Understanding the System
The flap control system uses a handle to pull on a cable, which, in turn, pulls the flaps down. Springs in the wings hold the flaps up, when sitting on the ground. The primary flight controls are pull-pull systems, the flaps are pull only. The flap setting is held by notches in a quadrant on the cabin floor.
Assembling the Flap Handle
Study the assembly shown on plans page 28. Note that a spring inside the handle pushes a pin into notches in the notched quadrant. The spring is a type commonly available at hardware stores. Buy one that is quite a bit too long because you can fine tune the button’s resistance by cutting coils off the spring.
The Quadrant
The half-moon shaped bracket with the notches in it is attached at the top and the bottom with bolts. It slides through a slot cut in the handle. Be sure to install the spring before assembling these two parts.
Routing the Cable
The cable that activates the flap runs aft through the far right (looking forward) pulley of the three pulleys underneath the fuselage, on the Bearhawk 4-place (on the Bearhawk Five, the cable runs in the far left pulley). The cable continues to the back of the baggage compartment where it turns upward through the pulley located there. As it turns up parallel to the back of the baggage compartment, it attaches to the bottom point of a triangular piece of 1/8” steel (supplied). A turnbuckle is attached to each of the upper corners with a cable going to pulleys located in the upper, rear corners of the baggage compartment. Make sure you are using forked turnbuckles that go on each side of that triangular plate. The vertical position of this triangle plate is not trivial, and moving the plate may lead to undesirable flap rigging. This is an area where it’s especially important to stick to the plans. When the flaps are fully down, the 1/8″ steel triangle should be as close as possible to the floor-level pulley.
If you are working on these parts without the wings attached to the fuselage, you can run your cables through the upper pulleys, but can proceed no further until installing the wings. Leave the cables long enough that you can trim them and install thimbles when hooking up to the flap actuation arms. Run them forward to the rear spar, add twelve inches, trim and then coil them up and tape them out of the way until you’re ready to work with them.
When it comes time to set up and adjust the flaps, start by installing the return springs in the wings. The return springs are an important part of that assembly because they provide the tension necessary to pull the cable system tight so you can see if everything is adjusted properly.
Connecting the Cable at the Wing
Once the wings are on, and the flaps are on, it is time to drill the inboard arms of the flap torque tubes. At this end of the cable farthest from the flap handle, the geometry of the lever on the inboard side of the flap torque tube is significant. The drawing on the plans shows the angle of the flap lever when the flap is all the way up. You can measure the angle of the lever shown in the drawing in relation to the bottom wing skin. In this area the main thing to watch out for is interference of the flap cable or lever with the wing rear spar plates. It is a tight fit. This is the place to use larger (1/4”) cable shackles. Sometimes this alone will solve the problem and eliminate any interference. The long flap torque tube can also be moved inboard or outboard a little (before drilling the flap arms), but the cable alignment in the closest pulley must also be taken into consideration.
A few builders have used two little strips of .062 steel to connect the cable to the flap lever when interference was a problem. Two little strips of 4130 of 3/8” – 1/2” wide x 1.5”-2” long. The cable fit into this closest pulley is worth a comment. When the flaps are all the way up – normally the cable will be on one side of the pulley. As the flaps are lowered, the cable centers in the pulley. Then when the flaps are all the way down – the cable is on the OTHER side of the pulley compared to when the flaps are up. This is how Bob designed it and has worked well.