Thread by @Indrani1_Roy

Here's a thread on ALH/LCH and LUH main gear box. The MGB is the magic around which the engineering is done. It is the primary reason behind ALH/LCH's phenomenal high-altitude performance.

@AnilBhambhani11, @unnipillai18,@realkaypius, @BahadurManmohan: please correct any errors.

First let's have a look at the ALH's main gear box (MGB) and rotor head in the first row.

The second row is that of AW139, a fabulous helicopter of roughly the same size and power

Notice how much more compact and clean ALH's MGB and main rotor head are?

This simplicity and cleanliness is because of a few design choices. Let's examine one by one.

The rotorhead. As a rotor blade goes around, it needs to move around its own pitch axis and its lead/lag axis. I won't go into details here.

Aw139 employs a fully-articulated rotor ..

which uses two hinges to allow for these motions. The benefit is that this reduces the stress on the rotor blades allowing use of more common materials as well as prolonging blade-life. The problem is that it adds these hinges are extremely rugged, adding parts, cost and weight.

More importantly, because the hub become heavy and large, the hub moment increases significantly slowing down control response. This is not a luxury that ALH/LCH enjoys. It must sharp controls at 20,000 feet to land on a postage stamp sized helipad with sizeable payload!

The maneuverability requirements of ALH/LCH at high altitudes is also very high. That is why you see the Sarangs snap into those extremely tight and precise turns. There are two other helis that have such rotor systems.

Tiger and Bo-105.

ALH, LCH, LUH, Tiger, Bo-105 all use hingeless (or rigid) rotor system. Instead of hinges, they use flexible elastic joints. All the "flaps" and "drags" of the rotor blade are accommodated through the flexing of the wing and the flexible sections of the root.

This has many advantages. It is mechanically simpler, lighter and most of all gives very sharp responses. This is one of the reasons of ALH/LCH and LUHs phenomenal high-alt performance.

But it has its disadvantages.

Recovery from some conditions is not possible. The Ecuadorian airshow crash was because of this.

Another huge problem is that the one needs exotic materials for the flexible wings and joints. Although MBB pushed for this design, the initial designs were "spectacular failures".

According to Hari Nair sir, by the time MBB left, the life of these joints was ~10 hours! It took a *lot* of effort to figure this out. There is no other helicopter in the world which transfers as much power through such joints. This is the 2nd reason for their extreme perf.

However, a lot of vibration is still passed from the rotor blades to the main rotorhead. These vibrations are eliminated at the MGB by mounting it on a ARIS system which dampens vibrations along 3 axis. No other helicopter in this class has such a system. Tiger uses 2-axis system

Now let's come to the MGB itself. The main jobs of the MGB is to transfer power from the engine shaft to main rotor, tail rotor, and an auxiliary gear box used for powering other systems and cooling the MGB.

But the main job is the first: transfer the power to the main rotor.

The MGB is the keystone of an helicopter's design. It needs to be extremely robust. In case of ALH, it needed to be extraordinarily compact and light weight. The power shaft of ALH engines rotates at 6000 rpm, whereas the optimum rotor speed is ~314 rpm. A reduction of 20x.

To keep the weight and volume low the number of stages were kept to just 2!

Stage 1: gear ratio-> 27:79
Stage 2: gear ratio-> 17:111

I don't know of any other MGB which transfers so much power is such few stages.

These came with enormous challenges.

Transferring so much power at such high gear ratios meant the teeth of the gears were grinding and filing at an alarming rate. The consultants couldn't solve it and left. This is probably the single biggest challenge that HAL overcame on its own. Teeth ratio, shape, size, area ..

Material, extremely low manufacturing tolerances, limited testing facilities: It was a nightmare. To make matters worse, the engines changed. The TM333-2B was found to be underpowered, then the LHTEC CTS800-4N was embargoed, the TM333-2B2 was close and powered the Mark Is.

But it was not until Mk2, that things started falling into place. Anybody who has pursued a serious technical product development knows how difficult things can be when one has many floating pieces. But once one of these pieces stabilized, everything starting falling into place.

By Mark III, HAL had solved most of the big technical challenges and stabilized the production line. Many of the smaller problems of Mk I/II like bad workmanship started getting mitigated as producing scores of helicopters per year required more automation and lower tolerances.

The mastering of these technologies were tough. But solving them have given the ALH the performance that the forces desired.

Now, let me ask a trick question. See the pitch change rods here. Sometimes they are called control rods. You can see them even on the LUH, but not on ALH

This was another challenge thrown up by the 2-stage transmission design. There was no place for the control rods to go around the large diameter stage-2 collective gear. So, the control rods pass through the stub shaft to the main motor.

There is some suspicion that the control rods of ALH are mal-functioning. I don't know enough. But say that they are: Why are they failing in the field? They didn't on the test bench or during certification. manufacturing issue with certain lots, design deficiency, unseen corner?

Difficult to say. *If* the last 2 naval ALHs' were downed by this problem, one great thing is that they have two near intact helicopters to examine

Also, may I say this is not uncommon. 25 years after first production, Boeing 737s suddenly started to plummet to the earth in 1990

Starting in 1990, 3 accidents were confirmed. 4 were suspected including the Sahara accident in 1994. But there was not enough evidence of a failure. It was only when in 1996, that a 737 momentarily lost control, but landed, could the airplane be studied in detail.

Also, the pilot(s) had lived and could tell the tale. The problem still could not be replicated easily. It will take 6 more years when a 747-400 with two rudders suffered a hard-over on one. It survived and the problem was finally solved in 2002

That's Boeing. With 10 of thousands of 737s flying everyday. They suspected for the longest time that these cases were that of "pilot-suicide". Where they saving their skin. May be. But was it also very difficult to replicate: yes. Rudder hardovers haven't happened on 737s since.

Wow! this became a long long thread. @zone5aviation I actually wanted to answer your question about the the similarities of the LUH and ALH rotor head and transmission system. I will do so quickly now.

@zone5aviation Both LUH and ALH have rigid rotor heads with 4 shafts. Both have oversized engines for the MGB. This allows MGBs to be lighter. They can use the full engine power above a certain pressure-altitude. But the MGBs can use the full power at sea level in emergency (I think).

LUH also simplified a few things. LUH has gone for a 3-stage reduction MGB. The control rods no longer pass through the hub shaft. Consequently, the MGB is not as compact as ALH.

I believe they have simplified the ARIS system too.

Also they have moved the tail rotor to the base of the fin. This eradicates the intermediate gearbox at the base of the ALH and also the shaft that joins the IGB to the tail bear box at the top of the fin.

This makes the heli lighter and reduces part count. but it might come with a slightly uneven deck at hover and increased risk of tail strikes.

But LUH is a light helicopter, so its manageable.