Movement Geometry by Servos for RC Planes
I’m sure that by this time you are aware of the purpose and use of servos for rc planes. When you buy an ARTF model kit the way that the servos are arranged and operate the controls should have been thoroughly researched and correctly positioned. There should be no resistance to their operation and to the way they move the control surfaces or throttle.
Having said this, there is no reason for not understanding the way their positioning and settings have been decided. The greater your knowledge of what makes your plane function correctly, the better it will be for your future progress.
In the diagram to the right you will see that the output arm is shown at its neutral central position as well as the extremities of its movement range. The full significance of these positions will become clear as we progress through this post. I will be referring back to this diagram to clarify points we will be considering.
Making the Most of the Servo Motor Output
Most servos made for radio control applications have a rotational range of approximately 90 degrees. This represents the most efficient operating movement and we should try to use as much of this range as possible in order to get the best out of the servo. The majority of modern radio control transmitters have the programming capability to adjust the range of this movement. The servo can be made to move slightly more than the default setting or be taken to virtually zero movement.
Although these adjustments are possible via the transmitter, they should not be used as the normal way to set the range of movement for the various controls. If you look at the diagram above you will notice that the output arm has a series of holes to which the pushrod can be connected. These are provided so that the range of pushrod movement can be varied.
Now let’s look at the diagram over to the right here. You will see that although the servo motor output arm rotates in a circular motion, the actual range of linear movement is the solid line between the ends of the two radii created.
Control Surface Travel
Below this diagram is a photograph of a pair of control surface horns. You will note that the pushrod linkage holes are offset from the mounting plate. This is necessary so that the plate can be positioned close enough to the hinge line of the control surface to bring the holes perpendicular to the hinge axis.
The ideal situation is for these hole to line up with the hinge axis so that the transfer of movement is as accurate as possible and completely linear. The diagram below will clarify this requirement.
It will be evident from this diagram that the actual travel of the pushrod connection at the control horn will be considerably less than the travel of the trailing edges of the control surfaces. What we now have to do is to match the full movement of the servo output arm to the surface control horn so that the control surface movement meets our requirement.
The way we do this is to select the hole in the servo output arm that will give us the same amount of travel as that provided when the pushrod is connected to the outermost hole of the control surface horn. The reason for choosing the outer hole at the control surface horn is to minimise any effect of play or slop due to a lose fit of the clevis pin in the horn hole. This will represent the smallest percentage of overall movement compared to maximum pushrod travel.
Matching Control Surface Movement to Servo Output
With the ideal outer hole selected at the control surface horn we now need to measure the actual linear travel of the pushrod at its connection to this horn. Once we have this measurement we can find the hole in the servo output arm that gives us the closest linear movement match. This will be the best point to connect the servo end of the pushrod.
Using this method ensures that the maximum output efficiency of the servo is employed. You will need the help of someone to take this measurement. Someone will need to move and hold the control stick on your transmitter at each maximum throw whilst you measure the full travel of the servo arm.
Using A Servo Tester
Alternatively, you could invest in a servo tester as shown in the photograph right. This is a very useful little device that makes setting up and adjusting servos much simpler. There are three options with this tester:- a) Automatic Centring of servo. b) Cycling of servo to maximum travel either side of centre. c) Manual Control of servo travel to any position within the full range either side of centre. This feature is most useful for determining the full linear travel of the output arm as the arm will remain where it is set until its position is changed manually by the operator.
The automatic servo centring (a) is most useful for ensuring the servo output arm is fitted in the correct position on the spline for the centralised neutral.
If you would like to purchase this tester, USA readers can click on the photograph. UK readers can purchase by clicking the following link:
The power for the tester comes from a 4.8V or 6.0V NiMH battery plugged into the right hand side of the case whilst up to three servos can be attached on the left side. The selected operating mode is indicated by bright blue LEDs above the control knob.
Control Surface/Horn Considerations
As explained previously it is important that, whenever possible, the pushrod holes on the surface control horn are lined up with the hinge axis line. This ensures that the travel of the trailing edge of the control surface is equal either side of the neutral centre line.
When it comes to driving Flaps this rule is not so significant. Flaps generally only travel downwards so the alignment of the pushrod holes is not so critical. Admittedly at this early stage of your learning programme you will not normally encounter flaps but it is important to understand the way they operate.
In general the pushrod will hold the flap in alignment with the wing trailing edge and neutral aileron position. This represents the least load on the servo mechanism so ideally the servo output horn should be close to the normal neutral position (see illustration below).
Because we still use the full range of travel from flaps up to flaps down the ideal position of the servo output arm should be close to right angles to the servo body when flaps are up and in line with the servo body when flaps are lowered. Generally the flaps are operated by a two position switch on the transmitter that will cause the servo to travel from one extreme to the other.
The way we set up this positioning is to find the true neutral centre position. This can be done through the transmitter/ receiver link or using our little friend the servo tester discussed above. Once we have this position remove the servo output arm from the spline and reposition it to a position approximately 40 degrees further round the rotation away from the flap hinge line.
When the flaps are lowered the maximum load on the servo occurs when they are at their lowest position. This is due to the pressure of the air being deflected downwards. The greatest load bearing strength within the servo is when the output arm is virtually in line with the pushrod. This just happens to be when it is at its maximum throw when set up as described above.
You will see that the right angle is slightly offset from the servo body. The reason for this is to ensure that the clevis/pushrod does not bind on the servo arm spindle when in the lowered position. As and when you get to the situation of fitting flaps to a plane, I suggest you take advice from a more experienced modeller to ensure flawless operation of the system.
I hope this post has helped you to understand why servos for rc planes are installed and connected to the various controls the way they are. The guidelines I have described cover the majority of installations. There are certain circumstances under which these are changed to accommodate special requirements. For now, so long as you understand the above, you won’t go far wrong.
Don’t forget to visit my website www.rookiercflyer.com for a comprehensive learning programme on getting started in model plane flying.
Till the next post, happy flying.