Flight Controls

This shows the rudder cable guides routed through the first fuselage bulkhead. Note that the right pilot rudder cable crosses through the center of the bulkhead, where it will be joined with the right copilot rudder cable, and later connected to the right side of the rudder itself. Similarly, the left copilot pilot rudder cable passes through the center of the bulkhead to be joined to the left pilot rudder cable, and later connected to the left side of the rudder.

The throttle/choke assembly and elevator trim assembly installed. The throttle is installed permanently with red Loctite 262.

I have installed the elevator trim temporarily until I ask the factory how the trim cable housings are secured to the ends of the trim assembly. I also discovered that I need to shorten the threaded stud on the trim knob. It now bottoms out in the slider before it clamps down on the washer to hold the trim at the desired position. Once I have these things sorted out and the assembly mounted permenantly with Loctite, I will grease the slider assembly with white lithium grease.

I received an answer from Tomaz, explaining that the elevator trim cable outer tubing does indeed fit loosely into the fittings at either end of the cable. One fitting is on the trim adjustment slider mechanism and the other end is at the elevator trim spring assembly on the fuselage floor behind the cockpit.

The terminology is Bowden cable, meaning a flexible cable running inside an outer tubing sleeve, sometimes used for push-pull operations, but always in tension for this particular application.

When the trim cable is attached at both ends and pulled tight, the Bowden outer tubing is pulled into the fittings at each end by the tension on the trim cable, so that it cannot escape once assembled. So, all is normal at this point.

I'm going to wait and install the elevator trim permanently with Loctite until I get the complete elevator trim mechanism installed behind the cockpit, to ensure that it all hooks up OK. Then I'll put Loctite on the mounting screws and grease the slider.

After study of the rudder pedal and brake line installation, and examination of various pictures in the kit manual and the supplemental photo album, I determined where to cut a slot in the right side of the fuselage for the right brake line, shown here. The brake line from the brake master cylinder for the right wheel brake passes through this slot, which allows for rudder pedal position adjustment. The brake line is protected from chafing by a length of 12mm heavy fuel line that goes over the brake line from the master cylinder through the slot. I drilled a few large holes to start the slot, then finished the job with a small sanding drum on my rotary tool.

I then installed the rudder pedal assemblies permanently with Loctite 262. Note that the pedals on the left have the brake fluid reservoirs.

Next I installed the velcro strips for the seats and carpet, described on the Fuselage Interior page

This shows the cable feeding through the S-curved tubing on the rudder pedals. This arrangement allows the cable to run through the tube when the pedals are adjusted forward and back to fit the pilot and copilot. It is a very ingenious design that has been around for decades, and is commonly seen on sailplanes.

I moved the fuselage part of the way out of the garage to get access to the aft end of the fuselage and vertical stailizer. Now that it's on the wheels, that was easy. The first challenge was connecting the main fore-aft elevator push rod to the bell crank assembly just forward of the rudder. The aft end of the push rod is slotted, and there is a bolt that goes through the push rod and captures the bell crank in the slot. The tricky part is that two 1 mm washers also go in the slot, over the bolt and between the sides of the slot and the bell crank arm.

After fumbling around a bit, convincing myself very quickly that it was impossible to accomplish this with fingers or ordinary tools, a few minutes thought brought to mind a quick temporary tool shown here. It's just a short piece of 14 gauge copper wire that I had laying around the work shop, bent into the shape you see, and pounded flat with a hammer, so that it's about the same thickness as the washer. The technique I used then was to insert the bolt, slip the curved wire into the slot under the bolt, and drop the washer into the slot. The washer is now resting on the wire and the side of the bolt. Then I backed the bolt out until the washer dropped into place (constrained by the wire) and pushed the bolt through it. I then repeated the process in the slot on the other side of the bell crank. Maybe there's an easier way to do this, but it worked with materials that I had on hand.

Then I installed the outer washer and nylon lock nut, secured with Loctite 262.

