Many modelers assemble their model airplanes without ever giving a thought to their control system. They just use whatever they have, but really, this isn’t the best plan of action. To achieve the optimal performance for your aircraft, you need to use the servos best suited for your aircraft.
Servos come in a wide variety of sizes and power ratings, and you need to match them to the size and performance of your airplane. In general, the larger and more powerful your airplane is, the larger and more powerful your servo needs to be. Large airplanes require more strength to move the control surface, so they usually require large servos. Also, in the case of high-performance 3D airplanes and pylon racers, which fly at very high speeds, you need servos with the power to properly control the model. It is not always the size that matters. Some mini servos can produce more torque than a standard-size servo; this is especially true when comparing analog servos to digital servos. When it comes to power, the servo’s torque output and the type of gear train that it has is far more important than its size.
The function you are asking the servo to do is another consideration. Throttle servos and servos that activate switches and valves for retractable landing-gear systems need not be powerful. To save space, you can use mini servos to do the light-duty work. When it comes to aerobatics, travel performance is also important. A powerful servo that can move a big rudder on an aerobatic plane needs to have precise travel and centering functions for the optimal performance of the airplane.
“For complicated projects, like jets, you want the most reliability in torque servos for the flight-control surface.”
Spektrum brand manager
Spektrum brand manager John Diniz notes, “First, it comes down to the size of the aircraft, then the output needs. With a 3D aircraft, you will need speed along with torque. I fly sailplanes, which have very thin wings, so I am looking at the best-performance servo that can fit in a small space. For complicated projects, like jets, you want the most reliability in torque servos for the flight-control surface. But on the accessory, you want the smallest, lightest servo that will work. It is just a hierarchy of what am I putting it in, what do I want it to do, what does it need to do, and how precise does it need to be to get the job done.”
THE DIGITAL ADVANTAGE
Digital servos are quickly becoming the standard offering from most manufacturers.
There is no physical difference between the two types of servos: analog and digital. The servo cases, motors, and gear trains are exactly the same, and both have the same feedback potentiometer. The digital servo’s microprocessor circuit, which interrupts the incoming signal, is what makes the difference. A conventional analog servo compares the receiver’s command to the actual position of its output shaft each time a new pulse command is received. The pulse rate for an analog servo is anywhere from 40 to 50 times per second, depending on the brand of radio and the number of channels being transmitted. The digital servo’s microprocessor monitors the position of the output shaft more frequently, typically 300 times per second (or roughly six times faster than the standard analog servo). And it is this rapid updating that gives digital servos their quicker response times compared to analog servos.
Rapid updating also creates stronger servo-holding power. When a force is applied to a digital servo’s output arm, it sends corrections six times faster, developing maximum torque to resist the servo arm’s load. Analog servos do not develop maximum torque until their output shaft has been displaced several degrees from their desired position. In this case, the advantage of the digital servo is greater centering precision and power.
Hobby People product manager Craig Kaplan has this to say about digital servos: “In most cases, digital servos will outperform analog servos in multiple ways. Digital servos center better, provide better torque throughout the movement of the servo, and also have better holding power over analog servos. Some radio manufacturers insist digital servos be used as analog servos, but they are not compatible with the newer, faster-processing radio systems.” John Diniz agrees, “Digital servos are now the standard.”
There is only one disadvantage to using digital servos: power consumption. Digital servos transmit power to the servomotor more frequently, and therefore, the power consumption is greater. It is important to use larger-capacity receiver battery packs when using digital servos. It is recommended to use one that is at least twice the size of your normal battery capacity. If you use a 1000mAh pack, switch to a 2500mAh pack when using digital servos.
“Digital servos center better, provide better torque throughout the movement of the servo, and also have better holding power over analog servos.”
Hobby People product manager
The new HV (High Voltage) servos can run directly off of lithium-polymer (LiPo) batteries.
The benefits of high-voltage (HV) servos come from being able to run them off an unregulated lithium-polymer (LiPo) battery pack. This eliminates the need for a voltage regulator, which can be a failure point. Also, LiPo packs provide a more consistent voltage level throughout discharge as compared to nickel-cadmium (Ni-Cd) packs, where voltage drops off continually from the start. LiPo packs provide better servo performance throughout the entire flight.
