Step 6.7, Aileron bellcrank

A bellcrank is a mechanical contrivance the changes the direction of travel in some kind of rigid linkage.  It is analogous to how a pulley can change the direction of travel for a rope.  Bellcranks are often used in aircraft to link the various push tubes that transmit the force from the control stick in the cockpit to the control surfaces on the wings or empennage.  In the RV series of aircraft, each wing contains a bellcrank the turns the linkage connecting the ailerons.

So this is a bellcrank, I realize it would be pretty much impossible to deduce its function from this photo, but I didn't stop to take many photos during the assembly, so here it is.  What I am actually working on though, is a part of the autopilot.  The aileron roll servo control arm.  In the photo above, the aluminum tube with the ball-end bearings allows the roll servo to move the bellcrank.  Which is fascinating of course, but just how does the autopilot control an airplane? 

Perhaps you are thinking that an autopilot is just a computer and so is my Iphone.  And further, you are 85% certain that your phone can't change the direction of an airplane.  True enough.  But an Iphone could make an effective autopilot if it could transmit its computed flying solution via servos to the airplane's control surfaces.  A servo is just a special motor that can convert a number from a computer, into a force that can move the aileron.

In the right wing, the aileron servo is attached by  the control arm to the servo.  The installation manual specifies 5.00" as the nominal length for the adjustable arm.   As you can see, I've clearly blown it as the length measured 5.002".  Oh well, I guess its not too late to start over on the airplane.

And at the lower right is the servo itself.  It looks pretty much like a servo for a RC model airplane, this is just a bit larger and a lot more expensive.

And here is the bellcrank and roll servo assembly.  The push tubes are not yet attached, but the servo control arm is clearly visible in the forefront.  In this view, the push tube from the cockpit would come in from the left and the push tube connecting the aileron would exit at the bottom.

There is still some plumbing to run through the wing to the Pito tube and some wiring for the roll servo, Pito heater, and for the stall warning device.  Then it will be time to close up the wings.