Bell 412 questions
 

Fellow old farts, I have been out of the flying game for a good while now and it looks like I'll be seating in a 412HP again shortly.
I remember the wealth of 412 vets in this forum and while I was getting re-familiarized with the airframe a few questions -I swear I used to know the answers :ugh:- popped in my brain.
Here we go:
1- Why is there a door(s) open speed limitation? I recall it might have to do something with rotor resonance and CG shift but it's all foggy :*
2- While preflighting on top and manually spinning the blades you hear a 'Click" sound. Again I remember was a "healthy" sound that had something to do with the hydraulics but I'd like to hear the detailed/correct explanation
3- Why the VNE of 105KIAS when operating above 81% mast tq?
Any takers?

I'll have a shot at (3): when you are trying to go fast, you are poking the stick (and disc) forward, while the fuselage tries to maintain a level-ish attitude. The mast is trying to bend forward, taking the transmission with it to a certain extent. But the engines are fixed to the fuselage, and the driveshaft connects from the C-Box to the Txmsn. If you are pulling lots of power while trying to go faster, the bending forces on the driveshaft can be quite high.

Hence if you are pulling big power, keep the speed a bit lower to reduce the bending forces on the driveshaft.

The doors limitation is to avoid them from getting ripped of the airframe by the wind...

Of you have a close look, there is only a smalll "nubsy" (as we called it) holding it onto the frame...


The "klick"-you mean the idle stop?

That would be the pin pushed back for 5 seconds when you push the "idle stop release" button on the pitch...you check it before start-push, it klicks once when it releases...then klicks again after 5 seconds when it locks...

I'll have a go at Number 2; "While preflighting on top and manually spinning the blades you hear a 'Click" sound. Again I remember was a "healthy" sound that had something to do with the hydraulics but I'd like to hear the detailed/correct explanation."
I believe you mean a "clicking" sound while the rotors are being turned. Yes, this is from the slowly turning hydraulic pumps and is perfectly OK. 'System 1' and 'System 2' pumps are very different and each turns at different rpm.

Vne is 140kts, but Vne in autorotation is 105Kts. Why the difference?

Large amounts of drag in auto can reduce RRPM too much and it becomes dangerous. Rate of descent is pretty high at that speed too.

I'll take a stab at 3, I hope I will be corrected if I'm wrong.
The tq limit at a high A/S may be to do with RBS, i.e. Bell have worked out if you are over a certain airspeed and a certain tq limit that you are in the RBS area
I think the 206 has a tq/AS limit for the same reason.
Similar but different reason for power on vne being higher than power off (auto) vne.
If the RRPM was at the bottom of the power off range you would get RBS at power on vne hence the lower vne for the auto (power off)

Sorry, Fadec, it is definitely not RBS, it is mast and driveshaft bending.

Rbs doesn't happen within the flight envelope.