By Noel Drindak • Originally published in Stunt News, July/August 2009
Editor’s note: This article was originally published in Stunt News and has been reformatted for digital readability. Original terminology, measurements, and technical details have been preserved where possible.
Moving my plane at contests, I frequently have someone carry my handle while I get the plane. Often, for no particular reason they will wiggle the handle. When they do this, a frequent reaction is: “Wow. That’s really smooth. How do you do that?” I work hard to build smooth control systems, and in this column I’m going to tell you how to do it. First I’ll discuss hardware, and then I’ll cover putting it all together.
Here are the key elements of a super smooth control system—Rocket City #87 Ball Links, Central Hobbies pushrods with titanium ends, and Tom Morris ball link control horns.
We are lucky to have available today high-quality hardware that is specifically designed for our control systems. This wasn’t always the case. I’m sure that many of you remember the days when you had to build things for yourself—making pushrods from fiberglass arrow shafts, making inserts from dowels, bending ends from music wire, gluing it all together, and soldering the ends. (How many planes were lost because of bad solder joints?) The end result was often a control system that was stiff and had lumpy spots. Today, building a smooth control system is much easier. Mostly it’s just a matter of screwing things together.
The key to smooth control systems, the thing that started all the changes, is the Rocket City Ball Link. (I call it a ball link because Rocket City does, but technically it’s a spherical bearing rod end.) Ball links offer a number of advantages over music wire pushrod ends:
1. Ball links accommodate the motion of the flap pushrod. Music wire pushrod ends use sleeve bearings (also known as journal bearings) on the pushrod ends. (A wire pushrod end in a bushed control horn constitutes a sleeve bearing.) Sleeve bearings are good for simple rotation that is confined to one plane. They are well suited for use on the elevator pushrod since both ends (one attached to the flap horn and the other to the elevator horn) rotate in the same plane. Sleeve bearings are not well suited for use on the flap pushrod because the two ends (one attached to the flap horn and the other to the bellcrank) rotate in different planes. In fact, this is the worst case for using sleeve bearings—the two planes are actually perpendicular to each other.
Consider the bearing on the end attached to the flap horn. It provides rotation in the vertical plane of the flap horn with no problem. The problem comes when the pushrod tries to rotate in the horizontal plane as the other end follows the motion of the bellcrank. The only way the bearing on the flap horn can accommodate this motion (without something bending) is for extra clearance to be provided in the bearing. Extra clearance means free play in the controls and results in rapid wear of the bearing.
The bearing on the bellcrank end has the same problem as the bearing on the flap end, so there are two bearings with extra clearance contributing free play to the system.
The two bearings are also subject to rapid wear. Back in the day it wasn’t uncommon for fliers to cut open one- or two-year-old airplanes to rebush the flap horn bearing. It’s ironic that in a model made of balsa and tissue a metal bearing wore out before the plane.
2. Ball links are self-aligning. Because ball links use a spherical bearing, they can accommodate motion in more than one plane. When ball links are used on both ends of a flap pushrod, no extra clearance is required to allow free motion—the ball links give minimum free play and are self-aligning.
3. Ball links are easy to adjust. If you’re using a ball link with 4-40 threads in the barrel, it can be adjusted in increments of 1/80 (0.0125) of an inch, certainly adequate for most fliers. If you want finer adjustment, ball links are available with left-hand threads. Make your pushrod with right-hand threads on one end and left-hand threads on the other end, and it works like a turnbuckle, giving infinitely variable adjustment.
4. Ball links require no soldering. Making reliable soldered connections is a skill. You must practice to be good at it. It’s a lot easier to make a reliable threaded connection than a soldered one. Also, threaded connections are many times stronger than soldered connections.
5. Ball links don’t wear out. I’ve been using ball links for a decade now, and I’ve never seen any signs of wear.
6. Ball links don’t come loose. I’ve crashed a few planes with ball link control systems (one straight in on the tarmac), and in every case the pushrod with its ball link ends wound up holding the pieces together.
Rocket City no longer markets its ball links. However, they’re available from Control Line Central and also from Don’s Hobby Shop.
Ball links require pushrods with threaded ends. The first manufacturer to provide push rod kits with threaded ends was Central Hobbies. I’ve been using them for 10 years, and I still think they’re the best. They are stronger, lighter, and easier to assemble than any other option. Whether you use ball links or clevises they’re the best choice.
Central Hobbies kits consist of a pair of carbon-fiber tubes with four threaded titanium end fittings. The tubes come in three sizes—1/8-, 5/32-, and 3/16-inch diameters. End fittings are available with 4-40, 6-32, or 8-32 threads. I use 3/16-inch rods with 4-40 ends. The end fittings are attached to the carbon-fiber rods using slow-cure epoxy (J.B. Weld is recommended). Although end fittings are also available in stainless steel, I recommend against using them. Titanium is both stronger and lighter than stainless steel. Speaking of strength, I once load-tested a 3/16-inch Central Hobbies pushrod to 400 pounds. If you don’t think that’s strong enough, try hanging 400 pounds on the tail of your Stunt ship.
