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Comment by gavinhoward | Hacker News Reader

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.)

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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...

(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!)

That's the cyclic, not the collective.

Oh right, thanks! Good catch. :)

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.

Yep.

And yes, they are complicated.

Boy, is it a rush when you get good enough to fly one by feel.

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.

I'm not sure I agree with him.

Yes, Robinsons are known for mast bumping. On the other hand, they have the best tail rotor authority.

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__./

Due to lack of centrifugal force tensioning the blades. Yes, the will fold up like in cartoons.

Neat. Apparently it's called 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.

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

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.

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/jBXi6uK1P...

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_...

The wikipedia article has a link to an accident investigation report where unfortunately it was not put back onto the craft after maintenance.

Ever pray to Jesus?

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