Source: 2018 Q3 Beartracks, Kevin Deutscher
The purpose of this series of articles is to help the builder understand the importance of attention to detail while rigging for a safe, predictable airplane. Performing rigging on essentially 4 Bearhawks has provided a bit of insight to the methodology that yields a plane that is pleasant to fly, predictable and safe, and free from odd quirks. Straight and level, hands off, with uniform movements and pressure gradients is the end goal, as well as a smile on your face. First, we’ll look at geometry and alignment and basic foundational setups. Second, will examine several areas of the flight envelope and note existing behavior when controls are deflected. Finally, we’ll assess what to do to correct any undesirable behavior.
Years or decades have been invested in building your dream and the desire is to have an aircraft that handles and behaves seamlessly and with anticipated control harmony- a pure delight. Well, if that is not the outcome, what do you do? How do you correct the discord?
A bit of background and full disclosure: I am still building 4-place #272 and have had the pleasure of flying in numerous different Bearhawks. An eye opener for me was, WOW they are all so different in handling, what will mine fly like? Will it be a joy or a Mack Truck? There is one out there that is velvet perfect, a different one, a constant effort to fly. Why this big disparity?
I want to be perfectly clear right up front, Bob did a great job with the airplane design and execution. The difference in handling was all in the finite attention to detail and how correcting an issue was executed. No good deed goes unpunished, a faulty analysis and a correction gone bad with another correction to fix the other and so the snow ball rolls. Tribal knowledge says if your wing is heavy, pinch the trailing edge or add a washer. Really, a .040 washer raises a heavy wing? That is a tiny adjustment. Oh, why did you raise the heavy wing rather than push the light wing down? Everyone has their personal preferences as to how the plane should feel and respond and within reason that can be accommodated.
So given that, let’s layout some knowns for those who are still building and those flying to review and contemplate.
Keep in mind that we are talking about connecting and disconnecting elements of the control system and potentially making a change or adjustment. This requires the utmost caution and cross check for integrity and security of turnbuckles, fasteners, access plates. All controls should be checked and rechecked for security, positive control, correct direction of motion. Have a qualified assistant double check all routings and attachment before closing up the aircraft for flight.
Take your time. This will take time. Much of the process is iterative and incremental.
The big one first: Friction is your enemy. “Play and slop” is friction’s kissing cousin. The only place friction is your friend is in the trim system, more on that later. Everything in the control system should be smooth, lubricated, free of binding when operated thru the entire range of motion of the control. The stick assembly should be free to flop over to the floor without the cables attached and lateral motion of the ailerons should be obtainable with no undue pressure on your little finger. Rudder assembly should be free with a rattle fit between the members. Brake hoses should not overlap and intertwine with the rudder mechanism. The rudder return spring must not interfere with pedal assembly. Elevator and rudder shall move stop to stop when the control cables are not tensioned with just the slightest push from a single finger. Elevator and Ailerons should hang freely, leading edge/counter balance weight down toward earth when released from a neutral position (see note below). The challenge is to keep this system as friction free as possible when the control cables are attached and tensioned.
With respect to the horizontal tail, there are numerous binding points that come in to play and create friction that progressively creeps up and bites you. I have spent almost a day on a horizontal tail assembly. The objective on the horizontal tail is that the elevator will fall freely with its nose down, counterbalance towards earth, without any external human assistance and maintain this function as the cables are connected to the stick assembly. The least painful method is something like this: go to the cockpit, center the stick at neutral, and release the stick. If stick freely travels aft, ending with the elevator counter balance weight down toward the earth, consider yourself a magician! If not, read on. First concern is that the aft spar tube of the horizontal tail must be perfectly straight along the aft edge, both fore and aft and up and down. This must be maintained when the flying and landing wires are tensioned. In order to achieve this condition it may be necessary to add small ring shims over the tube extending out of the fuselage that the horizontal tail slips over. It may also be necessary to re-drill the AN3 bolt on the leading edge. Often this process is easier to perform with the elevator assembly removed from the aircraft. Verify that the LE of the horizontal tail is -4 degrees nose down with respect to the datum of the fuselage fore aft level measurement from the cockpit floor. Keep in mind that changing the tail angle of incidence will effectively move the neutral center position of the stick and you will have to re-assess the fit/feel to the pilot.
Next mount and reattach the elevators, one at a time, with a bolt slipped thru the hinge strap. Each elevator should fall counterbalance nose down freely. If not there is friction, or the elevator is not adequately counterbalanced, more weight is needed. Binding comes from the hinge points being misaligned, twisted, forced inboard or outboard. At this point each elevator should be independently assessed and remedied. The implication of this is that the elevators are not connected to each other at the root end yokes. They should be free with a gap between the two interior faces. Now is the time to make the shim that was never made to fill the space exactly between the two yoke faces. This shim should cover the entire face and the have four holes, two at the cable connect and two to join the elevator halves together. When the halves are joined, if correctly shimmed, the elevators will have picked up no friction. Success. Connect the control cables and cotter pin the castle nuts. Secure the two bolts clamping the elevator yokes together. Tension the elevator cables and check for friction and binding. They should still be free moving, falling to counter balance weight down toward earth, aft stick position with little encouragement.
Secondly: Control gap seals and Aileron Pocket Configuration. The Bearhawk benefits greatly from sealing the gaps between the elevator and horizontal tail and the rudder and vertical fin. In flight the control surfaces center more accurately and have greater control authority. Commercially available gap seals are available from ACS, Wicks, Wag Aero. All rigging and adjustments are performed with the gap seals removed, they are the final icing on the cake. During assembly and rigging it may be necessary to shim the location of the aileron for one of a number of reasons. Monitor the fit of the ailerons to the aileron pocket or cove. The gap between the aileron and wing should be uniform top and bottom and consistent from one side of the aircraft to the other. The absolute size of the gap is not that critical but there should be reasonable symmetry between features on both sides of the aircraft.
