Running through the rules of rig
We've all piloted airplanes that just don't seem to fly right. And this might be the scenario that gives you the clue: As you trim out for cruise and loosen your grip on the yoke, the airplane immediately dips a wing to begin its own uncommanded version of turns around a point. Then you notice that, after rolling the wings level, the ball will not stay in the center without pressing on a rudder pedal. Assuming that the airplane has no cockpit-adjustable aileron or rudder trim — and a great many airplanes do not — and that you don't have excessive weight in fuel or passengers on one side of the airplane, then you must conclude that the airplane is out of rig. Moreover, if the control pressures are anything other than minor, you are flying an airplane that is seriously out of rig.
Rig refers in the strictest sense to the adjustment of cables and links between the yoke or stick and the control system. But in a larger sense, rig has come to mean the overall alignment of the airplane — the relative incidence of the wings; the full-up adjustment of the flaps; and the relationship of the ailerons, elevators, and rudder to the airframe itself. Improper adjustments or airframe damage can cause the airplane to want to fly sideways in cruise or require noticeable control inputs to keep the wings level and the ball centered.
Uncommanded or undesired turning tendencies come from three major areas — a yawing input from the airplane, which forces a turning motion, in turn creating more lift from one wing than the other; a load imbalance, like excessive fuel in one wing tank or a single heavy passenger; or simple lift asymmetry from the wings caused by airfoils of slightly different shape, incorrect alignment of the wings to the fuselage, or control surfaces in the wrong state of rig.
Worse, an out-of-rig airplane wastes performance. If the control surfaces are all akimbo, some of the lift normally given to holding the airplane in the sky is sacrificed to counteracting undesired turning or crabbing tendencies. It can also mean that the fuselage is not parallel to the slipstream, but rather cutting through at an angle — a sure way to dramatically increase drag and kill speed. Finally, an out-of-rig airplane may have unpleasant or outright dangerous stall characteristics — remember, the airplane is tested and certified in perfect rig. Any deviation from that form promotes you to the rank of test pilot.
Every airplane will seek equilibrium at some airspeed, even if technically out of rig. The design point is usually cruise, which makes sense because that's where the airplane spends the most time. As such, fixed trim tabs are intended to be positioned so that the airplane flies hands-off at normal cruise speeds and altitudes. It should not turn in any direction, and the ball should be in the center of the instrument with no rudder input. Many airframe designers, when given the dictate to use only fixed tabs for roll and yaw, allow the airplane to require rudder and/or aileron input during the climb and while in a high-speed descent. It's a common trade for having a hands-off cruiser and may, in some cases, call for offset engine mounts or vertical fins that aren't exactly on the airplane's centerline.
Why are there fixed trim tabs in the first place? Try as they might, the airframe manufacturers can't always churn out precisely, exactly true airframes. Production variances and the toll taken by everyday wear — to say nothing of post-accident repairs — mean that not every airplane will fly straight and true throughout its lifetime. So the manufacturers use fixed trim tabs, typically on the trailing edge of the rudder and one or both ailerons. These tabs do nothing more than to force the entire control surface in the desired direction, relieving the pilot of having to exert the pressure to do so. Referring to these devices as fixed merely means they cannot be adjusted from the cockpit and that tedious trial-and-error testing is needed to determine the exact settings to get hands-off flight in cruise.
Many airplanes have inflight-adjustable tabs in two or three axes; all have trim control in pitch. Some, like the Piper Cherokees, have rudder trim, while others, like the short-body Beech Bonanzas, have just aileron trim. Moreover, this trim control can be implemented with a movable tab on the control surface, or through spring tension — called a bungee — exerted somewhere in the control system. Designers like the bungee arrangement because it reduces the chance of flutter. If allowed to come loose, a movable trim tab can induce flutter into the control surface to which it's attached. Admittedly, the bungee trim system is less exact and does not respond proportionally to airspeed, a characteristic typical of the movable tab.
When you walk up to an unfamiliar airplane, the positioning of the trim tabs will give you an indication of its state of rig. A rudder tab that's cranked hard over, for example, usually means that someone has tried to compensate for other rigging maladies — typically manifested as a "heavy" wing — by cranking in a lot of rudder trim. Similarly, an aileron trim tab with more than about 10 degrees of deflection is an indication that there's something amiss in the way the airplane is rigged or assembled.
Adjustable trim tabs can mask an out-of-rig condition, so be looking for excessive tab displacement in hands-off level flight. (If you can't see them in flight, leave the tabs in the cruise configuration and check them after landing.) Generally, the ailerons should be faired flush with the wing tip and outboard flap skin, and the flaps should align with any fuselage fairings. If an aileron is displaced more than about one-quarter inch or the flap in the fully retracted position doesn't line up with the fuselage, then you've got larger problems.
