CL Aerobatics
Frank McMillan, 12106 Gunter Grove, San Antonio TX 78231
Power vs. characteristics
If you are thinking about becoming a competitive Stunt flier, you must be able to make your power package run to the optimum combination of characteristics and power.
Note that I make a distinction between power and characteristics. It is relatively easy to detune any of the suitable engines by lowering compression and/or reducing the venturi size. Detuning can produce a docile engine that yields very nice runs. However, this is a closed system—detuning also takes away power.
There must be some compromise.
There is light at the end of the tunnel! The good engines available now usually have a broad, linear range where you can make adjustments without significantly decreasing power. As with any adjustments, you need to find the useful range.
I learned that with the stock head on the OPS-piped .40, .025 inch deck height was where the characteristics continued to soften, but power fell off dramatically. I've noticed the same circumstance with other engines; there is a compression setting where the "edge of the cliff" is reached.
Likewise, there is the other end of the linear range, where more compression will make more power but will cause the engine to hit harder and hold the break longer.
You have to determine this empirically—or better yet, listen carefully to recognized engine builders who have their fingers on the pulse of the optimum settings.
Compression and venturi
Working with the compression ratio, the venturi diameter also governs power and characteristics. Too small a venturi will not allow the engine to breathe, thus restricting power. Too large a venturi liberates a great deal of power, but drastically affects characteristics when taken to excess.
There are combinations at the extremes that seem to work: low compression with an open venturi, or a restricted venturi with high compression can be effective. However, these setups "push the envelope"—they work in a specific environment (certain location and weather conditions). Since these setups are on the edge, they require tuning to travel well.
This is not to say that other setups don't require adjustments—they do—it's just that the extreme-range setups are more sensitive to location changes.
There are other factors in the equation: airplane drag and propeller load affect the run characteristics, as does power-absorption effect. These variables interrelate; the point of this discussion is understanding the factors and being able to try different adjustments to make your package work.
Down the road, I'll look at tanks and engine heat. For now, here are a couple of points you might not have considered that also affect your engine run.
Fuel filters and fuel routing
There is no question that you should run a fuel filter, just as you should filter your fuel as you fill your tank. But did you know that the filter can seriously alter your engine's run?
Engines react to the work they have to do. Pulling the airplane around is one factor, but how about drawing fuel? You really want the engine to react to specific things when determining the characteristics.
Remember the two-cycle/four-cycle break? That reaction is the drag on the model caused by a change in aircraft attitude and/or control deflection. The engine may not react properly if it has another undesired load.
- The screen in the filter can cause the engine to work hard if the mesh is too fine. A fine mesh can also cause the filter to block more quickly.
- Beware of very small filters; they may work okay for a while but have a limited number of flights before they must be backflushed.
- Very large filters work well because of size, but they present a restriction because of their length; the engine must draw fuel through a longer path, and this affects the mixture.
A compromise in filter size is best. Also, route the fuel line to minimize loops, high spots, and pinches that may trap air or fuel. Finally, make sure all fittings are tight; any air leaks will change the mixture and the characteristics.
They may cause adjustment difficulties (they can act like a small tank/chicken hopper).
One filter I know that works well is the blue plastic Fuerstnost. You have to be careful to seal the halves together so they don't come apart or leak. A silicone seal works well. These filters need to be replaced periodically because the case gets brittle and shrinks.
Muffler/pipe pressure
To get your engine in its "comfort zone," you need to look at whether your particular engine needs muffler/pipe pressure. I've seen many superior fliers change day to day on this subject. The best I can offer is that it's a real choice—especially if the engine seems unsettled.
The older engines react well to muffler pressure. (Remember to drill out the tap to 1/16 inch for uniform pressure.) The newer engines—pipe especially—do well on straight uniflow. One engine that seems to want slight muffler pressure is the Aero Tiger conversion.
Stanzel Museum and Victor Stanzel & Co.
Last year I wrote an article on the opening of the Stanzel Museum in Schulenburg, Texas (a wonderful stop if you're in the area).
In the museum are wonderful examples of the long list of Stanzel designs—many recreated by famous MonoLine™ flier Dale Kirn. Dale is also remembered for his champion MonoLine™ Speed designs and the world-famous MonoLine™ demonstration tours. Great memories are also recreated at the Vintage Stunt Championships.
Dale sent me a flyer from the Victor Stanzel Company, and the list of products goes back to the days of the Stanzel and MonoLine™.
Seven kit types are available in original boxes, ranging from the Tiger Shark to the MonoLine™ Lil Rider. Full-size plans for the Shark designs, MonoLine™ and several Fleetflight designs are also available. Included are scale model plans in several scales, which formed the basis for the original business.
Contact Victor Stanzel & Co., Box 28, Schulenburg TX 78956; Tel.: (800) 422-6823, Fax: (979) 743-4870. This is a rare opportunity to acquire parts of our history from the source.
Gadgets: building board
One vice I admit to is a weakness for the newest gadgets.
While at the Vintage Stunt Championships, my good friend Keith Trostle mentioned a new building board he was using. I had not seen an advertisement, but I ordered one on his recommendation. My board had seen a lot of use, so I was looking for something new, flat, and stable.
When the board arrived, I was pleasantly surprised by the careful packing. It was suspended inside a sturdy cardboard box, so the board was protected—even if the box was dropped.
I ordered a 60 x 24 inch board—large enough for any project. It is a laminate of composite synthetic material and cork, in 1/2-inch-thick layers.
With the size I ordered, there are extruded-aluminum boxes supporting the undersurface and stabilizing the surface. The cork surface is durable, pinnable, and dead true.
I recommend this high-quality product. Multiple sizes are available. Contact Wright Engineering, 11780 Terracina Blvd., Redlands CA 92373-4855; Tel.: (888) 422-6268, Fax: (909) 793-7003.
Photos
If you would like some great photos of Vintage Stunt Championships 12 on CD, contact Ricki Pyatt.
Ricki has a well-deserved reputation as a model photographer, and she has developed an excellent photo collection. Contact her at 9629 Hazard Ave., Garden Grove CA 92844-3139.
Transcribed from original scans by AI. Minor OCR errors may remain.
