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  #16  
Old 02-13-2017, 10:54 AM
Jean Claude Jean Claude is offline
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Originally Posted by bryancobb View Post
Forward flight tracking has not been fine tuned but The DSS MicroBalancer shows it bouncing around from 0 to 0.25.
My calculation says that the radial vibration at the teeter bolt at 66 kts for the mini 500 is about 2 IPS (18.2 Hz)
As Mike said, we can not compare vibrations if they are not measured at the same point.
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  #17  
Old 02-13-2017, 11:41 AM
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put the transducer/velomiter in a node spot in the air frame it will read zero as the rest if the machine is shaking itself to pieces... Oh the fun you could have.
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  #18  
Old 02-13-2017, 01:05 PM
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...shaking itself to pieces...
Avoiding that's the whole purpose for dynamic balancing in the first place.
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  #19  
Old 02-15-2017, 09:01 AM
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Brian
In your document about the Robinson, does it say when you use the accelerometer position “B”? It seems to be the one in the cabin. Does it say which direction the accelerometers are pointed? Is “A” pointing left/right or for/aft? I assume “B” is pointing vertically.
In your Mini-500 where is your accelerometer and what direction does it point? Can you post your polar charts, it might help others understand what we’re talking about

Hillberg
Do you have any copies of these documents,
“The Helicopter Directorate in Fort Worth will adopt general limits as a reference in Advisory Circulars ...AC 27-1A AC 29 2C ????”.
I’m in Europe so they don’t mean anything to me?

I agree if you put the accelerometer on a node it will read zero at the natural frequency of that part of the airframe but it’ll probably register lots of vibrations at other frequencies.
Brian
Dynamic balancing will only allow you to reduce the vibrations at the 1/rev frequency due to unbalance. Tracking also helps you reduce another vibration at 1/rev. All the other frequencies (such as your transmission vibration from a few threads ago) need to be addressed using the frequency analyser.

JC
2.0 IPS at 18.2 Hz!! I assume this is 2/rev for a 1/rev at 546 rpm. Does the mini rotor turn that fast?
Mike G
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  #20  
Old 02-15-2017, 10:21 AM
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All the documents are on the FAA web site available to anyone, Advisory Circulars, General Orders, anything at all .
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  #21  
Old 02-15-2017, 03:18 PM
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Brian
In your document about the Robinson, does it say when you use the accelerometer position B? It seems to be the one in the cabin. Does it say which direction the accelerometers are pointed? Is A pointing left/right or for/aft? I assume B is pointing vertically.
In your Mini-500 where is your accelerometer and what direction does it point? Can you post your polar charts, it might help others understand what were talking about

Hillberg
Do you have any copies of these documents,
The Helicopter Directorate in Fort Worth will adopt general limits as a reference in Advisory Circulars ...AC 27-1A AC 29 2C ????.
Im in Europe so they dont mean anything to me?

I agree if you put the accelerometer on a node it will read zero at the natural frequency of that part of the airframe but itll probably register lots of vibrations at other frequencies.
Brian
Dynamic balancing will only allow you to reduce the vibrations at the 1/rev frequency due to unbalance. Tracking also helps you reduce another vibration at 1/rev. All the other frequencies (such as your transmission vibration from a few threads ago) need to be addressed using the frequency analyser.

JC
2.0 IPS at 18.2 Hz!! I assume this is 2/rev for a 1/rev at 546 rpm. Does the mini rotor turn that fast?
Mike G
Mike,

All balancing I have done has been on helicopters. All I have done (except the Mini-500) involved one accelerometer mounted at the main transmission and pointed horizontally to the left (9 o`clock), and another accelerometer mounted to the left side of the instrument panel and pointing up. The transmission one was used for hover balancing and the instrument panel one was used for forward flight tracking.

On the Mini-500, the DSS Microbalancer only had one input so instructions direct you to install the one accelerometer outside, on the very tip nose of the canopy and point it straight up.

I`ll post the charts and instructions.
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  #22  
Old 02-15-2017, 03:28 PM
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Default Norm Serrano

When Fetters was getting the factory Demonstrators "dialed-in" and writing objective procedures for customers to use to balance their helicopters, he hired a consultant named Norm Serrano, who had extensive in balancing rotorcraft. If I remember right, he had worked for Chadwick-Helmuth for a long time and was one of their experts at developing balance procedures for newly certificated helicopters.

Norm and Dennis came up with the initial Mini-500 method. I`m not sure if Dennis used him to help write the "super-enhanced" method after the Mast Support Kit and the horizontal stabilizer trim-tab and stall strips on top.

The procedure and charts I will post are the latest and greatest after all those production changes.

