Author: Bob Hunt
Edition: Model Aviation - 2009/09
Page Numbers: 137,138,140
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Control Line Aerobatics - 2009/09

Bob Hunt [[email protected]]

Also included in this column:

  • Advantages of electric power for Stunt

You won't miss the "lovely" castor oil smell

I'm taking a break from the Project Hole Shot series this month. The model is ready for finishing and I'm contemplating trying something new, so it is sitting for a while. This break gives me the opportunity to write about something that has become important to me and something that is about to become significant in the CL Precision Aerobatics (Stunt) world: electric power.

I'm now a total convert to electric power for my AMA and FAI Stunt programs. (I'll still fly Classic and Old Time with glow engines.) However, I have not strived to be a "salesman" for electric power in print. I figured that it would eventually display its many merits without my help.

Besides, I'm not one of the pioneers of using electric power for Stunt. That distinction belongs to Mike Palko, Walt Brownell, and a handful of others. I have believed, up to this point, that it has been theirs to promote.

If you follow various threads on Stunt online forums, you might know that many pilots have made much progress in bringing electric power to fruition as a viable alternative to glow power. You can type "electric-powered CL Stunt" into the subject bar on an Internet search engine, and that will bring up dozens of videos showing the practicality and success of using motors in our event.

It is time to acknowledge the advantages of this power source and promote electric to the masses. Until now, I did not believe that the technology and component availability was such that the average modeler could achieve success without having a local "guru" explain things. Electric power for CL has fully arrived, and it deserves some ink.

Even though I had watched Mike Palko fly electric-powered Stunt models for a few years, my eye-opening personal encounter with it happened at the 2005 Nats. I was watching Walt Brownell fly his ArcAngel model through a pattern. At that time I didn't even know that it was an electric model.

I was walking toward the circle in which he was flying. As I got closer, I realized that the model was emitting no discernible noise, but it was doing a fantastic job of going through the pattern in a stiff wind. To that point, I had felt that electric power was interesting but probably still a few years away from being truly viable for top competition use.

Later that day, after the competition had concluded, I approached Walt and did something I rarely do; I asked him if I could fly the ArcAngel. Walt seemed pleased that I asked and said "Sure!"

During the takeoff roll on that flight, I knew that my model-flying paradigms had changed. The amazing amount of line tension, the smoothness, and the model's solid feel even before it broke ground convinced me that practical electric power had arrived. The rest of the flight confirmed my initial feelings.

On the trip home, I called my buddy, Dean Pappas, and asked him whether or not he thought that converting my glow-powered Genesis Extreme to electric power was possible. He asked a few pertinent questions about its weight and wing area and then did a few quick calculations. He determined that the airplane was a perfect size and weight for such a conversion.

Dean suggested an AXI 2826/10 motor and a 4000 mAh, four-cell Li-Poly battery. There were no great sequencers available then, so we used a simple timer that set the ESC to full throttle for approximately six to six-and-a-half minutes. Then we chose a propeller (pitch, number of blades, and diameter) that flew the Genesis at the desired lap time and absorbed the 120 watts per pound of all-up flying weight.

As I walked out to the handle, the initial power burst caused by the fully charged battery was "burning down," and the battery was warming up internally. The horsepower was outrageous up to the Horizontal Eight, and then as the battery voltage dropped the model slowed about one-tenth of a second per lap for the next few maneuvers.

Then as the battery voltage continued to diminish, the lap times slowed again about another one-tenth and the line tension dropped correspondingly—exactly where I really needed it the most. Clearly this was not an optimal setup. But let the record show that it was good enough to allow me to make the 2006 FAI team a mere three weeks after having flown Walt’s ArcAngel.

The next step in the program was to stabilize the lap times and maneuvering speeds, and improve the horsepower everywhere. And this would require some actual technology.

