Rotary Wing - Week 2

How does the rotor system continue to rotate during autorotation?

The engine disconnects from the main rotor gearbox/transmission via a freewheeling unit. Aerodynamic airflow upward through the main rotor blades and the rotor system's inertia keep the rotors spinning freely.

How would a correctly executed power back autorotation descent be indicated during flight training on the dual tachometer gauge?

It is indicated by a split in the dual tachometer needles, where the engine/turbine speed needle sits lower (70%) while the main rotor speed needle remains higher at flight speed (100%)

On a small piston helicopter that uses a belt drive system, when are the two instances when the rotor system drive would not be driven by the engine output?

During engine start up and during an autorotation

What do the T and R needles indicate on a dual tach

speed of the turbine and speed of the main rotor

How would an actual engine failure/shutdown be indicated during the autorotation descent on the dual tachometer gauge?

The engine/turbine tachometer needle would drop to zero RPM %, while the rotor tachometer needle would remain up in the normal operating green arc.

During the piston engine start on a belt driven rotor system, what unloads the engine to allow the starter to turn the engine easier?

Disengaging the belt tensioner (loosening the drive belts over the sheaves/pulleys) unloads the transmission and rotor system from the engine during start-up.

What device will disconnect the main rotor system from the engine during an Autorotation?

The freewheeling unit (or over-running/sprag clutch).

What causes the device used to increase the tension of the belt drive between the engine and rotor system?

An electric or mechanical clutch actuator switch located in the cockpit that slowly raises the upper pulley/sheave to flight position.

What is the purpose of the centrifugal clutch on helicopters like the Bell 47 or the Eurocopter Gazelle?

To unload the engine during start-up, allowing it to start and warm up without requiring power to turn the transmission/rotor system.

What is a caution relating to start-up of a belt helicopter engine with the belt tensioner disengaged?

Engines without a load are very easy to overspeed with small throttle inputs, which can cause severe engine damage and costly inspections.

On a belt driven helicopter, when does the operator engage the belt tensioner during the start sequence?

Once the engine is started and running (warmed up at idle).

On the Robinson R22, how are the belt tensions increased?

By closing the clutch switch in the cockpit, which powers an electric actuator to raise the upper sheave/pulley to tighten the belts.

On the Schweizer 300, how are the belt tensions increased?

By engaging a belt-tightening/tensioning mechanism that draws the pulleys apart to apply friction to the drive belts.

What causes the clutch shoes on the Bell 47, Hiller, or Eurocopter Gazelle to engage during start?

Centrifugal force acting on the clutch shoes as engine speed increases, which overcomes spring tension and forces the shoes outward against the outer drum.

What is the compound used in the Hiller clutch assembly to provide mass inside the clutch shoes?

Mercury

What holds the shoes of the clutch away from the drum on the Eurocopter Gazelle?

Return springs.

When are the sprag units inside the freewheeling unit locked up between the inner and outer races of the freewheeling unit?

When the input (engine) speed matches or drives the output (rotor/transmission) speed under normal power-on operations.

When the engine drives the transmission, what are the sprags doing inside the freewheeling unit?

They stand straight up and jam between the inner and outer races to form a friction lock so both turn together.

Where does the oil come from to lubricate the freewheeling unit on the Bell 206?

The main transmission oil system.

How do you inspect the freewheeling unit for damage in the field before flight?

By inspecting the magnetic chip detector for metal contamination.

What are the various locations for the freewheeling unit?

• Mounted onto or inside the engine gearbox

• Inside the main transmission case

• Inside an external combining gearbox

• Built into the upper belt pulley/sheave assembly

What type of metal does the chip detector in the Bell 206 freewheeling unit pick up?

Ferrous (magnetic) metal particles.

Where is the freewheeling mechanism located inside the Bell 47 transmission?

Between the first and second planetary gear stages (on the ring gear of the first stage).

What are the two types of freewheeling units?

• Sprag type

• Roller type

What are the two purposes built into the design of Main Rotor Gearbox/Transmission Driveshaft?

• Transfer torque from the engine to the main gearbox.

• Correct for misalignment and movement of the transmission during flight.

In which ATA Chapter will you find lubrication requirements and intervals in the Aircraft Maintenance Manual for the drive belt pulleys?

ATA Chapter 12 (Servicing).

What attaches directly to the engine output on the Bell 47 to drive the centrifugal clutch?

A gear adapter/spider assembly.

What attaches the engine to the Mercury Clutch assembly and absorbs torsional vibrations on the Hiller helicopter?

An elastomeric (flexible rubber and steel) coupling

How is flexibility obtained on the Bell 206 Main Rotor Transmission Drive Shaft?

Via flexible couplings on each end featuring crowned tooth/spherical coupling gears that ride within internal splines of the outer coupling.

What is the purpose of the flexible couplings at each end of the Bell 206 Main Rotor Transmission Drive Shaft?

To allow for momentary shaft misalignment caused by the movement/deflection of the transmission during flight.

What is the purpose of the coloured strips bonded to the surface of the flexible couplings at each end of the drive shaft?

They are temperature-sensitive "Tell-Temp" indicators that change color/darken to signal an overheat condition.

How is flexibility obtained on the Hughes/MD 500 Main Rotor Transmission Drive Shaft?

By circular spring steel plates welded together at each end of the shaft.

How do they attach the flexible plates together on the Hughes 500 drive shaft?

By an electron beam welding process along their outer circumferences.

On the Eurocopter AS 350 Astar & AS 355 Twinstar, how does the driveshaft assembly correct for movement of the transmission?

Through a coupling housing containing a universal joint / gimbal ring attached between the transmission and engine.

On the Eurocopter AS 350 Astar & AS 355 Twinstar, which end of the inner drive shaft has a flexible coupling assembly to accommodate for deflections of the transmission?

The flexible disc couplings are located on the inner shaft assemblies to absorb deflections without taking tension/compression loads.

On the Eurocopter AS 350 Astar & AS 355 Twinstar, what provides the flexible coupling on the internal driveshaft between the engine and main rotor transmission?

Stacks of flexible stainless steel discs bolted to the shafts.

Due to the use of the Free-power turbine engine, why is there no centrifugal clutch needed?

Because the power turbine is gas-coupled (aerodynamically linked) to the engine gas producer without any direct mechanical connection, allowing the engine to start without turning the rotor system.

In which ATA chapter will you find the Aircraft Servicing Requirements?

ATA Chapter 12

How does the Kaman K-Flex drive shaft modification eliminate the problem that caused Bell 206 driveshaft failure accident discussed?

It uses flexible metal straps that accommodate misalignment without friction, eliminating the need for grease lubrication and removing the risk of overheating/seizure from grease loss.

In a case such as the K-Flex coupling modification, where will you find any maintenance requirements for such a modification?

In the Instructions for Continuing Airworthiness (ICA) published by the holder of the Supplemental Type Certificate (STC).

The K-Flex modification discussed is a major modification as defined by the regulations, in accordance with these regulations what has to be accomplished to certify this maintenance activity?

An AME must complete and submit a Major Modification or Repair reporting form (Transport Canada Major Modification form / FAA Form 337) in accordance with regulatory standards.

What does Bell Helicopters call out for to lubricate the Original Bell 206 Main Rotor Transmission Drive Shaft Couplings?

Bell "204" Grease (P/N 204-040-755-05)