How To Get Into Hobby RC: Radios and Motors

By Terry Dunn

This is the second in a series of guides that will walk you through the wonderful hobby of radio-controlled (RC) vehicles. Specifically, this installment will deal with the radio equipment that controls your model and the types of motors that could power it.

This is the second in a series of guides that will walk you through the wonderful hobby of radio-controlled (RC) vehicles. We started off last month with an overview of the different types of RC vehicles, but still have some basics to cover before we get into the actual cars, boats, and airplanes. Specifically, this installment will deal with the radio equipment that controls your model and the types of motors that could power it. It’s crucial to have a basic understanding of both of these subjects before making your first purchase. You don’t want to blindly purchase something now that limits you down the road.

Putting the “R” in RC

No matter what type of RC vehicle you plan to use, you will need a radio system to operate it. The essence of RC is that you send radio signals from a hand-held transmitter to a matching receiver that is onboard the vehicle. The receiver translates those signals into commands. The commands are passed on to hardwired components that execute the orders either mechanically or electronically. Sounds simple enough, right? Even the most complex RC set-ups are just an extrapolation of this basic concept.

Each separate function that you want to perform on a vehicle requires a discrete channel within the radio signal.

Each separate function that you want to perform on a vehicle requires a discrete channel within the radio signal. Cars and boats typically operate with two channels; one each for throttle and steering duties. Most airplanes use four channels to control throttle, roll, pitch, and yaw. Discussing the radio needs of helicopters, multi-rotors, and robots will probably just cloud the issue at this point, so I’ll hold off on that for now.

For many years, RC equipment has operated in specific frequency bands allocated by the FCC just for that purpose. Surface vehicles (cars and boats) use the 27MHz and 75MHz bands while aircraft use 72MHz. Each radio set operates on a specific frequency within that band. For instance, a car radio may be tuned to 27.145MHz, while a helicopter radio operates at 72.390MHz. That all works fine as long as there is only one transmitter broadcasting on any specific frequency in the general vicinity (a few miles radius). If someone else turned on a 72.390MHz transmitter while that pilot was flying his helicopter, there would likely be a very sudden and very expensive crash!

RC race tracks and flying fields have developed various methods of frequency management throughout the years. Yet, the honor system is still the cornerstone of even the most draconian management policies. The good news is that most modern radios (surface and air) operate in the 2.4GHz band. The prime benefit of this change is that the radio equipment essentially has automated frequency control. When a 2.4GHz radio is turned on, it searches the airwaves for an open frequency (or two) and then communicates with its partner receiver on that frequency until the system is turned off. For the most part, you no longer have to worry about radio interference from other users. In fact, I once witnessed 99 RC airplanes flying in the same patch of sky using a combination of 72MHz, 2.4GHz and 50MHz (ham band…also legal, but rare for RC) radios. That set a Guinness World Record which still stands.

The older frequency bands are still valid for use and are standard equipment in some lower priced all-in-one packages. Thanks to the massive popularity of 2.4GHz equipment, the 27MHz, 72MHz, and 75MHz bands have never been less crowded! Of course, the same sacred frequency control rules still apply when using those bands at race tracks and flying fields. The only problem is knowing what frequencies are being used two blocks away when you are driving in front of your house or flying at the local schoolyard. For that reason alone, I strongly suggest 2.4GHz equipment. That being said, there’s no reason to pass up a good deal on the older stuff…it works.

Transmitters

Pistol grip radios such as this Hitec Aggressor are the preferred form-factor for most RC car and boat modelers. The trigger controls throttle functions, while the wheel is used for steering. (photo courtesy Hitec RCD)

Just about every modern car and boat-oriented transmitter is the pistol grip style. As the name implies, you hold the transmitter like a pistol (typically with your non-dominant hand) and the trigger operates the throttle and brake. You squeeze the trigger to go and push the trigger forward to stop. To steer, there is a wheel on the side that operates just like the steering wheel on a car. The only difference is that the wheel isn’t facing you as you hold the transmitter. Like most of the world’s consumer products, the default configuration for pistol grip transmitters favors right-handed drivers, i.e. you steer with your right hand. Many transmitters, however, can be reconfigured to accommodate lefties.

Most people find pistol grip transmitters comfortable and intuitive to use, but they are not the only option. You can still buy boxy two-stick transmitters for cars and boats. They look much like airplane transmitters, but each stick moves in only one axis. The left (throttle) stick moves up and down while the right (steering) stick moves left and right. It may seem clunky, but it works. World-famous RC racer (yes, there is such a thing) Masami Hirosaka raced with two-stick transmitters long after pistol grip controllers became available. As a parlor trick, he would even drive using his feet!

