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I briefly trained as a helicopter pilot. We do refer to it as the Jesus nut. Bit of trivia: the bigger danger to helicopters is actually the rotor slowing down

I briefly trained as a helicopter pilot. We do refer to it as the Jesus nut. Bit of trivia: the bigger danger to helicopters is actually the rotor slowing down too much. If that happens, the rotor will fold. Yes, centrifugal force is the major force keeping rotors in one piece. So lose your engine, and you must slam the collective down to keep rotor speed. (Nit: higher energy rotors give you more time.)

metadat 3y ago
Are you saying the rotors will fold and bend upwards too far due to wind resistance? ASCII diagram (the "o" is the j-nut): | | \.__o__./
- f1shy 3y ago
  Due to lack of centrifugal force tensioning the blades. Yes, the will fold up like in cartoons.
  - the_pwner224 3y ago
    Neat. Apparently it's called coning. https://en.wikipedia.org/wiki/Coning https://en.wikipedia.org/wiki/Coning > The tips of the rotor blades move faster through the air than the parts of the blades near the hub, so they generate more lift, which pushes the tips of the blades upwards, resulting in a slight cone shape to the rotor disc. This is balanced by centrifugal force. If rotor RPM drops too low, the rotor blades fold up with no chance of recovery. > Rotors are typically designed with washout (twist) so that lift is relatively uniform along the blades. However, because lift increases quadratically with airspeed, coning still occurs at higher RPMs. > Some helicopters such as the Bell UH-1 Iroquois are designed with "pre-coned" blades, which are curved downwards but lay more flat in flight.
IshKebab 3y ago
[flagged]
- epylar 3y ago
  You know, maybe it's OK to sound stupid sometimes. You're likely to learn more if you're more willing to make mistakes.
  - IshKebab 3y ago
    People don't say centripetal because they're willing to make mistakes and learn. They do it to sound clever.
    - meepmorp 3y ago
      You don't actually have to be this way, you know. You could show people a little more grace.
tjohns 3y ago
But don't slam the cyclic down too quickly, otherwise you'll get a "mast bump"... which is a nice way of saying "the main rotor just cut the helicopter's tail in half". This also doesn't end well. Good ol' Robinson Safety Notice SN-11: https://robinsonheli.com/wp-content/uploads/2015/12/rhc_sn11.pdf https://robinsonheli.com/wp-content/uploads/2015/12/rhc_sn11... (Though it does note that this scenario doesn't apply to an autorotation entry on engine loss, so I suppose you're okay in this example. Still terrifying though!)
- gavinhoward 3y ago
  That's the cyclic, not the collective.
  - tjohns 3y ago
    Oh right, thanks! Good catch. :)
    - geocrasher 3y ago
      I just watched a vid about mast bump. It's not the rotor cutting off the tail, it's the mechanical linkage hitting the mast because the rotor has been unloaded and the collective .... something. Heli's are complicated.
      - gavinhoward 3y ago
        Yep. And yes, they are complicated. Boy, is it a rush when you get good enough to fly one by feel.
- mikewarot 3y ago
  I have a good friend who used to be an instructor, mechanic, and pilot. He was type rated in a ton of things, but refused to fly Robinson helicopters, ever. Edit/Append: He explained because the rotor turns the opposite direction of most of the other helicopters he flew, his instincts would get him killed.
1-6 3y ago
Ever pray to Jesus?
dclowd9901 3y ago
To your knowledge, has one of these nuts ever actually failed? That thing looks substantial.
- gavinhoward 3y ago
  Not to my knowledge, at least in the US. All aircraft in the US have a manufacturer-decided, and FAA-approved max amount of time between overhauls, and such overhauls are so thorough as to make the aircraft new again. (Sometimes, they even change enough things that the aircraft has to go through airworthiness tests again.) These overhauls would definitely replace the Jesus nut if there's any hint of problems.
- kens 3y ago
  Here's an article from 1975 claiming that the Jesus nut came off the main rotor of an OH-6, causing two fatalities: https://www.google.com/books/edition/The_MAC_Flyer/jBXi6uK1PFsC?hl=en&gbpv=1&dq=jesus+nut&pg=PA19&printsec=frontcover https://www.google.com/books/edition/The_MAC_Flyer/jBXi6uK1P...
- semi-extrinsic 3y ago
  Not a jesus nut failure, but some versions of the Super Puma had a gearbox failure mode that caused several accidents with rotor separation from the helicopter. For example this one that led to grounding of the EC225 and several related models: https://en.wikipedia.org/wiki/CHC_Helikopter_Service_Flight_241 https://en.wikipedia.org/wiki/CHC_Helikopter_Service_Flight_...
- zzbn00 3y ago
  The wikipedia article has a link to an accident investigation report where unfortunately it was not put back onto the craft after maintenance.

