Control Line: Aerobatics
Ted Fancher
Starting Fresh
It seems like we so-called "experts" sometimes forget that many of the things we take for granted are simply not understood by newcomers to our event. In retrospect, there is no reason they should understand if they ain't never had it 'splained to them.
A case in point was a letter from Michael Martin of Albuquerque, NM, who was perplexed about why different engines of nominally identical displacement consumed radically different amounts of fuel and produced vastly different amounts of power. Specifically, what should he, as a budding Stunt pilot, do about it? Some edited excerpts follow.
"I have been flying a Carl Goldberg Shoestring equipped with a Fox .36RC and a 3.25-oz. Triangle Hobbies Combat tank using 10% Red Max fuel and have been getting three-minute flights. If I use 5% fuel I still won't get the eight-minute flights needed for CL Aerobatics. The same plane with a Fox .35 gets nine-minute flights but it doesn't seem to have much power. I don't know what to do! Is the Fox .36 such a gas-guzzler that it should only be used for Combat? Is the Fox .35 just not powerful enough for an ambitious Stunt pilot?
"I also have a Ringmaster with a Fox .35 and a two-ounce clunk tank. It flies beautifully right side up but goes rich when inverted. The centerline of the tank is in line with the venturi as the old pros have suggested. Triangle Hobbies says that their Combat tank must be located 1/4 in. above the centerline of the venturi. Can you explain why it is necessary to have this offset, and must it be done for every plane?"
As Michael suggests, fuels, engines, Stunt and profile fuselages often seem an insoluble mystery to beginners—especially when everything they read from the "pros" is full of gobbledygook like "nitro... castor versus synthetic oils... choke area... uniflow suction... etc." Let's break the power-plant questions Michael raises down to the basics that are important to beginning Stunt pilots.
How long do you really need to run?
First of all, you don't need an eight-minute engine run for Stunt. Eight minutes is the maximum time the rules allow from the time you signal to start until the time you end landing roll. Actually flying the pattern takes just about 5½ minutes from launch to the end of the Clover. Allowing one minute to start and set the needle and for protection against running short results in a maximum desirable run of 6:30 to 6:45.
OK. That solves the easy part. You only need enough fuel to run for about 6½ minutes. Now, how do you determine how much and what type of fuel you will use to achieve that?
Fuel Basics
Let's break fuel down to its basics. We need components to provide lubrication and combustion. That's all that is necessary.
A perfectly usable model engine fuel is a combination of alcohol (usually methanol) and oil (usually castor oil) in about a three-to-one ratio. In actual practice we use some percentage of power additive, generally nitromethane, to refine the amount of energy that is derived from our tank of fuel.
The jury is always out on which lubricant is best: castor oil or synthetics. I've used each by itself and presently use a combination — all successfully. Generally the amount of lubricant seems to be more important to the actual run than the precise ingredient. The absolute minimum I would suggest is 20%.
I use Omega fuel to which I add an additional four ounces of castor oil per gallon. The additional oil causes the engine to be less prone to break from a four-cycle to a two-cycle and to return to a four-cycle more reliably after a maneuver.
From a beginner's standpoint such a refined mix is not necessary. I would simply ensure that I was using a fuel with a higher percentage of castor oil than synthetic. I use a fuel from a major supplier that contains at least 20% lubricant. That really includes many commercially available fuels which are 20 to 50% lubricant (most blends are 50/50 castor/synthetic) for two reasons:
- Castor maintains its ability to lubricate at higher temperatures than synthetics.
- It reduces the tendency to run heavy on the bottom for plain-bearing engines (as opposed to ball-bearing engines). The Fox engines seem to run better with castor.
Alcohol and nitromethane — the power components
The other two fuel ingredients — alcohol and nitromethane — are the power. Both are combustible; the nitro releases much more energy while burning. Therefore, as the ratio of nitro to alcohol is increased, power will increase. Unfortunately, so will fuel consumption! Like lunch, there is no free power.
To make this as simple as possible, 95% of all competitive Stunt fliers use nitro percentages from 5% to 15%. For you as a beginner to even be worrying about nitro percentages is a hassle you don't need at your stage of development. (The exception to this is anyone who lives at high altitudes such as Denver, Albuquerque, Reno, etc. The less-dense air at these altitudes will rob you of some horsepower, and you should probably seek out fuel in the higher-nitro range, i.e., 15%.) Don't worry about fuel consumption at altitude, as the thinner air will both decrease power and rate of burn. Increasing the nitro content will approximately neutralize these effects.
The bottom line is that you should be looking for a readily available, high-quality fuel containing at least 20% lubricant (of which at least 50% is castor oil) and with anywhere from 5% to 15% nitro. (A hint: nitro is expensive, so unless you like to spend money, opt for the lower nitro.) Decide which fuel fits this recipe for you, and stick with it. By doing so you will have standardized a major variable in the fuel-consumption equation.
