By Lisa Turner, EAA Lifetime 509911
This piece originally ran in Lisa’s Airworthy column in the June 2020 issue of EAA Sport Aviation magazine.
The row of composite homebuilt aircraft gleamed in the glow of the setting sun on the second day of EAA AirVenture Oshkosh. The excitement of the day’s activities seemed to float in the air as spectators wandered with smiles on their faces through the displays and rows of aircraft types.
A man and woman made their way urgently down one row of airplanes.
“Cindy, this is the one here,” Kevin said, coming up to one particular composite aircraft.
“It’s gorgeous,” Cindy said. “What a phenomenal paint job and workmanship detail. So, did you talk to the owner? Is he willing to accept our offer?”
“He is,” Kevin said. “I made it contingent on a prebuy flight. Joe took a thorough look at the airplane yesterday and said everything looks fine. I can’t see how we can go wrong. This is the third owner, though, which I find interesting. There are only 250 hours on the aircraft.”
The next day, Kevin arrived early for the flight. The owner flew the aircraft away from the hustle and bustle of the field and turned the controls over to Kevin. As he took the stick, Kevin noticed a strange sensation that he could not name.
“Roger, I think the trim is off or something is loose,” he said. “The nose is wandering side to side. I can feel it in the stick. It’s like a cyclical yaw.”
“Oh, it’s always done that,” Roger said. “The previous owner said it was normal. I just put on the autopilot, and it seems to hold the heading okay.”
In the midst of the excitement, Kevin dismissed the strange motion as wind effect and a different aircraft than what he was used to. They landed and the deal was completed.
Three days later Kevin and Cindy left for home in their new homebuilt. As soon as they left the field and established cruise, Cindy said, “There’s something wrong somewhere.”
“Naw, Roger said that was normal.”
“It’s not normal,” she said. “The nose is wobbling left and right like it’s hunting for something.”
Kevin suddenly got a knot in the pit of his stomach.
“I should have paid more attention,” he said. “You’re right. This is not normal.”
When they got back to their home field, they had their mechanic, Joe, check it out. After the flight, Joe exited the aircraft shaking his head in puzzlement.
“That’s the weirdest sensation,” Joe said. “Let me go over the aircraft with a fine-toothed comb. At Oshkosh, I did check all of the components, including travel, trim, and cables.”
As they pulled the airplane into the hangar, they heard a sloshing noise at the right wingtip.
“What is that?” Kevinasked.“It can’t be fuel; the tanks are closer to the fuselage.”
A little later, with a newly cut inspection port open in the right wing near the tip, Joe was baffled to find a sealed compartment.
“Is that a third reserve fuel tank?” Cindy asked.
“No, there’s nothing running to it, and there isn’t a similar tank in the other wing to balance it. I’ll need to remove it and see what’s going on.”
They found that the strange handmade 3-gallon fiberglass tank held 2 gallons of fuel, creating a sloshing weight of 12 pounds. Once removed, Joe repaired the area and went flying. As he taxied back up to the hangar, Kevin and Cindy stood waiting patiently with great curiosity.
“Super left wing heavy,” Joe said. “Looks like the left wing was not installed correctly. No wonder we were getting that side to side action, with liquid sloshing around.”
After several days of complete disassembly and rerigging, the airplane flew perfectly.
“No more side to side hunting action,” Cindy said on the next flight.
Kevin just shook his head.
“No one will believe this story.”
This is actually a true story with a few details altered to leave some protection in for the surprised participants.
**
An A&P mechanic friend said to me, “You should write about rigging.”
“Why? Won’t that be pretty boring?” I asked.
“It’s not boring when a control cable or a flying wire breaks because it wasn’t installed right.”
“Okay, you’re right.”
I thought about it and realized that I’d seen enough mistakes having to do with rigging to create some advice around it. As a topic, rigging has taken up entire books. If you’re an A&P/IA, you’ve seen the outline called “Aircraft Rigging” that goes into great detail about hardware and controls.
Rather than launch into that level of detail, let’s pick out the most common issues and misconceptions that might help you avoid problems in the future.
Rigging an airplane is similar to rigging a sailboat. Surfaces must be the right size, be installed in the right places, and have controls to make sure the airplane or the sailboat move through the medium as smoothly and efficiently as possible. Just as a race car designer will drill down into the minute details of airflow and surfaces, we must fine-tune our aircraft to slip through the air with a minimum of resistance and a high measure of safety.