metadat 3y ago

Are you saying the rotors will fold and bend upwards too far due to wind resistance? ASCII diagram (the "o" is the j-nut): | | \.__o__./

f1shy 3y ago
Due to lack of centrifugal force tensioning the blades. Yes, the will fold up like in cartoons.
- the_pwner224 3y ago
  Neat. Apparently it's called coning. https://en.wikipedia.org/wiki/Coning https://en.wikipedia.org/wiki/Coning > The tips of the rotor blades move faster through the air than the parts of the blades near the hub, so they generate more lift, which pushes the tips of the blades upwards, resulting in a slight cone shape to the rotor disc. This is balanced by centrifugal force. If rotor RPM drops too low, the rotor blades fold up with no chance of recovery. > Rotors are typically designed with washout (twist) so that lift is relatively uniform along the blades. However, because lift increases quadratically with airspeed, coning still occurs at higher RPMs. > Some helicopters such as the Bell UH-1 Iroquois are designed with "pre-coned" blades, which are curved downwards but lay more flat in flight.

IshKebab 3y ago

[flagged]

epylar 3y ago
You know, maybe it's OK to sound stupid sometimes. You're likely to learn more if you're more willing to make mistakes.
- IshKebab 3y ago
  People don't say centripetal because they're willing to make mistakes and learn. They do it to sound clever.
  - meepmorp 3y ago
    You don't actually have to be this way, you know. You could show people a little more grace.

tjohns 3y ago

But don't slam the cyclic down too quickly, otherwise you'll get a "mast bump"... which is a nice way of saying "the main rotor just cut the helicopter's tail in half". This also doesn't end well. Good ol' Robinson Safety Notice SN-11: https://robinsonheli.com/wp-content/uploads/2015/12/rhc_sn11.pdf https://robinsonheli.com/wp-content/uploads/2015/12/rhc_sn11... (Though it does note that this scenario doesn't apply to an autorotation entry on engine loss, so I suppose you're okay in this example. Still terrifying though!)

gavinhoward 3y ago
That's the cyclic, not the collective.
- tjohns 3y ago
  Oh right, thanks! Good catch. :)
  - geocrasher 3y ago
    I just watched a vid about mast bump. It's not the rotor cutting off the tail, it's the mechanical linkage hitting the mast because the rotor has been unloaded and the collective .... something. Heli's are complicated.
    - gavinhoward 3y ago
      Yep. And yes, they are complicated. Boy, is it a rush when you get good enough to fly one by feel.
mikewarot 3y ago
I have a good friend who used to be an instructor, mechanic, and pilot. He was type rated in a ton of things, but refused to fly Robinson helicopters, ever. Edit/Append: He explained because the rotor turns the opposite direction of most of the other helicopters he flew, his instincts would get him killed.

1-6 3y ago

Ever pray to Jesus?

dclowd9901 3y ago

To your knowledge, has one of these nuts ever actually failed? That thing looks substantial.

gavinhoward 3y ago
Not to my knowledge, at least in the US. All aircraft in the US have a manufacturer-decided, and FAA-approved max amount of time between overhauls, and such overhauls are so thorough as to make the aircraft new again. (Sometimes, they even change enough things that the aircraft has to go through airworthiness tests again.) These overhauls would definitely replace the Jesus nut if there's any hint of problems.
kens 3y ago
Here's an article from 1975 claiming that the Jesus nut came off the main rotor of an OH-6, causing two fatalities: https://www.google.com/books/edition/The_MAC_Flyer/jBXi6uK1PFsC?hl=en&gbpv=1&dq=jesus+nut&pg=PA19&printsec=frontcover https://www.google.com/books/edition/The_MAC_Flyer/jBXi6uK1P...
semi-extrinsic 3y ago
Not a jesus nut failure, but some versions of the Super Puma had a gearbox failure mode that caused several accidents with rotor separation from the helicopter. For example this one that led to grounding of the EC225 and several related models: https://en.wikipedia.org/wiki/CHC_Helikopter_Service_Flight_241 https://en.wikipedia.org/wiki/CHC_Helikopter_Service_Flight_...
zzbn00 3y ago
The wikipedia article has a link to an accident investigation report where unfortunately it was not put back onto the craft after maintenance.