Bumpy Green Model Aerodrome: My First PSS Model

When I first started back into aero-modelling I bought a magazine (RC&ME 51) with a free plan for an electric jet. I had the sense to know that, however attractive this model was, at that time it would be beyond both my building and flying skills. I filed the magazine knowing that one day I would have a go at that model.

Now two years and six models later and thoroughly addicted to the hobby I found myself with nothing on the building board and I remembered that plan. It was a semi-scale model of the Aermacchi MB 339 that could be built as a PSS, electric or IC version, designed by Cyril Carr.

Why this model?..

The appeal was that this was a 43” span (not too big or small) aileron/elevator model that could be kept in one piece in the back of the car; Although only semi-scale, to the average person it looks, both on the ground and in the air, like the real thing (The main deviations from scale are that the fuselage is narrower and the wingspan/area has been increased); There are a wide range of possible colour schemes with lots of photos of both the full size and models available on the net,it should be easy to see in the air; It uses conventional built up balsa construction and film covering.(Lost foam and glass etc. are pleasures for the future)

Small enough to fit in the car assembled

Problems and their Solutions!..

Because of the publication process the plans in magazines are often drawn by professional draftsmen from rather sketchy retro-designed drawings from the original designer. Errors and inaccuracies creep in and this plan was no exception. A thorough study of the plan and building the thing in your head a few times before putting knife to balsa can save problems later. I am used to working from kit-plans which are usually accurate and it took a while to feel confident enough to make my own decisions about the fuzzy areas. I did this in a few places and even having only built a few models before, my hunches paid off .

One of the benefits of working from a plan is that you can easily alter the design to suit your requirements and building style. I really don’t like bending wire and Arthur (he of wondrous modelling knowledge) suggested that to avoid those horrible aileron torque rods I should put two narrow servos in the wings. I just happened to have a couple from a much crashed and unloved flying plank,so guided by various magazine articles, this I did and this made tape hinging the ailerons very easy. Keep reading and re-reading the mags....they are a gold mine for the beginner. One benefit from this is that when I get my new computer radio (when oh when??) I will be able to set differentials and spoilerons and all that stuff more easily.

The simplest way I found to cut all those complex shapes is to photo copy the plan and cut out all the pieces. (I usually make two copies, one to keep and one to cut.) Then use 3M Spray Mount to fix them to the wood and then it is simple to cut around them. The patterns can be easily peeled off before assembly. This technique is also useful for cutting complex patterns for covering film. Thin paper patterns can be made by fixing them to the model with the 3M adhesive and trimming to shape. When an accurate pattern has been produced the reverse side can be sprayed and it can be fixed to the back of the film or trim and cut out with a scalpel. This makes cutting easy and accurate as the pattern and film cannot move. Any glue residue can be removed from the model with cellulose thinners.

Magazine articles often assume a lot of knowledge about the details of the construction process. The construction of this model was quite straight forward except for the curved areas at the top of the fuselage that give the model its scale appearance. I discovered, by almost re-inventing the wheel, that sheet balsa up to 1/8” thick can be bent into quite tight curves by painting the outside of the curve with house hold ammonia solution. You paint or spray on a 1 part household ammonia / one part water solution and leave for a few minutes. The wood will soften and can then be bent around a former such as a dowel or broom handle and fixed with masking tape or elastic bands until dry. When dry the wood can be easily trimmed to shape and glued in position. Household ammonia can be difficult to find. I found a dusty bottle in a hardware shop in Curry Rival!

Was it worth it?..

This has been a good introduction into some of the rewards and problems of scale modelling.

You can see the results by comparing the illustration at the top of this article with the photo(s) of the model at the end. As yet I haven’t installed the radio and set the C of G etc. but as soon as I have, I look forward to Ted ( brave man!) helping me with the trimming flights and hope soon that you all will be able to see it on the slope for yourselves.

Although only a relatively small model it has taken longer to build than anything else I’ve done due to the amount of thinking time involved. I’ll leave you to decide whether it was worth it!

My Aermacchi Lives Again... (Update December 2004)

My Aermacchi 339 suffered a violent arrival recently. This was caused by me giving a wrong control input when I became disoriented as its silhouette flew across the sun while attempting a high speed, low level pass, during a Wednesday evening session on the West slope at the Beacon

Initially I thought that this would be the end of a much liked model but on examination it looked like a rebuild would be possible.

First step was to remove all the Film covering and then the process of putting the jigsaw back together could begin. This was more like mending a piece of broken china than building a model. Both wing tips were carefully rebuilt on the plan but, surprisingly, if you consider the mess in the picture, the rest went back together quickly, with a little CA here and epoxy there.

The result was a stronger (but heavier) model.

The original model scheme was that of the Italian national aerobatic team, The Frecci Tricolore, which was very eye catching and easy to see at a distance. The new version has got tip tanks and reflects the military trainer scheme I have chosen.

With this scheme I can have camouflage but also benefit from the visibility afforded by the fluorescent flashes on the tail group and tip tanks. I have also used my misfortune to learn some new modelling techniques. Firstly I’m beginning to get the hang of working with foam. Blue foam can be worked much like balsa. The new tip-tanks were made from a central rib of 1/32 ply and two pieces of blue foam cut with a knife and shaped with a flat Permagrit sanding tool. They were covered with light weight glass cloth and about five coats of acrylic resin. This is an easy product to use as it requires no mixing and dries very quickly so a lot of coats can be built up in a short time. A light rub down with fine wet and dry paper, between coats, gives a very smooth hard surface that takes paint really well. The big bonus is that brushes can be washed out under the tap! The next step was a coat of white primer and the fluorescent orange and matt black finish was applied with spray can and brush respectively. The test will be how they survive the inevitable impacts that will result from my dodgy landings.

