Control Line: Aerobatics
Frank McMillan 12106 Gunter Grove, San Antonio, TX 78231
Fuel
The subject of fuel has surfaced only occasionally in this column, perhaps because many of us take for granted the high-quality fuels generally available. That is, until something bites us — you know where — at the most inopportune time. In case you haven't experienced a dose of bad fuel — which likely means you haven't been around long — here are some thoughts on analyzing problems that may be caused by bad fuel and what you can do to get back on track.
First, realize that a true fuel problem — assuming all other components of your setup are correct and tested — may not manifest itself for as many as four to six flights after you change fuel. A good engine seems to hold its running characteristics and will gradually slip into unsatisfactory runs. Likewise, even when you change to a known good fuel, it may take several flights to settle down on the good stuff before solid runs return. I don't have a technical explanation for this, but I know it's true because I've fought through as many fuel problems as any modeler.
#### Recognizing fuel-related problems
Fuel problems will affect how your engine runs. To begin, you must have a solid baseline engine run with which you are satisfied. The engine should perform well and respond correctly to load changes — adding a little power going uphill and dropping off slightly going downhill. Sounds like the classic stunt run, and it is — normally aspirated, piped.
Common fuel-related symptoms include:
- The run gets thin and the overall power gradually drops, sometimes significantly.
- Maneuvers above 45° are affected by reduced line tension.
- It becomes increasingly difficult to fly round maneuvers and to hold shapes under 45°.
- A harder break to a two-cycle run and/or a tendency to remain in a two-cycle mode with diminished power and erratic running.
- Difficulty getting a stable setting on the ground with a tach.
Any or all of these symptoms can also result from other problems. Change the plug first — that's an instant fix. If it doesn't work, inspect fuel lines and fittings. But don't overlook bad fuel.
#### Procedures to reduce fuel incidents
- Keep a quantity of known-good fuel that you continually test, expend, and renew. Try to keep on hand an almost-full gallon of fuel with test flights flown from it, in case a suspect problem exists. That way you can run flights from known fuel and verify whether the problem is fuel-related.
- Once a problem is sorted out, test a new gallon for your baseline and burn the previous test batch. Rotate the "good stuff" so it doesn't go bad.
- If you need a reliable product quickly, Duke's Fuel used to be a problem solver for Al Rabe; today Doug Taffinder's or Sig's Champion mix are good choices. You can also borrow a few tanks at the field to check things out.
- I try never to fly a contest or arrive at a contest site to practice with fuel that hasn't been tested at home. Mix batches and fly with a gallon from the same set of ingredients to verify everything is correct.
I've been bitten before. Once I switched to 28% all-castor fuel on a SuperTigre .60. It worked great for a few flights but became very erratic at the Nats. As soon as I went back to my old mix (20% oil: half Klotz/half castor), everything was fine. Incidentally, this is very close to the Sig Champion 10% mix.
Al Rabe used to claim he could detect differences between gallons of fuel. He would fly several flights on each gallon he took to the Nationals; the best was reserved for the final five, the next best for top-20 day.
#### Temperature and handling
Be mindful of temperature effects on fuels. Try to avoid extremes where possible. At the Nationals, I bring all my fuel in at night, especially when nights are cool, because castor can form globules. Heat in a closed vehicle can do funny things to "prop" (propylene oxide) and to methanol, especially if you use plastic jugs. I try to use metal cans religiously. This also applies to decanting fuel.
Since I mix my own fuel, I keep ingredients separate as long as possible. I mix only enough for immediate needs and find I have fewer problems. One ingredient I use is prop (propylene oxide). As many of you know, the flash point of pure propylene oxide is 55°F. The best way to keep it relatively stable is a 50/50 mix with methanol. Doug Taffinder sells it this way and has no trouble. Be cautious with pure propylene oxide — it is toxic, and the low flash point can cause problems.
On safety: be prudent and observe all published safety rules on fuels. The mixes are relatively benign, but most ingredients carry cautions in certain circumstances.
Recommended products
Last year Bob Hunt recommended I try Hobbypoxy's new product, Smooth 'n' Easy, a two-part epoxy finishing resin. I purchased a kit; it's a one-to-one mix that comes in a two-tube set.
The first application I tried was fuelproofing the inside of a tank compartment. The characteristic that appealed to me was the thin viscosity of the mixture, which made it easy to brush on. I poured about 2 cc into the area, spread it with an acid brush, and let it soak in. It penetrated nicely and, as it set, formed a hard, smooth surface perfect for fuelproofing. For insurance, I recommend a light sanding followed by a light second application, wiping off excess with a paper towel.
Until recently I had not tried another application Bob was enthusiastic about: covering foam wings. For years I'd used Hobbypoxy Formula 2 heated to give a nice consistency. I was satisfied with the results, but it was messy and required quick work. I was also concerned about fumes from the heated epoxy, so I always wore a double-filter mask.
With Smooth 'n' Easy, fumes are minimal; the thin consistency works without tricks. Use a spreader with small notches (1/32 in.) to work the resin into the balsa skins, leaving only the ridges from the notched spreader. The objective is to minimize epoxy remaining on the surface. Apply the wing skins to the foam as you normally would and weigh them down on a flat surface. The surface becomes very rigid, adheres well, and remains light.
I've also used the resin to glue on wingtips. The resin sets hard and sands easily, so the seam works well with normal sanding. Unlike other glues, the joint remained invisible after a summer in the hot Texas sun.
Tech tip: Nylon tie wraps
I'm no expert on nylon tie wraps, but with the era of the pipe upon us, some knowledge of electrical tie wraps is necessary to make pipe-tuned systems seal properly.
- Where to buy: If you're starting out, you'll use a bunch. Forget Radio Shack or your local hobby shop as long-term sources. Find an electrical contractor supplier and buy in quantity. As a comparison, Radio Shack sold a pack of 25 for just under $3, while my supplier had a pack of 100 for $3.50. You will also have a better selection of types and colors.
- Which size: Don't get too narrow a band because it probably won't seal as well as a wider band. I use 1/4-inch-wide by 10-inch-long nylon, which works very well.
- Leak reduction trick: Use two tie wraps on each end of the coupler. This may seem wasteful, but since starting that procedure I haven't noticed any leaks. With the heavy, dark blue coupler you need extra insurance.
- Installation tip: Installing the tie wrap and getting it snug without tearing things up can be tricky. Watch how others do it. Grasp the end of the tie wrap (after securing it loosely around the coupler). With needle-nose pliers, rotate the tie against the fixture end, using it as a leverage point. You may need to reposition the pliers several times to maintain adequate leverage. Once you get the hang of it, the procedure is simple.
Join PAMPA
A reminder to join PAMPA (Precision Aerobatics Model Pilots' Association), our organization to promote precision aerobatics flying. Contact Tom Morris at PAMPA, 1019 Creek Trail, Anniston, AL 36206.
Transcribed from original scans by AI. Minor OCR errors may remain.
