Out of Track Vibration

[helisphere]

When a helicopter is out of track it typically produces a vertical vibration. Can anyone explain this in detail? It seems to me that it would produce a more horizontal vibe similar to an out of balance rotor.

[FSXPilot]

For simplicity think of a two bladed system. If one blade is higher than the other then the effective centre of mass of the rotor system will not be in line with the top of the mast. This imbalance will cause both vertical and horizontal vibration as the rotating mass tries to move the mast. In a three bladed system having one blade out of track may be the smoothest solution.

[helisphere]

First of all two bladed rotor systems are pretty much always underslung so that the teetering hinge is on the CG of the rotor so that this will not happen. However I do understand that this varies with different flight weights and certainly can cause some vibration but again I see how lateral displacement of the rotor CG would cause a horizontal vibe but I don't see where the vertical comes from.

details details... I need details...

[RVDT]

In the most simple terms -

Vertical -

Track

Understand this (http://en.wikipedia.org/wiki/Flap_back) and then think of the net result of the blades not being the same relative to each other.

More prevalent in forward flight due to the above than in the hover.

Lateral -

Mass balance

The lateral or roll axis is the "softest" plane relative to the centre of mass of the aircraft so naturally that is where it becomes more apparent.

More prevalent in the hover as the roll plane is damped in forward flight due to the airflow over the appendages.

[helisphere]

I'm asking this because I was test flying an experimental helicopter last year and we had some tracking issues due to blade mismatch, and even in a no wind hover there was a pronounced vertical vibe and not much lateral.

As I analyze this out of track condition for a hover I see the vertical lift vector at the teeter hinge being tilted and rotating with the rotor at that tilt causing a lateral oscillation of force but the vertical force staying constant. In other words even though one blade has more lift than the other, the total lift looks like it would remain the same throughout the rotation.

However, as I specifically considered our helicopter, which has blades that when tracked at one collective setting would be way out of track at a different collective setting, I thought... The blade that tracks high will not track the same amount higher when the lift coefficient is changed. This made sense to me that the total vertical lift was changing through the rotation because of cyclic pitch variation.

Example: lets say that nominal pitch at hover was 5 degrees, but due to CG it takes one degree of cyclic pitch to maintain hover. And lets say that the blades track well at 4 deg but the red blade goes higher at higher collective pitch angles. This means that as the blades are going from red 5 and blue 5 degrees pitch as they rotate the red blade is slightly high (because its above 4deg nominal) but when the red hits 4 and blue 6, the red is at its "in track" reference but the blue is BELOW track at 6 deg then when the blue hits 4 and the red 6, you get the blue at its "in track" reference but the red is ABOVE track. So if the in track reference (4 deg) makes say 500lb of lift per blade and 6 deg makes 550lb then when the red blade is at 4 deg its making 500lb but the blue is at 6 but flying lower (so maybe 530lb) than it should be and then 180 deg later the blue is at 4 deg making 500lb but the red is flying higher than it should be for 6 deg (so 570lb). You can see how the total lift would be changing cyclically causing a vertical vibe.

But then I thought: when the swashplate is tilted 1 degree and the tip path plane is matching the no feather plane (control plane), the blade angle of attack will be constant all the way around even though there is 1 degree of cyclic.

Ok I am still stumped. If a helicopter has perfectly matched blades and is in a no wind hover but out of track, is there a vertical vibe? If so please explain by showing me how the lift (vertical) vector is changing with rotation to cause a vertical vibration.

[Phoinix]

Reading through your explanation I pulled a parallel line with model helicopters, as we do a lot of tracking and balancing troubleshooting all the time.

Although you have a set of perfectly balanced blades (mass wise, static balancing), if one blade is a bit twisted or airfoil is slightly deformed (factory error) you will never get the two blades tracking right. Now, being an experimental helicopter which lacks certification criteria, is it maybe possible that the two blades are not aerodynamically the same?

Although, you are starting to get my brain going... the cyclic lift change due to tracking issues - how do these effect the lift vector?
My thought is the lift vector also becomes a cyclic variation, as you have two blade tip paths to which the lift vector is perpendicular, but than you would also get vibrations in lateral and longitudinal axis.