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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,
Would someone be kind enough to share with me a weight and balance spreadsheet for my 2008 CTLS that I can use on the iMac with all the proper arms. I'm having difficulties finding this information for pilot, baggage, etc. POH doesn't help. Thanks!
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The Anatomy of a Carb Sync
How do you know which one to adjust?
The carburetor sync on a 2 stroke or a 4 stroke is one of the most important functions to keep up with for the health of your engine. Let’s take a look at performing a carb sync on a 912 series engine. The carb sync is nothing to be afraid of and with a few times at bat performing this function it will become fairly easy. First, why is it so important? The carb sync should be done anytime the carbs or throttle cables are removed or adjusted and at the 100 hour or Annual Condition Inspections. The reason for this is cables stretch, the pulley system wears, cables slip and parts wear and have more tolerances. The carbs are almost always out of sync at each 100 hours or the Annual. If you did a carb sync back at the last inspection then they may not be out of sync much, but they will be out and then your sync job should be easier. The sync instrument should also be used to set the idle sync if you change idle settings. Let’s start off with thinking of the engine as two engines, a left side and a right side. Two carbs controlling different sides of the engine. You don’t want one side trying to operate at 5000 rpm while the other side is trying to operate at 5100 rpm. These opposing rpms will cause excessive stress and wear on your engine over time and possible damage. You say there is a balance tube in between to help balance them out.The operative word in that sentence is “help”. The balance tube can correct and help with small differences between the two carbs, but it is not a cure all and it is there to help make the system run a little smoother than if there was no connection or correlation between the two carbs.
So which sync instrument to use? Well that is up to you, but here are a few considerations. You might use an electronic sync instrument like a CarbMate or a Syncromate or a set of gauges. Here are a few pros and cons of each sync instrument. The electronic instrument may have the capability to split hairs and give you a very fine adjustment, but they are harder to interpret as far as knowing which carb you want to adjust to achieve a specific goal to bring the two carb vacuums together. It takes more time and going back and forth to get this accuracy. You also need a power supply like your battery to attach electrical leads to operate the instrument. There is nothing wrong with this, it’s just different. The gauges (liquid filled are better for dampening and needle valves in line to assist for dampening needle pulsation) allow the user to see immediately which carb he needs to adjust and how much he may need to make this adjustment. This writers’ one thought here is; does the accuracy of an electronic device to split hairs that fine over a gauge really make a difference and can the carbs and engine really tell a difference? If you pay attention to detail and use good gauges you can be very accurate. The drawback to gauges may be not as an accurate setting as the electronic device. Picking one of these sync instruments is strictly up to the end user and their personal preference.
Let’s move on to the actual anatomy of the sync and what to look for. I would like this discussion to be on the use of the gauges because it will offer some visual numbers to work with. First the engine should be up to operating temperature. Safety first so put in place; wheel chocks, hearing protection, eye protection and a person at the controls for safety. Now you need to separate both carbs. You can use hose pinch pliers to clamp off the rubber hose between the carbs or just remove one side rubber hose off the air intake 90 degree nipple and plug you gauge into the rubber hose end and the other over the metal nipple. There are two small screws on top of each air intake you can screw your sync instruments into also, but you still need to address isolating the carbs. This writer prefers to slide the rubber hose back off one carb since it makes sure the carbs are fully isolated and no leak from the hose pinch pliers could occur. This is only what I prefer; it’s up to you to choose your method.
There are two syncs to perform, the mechanical sync and the pneumatic sync. The mechanical sync is shown in the Rotax Owners video (http://www.rotax-own...-exp-si-912-018) and described in the Rotax Line Maintenance manual and it’s quick and easy to perform. So now you’re all set in your safety gear so have your safety cockpit operator start the engine. (Don’t forget to advise them that if they see you spin more than three times in the prop to turn the engine off and make sure your cockpit manager likes you and don’t use your wife right after an argument. )
Now we have the engine running and we take a look at our gauge set. If the needles are pulsating some then close the needle valves slightly until they stop and become smooth. The Rotax manual uses 2500 rpm for a sync reference for the higher rpm setting, but I will tell you from years of experience that if you do that they will be out of sync when you advance the throttle on up to 3500-4000 rpm. Just like many instruments or devices we use in life you are usually advised they are not accurate in the lower percentages or the extreme high percentages of their operating range. That puts 2500 rpm too low on the scale for fuel and air flow (needle position in the jet) to be accurate and we don’t fly near the idle side of that rpm, so why would you sync your carbs for the higher rpms at such a low rpm. Let’s mention here to that to adjust the higher rpms you adjust the Bowden cable screw either in or out which will add or subtract some rpm. You use the idle adjustment stop screw to affect the engine idle only. You do sometimes need to adjust the Bowden cable length to get the idle screw to have enough affect, but we can cross that bridge later.
