Mike Goodrich Visit to FD92

[Joe Pires]

Mike Goodrich spent a couple days with Laura and I during his US visit earlier this week. Those of you who are chasing vibration issues might want to find him at Bensen Days this week, I think he will be at Silverlight's display. He hooked up my Aviomania to his life support equipment and we did some flying, then he hooked up a neighbor's MTO and did his diagnostic. In the end he thought that the Aviomania with Ernie's Slider head and the heavy lift blades had so little vibration coming down to the pilot that there was nothing worth doing. I too have felt it is the smoothest 2 place that I have flown in.

My neighbor Jeff, however had a noticeable panel shake in his 2018 MTO Sport, after a few test flights they determined that one of his blades was 5 grams out of balance. They put 5 grams in the offending blade and the panel shake was resolved.

 

[All_In]

Very cool Joe.
Where on the blade did he put the 5 gram weight?

 

[Abid]

Did he show you what 1 per rev and 2 per rev were for your gyro at the rotorhead? That is where he recommends measuring it and that is where it should be brought down to below 0.18 ips.

In the cockpit there there is a different story. Many connections and joints to pass through.

 

[Joe Pires]

John, the MTO had a cap on the butt end of the blade. He took it off and slid the washers in. Fara, according to Mike, the purpose of removing vibration is to increase comfort and reduce stickshake. If reducing a vibration has no resulting benefit it is wasted effort, or at least that is how I understood his comment. You should ask him.

 

[All_In]

John, the MTO had a cap on the butt end of the blade. He took it off and slid the washers in. Fara, according to Mike, the purpose of removing vibration is to increase comfort and reduce stickshake. If reducing a vibration has no resulting benefit it is wasted effort, or at least that is how I understood his comment. You should ask him.

Thank you Joe. I've never seen it done so I wondered where they put the weights.

 

[Abid]

John, the MTO had a cap on the butt end of the blade. He took it off and slid the washers in. Fara, according to Mike, the purpose of removing vibration is to increase comfort and reduce stickshake. If reducing a vibration has no resulting benefit it is wasted effort, or at least that is how I understood his comment. You should ask him.

I think there is definitely a lack of understanding or misunderstanding there. That is not the purpose of balancing. In fact after balancing the blades for 1 per rev you may still have vibration in the cabin that is 2 per rev or others coming from the engine or prop or tail or rudder etc. He clearly instructs this in his intro course so everyone understands why we balance rotors and what should we expect and what we should -not- expect. I and a few others just took the intro as well as the whole course from him and I ate his brain out in the other days he has been here. Very very informative and actual data driven class. There is a science and method to it and its important to get the X (lateral) and Y (longitudinal) IPS down at the rotor below 0.2 IPS. If you'd like I and Greg can fly there one day and make the move lines for your gyroplane and show you what its doing or if move lines can even be made.

Balancing and smoothing using balancers is usually acting on 1 per rev which is either vertical or lateral imbalance of lift being created by each blade at a given point.
1 per rev can sometimes greatly mask 2 per rev vibration but when 1 per rev smoothing is done and accomplished, sometimes 2 per rev remaining vibration is now perceived by us as more annoying when the same was masked before and we never knew it was bad. These balancers do not work on n per rev solution, mostly 1 per rev solution only.

2 per rev and its causes are debated all the time. There are different theories of 2 per rev thrown around here on this forum as well. Rotor blade flapping to equality is likely a big contributor but there are other factors as well I am sure.

As an article from NASA (?) states:
" It is true that the smoother a rotor system is, then it tends to unmask the Nper rev vibration such that they appear a lot worse to the aircrew inside even though no adjustments were done to the absorbers. This is because the higher levels of 1 per rev vibration tends to mask the N per rev vibration. As the 1 per rev levels are reduced toward zero, the N per rev vibe becomes APPARENTLY more noticeable. If you were to take actual IPS level readings, you would see that the actual IPS magnitude has not altered – merely become more noticeable."

This shows you that just having your cockpit "feel" good isn't doing anything to some of the vibrations that may be highly damaging to components and shortening their lives.Sometimes perfect balance in 1 per rev has to be sacrificed a little to mask the 2 per rev so your cockpit feels acceptable while also improving 1 per rev smoothing to an acceptable level and not leaving it at a dangerous level. Things like spring suspension mounts on transmissions in 206, 205 etc. are serving similar purpose as the so called "slider" in your rotor head on a similar principal I think except not on the R&P block but further down

Hope to see you at Bensen Days.

 

[Mike G]

First a very big thank you to Joe and Laura for being such wonderful hosts, a truly exceptional couple in an exceptional house and setting.

While I’m logged in I think I should try to clarify the apparent difference between what I told Joe and what I taught Abid during the training session at Silverlight.

