ALH Naval variant in trouble?

Is the Navy having problems with the ALH Dhruv? Well, I bumped into a senior serving Naval officer recently at a party, and he assured me that the Navy had "cut" its order for the Dhruv by a "sizeable" number because "HAL had failed to adequately address vibration issues". He said that while the Army and IAF variants had proved extremely satisfactory, the Naval version was still afflicted by vibration problems especially since the choppers were flown at a much lower altitude.

As it stands, the current order book for the ALH looks like this: 159 new Dhruvs on order, out of which 104 are for the Army and 55 for the IAF. I sent a brief questionnaire to HAL asking them about what the Commodore had suggested. The replies don't answer all of my questions, but here's what I asked them, and what they had to say. Inputs on this from readers or those in the know would be very welcome. It would be very sad if something like this actually hampered the ALH programme – it's had its fair share of bad luck with the hard-landings and the crash earlier this year.

Q. How many ALHs have been manufactured so far? Give me the break-up of how many have gone to Army/Air Force/Navy/CG. The current order for 159 choppers is 104 for Army and 55 for IAF. Is that correct? Does this mean that the Navy's order is complete? What was/is the Navy's total order for ALH Naval variant? Has this been reduced in any way?

HAL's answer: As of end March 2007, HAL has produced around 70 ALH for the Indian defence services. More number of ALH for the Indian defence services are in the pipeline. The exact number of aircraft delivered to the Defence Customers is a classified document. Hence, it is not advisable to mention the number of helicopters on order and the delivery status. Supply of ALH utility variant to Navy with conventional cockpit has been completed. Supply of helicopters in glass cockpit configuration will be taken up in 2008-09.

Q. Give us specific details about the vibration issues of the ALH Navy variant, which have apparently been sorted out.

HAL's answer:
Vibration reduction on ALH was initially proposed to be achieved by passive ARIS (Anti-Resonance Isolation System). The ARIS units interposed between the rotor and the fuselage isolate vibrations arising from the rotor. ARIS is a passive system which can be tuned for a desired speed band. HAL has carried out extensive tests and analysis to carry out fine tuning of ARIS to achieve optimum reduction in vibrations.

Stiffening of fuselage has been carried out to reduce vibrations and to remove visual cues in the cockpit. Retro-modification for stiffening of fuselage has been completed on all the helicopters delivered. New production has been taken up with the stiffened fuselage in the series production. Acceptable level of vibration has been achieved for Ops roles with fine-tuned ARIS and fuselage stiffening.

Further reduction in vibration is planned through Frahm dampers (mechanical spring-mass system tuned to a dominant excitation frequency) are mounted at specific areas where vibration reduction is required. These are effective over a wider speed range. Frahm dampers will be introduced based on the specific requirement of the customers.

Active Vibration Control System is also being introduced in the transmission deck area to achieve further reduction in vibration.

Copyright Arun Vishwakarma via Bharat Rakshak

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