Oil vs. performance
More oil equals a richer mixture and reduces top-end performance somewhat, but prevents engine damage and helps keep the engine running through inverted flight. For most Stunt pilots the compromise is to use more oil than a pure race blend would have, accept a little loss of peak rpm, and enjoy greater reliability and smoother idle and transition behavior.
Engines and consumption
This still doesn't solve the problem, does it? Half of the story is the engine. This part is a little more complex.
As you are probably aware, some engines burn more fuel than others even though they may be of identical displacement. This is an inherent result of differences in design. Again, power is never free. If an engine is designed to produce maximum horsepower—even if we operate it at significantly less than its maximum—it will still burn more gas than its more conservatively designed counterpart.
This is what's happening with Michael's two Foxes. The .35 is an older, conservatively tuned engine of moderate power at best. It won't burn much gas—but neither will it put out great gobs of power, no matter what you do to it in terms of fuel, props, etc. The .36, on the other hand, is a relatively newer design with less-conservative timing and modern Schnuerle intake ports which allow it to produce more power…and burn more gas.
Because these two engines have fixed venturi sizes, there is not too much that can be done to adjust them to the available amount of gas. Going to low- or no-nitro fuel in the .36 may gain a half minute or so of run, at most. Going to higher nitro in the .35 would only affect power because of the conservative basic design of the engine. In neither case are you apt to produce a successful combination. You would simply need to use a larger tank for the .36.
By the way, Michael's description of the poor output of his particular Fox .35 makes me believe it is probably either worn out or damaged in some way. A Fox .35 in good condition should provide adequate power for a Goldberg Shoestring Stunter under almost any conditions.
Removable venturi inserts
You do have some increased flexibility with engines equipped with removable venturi inserts, such as OS and Super Tiger. These inserts are easily duplicated so that having a variety of them is easy. Simply by making the bore larger or smaller you can have a direct effect on power and fuel consumption.
- Enlarging the bore will increase power (and fuel consumption) and vice versa.
- Always be deliberate when changing venturi size. A very small difference in bore will make a noticeable change in performance.
- Enlarging the venturi will cause a reduction in fuel draw and vice versa. (Fuel draw is a measure of the engine's ability to suck fuel from the tank. The smaller the venturi size, the better it will draw and the more consistently the engine will run regardless of airplane attitude.)
- As the intake is enlarged the suction is reduced, and the engine will have a greater tendency to shift from a four-cycle to a two-cycle as a result of aircraft attitude.
Expert fliers use combinations of intake size and nitro contents to fine-tune all of these variables—power, fuel economy, and fuel draw (and, therefore, how and when the engine "breaks")—to achieve precisely the type of engine run they want.
Practical advice for beginners
As with fuels, if you are just getting started in the "Stunt biz" I suggest you ignore such refinements in the engine department. Settle on an engine of high quality, break it in properly, determine its fuel consumption rate using your "standard" fuel, and then simply ensure you have a big enough tank to allow 6½- to 7-minute flights.
General tank-size guidelines:
- Most .35s will need no more than four ounces.
- .40s and .46s should probably have room for six ounces.
- .60s need up to eight ounces.
Once you have determined the consumption rate for your particular fuel, the tank size is easy. Fill the tank with a measured amount. Use a Sig five-ounce syringe and know precisely how much fuel is in the tank and, to within 15 seconds, how long it will run.
Tank levels on profile Stunters
On the subject of tank levels on profile Stunters, the following can be considered almost gospel: the centerline of the tank (or, more precisely, the fuel-pickup tube) must be above the centerline of the engine to avoid enriching the mixture during outside maneuvers.
Proper tank height is one of the fundamental trim adjustments that must be made on any airplane for competitive engines. The builder must be aware of this and provide a means to adjust the height of the tank. A good place to start is 1/4 in. above the engine centerline, but final adjustment must be done by trial and error.
This, by the way, is not one of the refinements which I have been suggesting can be put off until you become more skilled. You can't fly even a good beginner pattern if the engine is half-quitting every time you invert.
Community notes
Good news. Once again, it appears that the rumored demise of Control Line was premature. I'm receiving more and more reports of greatly increased participation, particularly in Stunt. Tom Dixon's Atlanta group is up to 30 from 10 this year and almost identical numbers are reported for the Clovis Controllers near Fresno, CA.
Let me know if your area is seeing such a rebirth. You can't keep a great idea down!
Advanced Stunt at the Nats
Last item. If you've been using the lack of an advanced skill-level class in Stunt at the Nats as an excuse not to attend, you're out of luck. Contact Roland McDonald, 28746 Westfield, Livonia, MI 48150 for info on this year's unofficial (but nonetheless very real) Advanced Stunt. Rollie and Jack Sheeks have taken the bull by the horns and are providing the methodology, the groundwork, and the awards! If this is your bag, come on out and support them and your hobby. See you in Lincoln.
Ted Fancher 158 Flying Cloud Isle Foster City, CA 94404
Transcribed from original scans by AI. Minor OCR errors may remain.