At this point, Dean convinced himself that if we could govern the motor rpm to a fixed value, even as the battery voltage drooped and as the airplane climbed and dived, we would have something that roughly simulated a classic two- or four-stroke Stunt run.

An electric-model pioneer, Bob Boucher of AstroFlight, suggested to Dean that we try to optimize the helicopter rotor-speed governor that was designed into the Castle Creations Phoenix-series ESCs. This was a bit of a leap, because helicopter rotor-speed controls are slow compared to what we needed.

But sure enough, the required control range was available in the Castle unit. Now we would have to build a timer that produced the "throttle setting" that would yield the desired rpm and lap times.

Dean installed an RC receiver in my Genesis Extreme, and I flew it with him sitting at my feet in the center of the circle with a transmitter. He adjusted the throttle and made notes of the pulse-width numbers so we could produce a "script" of throttle pulse-width settings (as with a radio-receiver output). Our initial "script" read something similar to the following:

  • Run up to almost 9,000 rpm for several seconds and then to a 4,000 rpm idle for 30 seconds. This would allow time to walk out to the handle while saving battery capacity.
  • Next, set roughly 9,000 rpm for five-and-a-half to six minutes for the in-flight run.
  • Five seconds before the end of the run, drop the rpm to half for approximately one second and then recover to the 9,000 rpm setting for five more seconds as a warning so I could safely return to level flight before the power shut off.

Now all we needed was a timer that could be programmed to follow that script with fine adjustability in every parameter. Dean and I decided on the features we wanted (okay, he told me what features we wanted), and our good friend, Andy Kunz, who is a gifted code writer, took on the task of making our first-generation timer.

I’d like to tell you that this process immediately yielded the ultimate device, but that wouldn’t be the truth. There wasn’t enough time to fully develop the system before leaving for Spain the next year for the World Championships. The results were predictable; I didn’t do too well.

The good news is that a new model timer resulted from all of our trials and tribulations, and it has an abundance of neat features. I’ll invite Dean to write about that timer in a future column. Thanks to him for staying up late and helping me hammer out the preceding in proper order and terminology.

Advantages of electric power for Stunt

What I really want to write about in this column are the advantages of electric power for Stunt flying and the reasons why you should consider it. They are as follows.

Noise (or lack thereof)

The most obvious advantage of electric power is the fact that it emits far less noise than a glow engine. It emits no combustive noise. Yes, there is propeller noise, and that will vary from minimal to almost imperceivable in our application, depending on what rpm you are running.

The real advantage is that you will be able to fly at times and at places that were never possible with glow power. The noise issue is what really got me interested in electric power to begin with.

I have a field one-quarter of a mile from my house that is surrounded by homes. I could now fly there at daybreak and no one would have a clue I was there unless he or she looked out the window! New fields will, and have, opened up to many of us strictly because of the lack of noise emission.

No vibration

Motors simply don’t vibrate. Your airframes will last much longer, and they won’t have to be built as robustly, thus allowing a significant reduction in airframe weight in the bargain. Also, your model will not soak up weight from fuel residue.

Constant CG location

In glow applications, the full tank of fuel—which is situated ahead of the model’s CG—burns off as the flight progresses, effectively shifting the CG aft. There is no CG shift when using a motor! Your model will feel the same to you throughout the flight.

Have you ever noticed how difficult it is to perform Overhead Figure Eights in a wind? The model does not want to track properly throughout the maneuver and is significantly "pushed" down on the sides.

That is mainly because the CG has shifted so far aft by that point in the flight that the model has no drive. I’ve noticed that Vertical Eights, Hourglasses, Overhead Figure Eights, and entries to Four Leaf Clovers in the wind are vastly improved with the constant-CG electric setup.

Weight concentration

We have found that an electric setup’s all-up weight is virtually the same as that of a glow setup, providing that you factor in the weight of the fuel when considering the glow setup. Fuel weighs approximately 1 ounce per fluid ounce. But the concentration of that weight is significant.

