nuts & bolts

technical counselor

Fixing Flutter

How Has a Tech Counselor Helped You? Sharing information is the ulti-

Making an airplane solid as a rock Dick Blevins, EAA TC 4518

F

lutter is the uncommanded flapping motion of a flying surface that can result in structural failure if its cause is not corrected. Many things can cause flutter, even in a proven design with no history of this problem. Because subtle changes can sometimes result in flutter, the cause sometimes cannot be clearly determined, but what is certain is that it must be remedied. In my case, it reared its head some 75 flight hours after my homebuilt airplane’s first flight. It happened during a flight to EAA AirVenture Oshkosh 2001 as I accelerated to catch another air-

plane I was following. First came a violent Dutch roll. As my airplane rolled to the left, I shifted the stick and rudder full right and reduced the throttle to idle. As soon as the controls responded, I reversed them and leveled out. When I throttled back to speed, however, the whole scenario repeated itself. By then I decided it would be wise to fly at a slower speed until I could pinpoint and correct the problem. Flexing motions of two or fewer per second are of little concern as long as the distance of the flex is not large. The longer the flying surface, the larger the flex distance. At least that’s the norm. Think of a flapping The longer the flying surface, flag. In the same breeze a large flag the larger the flex flexes at a slow cycle while a short distance. At least that’s the norm. flag simply vibrates. Think of a flapping flag. In the same Installing a flying surface that lags breeze a large flag flexes at a slow behind a rising wing—and in cycle while a short flag doing so creating an even greater simply vibrates. force to that rising wing—adds to the

mate goal of this new department in EAA Sport Aviation. Whether you’re a new or experienced builder, or a Tech Counselor, on these pages we want to know how the team of a TC and builder met a particular challenge. You can either submit a fully written story, or just the “who, what, when, where, and why,” and we’ll have a staffer contact you for the rest of the details. Please don’t forget pictures, because they often tell a story better than words. E-mail your TC success stories to [email protected], with TC as the subject. flex, just like the flapping of a flag. A lagging aileron creates an additional lifting force as its aircraft moves forward through the slipstream. Compound that by raising the opposite aileron and forcing down the opposite wing. It all happens with no movement of the control stick, and with no warning you’re in a violent Dutch Sport Aviation

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technical counselor roll, shifting from level flight to 45 degrees left and then to 45 degrees right in a fraction of a second. I now believe that every flying and control surface should have at least three hinges. With three hinges, a wing or any other flying surface creates an arc when it flexes. That arc prevents the control surface from lagging behind the flying surface. The arc would also increase resistance of the wing to the arc, and the misaligned hinges would prevent the aileron from lagging behind the rising wing. It’s also important to note that vertical flying surfaces are not immune to flutter. As a wing rises on its uncontrolled journey, it becomes highly stressed, springs in the opposite direction, and reverses the cycle—if it doesn’t break. Any person with a minimum understanding of this principle must surely recognize the

Ironically, I never realized that anything was wrong because my piloting skills improved as I gained experience in the aircraft.

potential for a rapid onset of catastrophic structural failure. Immediate counter actions must be taken. Here is what worked for me: ■ Slow down immediately. ■ As you are slowing the plane, yank the control stick in the opposite direction to break the cycle. ■ As you continue to slow the plane, use rudder, elevator, and aileron to correct the angle of the flying surface. Are you getting the idea? There is a correlation between speed and flutter. To use our flag as an example, slow down the breeze. I tried several methods for eliminating my airplane’s flutter problem. The aileron was set at anywhere from 15 degrees below the wing to 15 degrees above it. I discovered several contributing factors: 1) Aileron balance, 2) increased leverage of the control

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system, and 3) elimination of most of the friction in the aileron system. The flutter cycle started with turbulent air, and the increased VNE at which the airplane is certified was also a factor. One aileron was a half-ounce out of balance. Ultimately, I traced my airplane’s flutter problem to the nylon blocks upon which the control sticks are mounted. The blocks are secured to a steel-tube cage, and just one of them is used to prevent left-to-right movement (or aileron control). The ears, welded onto the tube to stop the left-right movement of the tube, are at the extreme outer edge of the nylon block. That makes the tube more sensitive to the flexing of the block. The block was free to flex left and right, and the tube slid through the second block. Since it only happened during flying loads, it was difficult to discover during inspections. The flexing of the block, however, explains the squirrelly effects of the ailerons and the solid autopilot aileron control. I often said that the autopilot could fly the plane better than I could. Since it effectively removes the nylon blocks from the equation, my sentiment proved to be true. The squirrelly effects increased as my airplane’s flight hours progressed. But I never recognized that the condition was worsening. Ironically, I didn’t realize that anything was wrong because my piloting skills improved as I gained experience in the aircraft. As I inspected my airplane, I found three sheared rivets on the nose rib that attaches to the outboard side of the right aileron/pushrod. And the eyes were elongated on the rods that connect the right and left pushrods to the bell crank. With the airplane apart, I decided to remove the aileron trim tabs, which I found to be

unnecessary after 75 hours of flight time. I replaced one of the nylon blocks with a brass block, installing an angle aluminum between the lower bolt-head ends and securing them to the center tube of the cage. I encapsulated the other nylon block in metal to help retain it should it break in cold weather. Flight-testing has proved that I

found and corrected the problem. No more squirrelly ailerons. My airplane is now solid as a rock, the flutter is gone, and I can fly it better than the autopilot. At first I thought that some sort of aileron snatch was happening. Obviously I was wrong. Stabilizing the block to prevent it from flexing has improved aileron feel.

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