In this video I plan to show how a helicopter
drive system transfers power from the engines to the rotors. There are multiple stages of gear
reduction and direction change involved this drive system reduces the speed of rotation from 14,000
RPM at the engine to 256 RPM at the main rotor and the main rotor has a 95° direction change from the
engine output to the rotor input. The tail rotor in this model rotates at, 1400 RPM these rotation
speeds are representative of typical large, turbine powered, helicopters. The overall gear
reduction from the engine to the main rotor is 55 to 1. Because there are so many components needed
to create this gear reduction and direction change it'll be easier to visualize if I first hide all
of the components and then bring them back one at a time to show the gear reduction and function of
each component. I'll start by hiding the flight controls and the hydraulics. Then I'll hide all
of the drive gears, tail rotor gears, tail rotor shafts, and the tail rotor itself. Finally I'll
hide the engine cases so you can see both the gas generator and the power turbine. Helicopters
generally use turboshaft engines with two rotating spools. The front spool is the gas generator and
the aft spool is the power turbine. These spools are not mechanically connected. The gas generator
converts air and jet fuel into energy in the form of a high velocity flow of expanding hot gas.
The power turbine extracts power from this flow of hot expanding gas and this is the only part
of the engine that is mechanically connected to the drive system. The gas generator in this model
turns at 14,000 RPM and the power turbine turns at 12,800 RPM. These speeds are representative of
actual turbine engines, although the animation appears slower, because I've slowed down the
playback speed. For better visualization of only the mechanical path to the rotor, I'll hide
the gas generator spool for the remainder of this video. Now we'll start connecting the power
turbine shaft of the engine to the rotor with various drive-system components and explain the
gear reductions as we go. The flexible couplings at the front of the power turbine shafts are
there to allow for minor misalignment of rotating machinery. Misalignment is expected during flight
of the helicopter. The 28 tooth pinion gears, at the front of the power turbine, allow a
connection to the next reduction stage of the drive gears. For now these parts have not gone
through any gear reduction so everything here is rotating at 12,800 RPM; the same speed as the
power turbine shaft. The next part is a 56 tooth gear attached to a one-way clutch. This assembly
is called a freewheeling unit and it serves two purposes. The gear part of the freewheeling unit
provides a gear reduction and the clutch part automatically disconnects a failed engine from
the drive system. Without the freewheeling unit, a failed engine would drag the rotor system and
make it incapable of an engine-out autorotation. This is an an important safety device as it allows
the helicopter to autorotate, and land safely, following an engine failure. The 56 tooth gear,
on the freewheeling unit, driven by a 28 tooth pinion and this gives a gear reduction of 56 to 28
or 2 to 1 since the power turbine turns at 12,800 RPM the freewheeling unit now turns at 6,400 RPM.
The next step in the gear reduction comes from two 28 tooth gears driven by each of the freewheeling
units and powering a 56 tooth common bullgear. This is where the power output of two engines is
combined into one input to the rest of the drive system. Because the 56 tooth bull gear mates with
a 28 tooth pinion, this gives an additional 2 to1 gear reduction. The bull gear output is 6400 over
2 or 3,200. In front of the bull gear there's a 28 tooth bevel gear and this mates with a 70 tooth
bevel gear at a 95° angle. This gives a 5° forward tilt to the rotor mast and this is a common
feature on large helicopters. The 5° forward tilt gives a level fuselage attitude in forward
flight. A 28 tooth gear meshing with a 70 tooth gear gives a 2.5 to 1 gear reduction and the main
bull gear is now turning at 3200 over 2.5 or 1,280 RPM. At this point the combined gear reductions
have slowed the animation so much that it's hard to visualize motion. I'm going to speed up the
animation, but keep in mind I'm not changing any of the gear ratios, I'm only changing the speed
of the playback. The next step in the drive train is a planetary gear set. I'll start by adding a
24 tooth sun gear and then four, 36 tooth, planet gears. These four planet gears turn inside of a 96
tooth ring gear. This arrangement gives a 5 to 1 reduction and the output power comes from the four
planet gears. To access this power we'll need a device called a "planet carrier" that connects the
four planet gears directly to the rotor mast. The input rotation to the planetary sun gear was 1,280
so after a 5 to 1 gear reduction we're left with 1,280 over 5 which is 256. This is the final gear
reduction and the main rotor speed is now 256 RPM. The tail rotor is driven from the same gear
set that drives the main rotor. Looking at the back of the main bull gear there
are three 16 tooth gears and a final 30 tooth tail rotor drive gear. The multiple
16 tooth gears are idle gears and they serve the purpose of lowering the tail rotor drive
shaft to an appropriate water line on the helicopter the 16 tooth input gear is driven by
the bull gear that turns at 3200 RPM connecting the tail rotor drive shaft with the 30 tooth
gear gives a gear reduction of 30 to 16 or 1.875 to1. The rotation speed of the tail rotor drive
shaft is 3200 over 1.875 or 1700 RPM. The tail rotor drive shaft is a three-piece drive shaft
with flexible couplings along the length of the shaft the tail rotor drive shaft has
an intermediate gearbox that changes the direction of the drive by 120°. A 25 tooth
input gear meshes with a 30 tooth gear to reduce the RPM of the intermediate drive shaft. The
gear reduction is 30 to 25 or 1.2 to 1 so the intermediate drive shaft turns at 1,700 over 1.2,
or 1400 RPM. The final set of gears in the tail rotor drive system are two 25 tooth bevel gears.
Because both gears are the same tooth count, this doesn't change the RPM of the tail rotor from
that of the intermediate drive shaft. These gears are beveled to give a 20° upward cant to the tail
rotor, and this serves the purpose of providing some lift from the tail rotor in addition to the
tail rotor's main purpose of countering main rotor torque if you made it this far I'd like to
thank you for watching as we went through how engineers take the high-speed output of a turbine
engine and transfer that power to the relatively slow rotation speeds of helicopter rotors. When I
started this channel, a a little over a year ago, I had no idea that so many of us geek-out
on how machines, helicopters in particular, operate. I realized 10K subscribers and 3 million
views are relatively small by YouTube standards, but it's still kind of a thrill for
me that so many of you are interested in this content. I sincerely thank you
for watching and I hope to see you next time
