Author: Mike Riggs
Edition: Model Aviation - 2014/01
Page Numbers: 117,118,119
,
,

RC Aerobatics 2014/01

by Mike Riggs [email protected]

Using a sport radio for Pattern

F3A World Championships: Congratulations to Team USA—Andrew Jesky, Jason Shulman, Brett Wickizer, Joseph Szczur, and team manager Mark Atwood—for its first-place team finish in the 2013 World Championship in South Africa.

  • Joseph Szczur entered his first contest at age 7. He won the Advanced class at the 2012 US Nats, and in 2013, at age 13, Joseph was the first-ever US Junior World Championship competitor.

General setup

I have often been asked, "How expensive of a radio do I need to fly Pattern?" A simple four-channel radio would technically work; however, achieving the precision setup required for Pattern flying would be challenging using Y-harnesses for dual aileron and elevator servos.

After shopping for a six-plus-channel sport radio, I purchased a new Futaba T8J for less than $250. As a believer in flight-testing new radios, I used my Insight as a test bed. The following setup ideas will also work with non-Pattern airplanes.

The T8J lacks flight-condition settings, but it allows you to assign dual rates (D/R) and exponential (EXPO) of different control surfaces to the same two- or three-position switch. For elevator and ailerons, I used a three-position switch for regular, snap, and spin rates and throws.

Before setting D/R and EXPO, all endpoints except throttle were set to maximum (140%). Travel (volume) adjustments were then set as a percentage of the endpoint. This technique provides the highest stick-to-servo resolution the radio is capable of.

I find linkage and setup much easier using two elevator servos compared with other pushrod systems. The ailevator function, with aileron travel set to zero, links the two elevator servos. Servo travel of one elevator half is typically set at 10°–12° using a throw meter. Bamboo skewers are attached to each elevator half; their tips meet on the centerline behind the rudder. The linked servos' subtrim and travel percentages are then adjusted until the skewers move at exactly the same rate and distance. With elevator halves matched throughout their range of motion, D/R and EXPO are fine-tuned with slightly less throw for snap and slightly more for spin.

The T8J manual recommends using aileron differential (AIL-DIFF) for two-aileron servo setups. The flaperon function may also be used, but it complicates fine-tuning the throws. Travel is set for one aileron, then the linked aileron is matched using throw meter(s). Regular deflection is set to 6°–8°—slightly more for snap and slightly less for spin.

The T8J allows any channel's D/R and EXPO assignment to a single switch. Ideally, rudder rate for regular, snap, and spin would be assigned to the same switch as elevator and aileron, with a separate maximum rudder-rate switch assigned for stall turns to override the other switch. However, multiple switch assignments of any channel are impossible on the T8J. Low regular-rate rudder is desirable for smooth, tail-wagging heading corrections, while maximum high rate is required for stall turns. My original setup put rudder rate on a separate three-position switch (snap, spin, and maximum), but this proved too complicated in flight; ultimately a simple high/low rate switch was assigned for rudder.

As a starting point, regular aileron and elevator exponential is set at -20%. To ensure that flipping the rate switch is not telegraphed in flight, exponential for snap and spin is adjusted until little change in surface deflection happens when switching dual rates while holding a small stick input. This prevents the airplane from jumping when a rate switch is flipped while holding (line) corrections.

How much or little rates and throws you set for regular, snap, and spin are determined by the airplane. Fly it, try it, adjust it, and repeat. Just when you think you have it where you like it, some bright idea will pop into your head and you'll change it. Tip: Copy the model from one memory location to another before making changes so restoring the previous setup is easier.

Motor

I have a rule in my workshop: I do not arm any motor with a propeller attached. I often questioned this rule until one time while I was programming a radio I reversed the throttle channel and the motor started to spin at full speed. Be careful!

Without a propeller mounted, arm the ESC and adjust throttle endpoints for off and full throttle. For safety, enable the T8J throttle-cut function and assign it to a switch. When switched on, the throttle stick is disabled. Best practice is to disable the throttle stick whenever the airplane is armed and not in the air. Failsafe should also be enabled and tested without a propeller.

Motor setup is a personal choice. In case of a propeller strike, I prefer a freewheeling motor during takeoff and landing. In the air, a motor idle is preferred over ESC brake. Idle is switched on after "takeoff complete" and switched off after "exiting the box." On other radios, I've set up a switchable idle using a negative value in the IDLE DOWN function. However, the T8J does not allow a negative value in IDLE DOWN.

The throttle trim lever was originally used to turn the idle on and off, but it was a pain. Later, a programmable mix (PMIX) was used, mixing throttle to throttle and assigned to a switch.

Throttle curve is another "fly it, try it, adjust it" matter. Throttle curve is set to a best-guess setting and is subject to change. Making a nice, smooth throttle transition with the T8J’s five-point curve was challenging. I struggled until Don Atwood suggested using EXPO instead of a curve. Mine is set at approximately 40% throttle exponential and I couldn’t be happier.

For dead-straight downlines, nearly every airplane requires a mix of down-elevator slaved to low throttle. Roughly 3% down-elevator is typically mixed into the lower 10% of throttle stick movement, necessitating a curved mix. In addition to four programmable linear mixes, the T8J has two curved, programmable, five-point mixes—perfect for a downline mix.

Radio manufacturers increasingly offer more flexibility in their products, allowing us to explore many ways of achieving a well-trimmed airplane. I had great results using this setup at the last contest I attended. The T8J is only one of many Pattern-capable radios on the market today.

This was my first try at setting up a sport radio for Pattern use. Your radio setup may—and probably will—be different. Do not be afraid to experiment. It’s another part of our wonderful hobby.

FYI: The T8J transmitter uses a 1.3 × 3.45 mm charge plug. See "Sources."

Go Fly!

I’d like to encourage a friend (he knows who he is) to put down this magazine and go fly. Practice is good for the soul!

Flight complete.

Sources

Transcribed from original scans by AI. Minor OCR errors may remain.