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The basics of wing fillets

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Old 12-09-2002, 01:00 AM
  #1
Chad Veich
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Default The basics of wing fillets

I've noticed recently a number of members who have posted questions regarding how to create nicely formed and blended wing fillets. Since I happened to be at that point in the construction of a Ziroli P-47 I thought I would shoot some pics and share what I know on the subject. The following steps show ONE way and certainly not the ONLY way to do this particular job. My hope is that there is some info here that everyone can glean from, no matter what procedure you prefer. Please note that I prefer to add the fillet AFTER glassing the wing and fuselage. This is because I use epoxy resin and micro balloons for shaping the fillet and it is much harder than raw balsa. If you try and blend it into an un-glassed fuse you will find the balsa sands much faster than the resin and blending the two together becomes an exercise in futility. Here's a list of the materials that you will need:

-1/32 plywood
-epoxy or polyester finishing resin
-micro balloons (I prefer Sig)
-poster board or card stock
-denatured alcohol
-sandpaper of various grits (I use 80, 100, 220, and 320)
-5 and or 30-minute epoxy
-wax paper

Step one is to create a properly shaped fillet base from thin plywood, I prefer 1/32. If you're working on a kit or from plans this may be established for you. If not, get out the trusty 3-view drawings and start scaling them up! The fillet base will attach to the wing saddle area of the fuselage so don't forget to extend it inside the fuse enough to have sufficient glue surface. Study the pic to get a better idea of what is needed.
Once you've created the ply fillet base the next step is to attach it to the wing saddle. I usually use 5 or 30-minute epoxy mixed with a bit of micro balloons to take up any gaps. You may want some time to adjust position of the fillet bases so don't be afraid to use the slower drying epoxy. Apply the glue to the wing saddle making sure not to get it anywhere that it can come into contact with the wing. Lay the saddles in place on the fuse and bolt the wing on, forcing the ply fillet bases into position. Take care not to let the fillet bases slide out of position and make sure both sides are symmetrical. If you have a particularly large gap between the saddle and wing it may be necessary to tape the plywood bases to the wing once everything is in position. The point is to force the bases down against the surface of the wing as tightly as possible and let the glue/micro balloons mixture take up any gaps between the base and saddle. Note, if there is anyplace where glue may squeeze out and come in contact with the wings' surface make sure to put some wax paper in place to prevent gluing the wing on! On the P-47 I left the aft portion of the fillet, that which glues to the side of the fuselage, loose for the time being. This part of the fillet is often easier to create from solid balsa (as we will see) and it may not be necessary to even have it as part of the plywood base. Every airplane and modeler is different so tackle this as you see fit.
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Old 12-09-2002, 01:01 AM
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Default The basics of wing fillets

Once the base is attached to the fuse we are ready to create the fillet between the fuse and the wing. I recommend using balsa to form a good portion of the fillet if it has any size to it at all. You can make it from solid resin and micro balloons but the thicker this mixture is applied the longer it takes to dry and the harder it is to keep it where you want it. As the picture shows, I started with some 1/2 inch tri-stock in the corner for the Jug. The idea is to take up some space beneath the finished surface of the fillet so less resin is needed. However, it is preferable not to sand through the resin thereby revealing raw balsa on the finished fillet so don't get too carried away. Up near the front where the fillet gets very thin it may be best not to use any wood at all. Also note that it is necessary to keep the wing bolted in place while adding the wood so as not to force the saddle out of shape.
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Old 12-09-2002, 01:04 AM
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Default The basics of wing fillets

Next I add a small piece of wood at the trailing edge of the wing to act as a Dam when applying the resin mixture. It needs to fit into the corner of the fuse and fillet base fairly snugly and come up on the fuselage to the finished height of the fillet or slightly more. No need to make this part to the final shape of the fillet as it will be sanded to shape as the rest of the fillet is formed. The fillet aft of this "Dam" will be built up from solid balsa on this particular airplane.
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Old 12-09-2002, 01:15 AM
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Default The basics of wing fillets

