Two engines, one main rotor, one Fenestron, hydraulic pumps and oil-cooling fans all need mechanical power. Sitting at the centre of this arrangement is one of the most important components on the H135: the main transmission.
It does considerably more than simply reduce engine speed.
The main rotor drive transmits power from both engines to the main rotor, tail rotor and auxiliary systems. At the same time, it forms part of the aircraft’s structure, transmitting static and dynamic loads between the rotor system and the fuselage.
What Does the H135 Main Transmission Do?
The transmission concept was designed by ZF (Zahnradfabrik Friedrichshafen). Its fundamental job is to accept power from the two engines and reduce their high input rotational speed to the speeds required by the main rotor, tail rotor and accessories.
Internally, the transmission can be considered in four main areas: the input stage, freewheel clutches, collector stage and accessory drives.
The arrangement allows either engine – or both together – to drive the rotor system.
From the Engines to the Main Transmission
Each engine is connected to the transmission by a driveshaft. Flexible diaphragms at the driveshaft ends accommodate misalignment between the engine output and transmission input.
Power then enters the transmission through separate left and right input drives.
Each input incorporates a freewheel clutch. During normal operation the clutch locks, allowing engine power to pass into the transmission. If one engine becomes inoperative, its freewheel clutch overruns, preventing the transmission from driving the failed engine.
This becomes even more important if both engines stop. Both freewheel units overrun, allowing the main rotor and transmission to continue turning independently of the engines during autorotation.
So the freewheel units are fundamental not only to normal twin-engine operation but also to the aircraft’s behaviour following an engine failure.
Combining the Power of Two Engines
Inside the transmission, each engine input drives an intermediate gear.
The two vertical intermediate gears change the direction of power flow through 90 degrees before transferring power to the collector gear. It is here that the power from the two engines is effectively brought together.
The collector stage then transmits the combined engine power to the main rotor and tail rotor systems.
But it works in both directions mechanically. Rotor forces and moments are also transmitted back through the hub-shaft and transmission structure into the aircraft. The main transmission therefore acts simultaneously as a gearbox and a major structural load path.
This is one reason the transmission occupies such a central position in the aircraft.
Driving More Than the Main Rotor
The H135 transmission does not use all of its mechanical power simply to turn the main rotor.
The collector gear also provides drive for the Fenestron tail rotor, while accessory drives power the oil-cooling fans and hydraulic pumps.
This creates an interesting feature during autorotation.
Following a dual engine failure, airflow through the main rotor keeps the rotor and transmission turning. The collector gear therefore continues to drive the tail rotor, transmission oil pumps, oil-cooling fans and hydraulic pumps even though neither engine is producing power.
The transmission consequently becomes the mechanical link that allows several important systems to continue operating during autorotation.
If you’ve read our article Inside the Airbus H135 Bearingless Main Rotor, this is the component delivering power directly into that rotor system. At the other end of the drivetrain, our article [How the Airbus H135 Fenestron Works] explains how transmission power ultimately produces anti-torque thrust.
Keeping the Transmission Lubricated
A gearbox transmitting the combined power of two turbine engines requires continuous lubrication and cooling.
The H135 main transmission has its own independent oil circuit. Pressurised oil is filtered and routed through two oil coolers before being distributed through spray nozzles to critical areas of the transmission gearing. After performing its lubricating and cooling functions, the oil drains back into the sump.
Cooling is provided by fans driven mechanically from the main transmission. The two oil coolers each contain a section dedicated to transmission oil, while separate sections cool the engine oil.
Oil pressure and temperature therefore provide important indications of transmission condition.
Looking for Trouble Before It Becomes a Failure
The lubrication system also provides a simple but important means of detecting internal deterioration.
Oil returning to the transmission sump passes a magnetic chip detector. Ferrous material carried by the oil is attracted to the detector. If sufficient material accumulates to bridge its electrical contacts, the aircraft generates a transmission chip caution.
Preflight inspection includes checking the transmission for leakage and foreign objects, checking the oil-cooling installation, inspecting the filter clogging indicator and confirming the transmission oil level.
These checks provide different ways of looking at the same question: is the transmission receiving adequate lubrication and is there any indication of deterioration inside it?
The Transmission Is Also Part of the Airframe
The main transmission is mounted ahead of the two engines on the transmission deck above the passenger compartment.
Its mounting arrangement includes four Anti-Resonance Isolation System (ARIS) mounts together with three structural mounting struts. The ARIS arrangement helps reduce the transmission of main rotor vibration into the fuselage, while torque and lateral load struts react loads between the transmission and aircraft structure.
This illustrates why describing the component simply as a gearbox understates its role.
It combines engine power, reduces rotational speed, drives the main rotor and Fenestron, powers accessories, supports rotor loads and provides a major structural connection between the lifting system and fuselage.
At the Centre of the H135
The main transmission provides a useful way of understanding the H135 as a complete aircraft rather than as a collection of independent systems.
The engines provide the power. The transmission combines and distributes it. The bearingless main rotor converts that power into lift, while the Fenestron provides anti-torque and yaw control. Hydraulic and cooling systems also depend upon mechanical drive originating within the transmission.
And that brings us naturally to another major part of the powertrain: the two different engine families that have powered the EC135/H135.
Next in the series: Airbus H135 Engines – Pratt & Whitney PW206 vs Safran Arrius.
Internal Links to Add
In WordPress, I’d make these phrases clickable:
Inside the Airbus H135 Bearingless Main Rotor → Post 3
How the Airbus H135 Fenestron Works → Post 2
I’d also add a link from the opening or closing section to Airbus H135: From the Bo 108 to a Modern Light Twin Helicopter → Post 1.
And use Airbus H135 General Familiarisation Course as the anchor text for your H135 course/product page.
Once Post 4 is published, I’d also go back into Post 3 and make its final sentence, “How the Airbus H135 Main Transmission Works”, link directly to this new article. That gives us links travelling in both directions through the series.
SEO
Focus Keyphrase: Airbus H135 main transmission
SEO Title: Airbus H135 Main Transmission: How It Works
Slug: airbus-h135-main-transmission
Meta Description: Discover how the Airbus H135 main transmission combines engine power to drive the main rotor, Fenestron, hydraulics and other aircraft systems.
Image Details
Image Title: Airbus H135 Main Transmission
Alt Text: Airbus H135 main transmission showing the gearbox that transfers power from the two engines to the main rotor, Fenestron and accessory systems.
Short Title / Caption: H135 Main Transmission
