VACUUM BAGGING WINGS:

It is very evident if you are going to succeed  in pylon racing you should ensure you have a perfect and fast aircraft to fly.  I am sorry to say that Vacuum Bagged wings - is almost a necessity now days.  Why? - Because the finish on these wings are superior to the finish on the old balsa covered foam wings.  Also the airfoil systems are more accurate.  In other words the Vacuum wings are more efficient.

I have often thought about writing an article about Vacuum Bagging of wings.  Some time ago I was given the opportunity to get the chance when I visited Vernon Gibson's home and he offered to give me a demo.

This is really a follow on from the article that I wrote about the  Cutting of Foam Core wings using a system controlled by a Computer - Which is detailed in this web site.

Vernon is involved with this cutting - and as a logical extension to this he just gets these perfect cores and then adds some Carbon Fibre to the outside of these cores to finish them off.  This method ensures that you maintain a perfect airfoil.

The most common method is to to make two female moulds and lay up the Carbon Fiber into these.  After which you add some spars and join the two halves of the wing together to make a hollow wing.

Vernon has a different approach - he just bags his cores. To do this he utilizes the Male and Female bits of original cutout from the foam core wings that he is using.  He reinforces these to make a male and female mould. Incidentally he uses blue foam instead of white.  I thought that this would be too heavy - but after inspection I found that there was very little difference in weight between the two foams and the blue foam is so much stronger

There are arguments for and against this method.  I personally like it because with the other method you have to make a mould.  History will tell us that no one is happy with the wing that you make - as you are constantly trying to improve your performance.  Especially when it is not unusual that the wing skins or mould will warp, during manufacture or over time.

SO WHY GO TO THE TROUBLE OF MAKING A NEW MOULD WHENEVER YOU WANT TO CHANGE ANYTHING.

Detailed below is the method that Vernon has developed and is currently using

THE EQUIPMENT THAT IS REQUIRED: 

The picture in this article is the set up that Vernon uses.  He has a rather expensive “Gast” Vacuum Pump, which I think cost about $600.  I believe that you can use an old Refrigerator pump and run it in reverse.  I was also told the you can use and 12 volt car tire pump – which can be picked up very cheaply at and any auto store.

A Vacuum Pressure switch that will switch on the pump and turn it off when the pressure gets above and below a certain point.  The setting is 17 inches of mercury.

A vacuum gauge to set the desired level of pressure.

Some bag material – 600 mm poly tube – in roll form can be purchased from a major hardware supplier eg Bunnings.

A pressure chamber – which you can make up from a 150mm Poly Sewerage pipe – cut to about a meter long and block off ends of the pipe with end sealers. 

A bagging valve that is simply a rather flat headed bolt with about a 4 mm hole drilled down the center.  If you make one make sure that it is capable of being sealed into the bag without any air leaks - using some soft washers.

Some Mylar sheeting about 1.5 mm.

Carbon and Glass Cloth.

U section aluminum channel which is used in conjunction with rubber bungee cord to seal off the bag. You simply fold the bag and then place each end in the aluminum cannel and then push in the Bungee Cord.  This seemed to work fine for Vern.

Some pressure tubing to run from the compressor to the bag.

Some epoxy resin – West Systems 501 seems to be rather popular – use the slow cure version – that will give you plenty of time to do the job.  You cannot use Polyester Resins as they melt the foam.

This is the procedure that you should use to get a good result:

 

It should be understood that at this stage Vernon is bagging the wings in two processes – firstly the top of the wing and then the bottom.

 The two top and bottom shells of the wing must be joined together and strengthened.  The best way to do this is with Glass reinforced pressure sensitive tape.  Just run it along the full span of the wing along the edges.  Also on the corners etc.  Remember that you can use these shells over and over again.  The accuracy of these shells is very important, as they are essentially the moulds for your wing.  Also you must join them together as you will be molding a full wing – not two halves.

 You then cut out a Mylar shape the same size as the actual wing shell.  Then hinge this to the front of the shell for convenience purposes.

 This will be done for both top and bottom shells.

 Then comes the part of placing the cloth on the wing.

·        Spray Mylar with spray putty that can be purchased in spray pack form from any motor shop. This gives the epoxy resin something to stick to

·        Mix up some Epoxy resin.  About 80ml(400square inch wing) should be enough to do the top of the wing.

·        Place a small amount of resin onto the spray puttied surface of the Mylar sheet, and smooth it out with a plastic card.

·        Lay ¾ oz glass cloth over the wetted out surface.

·        Then place some more epoxy over it and again smooth it our with the plastic scraper – making sure that you remove all the wrinkles.

·        Lay 6 oz Carbon Fibre cloth over the wetted surface

·        Then place the remainder of the resin on the cloth and work it into the weave of the cloth with the scraper.  Remember that you have only mixed up 80ml and that should be sufficient to do one half of a 400 sqin or 2600 cm2 wing.   Naturally you would use less for a smaller wing.

·        Then place the foam wing core on top of the Epoxied soaked Fibre Glass resin.

 The next process is the bagging. (Note the lovely hands of Vern)

     Cut the plastic tube to size so as to run the full length of the wing.

·        Insert The Valve in the bag – make sure that it will not leak air.

·        Place the wing assembly in the bag.

·        Then seal the ends of the bag using the Aluminum U Channel

o       Folding the plastic material over a couple of times

o       Insert into channel

o       Then push bungee rubber into place over the plastic bag material and into the channel.

o       This should make the bag completely air tight.

·        Connect up the vacuum pump and start.

·        Stop pump after a while to smooth out wrinkles – otherwise when fully vacuumed resin will flow into the wrinkles in the plastic.

·        Then complete the full vacuum to 17 inches of mercury

·        Keep the vacuum alive and working overnight, this allows the resin to cure.

·        Then in the morning the wing can be removed from the bag.

TThis is the time when you install all the components that is required in the wing e.g. landing gear blocks.

Then you lay up the material over the bottom part of the wing, Naturally you use the bottom shell of the mould – Instead of the top –, which you used for the process as described above.

 One thing is different, and that is you have to fold over some of the excess fiberglass from the bottom of the wing over the top of the wing to form the leading edge of the wing.  Just pull it over and place and dampen it with resin.

Vacuum bagging again takes place as described above.

To make the ailerons  leave the top skin of the wing in place, and use it as a full-length hinge.

IMPORTANT: Vern found that the Wing core needed to be reinforced around the wing seat because it was creasing under load, and could cause a wing failure.  Because of this he adds some small spars in that area - You should definitely do that.

I Just cut two small slots in the foam and inserted two - 2mm ply full depth spars -  to about 5 cm outboard of the wing seat and that was sufficient. This was done prior to adding the glass and Carbon to the wing.

I hope that this gives you some idea about what to do.  I can tell you that the more wings that you make the better you will become!!