Welcome Back! We Missed You! Part One

Welcome Back – We Missed You!

Part One

Coming back to Model Aviation is like riding a bike.

Except, the bike has changed!

By: Frank Granelli

We are glad you have decided to return to the exciting world of Model Aviation. We have missed you and have kept a lamp in the window, hoping you would come back someday. I know how you feel right now; eager to get started again but concerned about the many changes that happened while you were with us only in spirit.

A lot has changed since your last flight. The photo above is but a small example of those changes. The green/yellow 50-inch biplane on the left is a Lou Andrews designed Aeromaster. I built it in 1980. Notice that it is glow powered with an HP 61 engine, one of the first Schnuerle ported .61 engines. The wings are held on using rubber bands, there is no rigging and the airplane weighed 7.25 pounds. It took 4 months to build. Despite its lack of sophistication, this airplane won first place in the 1980 WRAM Show Biplane category! Including the kit, covering, paint and hardware, its airframe cost about $165 1980 dollars.

The 50-inch Jenny on the right is an Almost–Ready-To-Fly ARF from Green RC. Its review is in the January issue of Model Aviation. The airplane required only a few hours to assemble, is E-powered with a strong outrunner, will out climb the Aeromaster and has full rigging. The wings arrive attached with struts and all rigging in place. The rudder and elevators are controlled by pull-pull wires as on the full-size airplane. It weighs just 2.38 pounds. For me, it is near a scale masterpiece. Yet its airframe cost is just $220.

These and other changes make this a most difficult time to return but also the most rewarding. As I said above, I had to stop flying models, except for one or two flights annually, for seven years; 1986 to 1993. During those awful years, my job responsibilities grew exponentially as did my income. But always, something was missing.

Maybe it was the great people in modeling, or was it the satisfaction of building and flying your own creation? Was it just the missing relaxation (nervous exhaustion?) of the sport or the sense of accomplishment felt when meeting self-imposed challenges such as the first flight on a new airplane or flying that perfect loop? For whatever reason, those few annual visits to the flying field, with my single remaining airworthy airplane (a SIG Kougar given me by a sympathetic friend), were always my annual highlight.

Photo 1

Finally, my great wife Ann could stand it no longer. She strongly suggested that I start flying again, quickly. So, I started back flying that old Kougar in 1994. But those years that I had missed were model aviation’s “transition period” and there had been a lot of changes.

ARF’s had started and had become ever more common during those years. Computer transmitters began changing the way airplanes were designed and flown. Servos became powerful and airplanes got a LOT bigger. Engines followed the march to larger sizes while increasing reliability made the bigger, more expensive airplanes practical. Pattern competition airplanes went from 60-powered ballistic missiles to larger, slower flying, true aerobatic precision instruments.

There was a lot of catching up to do and doing so required at least a year, maybe a little longer. Today, changes are even more numerous and the learning curve is steeper. Even three years of “MIA” in the modeling world today means that the returning pilot has basically missed the E-power revolution and the practical revival of neighborhood Park Pilot flying.

Just less than half of Sport Aviator readers are pilots now returning to the air. Many have been missing for five or ten years. Our record so far (based upon reader letters), is one pilot coming back to us after having been away for 47 years! The average time away, at a guess from the letters received, is about 12 years.

A lot has changed in the RC world in 12 years. There have been technical changes of course. But also whole new styles of flying, new competition classes, new trimming requirements and a sea-change in attitude have also become dominant. We’ll try to delve into each change as much as possible. Doing so with a few of the less objective changes, such as attitude, must contain some opinions. I will identify those instances as such and try to present all sides of a given change as much as possible.

Technical Equipment Changes: Radio Systems

Transmitters:

Photo 2

The most obvious change in the last 12 years is in the equipment. Engines, radios and airframes have undergone gigantic developments. Let’s look at radio systems first: Ignoring the presence of a lot more buttons on the newer JR 12X transmitter than on the 1974 Kraft KP7Z unit, check out the antenna differences. The Kraft transmitter has a retractable metal antenna about 3-4-ft. long. The 12X’s antenna is a non-retractable, composite unit just a few inches long. This is because the 12X transmits on 2.4 GHz while the older Kraft sends out its orders on the 72 MHz with which we older pilots grew up. But the real difference is in the way the units work.