When taking the vibration data from Joe’s Aviomania I was struck by a number of differences between the vibrations I was measuring on the Aviomania and those I usually see:

1) At the rotor head the left/right vibration was fairly high but the fore/aft vibration was very low, this is unusual because I usually see the reverse, the for/aft vibration at the mast tends to be the higher of the two. I assumed that this was because the slider (which operates in the for/aft direction) was doing its job.

2) Despite the relatively high left/right 1/rev vibration at the rotor head the 1/rev vibration in the cockpit was very low, usually the cockpit vibration increases with rotor head vibration.
I then took the cockpit accelerometer out of the cockpit and fitted it to the mast just below the slider.

3) Surprisingly there was little or no difference between the vibration above and below the slider and hence I didn’t understand why the cockpit vibration was so low if the rotor head and mast vibration was relatively high vibration implying that the slider wasn’t absorbing it.

4) Then in a subsequent flight the next day the angle given by the PB4 suddenly reversed through 180° for no apparent reason. A 180° phase reversal, as this is known, usually implies passing through a critical speed and that I couldn’t explain unless it was something to do with the natural frequency of the slider.

This was over the weekend at Joes and I had planned with Chuck to have dinner with him and hopefully Ernie on the Tuesday and I really wanted to discuss this with them both before messing about with Joe’s gyro. I knew that Joe was flying down to Bensen Days and I hoped we could look at it together there.

Also because the cockpit vibration was so low and I knew from experience that even if I got it a bit lower the pilot wouldn’t feel the difference I was prepared to leave it as it was. And to be perfectly honest I was completely at a loss regarding how to balance a rotor with such a wildly shifting phase angle.

As it happened the dinner with Chuck was delayed and Ernie was already at Bensen Days. In the calm of the hotel room I studied the data yet again and realised that the only explanation that made any sense was that for the flight the following day I cross connected the two accelerometers and that would explain the 180° reversal.
I prefer a simple explanation like “You f..ked up” to a complicated theory around critical speeds.

Looking at the data in this light my first conclusion is that the slider wasn’t doing its job properly, I suspect the springs being too stiff but that would really require a lot more testing to see if I was right.

The second conclusion is that the Aviomania mast&frame absorbs vibration much better than the classical Eurogyro. This I put down to the fact that it’s a bolted aluminium frame and aluminium has a lower modulus than steel and each bolted joint tends to act as a damper absorbing vibration as it travels from the mast to cockpit much better than a welded steel frame.

So in conclusion both Joe and Abid are correct, I did say that there was little point in balancing his gyro because the cockpit vibration was so low (also the rotor head vibration wasn’t disastrous either) and yes ideally we should balance the rotor because any reduction in vibration is good for the health of the machine. Interestingly enough the 2/rev at the rotor head was also quite high and again was unusually low in the cockpit.

Abid is proposing another training session at Sebring in January, if it happens I would definitely like to balance Joe’s rotor (if he’ll let me).

Mike G

 

[Joe Pires]

It was our pleasure to have your company. I will be happy to let you have a go at it.

 

[C. Beaty]

An isolated rotor moving through tha air must have a 2/rev vibration as a result of drag variation broadside to endwise.

Bolted to a bulldozer, there would be no 2/rev vibration but something would eventually break. That’s the reason isolation is required.

 

[Abid]

Mike if accelerometer measured no difference just above and below the slider, to me that means the slider as at least implemented didn't do much.
Also if 2/rev were quite high at the mast and rotor-head to me again that means the stiffening of the cross section of the hub bar with tie rods or anything else didn't do anything to reduce 2/rev. The cockpit vibrations are low simply because of natural isolation of a bolted together looser frame and softer Aluminum material by process of elimination because the other factors were measured. Should probably be measured again more carefully and precisely to verify this but if measurements are repeated, it is what it is in my view. I was hoping for some scheme to truly reduce 2/rev vibrations and will keep my fingers crossed.

 

[Jean Claude]

The slider seems to me ineffective on the 2 / rev because of the friction under load. It would take a slider on balls

 

[C. Beaty]

A slider is redundant when used with a sufficiently flexible mast but is essential when the rotor is mounted on a rigid pylon.
My tail boom on top gyro, built in 1989, had so much 2/rev vibration that it was dangerous until I came up with the slider.
The bearing surfaces of the rotorhead roll pivot were made from Delrin AF*, Delrin loaded with Teflon fibers.

*https://www.plasticsintl.com/datashe...lrin_100AF.pdf

 

[Doug Riley]

The slider on my Dominator tandem showed a definite wear pattern, indicating that "sliding" was, in fact, occurring. The tandem Dominator has a 3-tube, bolted pylon structure that is rather rigid in the fore-aft direction.

It's more limber side-to-side and vibratory movement of the rotor head was very obvious any time I looked up (or saw the shadow of the head on my dashboard).

Allowing aluminum to flex to isolate vibrations makes me uneasy. Aluminum has very poor fatigue qualities. Steel is better in that regard, but, in my opinion, rubber isolation is better still.