Craig Kaplan states, “High-voltage servos can [provide a] benefit in many ways but may not be required or compatible with some equipment. High-voltage servos are meant to operate directly from a 2S (7.4V) LiPo pack (or even higher in some cases). While the servo can accept the higher voltage, your receiver may not. Additionally, some ‘non-HV’ servos may out perform HV servos, so it’s good to spend some time researching what is best regarding your own radio equipment.” Because of the popularity of HV servos and LiPo packs, most of the major radio manufacturers are now making their new receivers compatible with HV servos.
Programmers like the Hitec HFP-25 make it easy to customize your servos.
Though digital servos have a lot to offer with regard to precision, power, and performance, some digital servos allow you to reprogram their microprocessors. Hitec RCD offers digital-servo programmers that work with their digital servos and give you the ability to change the travel direction, servo speed, neutral point, and endpoints of the servos. On some digital servos, you can also program overload protection and resolution-mode setting.
The advantage of programming the servos themselves is that you will need less equipment inside the aircraft, thereby saving weight. Let’s say that you need dual flaps or dual elevator surfaces for your plane. To operate with the same mechanical advantage, one servo needs to rotate clockwise while the other rotates counterclockwise. By using a servo programmer, each digital servo could be programmed with the proper rotation, identical deadband width, neutral points, and endpoints. These two servos can then be plugged into a Y-harness into a single receiver channel without any computer radio mixing required.
“Programmable servos’ biggest advantage is making two different servos digitally identical. This really comes in handy when you are using multiple servos on the single control surface.”
Hitec RCD sales manager
This same programming advantage can be used when “ganging” digital servos together for a single control surface. Each of the servos can be fine-tuned to have identical movements. This allows the servos to be slaved together for a comment task, without them fighting each other at the neutral-point and endpoint positions.
Shawn Spiker, Hitec RCD sales manager, explains: “The biggest advantage is making two different servos digitally identical. This really comes in handy when you are using multiple servos on the single control surface. If you need to slow a servo down for a certain application, you can program that into the servo itself even if your transmitter does not have that capability.”
S.BUS AND XBUS
The Bus systems are unique in that the servos (or converters) can be programmed to listen to the commands of only one channel. This allows a single wire to carry the signal protocol for all channels, and each programmed servo, when plugged into this cable, will listen only to the channel for which it is programmed. These systems also allow two-way communication, so it is easy to set up telemetry from sensors on the aircraft. In Bus systems, individual servos can also be programmed with different settings in much the same way as programmable digital servos.
Craig Kaplan has this advice for servo installation: “Before installing a servo, it’s wise to use the included grommets and eyelets from the manufacturer. The flanged part of the eyelet faces the platform that the servo is being mounted to in order to prevent the servo from ‘digging in’ to the material. This provides a more solid installation while allowing the rubberized grommets to absorb unwanted vibration. Also, when installing a servo, make sure that its neutral points and subtrims on the radio are all at zero and that the servo is centered before installing the servo arm. If the servo is off slightly, simple inputs with the subtrim function can correct it to the desired position.”
S.Bus servos can be programmed from the transmitter, a computer with a USB plug, or from an easy to use programmer like this one.
With regard to the XBus system, JR Americas brand manager Len Sabato says, “Installing the new JR XBus is like installing your own unique local area network. The Serial Data System allows for independent digital channel assignment and adjustment of up to four servos per channel.”
“A warbird that has two different flap and aileron servos will usually have four different wires coming out, which need to go into the correct channels on the receiver. With the S.Bus system, there is only one wire coming out.”
Hobbico product manager
Dan Landis, Hobbico product manager, puts it this way: “The main benefit to using the Futaba S.Bus system is simplicity to all of your wiring. A warbird that has two different flap and aileron servos will usually have four different wires coming out, which need to go into the correct channels on the receiver. With the S.Bus system, there is only one wire coming out that can be plugged into any open plug along the system. By having one lead, you just plug it in; it is faster and easier and works every time.” He goes on to say, “Another advantage is power distribution: You can separate and have different batteries on different hubs so that all the servos are not drawing off of one flight battery pack. For example, I can set my elevator to draw off of one battery, the rudder off another, and the right and left wing can each have their own battery. The receiver now has its own separate battery.”
As you can see, there are lots of things to consider when it comes to choosing servos. The norm for most RC pilots today is to use digital servos because of their falling prices and all the inherent advantages that they have over the older, slower analog ones. But you need to evaluate your model and its intended purpose and performance. As a start, ask your flying buddies what they recommend, then go to the local hobby shop and see what’s available. Large or small, fast or slow, the servos that you need to use are the ones that are best suited to get the job done to achieve the optimal performance for your aircraft.