Stringers, formers and pull/pull systems

Model Airplane News, Feb 2002 by Yarrish, Gerry

In my December 2001 column, I described how to about properly splice wooden soars and longerons and how to use gusset plates to reinforce glue joints. Staying in the wooden-structure mindset, let's look at stringers and formers.

Unlike spars and longerons, which bear structural loads, stringers only support an aircraft's outer covering. They improve an airplane's looks and aerodynamic sleekness. They contribute very little to its structural strength so we should make them as light as possible while keeping them stiff enough to adequately support the covering. Stringers should be about 3 or 4 times as deep as they are thick. For a 1/3-scale model, I like to use 3/32x1/2-inch stringers. Also, for additional rigidity in fairly large models, you can use spruce instead of balsa.

Stringers are usually very long and are exposed to knocks and bumps; they should therefore be properly supported. You can use either balsa sheet or lite-ply formers for support, but fitting them so that the stringers are straight and parallel to one another can be a challenge.

SCALLOPED FORMERS

For a cleaner appearance, make sure that the formers don't touch the covering material anywhere. Only the stringers should touch the cloth. To accomplish this, cut away (scallop) the formers between the stringers. Do this before or after you've glued the stringers into their notches (see the illustrations for details). If you splice your stringer material together (see December 2001 "Thinking Big"), put the splices over the formers to strengthen the joint.

As the stringers reach the aft portion of the fuselage, their spacing will begin to get tighter and tighter. To prevent them from interfering with one another and with the underlying structures, you may have to taper their ends. Simply cut their undersides at an angle, but leave enough of each stringer to maintain the model's outer profile. A little cutting and whittling comes in handy when you have to make the stringers fit properly at the airplane's tail. Also, to prevent the fuselage from being bowed or warped out of shape, to balance the stresses, add the stringers a few at a time and then add a few on the opposite side of the fuselage. Once you've installed all the stringers, you'll be surprised how sturdy the fuselage will be.

PULL/PULL CABLES

A great way to save weight in the tail of an aircraft is to replace those heavy pushrods with pull/pull cables. This is frequently done for rudder control and is often done for all tail-control surfaces in scale WW I and early vintage airplanes. If it's to operate properly, the geometry of the cable control system must be balanced to ensure and provide equal travel in both directions. As in full-size aircraft, a model's pull/pull cable setup is a closed circuit and to maintain proper control, the entire system must remain taut. The cables will slacken if the servo-arm or tiller arm and the control-horn throws are not equal. When a control surface is deflected away from neutral, one side of the cable feeds more than the other side pulls and this can lead to unwanted flutter; by equalizing tension on both sides of the cable control system, you keep the control-surface movement very stiff.

One way to keep the tension equal is to keep the control-horn attachment points for the cables directly in line with the hinge centerline. If this is not possible, such as is often the case with a scale model aircraft, you can maintain the tension by making the tiller's arm geometry the same as the control horn's layout. If the hinge line at the control horn is 1/4 inch in front of the horn attachment points, make sure the tiller-arm pivot point is also 1/4-inch in front of the cable attachments. Either way, it is the geometry of the two connected control input arms that makes the difference (see illustrations).

CABLE ATTACHMENT

Another very important part of a pull/pull cable control system is how the cables are attached. For common, sport-flying models, you can simply hook the cables directly to the servo arm and let the servo act as the tiller arm. This works OK if you don't overtighten the cable tension. Too much tension will quickly wear the servo's output shaft and increase the servo's current drain. For large-scale models and for powerful aerobatic aircraft, it is best to install a separate tiller arm and drive the tiller with a short, stiff pushrod from the servo. In this setup you can also use two or more servos to drive the same control surface.

It is also important to have some way to adjust the cable tension between the control horns and the tiller arms. A simple way is to use a clevis and a threaded rod that has a hole drilled in it to accept the cable connection. For scale models, turnbuckles are preferred for a couple of reasons: it looks real, and you don't have to remove the cable from the control horn to adjust the tension.

It's important not to attach cables directly to metal control horns and tiller arms. You should use some sort of bushing material to isolate the attachment-cable loop from the metal control-horn's surface to prevent the metal from chafing and even cutting through the cable. For smaller models (1.20-powered and smaller), I use 0.018- to 0.020-inch stranded-steel cable (U-control leader); I simply loop it through the servo arm and swage a crimp tube onto the cable to secure it. For larger models, I use 0.032 Berkley nylon-coated steel fishing leader and metal clevises for all the attachment points. Berkley fishing leader is available at most deep-sea-fishing shops and can also be found on the Web. Short lengths of brass or copper tubes can be used to secure the cable loop ends, and they require a couple of crimps (using an electric crimping tool) to secure them. Don't use a flat pair of pliers, as it will flatten the tubes and can cause them to crack. I like to use a 3/8-inch-long tube with an inside diameter that's twice that of the cable I am crimping (1/16-inch-i.d. for a 1/32-inch cable). I then crimp it in the center of the tube and again on either side of the first crimp. You can add a few drops of thin CA to the tube if you like, but so far, I have not found this to be necessary.