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Rigging and Trimming Part 1
By Tony Bingelis (originally published in EAA Sport Aviation, August 1990)
Very few homebuilders know or understand what the term "rig" ("rigging") has to do with airplanes.
Most of us do know it is used in reference to things nautical done aboard ships and boats. However, the term is equally applicable to aircraft and the realm of flight.
According to my dictionary, when you rig anything, you "construct," "adjust" or "arrange" (it).
As a homebuilder you are probably more familiar with your construction manual rigging words like "jigging" and "alignment."
Technically speaking, as you assemble your airplane you are, at the same time, also rigging it. Nevertheless, even after you have completed the final assembly, other, smaller, rigging jobs will still have to be completed.
Rigging Is Not Just For Biplanes
Mention rigging an aircraft to most any pilot and he will probably assume you are talking about biplanes and things like the assembly, installation, and adjustment of their landing and flying wires. Could be, but the term "rigging" has broader application than that.
All aircraft, not just biplanes, must be rigged properly to fly right . . . that is, to fly both straight and level . . . hands off.
Let’s assume you have just completed the construction of your airplane and are making the final preparations to launch it soon on its first flight.
Here is a brief summary of the rigging essentials you have probably had to cope with up to this point.
- The Structural Checks including:
- The completion of the precise rigging and alignment of each of the aircraft’s major fixed structural components (engine, wings, fuselage, landing gear, and tail surfaces) during that long challenging construction phase.
- Verifying the alignment of each of the major components with each other as you assembled them.
- The Control System Checks including:
- The correct alignment of each movable control surface with the adjacent fixed component (wing, stabilizer, and vertical fin) to which it is attached.
- Setting and verifying the minimum travel (angular deflection) of each control surface.
- Visually proving to yourself that each of the cockpit controls, which move these surfaces, are properly installed and move the control surfaces in the direction they should move.
- The correct adjustment and safetying of the push-pull tubes, bell cranks, control cables and control stops.
After applying careful attention to these rigging essentials you should now have a perfectly rigged airplane at your disposal.
What Perfect Rigging Will Get You
A perfectly rigged airplane is in its best aerodynamic configuration and will provide you with the most efficient flight characteristics possible . . . and it will require the minimum or no additional trim to fly straight and level, hands off, at normal cruise.
I can visualize your gleeful reaction after your first couple of flights.
"My airplane flew hands off right from the first flight," you say. "How about that? I must have it rigged just right!"
Well, that may not be necessarily so, amigo.
It is possible for an airplane to be somewhat "out of rig" and still fly hands off.
Let’s see how that can happen. Even though your wings might be level in flight, the airplane may not be flying (tracking) straight. That is, its nose may be drifting slowly off to the left, or to the right... and yet, there it is flying hands off. If this is so with your airplane, there is apparently a slight rigging problem, a trimming problem... or both.
I don’t mean to be picky, but I must point out it is not at all uncommon for a first test flight to be proudly declared a "hands off," trouble-free perfect event, when it is only close to it. It is an understandable assumption.
However, it is not until after a few more test flights are made that the average builder/pilot is no longer clutching the control stick so hard, and has become a bit more relaxed.
So relaxed, in fact, that the euphoria is wearing off and he is becoming more discerning... and, yes, picky.
For example, now he may notice that the ball in the turn/bank indicator is skewed off a bit, or that the airplane tends to drift off, ever so slowly into a steepening turn when the control stick is turned loose, or maybe, that the airplane’s behavior in some stalls is quite breathtaking because the bird wants to tuck a wing under.
After a while, he reluctantly admits to himself that the controls are a bit stiff and, maybe, just maybe, the oil temperature is running kind of low (low?) . . . and so it goes.
I’ll admit, there are perfect, gripe-free, test flights but most any homebuilt will need a slight adjustment, here and there, to fine tune its performance. Minor rigging and trimming adjustments, therefore, are to be expected.
When we refer to an aircraft as being "out-of-rig," we assume its components are not properly aligned due to inaccurate construction, or final assembly.
