Infield Engineering

infield engineering plane

Written by Paul Kohlmann 50-inch Volmer VJ-22 Sportsman

As seen in the October 2020 issue of Model Aviation.

Part 1: Design, tail group, and fuselage

the 50 inch volmer vj 22 is a scale model
01. The 50-inch Volmer VJ-22 is a scale model of a popular kit airplane that was designed in the 1950s. Photo by Charles Kennemore.

Flying from water is one of my favorite things about RC. There are few sights prettier than a silky landing on the mirrored surface of a mill pond.

I was contemplating this while I was flying my prototype Grumman Goose from the May 2012 issue of Model Aviation. That airplane was still going strong, but a er seven seasons, it was time to think about a replacement "just in case."

The Goose is an elegant model of stick-framed construction. It wasn’t a difficult build, but there were many parts, most of which were complex in shape. The resulting model is lightweight, but too fragile to simply throw in a trunk without care.

This time around, simplicity would be the order of the day. The design requirements were that it be scale, use a single motor, have a low parts count, could easily be cut by hand, and it needed to be durable. The resulting model has a 50-inch wingspan and is built from balsa and a little light plywood. For even faster building, a laser-cut kit is available from Manzano Laser Works.

The Sportsman Story

A er a little searching, an unexpected subject popped up. The VJ-22 Sportsman was a homebuilt amphibian designed by Volmer Jensen in 1956. Volmer, well known in the sailplane and hang-gliding worlds, built the first Sportsman for himself as a means of accessing diving spots.

After fielding numerous requests, Volmer began selling plans to pilots who wanted to build their own waterplanes. Approximately 1,000 plans have been sold throughout the years, and more than 100 aircraft have been completed.

The qualities that made the full-scale Sportsman successful were aligned with the goals of my smaller project—it was inexpensive and easy to build. The full-scale Sportsman was designed around the rectangular wing of an Aeronca Champion or Chief. The fuselage lines are simple and smooth. A small motor housed in a simple cowling is perched on struts overhead and is out of the vicinity of the water spray.

Also attractive are the myriad modifications that full-scale builders have applied to their Sportsman aircraft. Most feature pusher-props while others are tractors. Tail feathers vary in shape, with some having T-tails. The standard configuration has hand-operated landing gear, but skiing Sportsman can be found. And, of course, the paint scheme on every hand-built Sportsman is unique.

the fin and rudder have been roughed out
02. The fin and rudder have been roughed out. The extra bits at the corners will be sanded into shape later.
the fin has been sheeted with 116
03. The fin has been sheeted with 1/16-inch balsa and the outlines have been sanded to shape.
the horizontal stabilizer is fitted into its slot
04. The horizontal stabilizer is fitted into its slot in the fin, and the LEs and TEs have been shaped.
the left side former halves are glued
05. The left-side former halves are glued to the vertical 02 keels.

Building the Tail Group

The plans show the original Sportsman outlines, but with one significant modification—the bottom of the rudder has been extended so that it functions as a water rudder. As with full-scale Sportsman fans, builders should feel free to customize their tail feathers as they see fit.

Get started by pinning the parts that make up the tail group outlines to the building board. The outlines are composed of simple strip-stock in three different widths. Don’t worry about shaping the parts perfectly before assembly. Just make sure that the joints are tight and let the strips run long for now.

Note that the fin and the horizontal stabilizer are made from 3/32-inch balsa while the rudder and elevators are made from 3/16-inch stock. That’s because the fin and stabilizer will be built out by sheeting them with 1/16-inch balsa.

When the sheeting is in place, sand the outlines into shape then sand a radius at the leading edges (LEs) of all of the tail group parts and a taper at the trailing edges (TEs) of the rudder and the elevators. Join the elevators with a bit of music wire. Connect the rudder and elevators with your favorite hinges. I used 1/8-inch pinned hinges from Robart Manufacturing on the prototype.

here the left side panel is curing while the hull
06. Here, the left-side panel is curing while the hull planking is going on.
the left side is ready to unpin
07. The left side is ready to unpin.
the nose sheeting is complete and the soft balsathe windscreen was made from balsa sheetingthe servo tray tail group servos and control rods are in place
10. The servo tray, tail group servos, and control rods are in place.
a bellcrank isnt the stealthiest
11. A bellcrank isn’t the stealthiest way to operate the elevators, but it is simple and effective.

Adding the Servo Tray and Tail Group Controls

Cut the servo tray from hard 3/32-inch balsa. Fit two servos of your choice into the tray. One will operate the rudder and the other the elevators.

