Quality of Flight - vibration

Vibration

To establish a staring point in discussing vibration in an aircraft, we need to define the primary vibration frequency and the source, which is the engine.

For an engine running at 2400 RPM, the primary vibration (1P) frequency is 40 Hz. (2400 RPM divided by 60 seconds per minute to get the primary vibration pulses per second.) This is important in order to establish the various other sources of vibration acting on the airframe.

Dynamic Balancing

Dynamically balancing can greatly reduce an imbalance in the engine/propeller assembly. This is normally done after the rings have had a chance to seat in a new engine. Usually when the engine has around 50 hours of running time.

Dynamic balance of the engine/propeller can significantly reduce the primary vibration (1P). Reducing the 1P vibration amplitude to 0.04 inches per second (IPS), or below, can add life to the engine. With the 1P vibration below 0.04 IPS, there will not be enough movement in the crankshaft to allow it to contact the crankshaft bearing through the oil film.

There are some vibrations that are not affected by dynamic balancing. (This is basic physics, so don't shoot the messenger.)

1/2 P vibration.

This is a vibration that has a frequency of 1/2 of the primary frequency. For an engine running at 2400 RPM (40 Hz), the 1/2P vibration would be at 20 Hz.

The 1/2P vibration can best be felt in flight by resting your head against the airframe.

The weight of the propeller blade is directly related to the 1/2P vibration. The lower the blade weight, the lower the 1/2P vibration.

2P vibration

This is a vibration that has a frequency of twice the primary frequency. For an engine running at 2400 RPM (40 Hz), the 2P vibration would be at 80 Hz.

The source for the 2P vibration is the wake of the blades on the airframe from a 2 blade propeller.

This vibration can best be felt by placing your finger tips on the windshield.

3P vibration

This is a vibration that has a frequency of three times the primary frequency. For an engine running at 2400 RPM (40 Hz), the 3P vibration would be at 120 Hz.

The source for the 3P vibration is the wake of the blades on the airframe from a 3 blade propeller.

Again, this vibration can best be felt by placing your finger tips on the windshield.

2P vs. 3P vibration

For a 3 blade propeller conparison to a 2 blade propeller, the 3P vibration will be less than the 2P vibration.

For a given engine horsepower, the wake from each of the blades on a 3 blade propeller will be about 2/3 as strong as the wake from each of the blades on a 2 blades propeller.

Also, the human body doesn't feel the higher frequency virations as much as it feels the lower frequency vibrations.

Regards,
Jim Ayers

RVjim said:

For an engine running at 2400 RPM, the primary vibration (1P) frequency is 40 Hz. (2400 RPM divided by 60 seconds per minute to get the primary vibration pulses per second.) This is important in order to establish the various other sources of vibration acting on the airframe.

Click to expand...

Could you explain this number? Why only one vibration pulse per crank rotation?

Prop imbalance

Tomasz said:

Could you explain this number? Why only one vibration pulse per crank rotation?

Click to expand...

That is for a prop imbalance.

Dynamic Balance terminalogy used

DGlaeser said:

That is for a prop imbalance.

Click to expand...

Yes. I am using standard frequency terminalogy used in dynamic balancing.

I've seen various references using frequencies between 1/2p to 6P.

Jim Ayers

Good writeup Jim.

Could you expand on this one a bit?

<<The weight of the propeller blade is directly related to the 1/2P vibration. The lower the blade weight, the lower the 1/2P vibration.>>

And a minor thing, not really vibration related.

<<With the 1P vibration below 0.04 IPS, there will not be enough movement in the crankshaft to allow it to contact the crankshaft bearing through the oil film.>>

Have you seen some evidence that a crankshaft journal will contact a bearing shell due to any flyable 1st order vibration? I have doubts.

All bets off in the case of the extraordinary....losing a blade and the like.

Quality of Flight - 1/2P vibration data

DanH said:

Good writeup Jim.

Could you expand on this one a bit?

<<The weight of the propeller blade is directly related to the 1/2P vibration. The lower the blade weight, the lower the 1/2P vibration.>>

And a minor thing, not really vibration related.

<<With the 1P vibration below 0.04 IPS, there will not be enough movement in the crankshaft to allow it to contact the crankshaft bearing through the oil film.>>

Have you seen some evidence that a crankshaft journal will contact a bearing shell due to any flyable 1st order vibration? I have doubts.

All bets off in the case of the extraordinary....losing a blade and the like.

Click to expand...

1/2P vibration

The 1/2P vibration is a characteristic of the engine, propeller, engine mounts and the airframe in which they are mounted.

The 1/2P vibration level is related to the weight of the propeller blades. On a Cessna 182, the following 1/2P vibrations levels are provided for the three propellers available for this aircraft:
Hartzell 2 blade ? 1.2 IPS
McCollough 2 blade ? 0.6 to 0.8 IPS
MT Propeller 3 balde ? 0.4 to 0.6 IPS

This type of data is not normally available, since dynamic balancing does not affect it. This data was obtained during the STC effort by the MT Propeller on the Cessna 182. All three propellers mentioned are used on the Cessna 182.

1P vibration level goal of 0.04 ips


As to the 0.04 ips level for the 1P vibration, you already know the answer to this. "Normal wear" is the evidence of the crankshaft contacting the bearing.
The 0.04 ips level for 1P vibration is the dynamic balancers insurance policy that "normal wear" is only occuring from other causes; like during start up before the oil pressure rises.

Regards,
Jim Ayers

<<The 1/2P vibration level is related to the weight of the propeller blades. On a Cessna 182, the following 1/2P vibrations levels are provided for the three propellers available for this aircraft: Hartzell 2 blade – 1.2 IPS McCollough 2 blade – 0.6 to 0.8 IPS MT Propeller 3 balde – 0.4 to 0.6 IPS>>

Yes, don't doubt it exists. I was interested in the mechanics. End of a long day; I've since remembered (duh!):

http://www.vansairforce.com/community/showpost.php?p=155608&postcount=39

Quality of Flight - vibration physics

Vibration = Fatique

Lower vibration means less fatique on the pilot and passengers.

So far we have discussed the wake vibration on 2 blade and 3 blade propellers.

The 3 blade propeller will always have less wake vibration than a 2 blade propeller and it will be at a higher frequency.
The higher frequency is significant simply because the human body doesn't feel that vibration as much as it would feel the lower frequency vibration.

Let's take this one step farther to a 4 blade propeller.

The wake from each blade of a 4 blade propeller will be about 1/2 of the wake from each blade of a 2 blade propeller.
And this would be a 4P vibration. At twice the frequency of the 2 blade propeller.

If the 4 blade propeller weighs about the same as the 3 blade propeller, then the 1/2P vibration remains about the same for both the 3 blade and 4 blade.

So a 4 blade propeller will be even smoother than a 3 blade propeller.

Regards,
Jim Ayers