Tuesday, 14 June 2016

Making Polyester Plastic Fuselages

By Pham Anh Tuan

This article was lifted from the Aeromodeller Annual 1972-73 - advanced at the time and still a good reference today.

The modellers who are prepared to make their own polyester fuselages know the type of aircraft they want to build, so we will assume they can carve the master and cast the female mould. We must now consider the question of actual lay-up of the fuselage.

The aim of this article is to give readers as many hints as possible to enable them to go ahead without further undue difficulty.

Moulds are made of the same material as the fuselage, i.e.
1. a base resin,
2. the gel coat,
3. the release agent,
4. the glass-fibre (medium weave: about one mm thick).

The fuselage should be built in a well-aired room. A workshop is the most suitable, but if one is not available, use a room with large windows which can be opened fully.

The principle of construction is as follows:

The fuselage mould is in two halves. The two halves are joined together with bolts, and a band of glass-fibre matting is used to reinforce the seam.

The object is to put a smooth piece of cloth (or mat) over the release agent inside the two halves of the mould. First spread the release agent generously over the joint area, making sure the surface is coveted with release agent and that no excess builds up inside the mould. Leave it to dry for at least one hour. The release agent must be completely dry.

After this, prepare a dose of gel coat and add to this the catalyst. Using a paint brush, apply an even coat of gel over the dry release agent. Too thick a layer of gel coat will impair the solidity of the fuselage.
Leave it to dry for about half an hour, until the gel coat is tacky. While waiting, cut the fibreglass into two pieces sufficient to cover each of the insides of the moulds, leaving a few inches for overlap.
The fibreglass is then applied to the gel coat in the two halves of the mould. Gently rub the fibreglass to ensure perfect adhesion to the gel coat; this will determine the final finish of the fuselage.

Now, prepare a larger quantity of base resin, adding less catalyst.

Using a paint brush, spread the base resin evenly over the fibreglass, paying particular attention to the seam (make sure the fibreglass does not bubble at this place), and other areas such as the wing supporting flairs, fin leading edge, etc.

Reinforce the fibreglass in those places which are more susceptible to damage, i.e., the nose of the fuselage, the wing supporting fairings and the fin, etc.

Have a sharp knife at hand. When the resin has almost set (when it is firm] cut away the excess fibreglass. The two halves of the fuselage, though stuck together in the moulds, are now ready.

Now, the two halves must be joined securely.

We have already said it is very difficult to make at clean join at the fin without taking special precautions. Cut bands of fibreglass approximately one and a half inches wide; these are for the seams.

Cut a small and of fibreglass for the tail fin leading edge. Dip the band in base resin, put it on one side of the tail fin and curve it along the leading edge.
Do the same with the other inaccessible parts, i.e., the inside of the fuselage near the tail fin.

Work through the apertures left at the base of the fuselage for the wing, and at the rear of the fin, using sticks as spatulas and a long-handled paint brush. The fibreglass band must be moved into position while the resin is still fluid.

With the two halves assembled, the job is now three-quarters completed. All that remains to be done is to apply the fibreglass strip to the other seams.

This is a difficult operation due to the lack of access. The band must be rolled up, soaked in resin, placed on the accessible pan of the seam, and unrolled with the aid of a long and curved spatula.
The second “trick” is to put the resin impregnated strip on to a piece of balsa, slide this into the fuselage and tum it upside down so that that strip drops onto the seam.

Once this strip is in position, cut off any excess material.

Leave the fuselage for several hours so that the resin is perfectly dry before taking away the moulds.
Unscrew the mould retaining bolts. With the aid of a screwdriver gently pride the two halves of the mould apart, paying careful attention not to damage the joining surfaces of the moulds.
Gently press away any excess fibreglass along the seam of the fuselage, then wash of the release agent with water.

