The Oz Report

WAG - addresses

(This topic is in: 3.133 3.130 )

I'm looking for the addresses of foreign pilots who attended the 1999 Spanish National Championships.  If you have an e-mail address for any of the following pilots, please send it to me:

Betino Schmitz, Oleg Bondarchuck, Allan Barnes, Robin Hamilton, Lars Johansen, or Andreas Ohlsson.

I'd sure love to know what each of them thought of the site.  I've got an interesting article regarding CIVL and WAG and I'm just waiting for permission to publish.  My address is <davis>

Competition survey

G.W.  Meadows and Dave Sharp are looking for input re US national competitions.  They are asking for hang glider pilots to fill out their survey at http://www.justfly.com/compsurvey.html

Results of US National Team Raffle

Vicki at Moyes was pretty excited to tell me that Bill Watters, the US pilot that won the raffle, and had the choice of a Wills Wing, Aeros, Seedwing, Altair, Laminar or Moyes glider, chose a Moyes Litespeed.  Okay you other manufacturers out there, what's the good news on your front?

More on carbon fiber

My previous article on pre-preg carbon fiber contained an open invitation to hang glider manufacturers to write in about their approach to carbon fiber construction.  Fortunately, Dave Swanson, who works on the Light Hawk project, and who is one of my more enthusiastic readers, wrote in to give us all the benefit of his extensive experience.

The Light Hawk is a "foot-launchable" sailplane. It has been a long time in development, and we are looking forward to seeing it in action next year.

Here is what Dave had to say:

In pre-preg cloth, the epoxy resin and hardener are mixed together in their proper ratios.  This mixture is placed in the resin bath of a pre-impregnating machine.  The fabric is run down into the resin bath to fully coat the fabric.  A series of rollers and squeegees assure complete wet-out of the fabric.  The resin is partially cured in the factory. This "B-stage" fabric is then placed against a carrier plastic and cooled to prevent complete curing.  "A-stage" is the fluid resin and hardener mix.  "B-stage" is the partially cured, gelled mix. "C-stage" is the cured epoxy.

There are many different types of pre-preg.  Hexcel, Ciba-Geigy, Advanced Composites, Dexter, Ashland, and hundreds of other companies around the world, have a resin system that is then impregnated into the fabric.  The fabric can be a fiberglass (E-glass or S-glass), Kevlar, carbon, boron, ceramic, etc.  The weave can be whatever the composite fabricator or engineer wants.  Each resin system has its own physical properties of tack and drape, cure cycle (including need for post cures), as well as strength, impact, shrinkage, temperature, and modulus characteristics. Pre-pregs can be ordered with almost any degree of softness, drape ability, and tack.

Re storing cloth at room temperature for up to six month, that sounds scary.  All epoxy systems are thermo set resins.  They give off heat as they cure (exothermic).  It was stated: "The cloth is keep in the freezer until cut and formed, and then heated to cure the epoxy." Even a room temperature cure pre-preg resin system needs >120°F to cure properly.  Any elevated cure system will never reach its full properties unless it is brought through the proper cure cycle.  Any deviation from the proper cure cycle will reduce the cured resin physical properties and affect the end products' fitness for service.  All epoxies that sit at room temperature are curing but not in the proper fashion that the manufacturer specifies and this affects the final strength.

Every aircraft company that uses pre-pregs in a certified aircraft keeps track of the time out of the freezer and the time before starting the cure cycling as it affects the final physical characteristics of the laminate.  If this out of freezer time exceeds the limits, the fabric is thrown away or not used for structural parts.  Resin left at room temperature will never cure properly even if it is run through a proper cycle after 6 months at room temperature.  It will never reach the full structural properties.  Every epoxy resin system goes through a cross-linking of molecules to obtain the end physical properties.  (Polyester and vinyl ester resins polymerize.) Any deviation from the manufacturers recommended cure cycle means that the end product is less than what was originally intended and who wants to risk their life and company on such an unknown.

The proper cross-linking of molecules is very much dependent upon temperature.  The resin is stored frozen to prevent cure and minimize degradation.  If the resin system was intended to be stored frozen, its best to do so, as anything else causes deterioration of its physical properties.  The final flight part's structural properties are highly dependent upon the resin's strength.  Please don't use elevated cure resin systems that have been stored at room temperature for any structural component.

