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Jack
20th May 2008, 17:07
Hi been a while since i last posted and have come a bit further with my design :smile:
just a couple of questions can any type of fibre glass be used in the construction? or does it have to be BID ( i think thats what it is :confused: ) and does anyone know if theres places out there that do a day course or something similar on how to use fibre-glass?? :confused:

Any help appreciated

Jack

mig29fuk
20th May 2008, 18:01
Jack
Do not approach fibre glass or carbon fibre building lightly.
You need to manage heat and humidity, measure very accurate the mix and ensure the wetting out techniques are well practiced. The various cloths give strength and shape properties through directional weave.
Very briefly. Bidirectional or BID allows shaping and by angular application certain types of strength. Unidirectional or UNI allows strength but less flexibility. Both can be buggers handle and cut wet or dry.
Resins are the next considerations. For lightweight Aircraft building Epoxy resins tend to be used. Polyester resins are used on larger items like cowlings but are more widely used in boat building.
If you are considering a prototype build then some experience is essential as a warm up through making test pieces. If you live any where near Bath I'd be happy to get you started. There is a Company in Bristol that may give instruction if that's of interest and I'll talk to someone near here who runs a Fibre glass Aircraft repair company. He may help but would probably need payment. I know he has trained CAA Inspectors. Let me know. I've built one 2 seat LAA Group A Aircraft and modified and helped with others and still produce items as required in this very capable material.
Regards
Gerry

sted65
20th May 2008, 19:08
Hi Jack,

Some very good advice from Gerry there, but one added piece of advice which may help you make the choice between epoxy and polyester.

Polyester resins never really stop moving in the normal aging process. This is mainly due to panels that are made in this way being unable to take the temperatures, with stability that Epoxy can, as evidenced by the wavy nature of parts that have been subject to sun heat, for any length of time. Epoxy, on the other hand, is used to make exhaust cans, and can be charred before the component becomes unstable. I usually lay up a small supply of GRP sheet, made with polyester, because it is usefull and can be cut to shape with shears - it can also be bent up, like thin aluminium, by the application of a hot air gun and held to shape with the fingers while it cools - I havn't measured the temperature but it's not very hot.

Think about the above when selecting the resin to make any part for an aircraft and you will concude that there is not much that is best made with polyester. My Thruster nose is made from GRP matt and epoxy, and it is very fragile - and very wavy - still I suppose that it was made to a price, but in my view, matt and polyester should never have been used for that application

Wally Hayward
21st May 2008, 07:30
One "trick" which I used many moons ago when making lighter, stiffer racing panels was the usual bonding in of stiffening ribs, but in this case hollow ribs.

Use a thin wall flexible rubber tube as a former, bond one or more layers of mat over it and before it goes off, pull the tube end, which will shrink the tube diameter and allow it to be pulled out leaving a hollow rib.

Lauriehurman
21st May 2008, 08:19
Jack

Before you put on the rubber gloves, do the sums.

I expect your design calls for the lightest weight component that you can manage to make. If you start making some test pieces you will quickly discover that you canít make Chopped Strand Mat (CSM) at less than about 800 gms / sq meter. I think the lightest CSM you can buy is 200 gms/M and you will see that you will need a couple of layers because of the "random" nature of the cloth. Using wet lay up you will certainly not get better than 50/50 resin /glass ratio hence 800gms/M is a (optimistic) minimum. CSM if fine for making moulds - it keeps the cost down and is easy to work around corners but if you are interested in the weight of the finished article you wonít be using it for components.

All of the above will lead you to using woven glass cloth. In my experience BiDi (which we always used to pronounce "biedie") is a word that I am used to hearing in reference to heavier weaves of cloth. The lighter weaves tend to be referred to as "woven" or sometimes "twill" (depending on the actual weave). The very lightest cloths that you can buy are usually called "satin".

It is normal in the average lay up to use two layers of woven in an "0/90, 45/45" orientation so that no application of force is ever more than 22 degrees off the axis of one set of strands. This produces a material which has the same properties in all directions (isotropic). You will see from this that the finished piece is going to be four times the weight of what ever cloth you use. Unidirectional cloth - in other words a mat in which all of the fibres go in one direction - is used to add strength in one particular direction, for one particular purpose. The problem with using Unidirectional is that - as far as I know - it is only available in quite heavy weights (as dry cloth) because of the difficulty of holding the cloth together (you would be amazed at prepreg uni which looks exactly like hair).

I expect that you are about to make some test pieces, if you havenít already. What you will find is your next problem which is the the sheet you make from your Epoxy resin and woven cloth is incredibly thin and hence very flexible. Two layers of 50 gm woven at 50% resin produces something with the thickness and rigidity of kitchen foil. It is strong but it is much too thin to be rigid enough to, for example make a wing out of (imagine making a wing out of kitchen foil). This is why everybody uses foam cores. They make two sheets of epoxy glass with a sheet of plastic foam between them. The glass provides the strength and the foam provides the thickness which gives the rigidity. But - it all has weight and you end up with a piece that is plenty strong and rigid but has a weight of 16 times the weight of the cloth you started with. The lightest cloth you can buy is 50gms/M so your component weighs 800gms/M. The end result is a construction method which is not overall a great deal better than any of the other methods available to us home builders.

Anyway, I donít mean to discourage you. I have been out of the industry a long time and Iím sure others on here know a lot more than I do. Make lots of test pieces and have fun and gain the experience that you will need to make your aircraft. Do a search of the posts on here and the Yahoo group because this topic comes up from time to time and Iím sure a lot has been written in the past.

Good luck.

Laurie (2)

P.S. Stay away from polyester, itís nasty stuff.

Jack
21st May 2008, 14:39
First of all thanks for all the information you provided has definetly giving me more to think about :smile:
It may be alot heavy than i initially thought especially as a novice it wont be quite the perfect layup :lol: I reckon on a 50 sqmetre surface (including wings) if thats true and at 0.8kg a square metre for a good layup that would come to 40KG minimum without anything else :shakehead:

Thank you very much for the offer of helping me start Gerry but unfortuantly i live the otherside of the country in surrey :sad:

thanks for the replies and ill keep you guys updated

cheers Jack

Lauriehurman
22nd May 2008, 08:18
Jack
I hope it's not too much of a herasy to point out that the PFA (the LAA) run courses on Composites for the homebuilder. Info on their website.
Laurie (2)