You now have a fibreglass fuselage.
The back of the fin should be blocked with a piece of balsa and fixed with resin. This will provide a firm mount for fixing hinges. On the inside of the fuselage, cross ply the matting to aid strength
One might imagine a great deal of work being involved in all this. In fact, the opposite is true!
Once the formers and engine mount are installed,  the rest is easy. Bearing in mind the precision of the mould, the formers can be cut and pierced beforehand - this is, of course, a great advantage.

The fuselage should be rubbed down with a fine wet abrasive material (grade 400) before being painted with Epoxy.

With a little experience. it is possible to build a fuselage in one evening. Cost is relatively little. With this type of material it is possible to build models weighing less than 6.5 lbs, quite suitable for competition flying. The two main advantages, therefore, are speed and cheapness, and also versatility of fuselages thus made. Rounded curves, aerodynamic forms, wing supporting flairs, etc, are difficult to realise: when using only balsa for construction.

Many people decry the use of polyester plastics for fuselages. They say that this type of material gives rise to vibration and interferes with the radio equipment. We do not agree with their views. Vibration has never given any trouble. During many months of flying the author had no trouble at all. The servos were simply stuck to the side of the fuselage on rubber mounts. Reasoning is quite simple: the sides are fairly resilient and quite elastic in themselves.

Gliders, Control-Line and other types

Although the text has so far dealt exclusively with a radio controlled subject, application of the same method could produce fuselages for other types ranging from team racers to slope soaring gliders. In fact, the control line models have a lot in common with the radio example except that they are in general smaller. For some subjects it will he necessary to add extra reinforcement, around tho nose of a glider for example, and also to prepare for removable hatches which will have to he cut away.

As a club project, the polyester fuselage has a lot to commend it. Shared cost of the original carved or plaster shaped master, and the preparation of the moulds, will result in production line procedures which bring satisfaction to a whole group of clubsters. Moreover, it introduces a club “shape" of model - a uniformity that some people like to adopt for identification on the field.

Wednesday, 1 June 2016

Mystic 40

This is Hanno Prettner's 1993 World Championship model - scaled down for a piped 40 or possibly a 60 sized 4 stroke,

It results in a model of about 56" span and hopefully quite light.

 The design was actually pulled together using DevFus for the Fuselage and Profili for the wing. This enabled me to get the shape of the formers exactly right without needing to guess or manually interpolate.

A very slender fuselage and surprisingly deep. Its not really obvious from the until the size of the fin and wing is considered.

A detailed build blog exists on the Model Flying Forum  which covers many of the constructional aspects of this model and will not be repeat here

The output from Profili and DevFus were combined into a single file and edited using DraftSight. The parts were transferred to DXF and sent Dylan at LaserCraft for laser cutting - Liteply and Birch Ply wing ribs and fuselage formers

So rather than repeat the build blog, just a few progress photos here:

Tailplane construction is quite simple, the trick is to select good quality light wood. Elevator hinges using Kevlar cloth which is what I tend to do nowadays

The fuel tank area and Firewall are assembled as a crutch and the fuselage built up around it

Fuselage construction is quite straightforward

Upper decking is 1/16" balsa sheet

Wing construction is traditional. Building tabs used to make sure the wing stays flat

Retracts are HK copies of e-Flite and a cheap MG996 servo used as the retract servo.

And a video of them working

Leading edge sheeting added and a dry fit of the model - looking OK :)

Next bits were the tuned pipe tunnel, ailerons wing tips and rudder

 Wing tips hollowed using a dremel and sanding drum after the external shaped had been roughly carved and sanded to shape. A lot more wood than that came out.

The lower fuselage was crafted using lite ply formers and soft balsa sides

 Rudder laminated from 3mm balsa. The fin cap has a 3mm lite ply base to help keep it nice and straight and importantly, add a lot of strength. Balsa on its own is just too fragile

Ailerons laminated from 5mm balsa sheet over the Kevlar hinge. The ends are capped with 3mm lite ply to maintain nice crisp edges

 The Canopy came from an old defunct model that lost its life when an aileron linkage broke..

And so far, it looks like this.