The 72 MHz transmitter sends out information on one of 50 different frequencies. The frequencies are closely spaced but discrete. Some are on the FM band and some transmitters use the AM band. Some (not this Kraft unit) are the newer Pulse Code Modulation (PCM) digital conversions of an analog signal that provides clearer signals and best utilizes the new digital servos.

But the old problems of interference and getting an available frequency remain. PCM has the added disadvantage in that the conversion from analog to digital and then back again in the receiver requires extra time which results is increased latency periods. The latency period is the time between when the transmitter “stick” is moved and the servo starts to respond. However, PCM gives a very tight transmitter-receiver bond. This bond can be interfered with but only with some difficulty.

Photo 3

Enter the new 2.4 GHz systems. They operate on numerous channels is the 2.4 GHz band, probably around 80 channels. But the pilot has no say in what channels are being used. Futaba, Airtronics and HiTEC transmitters “frequency hop” through all the available frequencies during a flight. Each frequency is “used” for only about 0.6 seconds. Federal regulations suggest that any 2.4 GHz frequency currently in use be left out of the hopping rotation, but that does not always happen. Even if two transmitters happen upon the same frequency, the “conflicting” time unit is so small that interference is unlikely. Also, each transmitter sends an eight or ten digit code to the receiver and the receiver will listen only to signals employing that code. Interference is virtually impossible.

Photo 4

Spektrum and JR transmitters do not hop frequencies but grab two unused frequencies and transmit on them. These transmitters do avoid “channels” in use and will only lock onto open frequencies. These transmitters use a 10-digit identification code. They both use a technology called Model Match® which allows the receiver to obey its transmitter’s commands if, and only if, the pilot has selected the correct model from the transmitter’s memory. This feature has saved me twice in the past few years.

For complete information about 2.4 GHz, read the Sport Aviator articles “2.4 GHz for the Common Pilot” “2.4 GHz Radios” and “Spread Spektrum – Are you ready for full range” in the Flight Tech Section. For now, know that 2.4 GHz eliminates interference, shortens latency periods, eliminates frequency congestion and is fast becoming the norm in RC. I am having difficulty even giving away relatively new and operating 72 MHz systems.

Photo 5

Computer transmitters were around 12 years ago but not as sophisticated and capable as they now are. They also cost a bundle in 1997. Almost all transmitters sold today are computer driven to some point. Even budget-priced 4-channel units are available and very inexpensive. 2.4 GHz transmitters like the Spektrum DX5i and DX6i, Futaba 6 and 7 FASST units and Airtronics 8000 are great 2.4 GHz units that cost less than $200. A little known fact is that transmitting on 2.4 GHz requires much less transmitter power than does 72 MHz. Therefore, transmitter batteries can be only 6-volt units (like the old Royals from the early ‘70s) and use dry-cell batteries. My DX6i transmitter has been using the same four dry-cell batteries since last year and, after 100+ flights, still produces 5.9 volts under load.

Basic computer transmitters allow choosing servo directions, servo travel, a thing called “sub-trim” which allows servo center point adjustment, dual rates on elevator and ailerons that limit how far these servos move at full deflection, exponential which permits smoother flights by limiting travel around the neutral point while still allowing full control surface deflection, V-tail and dual aileron servo mixing. Some, like the Airtronics 8000, even allow for mixing control surfaces such as the rudder with aileron, flaps with elevator or elevator with rudder to fly those straight knife-edges.

If you are not a computer “genius” as I certainly am not, do not be concerned. The menus take the pilot step by simple step through every setup procedure. Today’s basic computer transmitters are simple to operate and represent about 90% of the new transmitter market. They are a must for any returning pilot. Learning to program even the most complex computer transmitter, such as the Futaba 14Z or the JR 12X, is about the simplest computer task possible. It is certainly easier that working Excel!

Recommendation (my opinion)? If you are going to buy a new radio system, choose a basic 6-8 channel, 2.4 GHz system that includes sport digital servos. The street prices for these systems are less than $250 and they are designed for those new to RC. They are easy to set up and program. The extra channels allow for growth when it is time for retractable gear and flaps. The sixth channel also allows for using a separate servo on each aileron (more later).

