ARTICLE ON THE USE OF CARBON FIBRE/COMPOSITES IN MODEL AEROPLANES                                                                                                                                                                                                                  BOB HORNIDGE

I have been using fibreglass, kevlar, and carbon fibre products for a few years.  I am not an expert on the use of, or construction with, these products but can only pass on some of my experience with them.  I built a few pairs of speed skates in the past using kevlar and carbon fiber cloth (with epoxy), and have used composites as building materials for free flight and controline planes in the last few years. 

Fiberglass, kevlar and carbon fibre are available as a normal cloth, as a cloth with a fibre layout like silkspan, as tow (threads), and sheets of unidirectional fibres (perhaps CF only).  These materials have no structural strength by themselves.  When treated with epoxy, they gain considerable strength and are called composites.  Fiberglass cloth/epoxy is a good all around reinforcer and is cheap and available.  Kevlar cloth/epoxy has great impact strength but the cloth is hard to cut .  Carbon fibre/epoxy is easy to use, very light and is very rigid but is not crash resistant.  All of these materials are readily available at model airplane stores or composite materials outlets.  In Vancouver there is a company called Coast Fibertek Products where I have purchased some supplies.

Composites are also available in made up form from model supply stores and other places.  CF arrowshafts are good and fairly cheap structural members (available where archery equipment is sold).  Tailbooms for Hand Launch Gliders are available and are virtually indestructible under normal use.  Larger tailbooms are available for large sailplanes.  Our local model store has made up structural members (CF).  Our local canoe manufacturer in Abbotsford, is a good source for kevlar (they sell scraps cheap), and also West Systems epoxy (not particularly cheap).  I have purchased some silkspan type (CF) cloth from Great Hobbies.  Carbon fiber fishing rod sections can be used for tailbooms on larger sailplanes and might be suitable for wings on controline planes.  Smaller diameter round CF tubes are great for pushrods.

The picture below shows scraps of (from left to right), kevlar cloth, carbon fibre cloth, and carbon fibre tape.  Below that is carbon fibre tow (threads pulled out of cloth is the simplest way to get tow).  Tow can be useful to wrap around round things like pushrod ends where they need reinforcing.

The picture below that shows (top) unidirectional CF.  It comes backed with very light fibreglass cloth.  The fibres are one directional and it is very easy to slit with a razor blade and straight edge.  I have found this stuff to be very useful.  (Bottom) shows the CF with silkspan type fibre.  I have found this to be useful for wrapping rubber powered free flight fuselages (hollow tube construction).

 

The picture above shows (top) a large tapered CF tailboom for a sailplane, (middle) a small tapered CF tailboom for a hand launch glider.  Below that is a CF arrowshaft (and the toe of my slipper).  I have found arrowshafts up to 30” in length.  Other types of made up composite members are available.

Anytime you want to stiffen something without a great deal of weight penalty, you should consider using composites.  You can even make up your composite with glue instead of epoxy.  It will not be as strong but may be lighter and easier to do.  I have used water based glues and model airplane glues with these cloths.  It is also practical to use polyester lining material (from a yard goods store), instead of fibreglass, kevlar or carbon fiber.  It is not as strong but sometimes it is just as good.  It also can be heat shrunk or sucked onto a surface by dope and silkspan.

I presently use 2 or 3 layers of CF cloth and epoxy to replace the 1/8” ply doublers on a controline airplane.  I also use the same cloth on some of the ribs. 

The 2 pictures below show my new controline plane fuselage.  The ribs were salvaged from the wreck of the previous version.  I have shown the arrowshafts through the fuselage where the wings will be installed complete with the arrowshafts the wings were built on.  Actually the wings were salvaged from the previous wreck too and they are in pretty good shape.

 

 

On my bigger ukies, I have started using CF arrowshafts as wing leading edges and I put one down the wing ribs at the midpoint of the chord.  This arrangement does not interfere with the controls at all and provides quite a bit of torsional strength.  It also permits using the CF arrowshafts as jigs to build a very straight wing.  I built one plane with this arrangement and no spars but in flight the wing flexes up and down quite a bit.  So, I now use spars as well.  I also have used unidirectional CF as cap strips from trailing edge around the rib, over the leading edge and around the rib back to the trailing edge.  This requires a bit of learned skill to make a nice job. 

When I do the CF fuselage doublers, I lay the parts up on a plate glass surface.  I put my layers of CF on the fuselage side.  I weigh the CF cloth down with glass above in order to get good adhesion.  I use a brick on the top glass and I use peel ply (available at composite stores), in order to avoid everything sticking together.  If you use 1/8” ribs, you can apply CF to the ribs under the same glass/peel ply sandwich.

 

An important consideration in using epoxy is the fact that if you wait a long time in between layers or joints of CF/epoxy, you will end up with a mechanical bond between the layers or at a joint.  If you put in the next layer or joint soon after initial cure, you will get a chemical bond which is much stronger.

I probably should go all the way and produce complete fuselage shells.  I have the vacuum bagging device to do it but don’t have the interest.  Certainly could make light cowls etc. pretty easily.

On free flight airplanes, I use a lot of CF in strategic places to make for a light and strong airplane.  I have a large towline glider with CF (arrowshaft), type spar.  I use unidirectional CF for fuselage strengthening on hand launch gliders.  I have a couple of HLGs with CF tailbooms, and three bigger sailplanes with CF tailbooms.  On rubber powered planes with a hollow fuselage, CF can be used to make a light balsa rolled tube fuselage very stiff but still very light.

I believe that Composites have limited use for controline.  For the best performance, some of our PA airplanes need not be any lighter than can be constructed by traditional methods.  Also, they are plenty strong for flying and crashes are the big destroyers of these planes.  And if you make them very strong in one area, they will just break somewhere else.  For some of the other events, composites may be more useful.  If however a complete fuselage was formed and mass produced (by an individual for themselves and this is practical), then you might have something useful.  It would be possible to have a generic

fuselage and bolt in wings and empennage.  That would provide for a lot of practical experimenting and lots of backup models.

If anybody is contemplating trying some composite construction, I would be happy to help with whatever answers and suggestions I may have.  My construction methods have evolved to not running the wing through the fuselage anymore.  Now, I run the arrowshafts through the fuselage and the wings plug in by the arrowshafts.  It is different but I like it.  Probably bolt in wings should be the next step.

I have not gone into the health hazards of epoxies and sanding dust from CF and other composites but I would suggest that anybody using these products do their own checking into the risks.

This last picture shows (top) a rubber powered free flight model (less wing and stabilizer), using light CF wrap on the motor tube.  Below that is a discus launch RC sailplane (wingless), with CF tailboom and below that is an FIA free flight (towline glider) with a CF tailboom.  It has a CF boom (painted white), and the wing has CF spar, trailing edge and rib cap strips.

 

 

 

 

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