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Written by Dan Grotzinger
Take your favorite model beyond the circle
As seen in the August 2015 issue of
Model Aviation.

Why would anyone want to make a Control Line (CL) Aerobatics-to-RC conversion? For me it was the challenge, and because I have experience in both disciplines, it wasn’t too difficult.

If you are a builder, a reason might be that there are not too many kits available anymore unless you spend your time mining eBay. However, a variety of CL Aerobatics or Stunt kits is available thanks to the Builder of the Model Rule and John Brodak’s dedication to keeping CL alive. There are also kits by such suppliers as Sig Manufacturing Co. and RSM Distribution. Another reason is that there are a lot of good-looking Stunt aircraft!

This conversion business is not new to me. I converted a Midwest Panther Stunt model for RC in 1973, using an early Citizenship proportional radio with kit-built World Engines S-4a servos. They were small servos for their day, equivalent to today’s standard servo. To make it work I had to cut holes in the W-1 ribs to mount the servos with only the output of the servo protruding into the fuselage radio bay.

The Panther had been built and was half painted when I moved from Pennsylvania to Utah and sold it to my friend, Pete Carr. Pete finished it and flew it for six years. I’ve had a kit of the Panther’s sister model, the Cougar, and I wanted it to be an RC conversion as well because it has better geometry than the Panther.

Many will remember the RC Nobler. That was more a redesign than a conversion. I wanted my Cougar to still look like an authentic Cougar Stunt aircraft. Today’s small radios make that goal obtainable. Let’s go to work!

Change 1: Equal Wing Panels

This is presented first because, other than creating ailerons from the flaps, this is the only indespensible conversion. Stunt models almost always have unequal wing panels to eliminate extra lift in the outboard wing caused by that panel moving faster because of the circular flight path. There is also a need for more lift in the inner panel because of the effects of the control lines. Most CL Stunt models have some combination of outboard tip weight and wing imbalance to correct these forces.

Figure 1 represents a typical Stunt wing. The outboard wing is usually 1 to 2 inches shorter. Some wings, such as those on the Veco Thunderbird, have equal wings that are offset from the desired amount in the fuselage. Others, such as the Cougar, add one rib inboard.

Figure 1.

This rib could be spaced equal to the other ribs or be less. A few Stunt models use an equal number of ribs in each wing but space them differently in each panel. You will have to determine which method was used in your conversion model in order to equalize your panels.

On the Cougar, I added a 2-inch rib bay outboard. Later, I will explain why it might have been better to take an inch off the inboard wing and add an inch to the outboard, keeping the original wingspan and area.

Change 2: Ailerons

The only thing you have to do here is uncouple the flaps and oppose their function as ailerons. Strip ailerons have been in use for a long time; however, there is a difference on Stunt aircraft.

Stunt flaps are usually wider inboard than at the tips, lessening the outboard turbulence and making sharp maneuvers smoother. This same geometry used as ailerons is inefficient. The wide inboard area creates much more drag than rolling force, but the solution is simple.

On the Cougar, the outer 60% of the flap was used as ailerons and the inner 40% was fixed at neutral. The linkages were set up for differential to account for adverse yaw.

A hatch was installed on the bottom of the fuselage to provide easy access to the radio gear. A switch was installed in the cover.

Other options would be to use the inner flaps as operating landing flaps or coupling them to the elevators as stunt flaps. You can have this kind of fun when you “roll your own.” I felt these options were unnecessary, but the choice is yours.

If you want a somewhat difficult-to-fly model, you don’t have to make any more aerodynamic modifications. If you want a well-behaved performer, read on.

Change 3: Dihedral

To dihedral or not to dihedral, that is the question. It is a matter of preference. I don’t like airplanes with no dihedral and I don’t like airplanes with too much. What is too much? That depends on how you want to fly this project you’ve embarked upon.

For precise aerobatics, you want minimum dihedral. For a stable, Sunday sport flier, you may want more. Dihedral helps keep the nose up in a banked turn. It also gives the airplane a feel of right side up versus upside down. If you want complete neutrality, do not use dihedral.

The more dihedral there is, the more wind will rock the wings and disturb the heading, but the airplane will be more self-stabilizing. Less dihedral will make the rolls more axial. With this in mind, the Cougar was built with 1.5° dihedral as measured at the mean line of the leading edge (LE), not the upper or lower surface of the wing. This gave the model a good pattern-like performance.

Change 4: Tail Length

The Cougar’s tail is longer than an average Stunt aircraft. I went with a 11/2-inch increase, which looked good as far as the proportion of the nose moment to the tail moment.

Nose moment is propeller arc to center of gravity (CG) distance, and tail moment is CG to elevator hinge line. For smooth aerobatics, I like a ratio of between 2:1 or 3:1. Again this is a matter of choice. On the Cougar, this worked out well for pitch stability (up and down), but yaw was another matter. I did not give the wingspan-to-tail-length ratio enough consideration.

