rotor 1/ rev vibration and airspeed

Mike G

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Chris
Are the units on the polar chart IPS? Sorry I wasn't clear.
Mike
 

Mike G

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Back to the subject.

If blade 1 has a greater pitch (hence lift) than blade 2.

Between what angles (using the clock format assuming 12 o'clock for the forward direction and anticlockwise rotation looking from above) will blade 1 give the extra upward force that would be detected as an unbalance by the vertically orientated accelerometer and at what angle (time) would this force be at its maximum?

Mike G
 

bryancobb

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Hi Mike G,
Let me rudely butt-in here. Please don't get angry, I just feel like I can add a little to the conversation.

There is a number of things you can do to any rotor system that will create or eliminate a particular vibration. I.E. pitch or depitch a blade (incidence) : bend trim tab(s) (if present) : Add weight at any number of points : Shift chordwise CG by leading/lagging a blade or shifting the entire head with shims. These are just a few. I'm sure some ships have many more. And...keep in mind, a perfectly-tracked rotor MAY NOT neccessarily be the SMOOTHEST rotor.

Each one of these VARIABLES, makes the data-point on the polar chart move in a different direction, when you make changes. EVERY AIRCRAFT IS DIFFERENT. The only way another person's chart can be used to balance YOUR aircraft is if the two aircraft have almost identical rotating and controlling parts.

The clock position readout for where a vibration is occurring doesn't have any relationship to the nose of the aircraft. It is related to where the accelerometer is mounted and where the photocell pickup is installed, and which blade has the reflective tape on it. You can install the accelerometer pointed ANY DIRECTION and install the photocell ANYWHERE as long as the reflective tape triggers it. Just be aware, your move lines on the polar charts will only be correct when these two devices are installed as they were when the charts were developed for YOUR aircraft.


Let's say you install the accelerometer up at the rotor head, pointing left, to measure lateral vibration. Then let's say you put the photocell about 12" in front of the mast, with a piece of reflective tape on one (master) blade.

Now say you built the ship from scratch and it's the only one that exists. Say you want to determine how adding or removing tip weight to the master blade makes the data point move. You can determine the azimuth of a move line on the polar chart by taking 2readings. One at rpm as-is, then plot it, then add tip weight to the master blade and plot it. That gives you move DIRECTION in relation to the accelerometer and photocell.
You can then plot intermediate points with different amounts of weight so that you end up with A) Move-Line Azimuth .and B) Weight/Distance Correlation.
This procedure can be done with each thing you have the capability to change on the rotor.

The concept to "take to the bank" here is... Your move lines and amounts of change per unit, ARE NOT GOING TO BE LIKE SOMEONE ELSE'S unless your ship is and exact twin of theirs and you have the accelerometer and photocell installed exactly like they did.
 
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ckurz7000

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Thanks, Brian. That's what I tried to say a couple of posts back. To do a good balancing you need some theoretical understanding but mostly you try a "unit" displacement of each of your tuning variables and plot them on the chart. Then, as you already said, you see in which direction and how far each one takes you. And with some experience and these data you can figure out -- by linear superposition -- which combination of adjustments will get you the smoothest rotor.

-- Chris.
 

Mike G

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Brian and Chris
I really appreciate your input, please bare with me for a bit longer.
I actually do understand a bit about balancing but I like to keep my questions as simple as possible and sometimes it gives the impression that I'm out of my depth (sometimes I am).

http://www.mediafire.com/?erbl3v9n0xkgbjk

is my understanding of the signals you would get on an oscilloscope if it where monitoring:

1) a sensor that signalled the passage of a passing bolt (hall sensor) or the passage of a bit of reflecting tape (as Vibrex does).
and
2) a vertically mounted accelerometer orientated such that it read positive accelration when rising and negative when falling.

and assuming that blade 1 had a greater pitch than blade 2 with the direction of rotation and flight as shown.

I have shown the plan view of the rotor in 4 positions with the readings I THINK we should get and a final sketch showing the screen after a few revolutions.

