In today's hobby market place there must be 100's if not 1,000's of different servos available. We shall attempt to simplify the various designs and make some suggestions for different applications.

We shall also take a look at how to mount them and hook them up to the control surfaces.

The photo at left shows the JR Radio 537 Standard Servo. This RC servo has been around a long time and we can use it as a "standard" to compare other servos. It's size category is "standard" (0.73" widex1.52" longx1.32" high) and it is an analog type, with one ball bearing, on the output shaft. Analog was the original design and is rapidly being replaced by the later technology, known as Digital.

The components in both types are identical, the difference is in how the signal from the receiver, is processed and sent to the servo motor. In the analog servo, pulses are sent to the motor at a rate of 50 per second. The digital rate is 300 per second.
The digital types have a reduced deadband, a faster response and quicker and smoother acceleration and better holding power. The downside is that they do use more power and do tend to cost more.

Another point to consider is the type and number of bearings used on the output shaft. Here you will find plain bearings or ball bearings. The ball bearings give lower friction and less stick/slip and a much longer life. The higher quality (and cost!) servos have two ball bearings, and others may have just one.

The type of servo motor will also vary. Coreless motors are faster to respond because the rotating mass of the motor is less giving us faster response time and more available torque. The latest technology is that of brushless servo motors. Once again. response time, speed and smoothness are improved.

OK-let us take a look at the performance specifications for the JR 537 Analog servo. It has a speed of 0.25 seconds for a movement of 60 degrees at a voltage of 4.8V. The torque generated is 40 oz/in. This torque figure means that a force of 40 oz. will be generated at a 1" radius or 80oz at a 1/2" radius.

This servo, with those specs, is good to use in all RC aircraft up to say a 60 size model. If you are flying small park flyers, then this is not the servo to use- for this there are micro or small sizes available.

The Mounting of RC Servos

JR servo mountings
Use the mounting units that come with the servo. These are the rubber-like "grommets" that will isolate the servo from vibration. If brass ferrules are included also, make sure these are used as well. The flange or head should be place on the bottom, so as it contacts the mounting surface, in the airframe. When the screws are tightened down, the idea is to limit the compression of the grommet. If too tight, then the RC servo will not be isolated, or if too loose it will move about on it's mounting.

Drill holes of 1/32" diameter, before inserting the mounting screws. I always re-enforce the mounting plate in the airframe, as it is usually just 3mm thick lite ply. I cut 3mm thick rectangles of lite ply, about 3/8"x3/4" and glue them in the spot where the holding screws will go. This gives a longer thickness for the screws to "bite".

The Servo arm
Servo arm Dynamite.
The RC servo output arm is the next point to consider, and if this is a high stress installation (say a model, using a glow engine of more than a 60 size), it may make sense to substitute a stronger plastic one or even an aluminum type. One thing to make absolutely sure of, is that it is designed to operate with the servo you are using, from a manufacturer's view point. For instance, the splined output shaft of a Hitec RC servo looks identical to a JR, it will fit but it will also fail! Ask me how I know!

Please take care of the screw that holds the arm to the output shaft and make sure to replace it. in the same make of servo that it came from.

One point to consider is the length of the arm being used. If the length is doubled, the force available is halved and the distance moved linearly is also doubled!

RC Servo arms Dubro
One manufacturer of replacement arms, is Dubro. They make stronger, injection molded arms for a wide range of servo manufacturers. If you click on the photo at the right, you will be taken to the RC Planet website, on the page devoted to Dubro Servo arms. Maybe you will find exactly what you are looking for and you can buy it there too!
So now we have the RC servo, solidly mounted but isolated from vibration. We must consider the overall lever ratio of the system. If the length of the control horns is the same as the radius of the servo arm, where the pushrod actuates, then we have a ratio of 1:1. Thus if the servo moves plus or minus 30 degrees from neutral. then the control surface must move the same. This is a good starting point, with good resolution. If you are fortunate enough to own a computer RC transmitter, you can increase or decrease the servo movement, by using the transmitter computer. If you do not then you can only adjust surface throw by moving the push rod points at the servo and/or the control horn.

