Throttle Linkages for Model Aircraft Engines

Many times the throttle linkage is installed as an after-thought at the end of construction.  However, it is a vital part of the propulsion system and can be as trouble-free or aggravating as you choose to make it.

Important!  Always make sure you can positively shut down your engine from the transmitter.  On more than one occasion I have seen a plane taxi into the pits and chew up someone else's plane because the operator couldn't shut off the engine.

Taxiing in the pits is a really stupid thing to do and anyone who does it should be held entirely responsible for any damage they cause.

Assembling the Throttle Hardware

The throttle is as close to a no-load situation as you are going to get.  A small servo will work fine but you want the entire setup to be as mechanically positive as possible.  When you are landing your aircraft it is important to have fine control over the throttle near idle.  If there is any play or excessive drag in the setup then the engine will not respond properly to throttle commands.

Traditionally the throttle pushrod has been a flexible cable.  I have never been satisfied with cables because they flex when I don't want them to and don't flex when I do want them to.  The other problem with cable is that it is unnecessarily heavy.

If you choose to use a throttle cable it is most often hooked up with an EZ connector at the servo end and a solder coupler with a clevis or ball link at the throttle end.  The problem with EZ connectors is that they are inherently sloppy.  The plastic fitting that holds it in place will not stay tight for very long.

The way I set up cable is to use a solder clevis at the servo and a solder coupler with a ball link at the throttle end.  That eliminates the play associated with EZ connectors and it is a much more reliable.  EZ connectors have been known to become EZ Disconnectors at the worst possible moment.  The pilot is then in a situation where he has no throttle control.

Use sharp side-cutters or an emery wheel in a moto-tool to cut cable.  If you are careful you can cut it without fraying it.  The wire strands can usually be put back where they belong without much difficulty if individual wires in the cable unwind.  One way to avoid fraying is to solder the cable before cutting it.  If you don't solder the cable ends before cutting them then you should solder afterward.

Deburr the ends of the cable using a grinder or an emery wheel in a moto-tool.  The cable will be less likely to cut you when working in the engine compartment or radio compartment, will not fray and will slide more easily into place.

My preferred throttle linkage is simply a piece of 1/32" (0.032") music wire with a Z-bend at both ends if the engine uses a plastic (nylon) throttle arm.  With a little practice you can put a Z-bend exactly where you want it.  The only drawback to this is that you can't disconnect the linkage from then engine before you remove it.  That is not much of a problem though.  After you remove the engine mounting bolts you simply turn the engine to release the linkage.  If you prefer you can use a solder coupler and clevis on one end of the pushrod.

	This is the business end of the throttle linkage.  An inner NyRod houses a small diameter music wire pushrod.  A solder coupler is soldered to the end of the music wire.  A ball is bolted to the throttle arm and the nylon ball link is threaded onto the threaded coupler.  Dubro makes 1/32" solder couplers having a 2-56 thread that are perfect for this. The fuel line could probably be situated better so that it does not come into contact with the linkage but in this case it is not causing any problems. Also note the plywood thrust wedge behind the engine mount.  This wedge is probably about 1/2°.  If you pay attention to small details like this you can make almost any aircraft fly well assuming it was built straight to begin with.
	I like to use a Z-bend on the throttle end as I do on the servo end.  In this case the throttle arm is metal.  I decided against a Z-bend to avoid metal-to-metal contact which can cause radio interference as well as premature wearing of the parts.
	A simple Z-bend on the servo end saves money and simplifies the installation.  You only need one end of the linkage to be adjustable if that.  Normally you want to be able to make adjustments outside the aircraft so there is no reason to have an adjustable linkage here.

The advantage of 1/32" music wire is that is easier to work with, lighter and stiffer, yet is still flexible and much less expensive than a cable setup.  Overall it has every advantage of cable and none of the disadvantages.  I have found throttle control on my aircraft to be much more positive once I switched away from cable.  It is especially important to have good control authority around idle where you want the engine to respond to even the smallest commands.

The pushrod must be enclosed in some type of housing inside the aircraft.  The housing guides the pushrod and prevents radio components and the fuel tank from interfering with its movement.  I groove small balsa blocks to fit the outer housing and space the housing about 1/8" away from the fuselage side.

Setting up the Throttle Geometry

When setting up the throttle, hook up the servo to the receiver and turn on your radio.  Move the throttle stick to mid-throttle on the transmitter.

Put the arm you plan to use on the servo so that it is 90° to the pushrod.  Move the throttle arm on the carburetor so that it is half open.  Loosen the throttle arm on the carburetor and set it up so that it is as close to 90° to the pushrod as possible at half-throttle.  Make the throttle pushrod the length necessary so that it can hook up to the servo and throttle arm in these positions.

With the throttle lever pulled all the way back (low throttle) and the trim lever all the way down, the carburetor should be fully closed.  With the throttle lever all the way open and the trim lever all the way up the carburetor should be fully open.  There should be no binding or servo buzzing at either extreme.

If your radio does not have end point adjustments (also known as Adjustable Travel Volume or ATV), then you will have to figure out which combination of holes on the servo arm and throttle arm allow the full range of throttle without binding.

To get the absolutely fastest throttle response, attach the linkage to the hole farthest out on the servo arm and nearest the center on the throttle arm.  This is easiest to set up with ATV's.

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Copyright © 2003 Paul K. Johnson