"BALSA FREE" C/L RACIN G W IN GSor "a different approach to make a wing using a mould and (almost) no balsa"

by Enrico M AULETTI Jan - 2001

1st PART

THE M O ULD(first part - upper mould)

"Balsa free" wing means (for me) a glassed foam wing made using two devices:

- a two parts mould

- a simple vacuum bagging system.

In my opinion, there are several reasons to think about a "balsa free" wing:

1. Possibility to reach always the same result:

1.1. Profile: if the mould is O K, then all the moulded wing will be O K

1.2. W eight: using "constant density" materials as foam and fibre glass, it will be possible to have (almost) the same

weight

1.3. Stiffness: using "constant properties" materials and a standardised procedure, the final result will be always

the same

2. Possibility to reduce working time (accepting to spend time in the mould preparation)

3. Possibility to avoid balsa dust … and lot of "sanding hours"

M y final weight target for a STR wing is something around 80÷85 g (without internal bellcrank and lead-outs) … here after I'll

explain why I think this is possible:

About glassed foam wing and vacuum bagging: I read a lot on Internet about how to produce foam glassed wing for different

type of model glider … these information I collected convinced me that it's possible to follow a similar method for C/L racing

wing too … so I decided to have a go! (one of the more sophisticated, but really interesting web page:

http://www.delago.de/ariane/EHome.htm)

Here after, a section of the mould, the fibre glass clothes and the foam sheet during the construction (of course the same job

has to be done for both the moulds).

wing surface [dm^2] 12

fibre-glass cloth [g/m^2] 42no. fibre-glass cloth layers (upperand lower wing surface)

2

total fibre-glass weight [g] 20.16

carbon rowing (leading edge) [g] 10 ?

foam density [g/dm^3] 30average foam tickness [mm] 4foam weight [g] 14.4

epoxy resin [g] 40 ?

TOTAL WEIGHT [g] 84.56

WEIGHT DISTRIBUTION

The basic idea is to use 2 fibre glass cloth layers (one at 90°, the second at 45° for torsional stiffness) for each wing surface

(top and bottom); with the vacuum bagging technology is possible to obtain 2 separated shells, one for each mould, with a very

low resin quantity (that means "light and stiff").

To fill the space between the 2 shells, after the curing of the epoxy used to produce the 2 shells, two 4÷5 mm thick high

density foam sheets are glued to the internal part of both the shells (still inside the moulds); these foam sheets are not shaped

to any profile (just cut with the top view shape of the wing), but just pressed on the internal surface of each shell by the

vacuum.

After curing it is possible to sand (or to cut with an hot wire) the foam in order to have a flat joining surface between the 2

parts of the wing; as reference the flat division surface of the mould is used.

Before joining the 2 parts together it could be possible to put internal leadouts, but for the moment I think to make the first

test with external controls and bellcrank.

At least for the leading edge is possible to make a groove in the foam and to fill it with carbon rows before to join the two

shells.

In this first chapter it's possible to find some pictures and descriptions about the mould construction … this is where I'm now …

in other words: it's possible to follow step by step what I'm trying to do, but I'm not sure at the moment that it will be a

success … of course, I hope so !!!

This is my first attempt in making such a big mould … so surely there will be things that can be improved!

N ote: O f course a trapezoidal wing top view can avoid the necessity of a master wing like these: it's possible to cut a master

using foam, templates and an hot wire or, may be, it's possible to avoid any mould.

This is my STR (you can find it somewhere in the cl_group model plan collection):

As you can see it's trapezoidal … and it's really "different", but it's the best model I never had, believe me!

(I fly C_L racing profile models since 1978 …I stopped my activity for ~15 years and now I'm back since 4 years ◊ 7 years of

racing with any kind of model)

The reason why I decided to use a "traditional" wing shape is only a "marketing reason".

"Down under" someone made this model and another guy said him: "… why you decided to build this model, it's so different,

unusual … try a Challenger, this is really like a F2C model!", so if I'll have in the future any change to sell some wing (but this is

absolutely not my first and only goal; I'm just thinking / hoping to supply some RTF wing in order to catch some newcomers or

"returnees") I need a "traditional" shape that looks really like a F2C model … something like this "Russian wing"!

Picture 1 - The basic idea is toproduce a two parts mould (upper

and lower m oulds) using a

"commercial" wing as master.

The two moulds will be divided

along the l.e. and t.e.

The green hollowed sheet is 30

mm thick high density foam that I

will use as division surface.

The grey part is the base to

support the m aster wing.

The first problem to solve is how

to align the master wing in order

to have the l.e. and the t.e. at the

same height of what will be the

division surface of the 2 moulds.

