heliman500 said:
I'm looking for a real basic explanation of main rotor induced vibrations.
Here's my stab at a basic answer:
- 1 per rev (aka 1R, 1P, 1 Omega or first harmonic) is usually described as a once per rotor revolution beat.
Causes of the 1 per rev. can be: mass imbalance (lateral), unequal lift between rotor blades (vertical) or dynamic chordwise imbalance (fore/aft)
Causes of the 2 per rev. can be: N per rev. of a 2 bladed rotor or a pair of blades not producing equal lift in a four bladed rotor.
- 3 per rev ( aka 3R, 3P or 3 Omega) is usually described as a three times per revolution beat.
Causes of the 3 per rev. can be: N per rev. of a 3 bladed rotor or blade mismatch in a four bladed rotor (typically Sikorsky models)
- 4 per rev (aka 4R, 4P or 4 Omega) usually described as a four times revolution beat.
Causes of the 4 per rev can be: N per rev. of a 4 bladed rotor.
The list can go on, but the 1 through 4 are the primary frequencies most people are concerned with in general discussions. The better RTB equipment will measure typically up to the 12th harmonic of the rotor.
- 1 & 2 per rev. reduction is usually controlled by adjustments to the pitch links, trim tabs are hub weights. These adjustments have very little effect on N per rev.
- N per rev is not reduced by the method above but by adjusting Frahm Dampers, Spring Absorbers, Cantilever Absorbers etc. N per rev. may also be greatly affected by fuel load, loose cowlings, seats and broken structure.
- Many people think that tabs only affect vertical and hub weights only affect the lateral vibes. This is often not the case as there can be crosstalk between all of the axis. Therefore it is important to use an RTB system that can acquire data on all three axis at the same time. It is also important that the same system use diagnostics that provide a solution that take into effect the crosstalk between the different axis's. Not all systems are equal...
Regards,
Chuck