Control Surface Installation

Sept 8, 2006

Hinging the rudder

Moving back to the tail again, it's time to start hinging the tail feathers. Starting with the vertical fin and rudder, I determine the position of the four Robart hinge points running from top to bottom on the fin. Using the Robart hinge alignment tool, I drilled the holes just big enough to fit the hinges deeply into the fin and rudder. The hinge points get wider at the joint, so a knife is used to widen and "square off" the hole near the surface so that the hinges can recess fully into the holes.

The objective is to get the LE of the rudder as deeply into the recession in the back of the fin as possible, and get the hinge joint to be right at the trailing surface of the fin and leading surface of the rudder. With the rudder temporarily hinged in place, I'll wait until after all glassing and painting is done before I'll do the final gluing of the hinge points.

Rudder hardpoints

The rudder will need two hardpoints installed to facilitate the installation of the rudder control horns and steering horns. First I cut out a nearly rectangular hole in the bottom of the rudder, with the hole depth reaching through only to the sheeting on the opposite side.

I cut a hardpoint block from plywood and added a 1/16" balsa skin to the top side. This block has a hole drilled and tapped for a 6-32 steering bolt. With all fitting well, the block is glued in place with Gorilla glue and sanded flush after drying.

Next, a 7/8" circular hole is cut all the way through the rudder just above the first hardpoint. I then used a thicker piece of ply cut in a 7/8" circle to plug the hole. You can also use a dowel rod, but I didn't have one the right diameter handy... ply will work just as well.

The plywood control horn hardpoint is also glued into position with Gorilla glue and sanded flush. I sand these as close to finished thickness as possible before gluing in place since it is hard to sand hard wood and balsa to the same depth at the same time. Later this will get drilled for an 8-32 control horn bolt.

Closing up the elevator

Back on the elevator, I needed to cut them to final finished lengths and close up the inboard ends. After cutting the right amount off with a scroll saw, I go ahead and hinge them to the stabs using the same techniques as the rudder above. With the elevator hinged, I can fine tune the lengths of them and determine how much balsa I need on the ends to close them up.

I ended gluing some 1/8" balsa onto the inboard ends of the two elevators, then sanding them to finished shape. After putting them back on to the stabs, I could figure out where to run my pushrods and just where the elevator hardpoints for the control horns would need to be. I also determined the shape and size that the horns would need to be to provide the best mechanical advantage and alignment to the hinge joint.

Custom elevator horns

I prefer not to just use bolts for my control surface horns, and want to get the clevise joints to line up directly over the hinge joints. So, I decided to try out an experiment in fabricating my own custom control horns for the remaining surfaces.

To do this, I used a 1/16" thick piece of aircraft ply, sandwiching it between two thin pieces of my G-10 composite material. This lamination of wood and composite is very strong and light. I sketched out the shape that would give me good geometry for control linkage and cut the horns out of the laminated material. I then drilled holes for the clevise attachments, and also some glue transfer holes going through the tabs at the bottom. This allows the epoxy to seep through the holes and provide a better glue joint when I glue the tabs into the hardpoints (like pinning a hinge).

Next, I cut out a rectangular area in each elevator, all the way down to the balsa sheeting on the opposite side. After making rectangular hardpoint blocks from hardwood, I drilled out slots in each hardpoint to glue the control horn tabs into. Once the fit and alignment was checked and fine-tuned, I permanently glued the hardpoints into the elevators using Gorilla glue, then sanded flush. The control horns will be glued in later after all glassing and painting is done.

Tail feather linkage Next I figured out where I'd run my elevator pushrod and rudder pull-pull exits at. Obviously, I know where my control surface hardpoints are, and also had figured out where the rudder and elevator servos were going to be mounted. So, it was a matter of determining where the lines between the two would exit out the side of the fuselage. Taking a look at photos of other JDM 190 projects helped tremendously in double-checking what I had come up with. I used Sullivan heavy duty 1/4 scale nyrods for elevator pushrods, which exit about an inch below the stabs and about an inch and a half back from the LE. Using a long drill bit and file, I feathered the hole back through the sides of the fuse towards each of their respective servo locations. For the rudder, I'm using Dubro pull-pull system (with coated wire). These have really nice control horn bolts with great mounting features. The plastic tabs that thread onto the bolts are quite long, so I just re-drill the clevis hole about halfway back on the tab, then cut it to length and round the end back off. With a hardpoint hole drilled and tapped for the 8-32 Sullivan bolts, I could then position the plastic tab and clevise components such that the hinge joint of the clevise and the hinge joint of the rudder lined up well to one another. I then drilled out the exit hole for the pull-pull wires which is slightly below the pushrod exit line and about three inches behind it (pretty much right under the elevator hinge line). Next, I made some align & support tabs from AC ply and balsa tri-stock. These get glued inside the fuselage at a couple points so that I could run the pushrod sleeves through them and keep them in place. It also keeps the rods from flexing under load.
Tail servos With the servo and linkage locations already determined, I needed now to actually mount the three tail servos. This brings me to an important point... My goal throughout the interior installation is to make sure that all components are easily removable, including any trays or rails that they mount to. I want to be sure that I can easily "get to" any component, including back into the cockpit and rear fuse area for future maintenance. So permanently gluing in rails or trays is not an option. For this reason, you'll find that my method is to glue mounting blocks to the inside walls of the fuse, then have trays and/or rails that screw to these blocks. This makes for fast and easy removal and gives me wide open space to work in whenever it is required.
With that being said, you'll see the photos above demonstrate the plywood rails that I have glued to the left and right inside walls of the fuse for mounting the servo tray. The 1/8" AC ply servo tray is then cut out for the three servos and installed on these two rails using four screws. Alignment to the pushrods is then checked out and additional alignment tabs are again glued inside the fuse for the pushrods to slide through. This pretty much wraps up the tail surface installation for now. I'll do the actual linkage installation later after the plane is finished. There I will cover all connections and adjustments.

Aileron hinging and horns

Well, I think you've got the idea now as far as how I go about the hinging and control horns. The ailerons were treated the same way. Hinging was done with five Robart hinge points distributed evenly across each aileron. Hardpoints were marked, cut out and then plugged with slotted hardwood blocks.

Custom horns were again made from a laminated G-10 and AC ply. The harpoint blocks were again glued in with Gorilla glue and sanded flush. The custom horns will be permanently epoxied in after glassing and painting.

"tanks and throttle"