The tanks are fixed to the wings with Velcro so that hopefully they will knock off if necessary. (3) The fuselage has a little reinforcement with glass on the inside and the whole airframe is covered in Solartex and painted. I was so impressed with the finish on a club-mates Vulcan done in this way that I thought I would have a go. This has added a little weight but I don’t think it will matter as it is a light model. The total extra weight including the new wing-tanks is about 3 oz. The original model was 27 oz and it now weighs in at 30 oz

The whole model was covered in white Solartex. This is the first time I have used this covering and I found it easy to get good results around some of the compound curves

The areas that were to be fluorescent were masked off with tape and newspaper as these needed to remain white to act as a reflective undercoat.

Then the whole top surface of the airframe was given several light coats of Flair Spectrum Dark grey. Next all the under surfaces were sprayed light grey. When all this was dry the whole lot was re-masked and the white areas were sprayed with Plasticoat fluorescent orange paint bought from B&Q. I’ve used aerosols before for minor repair jobs on the car but I must say that the quality of these Flair ‘rattle cans’ is a cut above anything I have used before. They lay down the paint in a soft even mist that with several coats builds up to a perfect finish. In complete contrast was the B&Q paint that came out in gobs and blobs and would run as soon as you looked at it.

However, a word of warning, the Flair paint is lead based and contains Toluene so that even with a mask on and lots of ventilation I quickly felt light headed and quite nauseous when using it.

The paint job was completed by adding the Olive drab areas to produce the camouflage effect. For this I used a technique called a floating mask. The masks were cut from 120 gm A4 white paper. Using small balls of Blue-Tack these are stuck to the airframe in such a way that the mask ‘floats’ just above the surface. This produces a softer edge that has a more scale appearance than a tape masked line. The amount of over-spray can be controlled by the size of the Blue-Tack balls.

This was a time consuming process as each area was masked and sprayed separately, but I was pleased with the effect. The next part of the process was to decide on how much scale detailing to put in. As this is more of a semi- scale model I decided to go for just the roundels and identification numerals with the added touch of the ejector seat warning triangles. Anything else is really too small to be visible on a model of this size although I did think about scaling up a decal sheet from a 1/72 scale plastic kit…… possible…. but overkill on this model. All the markings are cut from Solartrim which is a fiddly job made easier with a computer. If you print out your correctly sized numbers and roundels as outlines in a program like MS Publisher they can easily be reversed. Its then an simple matter of sticking them with 3M Spray Mount to the back of some solar trim. They can then be carefully cut out with a scalpel. If the backing is pulled off in a bowl of water, with a few drops of washing up liquid added, the letters can be easily slid into the correct position on the model. Surplus water can be carefully blotted off with kitchen roll and in a few hours the piece will be firmly stuck. This takes a bit of practice but is soon mastered.

All that remained to complete the airframe was to hinge the control surfaces. This had been done with Diamond tape on the original model. I had not thought when refinishing with a matt surface how unsightly this would look on the new model. I had to find a different method. I had some fabric hinge material left over from a couple of Great Planes kits and this proved to be the answer. As the surfaces are top hinged it was slightly tricky to cut the slots at the correct angle so the knife stayed within the wood… but I manage it. A pin was placed through the centre of each hinge. This allows the hinges to be inserted evenly into the trailing edge and the control surface and ensures a small, even, gap so that the glue will not affect the operation of the hinge. The pins are removed before a few drops of thin CA are applied and Bobs your uncle! I would usually pin this kind of hinge with cocktail sticks but I didn’t so as not to mar the finish. I will keep an eye on them and hope they remain secure.

The Test Flight...

Before the test flight I installed the radio and checked the C of G and lateral balance. (unlikely to be out but worth checking because of the new tip-tanks) I added about 1/2 oz extra nose weight but this may well come out again after the initial flights.

It was a very cold northerly of that varied between 10 and 20 MPH that greeted us at Broomfield Hill . Not the best conditions to test fly a crunchy! The foamies flew well and Ted, a fellow club-mate, and I flew our Middle Phases and they seemed to cope well with the conditions so there was nothing for it but to give her the old heave-ho. Ted (brave man ) consented to be the test pilot as I wanted to get some early pictures in case of mishaps. A firm level launch and she flew well…. a little up trim and she gained height rapidly out into the blue. The stall was straight and docile. The dive test showed the C of G to be about right and the wing tanks didn’t seem to effect the handling too much. The flying attitude seemed good. She always looked to be flying a little nose down in the previous colour scheme but now she was sitting just right. As predicted the florescent markings made orientation easy and she looked really scale at a short distance. Only the wine of the Rolls Royce Viper engine was missing. The photographic session was over and I was just about to take the helm when a sudden gust seem to knock her out of the sky and she nosed in at quite a speed. Fortunately the ground was very soft so the damage was minimal. Just a little dent on the nose and one of her old wounds opened up a bit. I’m sure a bit more glass on the inside will fix that ……………………………...Such is life!

I’ve probably learned more about modelling techniques with this little plane than with any other model I have built. I believe the plan and canopy are still available from Cyril Carr, 3 Battlemead, Corfe Castle, Dorset, BH20 5ER for about a tenner. This is not a true scale model but I hope it will be the forerunner for a larger more scale 60” version that I plan to build next year..

Original Article by Colin Wavell
from West Blackdown Model Gliding Club 23rd May 2003
Updated December 2004