maintenance & restoration Aircraft Rigging

less than the numbers in your operations manual. While it's questionable whether any of .... A calibrated cable tensiometer is a must for this task, and the adjust-.
1MB taille 3 téléchargements 338 vues
nuts & bolts

maintenance & restoration Aircraft Rigging The key to speed and ef ficiency JEFF SIMON, EA A 478233

T

he term “rigging” has been around a lot longer than airplanes have. It’s one of the many carry-over terms from the sailing world that aided aviation during its infancy, and still holds them close together. After all, we use “nautical miles” for distance, “knots” for speed, and both pursuits have an uncanny way of emptying one’s wallet. Just as in sailing, trimming and tuning the way that we harness the wind is the key to performance and efficiency. Chances are that your aircraft’s performance is somewhat less than the numbers in your operations manual. While it’s questionable whether any of our planes actually ever flew quite as fast as the original advertisements boasted, there is a lot that you can do to get close.

Airframe Alignment Due to the handmade nature of general aviation aircraft, a certain degree of variation in the manufacturing process is inevitable. Manufacturers employ jigs and fixtures to get each part as close as possible to the ideal blueprint design.

Proper alignment and travel of the control surfaces is essential to safe operation.

112

AUGUST 2009

However, most designs also employ methods of fine-tuning the airframe such as spacers and shims to ensure that the proper dihedral is in the wings, angle of incidence is correct on all flying surfaces, and the airframe is as straight as possible. General aviation aircraft tend to lead long lives, and those lives can include all sorts of accidents, incidents, hangar rash, and other activities that can change the alignment of the airframe. In some cases, checking the alignment of the airframe is fairly straightforward; in others it can require specialized tools. However, there are a few simple checks that you can do to verify the basics. Begin the process by leveling the aircraft. Every aircraft has a level point at which the aircraft must first be leveled laterally. In many aircraft, the leveling point is between the two control yokes. This makes it easy to balance a bubblelevel across the yokes or the control shafts. Ideally, this process is conducted on a level surface, but as long as the parking spot isn’t too bad, you should be able to get the

Using the proper tools is essential for a proper rigging check.

aircraft level by simply adjusting the air pressure in the tires. (Don’t forget to re-inflate the tires before flying.) While you’re in the cockpit, the first thing to do is to ensure that the aircraft instruments are reading what the aircraft is actually doing. The turn and bank indicator and the attitude indicator should be showing the aircraft to be perfectly level. If they’re not, each instrument can be adjusted by loosening the front mounting screws and gently rotating the instrument to the correct position. I’ve heard of more than a few cases where pilots thought the aircraft was flying with a wing down, only to discover that the instruments were in need of adjustment. The next step is to ensure that the fixed surfaces of the aircraft are in alignment. Using a tape or string that does not stretch, measure the distance from each wingtip to a point on the centerline of the tail cone. The same technique can be used to check the horizontal stabilizer by measuring to a central point at the front of the aircraft. Do not use the front of the vertical stabilizer or the spinner as a measuring point because many aircraft are designed with asymmetric mountings to help reduce P-factor. The left and right measurements should be equal. If they’re not, the aircraft may have been improperly repaired following an accident. If your shop floor is perfectly flat, you may be able to measure the wing dihedral, but you’ll need to have access to the proper rigging tools to check the wing and tail angle of incidence. In most cases, you’ll find everything in order, but it always pays to check the fixed surfaces first, before spending time and money on the control surfaces.

(##EGTVKſGFJ[FTCWNKEQTGNGEVTKEEQPUVCPVURGGFRTQRGNNGTUHQTCNNJKIJRGTHQTOCPEGCKTETCHVU

+ORTQXGF2GTHQTOCPEG 7PNKOKVGF$NCFG.KHG .GUU8KDTCVKQP .GUU9GKIJV .GUU0QKUG

OH LODE D Y D  7&V 6  \ Q

0D

,I \RXDUHORRNLQJIRUSHUIRUPDQFHZHVHOOLW 755GTXKEG%GPVGT(NQTKFC /62TQRGNNGT75#+PE 2JQPG   KPHQ"OVRTQRGNNGTWUCEQO

*GCFSWCTVGTUKP)GTOCP[ /62TQRGNNGT'PVYKEMNWPI)OD* 2JQPG UCNGU"OVRTQRGNNGTEQO

Z Z Z P W  S U R S H O O H U F R P

Engine Alignment As we stated earlier, many aircraft designs set the engine thrust line at an angle to help reduce P-factor. In most aircraft, this alignment is built into the engine mount design. However, if corrections are required, washer spacers are inserted, as necessary, between the rubber dampers (LORD mounts) and the engine case. Most mounts consist of two solid rubber “biscuits” that encase a gelEAA Sport Aviation

