Thursday, 4 August 2016

Cheshire Cat - 100S Glider

The glider bug is quite infective. After not building a Glider for several years, this is my 3rd in fairly rapid succession.

This is a 100S contest glider, the class rules dictate that Rudder Elevator and Spoilers are the only flight controls permitted, i.e. no ailerons and wing span restricted to 100". This class of model is also eligible for the 'OPEN' class events as well - F3J and F5J if electric powered.

There are very few 'modern' designs around that are suitable for home scratch building, so this is my own design with a great deal of help from Jef Ott on the BARCS forum.

The full build blog is available here on BARCS:

To add a little extra, this model will be an 'E' version with an electric motor, height limiter etc. so that it will qualify for the Bartlett's league - if I ever manage to get down there.

Our local field isn't particularly brilliant for tow or bungee launching so I need to have an alternative method of launching models.

There have been several iterations of the design, starting from a simple 'Vee' dihedral aircraft in imperial units and ending up as 6 panel wing in metric units... Wing section is Mark Drela's AG35 at the root tapering to AG37 at the tip.

I used a spreadsheet to calculate the various stability values, that in itself was a very interesting exercise..

These are the 3 key stability values, I need a model that can fly by itself if I ever need to rest my eyes momentarily.

The latest iteration looks a little like this:

The construction follows that of a 'Bubble Dancer' that I built late last year, in fact the tailplane is scaled directly from the BC plan. Boom, tail mount and spars are all Carbon Fibre or CF Composite - again following the same type of construction method used on the BD.

Tailplane - a little heavier than hoped for at 16gms

 The covered tail assembly showing the Carbon boom and Vladimir tail mount - from Hyperflight

Wing construction (inner right panel) in progress. Lasercraft produced the wing ribs for me - beautifully cut as usual.

Updated: 2016-08-04

The wing went together in a similar manner to the Bubble Dancer, the spars are actually thick soft balsa webs capped with carbon fibre caps that go down the length of the panel. Thinner caps are used in the tip sections to reduce moments of inertia (weight!)

This shows the spar (web), lower carbon cap and a carbon strip reinforcement that goes over the dihedral break

The top cap is added (and in the centre panels) bound with Kevlar. Probably not needed for an Electric model, but I'll be able to use these wings on a winch launched towline glider version if needed

The spoilers were a bit tricky. I originally wanted to use wing mounted direct drive servos but I couldn't get them to fit so resorted to a bell-crank and fuselage mounted servo driving snakes through the holes originally intended for the servo lead.

 Like this.. The horn was bent (downwards - this is upside down) to clear the push-rod.

Next job was to get the fuselage pod made.

 Manufactured from Lite-ply with balsa triangle section corners, it looks very uninspiring

 But with a bit of work it can be made to look quite attractive

 I can never resist a dry assembly shot :)

The Front end - a 40mm Aluminium spinner and a reasonable fit, 5 degrees of downthrust, no side thrust

Servo's are mounted at 45 degrees so that the arms don't interfere with each other (the spindles are on the fuselage centre line. The 3mm Carbon wing alignment rod passes through the fuselage as you can see

A bit of work finishing the fuselage, making some nice curved hatches and covering the wing

And it weighs this much with batteries.

And a shot of the Cheshire Cat modelled by my beautiful No. 2 daughter :)

Just a maiden to come..

Thursday, 14 July 2016

Mad Hatter F3-RES

F3-RES is a new low technology class thermal soaring class. It's well established and popular - in German and USA making good use of modern wing sections, limited Carbon or Glass Fibre components. Models tend to very light and have excellent performance if built accurately.

A small group of us in BARCS are trying to get the class introduced into the UK, initially as a class for a postal event - and if this grows then we hope to encourage local, regional and national events.

Flights are flown in 9 minute 'slots' with a 6 minute maximum and spot landing bonus. Launch is via a standard bungee and towline which should get the model to about 80-90 metres before release. Models must be built from wood (predominantly) with composite materials allowed for the boom, tail and wing mounts, wing spars and wing joiner. EPP, other foam or moulded flying surfaces are excluded.

I am planning to build an own design glider, named 'Mad Hatter'.

 It's basically a condensed version of my Cheshire Cat 100S design - which at the time of writing is still being built. There is misplaced coincidence for you.

The wing section is the well proven Mark Drela AG35. This uses 10x0.5mm Carbon Fibre caps over 10mm wide balsa 'webs' (in reality the spar) and 6x0.6mm caps in the tip panels. No Kevlar thread spar winding will be needed in this model, it does not have to carry the load of a winch launch.

This is sheeted as far as the 'flat' sections of the airfoil with covering support spars at the rear

I am getting quite interested in carrying out stability predictions and using my spreadsheet I have the following:

The model will undoubtedly change as my thoughts develop over the next few weeks but this is my starter for 10.

Wednesday, 13 July 2016

Lavoie method of building Stick Model Aircraft Fuselages

Posting this for posterity so that I don't lose these links again. I use a derivative of this method and as time moves on I use more of the ideas presented.

Three videos - well worthwhile watching if you are building lightweight rubber power scale models for indoor or outdoor use.

Part 1

Part 2

Part 3

Highly recommended

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.

Monday, 9 May 2016

Sloping Off - a wonderful afternoon flying

It started with the customary email from Ian Jones. Weather forecast nice with a decent breeze from the East. My opportunity to try out slope soaring on Bosley Minn, a N-S ridge at the western edge of the peak district.

The weather was superb, bright sunshine with a 20-30kt Easterly wind.

I arrived first and helped the farmer fit a new gate - always a good sign if the landowner is friendly.  Ian arrived about 10 minutes later with a few slope slopers and we assembled and headed onto the field about 100 yards away

Ian went first

After a few moments of trepidation I flew my own design Glass Slipper - Ian now on the camera and then me back on the camera while Ian had a fly of her.

After a long and fun flight or three, I decided to try the Bubble Dancer on the Slope. By now, the wind had started to drop although it was still quite  gusty, so waiting for a lull and the model went up vertically by about 20 feet and with a little down elevator I manager to get her away from the slope.

Unfortunately the glider was over elevated for some reason and due to the amount of lift was a nightmare to try and land. However, I did get her down in one piece and breathed a huge sigh of relief.

Great Fun.

Another couple of flights with the Glass Slipper and I was away home at 1630. Absolutely wonderful afternoon :)