Mechanical Components

In the previous section we mentioned that the receiver will receive input from the transmitter and tell all the mechanical components how to move. So, what are these mechanical components and how do they control the movement of the plane? Lets take a look.


The servo is what physically generates the force to move the control surfaces (like the rudder, aileron and elevator). Most foam planes use small micro servos and many of them weigh around 9g. Since we concern ourselves with foam planes, we need to minimize weight. Due to this, we usually can't use larger servos without weighing down the plane. As you can see in the picture, the servo itself has a few different parts. The main body contains the gears and electronics, while the servo arm (on the right) is held on by a small screw. Its not uncommon to take the arm off to either make adjustments or switch to a different type of arm. In some cases, a servo will power more than one moving surface or part. That is where different arms really come in, like the two sided servo arm in the picture.

Some servos are capable of pushing with more force than others. Its important that you buy servos that are capable of pushing the size and weight of the surface you intend to use it on. Aside from strength, hardiness is also an issue. Most servos use plastic gears, and extra strain or impact can cause the gears to strip. Metal gear servos help solve this problem, as they are often much harder to strip down. Generally speaking, they will be more powerful and hardier servos.

Servos also come in a digital and analog variants. The internal parts of these servos are practically identical, but there is a major difference in how the process the signal from the receiver. The end result is that digital servos have a faster response time and smoother acceleration when compared to analog servos. The big drawback is that they do use extra power, which might be a concern for your BEC. The role and workings of the BEC will be covered in the electronics section.

Servos are positioned throughout the plane, but usually within a short distance of the surface they are trying to control. The aileron servos are typically embedded in the wings (see above picture), while elevator and rudder controls are usually embedded in the fuselage towards the back of the plane.

You may remember that ailerons operate off of one channel, yet the ailerons are almost always driven by two servos (one on each wing). How is this done? Well, the servo wires are hooked into a Y-cable, pictured above. Both servo wires are plugged into the two female connectors, while the other end is plugged into the correct channel on the receiver. At this point, both servos will move in tandem. Since ailerons need to move in opposite direction, one servo is rotated/flipped over, to face the opposite direction, when it is glued to the wing. Now the arms on that flipped servo will move in the opposite direction of the servo on the other wing.

Pushrods, Control Horns and the Celvis


The servo is mounted a decent distance away from the surface its trying to move, so it needs some way to reach it. The pushrod is the item that extends the reach of the servo. One end will typically have what is called a "Z-bend", while the other will have some sort of clevis. The clevis is screwed on to a threaded portion of the pushrod. The above left picture shows the pushrod and clevis connecting to a horn, while the picture on the right shows the pushrod and clevis in their entirety. In the picture to the right, the top of the pushrod has a little bend to it. This right-angle looking bend is the Z-bend. With some careful maneuvering, you will snake that Z-bend portion into a hole on the servo arm.

On the other end of this pushrod, we have the clevis. The clevis is actually screwed onto the pushrod and can be moved up or down the pushrod by rotating it. This movement allows for mechanical adjustments to be made in order to properly align the control surfaces. This particular clevis has a snap on connector, which makes removal and adjustment quite easy.

The clevis is connected to a piece called a "control horn". This horn is glued and/or screwed into each control surface. In other words, there is one directly mounted to your elevator, ailerons and rudder. The picture to the upper left shows another type of clevis connecting to the control horn. The clevis on the pushrod in the upper right picture could just as easily snap on to the control horn in the left picture. Normally, you will use the type of clevis that the manufacturer supplied with your plane. If you scratch build your own plane, the connector you use is really personal preference. Lets see if we can put all of what we learned together now.

Try to visualize this. The receiver sends commands to the servo, and the servo arm moves back and forth. That servo arm is connected to the push rod, and it stays in place thanks to the Z-bend. The pushrod extends out to where the control horn is, and is attached to the horn via a clevis. As the servo arm moves, the pushrod moves, thus causing the horn and the attached surface to move. Hopefully this isn't too confusing, it actually very simple when you see it in person.

Its worth noting that there are ways to adjust *how much* the surface will move. If you look closely at all of the previous pictures, you will see that both the servo arm and the control horn have multiple holes to connect to. On the servo arm, the hole furthest away from the servo will result in the most movement of the control surface. The innermost hole on the servo arm has the least amount of movement. On the control horn its actually the opposite of this. The hole furthest away from the base of the horn will move the control surface the LEAST. The innermost hole on the horn actually produces the most movement. I try to avoid the top most hole on the control horns for a different reason altogether - they tend to break more easily (especially on the cheap horns). I typically use the next hole down from the top of the horn.

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Supplemental Videos From the Web

How Servo and Horn Position Effects Movement

This is a very important lesson that is hard to show without visualiation. This video illustrates how the different positions (different holes) on the servo and control horn will change the amount of "throw" you get from your surfaces. Please keep in mind that its not necessarily better to have them move more. The maximum settings can make your plane extremely reactive, to the point that you won't be able to control it. For beginners, its probably best to keep the range of motion closer to the minimum.

Clevis and Control Horn Connection

The video in question is about making the connection more secure, but its the only video I could find with close up of the actual connection between the control rod clevis and the control horn. If you are having trouble understanding how the pieces come together, this may be useful.

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