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by Stephen Adams

Super Decathlon

Many pilots are familiar with the Decathlon as the aerobatics trainer for the masses. This small tandem, two-seater, with its meager but adequate 180 hp engine provided plenty of thrills and chills for the would-be daredevil. This high-wing monoplane reached its ability under a semi-symmetrical wing.

While researching the Super Decathlon I discovered a web site with accident listings covering many years. The most surprising thing was the number of accidents attributed to "low-level aerobatics" as well as alcohol. Maybe we need some new TV commercials, "don't drink and fly." If you can't walk straight what on earth makes people think they can fly straight, or upside down, or inside out, or in any other attitude. I suppose I better get off my soapbox, but the statistics were alarming. The Dynaflite Super Decathlon offers the RC pilot the opportunity to learn aerobatics in a real world aerobatics trainer. Among the features of this kit is the all-wood construction, high-quality plywood and balsa, a beefy ABS engine cowl and wheel pants, vacuum formed canopy and clear butyrate windows.

Thankfully the huge 89" wing is in two pieces for easier transport. The wing is a typical "D" tube construction with sheeted ailerons and wing tips carved from the largest block of balsa I have ever seen.

The wood is very high quality with the die-stamped part numbers on or near most parts. The die cutting was clean and accurate with no mis-cut parts. Both the balsa edges and hardwoods proved to be very straight.

Dynaflite recommends engines sizes from 1.08 to 1.80 2-stroke, 1.20 to 1.60 4-stroke, and 1.5 to 2.1 gas. The instructions provide firewall details to install an O.S. 1.20 and the U.S. Engines 25 cc power plants. Installation of servos, battery, etc. is designed mid-way back in the fuselage tail section to offset the weight of the larger engines.

Building the Wing

The kit begins with the all-too typical tail feathers first approach. However, this time out I decided to be independent-minded, a rebel, and start somewhere else. I chose the wing. The 89" wing is built over the plans with a simple piece of balsa stick laying diagonally across the plans to act as a jig and provide the washout for the wing tips. The wing is built in two panels with an aluminum spar tube to bring them together and then dowels and nylon bolts pin the wing to the fuselage.

The innermost ribs are first reinforced with die-cut light ply doublers that are needed to support the cardboard spar tube. The ribs are then aligned perpendicular to the bottom spar and glued with thin CA. As to be expected with an aluminum spar, there is no dihedral in this wing so ensure that the root ribs are perpendicular.

The balsa shear webs are glued into place ignoring where you have the spar pinned to the building table. The innermost shear webs are cut from aircraft-grade plywood. Again a requirement to support the aluminum spar tube. The rear upper and lower spars are glued into place. It is imperative that the tail ends of the ribs remain in contact with the jig at all times so that the washout is correct.

Next we follow the edge-gluing instructions to build the sheeting needed to cover portions of the wing. This includes the leading edge back to the center of the main spar, across the trailing edge of the wing, and the innermost section of the wing center. Again, it is important to guarantee that the ribs are aligned on the jig or else the wing will suffer from a twist. The wing is then turned over and then completed by repeating that section of the instructions. The wing tips are constructed from a large balsa block. With the help of a band saw, belt sander, and razor plane I turned that block into a nice wing tip as depicted on the plans. I then used a large drill bit in my press to "hog-out" the tip to shave weight. It may have saved only a few ounces but looking at that solid block of balsa I was compelled to do something. You know what I am talking about.

A Box of a Fuselage

The fuse is constructed using a light ply box technique with formers glued onto the box structure. The die-cut fuse sides are assembled and keyed into the tail section with longerons. After completing the first side, the second fuse side is built directly over the top of the first. This ensures that both sides are exact. The sides are glued to the die-cut bottom and the only two fuse formers are installed to complete the primary box. The formers in the tail section are built using stick balsa with the exception of the tail itself. The tail former is also die-cut to ensure proper alignment.

The plans call for the servos, receiver, and battery to be installed about halfway back into the tail section. Two of the stick-built formers are designed to support the die-cut servo tray. Engine size and balance will ultimately determine how many servos get installed in the tail. With a large capacity 6.0 volt battery I suspect that it will reside much closer to the front.

The top and bottom of the tail section is sheeted with cross-grain balsa to increase strength. The die-cut light ply servo hatch is installed with screws to the underside of the tail.

