CL Aerobatics
Frank McMillan, 12106 Gunter Grv., San Antonio TX 78231
In the last column I mentioned that late summer and early fall offer wonderful flying weather for experimentation and learning. Set the stage for further development by doing a detailed retrim of your current model. This gives you the best baseline to evaluate how effective your changes are.
Look at the engine to see if something better is available. Items that can be varied are the venturi, head setting, glow plug, and fuel. As with everything you do, these factors are interrelated. Your goal is to have the engine run with the optimum combination of power, characteristics, and reliability.
Fuel and oil
Fuel is an easy factor to change, and doing so can show useful results.
For many decades, the standard fuel for Stunt was a 5% nitro mix. Typically, Fox Superfuel or K&B 100 all-castor oil was the norm, until synthetics came along. This created a few problems, since Stunt engines generally didn’t run well with many of the new synthetic-oil fuels.
Remember that in your situation the oil in the fuel is not only the lubricant, but is also the coolant. This point really affects the cycling, which is sped up under load and returns to a lower rpm when the load is relieved.
Most successful fuel mixes for Stunt feature equal parts of synthetic and castor oil, at approximately 21% of the fuel by volume. Many fliers have been using the Aero Products oil additive at one ounce per gallon of fuel; I have used it for years and recommended it. Stick to these proportions for modern engines; too much oil will definitely cause your engine to be unstable.
Once I tried 25% all-castor oil in a SuperTiger .60. The engine ran well for a couple flights, but it wouldn’t take or hold a good setting after that. Another example was a chromed O.S. .35S, which would hit a two-cycle and not drop into a four-cycle. A change back to my standard 21% oil mix had the ST .60 and the O.S. .35S running normally again.
It’s all a function of heat and the materials used in the engines. Anything chrome is going to run best at approximately 21% half-synthetic/half-castor oil. Older glow engines will take the 25% castor-oil fuel mix.
Nitromethane (nitro)
Focus on the nitromethane content in the fuel. The standard used to be 5% nitro, but most fliers have tried as much as 10%. A 50/50 mix of 5% and 10% will yield a 7½% mixture. These are the first mixes you want to try in your model, to see how your setup works.
Does the 10% mix give your model too much power and cause the engine to hit and drive out of the corners? Sometimes this effect is subtle, but it can cause problems in the consistency of your model's bottoms. Backing off on the nitro content will make it go away.
Remember that the cooler the weather, the more power, so fold all this into the equation.
Sometimes conditions vary when you go to a contest in another part of the country. It's often hotter with a higher density altitude. One way to compensate for the change is to increase the nitromethane content. How you do this is up to you, but most fliers mix 25% nitro fuel with a lower-content fuel to bump the overall content.
Does this make a difference? You bet it does. Nitromethane is a powerful oxidizer that will supply more oxygen to compensate for the increase in density altitude.
This discussion may seem to contradict the earlier point about 10% nitro possibly being too much, but it really doesn't. The difference is where the heat robs you of the punch you had at home where it was cooler, or when you fly in the early morning or evening.
As you add nitro, power will start to go up and the engine will get smoother. The maximum should be 20–25% nitro, and although you'd like to keep the extra power, don't be tempted. The extra power you don't want will have your model accelerating at the wrong time.
That's a function of the temperature. A few ranges will prove useful:
- 65–85° F is a great margin, where everything really works well.
- Anything lower, and you'd better think about 5% nitro as a maximum.
As the temperature climbs above 90° F, at some point you may experience "the wall," where the drive of the engine won't get it through the tough parts of the pattern. This is where an extra "tip of the can" will help.
Consider the nitro option as one of the last to use, but don't forget that it's available. More of the top fliers have used this than you might realize.
Propellers
The last subject I want to explore is propellers, and there is real magic here.
One of the best ways to tell whether or not you have a good combination is to listen to your engine run; you can tell by the smoothness of the sound and the characteristics in the maneuvers. I pitched a prop for a friend with the same profile I used. His engine immediately sounded like mine. (Hopefully that's a good thing.) The point is, all of this is repeatable.
There are several areas you have to be concerned with, including the engine load and the "fit" of the prop. Each engine has an rpm range and a load where it "likes" to run. You can ask fliers who run certain engines and they can put you in the ballpark, but the refinement is up to you.
There are different schools of thought about load on the engine. A view that developed before we had the newer engines was to load the engine with so much prop that it couldn't possibly break or accelerate. This approach could work acceptably in special cases, but there were problems. When the wind came up, the engine might unload and hit very hard. If the weather cooled off, the engine might develop enough power to hit hard. You are better off considering the second option.
Where does each engine "like to run"? Most engines will "tell" you, if you're willing to listen. Determining the exact range requires that you try a few different props, to get a feel for where you want the engine to be.
You want the engine to be "comfortable": it should readily cycle upward in rpm when there is an increase in load—such as in cornering maneuvers or long vertical climbs. When the load decreases, the engine/prop combination should sense this reduction and drop back to a steady state.
Load on the engine with regard to the propeller is a function of diameter, pitch, blade area, and airfoil.
- Diameter: The more diameter your propeller can carry, the better the vertical line tension above 45° will be. Diameter is a major factor on load, and a small reduction in blade length can have a significant effect.
- Pitch and blade area: Large diameter slows the turn; shorter props allow the airplane to turn more easily. These two factors can be balanced with the center of gravity (CG) location to give your model an optimum trim.
- Feel: The airplane must turn easily but retain a positive feeling.
Be aware that as you move the CG to the rear with a large-diameter prop, you can get the CG back to the point where a subtle problem manifests itself. The line tension may not be solid as you turn the airplane. As you unload the airplane in turns, you're not moving the model on the CG, with the torque rotating to the inside of the circle. The fix is easy: add nose weight, and everything will lock in again. It might only take a few grams.
Picture how you want to control the engine speed versus the lap speed. As Bob Hunt put it so well, pitch is like the gears in an automobile. Pitch the prop so the engine runs in its best torque range and you get the best lap speed.
Realize that there will probably have to be some compromise. I can't tell you exactly where you should go for your setup, airplane, engine, prop, fuel, or other variable; there are too many combinations. But I have a couple ballpark numbers.
Recommended starting points:
- For the O.S. .35S and similar engines: 10½ x 4¼ blade at about 9,600 rpm.
- For .40 piped engines: 11¾ x 3.84 at 10,800–11,200 rpm.
- For SuperTiger .60 types: 12¼–12¾ inch diameter prop at approximately 9,200–9,600 rpm, with pitch right at or just less than 5 inches.
- For piped .60s: 13-inch props (2- or 3-blade) with 3.5–4 inch pitch, at 10,400–10,600 rpm.
It's fine if your experience shows different numbers. The point is that you will have to try different things to see what works best for you. Keep trying different combinations, and remember that fall is the time to experiment.
MA
Transcribed from original scans by AI. Minor OCR errors may remain.
