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TIGGERS' TOP MODELLING TIPS |
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| Vac Bagging Technique | |||
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Over the years I have used a various methods of wing construction covering on just about every aspect of the hobby, with the exception of helicopters (if you cannot test glide the thing there is something wrong); recently I have been building wings for gliders and control line team racers using the vacuum bag technique.
Many years ago when I made up foam veneered wings the whole assembly used to be glued up and placed under weights to dry. This required the use of a lot of weight and a very strong bench. Typically a total of 250 pounds would be used for each wing panel. With an example wing panel area of say 400 square inches this would equate to 1.6 pounds per square inch. The table needed to be very strong to remain flat under that amount of weight. However, by putting a wing inside a bag and evacuating the air by a vacuum pump you can reach a pressure of 14 pounds per square inch with very little applied weight (vacuum) and therefore little chance of introducing bends or warps. "vac-bagging wings is not difficult but merely different"
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| Wings and Skins In order to illustrate this method in we shall be preparing a foam and wood veneer glider wing, which has a 1.7 metre wing panel that has a triple taper on the leading edge with a straight trailing edge. (Genesis Wing). Most of you will be building from a plan therefore the choice of wing section and plan form will be specified in your plan. First contact your local wing cutter who usually has a CNC hot wire cutter and can produce foam cores to a high standard. The choice of foam density is dictated by the span and section. High density foam for long and thin wings (blue, pink or orange), low density for short broad wings (white). If you decide to produce the cores yourself there are many articles on the subject. However, when sourcing the foam, particularly the white foam, ensure that you obtain “Virgin Bead” , this is new foam, as other white foam is essentially recycled and is usually contaminated with all kinds of debris and has areas of different densities and in not as strong. You need to decide at this point as to what you intend to skin the wing cores with. The choice is usually between wood veneer, balsa or even plywood. Whatever your preference, ensure that the cores are cut slightly undersized to allow for the thickness of the wing skin. Prep the Wing Cores and Outer Foam As you can see the our wing cores have been joined to form the overall wing shape (multiple taper) and the wing joiner tubes are in position. Before commencing work on the cores join up the outer foam parts which were produced when the cores were cut. This will give you an upper and lower bed upon which to hold the cores. Prior to joining them you need to determine where your servos are going to be located and a route provided for the connecting leads. The hollows for the servos are cut out after the wing has been skinned. You can do the same for the leads but I find that it is easy and neater to bore these holes prior to skinning the wing. To bore the holes use a sharpened length of 6mm tubing and by measuring and by eye you can achieve very good results. Just remember to withdrawn and clear out the tube at frequent intervals. When joining the core panels be very mean with the 5 minute epoxy as you do not wish excess glue to migrate to the outer surface of the cores. To allow the joins to set, hold the cores in the outer sleeves beds to ensure that the wing cores remain straight and true.     
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| SPARS All thin wings require wing spars; for the Gensis wing top and bottom carbon fibre (C/F) spars with an integral 1.6mm plywood shear web are used. Mark the position of the spar and saw a suitable vertical slot in the core. Two hacksaw blades taped together will provide a slot of sufficient width. There is no need the take the slot beyond 70% of the span. Measure the depth of the slot both at the root and the point of termination. Cut out a plywood shear web of correct length but ensure that the depth is 1mm shorter that the depth of the wing cores. With PVA/aliphatic glue, glue the shear web in place ensuring that it is uniformly 0.5mm under the surface of the cores both top and bottom. Place and lightly weight the cores into the outer sleeves. Leave overnight as the foam slows down the evaporation process of the glue considerably. Once dry you can sand in the rebate which will house the C/F spars in this case a small block of wood with a section of coarse Perma-Grit is used and using a gentle stroking action sand the rebate down to the plywood sear web. This is the reason for the uniform 0.5 mm under the core surfaces mentioned above. Extend the sanded rebate for about a further 200mm beyond the end of the shear web. With a pen mark on the core the location where the servos will be housed. During the construction phases you will have without doubt caused slight dings and bumps in the core surfaces and, therefore, at this time, use lightweight filler to make good. Using a waste strip of paper, measure the top and bottom surfaces of the wing chords. Unless fully symmetrical they will be different due to the varying curvature. Add an extra 3mm for the trailing edge and note these measurements down. The last job with the cores is to sand a slight flat on the leading edge which will be used to locate the C/F tows for the Leading edge. Place the cores back in their sleeves and place to one side.
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Using the measurements mark and cut out the skins from whichever medium (veneer, balsa, ply etc) you are using. Depending upon the size of the wing panel, if using balsa wood, you may need to glue two or more sheets together. Wood veneer and plywood can normally be obtained in suitable sizes. For this wing example, the trailing edge is straight therefore mark a line 3mm in from the edge of the veneer, this is where the trailing edge of the core will come to. Place the top and bottom skins on the bench allowing a gap equal to twice the thickness of the wing skins trailing edge to trailing edge. Apply a length of masking tape to join the two skins ensuring that the gap is maintained. Once this has been done you now have two skins which you can fold together like a book with the masking tape acting as the spine/hinge. If you wish, you can now place the core between the wing skin “book” for a dry run and check of the measurements.
Now is the time to decide upon what, if
any, composite reinforcement you require under to wing skin. In the
majority of cases either a layer of light to medium weight fibreglass
cloth is sufficient (25gms-80gms per square metre). Always cut the cloth
at 45 degrees (on the bias) whilst this will make the cloth more
difficult to handle as it will easily distort from its intended shape,
but it produces a far stronger and stiffer result. I always find it
handy to place and store the cut cloth lengths in a folded newspaper.