 

[eddie]

To isolate the 2/rev shake on my RAF I have installed a motorcycle shimmey damper,,the damper is connected to the lower mast on a bracket and the other end is

connected to the upper mast that is allowed to move because of the urethane bushing installed in the lower mast attach point. Between the urethane bushing and

the damper the 2/rev shakes are just about completely gone,and the aluminium mast itself isn't absorbing the vibration.

It was only after installation of a smooth set of rotor blades did I realize the 2/rev shake was also a problem, happy days are here.

This all started with Mike G selling and training the rotor manufactor the PB-4 analyzer in Poland,when my blades arrived they were perfectly

smooth and needed just a slight cordwise adjustment. Thanks MIke !!!

 

[C. Beaty]

When the Sparrowhawk gyro first came out, they were precluded from using the magic rubber bushing because RAF Canada had a patent on it and with a 2x4 mast, 2/rev vibration was intolerable.

Jim Mayfield asked if I had any objection to AAI’s use of a slider and of course, I didn’t.

I don’t know if that completely solved their vibration problem because of the drag hinged rotor hub but suspect it didn’t since inplane flexibility allows the rotor CG to become misaligned with its center of rotation.

 

[Jean Claude]

Chuck,
Attached to a bulldozer the rotary drag looks 65N for a load of 2050 N at 65 mph
How can he slide with a coefficient of friction of 0.19?
It would take at least 390 N is not it?

 

[Abid]

The slider on my Dominator tandem showed a definite wear pattern, indicating that "sliding" was, in fact, occurring. The tandem Dominator has a 3-tube, bolted pylon structure that is rather rigid in the fore-aft direction.

It's more limber side-to-side and vibratory movement of the rotor head was very obvious any time I looked up (or saw the shadow of the head on my dashboard).

Allowing aluminum to flex to isolate vibrations makes me uneasy. Aluminum has very poor fatigue qualities. Steel is better in that regard, but, in my opinion, rubber isolation is better still.

This assumes that the theory of sliding actually alleviates 2/rev or reduces it significantly which is something to be shown by data, not just theory and paper analysis. In the end if there is an implementation of any mechanism within the rotor-head to significantly reduce 2/rev, its efficacy can only be proven by measurement right at the rotor. An accelerometer above and below the slider would satisfy the requirement of isolating measurement to slider's effectiveness. If it is doing its job the theory that says that something like a slider can help is correct and the implementation of the slider steel spring method is also correct. If its not then one or both these things are on the wrong path.
Measuring vibrations in the cockpit is nice but there are many other reasons you may get good results in the cockpit even while your rotor-head even in 1/rev is not ideal not to speak of 2/rev.
More testing and validation is needed to verify results IMO.

 

[C. Beaty]

Chuck,
Attached to a bulldozer the rotary drag looks 65N for a load of 2050 N at 65 mph
How can he slide with a coefficient of friction of 0.19?
It would take at least 390 N is not it?

I think you’ve underestimated the periodic aerodynamic drag variation of the rotor, JC.
In any case, my 2050 N gyro shows a polished area on the roll axle of 3-4 mm.

 

[Jean Claude]

Chuck
My calculation by 11 slices of blade gives:
Rpm: 372, drag 279 N, radial vibration: 65 N Sin (2wt)

The analytical method of Naca programmed by Jean Fourcade (GyroRotor) gives with the same Cd[SUB]min[/SUB] of blade :
Rpm: 380, drag 247 N, radial vibration: 109 N Sin (2wt)

This remains too far from the 390 N required to begin the expected slip.

Perhaps area polishing is makes only during the run, when the mu transitorily reaches values greater, while that the load is still low (???)

 

[C. Beaty]

JC, when I first built this gyro with rigid rotor pylon, 2/rev vibration was so violent that it was dangerous to fly. In searching for a solution, I tried a drag hinged rotor with a coning hinge at the center of rotation and no undersling.
The coning hinge looked like a door hinge with the hinge pin also serving as the teeter bolt.
https://www.youtube.com/watch?v=qvdPL1pcG1A

Such an arrangement had worked well on my first gyro; built to Bensen B-8 dimensions, except using round 2024 aluminum tube instead of square tube. But a drag hinged rotor was no solution with a rigid rotor pylon. I think 2/rev vibration was worse with drag hinges.
The solution came when I watched a television series on the Discovery Channel showing the film, “Birth of the Bell Helicopter” and saw Arthur Young’s solution to 2/rev vibration. Remember, this was at a time before the Internet was ubiquitous; I bought my first computer in 1997 when my only available Internet connection was via a slow telephone line.
In any case, Arthur Young’s solution for the Bell Helicopter, stiff inplane rotor mounted on a soft mast solved my 2/rev problem; the slider completely eliminated 2/rev vibration.