The document describes the rotor and transmission systems of the Bell 407 helicopter. It discusses the main rotor, tail rotor, gearbox, driveshaft, and oil systems. The main rotor uses a soft-in-plane design with composite blades to reduce vibration. A Frahm damper cancels out main rotor vibrations before reaching the transmission. The tail rotor has a two-bladed teetering design in its own gearbox. The transmission includes a freewheel unit, gear train, and oil system to lubricate components.

1. Rotor Systems and Transmission
By: Jordan Sisco

2. Overview
• The main rotor is a four-bladed, soft-in-
plane design system
• VNE- 140 Kts
• Main rotor diameter- 35 feet
• Composite hub and individually
interchangeable blades
• Tail rotor is a two-bladed teetering rotor
• Tail rotor diameter- 5.42 feet
• Engine power (6317 RPM) output
through freewheel unit
• KAflex main driveshaft
• Two-stage gear reduction of 15.29:1 to
main rotor mast assembly
• Tail rotor reduction 2.53:1

3. Gear Train Schematic

4. Freewheel Unit
• Mounted on the engine gearbox
and driven under power from the
engine power takeoff gear shaft
• Engine power transmitted to the
outer race
• Drives inner shaft through sprag
clutch to couple the engine to the
transmission input drive shaft
• Forces from main rotor drive
accessories and tail rotor drive
system

5. Drive System-KAflex
• No lubrication required
• Higher misalignment capability
• Integral fail-safe feature
• High Tq capacity
• Repairable
• Extended service life (6,000 Hr
TBO)

6. Main Gearbox
• Consists of input pinion and
bevel gear, a planetary gear
train, and accessory gear
drive
• Engine to transmission
driveshaft
• Transmission oil pump
• Oil filter housing
• Hydraulic pump
• Nr monopole pickup
• Two electric chip detectors
• 5 degree tilt

7. SAVITAD Mounts
• System for Attenuating Vibration of
Tuning and Dampening
• Decreases vibration transfer to the
structure after Frahm Damper
• Elastomeric corner mounts and
restraint springs have natural
frequency less than excitation
frequency made by main rotor
• Cancels out “4 per” vibrations
• Also cancels out inertia forces made
by main rotor pylon before transfer to
the transmission

8. Main Gearbox

9. Main Rotor Mast
• Mast assembly consists of pole
and bearing assembly
• Splines mesh with the planetary
pinion gears
• Splines mesh with swashplate on
top end
• Threads for main rotor hub
retention nut
• Absorbs torsion, tension and
bending loads
• 413 RPM

10. Transmission Oil System

11. Transmission Oil System (Cont.)
• Lubricates the transmission, mast, and freewheel assemblies
• Oil level sight gauge
• Oil pump driven by MGB at 6 to 6.7 GPM at 52 PSI that
scavenges oil from lower case sump through screen and chip
detector
• Sent to oil manifold and filter element which incorporates an
impending bypass indicator button and a total bypass feature
• Oil manifold also redirects oil to the oil cooler if necessary
• Sprayed onto gears and bearings before collecting in lower
manifold and being sent to the FWU
• Three chip detectors, two on the upper and lower case and
one on the FWU housing, alert of impending failure

12. Transmission Oil Filter

13. Transmission Oil System
Schematic

14. Main Rotor System
• Four-bladed soft-in-plane design
• Glass/epoxy yoke that acts as a
flapping flexure
• Elastomeric bearings and dampers
require no lubrication and low
maintenance
• Composite main rotor blades
incorporating glass/epoxy spar and
skins with nickel erosion strip and
conductive paint for lightning
protection
• High Nr (413 RPM) to provide
controllability and more lift at
higher altitude
• -13 degrees of twist for equal lift
across blade

15. Soft-In-Plane
• Traditional articulated rotor system allows blades to flap,
feather, lead/lag independently…as is true with soft-in-plane
• Rather than traditional hydraulic or pneumatic hinges and
dampers, composite material creates virtual hinges and
dampers via bending and flexing
• Lighter weight, less maintenance, longer TBO
• Combination with elastomeric bearings to create a rotor head
that requires little preventative maintenance
• Soft-in-plane video

16. Bell 407 Soft-In-Plane

17. Frahm Damper
• As the main rotor blades
encounter inflight
convective turbulence the
Frahm Damper absorbs
vibrations
• Steel block in the center
vibrates at a similar
frequency
• Cancels vibrations before
they descend to main
rotor shaft and
transmission

18. Main Rotor Blade Fold
• Optional blade fold kit available for storage
• Requires aftermarket bolts installed on trailing edge of two opposing
blades
• Reduces width to 7.3 feet when folded

19. Tail Rotor System
• Two-bladed teetering rotor
• Steel yoke assembly with
elastomeric flapping
bearing
• Composite glass/epoxy
blades with nickel plated
abrasion strip
• Yield indicators on yoke
static stop to provide visual
of limit exceeded
• Gearbox contains self-
contained oil system and
chip detector with two
spiral bevel gears for 90
degree change and 2.53:1
reduction.
• Delta hinge mounted at 30
degrees to control flapping

20. Pedal Stop System

21. Pedal Stop System
 Early production models had a series of accidents that the
NTSB attributed to the tail rotor hitting the tail boom
 It’s thought that under extreme conditions: low RPM, high
cross-winds causing severe flapping and full left pedal,
contact could be made
 Bell added the Pedal Restrictor Control Unit to limit left pedal
above 55 knots
 Emergency release cable in the cockpit

22. Limitations

23. Emergency Procedures

24. Autorotation Chart

25. Exam Questions
What is a “Soft-in-plane” rotor system?
A soft-in-plane rotor system is a fully articulated head that
incorporates flexing and bending at the hinge points instead of
physical dampers.
What is the purpose of a Frahm damper?
As the main rotor blade encounters inflight convective
turbulence, the Frahm damper steel block begins to vibrate at
a similar frequency, canceling the vibrations before they can
descend into the main rotor shaft and the fuselage.

26. References
 (2003). Bell Helicopter Product Description [Pamphlet]. TX:
Textron.
 Bell 407 Rotorcraft Flight Manual (BHT-407-FM02). (2011).
Fort Worth, TX: Bell Helicopter Textron Inc.

27. Questions?