Before you condemn an airplane as out of rig, follow some common-sense rigging guidelines. First, is the turn coordinator or turn and bank really centered? In other words, you need to determine that when the ball says that the airframe is leveled, it really is. Service manuals will outline the procedure that's correct for your airplane, but it's usually to place a carpenter's level across some lateral leveling points, usually the seat tracks. Compare the level to the turn instrument. You'd be surprised how many of these instruments are removed for overhaul and reinstalled without the benefit of a 10-minute calibration. (Some turn coordinators can't be rotated in the panel, so you might not be able to make the calibration.)
Let's say, then, that you have a properly calibrated ball but the airplane still wants to make turns in hands-off cruise flight. Start with the basics. Is there friction in the control system — particularly for the rudder — that might allow any surface to "stick" in position? All control movements should be smooth, without kinks, bumps, or grinds. Often, a badly flat-spotted pulley — the device that guides the control cables around the innards of your airplane — can create enough friction to hang a control in place, even though the aerodynamics are trying to force it in another direction. Now is the time to check control-cable tensions, too.
Also look very carefully at the control surfaces themselves. Are they straight and free of dents or bends? An incautiously placed heel on a flap skin can do a surprising amount of aerodynamic damage. Understand that if the controls' trailing edges and major surfaces are not straight and square, you can play with rigging until you've run out of safety wire and still not get the airplane to fly straight.
Some airplanes employ what's called an aileron-rudder interconnect, the purpose of which is to provide artificial lateral stability. Certification regulations specify that an airplane be able to pick up a wing with just rudder input, for example, and if the airplane lacks natural dihedral effect, springs or bungees are often employed between the two normally separate control systems. Make sure that if your airplane is so equipped, the linking cables or springs are set as specified by the manual. Also, if you've got a fixed-gear airplane, take a moment to see that the rudder and nosewheel are correctly aligned. The nosewheel can act as a rudder — because of its location ahead of the center of lift, it is destabilizing — and can create significant yawing moments.
Next, straighten out all of the fixed tabs — or go back to the service manual-specified baseline settings, if there are any — and go flying. (Use two small blocks of wood to capture the metal tab and isolate it from the flying surface so that you bend it evenly, and have a helper hold the control so that you don't impart any loads on the system while you're doing this.) You might be surprised to find that your rigging problem has changed; many would-be riggers make counteracting changes to the trim tabs that in fact fight each other. Fly the airplane at normal cruise speeds in smooth air and note the relationship of the control surfaces to the wing and fuselage. Are the ailerons both well above or below the trailing edge, or are they opposing each other — one high and one low? If you can see the rudder, determine if it's reasonably well in line behind the vertical stabilizer. Make a note of these conditions — as well as an estimation of the control pressures required to obtain level, ball-centered flight and whether the control wheels are level with the ailerons completely in trail — and land. Do your head-scratching on the ground.
You may have found that both ailerons were trailing high — drag your A&P away from the coffee machine and set the ailerons to spec. (Note that you will not have cured the turning tendency yet. The ailerons are a closed-loop system. Adjusting one slightly down to compensate for a turning tendency will only result in that aileron's going downward by half the adjustment, with the opposite aileron moving up by the same amount.) You need to have the control relationships set to the baseline before you can determine the real cause of the turning tendency.
Different airplane models have varying provisions for airframe alignment. Some models, like Mooney M20s and all of the Aerospatiale Caribbean series airplanes, have one-piece wings — the shape of the wings and, more important, the relationship of the left/right airfoils is set during manufacture. There's no way to individually adjust dihedral or angle of incidence. So these models use adjustments to the flap up-limit to set alignment; often, the up-limit is a small bolt threaded into a boss on the aft spar.
Still other airplanes have a means of adjusting the wings individually, which affords you the option of altering their incidence to cure the turning problem. (Some speed tuners have used this adjustment to better align the fuselage with the relative wind for better cruise performance; this scheme works best on older airplanes that have received significant power increases or drag reductions.) It's crucial that you find someone who has experience rigging your brand of airplane to do this.
Airplanes with strut-braced wings are another subject. Often, it's just a matter of adjusting the wing-attach points at the strut to get the wing in the correct relationship to achieve hands-off flight. This is particularly true for the older Pipers, which, like other models of this era, have double lift struts. There are many rigging possibilities with this scheme, but it's critical to make sure that the wings have correct washout and the right amount of dihedral — and that they are symmetrical; our Timeless Tri-Pacer arrived with one wing tip higher than the other. Washout describes an intentional twisting of the airfoil from root to tip, intended to ensure that the inboard sections are at a greater total angle of attack than the outboard areas for good stall characteristics. An airplane with asymmetric washout or twist will also have a turning tendency.
Through all this, keep a few basics in mind. Always have working with you a knowledgeable wrench who knows your type of airframe inside and out. (Here's where the owners' group maintenance seminars really pay off, at least in getting you started in the right direction.) Make small changes, one at a time, and fly to test the result. Never make more than one alteration at a time, unless it's to set the airplane to the manufacturer's baseline settings. Take your time. You can't hurry the process of trial and error. Just know that a day or so of your effort could well give you a faster airplane with better climb performance and improved stall characteristics — and make life much easier on you and your autopilot.
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