Another Huey question

[Roger Cain]

I received this message, and have no idea on the answer:

I am trying to find some info on an old UH1 model helo. If you could poll your sources, it would be much appreciated. Apparently one of the older models had an actual chain that DROVE the tail rotor and was later replaced by a drive shaft I am told. I do find this hard to comprehend as it spins at quite a speed 1600 RPM. I have asked around but no one has given me a straight answer. Once again I really enjoyed looking at all your cool pics. Thanks.

[EDowning]

Roger

It's been a number of years since I flew the Huey regularly, but as I recall the 42 degree gear box drove a drive shaft to the 90 degree gear box and a chain from the 90 degree gear box goes to the shaft that drives the tail rotor. 1600rpm sound low to me for tailrotor rpm.

The chain is mention in the follow accident report as well.

Helicopter UH-1H 68-15568
Information on U.S. Army helicopter UH-1H tail number 68-15568
The Army purchased this helicopter 0469
Total flight hours at this point: 00002032
Date: 04/04/71
Incident number: 710404021ACD Accident case number: 710404021 Total loss or fatality Accident
Unit: 135 AHC
The station for this helicopter was Dong Tam in South Vietnam
Number killed in accident = 13 . . Injured = 0 . . Passengers = 10
costing 395145
Original source(s) and document(s) from which the incident was created or updated: Defense Intelligence Agency Helicopter Loss database. Army Aviation Safety Center database. Also: OPERA (Operations Report. )
Loss to Inventory

Crew Members:
AC 1LT LAPLANTE NOEL CHARLES KIA
P WO1 KNIGHT TERRY VASCAL KIA
CE SP4 REYNOLDS HARVEY MICHAEL KIA
G SGT STEPPEE LARRY ELMER KIA

Passengers:
, CET, G; , PAX, A; , PAX, A; , PAX, A; , PAX, A; , PAX, A; , PAX, A; , PAX, A; , PAX, A; , PAX, A;

Accident Summary:

AIRCRAFT ^68-15568^ WAS ENGAGED IN A COMBAT OPERATION OF THE ^135TH ASSAULT HELICOPTER COMPANY^ AND FLYING IN THE TRAIL SHIP POSITION IN A FLIGHT OF FOUR UH-1H AIRCRAFT. THE FLIGHT HAD COMPLETED TWO LIFTS AND HAD PICKED UP THEIR TROOPS FOR THE THIRD LIFT. THE FLIGHT WAS IN THE PROCESS OF JOINING IN A DIAMOND FORMATION WHEN THE TRAIL SHIP WAS HEARD TO SAY "LEAD, YOU'RE SCATTERED." THIS WAS THE LAST TRANSMISSION HEARD FROM THE TRAIL AIRCRAFT. THE AIRCRAFT WAS FIRST NOTICED AS MISSING WHEN THE COMMAND AND CONTROL AIRCRAFT QUESTIONED THE LEAD AIRCRAFT CONCERNING THE NUMBER OF AIRCRAFT INHIS FLIGHT. AT THIS POINT CHALK 3 OF THE FLIGHT BROKE OFF TO LOOK FOR THE TRAIL AIRCRAFT AND THE REMAINING TWO AIRCRAFT IN THE FLIGHT STAYED TOGETHER TO ASSIST IN THE SEARCH. THE COMMAND AND CONTROL AIRCRAFT ALSO ASSISTED AND WAS THE FIRST SHIP TO DISCOVER THE ACCIDENT SITE. NO MEMBER OF THE FLIGHT WITNESSED THE AIRCRAFT DEPART THE FLIGHT OR CRASH. THE AIRCRAFT IMPACTED IN A NOSE LOW ATTITUDE WITH THE LEFT FRONT SIDE OF THE AIRCRAFT STRIKING FIRST. THERE WERE NO INDICATIONS OF SKIDDING. THE TAIL BOOM OF THE AIRCRAFT SEPARATED FROM THE MAIN FUSELAGE BEFORE STRIKING THE GROUND ANDWAS FOUND APPROXIMATELY 100 METERS FROM THE MAIN FUSELAGE. THE TAIL ROTOR WAS STILL ATTACHED TO THE TAIL BOOM BY THE TAIL ROTOR DRIVE CHAIN. THE MAIN ROTOR BLADES AND HUB ASSEMBLY WAS LOCATED APPROXIMATELY 50 METERS TO THE RIGHT FRONT OF THE MAIN FUSELAGE AND ALSO SEPARATED FROM THE AIRCRAFT IN FLIGHT. THE ENGINE WAS FOUND APPROXIMATELY 100 METERS TO THE FRONT OF THE AIRCRAFT AND HAD ALSO SEPARATED FROM THE AIRCRAFT IN FLIGHT.\\

