DISCOVERY JET
Construction - Session 8

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The time has come to finally vacuum bag the wings skins down to the foam wings. This is my first time to actually vacuum bag wings by myself so throughout this whole process I've been a little nervous. I know what needs to be done and I'm just taking it one step at a time. So in the picture below I have one side opened and ready to begin the dry run test.

I decided to put some carbon fiber across the wing following the spar path of the wooden spar I previously glued inside the wing.

For additional strength I cut out some fiber glass cloth, 6oz I believe, and positioned it over the wing spar and landing gear spar area.

After removing the fiber glass cloth I decided to go ahead and cut 2 more carbon fiber strips to place over the landing gear spar too. It probably will not add any more strength however I'm hoping it might spread the load a bit in case of a hard landing and prevent the spar from coming through the top of the wing.

Just another view, but from a different angle.

Here are all the items I will be using to complete the wings. I've used West System epoxy in the past with good results so I will be using it again to coat the wing skins and foam. I know most of us probably don't wear gloves but I do because of the easy clean up the offer. A disposable sponge roller will be used to apply the epoxy to the skins and wing. Keeping the epoxy spread over the plate will give me a large surface area to contain the epoxy and offer a longer working time. Lastly cabosil filler will be mixed with epoxy to glue in the landing gear mount.

I used about 6 or 7 pumps from each container for each side. Mixing was done using a popsicle stick and tilting the plate at different angles. At any rate just make sure the epoxy is well mixed.

I began by spreading the epoxy down over the balsa sheet first.

Then I spread epoxy over the foam.

Next I placed the carbon fiber strips in place and rolled epoxy over them. I made sure to give them a good top coat so that under vacuum the epoxy would fully soak through.

Then I laid down the fiber glass cloth and rolled epoxy to wet it out good.

I then flipped the balsa skin over and on to the foam wing.

Repeating the above steps I did the same thing for the other side.

Another angle to show the wing from the front.

Now comes the time to mix up some more epoxy in a cup and add some cabosil. I kept adding cabosil until I got a nice pasty mix similar to pancake mix.

Using a popsicle stick I liberally applied the epoxy mix into the landing gear cut out area.

After I was done I placed the wing skin over and onto the foam and installed the landing gear mount. I also checked the trailing edge for proper alignment and made a few adjustments as needed to get everything to align.

Since I will be vacuum bagging the wing I cut some breather cloth, which is just cotton like batting that you can find at Walmart fairly cheap. This batting will allow for air to be drawn out evenly across the surface of the wing during the vacuum bagging process. To help keep it in place I used some tape on all four sides.

With both sides having batting attached the wing is now read to be placed into the vacuum bag.

The wing is in the bag and everything is connected up. I'm using a wing vacuum bag from CST and a bag fitting from Aero Space Composites. Along with 6mm hard plastic tubing and a Festo 6mm ball valve.

Here you can see the wing being vacuumed down and everything is looking good. Notice I have a square looking piece of batting under the vacuum fitting. This is very important as it serves as a path for the air to travel as the vacuum is pulling the air out. You don't want the bottom of the fitting to vacuum down straight onto the plastic as it will not work very well.

This is by far the cheapest way to get started vacuum bagging. What you see here is a venturi style vacuum pump normally used to draw a vacuum on automotive A/C systems. This unit works off compress air from your typical shop air compressor. It says it's capable of drawing 20 Hg of mercury which is plenty for wings and other items you might want to vacuum bag. It sells at Harbor Freight for around $12 to $15 but is usually put on sale several times a year for $10. I purchased a Festo fitting previously from McMaster Carr which worked perfectly to allow me to re-fit the connector on the vacuum side of the unit for connecting directly to 6mm tubing. Operating this unit is quite easy and requires about 90 to 100 psi to make it work. You basically use your air compressor valve to give you 90 to 100 psi and after it vacuums down the bag use the Festo ball valve to close off the vacuum and then turn off your air valve on your compressor.

Here you can see how the wing is vacuumed down and everything looks good.

Another close up shot of the front of the wing.

And lastly a view from the top. You can see that everything is under vacuum nicely and this wing should turn out well. I will keep it under vacuum for at least 10 hours; however, the first 3 to 4 hours is your most critical since the epoxy is setting up during that time. After about 3 to 4 hours everything is pretty much bonded in place but plan on keeping the vacuum drawn for at least 10 hours until the epoxy finally cures completely.

I will post a few pictures later today after I remove the wing to show you the results. The other wing came out great but I forgot to take pictures to post here. So please check back later tonight.

Next session will show the joining of the two wing halves so stay tune for Session 9 to be posted soon!

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