By Noel Drindak• Originally published in Stunt News, January/February 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.
A recent thread on the Stuka Stunt Forum addressed solid leadouts. A number of questions were raised that I wanted to answer—I’ve always used solid leadouts, and I think I have a good system. I decided to share my thoughts in this column and show how I do leadouts.
Of course there are two different types of leadouts. I’m not going to say that one is better than the other. They both have advantages and disadvantages. I will discuss both types of leadouts, but you’ll have to make your own decision on which to use. Personally, I prefer solid leadouts, but I’m probably in the minority.
Here are the leadouts at the wingtip of My Super Kestrel after five years of flying—round loops with no sharp corners help keep them from catching on stuff and getting bent.
– All mechanical cables are fabricated from individual wires that are helically twisted into a strand. Cable is specified as the number of strands in the cable by (x) the number of wires in each strand. Aircraft grade cables are most typically 1 x 7, 1 x 19, 7 x 7, or 7 x 19. Flexibility increases with the number of wires—1 x 7 is the least flexible, 7 x 19 is the most flexible.
– The cable most commonly used for leadouts in Precision Aerobatics models is 1 x 7 0.027-inch cable, usually Type 302 stainless steel. The breaking strength of this cable is about 90 pounds.
– The major advantage of cable leadouts is their flexibility—there’s no need to worry about the leadouts getting bent at the wing tip (definitely not the case for solid leadouts).
– One problem with cable leadouts is their abrasiveness. I’ve seen a plane lost after a cable leadout completely sawed its way through a Delrin bellcrank. The most common way of dealing with this problem is to run the cable through a C-shaped piece of copper tubing where it passes through the bellcrank.
– The major problem with cable leadouts is making the loops at the ends. One method of doing this is to wrap the termination with soft copper wire. Many modelers use this method successfully. However, the strength of the termination is highly dependent on the technique, skill, and experience of the person making it. In addition, doing the wrapping is a time-consuming process.
– Another method of terminating cable leadouts is by crimping a loop sleeve onto the cable. This is the way that cables are terminated in the aircraft industry. Our largest airplane, the C-5A Galaxy, uses control cables terminated with loop sleeves. Terminations made with standard loop sleeves generate the full breaking strength of the cable. They work well and they’re easy to make. The key is to use the correct loop sleeve for the cable being terminated, and to crimp it with the correct crimp tool.
– Crimp terminations have a bad reputation in the Stunt community. This is because almost no one has the correct loop sleeves and the correct tool for that sleeve. People make sleeves from whatever material they can find or they use something like an electrical splice. Their tools are just as varied including electrical crimpers and all manner of pliers.
– Although I don’t use crimped leadout cables, I do crimp my control lines, so I have a lot of experience crimping loop sleeves. Loop sleeves and crimp tools are readily available (although the tools aren’t cheap). If you want to learn more go to www.savacable.com.
Here’s a simple drawing showing two different triangular bellcrank loops. The longer triangle greatly reduces flexing (which can cause failure).
– Solid leadouts are made from music wire. Music wire is a high grade, uniform steel wire originally intended for strings in musical instruments. It is widely used in the manufacture of spiral springs and mandrels. Spring wire has excellent fatigue properties and is extremely strong. It is made in accordance with ASTM A228. The strength of music wire varies with the thickness of the wire.
– The minimum strength of 0.004-inch music wire is 439 ksi (a ksi is 1,000 pounds/square inch). For 0.250-inch music wire the strength is 230 ksi. The size we use for solid leadouts is 0.032 inch and its strength is 327 ksi. When we multiply the strength for our wire (in ksi) by the area (in square inches) we find that each leadout will withstand a pull of 263 pounds. That’s about three times as strong as the 0.027-inch cable leadout. Some would say that this extra strength is overkill. I like what my friend Bill Hummel says: “Never undersell overkill.”
– The major advantage of solid leadouts is their strength.
– The major disadvantage of solid leadouts is their lack of flexibility. I’m sure that most of us have accidentally bent a solid leadout at the wingtip of a model. If you have, you know that it’s next to impossible to straighten the leadout completely.
– Always, when making solid leadouts, avoid sharp bends. Sharp bends deform the wire, cause a decrease in strength, and become failure points. A good way to avoid sharp bends is to use round nose pliers. Also avoid designs with bends that get flexed repeatedly (like an acute triangle at the bellcrank).
It’s your choice.
Solid leadouts and cable leadouts—both have advantages and both have disadvantages. I’m not trying to sell either type—it’s your decision. However, I started out to show how I do solid leadouts, and I’m going to do that now.
This is how I do the bellcrank loops. The triangular loop is 0.047-inch music wire, crimped to the 0.032-inch leadout. The 90° bends in the wire ends add strength.
This is the Du-Bro Kwik Twist tool positioned to make the wrap on a loop. The leadout gets captured in a slot and the tool is rotated around the leadout. A tab on the tool makes the wrap.
I’ve been doing loops the same way for many years. I make small circular loops at the ends of the leadouts. They’re neat and unobtrusive—just large enough for a line clip. The photo shows the leadouts on my Super Kestrel.
I use a Kwik Twist tool from Du-Bro Products to make the loops on my leadouts. It’s specially designed to make loops on 0.032-inch music wire. The directions that come with the tool are pretty good, so I’m not going to give detailed instructions. I’ll just mention two places where I deviate from their directions.
1. I find their loop size a little small for my line connecters, so I make the initial bend around an appropriately sized rod.
2. Instead of breaking off the extra wire I use a Dremel cutoff wheel.
I make loops at the bellcrank that are triangular. The corners of the triangle are a concern because they flex when a pull is exerted. Years ago, Windy Urtnowski pointed out to me that longer triangles flex less. In accordance with his suggestion, I make the loops 3 inches long. Adding a bit of overkill (never undersell overkill), I make the loops of 0.047-inch music wire, attaching them to the 0.032-inch leadout with a crimp splice. Another reason for separate loops is that with a 62-inch wingspan, you don’t have enough wire in a 36-inch length to make 3-inch triangular loops.
One more thing about my leadouts—I do all the bending before putting the leadouts in the plane. I do it this way because it’s easier to make the bends with the leadouts on the bench. Also, if I mess something up I can do it over until it’s right. I make a line guide with a 1/4-inch hole at one end. After installing the bellcrank and leadouts, the wingtip and line guide go over the leadouts. It’s easy to fill the 1/4-inch hole afterwards.
Regardless of what your leadout choice is, I hope I’ve given you some information you can use.
Good luck with your next plane.
This article first appeared in the January/February 2009 issue of Stunt News and has been preserved as part of the PAMPA digital archive project.
