RVjim
Well Known Member
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
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