In the photo the upper elevator push rod is installed temporarily until I get confirmation on the construction sequence. The top of the push rod goes through a hole in another rudder bulkhead, through which it appears that I will also have to reach to install some nuts attaching the upper rudder hinge bracket. I'm thinking that I will need to get the elevator push rod out of the way to make that installation less troublesome.

I installed the lower rudder support bracket, with Loctite on the attach bolts, which screw into nut plates on the other side of the bulkhead. I installed the rudder stop threaded studs and nylon lock nuts. Finally, I installed the rudder control arm on the lower support bracket, after greasing the brass bushing, using Loctite on the nut. I marked all nut and bolt heads with red paint.

I removed a bit of the inside lip of the fuselage side at the rear to clear the rudder control arm and the rudder cable thimbles. I greased the thimbles, inserted the 5 mm bushings into the control arm with the thimbles, and installed a bolt, washer and nyloc nut to secure each cable, using Loctite 262.

I then devised a way to secure the rudder control arm with a couple of scraps of wood and a c-clamp, to keep the arm from moving while hooking up the rudder cables inside the fuselage.

Then form a loop in the end of the cable by feeding the end back through the thimbles.

Paying attention to the mark on the cable for the thimble position, pull the loop tight around a thimble and slide one of the swage collars up to the thimble.

Take one bolt out of the swage tool, slip it around the cable, reinsert the bolt and snug up both bolts enough to clamp the collar and begin to put some friction on the cable, but not so tight as to begin compressing it onto the cable. Pull the cable tight and ensure that the collar is pushed up close to the thimble, with the cable tight around the thimble. Check that the collar is centered in the tool.

Now tighten the swage tool until the two halves are in contact with each other. This ensures that the collar is fully compressed and will hold the cable securely.

Then cut off the excess cable end with a cold chisel, shown here. To do this, I put a pillow in the rear fuselage to absorb the shock, and put my bench vise on the pillow. The pillow here is a contour foam material that does a great job of absorbing the shock cleanly with no bouncing around. After carefully positioning the cold chisel to cut only the free end of the cable, one or two sharp blows with a hammer will cut the cable cleanly. Actually, the heavy vise absorbs the initial shock and the pillow then distributes the momentum over a large area so as not to damage anything. It's like the trick where someone holds a slab of concrete on their chest while someone else breaks the concrete with a sledge hammer -- not magic, just physics.

Finally, I removed the cable slack by tightening the nuts on the front side of the firewall, removed all the clamps and checked the system for full pedal travel.

The cable adjustment doesn't achieve real tension in the cables -- tension is achieved when the pilot steps on the rudder pedals. When your feet are removed, the rudder pedal return springs take up the slack and keep the pedals from falling backward. The cable length adjustment at the firewall determines the fore-aft resting position of the pedals. I asked Tomaz about the proper position for the rudder pedals. He instructed me to tighten the cables until the tops of the pedals are inclined back about 5 mm from the point where the pedals are pendicular to the guide tubes.

After attaching the trim springs, I positioned the cables with the trim knob centered in the cockpit, and the trim lever also centered, ensuring that the outer bowden housings were fully seated in the fittings at each end. Then, with the cables pulled tight, I tightened the screw on top of the lever, using Loctite 262, securing the trim cables to the mechanism. I then applied three layers of heat shrink tubing to the end of each cable about three inches from the attach point, to keep the ends of the cable from fraying. After that, I cut the excess cable with my cold chisel, as described above for the rudder cables.

I then removed the bolts holding the trim slider mechanism in the cockpit, applied Loctite 262 and tightened them again for permanent installation. Finally, I applied grease to the interior of the slider mechanism and covered the slot with tape to keep dust and dirt out during the remainder of the construction.