I’ve seen situations where an owner installed two or three different trim tabs on surfaces trying to correct a problem. These solutions often cancel each other out or introduce new problems.
Problems that go beyond the issues I talk about below can involve components that are no longer symmetrical because of accidents, repairs, or other factors. Rerigging an aircraft can get involved and complicated. If you feel in over your head, I’d recommend getting a professional to do the job or help you as you work through it. Factors include dihedral, angle of incidence, wash-out, wash-in, and, in the case of biplanes, stagger and decalage, the difference between the angles of incidence of the two wings. Remember that on production aircraft, you must follow the instructions in the service manual.
In 1990, Tony Bingelis wrote two rigging and trimming articles that go into specific detail for homebuilders. If you are undertaking a complete rerigging project, I recommend following these instructions, along with the manufacturer’s advice and directions.
Rigging Mistake 1: Not understanding aerodynamics
No, I’m not hoping you’ll dive into aeronautical physics. However, you should return to the basics of flight that we learned when we were student pilots. This means you, or whoever is piloting the aircraft, understands what the control surfaces are doing when moving through the air.
Amazingly, there are pilots who cannot tell you what the aileron, stabilizer, and rudder should be doing when they push the stick or yoke around, or what’s happening with rudder pedals. This is not so much their fault as it is the training they received. We’ve seen enough (usually fatal) accidents where controls were installed backward after an annual or repair to know that pilots don’t always check this. Always check for correct rigging: stick to the right — right aileron up; stick forward — elevator moves down; rudder left — rudder moves left. Use a trick to remember, such as thumb up on right — aileron up. If you understand what the air is doing to the control in flight, it will become intuitive.
I also see trim tabs installed “backward” routinely. Pilots are thinking that they want to counteract P-factor on takeoff, so they install a tab that bends to the right, thinking this will naturally apply some right rudder pressure. In fact, a tab must be bent to the left to cause the rudder to move to the right. I’ve seen builders continue to bend these tabs and complain that the problem is only getting worse.
How to correct: Pull out the manuals that came with the aircraft or the build manuals that came with the kit. Double-check direction of operation and control travel. Review your understanding of controls and make sure you’ve got reminders about correct direction in your preflight checklist.
Rigging Mistake 2: Confusing rigging with weight and balance
“I don’t have full motion on the elevator,” said a fellow flyer at the airport.
“What’s the airplane doing?” I asked.
“I have to hold back-stick all the time, and especially after I fill the tanks.”
“Even when you trim it?” I asked.
“I run out of trim.”
“When did it begin happening?”
“It’s done this ever since I bought it.”
“Did you build it?”
“No,” he said. “I bought it. It’s a secondhand homebuilt.”
“The prebuy inspection didn’t catch anything funny?”
“No prebuy.”
After investigation, we discovered that the aircraft weight and balance numbers had been calculated incorrectly and the builder had added some lead weight to the nose and changed the location of the seats. We removed the weight, ran the weight and balance again, relocated the seats, and adjusted the control travel. Problem solved.
How to correct: First make sure the weight and balance is correct. If it’s a production aircraft, and no modifications have been added, then move on to trim checks. Realize that fuel use and loading will affect trim in flight. On a nose-heavy or tail-heavy condition, rule out a weight and balance problem before installing a trim tab on the elevator.
Bring the aircraft up to cruise and trim for level flight. With feet and hands off the controls, what does the airplane do? If it tracks straight and the ball remains in the center, you don’t need to correct anything. If the airplane turns, does it turn first and then drop the wing in that direction or does it drop the wing first? If the aircraft turns and then drops a wing, adjust the rudder trim tab in the direction the aircraft is turning. If the wing drops first, followed by a turn, then you have a wing heavy condition. In this case, a tab on the aileron of the heavy wing should help. An aileron tab should be considered only after you’ve made all of the other rigging adjustments recommended by the manufacturer. On cantilever wing aircraft with flaps, a slight droop in one flap may help.
If your aircraft seems to yaw and you’re holding in some rudder to keep it straight, check control travel and check for the correct placement and mounting of the rudder, the stabilizer, and the wing attachment points on the trailing edges. Construction mistakes are not uncommon and may change the flying characteristics, sometimes in dramatic ways. If the nose yaws to the left, install a trim tab on the rudder and bend to the left. If you’re adjusting fixed tabs, it might take a bit of experimenting to get it where you want it.