Futaba FASST Systems and Airtronics are frequency hopping systems. JR and Spektrum transmit on two available 2.4 GHz frequencies for the entire flight. Both systems have proven to perform equally well in the field. Which one is pilot’s choice. Check Sport Aviator’s Radio Stack Section for individual radio system reviews. The Airtronics 8000 review even features videos of some of the programming operations.

Photo 6

Photo 7

If you already have a more complex computer transmitter on 72 MHz such as a Futaba 9Z or 12 Z, JR 8103 or 10X, you may want to keep the transmitter and just convert it to 2.4 GHz as shown in the photo above. Any transmitter that uses a module, not just a crystal, can be converted.

Converting the transmitter allows keeping your complex programming, trim and setup for each particular model while still enjoying the benefits of 2.4 GHz. If you had been using such an advanced transmitter, that usually means you had been flying more complex, read more expensive, aircraft. It might be a good idea to extend 2.4 GHz’s interference protection to these more expensive aircraft.

Conversion sets include the transmitter module and one receiver. If the transmitter operates more than one aircraft, a separate 2.4 GHz receiver is required for each.

Servos:

Photo 8

Looking at just the specifications, sport servos don’t appear to have changed much over the last 12 years. But they have. True, 30 years ago the strongest sport servos produced only about 26 oz. /in. of torque while taking about 0.4 seconds to move 60 degrees (Kraft servo on left). But even by 12 years ago, sport servos routinely produced 45-65 oz. /in. of torque while speeding up to .22 seconds/60 degrees (JR 505). Today’s standard sport servo specifications are about the same, maybe a little faster at .20 seconds (Airtronics 94102) but no major differences.

The big servo advance in the past 12 years has been the introduction of the digital sport servo. Priced in the $30 range, these sport servos are extremely precise and more durable. The durability comes from the ball bearings, instead of nylon or metal bushings, that support the output shaft (where much of the stress is). The servos also use less current to move because of the ball bearings and they last longer as well.

But the big news is the increased, really increased, resolution and precision of digital sport servos (S 3152). 12-year old sport servos have a resolution, how many separate “steps” in 60-degrees of movement that the servo could sense or stop at, of 340 (guesstimate) . No matter how precise are the transmitter and receivers, the servo could not divide its movement into more than 340 steps and that means less precision. About 10 years ago, servo precision increased to 512. 512 at least matched the common sport transmitter’s and receiver’s resolution.

Today’s common digital sport transmitters and receivers have resolutions of 1024. Some, like the Spektrum DX7 Special Edition MD-2 have 2048 resolution but at a higher price ($320). The greater the resolution, the smoother and more precise is the flying experience. Especially important to all aircraft is the increased centering resolution.

For best results, a servo must center as exactly as possible. This is mostly a function of the system’s resolution. But the servo must be at least capable of that many discrete steps for best results. To take full advantage of a system’s increased resolution for the best centering, a digital servo is required.

A digital servo is capable of resolutions of at least 2048 and even greater if the system can produce that much exactness. They can “stop” anywhere (almost). Digital servos have precise centering; every time. This precision operation, along with its increased speed, provides the tightest pilot connection to the aircraft.

(Opinion) Considering how cost effective digital servos are for their increased performance and durability, there seems little reason to not use them. If I must scrimp, such as on 10-servo airplanes like A-26 Invader or big P-47D Thunderbolt, the older, non-digital servos go on the throttles and retractable landing gear switch (A-26). The flight surfaces, especially the flaps, all enjoy digital precision.

Receiver Batteries:

Photo 9

20 years ago, Nickel Cadmium (Ni-Cd) batteries were the norm for both the transmitter and on-board batteries. They were reliable, easily re-charged, light weight and had capacity ranges, (how much work they could do) in the 450 to 900 milli-Amp hour (mAh) range. Most sport airplanes of that time had just four servos and used less than 100 mA per flight. Today’s sport airplanes use 5 or more servos and draw about 150 mA per flight.

Computer transmitters, especially those on 72 MHz, use more power than the older units. The older transmitters used batteries around 500 mAh and would transmit for at least 90 minutes, usually more.

Radio systems today use on-board Ni-Cd batteries in the 1100 mAh range and computer sport transmitter batteries have capacities around 850 mAh. If you plan to use your older radio equipment, you must replace both batteries for maximum safety. But before you do, remember that Ni-Cd batteries have one major RC disadvantage.