At this point, the trim knob still will not clamp fully down on the slider, with the result that the slider moves fore and aft when the elevator push rod is moved. The kit manual clearly indicates that the slider is supposed to lock in position when the knob is tightened. I need to get an explanation from Pipistrel on how this is supposed to work. The trim knob seems to be used to both clamp the ends of the cables within the slider, as well as keep the slider from moving after the trim is adjusted. But I don't see how it can do both at the same time. There is something I don't understand here, but I'm sure Tomaz can straighten me out.

Here is the key piece of information that I needed. You can see in this end view of the trim slider that the hole through which the trim cables pass is eccentric. The slider is supposed to be installed with this hole toward the bottom. The trim knob can then screw far enough into the slider to clamp the washer against the outer tube and lock the trim setting. The trim cables extend all the way through the slider from each end, and have small beads formed on the ends that prevent them from pulling back through the slider, so nothing else is needed to retain the cables in the slider once tension is set in the trim control loop.

Today I was able to take a closer look at my trim slider, and fix the problem. First, the slider was inverted, so that the cables passed through the slider toward the top, rather than the bottom. This is what made the trim knob tighten against the cables before it clamped the brass washer against the tube to lock the trim setting.

Part of what confused me was that after I first installed the slider mechanism, I could see that the cables passed through the slider and extended beyond the ends by a few centimeters. They slid back and forth through the slider easily, so I assumed that the threaded shaft was what locked them in place. I never thought to check if the cables would be contained by the slider itself if I just pulled them tightly from the ends.

So, after rotating the slider 180 degrees within the tube so that the cables pass through the slider on the bottom, and then reinserting the trim knob from the other side of the slider, the knob locks the slider in position as intended. I could then pull the trim cables up tight into the slider, where they remained fixed once the slack was removed. After that, it was simple to loosen the cable clamp on the trim spring lever arm and pull the resultant slack out of the trim control loop. With that done, the trim works normally.

While reviewing the photos from Pipistrel, I noticed that the trim cable bowden housings should be routed through the center fuselage bulkhead, not draped over the edge. This will also be necessary to accommodate the baggage compartment hard enclosure that I intend to install later. So, I disconnected the cables from the trim spring mechanism, drilled 6 mm holes through the bulkhead, and routed the bowden housings through the holes. After that, I reconnected and re-tightened the trim cables, using Loctite 262 on the cable clamp. I then replaced the heat shrink tubing that I had to remove from the ends of the cables when I disconnected and rerouted them.

After studying the manual and the components, it was fairly clear how things are supposed to go together. First, I used my flex-shaft angle drill to drill a drain hold in the fuselage floor at the low point just forward of the landing gear strut.

I inserted the central control rod through the cockpit tunnel from the rear and bolted the supporting front rod end to the carbon support member between the control stick openings. I then bolted the aft end of the control rod to the flaperon handle assembly. I used blue Loctite 243 on all the connections in this section. I also marked all the nuts and bolt heads with red paint as I installed them.

The control stick support tube goes left-right under the cockpit floor. It is mounted at each end by a spherical bearing in a flange assembly. The holes for the flange assembly on the right side were marked faintly in the cockpit floor gel coat, but not on the left. The mounting flanges have to be removed from the support tube in order to slip it under the floor through the control stick openings. By mounting the spherical bearing on the right side, assembling the support tube to the bearings and taking some careful measurements, I was able to locate where the holes for the left mount were to be drilled.

I also had to relocate the fuel lines to keep them away from the control linkage.

There is no play allowed between the support tube and the mounting flanges, so if the holes are off even slightly, there will be some force applied to the spherical bearings when the mounting screws are tightened, making the controls stiff. I got my holes just about exactly right, but when I tightened the screws, I still got some binding, as the kit manual predicted. I discovered that the problem was that the left mounting flange was interfering with the radius between the cockpit floor and the fuselage side wall, so I ground off a bit of material from the end of the mounting flange.

Also as the kit manual indicated, I had to remove some material from the edge of the left control stick opening because of interference with the control stop.

