H34 Differences (VNAF)

While with the VNAF 4 years 1964-1968 we had UH34D, UH34G, Ch34C & new MAP UH34D . Wish I could find an old post where my memory was more current. There was a multitude of diffrent configurations. Some just between the series Bu numbers others by the service requirements that procured them & finally the H34T which came out modified with a PT 6 twin pack Turbine Kit. Briefly the UH34D & UH34 G were equiped with AFCS which had been a Navy requirement both as as a factor to aid the pilot & for Navy Sub mission. The Army model came out CH34A/C without AFCS primarily because of the lack of Avionic support capability & their mission concept. It had "Dummy Motors" attached where the AFCS motors were on the AUX Servos to maintain the timing function. The UH34A and older H34 helicopters had a clutch utilizing the engine oil function and the UH34D & UH34G had a hydraulic supply for the clutch. The older H34 Helicopters had fuel system of transfer pumps for cell to cell transfer whereas later models had fuel ejectors that used the diffrential pressure (Ventura) VS the electrical pump . The old model H34 Helicopter had the bent leg main landing gear which was prone to help develope ground resonance which could destroy the helicopter if not lifted out of touch with the ground surface. The later models had the wider straight strut landing gear. Center of Gravity also varied between the Helicopters due to location of these various components. Manual folding of the MRH was on Marine & Navy models while not on the Army CH model. The Navy UH34G model also had a servo on the throttle control which was not on any of the other models. Now you may be able to see why the USAF which didnt have an H34 Helicopter in their inventory (They were Advisors AFAG) Created a four year career for me checking out pilots & maintenance personnel on all these confusing mixtures of Helicopter in the VNAF inventory. They had about 130 H34 Helicopters (Danang, Nha Trang, Saigon & Delta Binh Thuy) with all these mixtures. I rotated 2 weeks rotating at each unit for 4 years.Created both a Pilot & maintenance nightmare. Was one of the most interesting & dangerous periods of my life. With the coinfusion & lack of Vietnamese expertise in maintenace & folks shooting at us possibley you can get the picture!!:eek: Semper Fi PM

Uh-34d

Our resident expert on Vietnamese '34's in Paul Moore. Hope he'll chime in here with the answers you seek.

If I remember correctly the U in UH meant a utility helicopter and the C was for Cargo Helicopter.

H34 differences

Joe, thought I had posted a lengthy explanation on that subject. ??? SF PM

VNAF H-34's

According to Military Aid Programme documents, the VNAF recieved

23 CH-34A
87 CH-34C
89 UH-34G
32 UH-34D

I remember hearing some of our pilots discussing the configurations between our "D" models and the VNAF 34's. Sometime after 261 arrived in Danang in 65, some of our pilots made training flights with the VNAF (using our a/c). They said something to the effect that they had to shut down alot of the avionics and other mechanical features in order to replicate the VNAF 34's. I'll leave it to Paul to explain that.

VNAF Avionics

As in previous post the CH34 A/C Army models had no AFCS. Just manual control through the two servo systems ; Aux & Primary, no imput for stabilization from AFCS motor, gyros etc. The UH34 Helicopters had AFCS incorporated but the VNAF had little or no Avionics personnel and no IMA bench test etc. I convinced the USAF Command that it was dangerous to leave the AFCS active. The answer was to null out the AFCS motors to nuetral and remove the AFCS Channels. This simulated the CH34 status where each channel of the Aux servo had a solid dummy motor that its only function was keep the Aux servo centered to maintain the timing. The only problem was although we had nulled out the AFCS motors they sometimes vibrated out of nuetral & resulted in " hardovers" or at least out of tolerance stick jump when the Aux servo was checked during preflight. The entire VNAF H34 fleet was entirely without AFCS & was strictly pilot induced manual control with only stick trim in the system. Made long flights more tiring with constant pilot corrections. SF PM

Thank you Sirs, Flag Track

Re: UH34D/CH34A differences?

Please can anyone give a description of how the clutches, hydraulic & engine oil work, I read somewhere that if you raised the collective with the engine unloaded the engine would almost thrash itself out of the airframe? Many thanks, Flag T.

Re: UH34D/CH34A differences?

Yes if you raised the collective without the clutch engaged the engine would overspeed & probably through the fan blades off & blow up the engine. The collective control linkage was attached to the throttle and increased throttle as you raised the collective. You engaged the clutch in old models and that run the clutch pump from the engine oil. In later models the clutch operated the same except it was supplied by hydaulics. I was once running the engine for a VNAF Mechanic. The engine was at idle. I did not know what he was doing. Suddenly the engine ran up full throttle. It was screaming . I almost fell over the controls to turn off the magneto swithch. He had disconnected the throttle linkage from the collective which I had in full low position. The throttle has a spring loaded linkage which takes it to full throttle if the collective link is disconnected. I had killed the engine before it blew up. However the overspeed stretched the fan blades from the excessive overspeed. Replacement of the engine & fan was required. You can also get engine overboost when flying by pulling in excessive collective pitch without increasing the throttle enough. That can result in blowing off cylinders or at least bending the push rods. That also called for an engine change. The VNAF had both overspeeds & overboosts resulting in our engines usually being in service 200 hours or less. SF PM

Re: UH34D/CH34A differences?

The VNAF had both overspeeds & overboosts resulting in our engines usually being in service 200 hours or less.

And those were just the ones you knew about!!!:cool:

Re: UH34D/CH34A differences?

I volunteer at the Evergreen Aviatoin and Space Museum every Friday. We have a
CH-34C, in Vietnamese markings, and for Paul's benefit an HO3S in prime condition.

KD Logue

Re: UH34D/CH34A differences?

KD,
Need some pics of them, please. Hope you and Rae are well!!

Re: UH34D/CH34A differences?

KD. appreciate those imputs. Could you post the BU Numbers ?? Good to hear from you after long silence !! Semper Fidelis Paul

Re: UH34D/CH34A differences?

So if I'm correct, it's engine start, clutch pump on (After warm-up) Rotor brake off, Main rotor turns(?) engine back to idle, advance to flight idle, freewheel/Sprag engages, clutch pump off, pump off otherwise you can't autorotate? Before engine start were the spark plugs removed to allow accumulated oil out of the lower cylinders? On the S-55 I am told there was a crank turned 99 times to cycle all cylinders before they started.

Many thanks for your replies and patience. Flag T.