Firewall Forward

The firewall is installed next. Depending on the your engine choice (gas or glow), the firewall is constructed with different size engine mount spacers. After installing the firewall the fuel tank is assembled, attached to the tank floor, and installed. Install the tank now because it will be more difficult (if not impossible) when the aircraft is completed. There are three models of 1.20 engines offered by O.S. The top of the line Surpass SP is designed for F3A competition and features a geared fuel pump and "Roots-type" supercharger. The Surpass III features a PD-7 pump and is a mainstay for flyers wanting a pumped engine. I chose to install the O.S. 1.20S-E Surpass on the Decathlon. It is a non-pumped, ringed engine that should put out near 2 HP. A local flyer is working on the same kit and chose to install a 1.80, which means that it should be able to take-off at two clicks above engine idle.

Using a Great Planes large engine mount you will find that the engine is located at the very end of the mount to reach the requisite 7" from the firewall. The firewall is built using laminated plywood with an additional 1/4 inch plywood engine mount spacer. If you have some extra plywood, you should enlarge the spacer so that you have more engine mount to work with later.

I also installed a Great Plains Easy Fueler and a homemade remote glow system since the inverted engine will be completely under the cowl.

Feather in my Hat

It was at this point that I skipped back to the tail section to build the empennage. The tail section is built using balsa stick and features an open bay construction. There are a number of angled cuts required so use a mitre box if you have one. In any case, make sure that the pieces are joined snugly.

The elevators are "framed" using balsa stick but the trailing edge is unique. Since the trailing edge sweeps around the end of the stab, it poses a large curve challenge. Dynaflite overcomes this by using a five-ply lamination of 1/8-inch balsa stick. Starting at the front of the elevator, glue one stick into place. Slowly wrap this stick around the elevator framing, gluing it as you go. Within the balsa frame "T" pins are inserted into the building surface to provide a complete jig to bend the stick around. You may need to wet the wood so that it does not break but the humidity levels of Oklahoma eliminate that need. This process is repeated four additional times to complete the trailing edge. It is light and very strong.

Both the stab and rudder make provision for the installation of dowel hard points to support the installation of flying wires. If you are going to fly aerobatics, then flying wires are highly recommended. The completed tail section is attached to the fuselage after carefully ensuring the stab is parallel to the wing, verifying the negative incidence, and aligning the fin with the thrust line of the fuselage.

Back to the Box

We left the fuselage looking like a box until we installed the tail feathers. This makes alignment easier since the fuselage will sit level on the work surface. At this point we need to go back and add some shape and character to the plane. Time to make it look like a Decathlon.

Die-cut balsa longerons are glued to the sheeting on top of the fuse to give it some semblance of aerodynamic "open bay" shape.

Make sure that all your plumbing and fuelproofing is complete in the fuel tank area. The fuel tank box is enclosed with a piece of die-cut light ply and formers. Stick balsa is used to interconnect the formers and then sheeting is laid over the top. The bottom of the fuse is also finished using die-cut formers glued to the light ply box bottom and interconnected with balsa stick. It is finished with balsa sheeting. There are a few compound angles as you sheet the belly but work patiently and it will turn out great.

We are now ready to cover our aircraft so I decided to do something completely new but yet completely old. I chose to cover the Decathlon in fabric and dope using the methods that have existed many years before the advent of the plastic films. I decided it was time to learn how to cover a plane the real way; with fabric and dope! Naturally, Roger Smith wondered if I had already been sniffing said dope but I assured him that this was to be an educational experience for me. And it was. I chose to use Sig Koverall and Sig Dope to cover, seal, and paint the Decathlon. If you have never done this before, then find the guy in your club who has been flying since the days of Moses and ask for help. Not that Jim Dross is old, but he has been in this hobby for decades and has always been a wonderful supporter and educator. That is exactly what I did and it made the task an enjoyable learning experience.

The results were not flawless but I was very satisfied. My future dope project will simply increase in quality. And yes, I will use this method again. How is this approach different than slapping some plastic film on the plane?

For starters. The costs are slightly higher than film and the time factor is longer. Don't let that deter you because the finished product is remarkable in comparison to the films. Other differences, in no particular order.

First, this method requires - PATIENCE. Something many of us do not possess but there are times you simply have to walk away and wait for the dope to cure.

Second, Koverall has a much higher shrinkage factor. This allows you to work around compound curves and corners like a dream; i.e. no relief cuts. Heat up your iron, cut a small piece of fabric, and drop it on the iron. You will see what I mean in a hurry.

Third, you can hide, almost completely, overlapping material lines or seams. A feature that the plastic films cannot touch. Fourth, a word of warning: work in a well-ventilated area. If not, there will be a big "dope" in your shop not including that in the can.

