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Mast moment measurement in hingeless rotors

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Mast moment measurement in hingeless rotors

Old 10th Dec 2014, 13:38
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Mast moment measurement in hingeless rotors

I want clarification with reference to the mast moment measurement in 4-bladed hingeless rotor (Example like in BO 105, EC135, BK 117 and ALH from India). The measurement of mast moment was done on rotating mast with reference to main rotor blade through strain gauge/slip rings. There were strain gauges put on the four lift rods connected to fuselage attachment point from MGB. When we compared the mast moment measurement from rotating mast and through lift rods (by taking difference between rear and front lift rod and multiplying by arm distance w.r.t rotor centre), the moments were different.Kindly specify the reliable and sensitive method to measure the mast moment either by using rotating mast (rotating coordinates) or by lift rods (stationary coordinates).
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Old 11th Dec 2014, 00:14
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Mast Moment Measurement in Hingeless Rotors

What is the object of this exercise? Are you troubleshooting the Mast Moment Indicator or is this an academic theory project? If you're troubleshooting, contact the manufacturer or local tech. rep.


By the way, the BO-105 and BK-117 main rotor heads are not strictly 'hingeless", although they don't have flapping and drag hinges they do have feather/pitch-change hinges.

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Old 13th Dec 2014, 13:09
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Feathering Hinges

Are there any rotor heads that don't use a feathering bearing, i.e. feathering is by blade twisting?
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Old 13th Dec 2014, 13:11
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EC135, maybe even MD900 from the looks of it.
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Old 13th Dec 2014, 14:42
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Tejas,

There is no comparison. The mast moment system measures "bending" of the mast pole. Trying to measure this at the P/C links would be impossible due to the values you are measuring versus the mechanical play of each bearing component in the control system, e.g. P/C bearings, swashplate bearing, feather bearing, etc. The MM values tend to be a great deal smaller than any single acceptable bearing play. Now if you are curious on the flight loads only put on the links then your plan works.

Also remember the MM system is optional equipment and a lot of people don't use it.

Last, the EC135 while it has 4 blades and lineage to the 105 and 117 is not part of the group mentioned. For one, it has no rotor head and is not considered a rigid rotor system as the others. Plus the 135 has flexbeams and dampers so would be closer to an articulated rotor system.

Good luck
W1
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Old 13th Dec 2014, 16:56
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Bearingless Rotor Heads

Paul,
The Model 430 (out of production) and the H-1 Upgrades, Yankee and brother Zulu all Bell aircraft that have feathering flexures eliminating the feathering bearing.
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Old 14th Dec 2014, 04:04
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Was your measurement taken in forward flight? For example, the EC135 MMI has no azimuth reference and simply displays the maximum moment. Are you sure the maximum moment is in the longitudinal plane?
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Old 14th Dec 2014, 13:16
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mast moment measurement

sir you are right mast moment was measured during forward flight and there was a reference w.r.t blade no 1 on rotating mast. We know that mast moment is resultant of pitching and rolling moments.What I mean to say is measurement being done on rotating mast but how about using lift rods i.e non rotating system to evaluate the same. Here lift rod means not rotating control rods(pitch links) but 4 rods which connect rotor,transmission to fuselage structure.
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Old 14th Dec 2014, 14:42
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I see no relevant link between what you measured and what MMI measures.

Anyway, what is the purpose of your measurements?
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Old 14th Dec 2014, 16:49
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My bad on reference to P/C links. But Iím still curious. How will the use of strain gauges on separate structures (lift rods) provide a correlated result to the gauge installed on the rotating mast?

If using a typical mast strain gauge bonded to the mast pole surface as the benchmark, how would you correct for the cyclic loading effects of any mechanical/elastomeric bearing component installed on the lift rod? Or the acceptable play tolerances on the internal xsmn gears/bearings that drive the mast?

I just donít understand the why of comparing the mast moment measurement at a limited number of stationary points farther from the center of rotation, with multiple variables; to a measurement taken through a rotating system at the center of rotation, with one variable (bonding integrity of the strain gauge on mast).

If the intent is purely a theoretical exercise, well OK. But if the intent is to develop a new way to operationally measure the MM, then I would recommend staying with the current system and improving the reliability of its components.

