Vibration with Reduction in Power

[mocfly]

Yesterday while on my way to and returning from Peoria I got a pretty strong vibration when reducing the power from high cruise(5250) to 4000. It was so pronounced that I actually had to increase it a little and then wait a few seconds before reducing it again.

 

I am not quite sure where to start.

 

Ideas?

 

I was gonna start with a check of the prop pitch.

[Tip]

Make sure your balance tube that passes through the air box has the holes inside the box. I have had this happen on my LS several times and I had a balance hose come off the left carb both times.

[Duane Jefts]

Any secondary indications? For example low Cylinder Head Temperature on one side or the other?

[Roger Lee]

The vent tube that passes through the airbox would cause a steady rough engine and not intermittent. may need an carb sync, air hitting the prop at an odd angle, one carb not responding as quickly as the other at the throttle, gear box could need re-shimming (regardless of hours , it's more calendar time), loose engine mounts to the firewall, rubber engine mounts needing replacement. these are the most common.

 

Prop blades being unequal could cause this at certain rpms, but if they were good before I highly doubt they moved. It only takes 5 min. to check.

[mocfly]

Vent tube is ok. Verified.

 

Carb Sync may be it. Here is the reason. When I was in NJ I had experienced a momentary shudder during operation which was a cause for concern. It was recommended that I look in the carb bowls for debris and other abnormalities. I did this and while there were abnormalities discovered the one change noticed was a reduction in idle rpm. I was at roughly 1700 before the inspection and it went to 1300 after. I just recently purchased a set of vac gauges to make sure that the carbs were in sync. I will be checking the balance in the next day or so. Let you know what I find.

 

I will double check the engine mount torques and advise.

 

CHT is only measured on one cylinder in my SW

 

Speaking of engine mounts does this look like to much of a droop?

 

ADS-B gave me enough info to go around this little cell, nice to see the result of some of my tax dollars.

It never ceases to amaze me how beautiful these types of weather cells look from above and afar.

 

 

[Roger Lee]

It's not so much the droop. use a 8mm ball end Allen wrench and check the 4 Rotax ring mount bolts for tightness. Are the rubber engine mounts cracked or split or older than 5 years. If the idle dropped that far then the carb bowl could easily be clean, but still have a blockage in the idle circuit which isn't operating on the main jet. Personally I would do a carb sync and check both carbs with gauges and not an electronic sync tool. You can not diagnose with an electronic tool. Depending on what I saw I would pull both carbs, strip them and flush every orifice with carb cleaner backed up by high pressure air. From the additional ifo this sure sounds like a carb issue and is where I would focus my attention up front.

[mocfly]

Roger,

If imam gonna get that deep into the carb are there any instructions on proper tear down and re-assembly technique? And being that far into it should i go ahead and replace some of the wear items for good measure?

 

[FastEddieB]

I have overhauled nearly identical BING carbs on BMW airhead motorcycles. It's not rocket science, but again...

 

...until you get your LSRM-A, or A&P, or go Experimental, you pretty much can't hardly touch your carbs without an authorized mechanic supervising and signing off on the work.

[Tom Baker]

I worked on an airplane that was doing something like this and it had some black gooey stuff in one of the carbs causing it to lag behind the other. Take the 3" hoses that go to the airfilter off and reach in the carb and lift up on the slide. Check to make sure they are moving OK.

[mocfly]

Tom,

Thanks for that easy check

 

Eddie,

Thanks for the recital.

 

[Roger Lee]

If the carbs are more than 6 years old and have 800 plus hours on them and you are going to strip them totally down then I would do a re-build and then not mess with them for a long time to come. If you just clean them which I would do under many circumstances then just know at some time you'll have to do it all over again. Do you want to do it once or twice?

 

Give me a call and maybe I can give you some better parameters to make your decision.

[CT4ME]

At least on my aircraft, reducing from 5200 to 4000 has always given me "more vibrations" than at cruise. Nothing dramatic, but noticeable. I've always chocked it up to the Rotax lore that the engine is tuned to run at 5200-5400, and that 4000 with just not optimum. Anyone else?

[Roger Lee]

Hi Tim,

 

It could be the flex in the prop unloading.

[sandpiper]

Mine has done that in the winter. Barely noticeable but it's there. A carb balance has taken care of what was happening in my case.