Lastly, no more tugging, pulling, cursing, and holding the heat gun in your teeth to stretch film around a plane. I will NOT provide a definitive "how-to" because I could not do it justice as a beginner but I will provide some observations so that you understand the process. First you should visit your local hobby store and buy a Sig catalog because there is a "how-to" in there. Read this well and find a Jim Dross to teach you. Otherwise, you will be throwing your money away in frustration.

The Ground Rules

We will be working with Nitrate and Butyrate dope. Thin this down in a 1:1 ratio with Sig Thinner. You can clean your brushes with lacquer thinner but use dope thinner for its intended purpose. All references to dope will be Nitrate and in the thinned format until otherwise stated.

Get a "good" pair of scissors and plenty of razor blades. The polyester fabric will eat up blades and you will learn that cutting the raw material can be a chore. Cutting Tip: Fabric that has been doped is easy to cut so always brush the dope past the edges of the work surfaces. When the dope is dry, this fabric will cut extremely easy. Try this with a piece of scrap and you will see and hear the difference in the cutting.

Get a fan. Yes, a fan. Set it up so that air is moving past your face at all times. This will help keep the fumes moving on down the road. And open the windows and doors so you don't trap the fumes in your shop. This is not the kind of high that us flyers typically want to experience.

Get an old iron. If you choose to return to plastic films after this project you will NOT want to use this iron. It does get trashed out. Dope has a shrinking characteristic so do not be afraid to put on multiple coats. You will find that dope is extremely light compared to the films.

Covering the "Right" Way

Step one is preparing the aircraft's wood surfaces. You begin sealing the wood by brushing on three coats of nitrate dope. Do this only on the wood surfaces that will have fabric in contact with it. Sand with 400 grit to remove any texture that may surface after the second and third coats. Dope has a characteristic by which "dry" dope will soften when exposed to "wet" dope. In a simplified sense, we have applied the "glue" for our fabric.

Let's start with something small such as an elevator. Cut out a piece of Koverall leaving about a reasonable margin around it. Lay the fabric over the elevator and start brushing on the dope around the perimeter of the wood surfaces. After wetting the perimeter, lightly pull the wrinkles, excess, slack, etc. out of the material. If the perimeter starts to dry and it does not pull then apply some "wet" dope to soften the work. The goal is to pull the fabric snug...not tight.

As you work your way around the elevator "digitize" (use your finger) and rub the wet fabric down onto the wood surface. This adheres the dope and fabric firmly to the wood. If you find that it is drying too quickly, simply "wet" the dope again and keep going.

At times you will encounter a crease in the material due to its being folded into the package. If so, you need to use your iron to shrink it out before the dope dries. You can use your iron over the wet dope hence the trashing out of the iron but only shrink out the crease that is over the wood surface area you are working with. Ignore any creases that are in the open bay areas and between the perimeters for now.

Next, "wrap" the material over the edges and around the corners and such. We start by "training" the fabric. Simply take your iron and smoothly push the fabric over and around the edges. Do not linger too long or excess shrinkage will become a factor. You will see that the fabric will quickly conform to the necessary shape. In some places you will need to apply additional heat to shrink any excess fabric. This is where you get to avoid the dreaded relief cut. Now go back over these edges with a doped brush and digitize until the material is adhered. After the material dries, trim where necessary.

This completes the perimeter application. Hopefully, you thought about the overlapping materials so you can hide any lines/seams underneath the aircraft or around the edges. Example, cover the bottom of the elevator first so that top will overlap on the underside of the elevator.

Now you will turn the elevator over and repeat the process for the topside. Do not shrink the fabric until the surface is completely covered and the dope is cured. WARNING: Due to the high levels of shrinkage you can crush small structures such as stick-built elevators. Use an iron thermometer and follow the recommendations in the Sig article. This level of shrinkage can also result in warping and twisting of small structures so apply heat judiciously and evenly.

Now that the fabric is attached and shrunk we need to brush dope over the remaining fabric. This process encapsulates to fabric and begins the process of "closing the weave". Prior to moving to Butyrate Dope, we will brush (or spray) on another two coats of Nitrate.

When brushing the dope over open bay areas, be cautious to not wet the fabric so much that the dope seeps through. This will impact the final finish. If this happens, and it will, dry your brush and press the side of your brush up against the fabric. This helps to wick the dope back through the fabric.

This process of perimeter coverage is repeated for every part of the aircraft from wing to fuselage. This includes sheeted as well as open bay structures.

It is always the same...apply to the perimeter, pull snug, wrap the edges, let dry, turn over and repeat. Then you shrink the fabric and brush dope over the remaining fabric surfaces. Start on small surfaces and work your way up to the wing and fuselage.