The "freshness" of the pre-preg resin system is judged by the tack and drape of the pre-pregged cloth. The tack is the stickiness of the resin system at room temperature.  The drape is the ability of the pre-pregged fabric to contour to the mold surface.  If the pre-preg is old or has less tack, the temperature rise of the cure cycle will allow the resin to soften fully and flow into the adjacent fabric so that there is bonding between the different layers.

If the vacuum is not adequate, or the temperature rise is too rapid, or the oven temperature too low, there will be a poor bond between the fabric layers.  If the pre-preg fabric has poor drape characteristics, it may not fully contour to the mold surface, resulting in a discarded part.

The strongest structures are made with the optimum fabric to resin ratio.  The exact optimum percent of resin will vary with the style of fabric and the nature of the fiber orientation.  Ranging from 25% resin for pre-cured unidirectional lay up made under pressure and tension, to about 50% for B-staged cross-plied laminate using open weave fabric styles.  With resin content above 50%, the structure is almost always less than optimum unless one is using Kevlar fabric.  The pultruded, pre-cured uni-directional carbon rods easily out perform any and all pre-pregged spar cap strips.

Pre-pregged fabric gives a much better consistency of fabric to resin ratio and a better consistency from part to part.

Almost any elevated cure resin system will allow for ease of assembly at room temperature.  Some resins will never fully cure at room temperature, while others will cure at room temperature and then, after a suitable post-cure, exhibit high structural strength.

The temperature at which the strength of the resin degrades (softens) is called the Tg (T sub g) (glass transition temperature).  Room temp cure resins like: Aeropoxy has a Tg of 194°F (90°C), or Dow Derakane 411-45 vinylester's Tg is 210°F (98°C), while polyester is <150°F (65°C).  Some of the elevated cure (>250°F 121°C) epoxy resin systems have a Tg in excess of 450°F (232°C).

Not only must the oven used to cure a pre-preg part be large enough, but the temperature must be carefully controlled over the entire part and tool surfaces.  The rise time (°'s per minute), the plateau (temp vs.  time), and the fall time (°'s per minute) must be accurately controlled and documented if it is to be a structural flight part. Scrap pieces are also cured with each part to document craftsmanship.  Before the cure cycle is started, the part is put under vacuum bagging to squeeze the fabric layers together and start the flow of the room temp resin.  Then the part and tool are placed in the oven for the proper cure cycle.  This costs time, materials, and labor.

The carbon fiber is the same material used in wet lay-ups, so the fabric costs the same.  Depending upon the resin system and the availability, costs can vary widely.  Remember, the pre-pregger will charge for his services no matter who formulates the resin system. Obviously, the larger the order, the better the price.  The tool (mandrel?  in Ian's case) must be able to take the change in temperature too and do it without deformation.  The costs of freezer and oven add little to each part's overall cost if done in sufficient volume.

Congratulations to Ian for bringing composite hang glider parts into the 1990's.  Maybe some readers would be interested about Ian's comments on how much easier it is for him to do the actual pre-preg lay-up around the mandrel as compared to the wet hand lay-up techniques.

We use the PTM&W Aeropoxy (they are one of our sponsors) resin system (room temp cure) and hand "pre-preg" the fabric before doing the lay-ups schedules on the Light Hawk and consistently get 48-52% resin content.  Aeropoxy is easy to use, has a relatively low cost, low toxicity (we always use gloves), and high performance numbers.

A black part, like unpainted carbon fiber, can have a surface temperature >200°F (93°C) when exposed to the sun.  Ian is totally safe in allowing his parts to heat up in the sun because he has a resin with a still higher Tg.  In the Light Hawk, we use Aeropoxy with a <200°F Tg thus requiring us to paint everything white.  For us, any exposed composite material is covered with a UV blocking acrylic white paint.  Ian doesn't need to do this.  He lets the beauty of the carbon show through!!

Proud Supporter of:

1999 US Team

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Davis Straub
co-author of Windows 98 Secrets, and More Windows 98 Secrets, IDG Books
"The Jaws of Life for Windows 98 Users"
<davis>
http://www.davisstraub.com/secrets