In flight tests, the long, broad wing was creating hunting in the yaw axis (right and left) in wind or turbulence. This was partially because the wing was blanketing the tail because the wing was too long, the tail was too short, or the fin and rudder were too small in area. This is where I should not have made the wing longer by adding two inches, or I should have made the tail roughly 21/4 inches longer. The other possibility would have been more fin and rudder.

What to do now? The airplane was built, but technology provided an easy fix. A gyro in the rudder channel solved all of the unwanted characteristics, and as a bonus I got to keep authentic appearance. The gyro also improved ground handling.

Change 5: Incidence/Force Arrangement

This is important, but optional. Stunt models are set up for neutral stability. The mean chord lines of the wing and stabilizer are parallel with the thrustline—that is, they are all at zero incidence (see Figure 2).

Figure 2. Wing mounted below thrust reference line and stabilizer mounted above thrust reference line.

Many Pattern designs, as well as most 3-D designs, use this force arrangement. However, there is a tradeoff. These airplanes track more accurately through complex maneuvers, but they also increase the pilot’s workload.

With the elevator at neutral and a properly located CG, the neutral-stability airplane will fly a parabolic arc into the ground like a yard dart when starting from level flight with no control inputs. That is gravity trumping zero lift. To fly, you must input some up-elevator.

To further complicate matters, the amount of input changes with airspeed. In slow flight, the aircraft will fly noticeably nose-up and tail-down. Some become unstable, depending on other factors. The pilot may have become accustomed to it, but he or she is working that elevator.

Having been involved in Vintage RC, I have developed a preference for what I call the vintage force arrangement. The most common example of this is the motor at zero and the wing and stabilizer at +2°. Goldberg Falcons and Skylarks are set up this way, as are the Dave Platt Models 1/6-scale Spitfire and Fw 190-D. The Top Flite “red-box” fighters are also this type of setup.

The difference between the thrustline and wing creates lift. The more speed, the more lift. Without tail incidence, these airplanes would climb sharply or even loop. The stabilizer incidence lifts the tail and keeps the flight path fairly level. These airplanes are effectively self-trimming over a wide speed range, only needing a little up-elevator for the landing approach.

The Cougar was set up with -1° thrustline and +1° wing and stabilizer incidence. It was done this way to easily fit everything into the fuselage profile. The airplane will present slightly differently in the air. I’m comfortable with this vintage setup.

Change 6: Elevator Area and Hinge Line

This is an option worth considering. The Cougar has huge elevators, as do many Stunt aircraft. They must turn on a dime and work in a tight space. The Cougar’s hinge line was moved back 5/8 inch, increasing the stabilizer and decreasing the elevator areas. One could simply decrease servo throw, but this change would likely decrease the possibility of elevator flutter and load on the servo.

Change 7: Fuel Tank Area

This is a structural issue rather than an aerodynamic one, but it is important. Stunt models have a small fuselage profile and only need fuel for roughly 7 minutes. I modified the nose to accommodate a 6-ounce rectangular Du-Bro clunk tank.

I cut off the motor mounts approximately 3/4 inch behind the firewall and added a set of maple rails below the originals. These only extend forward far enough that the rear motor mount bolts engage both sets of rails. This makes the nose easier to carve (see Figure 3).

Figure 3.


An O.S. 32SX was used for power. Hitec HS-225MG mini servos were installed with a feather servo attached with double-sided tape to the back of the former behind the cockpit for throttle. The battery sits in a cutout between the wing’s LE and the main spars. The spars are doubled with spruce to make up for the strength lost by cutting out most of the center sheeting for radio equipment.

The rudder and elevator servos are between the W-1 ribs behind the main spar and the receiver is above them in the turtledeck. The wing is not removable for structural reasons.

There is an access hatch made of cross-grained, two-ply balsa that you must fabricate. A light finish was obtained using Polyspan, five coats of AeroGloss clear, and two coats of Rust-Oleum Painter’s Touch flat white primer. The weight came out at 4 pounds.


The airplane that is my gold standard is the Dirty Birdy. The Cougar isn’t quite that good, but it’s better than some popular sport airplanes I’ve seen. The gyro solved the minor hunting in yaw and difficult takeoffs.

Flight times on the Cougar are approximately 7 minutes with the O.S. 32SX engine and a 6-ounce rectangular Du-Bro clunk tank.

This was a gratifying, successful experimental project. Yours is waiting for you. If you have any questions or feedback on your project, call me at (317) 826-8106. I don’t often get to a computer.

—Dan Grotzinger


Brodak Manufacturing

Sig Manufacturing
(641) 623-5154

RSM Distribution
(951) 678-1406

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