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11.2 MD 520N® On Patrol
Four doors are installed on the MD 520N® - two on each side. The two forward doors permit
access to the forward compartment for pilot and copilot/passengers. The two aft doors allow entry
to the passenger/cargo compartment. The design permits flight with doors removed without major
speed restrictions. Transparent tinted windows are contained in the doors.
A Rolls-Royce Model 250-C20RS gas turbine engine with a takeoff power rating of 450 shp powers
the MD 520N®. Only 425 shp at l00 percent N2 rpm is used for takeoff; 375 maximum continuous
shp provides power for all other flight modes.
An overrunning clutch transmits power from the engine to the main drive shaft. The clutch has no
external controls and disengages automatically during autorotation and engine shutdown. The
main drive shaft connects to the main rotor transmission input shaft. The engine oil cooler blower is
belt-driven off the main drive shaft and draws cooling air from the air inlet fairing to supply ambient
air to the engine and transmission oil coolers and to the engine compartment. The main rotor
transmission is mounted on the airframe structure above the passenger/cargo compartment. The
transmission is lubricated by its own air-cooled oil lubrication system. The main rotor static mast is
non- rotating and is rigidly mounted to the fuselage mast support structure. This static mast is used
to separate the lift and torque loads of the rotor.
Torque is transmitted independently to the rotor through an internal main rotor drive shaft. Lifting
loads reacted by the static mast are prevented from being imposed through the drive shaft onto the
main transmission, thus eliminating thrust loading of transmission parts. This accomplishes a
design that reduces main transmission weight and reduces the probability of injury during a crash.
The MD 520N® utilizes a five-blade, fully articulated main rotor assembly. The MDHI strap pack
design provides restraint and allows all three degrees of rotor hub and rotor blade travel. The strap
configuration, while secured firmly to the hub, allows the centrifugal load exerted by one blade to
be countered by the force exerted by the opposite two blades.
SYSTEM DESCRIPTION
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Thus, very light centrifugal loads are sensed by the hub. The “V” legs of the strap pack rotate
as driving members to turn the blades and allow feathering, lead, lag and flapping of the
blades. The main rotor blades are secured to the hub with quick-release lever type pins.
Adjustable pedal controls are provided for rudder/thruster control. Adjustable friction devices
are incorporated in the cyclic, collective and throttle controls. In addition, electrical cyclic trim
actuators allow flight loads to be trimmed out. Control forces are low; hence, a hydraulic boost
system is not required.
The landing gear is a skid type attached to the fuselage at l2 points and is not retractable.
Aerodynamic faring cover the struts. Nitrogen charged landing gear dampers act as springs
and shock absorbers to cushion landings and provide ground resonance stability. Provisions
for ground handling wheels are incorporated on the skid tubes.
11.2.1 Maintenance made easy
The MD 520N® incorporates proven engineering features that are unsurpassed for design
simplicity, safety, reliability and ease of maintenance. There are no hydraulic boost systems,
intermediate drive shaft bearings or grease fittings on any MD 500 series helicopters.
Blade Retention System:
The mechanical simplicity of the main rotor system provides high reliability at low cost. Main
rotor blades are retained by an exclusive strap pack system that accommodates main rotor
blade flapping, lead-lag and feathering motions. Fewer parts result in higher reliability.
Main Rotor Transmission:
The main transmission has four gears and two gear meshes. It is light but rugged for maximum
reliability. The static mast design allows one mechanic to easily change the transmission
without removing any other component on the rotor head.
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11.2.2 Interchangeability
Parts and assemblies listed below are interchangeable:
1. Main rotor blade (tracking of new blades may be required)
2. Main rotor hub assembly
3. Lead-lag damper
4. Main rotor drive shaft
5. Main rotor mast
6. Main transmission
7. Overrunning clutch
8. Main drive shaft
9. Engine assembly
10. Engine mount
11. Engine exhaust duct
12. NOTAR drive shaft
13. NOTAR gearbox
14. NOTAR blades
15. Control system components
16. Oil tank
17. Oil coolers (engine and main transmission)
18. Tail boom assembly
19. Vertical and Horizontal stabilizers
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MD500 Series TechDescription Feb09(14)
