At this point in the build I'm installing boxes and getting ready to begin wiring. The last two servos to install before beginning the wiring are the roll trim and the pitch servo.
The roll trim servo allows one to compensate for lateral imbalance. This could be caused by uneven fuel use between the two fuel tanks which are located in the wings or even just flying without a passenger. In the extreme, one tank completely empty and one full, the roll torque applied to the aircraft would be on the order of 21 gallons x 6lbs x 4' = 504lbs! That's quite a lot of force trying to roll the airplane. Those interested will note that aviation fuel weighs 6lbs per gallon and the 4' figure is just an estimate on how far it is from the center line of the fuselage to the center of the fuel tank. Because the passenger sits so close to the center line of the aircraft he makes much less of a impact to the balance: A 200lb passenger x 1' from the center is only 200lbs roll force applied.
Whatever the cause of an imbalance, the roll trim servo is designed to apply and opposing force by moving the ailerons in the opposite direction. Fortunately for the servo, it doesn't take that much force to move the ailerons -- very little force at all. Partly because of mechanical advantage in the linkage, and partly because of aerodynamic forces acting on the ailerons.
The roll trim servo allows one to compensate for lateral imbalance. This could be caused by uneven fuel use between the two fuel tanks which are located in the wings or even just flying without a passenger. In the extreme, one tank completely empty and one full, the roll torque applied to the aircraft would be on the order of 21 gallons x 6lbs x 4' = 504lbs! That's quite a lot of force trying to roll the airplane. Those interested will note that aviation fuel weighs 6lbs per gallon and the 4' figure is just an estimate on how far it is from the center line of the fuselage to the center of the fuel tank. Because the passenger sits so close to the center line of the aircraft he makes much less of a impact to the balance: A 200lb passenger x 1' from the center is only 200lbs roll force applied.
Whatever the cause of an imbalance, the roll trim servo is designed to apply and opposing force by moving the ailerons in the opposite direction. Fortunately for the servo, it doesn't take that much force to move the ailerons -- very little force at all. Partly because of mechanical advantage in the linkage, and partly because of aerodynamic forces acting on the ailerons.
Because not much effort is required to trim the airplane, the roll trim servo (pictured above) is not so much larger than a large RC model servo.
The next servo to be installed is the pitch servo which is controlled by the autopilot. The pitch servo moves the elevator via a connection to the elevator push tube.
The control arm length is adjusted such that the servo is at its center position when the elevator is in its neutral position.
The pitch servo is installed behind the baggage compartment. Pictured below is the servo and the push-tube that connects the elevator to the control column via the elevator bell crank.
With the pitch servo installed the next step is to attach it to the bell crank with the control arm.
Now the wiring begins. Having the fuselage on the rotisserie makes accessing the interior a lot easier.
Pulling all the wires is a pretty big job which is made more difficult by the very small spaces that must be accessed.
At this point in the build I found it necessary to stop pulling wires momentarily to step back and complete the airplane's overall electrical schematic. Although proceeding without a schematic is much faster, it will inevitably lead to me leaving out some necessary circuit.
And so, a week or two elapses without much apparent progress as I pound out the schematic in my office. Its a good time to think through how the various systems tie together. Fortunately, I don't have to figure out everything myself as I bought some of the harnesses for the Garmin boxes from Stein Air to save some time.
But a lot of the wiring goes to the panel which has yet to be designed. I can't finish the schematic until I finish the panel designed. Next time: It's panel time.
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