RADIO CONTROL SCALE AEROBATICS
Paul Kopp, 1013 S. Sedona Ln., Anaheim CA 92808
Introduction
The last column focused on installation of rear-mounted dual-elevator servos. I'll continue the discussion of airplane setup by looking at battery type and propeller selection.
Propeller discussions are becoming more common, as more choices have become available. Two common battery issues are determining sufficient size and capacity, and the pros and cons of using a five-cell, six-volt pack.
Four- or Five-Cell?
Widespread use of five-cell packs originated several years ago in Pattern (RC Aerobatics). It was quickly adopted by competitive Scale Aerobatics pilots, many of whom were also Pattern pilots.
The extra cell increases the nominal voltage from 4.8 to 6 volts, resulting in faster servo resolution and increased torque. The "cost" of the extra cell is an ounce increase in weight and a slight decrease in usable capacity time, because of increased current draw.
For maneuvers that require a quick entry and exit (such as snap rolls), faster servos provide the pilot with a higher degree of control and a crisp response. The same is true for other non-precision maneuvers that require immediate corrections, such as torque rolls.
However, there are caveats. In some installations, the high surface voltage of a freshly charged pack causes the servo motors to chatter at idle. The chatter ceases after the surface voltage decreases—usually after one or two flights. But as the voltage decreases, the servo characteristics change—speed and torque also decrease.
Small onboard voltage regulators solve both problems. Even with a freshly charged pack, the servos are held to the regulated voltage—roughly six volts. And because regulated voltage is reasonably linear, servo performance is more consistent. The airplane "feels" the same after five flights as it does after one or two, and the negative side effects are eliminated.
There are a number of medium-priced servos whose speed and torque performance using a four-cell pack far exceeds what was available a few years ago using a high-end servo and a five-cell pack. In many installations, there is only a marginal benefit—if any—to using a five-cell pack.
However, many pilots—primarily those flying 25% and larger Scale Aerobatics airplanes—opt for five-cell packs and voltage regulators, and five-cell packs remain the rule in competition. Many Giant Scale and competition pilots want the extra punch of a fifth cell—especially for airplanes with oversized control surfaces and/or long servo extensions.
For sport flying, a five-cell pack may result in undesirable flying characteristics. Pilots who don't have exponentials on their radios may find that the added speed of a five-cell pack makes the airplane jittery and oversensitive. Conversely, pilots who like to hover and perform other low-speed 3-D maneuvers prefer the crisp response of the five-cell pack.
Duralites and Alternatives
Duralites differ significantly from Ni-Cd's in their weight and capacity; Duralites are much lighter, and have significantly higher capacity. Moreover, the cell discharge is relatively flat, negating the need for a regulator. Thus, Duralites provide the benefit of high voltage and the added protection of high capacity without the weight penalty of an additional cell or a second pack.
Duralites are not as forgiving as Ni-Cds, so one must pay close attention when using them. Duralites cannot be charged or cycled on regular chargers, so using them requires investing in the batteries and dedicated charging equipment. However, Duralites are a good alternative to Ni-Cds—especially in installations where weight is an issue.
Choosing a Propeller
Choosing the right propeller is a concern when trying to maximize airplane performance. However, proper selection is usually a matter of trial-and-error.
Are there real differences in props? Yes. Are carbon-fiber props worth three times the cost of good fiberglass-filled props? Are fiberglass-filled props worth twice the price of lower-priced wood props? I'll try to answer both of those questions.
A less-expensive wood or fiberglass prop lacks blade rigidity—especially at the tips. The result is that even though the rpm compares (sometimes favorably) to a more-expensive prop, the actual thrust is less and the airplane's vertical performance is compromised. A more-rigid quality prop generally produces higher thrust at the same, or even less rpm than a less-expensive prop. So the proof is usually not in the numbers, but in the flying.
Most propellers fall into one of three price ranges:
- Lower-priced wood or fiberglass-filled
- Medium-priced wood or fiberglass-filled
- Higher-priced carbon-fiber
There is a definite advantage to paying a premium for the more-rigid medium-priced props, such as the Menz wood props from Germany or the popular APCs. The next step to carbon is more subjective. Many pilots who fly 1.8 and larger engines prefer high-quality wood props to carbon fiber, but just as many swear by carbon fiber—usually Mezjlik or Bolly brands.
Some of the newer carbon props, such as the lightweight hollow-core Mezjlik, are closer in price to the better wood props, making it less costly to compare. And wood and carbon are widely used in competition. To a large extent, prop selection depends on flying style and preferred "feel." If one is happy with the performance of his or her prop, there is no reason to upgrade.
However, an airplane with marginal vertical performance may come alive with a high-quality prop. An airplane that has good vertical performance may have top-of-class vertical performance with a better prop. And as expensive as some props are, using even a good one is easier, cheaper, and probably lighter than swapping engines.
Select the appropriate prop diameter and pitch—not just the right construction. Diameter and pitch are critical in achieving maximum performance. The YS engine factory recommendations are low-diameter, high-pitch props, reflecting the Pattern development and intended application of the engines. For Scale Aerobatics applications, pilots favor higher-diameter, lower-pitch props. The result is slower horizontal speed and better vertical penetration.
Whereas a 15 x 10 might be recommended for a YS 1.2 in a Pattern design, a 16 x 8 is a popular prop for the same engine in a Scale Aerobatics airplane.
This Month's Building Tip
When building kits with "slick" tail surfaces, glue a piece of triangle stock to the leading edge of the control surface instead of beveling the square stock. This saves a great deal of sanding!
If you write me, please include a self-addressed, stamped envelope. If you have E-mail, please include your address. As long as the volume is manageable, I'll try to keep up with the questions. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
