Leaderboard

Vibration Common Causes and Fixes I get many phone calls on vibration in owner’s aircraft. Here we’ll talk about some of the common causes, where to look and what you can do to help mitigate these vibration issues. Here is a common list, but is not all inclusive; 1. Carbs not synced properly. 2. Carb vent hose improperly placed or removed. 3. Carbs not opening equally or fully. 4. Prop blades not the same pitch or out of track. 5. Prop out of balance. 6. Aircraft wheels not balanced. 7. Old rubber engine mounts. 8. Mag drop difference too wide between ignition modules. 9. Trigger coil air gaps too wide. 10. Gearbox worn, damaged or in need of maintenance. So let’s address each of these. The carb sync (#1) should be fairly obvious to most now. The carbs should be synced at each annual / 100 hour inspection or anytime they have been removed for maintenance or you suspect a problem like vibration and you need to rule this in or out as the problem. Carb sync is vital to a good smooth long lasting running engine. You don’t want one side trying to run at 5100 rpm while the other may be trying to run at 5200 rpm. Sync those carbs. Once done it’s easy to keep them there. The carb vent hose (#2) that may be attached to the standard Rotax air box, a small clear plastic tube on the side of the carb under the carb bowl bale or some others have them routed to different places. These hoses should be as close to equal length as possible and be routed to the same area of pressure. If one hose has fallen off the side of the carb and the other is still attached it will cause the carbs to become unbalanced which will cause your vibration. Do not place these hose ends in the air stream outside the cowl. These only take a minute to confirm their attachment and placement. Check to see if the carbs open equally (#3) by moving the throttle from idle to wide open when the engine is off. You may see some signs of this during a carb sync, but most people don’t go above 3500 rpm for a carb sync so you need to double check this while the engine is off to see if they do in fact reach WOT at the same time or if one hangs up slightly. The prop blades all too often are not the same pitch (#4) from blade to blade. This is easy to double check and can be done with either a prop protractor and or a 12” digital level. Measure back from each tip 8”- 9” and put a mark on each blade. Make the blade out to your right level with the floor and then put the level on the back of the blade where you made the line from the tip. The blades should be no more than one tenth of a degree out from each other. That measurement seems small, but it is quite easy to accomplish. The Sensenich prop gauge pins are not accurate enough. Check them by hand with a prop gauge or level once you are close. To check tracking place a box underneath the bottom tip of a blade pointing straight down. Put a line on the box where that tip just barely touches the box. Then swing the other blade(s) around and see if they all cross at the exact same mark. If they don’t you’ll need to loosen the prop flange bolts and re-torque them to get the blades to all track over your line on the box. Prop blades now days are much better in balance (#5) than they were decades ago, but all props should still be dynamically balanced. All wood blades in humid climates can change due to moisture absorption. With all the new composites that aren’t susceptible to this anymore I’m not a fan of all wood blades. Even the main bolts change torque with humidity changes. A dynamic balance will not only help vibration, but will help save your gearbox from wear or damage. The heavier the blades i.e. long Warp Drive props the more important this becomes. I have never found an aircraft wheel (#6) in balance. Most do not ever think about the smaller aircraft wheel being out of balance as a vibration cause, but over the years I have cured many a vibration just by balancing the wheels. I always balance all new wheels I install. I see some occasionally that would need up to 20 x ¼ oz. weights to bring them in balance. If you failed to balance your wheels you would never find this huge disparity. These come off and go back to the distributor. What I normally see is 2 – 8 x ¼ oz. weights per wheel. It usually takes me about 3-5 minutes to balance a wheel after it’s off the plane. Don’t disregard this when you are looking for a vibration cure. Old rubber engine mounts (#7) are a common problem. Rotax wants a 5 year rubber replacement which I’m a fan of. This includes the rubber engine mounts. Rubber can get hard or soft from repeated heating and cooling cycles plus chemical exposure and just the ozone in the air. I replace these every time I do a rubber replacement on an aircraft. It usually isn’t hard or expensive. The mag drop vibration (#8) should be obvious when you do your mag drop check. Most see anywhere from 40 rpm – 100 rpm as a normal drop and usually both mags are within about 10 rpm – 30 rpm of each other. If you experience 300-1000 rpm drop then it’s time to troubleshoot your ignition system. There are documents out there that tell you how and where to look for ignition issues. It could just be a bad plug, too wide a plug gap, a bad plug boot, a bad connection at the plug boot where the wire screws in. If it is a large drop like 800+ rpm it may be