Now we're ready to lay in the first layer of resin and micro balloons. It helps if you have some sort of stand that will allow you to rotate the fuse in order to find a position that will keep the mixture from running out while it dries. By the way, you will want to remove the wing for this part of the operation. Getting the thickness of the mixture just right is a matter of trial and error. Basically, you want it just thick enough that it will stay where you put it and no thicker. I usually mix in micro balloons until the mixture will just barely run out of the cup. Some people make it thicker but I have had problems with it not curing if too much micro balloons are added. Pour the mixture into the fillet area and spread it with a piece of card stock or a finger. I find I can rotate the fuse back and forth to help level out the resin and get it to flow where needed. It will probably be necessary to keep an eye on the mixture and occasionally redistribute it until it starts to set up. Now comes the really important part. Once it begins to cure you can smooth it to near perfection with a finger dipped in denatured alcohol (rubber gloves are not a bad idea). If the mixture has cured enough it will easily smooth out to whatever shape you desire and stay there. You will find that you have to keep plenty of alcohol on your finger to prevent the resin from sticking to it. A bit of practice here saves lots of sanding later.
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Old 12-09-2002, 01:16 AM
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Default The basics of wing fillets

Once the mixture is completely cured it is time to start sanding. I usually rough shape with some 80 or 100 grit paper wrapped around a properly sized round object. I found that an old Zip Kicker bottle worked well for the Jug. In the end you will need several different sizes of round sanding sticks to complete the job. Be careful to keep the sanding on the fuse itself to a minimum or you may sand through the glass. As the fillet takes shape move to the lighter grades of sand paper until you reach the finished product. For myself I find that the round sanders work good to get the basic shape and then I hand sand to get the final shape. Practice makes perfect, just go slowly and try different techniques until you find those that suit you best. If you find that you have some low spots in your fillet these can be taken care of with a second coat of the resin/micro balloon mixture. If they are more like dings or pinholes I use glazing putty to fill them in. Here it is all sanded to shape.
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Old 12-09-2002, 01:19 AM
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Default The basics of wing fillets

Here's another showing the finished shape of the fillet and also the unfinished rear portion which is where we go next.
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Old 12-09-2002, 01:20 AM
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Default The basics of wing fillets

The aft portion of the fillet can be finished in the same manner as the rest if you wish. Most of the time I find it easier to carve this part from solid balsa and glass it. The choice is yours. On the Jug I elected to leave the aft portion of the plywood base attached and use it as a guide when shaping the balsa. Not much I can say about this step other than just start carving and sanding until you get the proper shape. This pic shows it after some rough shaping. By the way, the two different colors on the fillet come from using two different colors of micro balloons.
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Old 12-09-2002, 01:22 AM
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Default The basics of wing fillets

And here is a pic after final shaping and blending it into the fuse with some spackle.
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Old 12-09-2002, 01:27 AM
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Default The basics of wing fillets

Depending on the shape of your fillet it may be necessary to add wood to the underside of the rear fillet in order to blend it in with the trailing edge of the wing. On the Jug this was a simple matter of some 3/32 balsa sheeting. The last step will be to glass it with some .6 oz cloth and final finish it. It is not necessary to glass over the resin/micro balloons, just those areas that remain as raw balsa.
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Old 12-09-2002, 01:33 AM
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Default The basics of wing fillets

If all went well we now have a nicely formed fillet that fits tightly against the wing. This is where most people stop and that is fine if you are satisfied with it. However, if you want to take your wing fillets to the next level of realism keep reading.
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Old 12-09-2002, 01:35 AM
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Default The basics of wing fillets

The next few steps can take a good fillet and make it fantastic. As I'm sure you know, the wing fillet of a full size airplane does not rest on the top wing skin but blends the fuse and wing together seamlessly. If you want to spend the time it is really not too difficult to make our models appear the same. This is accomplished buy building up a ridge on the wing along the edge of the fillet. First step is to sand the edge of the fillet back just a bit to make sure it is at least 1/32 of an inch thick or a bit more. This means that the ridge that we will be building up on the wing will have to be at least 1/32. I recommend this because if it is any thinner the resin has a tendency to chip out when separating the fuse from the wing (hopefully this will become clear as you read on).

