Electric Ducted Fans -- August 2004

The building season is just around the corner. So, since EDF models and foam core wings go together so well, let's take a look at the vacuum bagging process this month.

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Hello EDF fans! With the summer fun-fly season well underway, I'm sure lot's of you are concentrating on flying more than building these days. However, the building season is just around the corner. So, since EDF models and foam core wings go together so well, let's take a look at the vacuum bagging process this month.

Bagging Foam Core Wings

I like foam core wings. For small EDFs, a foam cored wing tends to be about the same weight as a built up structure. Additionally, with the core beds I find it much easier to wind up with a dead straight surface compared with built up structures. Strength is also excellent, and if you glass your wings it is a great surface to paint. Silkspan and dope also works out well as a paintable surface. I have always attached my sheeting with Epoxy and weighted the core beds with several cinder blocks. However, you need to be careful using this method and apply a fairly generous amount of epoxy. I have had one or two occasions where I was a little too aggressive in scraping excess epoxy off, and wound up with skins that were not attached to the core all that securely. Polyurethane glue is used by some with good success, and this avoids the adhesion problems since the polyurethane glue foams as it cures. The foamed glue tends to squirt out gaps, though, and can be a bit messy!

In late February I attended the WRAM show in Westchester NY. One of the vendors was Aerospace Composites. Aerospace was selling a complete vacuum bagging kit for doing sheeting on foam cores. The kit cost $100 and included 9 feet of 18 wide bag, 9 feet of breather felt, a fitting to get the vacuum into the bag, two re-usable clip systems for sealing the end of the bag, and a small vacuum pump preset to the correct value for white foam cores (which is approximately 6-7 inches of mercury). You must not use too much vacuum on white foam cores as it is possible to actually CRUSH the cores!

Lets bag a wing! First prepare your skins. Either edge glue enough sheets together to do the job, or secure the sheets together with full length strips of masking tape on the outside. If your core is a feather cut trailing edge type of core, the sheets should be placed on top of each other and taped together at the trailing edge. Set the wing beds aside for now.

Cut a section of bag about 18 inches longer than your core, and cut a section of felt 12 inches longer than the core. Apply epoxy to the inside of both sheets using a squeegee. Place the felt on the table, cover with waxed paper, place the sheeting on top of the wax paper, place the core on the bottom sheet, fold the top sheet over the core, cover the top sheet with waxed paper and secure everything in place with bits of masking tape as needed.

Now, slide the whole assembly into the bag, making sure the area with the fitting at the end has breather felt under the fitting. Close up both ends of the bag with the long clips and hook up your vacuum pump to the fitting. Finally, place the bag on the bottom wing bed and turn on the pump. As the air is extracted, smooth the bag if needed and hold the core flat on the beds either with your hands or with some light weights. In a very few minutes all the air will be sucked out and things will start to creak and groan a bit the pressure exerted is quite amazing!

You leave the pump plugged in overnight and atmospheric pressure does the work for you - instead of a huge stack of concrete block or books. Leave it overnight, then the next morning disconnect the pump, remove an end seal and check out your rock hard wing panel. I was very happy with how my first bagged wing came out! The wing seems much harder and stronger than Ive achieved previously with the weight down method. With the vacuum bagging system it is also possible to apply glass skins to the core instead of balsa. Glass skins are a bit more involved and require the use of mylars and mold release wax which are also available from Aerospace Composites.

That's it for this month. Til next time, fly low to avoid the radar!

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