Keep in mind Mike, for a helicopter to be its smoothest in cruise flight, the mast needs to be as close to perpendicular to the tip-path-plane as you can make it. The Mini's horizontal stabilizer is rigged in these instructions so it does that very well over the cruise speed range. Some owners complained that in actual unexpected engine failures, the nose tucked badly when collective was quickly lowered.

Dennis added the stall strips (he called them vortex generators) on the upper surface to create an intentional stall of the stabilizer when rotor downwash disappeared after the pilot quickly lowered collective during an engine failure. This was to eliminate nose-tuck tendency.

All this has a lot to do with balance.


Now...for gyros, is it possibly practical to have the fuselage and the tip path be in alignment so no feathering is needed? This may be a place where gyro vibrations can be reduced significantly.????
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Last edited by bryancobb; 02-15-2017 at 04:06 PM.
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  #23  
Old 02-15-2017, 04:15 PM
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Default Start With Hover Balancing

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  #24  
Old 02-15-2017, 04:18 PM
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Default Now To Forward Flight Balancing

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Last edited by bryancobb; 02-15-2017 at 04:23 PM.
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  #25  
Old 02-15-2017, 04:25 PM
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Default Finally The Trim Tab and Stall Strips

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  #26  
Old 02-15-2017, 05:57 PM
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Originally Posted by bryancobb View Post
When Fetters was getting the factory Demonstrators "dialed-in" and writing objective procedures for customers to use to balance their helicopters, he hired a consultant named Norm Serrano, who had extensive in balancing rotorcraft. Norm and Dennis came up with the initial Mini-500 method. I`m not sure if Dennis used him to help write the "super-enhanced" method after the Mast Support Kit and the horizontal stabilizer trim-tab and stall strips on top.
This is correct. We paid Norm a good deal of money to stay at our factory and help us. Norm is a good egg. This was all before we found the real vibration problem with the poorly designed transmission mounting point of our airframe, so what results we got out of Norm were not so much able to solve problems with the Mini, but was very educational for me and my staff.

After I was able to finally solve the frame problem, then the dynamic balancing results started to make since and become stable. I was able then, thanks to all the teaching I got from Norn before hand, to come up with the latest and final dynamic balancing procedures.

Here is the basic rule that makes life in dynamic rotor balancing easier;

When you balance the rotor system in a hover, and blades are in perfect track.... then you are in balance, and no further need to balance in forward flight. Balance in hover is balanced in forward flight. Done. Just because you are moving forward did not change the principals of matter and motion and make it go out of balance against the rules of the universe.

Now, where the problem comes from to cause forward flight vibration is from blades going out of track in forward flight, and out of track causes vibrations. That is why its so important to have your blades in perfect track AND at the same time achieve acceptable balance in a hover.

Blades go out of track in forward flight because no two blades are exactly the same and twist differently from the forward flight aerodynamic forces, causing them to go out of track, and that causing vibrations, not something that suddenly became out of balance. You just need to adjust the trim tabs and bring the blades back into track to reduce the vibrations in forward flight, because the rotorsystem is not out of balance, its only out of track... and again, out of track causes vibrations. The trim tabs should be mounted inward towards the center of the disk enough where adjusting them will not effect hover, but the extra speed of forward flight will be enough to change the twist.

Yes, you can reduce forward flight vibrations by throwing out the balance to counter the track vibration, and make the vehicle FEEL smoother. But this is a mistake. Like taking two hammers of equal weight and pounding them together will reduce the forces you feel, the destructive forces are still being absorbed by the airframe. Better to solve the source of the vibration problem rather than using more complicated and less effective counter-forces.
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  #27  
Old 02-16-2017, 05:52 AM
Jean Claude Jean Claude is offline
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Now...for gyros, is it possibly practical to have the fuselage and the tip path be in alignment so no feathering is needed? This may be a place where gyro vibrations can be reduced significantly.????
Bryan,

On two blades in forward flight, a 2/rev vibration caused by the rotating component of the rotor drag. Delete it at source is not possible. Just to avoid transmitting by mast design.

Other vibration 2/rev caused by the angular difference between the plane of the blade tips and teeter bolt. Can therefore be canceled when the swashplate is independent of the teeter bolt, because the two planes can then coincide (helicopter)

When the control is done by the orientation of the shaft the two planes not coincide due to longitudinal flapping a1. This affects especially the stick because it is a cyclic torque on the shaft.

It would be possible to cancel this flapping by a cm0 <0 which twists the advancing blade. Dangerous, however, because overspeed may be unrecoverable.


Quote:
Originally Posted by Mike G View Post
2.0 IPS at 18.2 Hz!! I assume this is 2/rev for a 1/rev at 546 rpm. Does the mini rotor turn that fast?
Yes Mike, 546 rpm due to the little surface of blades (rotor 5.8 x 0.2 m)

Last edited by Jean Claude; 02-17-2017 at 02:26 AM.
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