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gerry-42-1I still remember the second airplane in my control line career with great amusement. Using the wing of the crashed Number One I built a profile model with some of my own 'ideas'. Without much knowledge of how to do it I had to construct the control system. This caused some problems. Either there was too much friction in the leadouts, the bellcrank jammed, or the pushrod fell out of the elevator horn. Every possible defect happened. This system was really perfect: the model never crashed. It just didn't get into the air!

This must have been a healing shock for me. With one exception, I've never had a control system fail again. Maybe I went to the other extreme with some design overkill on dimensions and thicknesses. In recent years I have been steadily but carefully reducing dimensions in order to save weight. So far I haven't experienced any weak spots. Actually, when some of the airplanes which I had sold were crashed a few years later by their new owners, the control system, when examined, showed no notable signs of wear. I don't claim that I've found the optimum solution. During the years I've seen quite a number of clever ideas, but everyone seems to stay with a certain way of doing things, a method that is familiar or one which suits his abilities. Here I'm going to describe the kind of control system I use in my 46 to 60 powered aerobatic airplanes. It’s a general description of what I have without going too much into the details about reasons and construction (that’s beyond the scope of this story). Shape and dimensions are sometimes dictated not by function only, but by construction techniques, the way the model will be used, and of course, by personal preference too - so, please stay cool when you read the following! Let's start with the

BELLCRANK
There are probably more construction methods than control line flyers. In construction articles I've often shown a nut soldered INTO the bellcrank itself. Since the bellcrank must be prevented from tilting and sliding, there must be some means to keep it in place. It's possible to use brass tubes and steel bolts but these have to be fixed, too. With my method it takes a little longer to cut the hexagon-shaped hole, but once the brass nut has been soldered into the bellcrank the rest is easy. The bearing bolt (it's a high quality steel bolt which has a cleaner thread and never wears out) fits into the bellcrank platform. By turning the bolt with the bellcrank already mounted, you can precisely set the vertical position of the bellcrank. I cut the bellcrank from 2mm. brass sheet; a 1.5mm. sheet would be just enough, but I like the additional thickness at the holes where the leadouts and pushrod are connected. To compensate for the weight I prefer to make the shape a little more slim, or I have drilled lightening holes (see FIG 1). The bearing of the brass nut on the steel holt is very clean, tight, runs freely yet without slop, and lasts forever. In order to reduce weight in my last airplanes I have used bellcranks cut from 2 mm glass fibre plate (carbon fibre is still better). However I hate the complicated installation process (FIG 2).

fig-1
fig-2

HORNS
I'm always on the look-out for a suitable control horn. So far I haven't found one on the market which I dare to trust. Flimsy sheet pieces soft soldered to thin wire, or nylon pieces shrunk on might be sufficient for 1/2A toys. That’s why I make my own. Brass horns are silver soldered to piano wire. A “Camping Gaz” or “Ronson” burner is a very practical tool for many applications around the house so I own one anyway. I'm very careful not to heat the wire more than necessary, or rigidity will suffer. Also, be sure to make the horn wire wide enough so you can bend the wire on a spot where it hasn't been heated so much. If the wire is bent too near to the horn it is prone to crack. As for bearings, I prefer to use brass tubes. Don't forget to slide the bearings on the wire before bending (I do this now and then!). If you forgot, don't ever try to straighten the wire to use again. Throw it away! You'll save your  airplane.

There are two ways to connect the flaps and the elevator. You can run two separate pushrods. For this system you’ll need a bellcrank with two holes at different distance from the pivot point. .The one driving the flaps is hooked to the smaller moment arm, the one controlling the elevator to the longer moment arm of the bellcrank. If however the airplane has a detachable wing we need a way to disconnect the elevator pushrod. This is usually done by driving the elevator from the flap horn. The desired control geometry calls for a special shape for this horn (FIG 3). I prefer the solution with two horns which allow for a wider choice of  deflection rates for flaps and elevator   -  independently from each other. The elevator horn even has a slot instead of holes. This way the deflection can be changed gradually.

fig-3

LEADOUTS
There are two kinds of lead outs: flexible or stiff. Many enthusiasts like flexible cable. I don't, I’m still using 0,5 or 0,6 mm piano wire. Maybe I just prefer the simple method of bending the end loops. The wire is simply bent around a pin of appropriate diameter  -  say 3 mm  -  , tightly bent back and around itself in a very tight wrapping. No solder, no knots, no nothing. Just make sure you never have a sharp bend, caused by sharp edged pliers. I've seen this method used on those much thinner team race lines, so it should be sufficient for our purpose. Of course, the inner (wing tip) ends can only be made after the controls are installed in the wing with the lead out guide installed and the wing tip shaped.  It helps when you try to position the wing so the leadouts can be clamped in the vice. Grandma's sofa cushions can do a good job here to support the wing or airplane!  I will not be responsible for cleaning bills incurred as a result of following this advice).

