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so this may be the wrong place to be asking this question, but given the incident involved it might be appropriate... i have very limited engineering/design/mechanical smarts (so i expect a quick "duh" response from smart engineers :lol: ) but i find myself wondering if it would be possible to put some kind of freewheeling unit on the rotor mas itself, effectively keeping it seperate from the MGB? i guess it would need another swashplate, which would be added complexity/weight, and it would probably be less durable, and im sure it would need some interesting bearing system to be able to spin freely while supporting the aircrafts weight. but is this plausible? or is there a complete duh factor that im not thinking about here?

 

Not a "duh" question at all Emory, in fact it's a very good question. Personally I think it could be done quite easily on a machine such as a Bell Medium. However it would mean an entirely different MRG. Instead of the gears driving the mast directly, they would drive a sleeve that surrounds the mast. At the bottom of the sleeve would be your free wheeling unit that would in turn drive the mast from the bottom. The mast upper and lower bearings would be supported by bearings that are lubricated via an independent oil system. You would not need another swashplate. The freewheeling unit could be based on the old Bell 47 mechanical sprag clutch design or the old Sikorsky 55/58 hydromatic design. Both would work well.

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well, I too have never seen or heard of the shear point used (thankfully), nor had any of the teachers on my course 10 years ago, but I would think the torque loading from a blade to ground impact would be a lot different than a mgb seizure. (note: I am not a a/c designer, but have a small knowledge of physics). When a rotor hits the gound the torque loading would be transmitted to the "whole" aircraft structure, causing it to "flop all over the place", whereas in a transmission seizure, all that torque would be transmitted down through the mast into the weakest point (shear grove, if equiped). If you think of a treading tap, where does it usually break when it jambs, usually where the cutting flutes start, leaving you holding the thicker shank in the holder.

Now, that being said, Eurocopter has removed this feature from the 4 contact roller bearing mast assys, so they must have thought it was no longer neccesary (or thought having a mast shear point would scare customers away)

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"NOTE: The VMINI, MAX IFR Approach Angle, and G/A Mode Speed for a specific helicopter may vary with

avionics/autopilot installation. Pilots are therefore cautioned to refer only to the Rotorcraft Flight Manual

limitations for their specific helicopter. The maximum rate of descent for many autopilots is 1,000 FPM"

quote from http://www2.faa.gov/library/manuals/aviati...media/CH-07.pdf

 

Is it correct that they could have been down to sea level in about 9 minutes then ?.

 

 

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"NOTE: The VMINI, MAX IFR Approach Angle, and G/A Mode Speed for a specific helicopter may vary with

avionics/autopilot installation. Pilots are therefore cautioned to refer only to the Rotorcraft Flight Manual

limitations for their specific helicopter. The maximum rate of descent for many autopilots is 1,000 FPM"

quote from http://www2.faa.gov/library/manuals/aviati...media/CH-07.pdf

 

Is it correct that they could have been down to sea level in about 9 minutes then ?.

 

Why does it matter what the rate of descent on autopilot is? In an emergency, disconnecting the autopilot will be one of the first things I would do, barring the flight manual saying something different. Knowing now what we know about the 10 minute life, max, of a dry G/B, I wouldn't ever fly that machine above 5000 ft., and I'd be immediately slowing to 70 kts and descending at max rate to 50 ft ASL if an immediate landing wasn't safely possible. Frankly, any machine that will self destruct in 10 minutes should be flown VFR over land at all times.

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well, I too have never seen or heard of the shear point used (thankfully), nor had any of the teachers on my course 10 years ago, but I would think the torque loading from a blade to ground impact would be a lot different than a mgb seizure. (note: I am not a a/c designer, but have a small knowledge of physics). When a rotor hits the gound the torque loading would be transmitted to the "whole" aircraft structure, causing it to "flop all over the place", whereas in a transmission seizure, all that torque would be transmitted down through the mast into the weakest point (shear grove, if equiped). If you think of a treading tap, where does it usually break when it jambs, usually where the cutting flutes start, leaving you holding the thicker shank in the holder.

Now, that being said, Eurocopter has removed this feature from the 4 contact roller bearing mast assys, so they must have thought it was no longer neccesary (or thought having a mast shear point would scare customers away)

 

 

Hope this works

 

LS1897_62_08_REV1_EN.pdf[

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