W1
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Old 14th Dec 2014, 18:22
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Another View

Nick Lappos, if you read this, perhaps this group discussion would benefit from a description of your penultimate design for a Mast Moment Indicator ( but never patented ) and the history behind it!
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Old 15th Dec 2014, 09:57
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Bearingless Rotor Systems

Phoinix says:

EC135, maybe even MD900 from the looks of it.
and Otterotor says:

The Model 430 (out of production) and the H-1 Upgrades, Yankee and brother Zulu all Bell aircraft that have feathering flexures eliminating the feathering bearing.
Thanks guys. For people interested, I found a Google Book excerpt that mentions exactly these rotor systems: https://books.google.com/books?id=rv...20head&f=false

plus the Comanche rotor system which it claims was a derivative of the BO-108 (EC135) rotor design. Cool designs although visually the cuff reminds me a bit of a Brantly rotor system! :-)

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Old 15th Dec 2014, 13:41
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Mast moment is the bending of the main roor shaft due to the great capability of the rotor to develop pitch and roll moment (bending). In a "rigid" rotor these bending moments are two or three times more powerful than in an articulated rotor, and of course infinately more powerful than a teetering rotor, which has virtually no moment producing capability (look at the thin masts of a teetering rotor to understand how little bending it can or must transmit). Compounding the extra moment is the fact that the rotor blades flap less, and the tip path moves less, so the rigid rotor gives the pilot fewer cues as to the moment the head is imposing in the mast and on itself.
In flight, this moment produces eye-watering rotations of the fuselage (look at Chuck Aaron's displays in the Red Bull youtube videos for illustration!)
The reason for the moment gauge is that long term damage from high loads can occur on the ground. When the aircraft is restrained against the ground, the rotor moment is countered by the ground forces, so the mast can be highly stressed and the pilot has no cues. In a few minutes, much fatigue damage can occur, and the mast is a critical component, for sure. The "rigid" rotor helos also have cyclic restraint systems to help keep the mast from suffering from pilot inattention.
It is possible for the moment to be calculated from the summation of the mast moment and the forces imposed on the support legs of the static structure (or the beams cast into the main transmission housing, which serve the same function) . The question is whether the moment is a problem in the mast or in the rotor head and blades, or in both. For a specific design the moment gauge should receive its inputs from the proper place, mast and head, mast alone or head alone, whichever is the area of concern to the design team. Mostly, I think the design team is worried about the mast and its long term fatigue life, since cracks can be hidden between overhauls.

Here is a fun anecdote from back in the 1970's during the ferocious competitive flyoff between the two protptypes for the US Army contract for 1500 helicopters that became the Black Hawk:
The Boeing YUH-61A experimental prototype had a rigid rotor scaled up from the BO-105, and the Sikorsky YUH-60A had an articulated rotor (and still does). In an exchange of teasing "coloring books" that critiqued each aircraft, the test pilots let fly their comic sensibilities. Frank Duke of Boeing delivered their coloring book to the Sikorsky compass rose one day, dropped from their prototype with a small parachute. I saw it land. Inside a wooden box of bullshit and Sikorsky brochures (to establish equivilence) was a plastic bag and the coloring book, which had a number of great jibes at the Sikorsky design. One page showed a hydraulic damper standing with its back turned toward the viewer, looking over its shoulder as it pissed a red stream of 5606 against a wall. Their caption read, "I am a hydraulic damper, doing what hydraulic dampers do best, taking a leak. Color me obsolete!"
As a response, John Dixson had us make a coloring book, and John and I dropped it off a week later at Boeing's Calverton, Long Island test site. The crewperson in back was Lou Cotton (who later ran the Comanche program.) The Boeing helo needed a mast moment gauge, so we picked up on this new limit by drawing a mast moment gauge, and its needle and arc. The gauge started off on the left with a yellow scale that started out as "Uh-Oh.." and a top position that read "Oh Shit!" before it then swung to the right to read "Jesus Christ!" The caption said, "This is a Mast Moment Gauge, it tells you how much trouble you are in. Color it Dangerous Red!"

Today I am sure there is a Federal Law that would declare us urban terrorists for dropping boxes from helicopters.
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Old 16th Dec 2014, 01:40
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And here's a picture of a lab-coated Boeing engineer trying to figure out where to put the strain gauge...

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Old 17th Dec 2014, 03:19
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I can see where you might get different numbers between measuring strain at a hingeless rotor mast and at the MRGB attachments. There are both in-plane (lead/lag and drive torque) and out-of-plane (flap) moments produced at the rotor shaft. The rotor shaft is subject to a combination of tension, torsion and bending. The tension and out-of-plane moment forces acting on the rotor shaft are typically transferred thru the mast bearings to the MRGB housing and into the airframe. The in-plane moment forces at the rotor shaft are transferred thru the numerous gear meshes of the MRGB and back to the engine power turbine output shaft. The load reaction at the MRGB/airframe attachment from torque transfer between the gearbox engine input and rotor shaft output is quite complicated.

What probably matters most with stiff rotor systems are the combined bending and torsion stress levels produced in the rotor shaft. So it would seem logical to measure these in the most direct manner practical.
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