Vibration at lower RPM in Cessna 210?

Question

Since the increase in fuel I have started throttling back. I used to fly at 24-24 (MP-RPM), and now I have found that at a setting of 20 (MP) and 2200 RPM, which is 45% power, according to the POH, still gives me 130Kts. However over the last number of days, I have noticed increasing vibration at RPM's below 2400. What can be causing this? Should I be worried about engine problems (1200 hours on the engine)?

Answer 1

Check for harmonics....Is it variable pitch prop? But sounds like harmonics or out of balance in that rpm range.....Check your blades for nicks or strip tears.....It does not take much to have a vibration... I would check in the mid position on the blades....high speed vibes usually occur at the tips....mid rpm...mid blade ...and low rpm..is usually at the hub.

Answer 2

You should; 1) talk to a competent A&P, 2) look at your oil consumption and the filter -- is it going up or is there any metal?  3) Any streaks on the belly, are you trailing smoke?  And, 4) talk to a compentent A&P.

Answer 3

Michael -

It is hard to tell without knowing some more information. Certainly, if the vibration signature is changing, you should find out why, since it is an early indication of mechanical problems that can quickly become more severe at an inopportune moment. Can you get the vibration to occur on the ground statically? If so, it's a fairly simple procedure to get a technician trained in dynamics to install an accelerometer at a convenient location to measure the characteristics of the vibration signature. This can tell you several things:

1. By determining the frequency, you should be able to identify the vibration with a specific shaft rotation speed. Since there may be several shafts running at different speeds, this will isolate the problem to a very limited region. It can also provide other valuable information - for example, if the response frequency is roughly 1/2 x a shaft speed, then you may be seeing an early indication of bearing cage damage / wear (a bearing cage spins at 1/2 x shaft speed). If it is 1x shaft speed, it is usually imbalance.

2. As you accelerate the engine through the speed range where the problem occurs, does the response follow the shaft speed, or is it a fixed frequency that is excited by whatever shaft source is driving that particular frequency? This can be determined by obtaining what is called a waterfall trace during a slow acceleration. If the response follows the shaft speed, then the problem is usually within the rotating hardware. If it is a fixed frequency, then it is probably a resonance within some loose or damaged static hardware that is ringing like a bell at the fixed frequency.

If you can only reproduce the problem in flight, the measurements are a little more complicated, but they can still be done. Cessna has some very talented folks that are very knowledgeable in this area. I would call them today - not tomorrow.  

Answer 4

You could be running in a zone where the wear on high time engine fly weights will vibrate out of sync with the prop.

Don't baby a big Continental. Enjoy the ride. If you wanted fuel economy you would fly a C-150.