Create droppable wing tanks

Dec 17, 2012 5 Comments by

In this how-to, Charles White uses lite-ply, green floral foam, fiberglass and resin to make external wing tanks fora 1/6-scale TBM Avenger. You can use this method to create many aircraft components, such as wing or centerline tanks, bombs and cowls.

After competing at the Nats level in Pylon Racing for several years, I always enjoyed watching the radio control scale event between pylon racing heats at the Nats.  I even made several trips down to Top Gun to see how the big boys did it.  Now that I have stopped competing in the pylon event, I quickly jumped into the event that always drew my secret interest – RC Scale.

Like most rookie scale builders, I started out constructing several Top Flite models and a Vailly Aviation O-1 Bird Dog.  Now I was truly bitten by the scale bug.  Thanks Roy (Vailly Aviation) for all your advice and kind words on the Bird Dog build.  I then decided to get serious and build a model that I could compete with so I decided to build one of my favorite WWII aircraft – the Grumman TBM Avenger.  The particular designer I choose was Charles Kellogg’s TBM Avenger.

As a working scale option to the Charlie Kellogg 1/5.79 scale TBM Avenger; I wanted to add droppable external wing tanks.  A search of the commercially available wing tanks did not provide the shape or the size needed for the Avenger.  I heard about the use of white foam, resin, strips or layers of fiberglass cloth, and a final acetone wash to construct cowls and other fiberglass components; however, I ran across a chunk of green floral foam in a craft shop and I was terribly pleased with its ease of sanding and usefulness to this project.  I decided to take it home and experiment with it.  Please note, this stuff is extremely delicate so as the old saying goes: Handle With Care.

I used as a model, a wing tank from a 1/32 scale plastic kit of the TBM Avenger.  After taking multiple measurements, length, height, circumferences along the length, etc., from the model tank, I then converted those measurements to satisfy the construction of a 1/5.79 scale wing tank for the Avenger.

As shown in the photos, using the measurements taken, I drew paper top and side view outlines which I transferred to a 1/8” light ply/balsa frame to construct the foam support frames and attachment point to the wing.   I used the finished outlines to rough cut the floral foam.  Again, cutting and sanding the foam was unbelievably easy so be careful.  Elmer’s yellow wood glue was used to attach the delicate floral foam to the wood supports.  Gray primer was sprayed (from a can) onto the carved green floral foam to provide a hardened surface for the fiberglass mat and resin to adhere to without destroying the carved floral foam shape while the mat/resin is being applied and cured.

Once dry, I then mixed some West System and thinned it with alcohol to attach the fiberglass mat to the foam shell.  I used various size stripes of fiberglass mat to cover the foam shell.  I let the tank cure overnight.  Once the resin cured, I sanded the mat to get a more even surface.  I then mixed up another cup of West System.  This time I added microballons and completely coated the tank.  Letting that sit over night, I sanded the microballons almost completely off.  The tank was then cut in half, the green foam was scooped out and most of the wood frame is cut away, to save weight, except for the plywood part that will be used to attach the tank to the wing.  The tank halves are then re-glued back together.  I added the simulated seam of the tank halves and again primer the tank which now sits awaiting painting.  For scale details, I added a simulated tank filler and support lip along with the fuel line from the tank to the wing.

Hope I have offered another useable construction tip to the scale community.  I’m still learning and would surely enjoy hearing ways to improve this particular procedure.



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5 Responses to “Create droppable wing tanks”

  1. Nik says:

    Was there really that much weight savings to justify cutting it in half, shelling it and reassembling?

  2. Pete Juergens says:

    really glad to see someone that has tried the foam/fiberglas method of construction. I don’t need drop tanks at this time, but plan on using this method to fabricate a cowling for an older model 1/6 scale Cub that I recently inherited. Thanks again for the info.

  3. fhhuber says:

    Next installment needed:

    How to make a release mechanism.
    With pictures

    I like to use a pen spring, 3 bits of nyrod tube and a wheel collar. A 4th bit of nyrod tube on the bomb/tank … pull the pin with a string to drop. (pull the string put the droppable in place and let go.. its ready to go)


    Weight savings is VERY important for a droppable tank/bomb. Lower weight to drag = lower impact speed = the thing is reusable rather than destroyed the first time you droop it.

  4. Chic White says:

    Thanks guys for the favorable comments. My specific release mechanism will consist of brass cable running through yellow nyrod from a single servo in two directions out to both drop tanks. Weight is certainly an issue, especially when the tanks hit the ground. Will keep you posted on progress and how the tanks fly. Chic White

  5. Arthur Lowery says:

    Enjoy your reports and information. It is great for beginners and old timers also. A great many tips to take advantage of. I am looking for a method of attaching wing tanks that can be removed when I wish too such as transporting. I have not been able to find a way without having to glue them on the wing or under the fuselage. If you have any ideas I would appreciate a forward. Nothing fancy just a method to attach them. Thanks so much for all you do for us flyers young and old. Keep it up.
    Respectfully, Art Lowery

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