Thirdly: Springs. The tail wheel assembly and tail wheel steering springs DO HAVE an appreciable effect on in flight yaw behavior. If the tail wheel does not swivel freely and detent to neutral, that will be seen in the yaw behavior. It is a bit of discouragement to debug and tweak and adjust, only to discover that not only is the tail wheel not centering but the springs are of different strength on each side of the plane, deflecting the rudder an additional amount every time feet are removed from the pedals. Worse yet, having to hold left rudder on takeoff and climb out is a bit odd.
Datums and Relationships: Now what is neat for our work is that the Wing Chord Line is +2 degrees with respect to the level datum of the fuselage floor. The bottom wing skin is very close to level and parallel to the fuselage level datum. The wing chord line is approximately +2 degrees, nose up relative to the bottom of the wing. We are looking for differences side to side rather that true absolute values. Therefore we can use the bottom of the wing as a pseudo-level reference. Drum roll here… The bottom of the wing fore and aft, spar to spar, will be “Zero Datum” for all of our conversations and level measurements. Hence, the horizontal tail will be -4 degrees from whatever is read from the bottom of the wing (also the fuselage floor) while sitting on whatever gear or tires or ground slope. And this 4 degrees is usually the minimum.
To reasonably measure that which is not straight and level, a long straight bar is needed that will span the wing chord from the front spar to aft of the trailing edge. The good news is that it is not necessary to jack up the plane and level it to do this deed. My go-to is a square aluminum tube with the level placed on the tube. What we are checking is differences in the level datum one side to another at wing root, strut attach and just inboard of the tip. Both sides of the plane. This straight edge line may not lay flat on the ribs, you might have to place an equal sized shim at both spar locations. Usually ¼” is all that might be needed. This set of measurements should NOT include the flaps or ailerons at this time, only Front Spar to Rear Spar.
Beartracks Newsletter—Third Quarter 2018 5
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Set the digital level to indicate level ( 0.00 ) when placed fore and aft on a bubble level or referenced to a bubble level. Now measure the angle of the bottom of the wing. Check the reading between the wing bottom and the fuselage floor just in front of the sticks. The readings might be between 9 degrees and 14 degrees (leading edge up), depending on wheel sizes and ground slope. This is to be considered a positive reading. Floor to bottom of wing should be very close to the same angle.
The angle of incidence of the horizontal tail needs to be measured at the root near the fuselage and outboard from the flying wire attach point to a point on the centerline of the leading edge. Note, this is not a measurement that is referenced to the ground as the leading edge gets lower to the ground as you move from the root outboard due to the aft curvature of the leading edge.
Ultimately the difference between the Wing Bottom Datum and the Fuselage Datum should be within about half a degree and the Horizontal Tail Should 4 degrees less than the bottom of the wing datum. With a level fuselage the plans show the wing incidence at + 2 degrees and the tail at -4 degrees, a 6 degrees difference! The wing chord line is approximately +2 deg, nose up to the bottom of the wing.
Fourth: Measurements. Everything will be different for everyone. I have not found a basic structure yet that was so far off that the plane rigging had to be adjusted for the misalignment of the airframe. That said, you do not know ‘till you measure. The good news is that it is not necessary to jack up the plane and level it to do this deed. What we are looking for is differences between sides and conformation that what will use as references are fairly similar. Laterally the plane should be leveled across the front floor or the top of the windshield. Yes, that 24 or 36 inch digital level will be perfectly fine. Grab a note pad and obtain the following dimensions. The reason for these measurements is to have a known condition and reference plane to compare the observations that what will be made during flight testing. Aileron and flap measurements are not taken now as they are mostly meaningless until post-flight analysis.
 Dihedral of wing: Measure on the front spar line, just outboard of wing strut, on both wings.
 Distance from Tail Post: At the hinge line, top of the vertical fin to a point on the rear spar rivet line, bottom wing skin, near where the flap transitions to the aileron. Mark this spot with a piece of tape both sides of the airplane as it will be used several times.
 Distance from a horizontal prop blade: To the above tape spot on the bottom side of both wings.
 Distance from the horizontal tail: Topside flying wire attach to the horizontal stabilizer at the attach bolt. To the above tape spot on the bottom side of the wings.
 Distance from the horizontal tail: Measure the aft spar line down to a LEVEL 2X4 spanning the entire width of the horizontal tail. Check level of ground based 2X4 against the front fuselage level point. Verify this is level with the front fuselage datum.
Before changing anything, stop, think, organize, proceed with caution and double check everything incrementally each step of the way. One or two turns misadjustment of tail flying wires can bind up the elevator. Friction will impede the control surfaces ability to self-return to a neutral position. Friction and the associated hysteresis will impede the ability of an autopilot to perform properly. And, it actually makes hands off flying difficult!
While waiting for the next installment, review the above, assess the aircraft, remove friction, measure geometry. Those flying, what are your observations, concerns?
We’ll tackle flight assessment one axis at a time, Pitch, Roll, Yaw.
Final installment will be adjusting for optimal handling.

Editor’s follow-up note:
There was some discussion after this first article was published about whether the tail should be balanced neutral, or stick aft. This in itself is not extremely critical. Having the surface balanced is good, and having it slightly overbalanced is good.