Creating Long Leads and Tow Plane CG

The ISSA tech columns are intended to answer questions for new entrants into scale gliders, with a focus on sailplanes in the three-meter to six/seven meter range. For each set-up question, there are usually a multitude of different methods, and no one has the monopoly on which is “best”. The objective here, however, is not to overwhelm anyone with multiple solutions to the same problem, but instead to offer solutions that are simple and effective.

Compared to other RC airplanes, sailplanes use very long leads. A few years ago, builders were relegated to cutting and soldering leads because commercially made “long” leads were not available. Some pilots also opted to make their own connectors. Either way, the process was both laborious and tedious. In this column we’ll look at alternative to soldering and splicing.

The inspiration for this column comes from a friend who recently acquired an EMS six-meter DG-800. The 800 is an unusual bird in that it has five servos in each wing – one for aileron, one for spoiler, one for water release, and two for flaps. The total number of servos is 14! With a project of this complexity, installation planning, simplicity, and efficiency are the cornerstones for finishing. With a simpler wing, the differences in time between competing building/finishing methods may be slight – with a more complex wing, it may be significant. Our goal is to minimize the time to completion while also making the wing (or fuselage) easily serviceable.

The aileron servo on the DG-800 sits way out on the wing, with one of the flap servos towards the end of the flap, and the remaining servos inbound, towards the root. On this particular installation, all the servo leads were cut, and wires spliced in, and a connector soldered to the lead exiting the wing. Each servo required six solder joints, and in total, there were 60 solder joints – ten servos times six joints each. That’s a heck of a lot work, and a lot of time was spent just preparing the leads. Now imagine one (or more) of the servos going bad – which one of them did – and how much time was need just for that servo to be replaced. Furthermore, because the leads were cut, the warranty on the new servo was voided. So there is both expense of a new servo and the time to replace it.

Now, it should be stated clearly that there is nothing inherently wrong with the method. It just eats up a lot of time. Some modelers may prefer soldering, and that’s a personal preference. What are the alternatives? The simplest alternative is to use long connectors, jumped together for the servos that are outboard on the wing. The advantage to using connectors is that it eliminates the need for soldering, doesn’t require modifying the servos, can be done in a short time. Jumpers are a little more expensive, since they rely on pre-made connectors, rather than cut-to-size three-wire lead. But in my opinion, the reliability of the connection, and more so the time saved is well worth it.

At least one company, Cermark Model Company, has both 36″ and 48″ leads, with different connectors, and in five different connector colors (black, red, yellow, blue, and green), and various lengths. In addition, the Cermark connectors are also gold plated, and use “heavy duty” 22 gauge wire. The long connectors can be jumped together, and even on a six-meter sailplane, the ailerons can be connected using only two leads for each wing.

An additional advantage is that because the Cermark connectors are color coded, the puzzle of what lead goes to what servo is simplified, and labels aren’t needed. On the 800, the two flap servos leads both exited the wing, and will require the use of a Y connector, either attached to the wing leads, or to the receiver. Another option would have been to use shorter leads and have the Y cord in the wing, thus only having a single lead exiting the wing. Think simplicity.

My own 6-meter ASH 26 is another good example. The wing of the 26 is a common set-up, using a single aileron servo, a single flap servo, and a spoiler servo. The aileron servo is a little less than 70″ from the wing root, the flap and aileron servo are about 30″. The aileron connectors are a 48″ mated to a 24″. Including the servo lead, a small length exits the wing, which then plugs into a short lead from the receiver. The spoiler and flap servos both use a 36″ lead and are tied together with the aileron lead. Color-coding makes hooking them up an idiot-proof exercise.

Planning is important. Whereas soldered connectors can be cut to length, using commercial connectors requires measuring the lead lengths before starting the installation. Once the leads are measured, check each connector joint to ensure that the wires are properly seated in the connector. Also, make sure that the three wires on the connector are properly oriented, as some connectors “go both ways”. Once the leads are made up, check the movement of the servo before running the wires through the wing. This is an especially important check if the servos are mounted permanently in the wing.

The single caveat with using connectors is ensuring that the leads do not separate. There are a couple of commercially available servo connector doo-dads that absorb the tension if the servo lead is pulled. Some pilots simply use dental floss to tie connectors together, and that also works just fine. The floss has the advantage of sometimes being easier to pull through tighter cavities, (wood wings) and it is very inexpensive. There are various methods of securing the floss; I use a small knot with a drop of CA. A couple of wraps of good tape also works.

One can also vary how this is applied, for example, using two 36″ leads and soldering the ends, creating a continuous 72″ or so lead. Again, it’s personal preference.

Tow Plane CG

There was a recent thread about optimizing the CG of a tow plane for aero-tow. The ISSA Pegasus with the Brison 5.8 started out extremely nose heavy, but towed everything just fine. The airplane was a little tough to land, as the nose dropped when the power came off, even with flaps extended. That had nothing to do with the design of the Pegasus, however, but is a common trait of any nose heavy power plane. Tony Elliot’s 3W 120 powered Peg, which I spent the day flying at Visalia, however, landed like a trainer – and didn’t have flaps. The difference was in the CG – Tony’s was much further rearward than the ISSA’s tug, and it also towed the big stuff just fine. When the ISSA Brison 5.8 was replaced with a Desert Aircraft DA-100, almost three pounds of engine weight was taken out of the nose. Immediately the airplane flew better, and landed very much like Tony’s. The CG was about 40% of the wing chord, (waaay back) but the airplane remained predictable and stable. At 40% we towed 4-meter “slippers” with no problem. But the larger sailplanes create far greater loads on the tow plane, and 40% is probably too far rearward for good tow stability, so we need to add some weight. (All the batteries are already forward). The consensus from the more experienced tow pilots is that a forward CG helps under tow. And we know from our experience that even a really forward CG tows fine, it just requires a little finesse when landing. So with those ideas in mind, we’ll move the current CG of the Pegasus forward while trying to retain good landing characteristics.

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