With very few exceptions, airplane transmitters are of the two-stick variety. In this case, each stick can move in two axes. In the United States (it is different overseas) the left stick moves up and down to control throttle, and it moves left and right to control yaw. The right stick commands pitch with up and down movements, while left and right inputs command roll. You’ll notice that many airplane transmitters have all sorts of switches and knobs scattered about…maybe even an LCD screen. To a newcomer, all of that stuff may seem as complex as a 747 cockpit. It isn’t so bad when you break it down piece by piece. I’ll cover what those switches are for in a future article. For now, the two control sticks are all you really need to fly.

Many modern transmitters are referred to as “computer radios”. What that means is that there is some sort of interface that lets you make changes or adjustments to the features of the transmitter. The simplest computer radios use stick movements for inputs and provide feedback through audible beeps. More advanced radios have menu-driven, full color, touch screen interfaces. The prime advantage of most computer radios is that they have built-in memory to record the individual control settings for multiple vehicles. This allows you to use one transmitter for several different models. Gone are the days of having to buy a complete radio system for each RC vehicle.

These varied aircraft radios offer a wide range of features, capabilities and price points. However, they all share the basic two-stick layout that is the standard for aircraft transmitters.

Receivers

The outward appearance of RC receivers has changed little over the past two decades. Sure, like most other electronics, they’ve become relatively smaller, but not that much. Perhaps the biggest change is with the length of the requisite antennas. Since antenna length is a function of the wavelength of the radio signal, the 27MHz, 72MHz and 75MHz systems have substantial antennas that must be managed and routed properly. Newer 2.4GHz receivers typically have one or more antennae that are just an inch or two long.

Photo credit: Flickr user keoshi via Creative Commons.

The receiver must be equipped to handle the number of channels that you require, but it doesn’t necessarily have to match the number of channels output by the transmitter. For instance, you could fly an airplane using a 7-channel transmitter, but with a 4-channel receiver onboard the plane. The remaining three channels would just go unused.

2.4GHz receivers are typically only compatible with the same brand of transmitter.

2.4GHz receivers are typically only compatible with the same brand of transmitter. If you have a Hitec transmitter, for example, you will need a Hitec receiver. There are some aftermarket receivers that claim compatibility with various brands of transmitters. These discount clone receivers are obviously not endorsed by any of the radio manufacturers and results vary, so use them at your own risk.

It is important to isolate receivers from vibration. This is especially true for 27MHz, 72MHz and 75MHz units, which require fragile crystals to set the radio frequency. Models powered by internal combustion engines (i.e. lots of vibration) usually require a cushy foam wrap around the receiver. For electric powered vehicles, it is sufficient to simply mount the receiver using Velcro or double sided tape. Boats and some cars will house the receiver in a waterproof case--wet receivers do not work well.

Servos

The most common actuator used on RC vehicles is a servo. It translates your inputs on the transmitter into rotational motion of the servo’s output shaft. That rotation can then be used to move the rudder on a boat, the steering assembly of a car, or the control surfaces of an airplane or helicopter. Servos are available in countless sizes and power levels to meet just about any requirement you can imagine.

Servo performance is typically a tradeoff between torque and speed. A car racer will likely want a fast servo so that their car responds quickly to their inputs. On the other hand, a monster truck driver will want a servo with plenty of torque to schlep its big, heavy tires to and fro. It pays to research and choose the right servo for each application.

These differently-sized servos are intended to perform varied duties. Choosing the correct servo for a given application is often key to success.

An electric car or boat will typically have only one servo (for steering). Throttle control is accomplished through a device called an Electronic Speed Control (ESC). If the model is powered by a fuel-burning engine, a second servo is used to control engine speed via the carburetor. Most airplanes have four to six servos onboard, yet some complex models may have twenty or more (there may be multiple servos operated by a single channel). As with surface models, electric airplanes utilize an ESC while fuel powered versions have a servo attached to the carburetor.

Most electric vehicles utilize a Battery Eliminator Circuit (BEC) to allow the battery which powers the vehicle’s propulsion motor to also power the radio gear.

The driving force behind servos is a small electric motor with lots of gearing. Like the receiver and other onboard radio gear, servos require a power source to provide the appropriate operating voltage (usually 5-6 volts). The simplest method for this is a small battery dedicated to the radio equipment. Most electric vehicles, however, utilize a Battery Eliminator Circuit (BEC) to allow the battery which powers the vehicle’s propulsion motor (usually much more than 6 volts) to also power the radio gear. The BEC may be an integral part of the ESC or added as a discrete component.