I then attached the threaded rod end bearing to the elevator push rod and the arm on the control stick mounting tube. I followed the suggestion in the kit manual to back off the stop nut three turns from the end of the threads and tightened the connection, after applying Loctite 243. I then marked the nuts with red paint.

I mounted one stick temporarily to check movement, but did not connect the linkage, since the sticks will be in the way of a lot of remaining work in the cockpit area.

These steps don't seem like they should take five hours to complete, and I could do it faster the next time, but I needed to install and remove the parts multiple times to get the mounting holes located correctly, adjust the clearances and complete the assembly. It is also very tight working under the cockpit floor, mostly by feel, with very limited room to get the nuts and bolts started and swing a hex wrench. The camera shots make it look like there is plenty of room, but they were also taken by feel, since with your hand reaching through the opening the view is blocked so you can't see very much.

Mounting and adjusting the control sticks requires that the flaperon pushrods first be jigged at the neutral position. The kit manual describe first installing the torque tubes, connecting them to the vertical pushrods, then jigging them in position. I found, however, that by delaying the torque tube installation and unscrewing the rod end bearings to nearly full length, the top bearings could be extended until they just cleared the fuselage top, where I used a 6 mm drill bit to jig them at that position.

This allowed me to install the control sticks and adjust the linkage until they were vertical with the aileron linkage fixed at the neutral position.

Three important points about the control stick linkage: (1) The two push rods that attach to the bottom of the control sticks are different lengths. The shorter push rod goes to the left stick and the longer to the right. (2) The spacers that go on the bolt at the bottom of the stick where the push rods attach are also different lengths. On the left stick the longer spacer goes to the back. On the right stick the longer spacer goes to the front. (3) The left push rod attaches to the front fitting on the center torque tube and the right push rod attaches to the back fitting on the center torque tube.

After mounting the control stick on the cross tube, the push rods are connected to the bottom of the stick. I used a suggestion from George Powers for that step. I cut a short length of 1/16" scrap wood to a width that made it a friction fit through the spacers and rod end bearing, cut to length so that it fits entirely within the assembly. The piece of wood then held the spacers in position while I slipped the rod end bearing between the legs on the bottom of the control stick. Pay close attention to the kit manual instructions for connecting the push rods to the left and right control sticks, and for positioning the spacers, which are different lengths. It will be necessary to disconnect the push rods and adjust the rod end bearings a few times to get the sticks vertical while the flaperon push rods remain jigged.

After the control sticks are connected and vertical, tighten the lock nuts on the control stick push rods. When you do this, take care to rotate the rod end bearings so that they allow the rod to twist freely on the spherical bearings throughout the full range of motion of the control stick.

Next I mounted the flaperon torque tubes. They are secured to the support bearing at the center of the fuselage. A bolt goes through end plate of one torque tube, then through the center support bearing. The end plate of the other torque tube goes over the bolt end and a special nut is screwed onto the bolt to hold everything together. The trick is holding the bolt in place with a 22" long 5 mm allen wrench through a torque tube from one side while screwing on the special nut with another 22" long 4 mm allen wrench through the other torque tube. I found that a 1/4" drive socket extension fits inside the torque tubes and I was able to find some metric allen bits that fit a 5/16" socket. By fitting the socket extensions together I was able to reach in from one end of the torque tube assembly to screw on the nut while my wife held the bolt in place from the other end. I used some instant glue on the joints of the extensions and sockets to that they wouldn't come apart inside the torque tubes, and I held the bolt and nut on the allen bits with some plumber's putty so they would not fall off while maneuvering them into place.

I found these socket head allen wrenches and removed the bits, which fit a 5/16" socket

Next I removed the drill bit that was jigging the flaperon push rods and screwed the rod ends into the push rods until a few threads showed. I then attached the top ends of the push rods to the flaperon torque tubes. Final adjustment of the push rod lengths will come after the wings are installed.