Airfield Models - Hardware for Flying Model Aircraft

About Pushrod Linkages for Model Airplanes

May 05, 2015



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Airfield Models (http://www.airfieldmodels.com/)About Pushrod Linkages for Model Aircraft

A linkage is a component that hooks one moving part to another.  That would include the entire pushrod system.  On this page, I am changing the meaning to include only those parts that are attached to the ends of pushrods to connect the pushrod to a horn (servo arm, control horn, etc.)

There are a variety of linkages available for an equal variety of installations.  The linkage you choose is often a matter of personal preference.

The three most important properties of any control linkage

  • Must align properly with the arm it connects to.
  • Must be strong enough to transfer and withstand the maximum load requirement for the entire pushrod/control surface system.  A $125.00 digital servo is worth precisely squat if the $1.25 linkage connected to it fails.
  • Must fit properly to avoid binding or introducing excessive play.

We do not have data on how much load is on a linkage or even how much load a linkage is supposed to withstand.  Instead, we are again making selections based on experience and examining the system for obvious faults.

Always keep in mind what the purpose of it is and try to imagine ways in which it might fail in your intended application.  If you think of something, don't use the linkage.  Again, I said, 'reasonable,' not 'any possible.'  Anything can fail with the right circumstances.

No matter what type of linkage you use, it should fit properly.  Over-sized holes in horns are asking for slop which can result in trim problems or devastating flutter.  Do not wallow out holes to make things fit.  If a hole needs to be enlarged, use the right size drill bit.

The "right" size equals the same size as the wire or pin going through the hole in the horn.  Most horns are made of nylon which is a slippery material.  If the fit is a little on the snug side, it will not cause problems.  If the fit is so tight that things bind then the hole should be enlarged.

Caution! Never use linkages on both ends of a pushrod that will allow the pushrod to rotate such that one of the linkages becomes unthreaded.  For example, having threaded metal clevises at both ends even with locking nuts is a bad idea.

Generally speaking, only one end of a pushrod needs to be adjustable.  Most people prefer to put the adjustable end on the outside so that it is convenient for making trim adjustments without having to open the airplane.

 
 

A linkage must align properly with the control horn and servo arm

Clevises should align with the control horn to prevent binding or damage.

Servo with clevis and threaded rod

Clevises, L-Bends, Z-Bends and Quick Connectors should align with the horn such that there is no binding or strain on the connection.
Misaligned linkages can cause binding or failure.

Servo with clevis and threaded rod

Linkages that do not align properly can cause excessive binding, wear and can fail.  Failure of any control surface linkage often leads to the demise of the model.

In cases where the pushrod is at an angle of more than a couple degrees, a ball link is a better choice.

 
 

Strength and failure rate of linkages

These charts is based on my experience and is not backed by any kind of data.  If your experience is different, then go with what you know and don't listen to me.

Strength

Strength in this case is a combination of rigidity and breaking strength.  Nylon has great breaking strength but is also very flexible which rates it lower.

Highest Strength (Best)

Lowest Strength (Worst)

Z-Bend when properly made L-Bend Metal Solder Clevis when properly soldered Threaded Metal Clevises Improperly made Z-Bend Nylon Clevis EZ Connector

Failure Rate

Lowest Failure Rate (Best)

Highest Failure Rate (Worst)

L-Bend Z-Bend when properly made Nylon Clevis All Metal Clevises Improperly made Z-Bend Improperly soldered Metal Solder Clevis EZ Connector

Note that EZ Connectors are the worst in both charts.  I have more to say about them later, but for now I'll just say don't use them on any control surface ever in any application unless your application is to find out how much they can take without failing so you can educate the world about how bad these things are.

 
 

EZ Connectors

EZ Connectors by DubroEZ Connectors are designed to allow maximum allowance for adjustment.  Whereas a threaded rod must be measured to the correct length, an EZ Connector allows the builder to leave the pushrod wire over length until everything is hooked up.  The excess wire is then clipped off.

EZ Connectors attach to a horn with a pin and snap-on retainer.  There is a hole in the connector that receives a wire which is then clamped in place with a set screw.  They are very easy to set up.

The downside of EZ Connectors is that they create inherently sloppy connections.  The snap-on retainer loosens over time and can fall off.  Even if the retainer does not fall off, the system will become sloppier the more the retainer loosens.

Many people will tell you that they've never had a problem with EZ Connectors.  Other people will tell you they've lost planes because of them.  I do not take the middle road on these.  I think EZ Connectors are dangerous and should be outlawed for use on any flight surface.

However, EZ Connectors are perfect for many non-critical situations.  For example, there is not room between the nose gear steering arm and the firewall for any other type of connector, so this is a good place to use an EZ Connector.  In fact, their inherent sloppiness will help absorb some of the shocks from the gear rather than transmit it to the servo.

EZ Connectors can also be used for the throttle.

Again, never use these connectors on any control surface.  I do not like them and do not use them unless I absolutely have to, but there is almost always another solution that is better.  I recommend that you learn how to get along without them as well.  They have the highest failure rate of any type of connector, are very sloppy and using them just gives you another thing to worry about.

 
 

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Pushrods and Pull-Pull Controls for Model Airplanes
L-Bends and Z-Bends

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Copyright 2004-2006 Paul K. Johnson