You guys have to remember something: Traditions die hard in aviation. Some of us are old enough to remember when things were done a certain way - like pulling collective to slow the main rotor. Like...remember when the Bell flight manuals used to allow pulling up to 1/3 of the collective pitch travel on shutdown? I do. And hey- if something worked in the past, why wouldn't it still be okay today? Because we learn from experience, that's why.
I once flew for awhile a 47J-2 with
no rotor brake. Sometimes, getting that thing stopped meant full left pedal and <ahem> as much collective as was necessary. Certainly slowing the rotor with the collective is no worse than doing a hovering auto. No?
In the good old early days of the 206A, with those weak old NiCad batteries and maybe a Ceco fuel control that wasn't "tuned" too well, you'd want as much juice as possible to get from the battery (way up in the nose) to the starter/generator (way back there in the engine compartment). So I would start with *no* boost pumps - no electrical consumers on at all in fact. Then Bell decided that in such a situation the fuel system is under suction (from the engine-driven fuel pump) and not pressure and could possibly suck a molecule or two of air in through a hole too small for a molecule of fuel to pass through.
Voila! Air in the system. Now *both* boost pumps have to be operating anytime the engine is started and flight is intended.
Batteries are better now, and Bendix F.C.'s start better than the old ones, but there are still a lot of dinosaurs out there who just refuse to keep up and change with the times (like maybe some Bell Helicopter 214 test pilots). Is there any real danger to staring a RR/Allison engine with *no* boost pressure? Probably not. But do I start a 206 with both boost pumps operating now? You betcha!
And sure, I used to slow the MR down with left pedal. A slow, gradual application...what could it hurt? Well I guess it does - to those feathering bearings Blender Pilot referred to. I believe that PHI even recently changed their policy to
prohibit using left pedal to slow the rotor, despite "Devil 49's" admission of still doing it.
But isn't it ironic that on the old H-500C I used to fly, the left pedal was
spring-loaded forward! Seems that Hughes didn't mind using left pedal to slow the rotor.
But to address Letby Avenue's actual original question - about torsional loads - I'm not an engineer (I must defer to Lu here), but I doubt it's an issue. The loads are not transmitted in reverse. The load against the Thomas couplings and t/r drive gear in the engine is always the same. The t/r is always providing resistance/drag whether the rotor system is under power or not. Let us consider that the drivetrain is capable of absorbing the application of full left pedal at 100% of operating rpm - in other words, however much h.p. that consumes. As the rpm is reduced, then the total loads are being reduced as well.
The bending moments on the blades however - well, that is another story.