The last point to consider, is the electrical hook-up of the RC servo to the receiver. If the servo is mounted in the fuselage, close to the receiver, then there should be no problem with the length of the servo lead. But servos can be mounted in the tail, or in the wings and lead extensions are required.

Use the highest quality you can find, make sure the plugs are compatible and use only the correct length required. Too long leads can cause radio problems and try to clip them to the framework, to stop vibration from causing problems. Wrap the joints with electrical tape or use a commercially available connecter, that keeps the plug and socket together. I have heard of modelers using dental floss to tie the plug and socket together, but I have found the electrical tape to work well.

RC Servo Control Linkage
Ailerons, flaps and tail mounted units generally have short connections to the servos. Piano wire is a natural method of construction, with the end connections to suit the type of control horn used. Some form of kwik-link can be used, at either or both ends, as long as there is a way to adjust length to optimize the set-up. All connections must be stiff and friction-free and easily handle the flight loads with no deflection. The smaller the RC airplane then the simpler and lighter the RC servo connection system can be. The giant scale RC airplanes are probably the most demanding, but specialized hardware is available.

RC Ball links Dubro
This photo shows Dubro 4-40 heavy duty ball links, that can be used in a variety of locations. If you click on the photo, you will be taken to a page on the PC Planet site that lists a lot of Dubro servo connection hardware. Note that Dubro has been around a long time and make an excellent product that can be depended upon, when used in the correct application.

For the longer connections, generally in the fuselage, we need a rigid connection, that is adjustable for length and easy to install. If the fuselage you are using, is completely covered in (such as a fiberglass job), then your choice may be limited. I am thinking of the plastic tube in tube types, manufactured by Sullivan Products and others.

I have used these for years, with complete success, in a wide variety of RC airplanes up to and including 1/4 scale RC. However, all the models were built from plans or self-designed and I had access to the fuselage interior, so I could support the outer tube, every 6"to 8", along its length. This is essential to prevent "bowing" and the subsequent control surface deflection. This support (generally a balsa stick glued crosswise, across the fuselage) can be very difficult to reach, inside of a closed in fuselage. The quarter scale Model used the carbon filled nylon core and worked to perfection.

Pushrod by Dave Brown
For the closed in type of fuselage, the type of pushrod that does not need any kind of intermediate support can be manufactured from hard balsa (smaller models) or wooden dowel rod (larger types). Dave Brown Products Inc. offers a tubular fiberglass hollow rod, complete with end fittings, that makes into a superb RC servo pushrod.

The Dubro Pull/Pull control system
The next type of system to consider, is the "Pull/Pull" system. This uses a horn on each side of control surface and a double sided servo arm. Plastic covered fishing wire or fine Kevlar is used to connect the horns to the servo arm and generally running through small diameter plastic tubes. When installing these systems, make sure the cable is tight, throughout the complete range of movement.

The photo at the right, shows the Dubro system.

The final system to look at is the throttle cable for the glow and RC gas engine. Sullivan Products make a range of products for this application, consisting of a plastic tube, with a flexible wire inner. The plastic tube must be well supported and keep the tube as straight as possible. I prefer to use a plastic nylink at the throttle end( If the throttle arm is steel.)

One tip I will pass along regarding setting up the throttle travel. Start with the RC servo and the throttle arm at the mid point of travel. First set-up the high throttle position by adjusting the radius point on the arm(s). If you have a computer radio, simply adjust the servo travel. Then set up the low throttle point and I prefer to use the throttle trim function to "cut" the engine. Make absolutely sure that the servo is not "bottoming-out" at extremes of motion. I find that by starting at the mid-point makes this set-up a lot easier.

If you are building ARF RC planes, then usually, the pushrod system is provided for you. Sometimes I find the system supplied leaves something to be desired, so I do not hesitate to replace a potentially defective arrangement and I will not compromise safety!