Okay back to our running engine. Have your cockpit operator advance the throttle up 3500 rpm. All joking aside give that prop a wide berth. (My unlucky partner “Lefty”has a hard time holding two wrenches) So with the engine running at 3500 rpm we look at the gauges and see that the left side is at 5” of vacuum (more fuel)and the right side is at 6” of vacuum (less fuel). (Vacuum is expressed in inches of water “H2O or inches of mercury “Hg) The higher the vacuum in our case (6”) the harder the carb is trying to draw in air and fuel, leaner , less fuel. The lower the vacuum (5”) the more fuel it is receiving (richer). Keep this in your head about vacuum; the higher number is less fuel (leaner)and the lower the vacuum number, more fuel (richer). Now let’s go to the left side and loosen the Bowden adjustment nuts and screw it back out toward the cable and shorten the cable which pulls the throttle arm and reduces the rpm and fuel flow. Adjust it back until its 5” moves to 6” like the 6” on the right side.Now they should both be equal at 6” of vacuum at 3500+ rpm. If you went to adjust this left side and the adjustment was already way back and you didn't have enough adjustment there to pull it back any farther then you have two choices. Go to the other side and adjust that Bowden cable adjuster forward to lengthen it and lower the vacuum towards the left side. The other thing you may need to do is shut down the engine,screw the Bowden cable adjustment in towards the half way position and then loosen the cable at the throttle arm screw and shorten it by 1/16” to give you more room to adjust the Bowden cable adjuster farther back on that left side. Sometimes because of how these are setup you may need to adjust one side back a tad and adjust the other side forward a tad to make them equal and not run out of adjustment on either side.
Now pull the throttle back to idle and see where it is. If you have a 912ULS a good idle is around 1750-1850 rpm because of the vibration and hammering from the higher compression of this engine. Now if your idle is too high after you pulled the throttle back then look at the gauge and see which gauge has the lower vacuum number. Remember the lower the number the more fuel it is receiving. Let’s say the idle rpm is 1900 rpm and you want 1800 rpm. The right carb gauge is at 12” and the left carb is at 11”. The carb on the left side is getting more fuel and the rpm is too high. So that is the carb we want to reduce the rpm on and raise the vacuum to get to 12” like the right side. So you back out the idle stop screw and the 11” of vacuum raises to 12” of vacuum like the right side. If that made your idle rpm 1800 and you are happy then you’re done. If your idle rpms were still too high then back the idle stop screws out on both sides a little more until the idle rpm is where you want it and the vacuum on both sides is equal. Always double check your work. Run the engine back up to 3500+ rpm and see if the needles are still equal and if not then tweak the Bowden cable adjuster on the side you want to affect. Then back to idle to check that vacuum setting and the idle rpm. If you idle for a long time making an adjustment then run the engine up for a few seconds now and then to help keep it cleared out and from loading up at those low rpms. If your idle rpm was too low (1600 rpm) then screw the idle stop screw in more on the carb with the higher vacuum 12” down to 11” until the vacuum number lowers to match the other side of 11”and the idle comes up where you want it.
After you have doubled checked your work then shut down the engine and make sure all the nuts to the Bowden cable adjuster are snug. Remove the gauge set and connect the carb balance tube setup. Even after a sync the engine may be slightly rougher with the carbs separated, but should be a little smoother when it is reconnected.
Two last parting comments. The throttle in your cockpit at idle should have a stop on it and when you pull it back to its stop at idle then the idle stop screw on the carb should make contact at the same time. If you do not have a throttle stop for idle then you will most likely bend the idle stop lever on the carb.You will over power it and if you do or have the idle set too low then you stand a high much bigger chance of stalling your engine from low rpm and it won’t be when you want it to quit.
