TITLE
DESIGN OF SINGLE ENGINE HELICOPTER GEARBOX CONFIGURATION ELIMINATING THE NEED FOR DROP DOWN GEARBOX IN THE HELICOPTER ENGINE
FIELD
The present invention relates to the design of a single engine helicopter Main gearbox eliminating the need for the drop-down gearbox in the helicopter engine.
BACKGROUND
In a conventional Main Gearbox (MGB) especially finding application in a low weight class single engine helicopter, providing drive to the tail rotor by power tap off from MGB is a technological challenge. The present invention involves the arrangement of gears in Main Gearbox for providing drive to main rotor and accessories. It also includes provision to drive tail rotor from power tap off from MGB through tail drive shaft running directly below the engine in a straight line. This concept has eliminated the heed for drop down gearbox in engine. The drive train arrangement is very compact as shown in Fig.-1.

DESCRIPTION
The gear arrangement in MGB is as shown in Fig.-1. The Input from the engine (6000 rpm) provides the drive to main rotor (367 rpm) through 3 stage reduction comprising of helical gear mesh (a), intermediate bevel mesih (b) and final planetary mesh (d). The power tap off to drive the tail power take off (TPTO at 4003 rpm) is taken from the intermediate bevel mesh through TPTO helical mesh (c). The drive to the accessories like cooler fan, hyd. Pump and alternator is taken through spur gear meshes (e), (f) and (g) respectively. Drive to additional hydraulic pump is taken through spiral bevel mesh (h). Freewheel unit is provided at the Input side of MGB to isolate the engine in case of engine failure to aid autorotation.
Normally in case of single engine helicopters where the tail rotor drive has to be tapped from the MGB, the tail drive shaft has to run below the engine and the management of configuration will have two possibilities:
1. Engine output drive axis to MGB will be very high compared to the platform, i.e., long engine mounts will be required to clear the engine envelope with Tail drive shaft.
2. The Tail Drive Shaft has to be inside the cabin or with an inclination, in other words gearbox height will be high resulting in low cabin head volume.
To manage these configurations normally in single engine helicopter, a drop-down gearbox is used wherein the freewheel is attached to the engine and then drive is given to both Main Rotor and Tail Rotor. To achieve this objective, engine will require major modification and re-certification.
The present configuration address all the above issues and meet all the requirements of low height gearbox, high head room, more cabin volume without any major modification in the engine. Here the MGB is driven by Engine located at 'Y0' of the Helicopter. The Tail drive shaft is also passing through 'Y0' of the Helicopter as shown in Fig.-2. The distance between engine axis and Tail drive shaft axis is considered for clearing the engine envelope / size. This is achieved by gear sizing of meshes (a) and (c).

The main advantages of this configuration are as follows:
(i) It can accommodate different engines, if necessitated with minimum changes to MGB.
(ii) MGB can be easily converted for twin engine application by changing gear meshes at (a) & (c).
(iii) MGB can easily adapt to the engines with drop down gearbox.
(iv) MGB can be converted as over speed gearbox configuration which allows the rotor speed to take to higher rpm to conduct qualification testing like over speed tests.

Claims:
1. Gear arrangement involving reduction of engine rpm to main rotor rpm with required direction through 3 stage reduction involving input helical stage, intermediate bevel stage and final planetary stage. Power tap off to the tail rotor side through helical gear stage. Also arrangement of accessory gear system.
2. Incorporation of Freewheel unit in input side of MGB to isolate the engine in case of engine failure to aid autorotation.
3. The configuration of Main Gearbox (MGB) for various advantages like:
(i) It can accommodate different engines, if necessitated with minimum changes to MGB.
(ii) MGB can be easily converted for twin engine application by changing gear meshes at (a) &
(c). (Hi) MGB can easily adapt to the engines with drop down gearbox.
(iv) MGB can be converted as over speed gearbox configuration which allows the rotor speed to take to higher rpm to conduct qualification testing like over speed tests.