The Types and Sizes of RC Servos Available
JR DS8711 RC servo
As I mentioned, at the beginning of this page, the range of servos available today is HUGE! The RC servo shown here is a large type, intended for giant scale RC airplanes that are fully capable of 3D RC aerobatics.

This class of model is probably the most demanding in the RC hobby. This RC servo is enclosed in an aluminum case and produces 480oz/in of torque. Transit time for 60 degrees is a mere 0.12seconds at a voltage of 7.4V. The high voltage means that a 2S lipo battery can be used, without a voltage regulator.

This model has all the preferable attributes- it is digital, has a coreless motor, dual ball bearings and metal geared. The total weight is 2.6oz.

The Item is available in stock,we will ship the package box within 48 hours after the buyers pay money via paypal.

Hitec RC servo HS-225MG
Here we have a mini size RC servo, primarily intended for electric RC planes. It is manufactured by Hitec and is analog with ball bearings and metal gears. Speed is 0;14seconds for 60degrees and has an output torque of 54oz/in. the weight is 1.1oz.

The propeller produces thrust and pulls the model airplane through the air, but what makes a model turn to the right or left, or rotate its nose up for a climb or down for a dive? The control surfaces��the ailerons deflect to make the airplane roll, the rudder moves to cause the aircraft to turn (yaw) to the right or left, and the elevator moves up or down to pitch the nose up or down. What moves the control surfaces? Servos.

All About Servos
The Hitec HS-322HD servo weighs 1.5 oz. and is 1.57 X .78 X 1.43in. in size. With the output horn and top of the servo case removed, the tough plastic gear train is revealed.
Servos are small, rectangular black plastic work-horses with wheels or horns (we will call them horns) on the top and wires that plug into the receiver in the airplane. Two wires in the three-wire strand power the motor in the servo and one carries the ��signal�� that tells the servo how far to rotate its output horn. In addition to moving control surfaces, servos can be used to lower landing gear, move a canopy back, release a bomb, flip an onboard switch to turn lights on or off or power up the motor system, and more.

Servos offer proportional control of the things they move. An example is pulling the nose up to enter a climb: Pull back on the right stick just a little, and the elevator servo will raise the elevator control surface a small amount, causing the airplane to pitch the nose up slightly. Yank back on the stick aggressively and the elevator will deflect upward decisively, sharply pitching the nose up. This shows proportional control.

Servos have gear trains in them that enable them to produce ample torque. Torque is rated in ounce-inches, which is a measure of the force exerted by the control horn an inch out from the output shaft. The $10 Hitec HS-322HD standard servo produces 41.7 oz-in of torque when powered by 4.8V and slightly more force if it is powered by a 6V power supply.

That is plenty of force to deflect the ailerons, elevator and rudder of a typical trainer. The output shafts in servos can be supported by bushings or ball bearings��standard servos typically use a bushing. A device inside the servo tells the motor exactly how far to rotate. That device is called a ��potentiometer�� and it decodes the signals transmitted to the receiver by the transmitter. The signals are referred to as ��pulse width modulation�� or PWM. The angle of deflection of a servo is controlled by the duration of pulses in the PWM signal coming out of the transmitter. Several great websites are readily available where you can learn more about the electronics and these pulses��just Google ��how servos work.��

All About Servos
Hitec HS-322HD servo.
Another metric often applied to servos is the speed it takes the servo to rotate the horn through a 60�� arc. The HS-322HD takes .19 seconds to rotate its output horn 60 degrees.

Standard ��analog�� servos like the Hitec HS-322 are affordable ($10) and reliable, and depending on the brand, only consume 250 to 700mA of power when in operation. ��Digital�� servos can provide much higher output torques for larger models��often in the 150 to 300 oz-in range. They consume a lot more power. When ganging a lot of digital servos, modelers will consider the total power consumption, but the use of three or four standard servos typically poses no power issues for a trainer. The best way to install servos, if you are scratch building, is with the shortest slop-free linkage possible��we will address that in another column! Good Flying!

The Item is available in stock,we will ship the package box within 48 hours after the buyers pay money via paypal.