LINE GUIDE
There's not much to be said about this. I like to stiffen the innermost left rib with a plywood half rib which has a slot cut in it for the guide. This can be made of metal, plastic or wood. I make mine of brass sheet since metal slides best on the plywood. Two small pieces of brass tube and a nut are soldered to an oval brass sheet. Those brass tubes run in a slot in the plywood stiffener. The line guide should be kept as small as possible to avoid unnecessary weight. With rounded wing tips (as on wings of elliptical planform) the distance from the innermost rib to the actual wing tip can be quite large, and it can be a problem to reach the set screw. In this case the guide can be made with the adjusting nut fixed at a right angle to the guide tubes. The guide is then mounted to the wing tip sheet (this means the set screw runs vertically) and the adjusting slot is cut into a plywood stiffener which is glued to the wing tip sheet instead of the rib (FIG 4).

fig-4

PUSHROD
The pushrod is an item about which it is well worth while to think over twice  -    or even more. On many plans a piano wire of 2 mm thickness is specified. To put an end to any discussion: this is simply not enough! It might possibly suffice for up to 15 engines, and only when the pushrod isn’t bent on the way to the elevator. In any case use some kind of pushrod guide, like small holes in fuselage formers. My own method has developed via 2.5mm diameter piano wire ends in fibreglass rods or aluminium tubes. Aluminium pushrods are more rigid and weigh less. Nowadays the only accepted solution is carbon fibre tubes. A tube pushrod demands more work to build in comparison to a simple wire type, of course. However, for aerobatic airplanes intended for serious competition there’s no other way of making a stiff, yet light pushrod. Because all my latest airplanes have detachable wings and tails and my control horns allow  to change deflection ratio (by means of three holes in the flap horn) I use a spring loaded keeper on the ends of my pushrods.

Start by carefully measuring the length of the pushrod. If you use detachable pushrods, I'd suggest about 6cm wire length at each end (outside of the tube). With fixed ends, 2cm is enough for the hook-up. Now you can figure out the tube length. I have used 8mm diameter tubes with a 6mm inner diameter, but a smaller diameter will do just fine. Two short pieces of 6mm hardwood dowel about 2cm long are drilled with a 2mm hole lengthwise. The diameter of the pushrod wire is 2mm. All measuring should be done extremely carefully, particularly if the pushrod is to be detachable. With fixed ends, construction is much easier. Make a right angle bend and cut off the wire so that you can insert the angled end into the  tube. Drill a hole a little more than 2cm from the end of the tube. Slide the dowel on the wire and make the other bend (where the pushrod is hooked in). Now put some slow-drying epoxy into the tube, on the wire and on the dowel. Insert the wire into the tube (the angled end must protrude through that little hole). Slide the dowel on the wire into the tube until it stops at the wire bend. This bend locks the pushrod wire end to the tube. It cannot come off  (FIG 5). If a spring keeper is used, solder this to the wire end before you install this into the tube. Soldering after installation might soften the epoxy bond.  For adjustable length pushrods we use available thread inserts glued into the tube. The RC market offers steel rods with threaded ends which serve very well as wire ends. Several diameters are available:  M2, M2,5, and M3 threads. For safety reasons I prefer to install two inserts at one end (FIG 6).

fig-5

fig-6

I forgot to mention that all holes in the bellcrank and in the horns are bushed. Short pieces of brass tube are sufficient. The complete control system is then checked for free movement. The weight of the pushrod alone should be enough to deflect the control surfaces when the model is held vertically with the nose or the tail up. If the control system doesn't work that freely, don't  continue construction until it does!  Of course, now you might say that in the same  time it took you to build the control system, you could easily build another stunter. That’s right. Quality has its price - but it isn't too high if you consider that you've avoided one possible way of losing your airplane.

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