Vibration can be very hard to find

I’ve been following the various threads on vibration and thought
the following would add some value. Note that my SR22 does have
vibration but it doesn’t seem as significant as many report
(but it is there).

I have a close friend who has a 2000 B36TC Jaguar Edition with a
balanced special-edition Continental turbo 540. Quite simply, it’s the
smoothest single I’ve ever flown and the engine has near turbine
balance. That said, about five months ago the airplane developed a
minor vibration after some engine and avionics work. The shop put over
50 hours into the problem before pleading to Continental to become
involved. Continental approved the replacement of the turbo charger and
some exhaust components. No joy. Finally, after an unrelated check of
an avionics problem, it was found that the transponder, at the very bottom
of the stack, was contacting a mounting bracket of the panel which transferred
(acting as a conduit) a vibration to the panel, then to the yoke which continued
the vibration across the control surfaces to the wing tips!!

So, in my mind, Cirrus has a point with this issue. Vibration can be very
hard to find and we have open minds about the root cause. We must keep
looking…

Chris SR22 N747SJ

One has to realize that vibration can be a very subjective thing, but it need not be. Vibrometers exist. Many are for just this kind of application. (Click here here.) But short of deploying these to the masses with detailed instructions and data collection systems, one can sympathize with Cirrus as to how to get their arms around the extent and severity of the problem in the field. Maybe acquisition of a vibrometer by the factory and a pass/fail test for production aircraft would be a first step, but this won’t address the situations where the vibration begins somewhat after delivery. Deployment to field service centers is a possibility I suppose.

Aerosance states that a FADEC-equipped engine “should run more smoothly” but at this point who knows whether this will address the problem.

Hi Folks! I am not an owner myself but specialize in prop
balancing and related work in my area the past 14 years.
I just heard about the many owner complaints. I checked a SR20 last year but couldn’t improve it prop balance wise.
What was unusual to me was the movement I felt in the cocpit as I began to crank the engine. As if a smooth kick was moving the engine’s aft end side to side.
I’ve never felt that in another aircraft except one homebuilt that had the prop indexed 90 degress from normal position.
My first gut feeling was index position or a very flexible motor mount.
Speaking from my experience, any aircraft well designed should be relatively smooth with only minimal vibration in the cockpit. Dynamic balancing helps 85% of all prop aircraft. How much it helps depends on how far out it was to begin with. Inches per second, IPS, is the common unit of measurement. 0.2 ips or less is the goal, though most pilots will not feel a big change unless the intial roughness is above about 0.35 ips then balanced down to 0.1. It is common to see .5-.7 intial readings. On the other hand, an engine will always vibrate, so anything that is attached and shakes with engine movement should be isolated from the airframe!
Exhaust pipe rubbing is the largest culprit. Beech barons are a good example because the exhaust system bends and twist in tight space. You’ll feel it in the cockpit as if somebody was knocking on the door.
I would be surprised that an exhaust system design itself would cause vibration if it is isolated from the airframe.
Anybody is welcome to contact me direct if you have questions about prop or engine balance.
BTW has anybody ever measured the frequency of the cockpit vibration? If it is 1/2 crankshaft speed than it is combstion related.
Kent Felkins
Tulsa

The vibration in our plane seems to be about 10-12 frequencies per second, although this is, at best, a guess on my part from just sitting in the plane (SR22). This vibration surges about 2 times per second at any RPM from around 2100 to 2700 with a small variance in frequency and amplitude. Unfortunately, it is worst at about 2500 RPM. I feel pretty confident my estimate of the surges at two per second isn’t too far off, but my butt is not very good at counting things happening much faster than that (regarding the underlying, higher frequency vibration).

Gordon, based upon my dialogue with Alan, Cirrus Design does have some instrumentation for that purpose, but regardless of how ‘good’ the results, Alan has personally decided to be the final test.

I agree with that. With a vibrometer and a derived power spectrum graph, it may be that one could isolate constituent elements as possible culprits. We did precision measurement systems for the disk drive industry that used this technique to isolate bearing-related problems, non-repetitive runout, etc. just by examining the FFT results. You could look at a particular frequency and know what caused the problem.

Re:

BTW has anybody ever measured the frequency of the
cockpit vibration? If it is 1/2 crankshaft speed than it is
combstion related.

That’s what I was talking about re running an FFT on vibrometer output. Another example might be that if you saw energy at 3x the crankshaft speed it could indicate prop-related vibration (pulses).

May be some of our Bay Area friends could have a ride in your plane and count the oscillations with their butts:) Just a joke, no offense intended.

Is this the vibrometer that Gordon was talking about yesterday… If so, how much does one cost? Have you patented it yet?? Maybe Radomsky & Son can manufacture a new and improved one that will run off of the panel power.

Michael,
Your probably on to something butt according to a message in this thread only Alans “Butt” is authorized to measure the vibrations. Not sure what is used to calibrate his “butt” however… Ok I’ll shut up, sorry if I offended anyone, butt he was kind of set up on this one.

I, for one, trust Alan’s butt.

The impression I got was that Alan’s personal approval was intended as an extra quality element, not a substitute for the analytical parts.

Andy