Integrating servos into a model can be somewhat of an art form. You must consider the geometry of the control linkages used, the mechanical advantage of the servo, how the servo’s placement may affect the overall weight distribution, and possible aesthetic concern. Most servos include built-in mounting lugs and rubber grommets that provide vibration isolation when everything is screwed into place. Depending on the application, it may also be acceptable to use glue, double-sided tape, or just friction to keep the servo in its proper place.

Radio Telemetry

A few of you have asked about radio systems equipped with telemetry features. Traditional radio systems only transmit signals from the controller to the receiver. Telemetry-capable systems, however, send real-time data from the model back to the transmitter. A variety of sensors can be utilized to capture all sorts of data. Maybe you want to know the voltage of your onboard battery(s), how many RPMs the motor is turning, how high/fast/far away the model is. The selected data may be relayed to the modeler via a screen on the transmitter, a voice module, or perhaps saved to memory for later analysis.

RC models were around long before telemetry was an option. So, I think it goes without saying that telemetry is not a requirement at all. Think of it as a tool for advanced hobbyists to intelligently tweak their models, or a “gee-whiz” boon for those who just like to be slathered with data (you know who you are). There are definitely practical applications for telemetry (battery voltage is a good example). I think that the average weekend enthusiast, however, will find limited usage of telemetry functions. With all of that said, I have a few telemetry-capable radio systems that I will showcase in a future article (the same one that will explain what all of those switches and knobs on the transmitter are for). You can decide if telemetry is a must-have for you.

Propulsion Choices

This O.S. 55 AX engine displaces only .55 cubic inches. However, it creates more than 1.5 horsepower burning specialized nitromethane-enriched fuel.

For decades, RC models were primarily powered by two-stroke engines burning custom-mixed fuel with nitromethane (aka “nitro”). Rather than spark plugs, these engines use a passive heating element called a glow plug; hence the common moniker “glow engine”. These motors are lightweight and very powerful, but the exhaust carries a greasy residue and they can be very loud if not muffled properly. Indeed, one of the main challenges to modelers over the years has been the loss of flying sites and race tracks due to noise complaints.

As electric power came of age during the turn of the century, there was a very real and sometimes heated rivalry between many glow flyers and those who chose electric power. Both electric and glow power have their advantages and drawbacks. It really boils down to personal preference. Like most silly feuds, this one seems to have fizzled out naturally, with many flyers now having both glow and electric power plants among their fleet of models.

Electric motors offer the advantage of quiet power for RC vehicles. When properly set up, they yield nothing to their fuel-burning cousins.

It is interesting to note that while electric power was blossoming at flying fields, race tracks saw resurgence in glow-powered cars and trucks. Nitro fuel is hard to beat for raw power, but modern electric power plants hold their own. These days, there seems to be a pretty even mix of glow and electric racers. If your ambition is limited to bashing around on the street, your neighbors will appreciate you choosing electric power.

While glow and electric are currently the most common RC power systems, they are not the only choices. Gasoline engines have enjoyed a surge in popularity in large models, where the cost of the necessary glow fuel or LiPo batteries would be prohibitive. Some of these gas engines are adapted from weed eaters and other yard tools. The most powerful versions, however, are designed and built specifically for RC use.

Turbine (jet)-powered aircraft have a huge wow-factor that attracts many newcomers to the hobby. These models look, sound and smell just like full-size jets. While not suited for rookie pilots, these airplanes and helicopters often fuel their dreams and give them a goal to reach for.

Turbine jet engines are now surprisingly popular among RC flyers. These expensive little jewels operate just like the larger kerosene-burning engines on airliners and fighters. The big advantage of turbines is that they provide a lot of power from a lightweight engine. There is nothing quite like the sight, sound, and smell of a speedy turbine-powered model making a fast pass over the runway. Turbines are obviously not for beginners. In fact, you need a special waiver to operate them at most clubs. They do, however, provide a lofty goal for speed-freak newbies to aspire to.

While many turbine powered models are capable of more, safety regulations in the US limit their top operating speed to 200mph. In countries where no such regulations exist, turbine models have reached 440mph--that's insane! As impressive as those speeds are, it is interesting to note that the fastest RC planes are not powered by turbine engines. In fact, they do not have an engine at all. Very specialized gliders use wind currents on mountain ridges to reach upwards of 500mph! Look up "dynamic soaring" and prepare to be blown away.

In Conclusion

If my previous “Getting into RC” article was a 10,000-foot-level overview of the hobby, then this one is more like 5,000 feet. There are still quite a few basics to cover that would probably justify another high-level article or two. However, my primary interest is holding your interest. So, I’ll delve into the other necessary topics while also introducing RC hardware at the ground level. Next month, I will look at a few RC car and trucks, while also explaining the differences between brushed and brushless motors, as well as the battery options that are available.

Photos courtesy Terry Dunn unless indicated.