[Runtoeat]

4 things to check for vibration per my experience smoothing out my CTSW. (first three "must do")

1. Equal pitch on all three blades (need good digital measuring method here for tenth of degree)

2. Carb Balance @ 3500 rpm

3. Insure equalizing tube inserted into airbox so holes are not exposed

4. Dynamic prop balance. (This is optional but still important)

 

Note: A rear quartering cross wind will sometimes set up a small temporary vibration while landing with power pulled or when taxiing. Maybe this pertains to Roger's "unloading" comment?

[FastEddieB]

 

Note: A rear quartering cross wind will sometimes set up a small temporary vibration while landing with power pulled...

 

As a minor quibble, there should be no effective "wind" as long as the plane is still airborne. Once on the ground, its another matter.

[Runtoeat]

Eddie, the wind I refer to is a cross quartering wind of any velocity which is greater than the velocity component of my CT in this wind direction. While taxiing, this velocity doesn't need to be much to be greater than the vectored side velocity of my plane. While flying, the wind that may buffet my prop may be any where's from less than or greater than the vectored velocity of my plane as related to the wind direction. The Dynon panel is neat because it gives direction and speed of the winds aloft while flying and this is found to be at oblique angles to direction of travel many times. It is during the times when I am slowed on final approach that this wind is sometimes greater than my vectored velocity in the wind direction and if the delta for this velocity is sufficient, buffeting of the prop is noticed in the form of vibration.

[Ed Cesnalis]

Dick,

 

Eddie is right, your plane including the prop flies in the relative wind, no matter the direction the wind won't effect the prop.

 

But your right too, if there is wind there is usually wind sheer and this does effect your plane and prop.

 

What do you mean by the 'velocity component of your CT'?

[Runtoeat]

Tim and Ed, I see your resoning and now see where I am incorrect. What I am refering to when I say "velocity component" is the components of forward travel and of the relative wind that are used to calculate the "resultant" vector one uses to set a heading that will provide the desired direction over the ground ("track"). I think I'm setting myself up for the vultures here. Tim, I think you may have hit on what is probably the cause for my occasional vibration. This could be caused by wind shear or by gusting, both of these present a momentary condition that could upset the prop.

[FastEddieB]

To avoid hijacking the thread, I'm going to start a new one later titled, "Stick and Rudder" Moments?.

 

In any case, my contention here is that the wind, per se´, cannot cause engine roughness.

 

Gusts conceivably might.

 

Wind shear conceivable might.

 

Slipping conceivably might.

 

But there is no "wind" to an aircraft in flight, and please address these assertions in my thread to follow. Its unlikely that wind is a factor here.

[cdarza]

CTSW 2006;      Took off today from sea level with 5000rpm on the roll out and climb. Was all good until I got to 1500ft, levelled off and once rpm sped up to roughly 5400 I reduced power to achieve 5000 cruise as I usually do.   Engine got really rough and only got better with full rpm.  Made a quick return to land but since I had lots of altitude I kept playing with the throttle and still got rough on most throttle settings except full power, which was smooth.  Landed and upon inspection I found the balance tube (if thats the correct name for it) had come off on one side.  Seems kinda obvious this is the culprit right ?  I did multiple static runups  2000 to 5000rpm on the ground (with the vent tube still unattached) and could not replicate the engine roughness.   Dumb question; is it  the altitude that would create a difference ?  I guess I don't fully understand the basic principle of how that works....  anyone anyone ??      Edit: forgot to mention; i replaced the gaskets on the carb bowls earlier that day.  I assume during that work i must have caused that tube to come apart and didnt notice.     

[Tom Baker]

It has to do with a pressure difference between inside the cowling and the airbox. On the ground the pressures will be fairly even. In flight there can be a difference in pressure.

[Roger Lee]

The carbs are vented to outside air. So long as they are equal all is good and the carbs stay balanced. These tubes may be vented to just the air along side each carb or go back into the air intake system, but the pressure on each carb needs to remain as equal as possible. 

Change the pressure inside the carb bowl on either carb changes the carb balance between the two carbs and you get rough running which is more pronounced at higher rpms than low rpms you use on the ground.

[Madhatter]

My CT always had some vibration when going from high cruise to reduced power until a positive thrust was available. The E-prop eliminated this issue almost completely. 

[Roger Lee]

It's normal to have some minor vibration as you throttle back and unload the prop in the pattern. Usually around 3800-4200 depending on the plane, wind and pilot. If there is vibration up around 4800 rpm that's usually carb sync.