Bell 212 Collective rigging question
 

This is a question for my fellow Engineers.

I am bored at work today and was re-reading the maintenance manual (yes I am that bored) and It has always bothered me that I cannot come up with a good reason for why the manual states in paragraph 67-2 step 5 to shorten the servo input rod 3-5 turns for the collective servo while for both cyclic servos you are to shorten the input rod exactly 3 turns.

Obiously this is to protect the servos from being bottomed out and damaged at their extremes of travel but why is there a range allowed for the collective servo and a specific number for the cyclic servos?

I have had conversations with co-workers about this and haven't really been able to come up with a logical explanation, anyone on here have a fancy reason I can impress people with?

Dumb pilot input here, but could it be anything to do with allowing a range for an auto rev adjustment?

Errrrrrrr.....that would be Pitch Change Links for that adjustment.

Yeah, just a thought that maybe there was some scope for adjustment at that end of the link too. Not particularly well founded, I'll grant you!

Maybe it is to ensure that the servo does bottom out and does not drive the collective sleeve into the stop and snap off the lever?

Read this

Albeit the 205/212 is slightly different with a 2 piece link.

http://i66.photobucket.com/albums/h2...s_removal_.jpg

http://i66.photobucket.com/albums/h2...z/IMG_1316.jpg

Having been involved in this broken 412 collective lever from listening to the pilot trying to land the machine and the subsequent Bell investigation, I can categorically state that this failure has nothing to do with the collective being mis-rigged but metal fatigue possibly caused by overzealous shot peening during manufacture, if you had included the electron microscopic photo's of the shear plane it would give you a better understanding of that event.

S

Stacey - operative word being "Maybe"

As to your categorical statement - are you familiar with the total life and history of the lever in question?

It pays to keep an open view.

Dont get me started on control bolt heads coming off!

...if that bit broke off in-flight, and the pilot has been able to land it in one piece, he's done a remarkable job :ok:

Clearly without doubt.

Having had a rotating control bolt head fly off and it remain only because of a tiny part of the radius under the head left it certainly makes you review things in depth!

I dont mind being a statistic - its the size that is the issue!

It's fascinating to see that picture, as I had one of the bolts in the linkage shear and the resulting twisting meant the other bolt couldn't be removed conventionally. I spent a lot of time trying to work out what the effect would have been if and when the second bolt had failed :uhoh:

Ask Dog Breath.....as Monty isn't around to answer any questions.:mad:

SAS, different bolt! I'm more than well aware of Monty's bolt (with C.C.C who frequents here). ARK got off Scott free from that one! My bolt would have given the same symptoms as the link failure in photo.

Different bolt...but much the same reaction if it had broken.

Scary stuff...."cast iron" failures in a helicopter!

That's scary to look at, but I do not think that failure has anything to do with the control system being mis-rigged.

Shortening the input rod end is without a doubt to prevent servo and airframe damage.

If the input rod was too long AND the servo's bottom rod end bearing was not adjusted to the proper length to fit exactly in the bottom fitting the servo would be exerting force against the airframe between its upper mounting ladder beam and the lower fitting when at its fully extended position. Or at least that is my understanding.

My question is as to why the cyclic system gives an exact number of turns to shorten its servos' input tubes while the collective allows you a range of adjustment (which isn't really useful for any other part of the rigging procedure.)

It's not really important I guess, I just hate reading the manual and doing things "because that's what the book says" and not for a logical reason that I can explain to myself.

Bell 212 airframe
 

Dear college


Anybody knows some web page or some person who has a complete 3D model airframe to Bell 212, like this details??
I need make a simulation in FEM to crash situation.
Thank´s for your time.

https://www.dropbox.com/s/n0ayufij6n57jnl/airframe.png