Picture 2 - The 3 "things" hereare M 3 screws with a small balsa

block glued on the head.

They are inserted inside 3 holes

and it is possible to adjust their

height screwing or un-screwing

the relative nut.

Adjusting the height of each

single screw is possible to align

the wing.

Picture 3 - W ith the 3 "things"

("aligning screws"?) inside the

holes, using double face sticky

tape, the master wing is fixed on

them (at this stage no alignment

is done).

In this way it will be possible to

extract the wing plus the 3

"aligning screws" without loosing

the relative position and the

alignment (if necessary).

Picture 4 - To check the rightalignment and height of the l.e

and t.e. I use a simple device

made with a pin fixed on the top

of a small foam block (the small

foam block is a piece of the

hollowed 30 mm high density foam

sheet so the pin is at the sam e

height of the division surface).

All around the wing perim eter,

adjusting the "aligning screws",

the l.e. and the t.e. must be at the

same height of the tip of the pin.

Picture 5 - This is the picture ofthe m aster wing after the

alignment.

The wing and the 3 aligning

screws are in position on the

support (the grey part in Picture

1) as the foam sheet too.

The hole inside the high density

foam doesn't fit perfectly the

shape of the wing, but is tight

enough to allow a supplementary

alignment check.

Picture 6 - This is another

picture of the master wing after

the alignment. As you (perhaps)

can see, the upper surface of the

foam sheet is coincident with the

trailing edge, being the "division

surface" for the m oulds.

But only 3 supports are not

enough to avoid deform ation of

the m aster wing during the m ould

construction … this is the second

problem: how to support the wing

in a better and well distributed

way in order to avoid as much as

possible any deform ations?

Picture 7 - In order to have abetter and well distributed

support of the m aster wing I use

around 50 M 4 screws. They are

screwed trough the support from

the bottom surface (see next

picture).

Picture 8 - Each screw has a nutglued with cyano to the bottom

surface of the support.

Picture 9 - Looking from theupper surface of the support it's

possible to have an idea about the

distribution of the 50 screws in

respect the shape of the wing.

In order to not damage the

master wing, on all the tips of the

screws a small plywood part is

glued on (see next picture).

Picture 10 - N ow the wing plusthe 3 "aligning screws" are again

on the support.

After this is possible to adjust

the height of each "supporting

screw" in order to have a contact

between the tip of the screw and

the lower wing surface (of course

it must be possible to reach the

screws from the bottom of the

support).

Picture 11 - The only differencebetween this picture and pictures

5 - 6 is what it's not possible to

see!

N ow the wing is evenly supported

by the 50 screws adjusted at the

right height.

Picture 12 - As I wrote, the holeinside the high density foam

doesn't fit perfectly the shape of

the wing.

In order to m ake the mould is

necessary to seal with some putty

all around the master wing

perimeter. The putty must be

coincident to the division surface;

I use a sharp knife to cut the

putty all around the wing

perimeter.

To have a good sm ooth division

surface I put all around the wing

some adhesive tape.

Picture 13 - This is the wingready for the gel-coat application,

but … remember the mould

releaser … and hope it will work

correctly !!!

Picture 14 - Around the wing Iput a fram e m ade with 20x20 mm

wood. I fix it on the foam using 8

nails (just to avoid any m ovement)

and then I add 2 other wood

pieces in order to limit the area

where the gel-coat is applied. The

gel coat I use is ARALDITE 419-1

with HV 2419 hardener. After 2

days, over the gel-coat, I apply a

layer of fibre-glass mat with

epoxy resin. Then, after other 2

days, in order to have a good

support for the m ould, I fill the

frame with plaster. The 2 white

arrows indicate 2 tubes I put in

contact with the wing surface;

blowing some air trough them I

hope to be able to have back my

master wing at the end of the

mould construction.

Sorry, no picture here

(I'm using a traditional cam era and the film was over at this point,

but I promise to continue m y reportage …)

N ow (12th January ’01) the first mould is m ade: it looks great!

O f course the wing must stay inside the mould to proceed correctly in

the construction of the second part of the mould.

In the next days I'll proceed in the sam e way to produce the second

part of the mould …

… then I hope to be able to take out from this "sarcophagus" m y

precious master wing … stay tuned and "m ay the fortune be with m e"!!!

As usual: English is not my

mother tongue … at least I hope

you'll understand the meaning of

what I wrote … let the pictures

help you!

Enrico Mauletti

Absolutely no copyright: I'll beglad to know that this has been of

some help for som eone … at least

to avoid to repeat my mistake …