CONTROL LINE AEROBATICS
Frank McMillan 12106 Gunter Grove, San Antonio TX 78231
Introduction
This month I'll talk about problems that can arise when checking out a new airplane or engine. I used the phrase "checking out" because you are asking for trouble if you attempt to do both at the same time. Sure, it's been done and will be done in the future, but it certainly isn't the most desirable situation. Use an engine system (engine, propeller, tank, and exhaust unit—tuned or muffled) that you know is checked out and working reliably. Having that baseline will prevent a situation from which you can't recover, such as the engine stopping in an attitude that will cause the untrimmed model to crash.
Engine checkout
- Check out a new engine on a proven airplane after you have sufficient time on the test bench.
- Monitor the engine running on the bench to establish a good-running fit; it's not exciting but it is vital.
- Follow your supplier/manufacturer's instructions—the company knows the engine.
- Be careful to use the recommended fuel type and mixture. You need to be in the engine's comfort zone.
Many pilots create their own issues by not following instructions or using common sense. If you have any doubts, recheck or call someone you trust.
Airplane setup / Preflight checklist
Before you leave the workshop, set all adjustable parameters based on the designer's recommendations. This basic procedure will save you many problems and gives you a reliable baseline. Make sure you have good, conservative numbers. You can get better advice about flight behavior when you know what the settings are.
Preflight checklist:
- Check and set the center of gravity (CG) in the right place.
- Position adjustable leadouts as per plan; as a minimum place them slightly behind the CG.
- Spend time getting a good handle setting and line adjustment.
- When you get to the field, recheck everything and run the engine briefly to ensure the system is normal.
- Set the engine slightly rich for the first flight.
Fine trimming and wing leveling
Once you have the engine running reliably and gross trim done, the next baseline is fine trimming — and the first fine-trim task is to level the wing. No matter how carefully you constructed and aligned the airplane, chances are good you will have to level the wing. This is extremely important and must be done before any other trimming.
The wing may not fly level for several reasons: a twist in construction, control surface alignment, or top weight. A non-level wing will affect every maneuver, probably adversely. Some fliers try to trim other problems by carrying an "out of balance" trim, but I wouldn't recommend it.
Do wing balancing on a good day with light winds and little turbulence. I can get close myself, but I prefer someone to fly the airplane while I watch from outside the circle — it's much easier that way. Setting trim on a windy day has cost me weeks of grief.
#### Long-wing vs. lower-aspect designs
Long-wing (high-aspect-ratio) models are more critical to leveling the wing. They manifest any out-of-trim condition more noticeably and have a narrower trim range. This is especially true with equal-span panels (a past fad). This relates to the so-called barbell effect and the leverage longer wings can exert. Once in trim, long-wing models fly very well. Lower aspect-ratio designs are easier to set level and seem less reactive to turbulence.
Diagnosing wing roll: tip weight vs. flap tweak
If from the outside of the circle the wing appears inboard-wing-low, do one adjustment at a time and observe. If it's a slight condition, don't start tweaking surfaces yet; a small increase in tip weight might be all that's required.
- Try adding a small amount of tip weight (for example, half an ounce) and fly again.
- If the condition was true tip-weight related, the behavior should be the same upright and inverted—i.e., adding weight should move the wing toward neutral in both attitudes.
- If adding tip weight makes the problem worse in one direction and better in the other, it's likely a flap tweak problem, not tip weight.
If the wing goes from inboard-low to slightly inboard-high after adding weight, cut back about half the added weight. If the wing remains inboard-low even after more tip weight, consider a flap tweak or tab.
My preference is to avoid tweaking flaps unless necessary. Flap tweaks can lead to "chasing the tail"—too much one time, too little the next. An adjustable tab on one flap area is a very nice feature: you can try it and go back and forth until you arrive at a good setting without destroying your initial alignment.
Your indication to consider a flap tweak/tab adjustment is when one control causes a roll to the inside and the other control causes an opposite roll. For example, if the airplane is inboard-low upright and an up control causes it to roll in while a down control causes a roll out, make small adjustments: slightly inboard flap down (or if the tab is on the outboard wing, tab up). Fly again and observe; the airplane's reactions should become more linear and the rolling diminish. Continue until you observe wings level from outside the circle.
Hinging, final tip weight, and other factors
After achieving a wings-level condition, watch for "hinging"—when excess tip weight combined with enough energy carries the tip in the established line rather than allowing a smooth commanded turn. Very light, built-up wing designs tend to require less absolute tip weight and less differential weight to show hinging.
Typical final tip weight I've seen is about 1/2 ounce, with as little as a gram or two marking the hinging point. Line diameter, length, and type also have a significant effect on final trim.
Don't rush to make many changes. Leveling the wings is extremely important and will save you time.
Differential of the flaps is another factor that affects rolling and is still debated. Many successful designers use a slightly wider flap—about 5/32 inch—at the outboard tip. Others add tabs to the outboard flap to fine-tune the adjustment.
Reader comment
Well-known Nationals competitor Windy Urtnowski wrote in response to a previous column:
"A thought on your last column—engine unloading into higher rpm (added G.P. stability) with lower pitch—this phenomenon is especially true with Big Jim set-up when using cut down 14 x 5 rev-ups and most pronounced with 4-5B carbon (more blades, lower pitch, more revs—lots more G.P. stability). I've found this true any time you go in that direction (i.e., 6 pitch to 5, 5 to 4, etc., assuming weight of prop is the same).
"Also—a slightly higher compression will do its same thing in any rpm—good thoughts, thanks for detailing this—we need more of your stuff to help some guys understand what's happening up in the nose."
Transcribed from original scans by AI. Minor OCR errors may remain.