Once you have cut sufficient glass cloth for the wing you will be left
with many off cuts. You can cut these into strips about 30mm wide and
various lengths place these to one side for later. Determine what
lengths of C/F you require for the spars and the leading edge - remember
that slight flat you sanded onto the leading edge.
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| Test the Vacuum Take a scrap piece of foam and place it in the vacuum bag, seal the bag and start the pump. The object is to confirm that you have enough vacuum available to ensure that the veneer skins are sucked down onto the cores without the foam cores being crushed by an excess of vacuum pressure. Adjust the vacuum switch as appropriate. Additionally, given a Vacuum reservoir, the test will also check the duty cycle (Pump Running -vs- Pump Time at rest when vacuum - pump should spend more time at rest than pumping) Everything in the process is determined by the pot life of the epoxy that you are using. The object is to apply the epoxy, add all the spars and leading edge wrap the veneer skins around the core and place the whole assembly into the vac-bag. Generation game and a one armed paperhanger are two things that spring to mind……… On small jobs I am confident to work solo. However, on larger or complicated wings I tend to use a friend to assist in the epoxy work, particularly for the first time when time and complication factor can be unknown. |
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| Let the
Gluing Commence So now you are stood with a friend, wondering why you have started something that you wish you hadn’t, but before you get immersed into the next task, think SAFETY and use gloves and if required by the epoxy that you are using, a breathing mask. Open up and lay the taped together veneer skins on the table. Onto these lay out the glass cloth and smooth down evenly. To apply the epoxy there are various methods to chose from. Some use a brush and an old credit card or similar type of plastic as a spreader to work the epoxy into the C/F and the cloth. I use a roller as I find that with a plastic spreader most of the epoxy ends up on the floor. Mix up sufficient epoxy for the job following the maker’s instructions. This can be a bit of a “black art” until you have done a few wings and obtained a feel for the quantities involved. Anyways best to mix slightly too much than too little. If you feel that the pot life time is short prepare two quantities and don’t add the harder until you have used up the first lot.
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In order to apply the
leading edge C/F it is far easier to have the wing on edge with the
front of the wing uppermost. To avoid any damage to the very thin
trailing edge of the core I push two kebab sticks through the core and
use blocks to keep the trailing edge clear of the bench. One of you can
then wet out the leading edge C/F, give the leading edge flat and the
first 20mm top and bottom of the core a wetting of epoxy.
 Using either a knife or scissors, cut through all ten strands of the C/F tow which will leave you with a wetted out tow the length of the leading edge place the tows into position on the leading edge. To ensure that they remain in place remember the 30mm strips of glass cloth that you cut from the off cuts. Lay these along the leading edge and smooth them over the top and bottom of the core. This will have the twin effect of retaining the leading edge in place and reinforcing the front of the wing. Whilst this is being done the second person can be wetting out the glass cloth on the veneer skins. Underside Now wet out the C/F tows for the bottom spar in the same manner as the leading edge, also wet out the spar rebate in the wing core. Cut the tows off the batten and arrange them to lay evenly in the rebate. Now give them a firm application of the roller which will cause the tows to spread out and fill the rebate. Should you think that more tows are required you can always place further lengths off the roll dry and roll some epoxy onto them. Once you are satisfied with this spar, coat the bottom surface of the core with a thin layer of epoxy. Using the off cuts of cloth you can reinforce the areas where the recesses for the servos will eventually be cut. When the task of wetting out the cloth on the veneer skins has been completed lay the core onto the bottom veneer skin ensuring that the trailing edge of the core meets the lines you had previously drawn on the wing skin. Upper Now repeat the task for the top spar in the same manner as the bottom spar and apply a thin layer of epoxy onto the top of the wing core. Once you are happy that the cores and wing skins have a sufficient covering of epoxy close up the “book” of veneer skins carefully around the wing core making sure that the core does not move in relation to the skins. If you wish a few strips of masking tape to hold the top and bottom skins together over the leading edge can be used. |
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| Into the Bag Place the whole assembly into your vac-bag. Due to the porous nature of the veneer, I find that the use of a whole layer of breather felt is not needed, however use some kitchen towel to provide a pathway from the wing to the point where the air is sucked out of the bag. Evacuate the air and ensure that the bag does not crease once all the air has been removed place the whole assembly into the top and bottom foam outer sleeves and with some boards and light weight placer on top of the whole structure to ensure that the wing is straight and true. Leave to cure with the pump on. 
 My pump equipment is set up with a vacuum gauge, vacuum switch and a reservoir. For foam cored wings I set the switch for 12 inches of mercury, which I run for about 10 hours with the epoxy I use. The pressure switch ensures that as the vacuum pressure reduces due to minor leaks due to inefficiencies in the vacuum connection, then the pump will restart to to bring the pressure back to the required vacuum. A typical duty cycle is around 30 second pump to 4 minutes off. 
Remember that it was best to mix too much
epoxy than too little. With what is left in the tub you can periodically
check as to the state of cure. Once the epoxy has set hard to the touch
switch off the pump and leave the wings for a further 12 to 16 hours to
fully cure. |
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| It’s a Wrap Like many of the modern processes connected with model construction vac-bagging wings is not difficult but merely different. The secret is in preparation, once the epoxy is mixed the pot life clock starts counting down therefore, everything needs to be ready and to hand. Please don’t believe that the process is beyond normal modellers, it isn’t. Should this short article have whetted your appetite to try vac-bagging wings and you are unsure please contact me (neil@slope-dudes.co.uk) as sometimes depending upon the wings, the method can vary. Also, you can use the vac bag method to apply a final external layer of glass cloth to the outer surface of the wing but that will be the subject of another article if desired. Neil T |
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