Photo 10

When Ni-Cd batteries were all we had, they were OK. However, their one flaw, as far as airborne RC is concerned, was a real killer. Look at photo 10. This is an actual discharge curve, made using the West MountainCB II battery analyzer, of a one-year old 800 mAh Ni-Cd battery pack that is in good shape. After the initial high surface charge dissipates, the voltage remains fairly flat for most of the curve. This is perfect for RC use as the receiver stays equally energized and the servos have basically the same response rates for most of the flights.

However, when time is up the Ni-Cd’s voltage literally falls off the proverbial cliff. In just seconds, the voltage drops from a useable, 4.6 V, to a “pick up the pieces” 3.6 V. In short, when Ni-Cd batteries reach the end of their capacity, voltage drops so fast it is nearly impossible to save the airplane. There is little warning given to the pilot.

Ni-MH discharge curve

Photo 11 is the discharge curve from a 6-volt, 1600 mAh Nickel Metal Hydride (Ni-MH) battery. When the Ni-MH battery reaches the end of its useful capacity, the voltage drops much more slowly. Instead of quitting entirely, the servos slow down. This warns the pilot it is time to land. Since there is still enough voltage remaining, even with a 4-cell, 4.8-volt Ni-MH battery pack, the receiver continues to work and the pilot has sufficient time to safely land the airplane.

Ni-MH batteries have more capacity for the same size cell than do Ni-Cd batteries. Notice that the smaller, 5-cell 6-volt Ni-MH pack on the bottom still has 720 mAh capacity despite how much smaller it is than the 4-cell, 4.8-volt, 700 mAh Ni-Cd to its upper right. Since the Ni-MH cells can be smaller than the Ni-Cd cells for a given capacity, the Ni-MH battery pack is also 1.8 oz. lighter. Ni-MH battery packs can use the same chargers used for Ni-Cd batteries including those included with most radio systems.

(Opinion) With more capacity for a given weight and the ability to save an airplane, my suggestion is to use Ni-MH batteries for all airborne packs. Since you have to buy new batteries anyway, and the costs are the same, use Ni-MH airborne battery packs.

I have real world experience about this saving the airplane thing. I improperly charged the on-board Ni-MH battery pack in my expensive Prophecy Pattern airplane. On the third flight of the day, the aileron response suddenly slowed dramatically. It took me 2-3 minutes to realize what was happening, and then 3 more to land the airplane. I still have that airplane but I would not if those batteries had been Ni-Cd’s.

For more battery information, read the Sport Aviator articles “Those Things We Call Batteries – Part 1”, “Part 2” and “Part 3” in the Flight-Tech Section.

Here is a special note on the voltage needs of 2.4 GHz receivers. Most such receivers need at least 3.8 volts to work properly. Older receivers would work down to about 3.4 volts, but with reduced range (so “they” tell me but I have never experienced or measured that). If all four servos (5 with dual aileron servos) are moved at once, as would happen when performing a snap roll, it is possible for a 4-cell battery, on the last flight of the day, to momentarily drop below 3.8 volts. The receiver could momentarily stop working and could require a short time (as much as 1 second) to reboot. That could be the longest second in your life if the airplane is headed downwards when it happens.

While this is not as critical as it once was, when reboot times reached 3 seconds, this situation should be avoided. For 2.4 GHz radios, try using the common 5-cell Ni-MH battery packs. They will never reach below 3.8 volts in normal flight. If they do, 3.8 volts will probably not be your major concern as something has just shorted out!

To sum up this radio section, the last 12 years has brought the following suggested changes (opinions again):

ª For new radio systems, purchase a 2.4 GHz, basic computer transmitter in the $150 to $230 range.

ª If you already own an advanced, 72 MHz transmitter with several airplanes in memory, convert it to 2.4 GHz when economically feasible. Protect your expensive investments.

ª If you have a good, narrow band 72 MHz radio system and want to use it, replace the batteries even if they have been unused for only 2 years.

ª Use digital sport servos for airplanes up to 9 pounds. Larger, more sophisticated airplanes use premium servos that cost around $80 to $120 each. Such airplanes are outside the scope of this introductory article.

ª Use Ni-MH airborne battery packs with the largest capacity the airplane will comfortably carry. This is usually 1650 mAh. Use 5-cell packs with 2.4 GHz systems.