After the aircraft is covered, you switch to butyrate dopes. Begin by spraying one or two coats of clear butyrate over the nitrate covered aircraft. This starts the butyrate process.

For color I selected Forest Green and Diana Cream from Sig. I realize that these are not scale colors but that was my preference. In addition the markings on the aircraft are considered semi-scale.

Always start with your lighter colors and progress to the darker colors. After masking the plane with a good tape then shoot the tape seams with clear butyrate. This helps to seal the seam and prevent bleeding. I had good success with a 3M "lacquer" tape. Test shots revealed a good mask line without sealing as long as the first couple of coats were light.

When you start shooting color you will see that this dope dries to the touch quickly just like our nitrate dope. This allows you to shoot subsequent coats in short order. Coverage can be gauged over open bay areas by putting a light behind it. If you see too much light then put additional coats of dope on it. You do not want the plane to look translucent when the sun is behind it.

Before switching colors, allow the dope to cure for a couple of days so the next masking tape session will not pull any paint. Once all the color is complete, then finish the aircraft with several good coats of clear butyrate. After allowing the dope to cure for a couple of months, the finish can be rubbed out to awesome results. Yes, you can fly the plane during this time but it does take time for dope to fully cure.

Flight Preparation

The instructions indicate to place the receiver, battery, and tail servos in the aft servo bay...do NOT do this unless you bolt something like a Zenoah G-23 or US Engines 25 cc to the front. Hobbico's web site states that the US Engines motor actually requires lead in the tail and at over twice the weight of the O.S. I can agree. I highly recommend a gas engine on this bird. I chose to place the elevator and rudder servo in the aft bay to minimize pushrod distances and placed a 1300mah, 6-volt pack alongside the fuel tank.

Lo and behold, it needed 30 ounces of lead on the firewall to reach the balance point. Significant lead weight is always disappointing but one must realize that the Decathlon has a short nose moment. A few flights will let me know if I can remove some weight to shift the CG aft for improved aerobatics.

Engine Prep

The O.S. 1.20 fired up at 2.5 turns in nothing flat. It was so smooth that it was rather anticlimactic. Something to be said for quality motors! I chose a Top Flite 16x6 PowerPoint wooden propeller as my starting point and ran the engine thru the O.S. prescribed break-in process. It is now time for a range check and first flight.

Breaking Ground

The day was clear and calm so after a range check I taxied the Decathlon around and made a couple of aborted take-off runs to check the ground handling. The gear is wide and stable and needed only a minimally amount of rudder to take-off the grass/dirt runway. The climb out was smooth and very scale in appearance. The O.S. 1.20 has plenty of power for scale aerobatics with the only limits in vertical thrust. The Decathlon will climb nicely but not for long. Once again...very scale.

The plane required a couple of clicks of right aileron and depending on how full the tank was an occasional click of elevator. Axial rolls on low-rates required a good amount of down elevator to maintain the nose. On higher-than-high rates the roll rate needed only a twitch of elevator. I found that I liked the higher rates for normal flying.

The elevator on low rates was perfect for loops but not enough for snap rolls. On higher-than-high rates the snap rolls were scale but the Decathlon would flip out of loops at the top. I turned down the high rates and that problem went away.

Use of rudder is needed to make scale / coordinated turns and there was plenty of rudder to go around. My radio did not have dual rates for rudder so I setup for maximum mechanical deflection. With the turns I only needed a little to make a beautiful coordinate turn. At the other extreme was stall turns and hammerheads. Given the limited vertical performance I would simply kick the rudder over as the plane stopped vertical travel. It was a nice sight to see.

Knife-edge maneuvers were straightforward and the aircraft would actually climb in a straight pass. The Decathlon tended to pull towards the top of the fuselage so a little bit of elevator is needed for a truly straight knife-edge.

With the elevators abilities tied in the plane would do awesome knife-edge circles. Prior to landing I made several low passes to see how well it would slow down.

The results were nice. I was able to three-point land all but one of my landings that day. The very first landing was harsh as I allowed the plane to stall about 2 feet off the ground resulting in splayed landing.

All in all the Dynaflite Decathlon project has been a joy. The construction is simple and straightforward without any surprises, covering with dope was a learning experience that everyone should try, and I will truly enjoy flying this bird in the future. A nice entry into giant-scale aircraft. Kudos to Dynaflite and the folks at Hobbico and O.S.

Reprinted with permission.
Nov/Dec 2001 & Jan/Feb 2002 R/C Excellence

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