The Not So Perfect Rigging Job
The consequence of a serious rigging deficiency is that the total drag of the aircraft will be increased, and its performance will suffer because trim tabs have to be installed. Often, when trim tabs are necessary, they may have to be severely deflected to achieve that elusive hands off condition in flight.
There is another concern. In some installations, the amount of control travel available may be insufficient to effect the correct response in flight.
In other instances, the amount of control travel may be too great, and a sudden deflection of the control surface by the pilot could fail the structure due to the excessive loads (stresses) imposed.
It is very important, for this reason, that your airplane be rigged in conformance with the designer’s specifications for: angles of incidence, dihedral, wash-in or wash-out, engine thrust line alignment, horizontal tail incidence, the amount of vertical fin offset (if any), and the amount of control travel.
Needless to say, your wheels should also be aligned, or you may experience excessive tire wear and, possibly, runway control problems during landing or takeoff.
Those Final Rigging Checks
Let’s go back to square one and review some of the last minute rigging checks and adjustments that will help get top performance out of your airplane. Here’s what you can do:
First, level the airplane laterally and longitudinally. In most homebuilts you can simply lay a long level across the top longerons to level the airplane laterally. Sometimes a level cockpit floor can be used.
- At any rate, utilize the official leveling point for your aircraft and level it as accurately as you can laterally.
- Do not attempt to make your final rigging checks until the airplane is in a level attitude.
- Next perform the following cockpit rigging checks:
- Climb in the airplane. If the lateral level bubble is now slightly off with you in the pilot’s seat, have someone let a little air out of the high side tire (remember to reinflate it after you have finished).
- Next, check your turn and bank instrument. Its inclinometer ball should be perfectly centered. Be sure you are looking at the instrument square-on (no parallax error, thank you).
- If the ball is skewed off to one side, however slightly, loosen the instrument mounting screws and twist the instrument a bit to get the ball perfectly centered. Re-tighten the mounting screws.
- Make the same check for your artificial horizon . . . if you are so equipped.
- Note: Most turn and bank instruments and artificial horizons have slightly elongated mounting screw holes which will allow you to twist the instrument slightly to help get the ball centered.
- If your instrument doesn’t have the elongated holes you can elongate the holes in your instrument panel instead with a small round file to accomplish the same thing. However, you had better think twice before you start filing or you might elongate the holes in the wrong direction.
- No matter how careful you may have been, it is quite possible to have inadvertently mounted one or two instruments in your panel slightly tilted. Naturally, if that happened, it probably happened to the very two instruments that cannot tolerate any lateral deviation.
- The slightest misalignment of either the turn and bank or the artificial horizon can result in a pesky in-flight trim problem.
- Center the control stick and rudder pedals. Verify that the ailerons, rudder, and elevator are streamlined with the fixed surfaces.
- With the flaps in their up positions confirm that both flaps are streamlined with the trailing edges of the ailerons.
- If you have the flaps rigged so that their up-position is slightly above the trailing edges of the ailerons, be sure both flaps are so, and are symmetrically rigged.
- Lower the flaps, and visually check that both are going down at the same time and to the same degree.
- If you have a cockpit flap position indicator check it for accuracy. Readjust it if necessary. Do the same for any external flap position indicator.
Check aileron operation.
- Move the control stick to the right. The right aileron should move up and the left one down.
- Move the stick to the left and the left aileron must go up and the right side down.
- There should be absolutely no binding or rubbing noises throughout the control stick travel. Aileron stops must effectively limit the amount of control stick travel.
- Check elevator travel. Pull back on the stick and visually verify that the external surfaces actually move up. Feel out the elevator stops.
- You should get full stick and control surface travel in both directions.
- Make similar rudder checks. Left rudder pressure must move the rudder to the left. And, as with the other control surfaces, check out the rudder stops.
- Be sure your cockpit adjustable trim control(s) works in the direction you expect it to work, and that you can obtain the desired amount of trim tab travel.
- There must be absolutely no binding or rubbing noises throughout the limits of the control stick travel and the same for the rudder pedals.
- Finally, have somebody hold each of the control surfaces while you wiggle and work the control stick and rudder. There should be little or no play in the controls. Excessive play in any of the control surfaces or trim tabs can result in control surface flutter in flight... a frightening experience nobody needs to experience.