The Mechanics of 3D Flight

12/10/2004 by Doug Cronkhite, Team JR

Copyright:© 2004 Horizon Hobby, Inc.

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3D flight is one of the fastest growing areas of our hobby. As these 3D airplanes like the Seagull Harrier and Hangar 9 Funtana become more and more popular, the ideas and concepts of a good mechanical setup become ever increasingly important. A good mechanical setup not only makes the 3D flight easier, but also decreases the chances of flutter.

A good setup starts when we're building the airplane. The following things apply to the new breed of ARFs as well so don't overlook them.

Hinges: Hinge lines should be straight and centered on the surface.

Pivot Point of Control Horns: Control horns should be installed such that the pivot point of the horn is exactly on the hinge line to avoid building in a differential.

Servo Arms: The arm on the servo should be exactly parallel to the hinge line. Servo arms should be switched around until you get the spline alignment correct. Always try to avoid using the radio to center the servos whenever possible.

Seal Hinge Lines: Hinge lines should be sealed so no air can pass through.
It doesn't matter how tight you think it is, seal it on the bottom with some covering.

Last but certainly not least is mechanical advantage. You might have heard this term before, but what does it mean? Put simply, mechanical advantage means giving your servos the leverage necessary to move these huge control surfaces in flight. Even the strongest servos can be rendered useless by a poor mechanical setup.

Let's start with the radio. Select the ATV feature and set all used channels up to 140% in both directions. Don't forget the flap and aux channels if your using multiple aileron or elevator servo setups.

This step allows for the maximum travel out of your servos and therefore maximum servo resolution. Most modern computer radios are 1024 radios, meaning there are 1024 steps of servo resolution for it's full range of travel. By running your ATV up to maximum, you utilize all 1024 steps to command the servos.

Now that we have the radio set up, we need to connect the pushrods. At neutral stick, you should always have your pushrod 90 degrees to the servo arm. Ideally, your pushrod geometry should look like the diagram below:

But what if you need more throw than your current setup is giving you? The best solution here is to use a longer servo arm. Use only the length necessary to give you the throw you need. Try to avoid moving the pushrod closer to the surface on the control horn if you can as this reduces the leverage applied by the servo.

If after changing the servo arms you find you need less throw, move the pushrod in closer to the center on your servo arm. Try to avoid using dual rates or ATV's to reduce throw whenever possible. You should always make large adjustments mechanically, and then use the radio to make fine adjustments.

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