RC Scale Aerobatics
Mike Hurley, 11542 Decatur Ct., Westminster CO 80234; E-mail: [email protected]
IT WOULD BE nice if all of us who move into flying the big airplanes did so on a slow and deliberate schedule, gradually experiencing the trials and tribulations of each advance and working out all the bugs before moving to ever-bigger aircraft.
Individuals who take this route usually end up being well-rounded and experienced modelers. A side benefit is that they usually experience less failures and have safer aircraft.
Just a few short years ago, only seasoned veterans owned and operated the big guns. But today big models are all the rage, and you can easily find third-, second-, or even first-year modelers tackling 35% aircraft!
Often, these Giant Scale rookies are unaware of subtle changes from their first airplanes that can make the difference between an outstanding-performing airplane and one that is destined to be an expensive pile of kindling.
Linkages are one of those subtle areas that are often overlooked but are critical to an aircraft's performance. It's essential that you install linkages that are up to the greater demands placed on them by these high-performance Giants.
The first thing that must be eliminated is any flex in the control rods (assuming that we are addressing rods that work in a push-pull fashion). So as a general rule, rods longer than roughly 10 inches are not recommended.
On most of my airplanes these days, I'm able to place the servo close enough to the control surface to keep the rods very short—on the order of four inches or less, not including the connection ends.
The 2-56 rods are unacceptable on any aircraft larger than approximately six pounds. Very short (three to four inches or less) runs of 4-40 rod using a soft metal, like the stuff you get at the hobby store, will work fine on a big model if the geometry of the linkage does not allow any side load to be placed on the rod. I'll discuss side loads more later.
Never use all-thread for a control rod. All-thread is very brittle and quite susceptible to breakage from specific frequencies of vibration alone. The airplane doesn't even have to leave the ground to incur a failure!
You may get away with all-thread on one model, but you may not be as lucky on the next.
Similarly, I'm skeptical about homemade rods tapped from various types of welding rod. There are so many grades and types of welding rods available that getting the right temper to hold up in our situation is a gamble, especially for those of us without a metallurgical background.
High-grade, hardened metal is not suitable for use in a high-vibration environment. I've used small-diameter, carbon-fiber rods like the ones sold for Pattern and Formula 1 models with great success, but there are a few things to be aware of with these too.
Carbon-fiber rods are actually tubes made from unidirectional carbon fibers. The specific construction of the tubes gives them strength, but the ends are susceptible to splitting with even a slight side load unless certain precautions are taken.
To keep the ends from splitting, I use a 0.5-inch length of aluminum tubing fitted snuggly over the carbon tube at each end and cyanoacrylate-glued (CyA'd) into place. Neatly wrapped cord covered with epoxy a la a fishing rod will also do the trick.
Cutting carbon tubes can be a challenge with the problem of splitting at the cut during the process. A bit of masking tape around the area to be cut, a high-speed cutting wheel, and a particle mask works well for me.
Threading the tube for a short length of all-thread weakens the tube ends by cutting the very fibers as you cut the threads. One manufacturer has created titanium ends with barbs on one side and 4-40 threads on the other. The part is simply epoxied into place, and the barbs keep the end from pulling out.
Carbon rods are quite stiff and can be used at much longer lengths. Unsupported, I would not go much more than roughly eight inches.
Lately I’ve been using thick-walled aluminum tubing for my pushrods. The outside diameter is slightly larger than the carbon rods, and the inside hole is just the right size for tapping a 4-40 thread.
These tubes are very strong and stiff, yet the material is soft enough that vibration does not affect it. The assemblies are cut to length easily and tapped with a screwdriver-type handheld tap.
They are not as susceptible to side-load damage, they’re light, and they can be polished to a mirrorlike finish. A 1.5-inch stud is threaded in approximately halfway with a tiny, aluminum lock nut used to keep the whole thing in check.
For added safety, some Loctite® is a good idea. These rods can go 10 inches without a problem.
Let’s jump to the servo side of the equation, then we can tie the two together.
I do not recommend using the wholly inadequate nylon servo arms that are supplied with most brands of servos in a large Scale project. In fact, I wouldn’t recommend their use in any airplane that has a total flying weight of approximately nine pounds.
There are inexpensive, heavy-duty nylon arms available that will work fine as long as the rod connection is supported on both sides of the arm, as with a standard metal clevis.
I recommend against using a ball-link-type connection with these arms. Ball links are attached only to one side of the arm. As the arm moves, all the energy generated through the arm is converted to a twisting motion because the force is offset from the center of the pivot point.
The nylon material is not designed to resist this type of offset geometric arrangement.
The twisting motion also causes the side load, that I’ve been discussing, in pushrods, and your likelihood of failure is high with both of these forces present. At the very least, you will have a less-than-positive arrangement, which will lower the precision at which your aircraft will operate.
Although still geometrically imperfect, the popular aluminum arms you see on many of the big airplanes are strong enough that they won’t twist under the offset force created with a ball link.
As long as the arm doesn’t twist, no side load will be generated on the pushrod. I use aluminum arms with ball links most of the time.
If you’re using them on servos with metal gears, it’s a good idea to use removable Loctite® on the threads. I have seen many of them lose the screw and come off.
I also swap the Phillips screws included with the servo for the appropriate cap screw.
Nelson manufacturing offers a more correctly engineered system that consists of two plates that bolt to the stock nylon wheels included with your servos.
The ball link is bolted in the desired position between the two arms, thus supporting it equally from both sides. The force applied is geometrically correct.
As with most anything, the Nelson arms do have their drawbacks. For very large control throw, some of the smaller Nelson arms are limited in that the arm itself interferes with the pushrod or the ball link, but I have not seen this as a problem on the longer arms.
Also, with the metal-geared, high-torque servos, the stock nylon wheel that the Nelson arm uses for a base has been known to strip with extreme use. In some cases you can buy a metal wheel to use as a base, but that can get expensive.
If you insist that all aspects of your aircraft’s setup geometry are correct, the Nelson arms are the way to go.
I prefer to use ball links to connect servo arms to the pushrods. They connect using a 4-40 bolt, allowing a secure link.
Rocket City makes the best hardware I’ve seen for Giant Scale aircraft. The company specializes in very heavy-duty components that are suitable for the demands placed on the big models.
I use Rocket City’s hardware whenever applicable, but when using the ball links I pitch the screw and nut in favor of an Allen-head cap screw and an aluminum Nylock nut. These steps are in no way overbuilding.
A failure in a connection could be the demise of your aircraft. If you do decide to use more standard hardware, I suggest Sullivan clevises. They hold up very well and have a locking clip for added security.
Always use a jam nut on the pushrod to lock the clevis in place once final adjustments are made. This step also eliminates metal-to-metal vibration that could cause interference.
The most common control horn in use on the big models is also made by Rocket City. It’s called the Super Horn Swivel Clevis. This system uses an 8-32 or 10-32 soft steel bolt that passes through a hardwood dowel epoxied into your control surface for the actual control horn.
A fiberglass-filled nylon clevis threads onto the bolt for superfine adjustability. The ball nut uses a bushing instead of a ball, and bolts between the two prongs on the clevis. The bolt secures everything in place, allowing very little chance for a failure.
I like to sweat the small stuff. It makes the difference between a trouble-free aircraft and one that constantly needs maintenance!
If you’re just starting your first large Scale aircraft, it’s a good idea to look at as many systems as you can from people you trust and know to have trouble-free airplanes.
Good luck with your projects. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