Cut two rails from scrap balsa to span the width of formers F4 and F5 to support the servo tray. Glue the servo tray to the rails. Run a bead of glue all the way around the tray to make it watertight.

To keep the a hull watertight, I ran the control rods through a nylon sheath all the way from F5 to where they exit the hull at the tail. Epoxy was used to seal the nylon at the tail end. Lubricate the control rods with plenty of silicone spray to keep them from corroding.

Because the elevator is positioned high on the fin, linking it to its servo can be challenging. I opted to use a bellcrank system. This decision was driven largely by the fact that I had a bellcrank laying on the workbench, automatically making it the easiest solution.

Fiberglassing the Fuselage

Similar to boats, the hulls of waterplanes are sensitive to impact. Adding fiberglass to the forward hull can dramatically increase its durability.

To be honest, I was intimidated by fiberglassing when I built the Goose, so I skipped this step. I’ve regretted that decision. If you have concerns, the Sportsman is a simple project on which to learn. I’m sure that you too will find that fiberglassing is nothing to be afraid of.

Start by gathering your supplies. Fiberglass comes in many weights, but for a small job like the Sportsman, 1/2-ounce fiberglass cloth is a good choice. My model has two layers of 1/2-ounce cloth, with the weave of the first running parallel to the keel and the second layer biased 45°. The bias provides maximum strength.

fiberglassing supplies include resin fiberglass
12. Fiberglassing supplies include resin, fiberglass cloth, a squeegee, brush, and a mixing cup.
fiberglassing the forward hull is cheap insurance
13. Fiberglassing the forward hull is cheap insurance for the part of the airframe that strikes the water—and whatever might be hiding in it!

There are also many types of resin. Your hobby shop might carry finishing resin or 30-minute resin. The slow curing time of these resins is what takes the scare out of fiber-glassing. You will have plenty of time to position the fiberglass and work out the wrinkles. If things get out of control, just pull off the wet layer and try again.

Small plastic cups are handy for mixing the resin. Small brushes can be used to spread the resin. The main tool is a squeegee. Credit cards work well, as long as there aren’t any nicks in the edges. Paper business cards are also good, but I prefer using the rubber squeegees that come with cellphone screen-protector kits.

Get started by cutting a piece of fiberglass fabric that is an inch or two bigger all around than the area to be covered. Use a very sharp blade or the weave will run like mad. This whole forward hull can easily be done with one piece.

Drape the fabric over the hull. Now pour some resin from the cup onto the middle of the fabric. The resin will quickly soak through the fabric and into the wood. Use a brush or squeegee to spread the resin out toward the edges. Take your time and work a little in each direction.

Keep an eye out for shiny spots. These areas are too wet and need more spreading. Add resin as needed and continue to work from the center out until the entire hull is coated. Now use the squeegee to gently work out bubbles and to remove any excess resin. Pull carefully on any wrinkles while working the squeegee.

Now for the hardest part. Set the fuselage aside in a safe place and don’t touch it until it is cured! It’s easier to give this advice than to take it. I normally can’t resist testing it too soon, but I’ll discuss how to remove fingerprints later.

After it has cured, sand the first layer with wet 600-grit sandpaper to knock down any dust or debris. Then add the second layer of fabric on the bias. The more daring might do both in a single sitting, but if you are a novice, there is less risk in applying two separate layers.

Wrapping It Up

Now that the fiberglass is curing, this project is at its midpoint. I’ll set the hull aside for now. Next month, I’ll tackle the wing and the motor pod and dunk the Sportsman into the lake!


Manzano Laser Works

Robart Manufacturing

(630) 584-7616

RCGroups: 50-inch Volmer VJ22 Sportsman



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Why not glass the whole fuse, instead of just the bottom of the hull? That's what I would do. I use finishing resin, but you can use 30-min epoxy. Just thin it out slightly with acetone, or rubbing alcohol.

Hi Steve, I chose to glass just the portion of the hull where I had concerns about impact and abrasion. I saved some weight by stopping there, but of course each builder is welcome to glass as much or as little as they want. My best, Paul

Love it! Thanks for adding build articles like this to the magazine. There’s not anywhere near enough as far as I’m concerned


Thank you for this article/plans. Been looking for an ROW for next summer at the lake and this fits the bill perfectly for a winter build. Probably will make it as a pusher. Looking forward to the 2nd half next month. Dave R

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