a bad ignition module. These are all items you need to rule in or out. Always start with the most common, easiest and cheapest first. Do not just throw money at everything hoping to hit the jackpot. Most ignition issues are simple common issues. The trigger coils (#9) in the flywheel compartment can at times have too wide an air gap between the pick-up and flywheel trigger point. These are checked by using a feeler gauge and checking the gap tolerances listed in the Heavy Maintenance manual and setting them to the proper gap. These can even be off from the factory so check them before installing a new engine when they are easy to get to. You not only are checking the gap, but the screw torque for tightness. Gearbox (#10) care is important. As you look for your vibration issue consider the gearbox. It has maintenance service times at either 600 or 1000 hours. Using an automotive oil over a motorcycle oil can cause premature wear and damage. At your 100 and annual inspections you should be doing a gearbox friction torque check. Normal measurements that I usually see in the field is between 425-490 in. lbs. There is a low limit, but I personally don’t like to see anything in the 300 in. lb. numbers. It only takes a few minutes to perform. Checking the magnetic oil plug for debris at every oil change is another check for gearbox wear and damage. Prop strikes should have the gearbox removed and sent to a distributor for a special inspection. Gearbox’s when taken care of tend to last a long time, but there have been a few with excessive wear in early run hours. There have been some with the 912iS engine. These are the 10 common causes for unwanted vibration. Most are easy to fix and find. When trouble shooting start with the cheapest and easiest to rule in or out and progress to the harder least common when you do your checks. Whatever you do be methodical and don’t jump all around to exotic areas to check. Most Rotax issues are easy to find when you start at “A” and then work to B, then C and so on. I hope this helps some reduce any frustration in locating an unwanted vibration. Signed your friendly, Safety Officer.

Attention all Pilots and planes. Come join us on our annual Page Arizona (KPGA) Fly-In this October 12-16, 2022. We fly together each day to various destinations around the Southwest while enjoying some awesome weather and camaraderie. We meet and stay in Page, Arizona then fly-out to places like Sedona, Grand Canyon, Monument Valley, Marble Canyon, Escalante, Bryce Canyon and more. Come out and meet some new friends and destinations. Hotel rooms are typically only $110 per night so stay as many days as you like. We all fly in the mornings then get together in the evening for Dinner, drinks and stories. Contact me at DJToddB@yahoo.com for more information. Some of us will also be starting the Page trip at Bryce Canyon for some Hiking and awesome Views from Oct 10-12, 2022. Damir Anteljevic.mp4 431092855_Bryce-2019.mp4

Have you ever dropped something down through the engine compartment and can't seem to find it. Here is a possibility. Just helped a friend with a Rotax 912 UL who reported a curious high pitched grinding at the aft of the engine while moving the prop. Turns out it was fragments of a wing type camloc and a whole camloc which he had dropped at some point in the past which had just recently found their way between the coils of the stator and the inner magnets of the flywheel. The bits were scraping the inner aspect of the flywheel as the magnets attracted them and the stator fingers kept them in one place while the flywheel moved by. His engine had no cover over the flywheel as some engines don't. Seems there was no harm done, just some grooves in the flywheel. Glad he noticed the sound on preflight.

PROCEDURES FOR MECHANICAL AND PNEUMATIC SYNCHRONIZATION OF THE 912 CARBS: This will get you through the basics and after you do a sync or two it will get easier and you will get better. This is not an official document and is only meant for your edification and will get you through a carb sync. Some experienced people that do carb syncs on a 912’s may vary some, but the basics ideas are the same. Mechanical Synch of the Carburetors – Do this First Bowden cables adjustment screws (cable housing adjuster) should be centered, but not mandatory. By having the Bowden cable adjustments in the middle of their adjustment range will give you a little more adjustment when you get to that part of the procedure. Adjust throttle cables so that both carb throttle arms move simultaneously to both extremes (open and closed). Ensure Idle Mixture Control Screw is properly set (on the bottom of each carb). Screw them all the way in then back out 1½ turns. Reverse the throttle arm springs so they pull toward “closed” throttle just to facilitate mechanical sync. Unscrew the Idle Stop Screw. Make sure the throttle is closed in the cockpit. With a .004” feeler gauge between the screw and the arm tighten the screw until it just touches. There should be minimal friction on the feeler gauge. Remove the feeler gauge and screw the Idle Stop Screw in 1 turn. Return springs to original configuration (i.e., pulling the throttle arms toward to the “open position”). Connect the Pneumatic Synch Tool It is imperative to have the engine up to the operating temperature. Remove one