STC for chain replacement here. http://www.garlickhelicopters.com/stc.html

[Roger Cain]

Eric,

Thank you very much for the quick response. I know my helos and aircraft fairly well, but fall in the dumb category when it comes to the mechanics of them............Roger

[RickH]

Eric, I believe the chain is the final mechanism that actually changes the pitch on the tail rotor. The 90 degree gearbox is shaft driven and the shaft that the tailrotor is mounted to is gear driven internally.

[EDowning]

RickH wrote:

Eric, I believe the chain is the final mechanism that actually changes the pitch on the tail rotor. The 90 degree gearbox is shaft driven and the shaft that the tailrotor is mounted to is gear driven internally.

Of course, you are right. I really seems forever ago (1982) that I was at Rucker, preflighting Hueys at 5am in the dark. I could never have imagined that I could ever be unsure of anything about them, but, time has a way of.........well, you know.

[muddyboots]

Eric, wasn't this actually a pretty short chain, almost like a band? I vaguely remember one coming apart at Polk while I was tehre in the late 80's but I wasn't connected to the issue, just heard it being talked about.

[mustangpilot]

Yes, this is the chain for the pitch change system. The old style was called a silent chain. The new style is called a bicycle chain.

[Roger Cain]

Thanks everyone for your replies, received this from an inside sou5rce at Bell Textron:

Most all Hueys up to and including the H model had what is called by mechanics a Silent Chain. All it amounted to was a sprocket mounted to a bearing on the back side of the 90 degree gearbox that when rotated by the chain would move the control rod in and out of the gearbox. The control rod ran all the way through the center of the gearbox output shaft and was attached to the center race of the crosshead bearing. The control rod did not rotate but the cross head did. By moving the rod in and out of the box you moved the cross head which through pitch links changed the pitch in the blades. I have attached a page from the maintenance manual, which gives a more elegant and precise description for clarification. The change to this was to attach a bell crank directly to the end of the control rod in place of the sprocket assembly. I don’t know about all the other various models but it changed on the N models, most civil models and I think the cobras. I hope this helps.

[sgt hawk]

The loss of the compensating idler rotor is not a good thing. The only thing that will sustain the aircraft, is forward air speed (I was a grunt so what the heck would I know) but one of the first cobras I ever saw up close came into the PSP runway at the SF compound at Loc Nihn without the tail rotor, and boy, did the sparks fly off of that PSP (Pierced steel plating). He must of been doing 85-90 MPH and had to land it like an airplane. He got it stopped in the grass at the end. But if he'd kept going another 60' or so, there was about a 40' drop off. It was just another day at the office.
Greg Hawkins
Woodstock, Ga

[jmkendall]

Roger,

I think the concensus is correct, in that someone is confusing the chain that controls pitch with the torque tubes that are the intermediates between the 42 and 90.

I looked back through all my manuals and could not find any reference to a chain replacing the torque tubes. I also worked on Alpha models back at Rucker many years ago.

I am a current 15M-Huey Maintenance Supervisor with the Army. I have a complete set o -23s, 23ps, and 209s. Feel free to send me any questions regarding the Huey.

Joe

[Roger Cain]

Thanks Joe,

So far if I have understood this correctly, the rotor is driven by a driveshaft, and the pitch is adjusted by a chain.