Next we must fill any gaps that are present between the fillet and the wings' top surface. Cover the center section of the wing with wax paper using two-sided tape or a very light coat of spray adhesive. Mix up some resin and micro balloons and spread a thin layer along the underside of the fillet base. Bolt the wing back in place on the fuselage and wipe off any resin that squeezes out from under the fillet base with some denatured alcohol. Once this is dry pop the wing off and clean up the edge of the fillet. Your fillet should now fit tight against the wing with no gaps. Notice that I have allowed the resin to wrap around the leading edge and fill any gaps there also. By the way, this whole process is simply to make a gapless fit of the wing fillet to the wing so that resin will not seep under in the following steps.
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Old 12-09-2002, 01:39 AM
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Default The basics of wing fillets

The next step is to wax the fillet with three or four good coats of Partoll or a similar parting agent. I actually use Johnson's Past Wax with good results. You need to make sure to wax the top and bottom of the fillet as well as along the edge of it. Once the fillets are waxed bolt the wing back in place.

Now mix up another batch of resin and micro balloons. Using a small scrap of card stock spread it onto the wing where the fillets contact the top surface. Don't worry about getting it on the fillet as the wax will allow it to peal right off. Make sure you pull the mixture out on the wing far enough to allow you to blend the fillet into the wing without a depression. Just as with the main fillet you can use denatured alcohol to smooth everything out as the resin begins to cure. Once dry blend the wing fillet smoothly into the wings' top surface by hand sanding.
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Old 12-09-2002, 01:47 AM
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Default The basics of wing fillets

A solid rap on the wing will be needed to free it from the fuse and that's all there is to it! Any minor imperfections can be taken care of with more resin/micro balloons or glazing putty and you're ready for primer. This last pic shows the ridge built up on the wing (if you can see it). Once I have the airplane reprimed I will post another pic to show the finished results. These are the kind of details that can turn an average airplane into a real show stopper. People may not even be able to pinpoint what makers your airplane different but they will know by looking that there is something that gives it "presence". Try it, you'll like the results.
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Old 12-09-2002, 02:07 AM
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Default The basics of wing fillets

An overall view of the Jug. This is the 70 inch Ziroli designed bird built using a kit from The Aeroplane Works.
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Old 12-09-2002, 02:09 AM
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Default The basics of wing fillets

Chad,

Thanks so much for the step by step on fillets. Great idea for the ridge. I'm just about ready for primer on my P-47, but I'm going back for the addtional detail!

Thanks again for the excellent post!

BTW, what primer/paint are you using?
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Old 12-09-2002, 03:54 AM
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Default The basics of wing fillets

Skypoint, the primer is Pactra Primer Prep which is a wonderful filler-primer that sands easily and fills glass weave and minor depressions very well. Plans are to finish the Jug with the Nelson water based paints so long as Nelson says there are no compatability problems. Best of luck with your Jug.
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Old 12-09-2002, 12:23 PM
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Default Fillets

Thanks for sharing your techniques with us Chad. Your description of the process was very concise, and easy to follow.
It's always helpful to see how other people do things!

Cheers, Nigel
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Old 12-09-2002, 02:29 PM
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Default Wing Filets

Chad,

Thank you for the great step by step How to demo. I used a very simular process on my Hurri but instead of using a wax parting agent between the fuse filet and the wing I Monokoted the filet edge. Once the wing filet had dried I pulled off the Monokote.

Rip
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Old 12-09-2002, 02:33 PM
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Default Filets

A second look from the front.
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Old 12-09-2002, 05:21 PM
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Nice work, Chad and Rip! Thanks for sharing.
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Old 12-09-2002, 06:21 PM
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Default The basics of wing fillets

Chad,

Great photos. Let me know how you liked the kit over all.

Mace Gill
The Aeroplane Works
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Old 12-10-2002, 03:00 PM
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Default The basics of wing fillets

Great tips Chad. I'm just about ready to do my fillets.
Don't forget about me when you do the vac form details and the fg tail cone we talked about.
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Old 12-10-2002, 03:16 PM
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Chad Veich
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Default The basics of wing fillets

Thanks guys for all the kind comments.

Mace, I will send you some pics of the finished bird. As far as the kit goes it went together well and I will not hesitate to build from your kits in the future.

Greg, I will lay up you tail fillets this week and I should have the waste gates done this week also. Stay tuned for pictures.
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Old 12-10-2002, 04:12 PM
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Default The basics of wing fillets

Thanks Chad,

Can you e-mail the pics to me? Is it okay with you if I put them on my web-site? I would consider it to be very helpful to my future customers.

Mace Gill
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http://www.theaeroplaneworks.com
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Old 12-10-2002, 10:12 PM
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Default The basics of wing fillets

Mace, feel free to use the pics as you see fit. Do you want construction pics or just those of the completed airplane?
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