by Josh of gently scraping the blade across the top of the paper. The remaining foam tends to come off easily. When scraping go slowly. If you get too carried away you will start scraping the paper away as well. Keep in mind that both the foam and the paper are white in colour so it can be hard to tell from the bits flying off whether or not you have gove too far. Again practice with some scrap and you will get the idea.

The extra weight impacts the finished model in two ways. Firstly, being roughly 30% heavier, sometimes you need to use a more powerful motor. Secondly the CG will typically move aft making the final balancing of the model more difficult. However it puzzled me as to why the CG seemed to move backwards. My assumption was that if the weight of the material used to construct the airframe increased by 30% then the entire aircraft would increase in weight by 30%. This assumption is, in point of fact, wrong. However I did not question it.

I built an FT-Flyer which was massively tail heavy. In the end it needed so much weight up the front to balance that I gave up on it. The next encounter was with the Mini Guinea. The solution here was to build a battery tray that extended forward into the nose. That fixed the problem so, even though I was puzzled by it, I moved on.

I built three FT-Spitfires and none of them suffered from the CG issue (one for me and two for friends). Problem gone or so I thought, its just one of those strange and random problems that will never occur again. I went on to build the aileron wing version of the Tiny Trainer. Again no problems. Now I am feeling really confident that the rearward CG issue is done and dusted.

Then I built the Bloody Baron and the rearward CG issue was back with a vengence. I decided to forget about why it had happened and simply fix it. I did so by adding some decoration in the form of dummy engine cylinders and machine guns which looked cool but didn't really help. I then decided I would move the elevator and rudder servos forward. However before I did that I tried putting a spinner on the front. Suddenly my Bloody Baron balanced beautifully. My success was dampened by why this was happening.

The next model on the building board was the Bushwacker and this was by far the worst example of the rear moving CG issue. At this point I could have simply done everything I could to get the plane balanced but I was tired of being haunted by this issue. I was sick of the pleasure of a good build being soured by the unpleasant surprise of having to move everything around to rebalance the plane. I decided that before I tried to fix the problem I really needed to understand why this was happening. I had not read anything about this CG issue happening with others so either the universe had it in for me or there was a sensible, rational explanation for why this was occurring and why it wasn't occurring with every FT model I tried to build.

Okay based on my initial assumption. If the entire aircraft increases in weight by 30% then this increase is uniform and therefore the CG should not move. In my case I had the evidence of the CG moving aft on some models (i.e. the assumption was not holding) and not on others (i.e the assumption is holding). Okay, given that the universe is currently not making sense I decided to question my intial assumption. If there is more foam behind the CG (and there usually is) then maybe that is why the CG moves aft. My daughter, who is pretty good at maths and physics, rolled her eyes when I suggested this was the cause and she told me that if the weight of the entire aeroplane increases uniformly in weight then there is no possible way the CG would move. When she said “the weight of the entire airplane” the lightning bolt struck me. I realised the error in my thinking. It suddenly hit me that the entire model doesn't increase in weight. Only the weight of the foam has increased 30%. The motor, battery and electronics have stayed the same. If a bigger motor is used then their final weight of the electronics would increase but there are no guaranties that the increase would be 30%. The most significant part of that weight is the motor and battery which usually reside in the nose. If the final weight of the motor and battery was significantly less than 30% then the CG would move aft. Eureka! Now it all makes sense. Now I looked back at all my past builds to see if my new understanding can explain what happened with each model.

Firstly I considered the FT-Tiny Trainer which did not have a CG problem. I used a 24 gram Hextronics motor with the Tiny Trainer. That motor is quite a bit heavier than the Emax one recommended by Flitetest and also stuck out further forward. By using a different motor I had accidentally resolved the problem.

I next considered the three Spitfires. Mine was pre-foam board and made from Depron. Weight wise it was only slightly heavier than the Adams foam board version so the CG issue did not arise. The other two I built for friends who both elected to use bigger and heavier motors. They both managed to balance their Spitfires with the battery as far forward as it could safely go. Again the use of a heavier motor meant the weight increase of the model is more uniform and so, again the issue was resolved.

The Mini Guinea puzzled me a bit. I used the same motors as the Flitetest guys so that should move the CG aft by a lot. However, being a twin, the motors are much closer to the CG than with a single engine model so that should reduce the amount aft that the CG would move. I decided that the evidence showed the lack of motor weight was more significant because the problem did in fact arise.

My Bloody Baron uses a different motor that the FT one but its about the same weight. Similarly with the bushwacker. At last what was happening to my models made sense! The universe and I were at peace again.

However I was still left with a massively tail heavy Bushwacker. If I had known this at the beginning then I would have known that I needed to move everything that I could forward. The fact that the model was finished narrowed my choices in this regard. I decided that moving the rudder and elevator servos forward would only help a little and would result in a lot of mucking about so I left them where they were. I looked at the power pod and decided to get rid of it given that most of its weight was behind the CG. Not much of an improvement but it was easy to do. Finally I elected to move the motor and battery forward by extending the nose. I was a bit worried about destroying the looks of my Bushwacker so I decided to start by moving the motor 10mm forward. This meant creating a suitable motor mount and a nose cone that covered the motor. I have a 3D printer so I banged away at Sketchup, fired up the printer and finally tried it all on my Bushwacker. There was an improvement but I was still way off. Okay time to forget about how the nose will look and really get serious here. Back to Sketchup and the 3D printer. My next attempt had the motor positioned 25mm ahead of its original position. It almost balanced but it was still tail heavy. Okay third time lucky. My final attempt had the motor 35mm in front of its original position. With the battery pushed hard up against the motor mount it finally balanced. Success! I had a balanced bushwacker. As an added bonus I think my nose extension looks okay so my Bushwacker didn't look like a hacked up mutant Bushwacker.

You can see the three nose cones for each motor position I tried.

I think my Bushwacker still looks okay with its longer nose.

So how does this help future builds? Next time around I can calculate the weight of the Adams foam board version by taking the published weight without battery and subtracting the weight of the electronics used. I can then multiply that weight by 1.3 and this will give me the estimated weight of an AFB constructed airframe. Then once I have decided what electronic components I am going to use I can determine if I need to compensate for the AFT CG shift. From there I could modify the plans to make the nose longer, use a heavier motor, bigger battery, move servos forward or whatever. At the end of the day its going to be a much easier journey if I am allowing for this before I put a blade to foam board. Okay its still going to involve a bit of guess work but I think I can get much closer to the mark and so there will not be any more unpleasant surprises.

So to conclude. If you are using a different foam board then you will need to look at the differences between the Adams foam board and your chosen foam board. You will find ways around the differences but they will be much easier to deal with before you start your first build than during your first build. As far as the AFB is concerned I think its great. My mistake was ignoring a confusing issue and then trying to fudge my way around it in the middle of the build.

Finally if your experiences and the universe seem to disagree then don't ignore it. If you think about it long enough the reason will come to you but only if you pay attention to the detail (e.g. an airframe is not the same thing as an airplane!).

I hope that AFB users and users of non Adams foamboard have found this article helpful.

Happy building and happy flying.