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Bell 47G Rotor Systems

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Rotor Bounce or vibrations - refer B47 Maintenance



MR47TIP 1: Pre-flight/M/R Hub/Trunnion

I have seen three cracked M/Rotor Hub drive trunnions. This is the splined part which slides onto the top of the M/R mast, is held on by the Jesus Nut/droop stop assembly and has two ďarmsĒ which fit into and turn the M/Rotor Hub. Wrap a clean lint free cotton cloth around the trunnion arms and wipe off any grease. The cracked areas occur where the arm goes into the M/Rotor hub gimbal bearings.


MR47TIP 2: Pre-flight/Controls/ Swashplate Horns:

The area where the cyclic rod end fits in the swashplate control horn does not have sliding bushes in the horn to take up any free space between the rod end inner ball and the horn when torque is applied to the attach bolt. If wear occurs in this area, over tightening of the attach bolt at maintenance can cause the swashplate horn to crack and detach on one side. The rod tends to stay attached to the remaining side of the swashplate horn but this canít always be guaranteed! In a well lit situation (reflect the sun off the metal back of your watch if need be), look closely for movement in this area when you move the cyclic push rod from side to side with your hand. If movement does occur determine if it is either a worn rod end or the rod end sliding on the attach bolt. Replace the worn rod end or replace the attach bolt (it will have stress marks on it) and shim between the rod end and horn with a shim similar to the Allison 250 compressor alignment shims. Of course any OVER TIGHTENED attach bolt wonít show up unless the horn cracks! Most good maintenance companies know of this, however it still occasionally occurs


MR47TIP 3: Pre-flight/controls/Stab bar:

Rock the weighted stabiliser bar from stop to stop while holding your finger against the bottom of the damper frame clamp and the M/Rotor mast. Any movement in the clamp area indicates loose attach bolts and could give erratic cyclic response, especially in the hover. The resultant "chasing" the cyclic stick may even lead to dynamic roll over.


MR47TIP 4: Pre-flight/controls/ damper frame clamps:

In a well lit situation (reflect the sun off the metal back of your watch if need be), look for fine black lines on the outside of the damper clamps. These can indicate cracks. Your friendly Helicopter maintenance company can show you what to look for.


MR47TIP 5:  An unusual vibration, seemingly associated with the tail rotor at engine RPM 2900 can some times be caused by a porous engine intake manifold back plate on turbo charged machines.

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