end of the rubber crossover compensating tube between the two intake manifolds or connect to the top of the intake manifolds or use the cross over tube rubber hose pinch off method shown in Rotax owner videos. All these attachment methods will work. Hook up gauges or electronic sync tool. Secure one end of a gauge or electronic device to the rubber end of the compensating tube and the other to the gauge at the 90 degree fitting coming out of the intake manifold where the compensating tube hose was removed. Secure both attachments with a hose clamps to prevent leaks. Leaks will affect the results of your sync process. You can hook up your sync tool at the small screw on top of the intake manifold, but then you have to pinch off the rubber hose between the carbs on the cross over compensating line. I find this is just more work and you need to make sure the rubber hose on the compensating tubes are long enough (usually newer engines) and you don’t allow any leaks between the carbs. Many people have to cut the tube shorter and make the rubber hose longer to pinch it if you have an older engine with short rubber connecting tubes. I think it is easier and faster by just pulling the hose off as originally described and it facilitates complete carb separation. Make sure you have good brakes, use wheel chocks and or if you have not so good brakes or no wheel chocks then tie your plane down and always have someone in the cockpit. Never do this alone while running an engine and no one in the cockpit.. “On Idle” Synch NOTE* Many people like to do the high rpm or off idle adjustment first and then do the idle set. I would do the high rpm sync first because it will affect the idle sync. Note* The gauge or carb with the lower vacuum number is getting more air/fuel and the gauge or carb with the higher vacuum number is getting less air/fuel. Bring the cockpit throttle lever to idle stop. Ideal idle RPM is approximately 1700-1800 rpm depending on your specific aircraft and needs. Adjust idle setting to this value. The 912UL 80 HP can handle a little lower idle rpm over the 912ULS 100 HP due to compression ratio. If the idle is high (say, 1900), adjust the carb with the lower vacuum number on the gauge because that is the one getting more air/fuel. Retard the Idle Set Screw until the vacuum numbers are the same. Example: If one gauge is at 15” of vacuum and one at 14” of vacuum retard the lower 14” until it drops to 15” to equal the other carb which will now lower your idle rpm. If idle is low (say, 1600), adjust the carb with the higher value, it is getting less air/fuel. Advance the Idle Set Screw until the vacuum readings are the same. Example: If one carb is at 15” and one at 14” turn the idle stop screw in on the carb with 15” of vacuum to match the one with 14”. This will raise the overall idle rpm. Needles should now match. “Off Idle” or high rpm Synch If while running the engine the gauge needles shake then you will need to install or close down the inline needle valves until the gauge needles stop shaking and pulsating. Now the needles will likely show different vacuum readings. Do this sync before the idle sync because this sync usually will affect your idle sync set point. Run engine at approximately 3500 RPM. Do the “Off Idle” synchronizations at this rpm. I prefer 3500 rpm to start since that is closer to where you run the engine. You may find that setting the balance at lower rpms doesn’t stay very accurate and that as you move up in rpm to 4000-4500 rpm they are out of sync again. Set them at the higher rpm or at least double check the higher rpm when you think you are done. There is a good chance they won’t be in sync if you used a low (<2500) rpm to set them. These adjustments will be made only at the brass Bowden cable adjustment for the higher rpm sync, where the cable housing meets the carb adjuster on top of the carb, do not touch the idle set screws. Note* The gauge or carb with the lower reading is getting more air/fuel and the gauge or carb with the higher reading is getting less air/fuel. Compare vacuum gauge readings. Normally, but not always, adjust the carb (retard) with the LOWER vacuum reading (getting more fuel) back down towards the carb with the higher vacuum reading. Adjust the Bowden cable screw outward (toward reducing the air/fuel) so the throttle arm backs off (decreased RPM) to bring vacuum reading to a higher vacuum number. When vacuum readings are equal, lock it down. If vacuum readings are the same, bring throttle lever back to idle stop then advance it again to 3500 rpm. Verify vacuum readings are still the same for both carbs. If readings are different adjust the carb’s Bowden cable with the lowest gauge reading to match the higher gauge. Once the gauges are aligned move the throttle back to idle and back to 3500 rpm a time or two to make sure the carbs stay equal. Throttle to idle. Re-adjust the Idle Stop Screw if necessary at this point. You are now done. Remove the gauges and hook the carb compensating tube back up. The engine will run slightly smoother after the compensating tube is reconnected and sometimes the idle will be slightly higher after the cross over balance tube is reconnected. If you know this then while doing the sync set the idle 50 rpm lower so when the balance tube is reconnected you be right on your personal target.