Second; You should balance the carbs at the high rpm and at idle. I have seen some back off the idle stop screw until it no longer functions and that means the carbs can only be synced at the higher rpms and not at idle. That means the engine is operating at idle at opposing rpms. If you thought it was important to sync your carbs at the higher rpms to keep them from opposing each other, reduce vibration and from hammering the engine why on earth would anyone not sync them at idle? This is a poor practice to get into. You spend a lot of time idling. Remember what our Dad’s told us; “If it’s worth doing it’s worth doing right”.
I know this was along article, but I thought it may be worth covering for some the Rotax owners.If you fell asleep half way through, print it out and take it to the airfield.
FLY SAFE AND FAR AND ABOVE ALL HAVE FUN LIVING YOUR DREAM!
Your Rotax engine will give many hours of trouble free operation. Just follow the Rotax manuals and provide it with the prescribed on time maintenance, but not necessarily your neighbor’s advice.
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Let's talk about how much runway do you need to land. What is the shortest you have ever landed on? Let's here from all you other CT pilots. <br style=""> <br style="">
Here are some considerations for a short runway landing for a Flight Design CT and will differ from plane to plane and even from person to person. Landing at these short runways are at your own risk.
Many people in rural areas or overseas in the UK and Europe land in these distances on a regular basis. A challenge from our UK friends is what spurred me to practice and play with these short fields.
This question is really one that has a few different facets. For me it is 300m or 1000'. We need to look at a few things here, your own flying skills, training, runway condition, atmospheric conditions, aircraft landing configuration, approach and some good judgment when you do pick a short runway. Let’s not leave out our buddy the wind. Let's look at a few variables.
First what is the runway made of: asphalt, dirt or nice energy absorbing grass. Grass a little long would be best. Plus something to think about while you are sizing this up is how wide is it? Will it allow for a little error to the left or right, something to consider. What is on either end of this runway? Is it flat off the end of the runway or does it have tress? Can I make a flat approach or does it have to be steep due to obstructions on the ends. If only one end has an obstruction that steepens your approach then maybe the other end is a better choice to land.
Ok, here is a big item, your own personal training or practice time. When at longer runways practice and practice again hitting a specific target. Learn to set those wheels down just as close as possible to a pre-selected spot every time. Then pick a different spot and learn to hit that spot. This type of practice will always pay off especially if you ever have an engine out situation and need to land in a short distance. Most of us aren't going to have an 8K' runway below us if the engine ever goes out. If I was that lucky I would buy a lottery ticket. So when you approach this very short runway you will be able to set those wheels down in the first 10% or less of the runway and hit your exact spot. Remember any runway behind you is a waste of your runway and if you land too long may add to you pucker factor. Speaking of this pucker factor or being in the panic mode is the worst thing that can happen. When in the panic mode all logical and rational thought ceases. If you practiced like you play then you shouldn't have this problem because your mind will already be set up for this eventuality. So now you are practicing and it may be on a very long runway, but pick out two markers or the beginning of the runway to a marker to stop short by. Now what about the plane? Well landing at zero flaps and at 60 knots is not the best choice. The better choice is flaps at 30-40 and 50 knots. This will keep you slow enough to dump some of that energy quickly on approach and once you touch down. Those 30-40 nice big flaps are good air drag surfaces. Now for the approach, you will need to approach this particular short runway a little flatter on approach than you would normally tackle a nice long runway. A flatter approach, landing right at the beginning of the runway with 30-40 flaps at 50 knots should pretty much guarantee plenty of room to land. One little thing to look at in addition to this is, am I really heavy at max weight or do I weigh 150 lbs and only have half a tank of fuel and solo? This is less kinetic energy to stop. Know your brakes!! Do my brakes stop me well or do they fade with heat and I just keep coasting? Do I have to pump them. These are not conducive for short runways.
So we have looked at runway length, type of surface, flaps, speed, approach angle, brakes, weight and your skill level. Landing in 1000’ isn’t hard, but does put more demands on being a better pilot and setting yourself up for success early.
Then the inevitable happens! What if I’m too fast, too long, too high, bounce and too much brake or it just doesn’t feel right? Recognize these early!!!
Try again and fix what might have been wrong with the first approach. Take as many approaches as necessary to get it right. Don’t sacrifice safety for your pride.
Practice, Practice, Practice. Hit your target and know your plane!