113

maintenance & restoration filled core. When they are in good condition, they do an excellent job of isolating the engine from the steel engine mount. However, as they age, they begin to harden and sag under the weight of the engine. In addition, it’s quite possible that the engine alignment may not have been properly done at the last engine change. The bottom line is that if your prop isn’t pulling the plane in the direction you

want to go, chances are that you’re not getting to your destination as efficiently as possible. Adjusting your engine’s alignment is not a difficult task. The first step is to determine whether you have an alignment problem. On most aircraft, this can be easily accomplished by examining the alignment of the spinner to the nose bowl. Even small variations left, right, up, or down can cause problems.

Fly-In Theater

Sunday, July 26 - Saturday, August 1 Bring your blankets & lawn chairs and join us for an evening of classic aviation films under the stars. Presented by Ford Motor Company

Sunday, July 26th | One Six Right (2005) One Six Right is an exhilarating documentary film that celebrates the unsung hero of aviation - the local airport. Featuring thrilling aerial photography, the film dispels common misconceptions and opposes criticism of General Aviation airports. Barnstorming (2009) Presented by writer and producer Paul Glenshaw & Andrew King. Barnstorming is the true story of an unexpected friendship that developed between a farm family and two pilots who literally dropped out of the sky. Monday, July 27th | The Right Stuff (1983) In 1947, a group of determined men gathered at a remote Air Force base in California and the Mercury 7 astronauts were chosen. Their goal was to break the sound barrier by using a small rocketpowered test plane called the X-1. Knowing the risks that they were taking, they were classified as having “The Right Stuff” to handle the challenge. Tuesday, July 28th | Cloud Dancer (1980) Presented by EAA President Tom Poberezny & friends. Despite recurrent blackouts, a professional competition flyer continues his career. After a number of near disasters, he is faced with a difficult decision.

Supported by Hamilton Watches, The American Brand Since 1892

Wednesday, July 29th | Six Days Seven Nights (1998) Presented by actor and Chairman of the Young Eagles Program, Harrison Ford. Robin Monroe, a New York magazine editor, and the gruff pilot Quinn Harris must put aside their mutual dislike if they are to survive after crash landing on a deserted South Seas island. Thursday, July 30th | Bat 21(1988) Lt. Col. Iceal “Ham” Hambleton is a weapons countermeasures expert. When his aircraft is shot down over enemy territory he works with an Air Force reconnaissance pilot to map out an escape route. Along the way he has to face enemy forces and the death of some of his fellow soldiers. Friday, July 31st | X-15 (1961) Presented by X-15 test pilot and Astronaut LT General Joe Engle. X-15 is a fictionalized retelling of the story surrounding the historic X-15 spaceship, introducing the men who risked their lives in the name of space travel, as well as the women who stood by their sides. Saturday, August 1st | Pearl Harbor (2001) The classic story of the Japanese attack on Pearl Harbor is told through the eyes of two boyhood friends. Rafe, selected to fly with the British in Europe while America is still not at war, is shot down and presumed killed. Danny comforts Rafe’s former lover, but when Rafe turns up alive, the two former friends become enemies. Through the turmoil of Pearl Harbor, the two may reconcile their differences.

Free shows begin at 8:30 p.m. daily. Located at the north end of Doolittle Drive behind the Camp Store. A celebrity presenter will introduce each night’s film. Come and enjoy an aviation classic & free popcorn!

114

AUGUST 2009

If you determine that the engine requires alignment, this is also a good time to change the mounts. At a cost of more than $100 each, they are not cheap. But, they do have a limited lifespan, and most begin to lose their vibration dampening capabilities after about five years or so. Keeping them in good shape will save your avionics and gyros and improve the overall experience of your passengers. Adjusting the alignment is fairly straightforward. The mounts are loosened and spacers are inserted, as needed, between the rubber dampers and the engine case until the alignment is correct.

