Control System Construction
Control System Construction |
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Date: February 2004 |
Chapter: 12.0 Control Systems |
Section: Fuselage |
Time to hook up the elevators. This was a pretty straight-forward operation, all things considered, the only real wrinkle was getting the push-tube connected at the same time as our Strong Pitch Trim system (below). One end of the tube was drilled and and finished by the factory, with the other end needing to be cut, fitted with the rod end sleeve, drilled, and bolted. Then the rod end is inserted into the center of the elevator counterweights. Also, the slot the tube passes through in the keel cover wasn't big enough when I originally cut it, so that took some grinding.
Elevator Push Tube, Pilot side
Elevator Push Tube, Copilot side
Here is another situation where we've decided to stray from the manual. The factory system is a candy-cane shaped fiberglass spring system, with a linear actuator pushing against it to provide pressure relieve to control stick's pitch inputs. This spring is reported to be of mixed effectiveness, and of sketchy reliability, so we've elected to go with Alex Strong's pitch trim system instead. This system attaches to the bottom of the control stick instead of up on the canard, which opens up space behind the radio stack. In our case, this mounting slight impinged on the housing we'd built for our fuel shut-off in the keel, but we sanded the foam out of the exterior on top and reinforced this portion of the box, alleviating the interference. The strong system consists of a drive motor that acts through a lead screw, which acts through a pair of springs on either side of the control stick in an 'override' assembly.
To install this system, first we had to drill a matching hole in the Strong control stick insert the same diameter as the Velocity aileron attach bolt. Since this is slightly undersized, I wrapped the attach lever with masking tape until it was a snug fit in the control stick, then the stick and Strong system were leveled on the drill press and match drilled. Then the stick was re-installed into the keel, and the strong system was leveled (with the plane leveled) and a mounting hole was drilled into the keel 14 1/2" aft of the neutral control stick position. Also, I had to modify the Infinity stick grip, to have the wires exit through a grommetted hole on the front of the stick rather than through the bottom.
Strong Pitch Drive
Strong Pitch Trim System
Pitch Trim Laid Out on Keel, Note the Fuel Shutoff Box
Drilling Control Stick Attach Hole
Keel Mounting Bolt Hole
Modified Infinity Stick Grip
In out final checkouts, we found that we could not quite get full aileron travel due to the Strong pitch trim system bottoming out on the sides of the keel, so we relieved the sides and made covers. It turns out the trim spring body doesn't actually protrude, but it need the thickness of the keel.
Strong Pitch Trim Bumpout
This is a odds-and-ends kind of deal, and since I was going to be doing some glasswork, I pulled it out. There's nothing complicated here, just cut these two templates alike (stack them on top of each other so they're identical), and micro them to the miniature wing. Then glass a couple of tabs to mount it to, so the mini-wing is upside down, and then RTV this to the co-pilot side elevator after final painting. What this does is apply a loading to the elevator that keeps the nose from bobbing so much when disturbed at cruise speed. It's like the cylinder in your shock absorber system on your car, in that it dampens the response by providing some resistance. Since it's a aerodynamic feature, it is proportional to speed. Pretty clever, that, even if it looks silly and has a funnier name.
Mircoing the airfoil together
Foam is used to hold the unit square
Once we've primed the strainer, it's attached inboard on the co-pilot side with silicone caulk.
Sparrow Strainer Installed
Here we're rigging the rudder system. First we've installed an AN111 bushing at each rudder and swage the timble onto the cable and attach it to the rudder belcrank with an AN3 bolt. Next we deflect the rudders out 4" from the wingtip to determine the amount of cable needed at the wingroot to get full deflection. Then we swage a loop on the wingroot end 1/4" outside of the maximum deflection length.
Swaging loop onto rudder cable at wing root end
Next we potted a bracket into the wing root for the rudder return spring, and swage an AN111 onto the rudder pedal ends of the cables, and attached them to the pedals with a spring return attached to the aft canard tab. Next, we adjust the pedals so that max travel is within 1/4" of the canard bulkhead, install a stop where the full 4" travel is achieved, and adjust the brake push block to start applying the brake at 2 1/2" of pedal travel.
Rudder cable attached to pedal, rudder stop riveted to fuselage side
Return spring attached
Finally we install a pulley over the rudder exits at the firewall to lead the cable to the wing, with a cotter pin to keep it from jumping the pulley, and a timble swaged and attached to the adjuster block.
Rudder pulley installed
Rudder adjuster block installed
The first step of this section is to drill holes in the gear bulkhead and firewall and route the aileron push-pull cables through to the wing roots. This is easier said than done, and I spent the better part of an evening getting these fairly ridgid cables to pass through. Argh.
Aileron Push-Pull Cables at Firewall
Now that the engine test runs are complete and we've installed the wings, we've started rigging the ailerons. First the cables were covered with firesleeve and then the two halves of the aileron bracket were fitted to the wing. The wing was trimmed back to the line where cowling flange was created, and the bracket is trimmed to where it fits with the inside of the bearing 2" in from the cowling flange line. The torque tube was also trimmed back to butt up against the bearing. Both pieces of the bracket need to be trimmed to fin into the wing root.
Fitting the Bellcrank
Bracket assembled
Bellcrank fitted
Next, the bracket is attached with two screws top and bottom. I installed these with nutplates rather than with nuts and washers for ease of installation, and added tinnerman washers to protect the fiberglass. Then the cable bracket is mounted to the aileron bracket. This bracket extends forward to clamp the cable at a set angle, 90 degrees, to the bellcrank arm when in the neutral position. We found that we had to force more of our cable inside the cabin that seemed appropriate to this point. The cable is then clamped to the bracket. Then an access hole is drilled over the torque tube through the upper wing skin to allow a bolt to be driven through the torque tube and the bellcrank, and allow it to be tightened. Once this is all done, the rod ends are adjusted to get at least 2 3/8" of travel on the ailerons.
Cable Clamp and Bracket mounted
Cable Bracket, Clamp, and Cable Routing
Bolt Access Drilled and Bolt Inserted
Finished Assembly
A support is added between the control stick and the mid-bearing support, 2 1/2" behind the stick on the pilot's side of the keel. It's attached with 2 10-32 screws
Torque Tube Support
We've purchased the dual axis digital autopilot from Blue Mountain. This is a really cool system, in that it can not only hold a course and altitude, it can climb to an altitude, or at a prescribed rate, or follow a flight plan that you enter into the EFIS. This is a very interesting system, in that the EFIS communicates with the autopilot control box over standard Ethernet cabling, and not only can it follow a GPS flight plan, or a standard VOR navigation radio signal, but 'virtual VOR', where the EFIS calculates where the VOR signal will be, when it is close enough to be received, and plots that course via GPS until the NAV radio picks up the signal, and then it switches to follow that signal.
BMA Digital Autopilot
Pitch Servo Arm to Elevator Torque Tube
Roll Servo Push Tube
Autopilot Roll Servo
Elevator Torque Tube Primed
~ End of Chapter ~ End of Section ~