Rotor Research Helicopter Info Site
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Bell 47 main Transmission, rear drive shaft and Tail Rotor Gear Box

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INDICATIONS:  There may be a bang as the drive system fails somewhere.  This will be followed by a slow yaw as the tail rotor comes to a stop.

ACTIONS: It is almost certain that the helicopter will have to be put into autorotation to prevent it rotating right.  How quickly will depend on how quickly the helicopter yaws.

(The tail rotor is only required to counter the torque produced by the engine driving the rotor.  Remove the torque.  Stop the engine and the requirement for tail rotor is not so critical).

IAS must  be kept high 50 Kts  minimum so the vertical fin will have some effect.

Power can be slowly reintroduced to slow the rate of descent.

If the yaw increases to the stage that IAS cannot be maintained reduce power slightly.  (If IAS reduces,  the effect of the fin to stop yaw is lessened and as yaw increases IAS will decrease with further loss of IAS etc. etc.)

Further nose down attitude will hold the IAS up but a higher R.O.D. will occur. 

The pilot has to assess the situation and act accordingly.

The helicopter can be landed two ways...

a)  Autorotative touchdown - the IAS is kept at 50 Kts the helicopter is decelerated lower than normal but harder to reduce the ground speed to zero in case the helicopter yaws off severely and rotates.

b)  Land with power on running landing.  Read loss of T/R control.  In the actual loss of T/R or drive the technique is similar but the touch down speed will be faster.



INDICATIONS: Vibration, bang and rapid yaw to right. The Helicopter may pitch  nose down if something falls off i.e. the tail rotor or gear box.

ACTIONS: Use cyclic to hold helicopter hover.  Immediately roll throttle off to prevent yaw rate increasing.  Carry out a hovering Auto.

If this occurs in an O.G.E. hover the pilot must decide whether to roll throttle off immediately/reduce height and then roll of throttle. 

Try to move a rapidly rotating helicopter to a more suitable area with cyclic. 

Try to get effective translation and reduce power to control yaw rate with lower power and IAS.



INDICATIONS: Bang and rapid yaw also some nose down pitching with the COG. change.  How severe the nose down pitch depends on how much falls off

ACTIONS: Snap the throttle closed and proceed as in loss of drive. 

The change in Centre of Gravity may make it difficult to decelerate as much of the aft cyclic will be used up countering the loss of weight aft.

BOTH  TECHNIQUES - autorotative touchdown and running landing - have been used successfully.  The terrain and All Up Weight will decide the technique to be used i.e.:  Rough terrain will require an auto.  High AUW may require an auto.



In this case the tail rotor is still driven but the cables have broken or pedals jammed.

INDICATIONS: (Broken Cables) A/C yaws slightly (how much depends on how well the helicopters tail rotor system balance weights have been rigged) and will not respond to pedal input.

ACTIONS: Keep the Indicated Airspeed high.  There is no hurry to land and if the rigging is at all reasonable no need to autorotate. 

About 100 yards of clean area is required.  Less can be used with proficiency. 

The helicopter can be flown fairly normally to a suitable area.  (If fuel is sufficient).

Left turns are easier and feel better than right turns. 

Ambient Wind Velocity is best from the left front next best from straight ahead - worst is from right. DO NOT CONSIDER A TAIL WIND LANDING.

A shallow running landing angle is required. Once on finals set 35 Kts  IAS as this will give a nose left yaw of about 30 degrees. Fly the approach as normal, i.e. ground track with cyclic angle with power.

The helicopter should cross the start of the landing area at 10 ft AGL. @ 35 Kts.  At this point reduce IAS and descent to 3 ft. AGL.  Hold 3 ft and continue to slow up, the nose may yaw further left - this is O.K.

As the helicopter is slowed up there will be some flare effect in that no power increase will be required to hold 3 ft. but the power must be squeezed in slowly to anticipate the loss of flare effect and prevent the need for a quick/large power input and its associated rapid right yaw.  As the power is squeezed in this will stop any further yaw left and slowly straighten the nose to straight ahead.

Depending on AUW and W/V the helicopter may come to a stop with no yawing rotation.  If this occurs - lower the helicopter to the ground with pitch and controlling yaw with throttle  With low AUW and good W/V this will occur.

With high AUW and poor W/V the helicopter will still be moving over the ground as the nose comes straight ahead. 

In this case reduce throttle is required to keep the nose straight ahead and lower pitch to run the helicopter on.

THROTTLE ON - Turns the nose right

THROTTLE OFF  - Turns the nose left.

Because the helicopter is in a decelerating attitude at 3 ft AGL the left rear skid will be close to the ground so  HOLD 3 ft until the helicopter stops or straightens up.


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