In the glow setup, the engine weighs more than the equivalent motor of the same power output range would weigh. In my setup, the engine that was replaced by the motor weighed approximately 10 ounces with the header attached. The AXI 2826/10 weighs 6 ounces. Yes, the battery weighs more than the full tank of fuel did, but, again, the two setups’ total weights were virtually the same.

The kicker is that the center of the mass of the weight that is now balancing the model at the desired CG location is much closer to that location! The fulcrum is shorter, thus making it easier to start that weight moving and easier to stop it. Thank you, Mr. Newton!

In fact, I was able to carry a CG location in the electric model that was a bit farther forward, therefore more "groovy" feeling, but still have even more corner and lock than with the glow engine and a farther-aft CG location. It’s akin to having your cake, eating it too, and then going out for a big dessert afterward!

Vertical CG location adjustability

With an electric setup, you can move the battery up or down within its bay to affect the vertical CG location. With a glow setup, you have to place the tank in a position that makes the engine run the same upright and inverted. You cannot use that weight to adjust the vertical CG for that reason and because the weight in the tank (fuel) will burn off gradually as the flight progresses anyway.

You may not think this is a significant plus, but it is. I’ve had many models that flew with the outside tip slightly high and assumed that it was a wing-warp issue. I tried slight flap tweaking to fix the problem, only to have other trim problems arise in the process.

The Genesis Extreme was always sensitive to roll trim, and I tried many combinations of tweaks, tip weight, and trim tabs before realizing that the real problem was the vertical CG location. Now I can simply move the battery (up in my case) to fix the problem. And since the battery weight doesn’t turn off, as fuel does, the fix is permanent.

Run consistency

Of all the pluses that electric power offers, the ability to achieve a consistent and repeatable run is perhaps the most important. Once you attain the run you desire, the system will repeat it for hours, days, weeks, and months. It’s almost boring!

The ability to have a consistent run means that your practice time will be optimized and your timing will be sharpened to a fine point. Glow power cannot even begin to approach the consistency of electric power.

Ability to use reverse-pitch propellers

Kaz Minato came up with another major advantage for electric CL fliers. He discovered that reverse-pitch propellers (pusher propellers) essentially give us right rudder trim throughout the flight, whereas standard-rotation propellers yield left-turn trim. Why? Spiral airflow.

Our normal-rotation propeller actually produces spiral airflow that circles the fuselage and pushes against our model's vertical fin's left side, forcing the aft end of the fuselage toward the outside of the circle. This means that the nose is being forced in toward the pilot. We have to trim our models to stay at the end of the lines.

With the reverse-pitch propeller, the spiral airflow is pushing against the right side of the vertical fin, causing the model to want to turn right, thus yielding more line tension.

Reversing the rotation of a typical outrunner motor is a simple matter of swapping any two wires coming from the ESC to the motor. This allows you to make A-B comparisons on back-to-back flights using normal and reverse-pitch propellers.

I was skeptical about this until I tried it. Now I'm a firm believer. The overhead and vertical tension have vastly improved, and weren't bad to begin with!

Ancillary benefits

Electric power has many not-so-obvious benefits. Your clothes won't smell of burnt castor oil anymore, and neither will your car's backseat!

You can test-fly your models before you paint them, because no fuel residue is emitted. The worst you might have to do is clean off a bit of chlorophyll from grass cut by the propeller before you paint. This advantage allows you to be sure that the wing and stabilizer are in alignment before finishing the airplane.

You can converse with your coach from inside the circle without having to raise your voice.

Finally, you can test-run your motor inside your shop. There is one caveat here; make sure your shop is clean before starting your motor. You will be amazed by how much air blast that propeller generates!

That's plenty for this time, but I will expound on each benefit of electric power for Stunt in the future.

Till next time, fly Stunt. MA

Sources

AXI

Castle Creations

Transcribed from original scans by AI. Minor OCR errors may remain.