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Wing-sheeting using vacuum bag.

I do all (well...90%) my sheeting of foam-cores with a vacuum-bag to press the sheeting onto the foam.  Below I will try to explain how I do it, together with how the pump and vacuum-bags are made.

For ease, in the text below I use the word vacuum, the precise description is pressure below atmosphere.

Advantages are :

  • An even force is applied over the whole area, no voids.

  • The available force is superior to weighting down with bricks, fuel-cans, car batteries, books etc.  Example : A normal F3A-model wing half has a area of 30dm2 = 3000cm2.  A pressure inside the bag of 0.2 bars below atmosphere is 0.2Kg/cm2, that generates a total force equivalent to a load of 3000*0.2=600Kg.  600kg is a lot of bricks etc!

  • The superior force combined with an even force makes it possible to safely use a very small amount of glue, minimizing the weight added to the wings.

  • No need for a very rigid surface to withstand all the weight during skinning, to avoid the danger of making bent wigs.

Disadvantages are :

  • Some equipment needed.

  • Can not be used together with honeycombed cores.

The vacuum pump

There are no need for a fancy and/or expensive vacuum pump.  I have used the pump from an old refrigerator for 15 years. In addition to the pump itself, one needs some kind of vacuum activated switch to keep the vacuum within a certain range.  The pump can easily crush the foam without it.  I have made the switch from the vacuum-bell from the distributor of an old 'analog' car, together with a micro-switch. A volume is useful, to give some hysteresis in the system that helps to avoid overheating of the pump.  With a volume it runs a bit longer each time it starts, but it also rests longer in between. My volume is an old jam-jar, simple and effective.  To be able to monitor the vacuum applied, a pressure-gauge is recommended. To connect the vacuum to the bag, a nipple is needed.  I have used a drilled out bras-screw, with a brass-tube soldered in to connect to tubing. On both sides of the bag, a silicone (or rubber) washer is used to keep it airtight. A plastic-washer between the nut and the outer silicone-washer makes it possible to tighten the nut without damaging the silicone-washer. The sketch and pictures below should explain the most of it.

The vacuum-bags :

The bags are easy to make from plastic bought at house building material stores.  A roll of 0.15mm is quite cheap, and the plastic is double on the roll.  Cut of a suitable piece, make a bag of it by wrapping tape around the edges.  Normal packing-tape is suitable.  The last edge is obviously taped shut after the wing is put inside. 

The sheeting process :

I describe my way of doing it step by step below, thee is a picture to each step at the bottom of the page.

  • Start by making the balsa-sheets. I like to cut it to have balsa parallel both to leading and trailing edge.  I don't glue them together, just holds them together with masking-tape.
  • I use low-viscosity epoxy, and applies it with a foam-roller. By adding some epoxy-color to the glue, it's very easy to get an even thin layer of glue.  With a roller, it's easy to get an even thin layer of glue,  FAST.
  • Pieces of fiberglass where cutouts for landing-gear and servo will be.
  • The sheet is 'painted' with epoxy.
  • The stack of balsa-foam-balsa is put into the bag.  A small piece of paper-towel is used to make sure of air-transport from wing to outlet nipple.
  • The wing in the bag is then put between the negatives generated when the cores was cut.  A flat board and some weight is put on top to hold the wing straight while the glue cures.