Control Surface Rigging Control surface rigging checks are probably the most neglected maintenance procedure on general aviation aircraft. If you don’t believe me, pick up your maintenance logbooks and look for an entry that says something like this: Control surface rigging checked and adjusted per manufacturer’s instructions. No luck? Don’t worry; you have plenty of company. It’s a well-known fact that drag reduction is the best way to increase aircraft performance. When we think traditionally about drag reduction, things like fairings and removing steps

and antennas come to mind. However, improperly rigged control surfaces top the list for reasons two aircraft of the same model can have very different performance numbers.

Control surface rigging checks are probably the most neglected maintenance procedure

adjustments following a rigging check. The rigging process for every aircraft varies by manufacturer. A good starting point is to check and set the cable tensions. A calibrated cable tensiometer is a must for this task, and the adjustment process can be quite complex on some aircraft. Every action has some reaction. For example, adjusting the cable tension at one point may cause the control surfaces to become misaligned with respect to each other. This

is why it is extremely important to follow the manufacturer’s rigging instructions carefully. You can save yourself a lot of time and frustration by using the right tools in the right order. Flaps and ailerons are probably the most common source of misalignment. Not only does each surface need to be set for the proper travel and alignment with the yoke, but each side must be aligned with its counterpart on the opposite side. Surprisingly, flaps are a

on general aviation aircraft. A properly rigged aircraft should be able to fly level and hands-off so long as the loading is balanced. If yours doesn’t, that’s a sure sign to check the rigging of the control surfaces. Other signs of rigging problems can include fixed trim tabs that are significantly bent to allow the aircraft to fly level, or the need to use regular aileron and rudder trim in straight and level flight. However, even if these symptoms are not present, it doesn’t hurt to go through a complete rigging check. It’s extremely unlikely that your aircraft is already properly rigged, and I have yet to hear of an aircraft that didn’t require

Adjusting the alignment is fairly straightforward. The mounts are loosened and spacers are inserted, as needed, between the rubber dampers and the engine case until the alignment is correct.

EAA Sport Aviation

115

maintenance & restoration very common cause of banking problems, and they can also have a significant effect on climb and cruise performance. Drooping flaps can induce a lot of drag. Depending on the aircraft, the flap-adjustment procedure may require applying upward pressure on the flaps to take out any play and properly check the alignment. Aircraft designs, such as the Maule, reflex the flaps upward to reduce wing drag during cruise flight. This is also a technique used by some air racers. Reflexing the flaps effectively reduces the chord of the wing and, therefore, reduces drag in cruise. However, be warned that setting the flaps outside of the manufacturer’s specifications is illegal on certificated aircraft and makes you an unwitting test pilot regardless of the aircraft category. Once all of the control surfaces are properly rigged, you should straighten any fixed trim tabs so that you will have a clean starting point from which to work. Then you can proceed to the flight-testing and trim-tab adjustment phase.

rudder pedals and check the ball to see if the aircraft is yawing left or right. If the aircraft has adjustable rudder trim, trim out any undesired yaw. If not, you will need to land to adjust the fixed-rudder trim tab and repeat the process. With the rudder trim properly set, you can proceed to the aileron trim. Again, set the aircraft for stable, level flight with a centered ball in the turn and bank. If the aircraft has adjustable aileron trim, you can complete the process in the air. But, for most aircraft, you’ll need to land and adjust the aileron trim tabs. Once the aircraft is perfectly trimmed, make a note of the position of the trim tabs. Only minor inputs should have been necessary to achieve coordinated flight. If the tabs need to be excessively bent to achieve level flight, it’s quite possible that something else is going on with the aircraft and further investigation is warranted. A well-rigged aircraft is a pleasure to fly. With a little effort and the right set of tools, you can eliminate the constant crosswind you’ve been fighting, gain a knot or two, and renew your love affair with your favorite bird!

Flight Test When flight testing, be sure to pick a calm day and begin by setting up a stable cruise speed. The aircraft should be loaded as it would be on your typical flights. Trim the elevator first so that the aircraft does not tend to pitch up or down when flying hands-off. Next, take your feet off the

116

AUGUST 2009

Jeff Simon is the president of Approach Aviation, a provider of educational products, tools, and supplies for aircraft owners. To learn more about aircraft ownership and maintenance, visit www.ApproachAviation.com or call, toll-free, 877-564-4457.