• Making Tee Joints

    MyCockpit ® Presents "Mikes Tips" by Mike Powell

    Making Tee Joints

    The ingenuity of the flight sim community never ceases to amaze. In this article, author Mike Powell demonstrates another use of a product that can be found in almost any garage and is nearly equally as popular. Its simplicity will leave you dumb-struck.

    When building a simulator with side-by-side seating, you will face the issue of linking rudder pedals. One approach is to adapt the methods taken by aircraft manufacturers. For example, the Cessna Skyhawk has a pair of parallel torque tubes mounted just above the cabin floor. The left pedals share one tube while the right pedals share the other. Each tube has a lever arm fastened to a cable which runs the length of the fuselage to the rudder. This approach can be used in a simulator but begs the questions of what materials to use and how to make those tee joints that join the pedal arms to the torque tube.

    A far as material for torque tubes and rudder pedal arms goes, thin wall electrical conduit (“EMT”) has a lot going for it. It’s available in just about all building supply stores; it comes in different sizes; and it’s cheap.

    As for as making those tee joints, welding works, but unless you’ve got the gear, it’s expensive and inconvenient. Fortunately, with a little care, epoxy can make a suitably strong tee joint.

    The first step is to prepare the end of the pedal arm to fit the side of the torque tube. These pictures show ¾” thin wall electrical conduit being used for both the arm and the torque tube. A 1” hole saw will leave a nicely curved end when used to drill through the conduit. (While the conduit has a nominal size of 0.75”, its outside diameter is actually 0.922”.)

    A little filing with a round file will dress the edges so the end fits against the side of the torque tube.

    Surface prep is crucial to creating a strong bond. Sandpaper with a 150 grit works well to clean and roughen gluing surfaces, specifically: the outside of the torque tube, and both the outer and inner surfaces of the pedal arm.

    “JB Quik” epoxy is one of many “filled” epoxies which work in this application. The filler thickens the epoxy and makes the joint stronger. Applying a lot of epoxy to the inner edge of the pedal arm, as well as to the curved end, will form a glue fillet inside the pedal arm to complement the outer fillet. While the epoxy doesn’t flow much, it does move a bit, so the work piece should be oriented with the arm pointing up as the epoxy sets. Any movement of the epoxy then serves to enhance the glue fillet inside the pedal arm.

    The joint at this point is strong, but not strong enough to hold up to a pair of enthusiastic feet. A short length of ¾” galvanized steel pipe hanger strap can provide a great deal of reinforcement for almost no money. This stuff is dirt cheap. (If you check the prices of potting soil, you’ll see pipe hanger strap is actually cheaper than dirt.)

    Cut and flatten a short length of the strap then roughen and clean it with sandpaper. (It’s much easiest to use the sandpaper on it before beginning to shape it.) Curve the ends so they will lie against the pedal arm side. Then bend the strap into a U shape so it will fit around the torque tube. Having all the forming and fitting done before mixing the epoxy makes the process go much more smoothly.

    With the reinforcing strap epoxied in place the joint is much stronger.

    If the joint were going to fail, it’s likely the failure would start at the ends of the reinforcing strap. Another piece of strap, wrapped and epoxied around the pedal arm and strap ends, makes this sort of failure much less likely.

    This is a very strong joint which should hold up well. However, if you anticipate overly enthusiastic side-ways kicks to the pedal arms, you can add triangular gussets to boost lateral strength. These gussets were cut from 1” by 1/8” steel bar and are something of an overkill. Smaller would be fine.

    A caveat: epoxies, even fast setting epoxies take tens of hours to approach full bond strength. “Testing” a glue joint early can permanently weaken the bond. It’s best to let things set undisturbed until the epoxy fully cures.

    Mike Powell, author of
    Building Recreational Flight Simulators,
    Building Simulated Aircraft Instrumentation, and
    Building Simulator Display Systems. (A work in progress)