To further clarify a bit about helicopter drivetrains, here is the most basic version below.
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Photo © Thomas Brügge
As you can see, the engine is standing on its accessory section with the "prop" shaft up. This drives a transmission directly above it hidden behind the fuel tank. The upper part of the transmission has two (I believe) sets of planetary reduction gears to reduce from engine RPM (which I don't recall) to rotor RPM of about 360 or so. (If you fly the -47 please correct me)
There is also a higher-speed driveshaft out the back of the transmission for the anti-torque (tail) rotor. You can see the shaft passing through hanger bearings along the top of the tailboom. It angles up the little pylon and makes a 90° to the tail rotor.
So long as the engine is turning, it will drive the transmission and the transmission will drive the main rotor through the "mast" and the tailrotor through this long driveshaft. Later single engine designs are pretty similar but have the mechanism faired in, hidden away.
If the engine quits, the engine will automatically uncouple from the transmission (sprag clutch or centrifugal clutch) the autorotation maneuver described in reply #1 will backdrive the transmission and through it, the tail rotor driveshaft. This helicopter in autorotation, without power can do anything but hold altitude or climb. It is fully maneuverable in the descent but descend it will.
Multi-engine, single-rotor designs like the Bell 212 and even CH
-53 and CH
-54 have this basic layout except their engines feed the transmission through separate drive quills. In some cases twin engine was a retrofit and may use a combining gearbox to allow both engines to drive one transmission.
Tandem rotor helicopters, or any layout with two "main" rotors will have those rotors shafted together. If they did not, and the RPM dropped ever so slightly you would die - right at that place and at that time. The rotors mesh and must be geared to pass blade-to-gap and not blade-to-blade.
The old Piasecki H-21 "flying banana" serves to illustrate this well enough:
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Photo © Alexandre Février
The engine was back near the tail, you can see the exhaust sticking out the side at the back end of it. This was a single-row radial mounted conventionally - with the "prop" shaft forward. From there a driveshaft ran to a little gearbox overhead just above the middle of the cargo door. From this gearbox a driveshaft ran at an angle up to the rear rotor. You can see this driveshaft in the uncowled area going up the tail. The shaft to the forward rotor is under the cowling along the top of the forward fuselage. There is a transmission under each rotor making the final RPM reduction.
Other tandem rotors like the Chinook are substantially the same, differing only in the point in the drivetrain where the engine torque is input. On the Chinook there is a combining gearbox back between the engines.
The important part however is that the rotors are splined together and cannot be run at differing speeds.
So, in a multiengine helicopter if you lose one engine you only lose that part of your power. If you have a heavy slingload on a mountain peak at the moment you will probably have to punch off the load or lose the whole aircraft. If you lose one of two engines in a tandem rotor helicopter the remaining engine continues to drive both rotors.
True for the Osprey too. Picture an engine failure in that thing if the prop/rotors weren't shafted together!
When the pilot pulls collective pitch at the bottom of the autorotation to cushion the touchdown he is using the kinetic energy stored in the spinning rotors to arrest the descent rate. This is why second generation helicopters have weights in the blade tips - more energy stored. If you squander that blade energy before landing it is gone.
If you do not put the collective down quickly enough the rotor RPM will decay very rapidly on some models, (shudder
I just had an OH
-23B flashback) and you are seriously hosed. Where you might
survive the resultant hard landing, the spinning rotors are also your only means of control. That last bit is important. On some helos with quite a lot of blade inertia pilots have been known to pull a tiny bit of pitch early, to "carry" them past some trees or powerlines but it is a very risky move and often does not work. I've lost a friend this very way.
Gee, I don't think I've used the phrase "sprag clutch" in thirty years.
Happiness is not seeing another trite Ste. Maarten photo all week long.