This is, of course, grossly over simplified and assumes no other scources of vibration except that cause by Blade 1 having a greater pitch than blade 2.

Do you agree with my sketches?

Mike G
 

bryancobb

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Mike,

You say you wish to keep your question in simplest terms. Well, your understanding that you used to develop that simple question is flawed.

If a blade with more pitch, passed the sensor, and caused an upward vibration that could be picked up by the accelerometer and displayed on an O-Scope, then moving the cycling would be changing blade pitch and thus, the vibration all the time. The vibration azimuth and magnitude would be all over the chart.

A vibration, by definition, is a back-and-forth oscillation. The algorithims and accelerometers that are used to balance rotors don't even detect the forces (caused by more or less pitch) you are ferring to.

I couldn't open your Visio sketches. Print them to a freeware PDF printer and I can open the PDF file or do a screen capture and post it as a JPG.
 

Mike G

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Bryan
I have understood from various threads on this forum that if you have a tracking problem (one blade pitch set greater than the other) the result is vertical 1/rev vibration "like a galloping horse". Of course I'm talking about two bladed gyros.
What I'm trying to clarify is the relationship between the magnitude of this vibration and the angular position of the blade that has the greater pitch.
I've converted my sketches to pdf and hope it makes more sense now
Thanks for being patient..
Mike G
 

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C. Beaty

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Out of track shake is quite simple, Mike.

Out of track: one blade has slightly more pitch than the other. It flies higher by an amount that can be determined by simple geometry. *

When the high blade is on the advancing side in forward flight, total lift is slightly greater than when the low blade is on the advancing side.

Viewed on an oscilloscope that responded to amplitude, the vibration would be a near sine wave with maximum amplitude when the high blade is at 3 o’clock. And of course, the faster you go, the greater the shake.

Out of track also means the rotor thrust vector scribes a conical path. This will stir the stick in a 1/rev circle with severe out of track.

********
*If the difference of pitch is 0.1 degree and the rotor radius is 14 feet, the blade tips will fly with a separation of 0.29 inches. (tan0.1 * 14 * 12)

That’s the reason shims made from beer cans are so useful for tracking rotors.

I don’t have the foggiest notion of how a Magni rotor is tracked but surely, there’s an eccentric bushing or similar surrounding the teeter bolt.
 
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bryancobb

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Mr. Beaty,

If I remember correctly, you are a highly experienced gyro pilot with an in depth understanding of gyro aerodynamics. I will now attempt to delete my previous post since it was obviously made from a position of100% ignorance.
 

Mike G

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Chuck
Thank you once again for putting me straight.
To conclude:
Does that mean that with out of track blades I can expect a vertical vibration at 1/rev frequency as shown in my attachment plus a horizontal 1/rev vibration that would be at 180° to the blade with the greater pitch (ie. when the blade of gretaer pitch is in the 9 o'clock positionj, the horizontal force will be in the 3 o'clock position?

From the other thread that, I started because I thought I wasn't getting your attention, never having galloped a horse, I presume that a galloping horse is at about 5 hz?

Mike G
 

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C. Beaty

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Mike, the rotor thrust vector, in the case of a seesaw rotor, is perpendicular to the hub.

With one blade high and the other low, which way does the thrust vector point relative to the rotorhead?

It is easily confused with out of balance.
 

Mike G

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Chuck
Thanks again.
When you say "hub" do you mean the bar that connects the two blades or the part that includes the bearings and teeter towers?

Mike G
 

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Mike G

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Chuck
Thanks

So back to the first question of this thread.
"What 1/rev vibration increases with airspeed?"

The answer seems to be that out of track blades give

1) a 1/rev vertical vibration that if plugged into a simple balance machine would look like the blade with the most pitch was heavier than the other.

and

2) a 1/rev horizontal vibration that if plugged into a simple balance machine would look like the blade with the least pitch was heavier than the other.

Both of these vibrations increase with airspeed.

Attached is a revised version of my previous attachment which had 2 errors in it.

Mike G
 

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