Commonsense control linkages

Model Airplane News, Nov 1999 by Hahn, Greg

WHEN LEAFING through my notes of frequently asked questions, I came across one that I get hit with all the time and thought that it might make a good subject for a column. Of course, I always seem to find controversial subjects, and-as usual-my way of doing things isn't always the norm!

The issue in question concerns servos and linkages: how you go about operating the many surfaces and control functions of today's giant-scale aircraft. As far as I know, there aren't any published rules of thumb on just what to do and how to do it, so I'll throw my standard setup opinions, equipment and procedures into the fiery depths of public scrutiny and maybe come up with some good rules to go by.

In the late '70s, Nick Ziroli and a handful of other visionaries pioneered the idea that model aviation could involve more than small "toy" airplanes. Now, giant-- scale airplanes, contrary to their humble beginnings, are adorned with everything that technology has to offer, from computer radios and onboard telemetry to high-horsepower, finely tuned gasoline engines. Yet even with all this techno stuff to the forefront, the all-important control linkages are still in question, and most of the available hardware is still a scaled-up version of a standard 40 to 60 linkage. A few companies, such as Robart*, Du-Bro* and Nelson Hobby Supplies*, produce hardware specifically for giant scale, but there are many questions on how to make everything work reliably and, above all, be strong enough for the job.

SHORT PHYSICS LESSON

Most full-scale aircraft have balanced control surfaces that, in level flight, compensate for gravity's pull on them. To achieve this balance, the surface usually has some type of weight and arm attached to it or somewhere in its linkage system.

The advantages of having a balanced surface are:

* Less force is needed to physically move the surface (easier on the servo).

* The surface resists inadvertent movement or instability, i.e., there's no chance of aerodynamic flutter. Unfortunately, at best, balancing our models' surfaces isn't easy, and it's often quite impossible, owing mainly to the large amount of throw used by most builders-including me! We rely on our servos and linkage systems to compensate for an out-of-balance situation and to be strong enough to absorb the sometimes enormous stresses to which we subject them. Keep in mind that when the airframe is stressed, the surface link is stressed even more.

BACK TO BASICS

Knowing your piloting skills and understanding the capabilities of your airframe can go a long way toward putting you in the ballpark as far as learning which equipment you need. Because of the lack of useful test data on components, most of us lean toward overkill. This is definitely adequate, but it can also be ugly and expensive. I assess the linkage needs of airframes keeping these four buzzwords in mind:

-short;

-simple;

-tight;

-leverage.

Short. I mean as short as possible. The longer the rod, the more flexing there will be in the system and, therefore, the greater the likelihood of slop.

Simple. This defines itself. Stay away from bellcranks and multiple rod links. Whenever you have to make a connection, you create slop. There has to be clearance between the drive pins in the clevis and the horn, and they can add up to a lot of play.

Tight. If you took care of the first two items on my list, then you have "tight" taken care of except for the servo. Use good, tight servos that have dual ball bearings and preferably a good solid gear mesh. If the servo is sloppy out of the box, it will only get worse-usually, in a very short time.

Leverage. Having proper leverage is very important-almost a subject in itself. It can also be hard to come by--especially if you build for scale competition and want to hide all of the pushrods. It's easy to get stuck within the confines of a cramped fuselage tail section or an outer wing panel where there isn't much room for a long control horn.

Short control horns provide little leverage and can lead to flutter and undue wear and tear on the system. This makes the servo work harder and puts it under load more often; this is also a big current drain on the battery. The rule for servo arms is exactly opposite that for control horns: farther out on the arm means less power. Here's a good rule to follow: couple a short servo arm with a long surface horn. This will provide maximum servo power and adequate leverage on the surface.

NUTS AND BOLTS

Most of the available "giant" hardware will do a fine job if used properly, but because I'm asked so often, I'll pass along my preferences. The pieces I mention are the ones I use in every project and have had good luck with over the years.

For throttle, I start with a good ballbearing sport JR* 531 servo and an 1/s-inch Sullivan* flexible Nyrod with a Sullivan clevis at the servo end and a 2-56 ball link at the carb end. Keep in mind that for consistent response, throttle linkages should be just as tight and neat as those for the elevator.

For aileron, I start with a medium-- power (90 to 100 oz.-in.) JR 4131 ballbearing servo and a threaded 4-40 solid rod with a Sullivan clevis at the servo end and a Robart clevis and ball-bearing hom (at least 3/4 inch long) at the surface end. After I've tested the setup, I solder both ends to eliminate the need for jam nuts.