Making pushrods with the Central Hobbies kit is a piece of cake. All you do is measure, cut, and glue. If you’re using Rocket City ball links, the tube should be 2 5/8 inches less than the horn-to-horn distance. This allows each ball link to be adjusted 1/8 inch in or out and still have a minimum of 3/8 inch of thread engagement. When cutting the carbon-fiber tube, be careful not to split the ends. Also, after cutting the tube, sand the end square so the titanium fitting will fit tight. Clean the components with alcohol or lacquer thinner before assembly. With J.B. Weld on them, the end fittings act like little pistons in a cylinder. When you push them into the carbon-fiber rod, air pressure wants to push them back out. I place the pushrod between a couple of building weights to keep the ends in place while the epoxy cures.
A set of pushrods, with ball links added, will weigh about 3/4 of an ounce. A kit costs $16.95. Again, they’re stronger, lighter, and easier to assemble than any other option—spend the money.
Ball links require control horns with 4-40 threaded holes. Tom Morris was the first to address this need. Tom Morris ball link control horns are available from Control Line Central.
Good bellcranks have been available for a long time. Control Line Central offers Tom Morris bellcranks and Brett Buck-style bellcranks. Brodak offers their Strong Arm bellcrank and PSP bellcranks from Windy. They’re all good. Mount ball links to bellcranks using a conical standoff to provide clearance so the ball link will have freedom of movement.
If you hold one half of a hinge horizontal, the other half should swing freely. If it doesn’t, it’s because of friction in the hinge. What’s a little friction, you ask? When you multiply it by 16 to 20 (the number of hinges in a typical Stunt ship), it’s a lot of friction. With the ball link control systems we’re using today, it’s the major source of friction. I don’t want friction in my control systems. Control force is limited (by line tension) and I don’t want to waste it to overcome friction.
Many will tell you not to worry about a little friction in the hinges—that they’ll loosen up after a few flights. Wrong! Nylon is very tough. The barrel of a nylon hinge rubbing against another barrel or against the hinge pin will not wear. That’s why it’s a good bearing material. The friction in the hinges you use is there for keeps. It won’t go away.
I use Du-Bro Heavy Duty Hinges. They come disassembled and there is lots of clearance between the barrels so they don’t require any modification. The only reworking necessary is to remove a little flashing and shorten the tab (you don’t need a 3/4-inch tab in a 3/8-inch trailing edge). I reassemble them with a Sig #20 pin, which gives plenty of clearance.
Be sure to use plenty of hinges. Randy Smith has observed that the flaps on many Stunt ships flex under load to the extent that there are visible gaps between the flaps and the trailing edge of the wing. I listen to Randy. My last plane used six hinges per flap and I’m going to use seven in the future.
There are a few key points to remember as you assemble your control system:
1. Always use high-quality alloy screws to assemble your ball link control system. A good source of quality fasteners is Micro Fasteners. My personal favorite is the big industrial supply house, McMaster-Carr. When I order stuff before 5 p.m. I get it the next day.
2. When you mount ball links to a horn, the threads in the horn are the primary fastener. Use a screw with enough extra length to accommodate a locking nut on the other side of the horn (a plain nut is all you need here).
3. When you do the final assembly of your controls, use a high-quality thread locker such as Loctite 271 (available from McMaster-Carr). It only takes a few drops on the threads to ensure your fastener will never come loose. Wipe off any excess and ensure that none gets in the bearing area of the ball link.
4. After the thread locker has fully cured (about 24 hours), I put a drop of 3-IN-ONE oil on all the bearings in the system. It’s the only lubrication I use.
5. The hardest part of making a super smooth control system is doing the hinges. All the hinges for the flaps (and elevators) must rotate on exactly the same centerline. The control horn must also rotate on the same centerline. A few key points:
a. You must acquire the skills and tools necessary to make precisely accurate hinge pockets in the flaps, elevators, and trailing edges. If your hinges are cocked, your control system is doomed to mediocrity.
b. When you glue the hinges into the flaps and elevators, you must ensure that they are all exactly in line.
c. Always be very careful that glue doesn’t get into the barrel of your hinges.
We are lucky to have available high-quality hardware that is specifically designed for our control systems. Building a super smooth control system is mostly a matter of choosing the right hardware and screwing it together. Doing the hinges is the hardest part—that’s where building skill and focused attention is required.
Good luck with your next plane.
It is possible, due to the timeframe in which this article was originally published in Stunt News, that some of the materials and/or services mentioned are no longer available from the source listed.
This article first appeared in the July/August 2009 issue of Stunt News and has been preserved as part of the PAMPA digital archive project.