If you have more serious problems, then you ought to take the aircraft through a full rigging check by a professional. A full rerig will include dihedral, angles of incidence, wash-in or wash-out, engine thrust line alignment, horizontal tail incidence, the amount of vertical fin offset, and the amount of travel on all controls.
Rigging Mistake 3: Not understanding or not following detailed construction procedures for building, restoring, or repairing an aircraft
“Get-there-itis” happens in building as well as flying. When the detail gets overwhelming, most builders stop and rest, back up a bit, and take another look. Where I see problems is when builders try to rush or don’t take the time to fix mistakes they discover in the process.
“It’s no big deal,” one builder said as he estimated wing twist specifications. “Everything will work okay.”
Not checking the plans or build manual, in this case, resulted in one wing not having the required twist, affecting the progressive stall characteristics. The airplane scared him so much in the air he sold it. A subsequent owner rerigged the entire aircraft after rebuilding one wing, correcting the problem.
How to correct: I can’t emphasize enough the importance of following the manufacturer’s recommendations on the precise alignment of the aircraft’s major structural components.
Double-check travel limits, free play, direction of travel, and angle on control surfaces.
Double-check adjustment on push-pull tubes, bellcranks, control cables, and control stops.
Rigging Mistake 4: Hardware installation or refurbishing mistakes
Hardware installation and operation has always been a weak spot for homebuilts. The issue is generally one of unfamiliarity on the part of builders rather than lacking the desire to get things right.
I see cables misrouted, missing fairleads, missing safeties, incorrect tension on controls and cables, wrong hardware, using an elastic stop nut where a castle nut and pin should be used for rotation, and automotive hardware where aircraft hardware should be. These omissions can cause some serious problems in the air.
In one instance, a restorer of a Waco F-2 gave the flying wire attachment fittings — clevis ends — to a local friend to be chrome plated. Later that year, while in flight, the front wire gave way at the end. The owner was lucky to be able to make an immediate emergency landing. He had not realized that metal plating is a certified process and not just anyone can perform it.
How to correct: Double-check all trim cables, push-pull tubes, bellcranks, torque arms, fairleads, bushings, and safeties for operational direction and condition. In hardware use and installation, your guide is AC-43.13-1B, Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair, Chapter 7: “Aircraft Hardware, Control Cables, and Turnbuckles.” Realize that with certified aircraft, many processes must be certified.
If you are the owner of a production aircraft, your rigging issues should be minimal but your hardware issues will be corrosion- and adjustment-driven. On older aircraft, the annual should include detailed checks for wear and tear in the control systems and a thorough check for impending failures due to corrosion. Include control travel and free play, two telltale signs that wear is progressing.
Rigging Mistake 5: Inadequate preflight testing and fine-tuning
Even highly experienced builders and restorers can fall into this trap. We can make assumptions about control travel and other setups and not realize that something is missing or misread the detail on the adjustments we are supposed to make.
How to correct: Put the control travel and free play detail into your preflight checklist and your pilot’s operating handbook. Your taxi testing after build, restoration, or repair should be adequate enough to really find out if the controls are working as they should. Over time, the controls will require fine-tuning to continue working in harmony. Don’t assume that it’s “set it and forget it” unless you are looking for excitement and surprise on your next flight.
If you’re a builder, have as many other qualified people look at your new airplane as you can. They will find what you missed.
**
Airplanes will fly in the most amazing of circumstances, including when they are out of rig. But the penalties include extra drag causing extra fuel burn; incorrect travel limits can cause a control to stop short of what you need, causing a safety issue; slop or too much free play can cause flutter, which is a very dangerous condition in flight that can deteriorate into an out-of-control situation; and hardware that can actually fall off in flight.
If you’re a builder, follow the assembly instructions to the letter. If you’re a secondhand homebuilt owner, be familiar with what’s in the build manual regarding rigging and be attentive to any issues in flight. If you’re a certified aircraft owner, be tuned in to anomalies caused by rigging and check for wear and corrosion. If you’re the owner of a biplane, make sure you or your mechanic are intimately familiar with correct rigging and flying wire attachments and tensions.
Lisa Turner, EAA Lifetime 509911, is a manufacturing engineer, A&P, EAA technical counselor and flight advisor, and former DAR. She built and flew a Pulsar XP and Kolb Mark III and is researching her next homebuilt project. Lisa’s third book, Dream Take Flight, details her Pulsar flying adventures and life lessons. Write Lisa at Lisa@DreamTakeFlight.com and learn more at https://DreamTakeFlight.com.