Tail Rotor Torque
Joined: May 2003
Posts: 1,032
Likes: 7
From: OS SX2063
The freewheel unit on a 206 is not part of the engine its part of the helicopter.
Any torque demanded by the drivetrain goes through the freewheel unit be it , from the Tail Rotor or the Main Rotor.
If the Tail Rotor where bolted to an engine power output it would either not turn in the event of an engine failure or the drivetrain would be turning the engine over in autorotation which kind of negates the need for a freewheel.
The splines at the back to the engine gearbox in a 206 are driven from a shaft in the freewheel unit not the engine.
Who knows what the torque limits are for the freewheel unit ? I don't, so I have a good reason to stay within the limits other than common sense / because the manufacturer says so and any legal requirements.
A great topic that got me thinking for more than 5 minutes !
Any torque demanded by the drivetrain goes through the freewheel unit be it , from the Tail Rotor or the Main Rotor.
If the Tail Rotor where bolted to an engine power output it would either not turn in the event of an engine failure or the drivetrain would be turning the engine over in autorotation which kind of negates the need for a freewheel.
The splines at the back to the engine gearbox in a 206 are driven from a shaft in the freewheel unit not the engine.
Who knows what the torque limits are for the freewheel unit ? I don't, so I have a good reason to stay within the limits other than common sense / because the manufacturer says so and any legal requirements.
A great topic that got me thinking for more than 5 minutes !
Last edited by VeeAny; 6th July 2010 at 11:14.
Thread Starter
Joined: Mar 2010
Posts: 220
Likes: 0
From: Florida
VeeAny, I'm glad that you took that 5 minutes to give it some thought. Thanks. You gave my brain something to work on before it exploded.
So true! For those that find that a little confusing, please see the attached drawing. The freewheeling unit shaft outer race (#14) is driven by the engine. The inner race shaft (#1) attaches to the main drive shaft and is driven by #14. The spline adapter shaft (#28) leads to the tail rotor.
So, we're in agreement that all the torque goes through the freewheeling unit. What I'm not sure about is that it all goes through the main transmission, counting against the torque limit.
Please note that I'm not saying to exceed any limits! Don't do that. I don't do that. I'm just using that as an example. I should have said 90% instead of using 100%. I was just trying to figure out if the engine torque demanded by the tail rotor goes through.... oh you know.
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The splines at the back to the engine gearbox in a 206 are driven from a shaft in the freewheel unit not the engine.
So, we're in agreement that all the torque goes through the freewheeling unit. What I'm not sure about is that it all goes through the main transmission, counting against the torque limit.
Please note that I'm not saying to exceed any limits! Don't do that. I don't do that. I'm just using that as an example. I should have said 90% instead of using 100%. I was just trying to figure out if the engine torque demanded by the tail rotor goes through.... oh you know.
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Joined: Nov 2004
Posts: 1,334
Likes: 0
From: Cambridgeshire, UK
Originally Posted by fly911
1. Hovering into the wind produces 100% engine torque.
2. Your helicopter is transmission limited on torque.
3. Increasing anti-torque pedal to hover crosswind produces a torquemeter reading of 105%.
2. Your helicopter is transmission limited on torque.
3. Increasing anti-torque pedal to hover crosswind produces a torquemeter reading of 105%.
Joined: Sep 2007
Posts: 600
Likes: 0
From: USA
fly911,
Let's take a look at the x-section of the Allison model 250 engine you posted (the Bell 206 powerplant):
The engine power travels from the two stage power turbine (N2) along a shaft to a pinion gear. The pinion gear drives an idler gear, which also incorporates the engine torquemeter. The idler gear then drives a power output gear. The power take-off for the tail and main rotors is split at this power output gear.
So based on your example given, the following can be deduced:
1) There are at least 3 gear meshes and 1 shaft that must transfer 100% of the torque produced by the power turbine, minus any efficiency losses in those 3 gearshafts and minus the parasitic losses incurred by driving the engine accessories.
2) The torquemeter measures 100% of the torque produced by the power turbine, minus the mechanical losses in the pinion/idler gear mesh, its bearings and the bearings on the power turbine shaft (these losses likely total less than 1.5%). Since the torquemeter is ahead of the tail/main rotor power take-off point, the torquemeter should be indicating the total torque transmitted to both tail and main rotors.
3) The overrun clutch is not shown, but is likely part of the main rotor gearbox proper. Since the overrun clutch is located after the tail/main rotor drive split, it is only subjected to the main rotor torque loads.
riff_raff
Let's take a look at the x-section of the Allison model 250 engine you posted (the Bell 206 powerplant):
The engine power travels from the two stage power turbine (N2) along a shaft to a pinion gear. The pinion gear drives an idler gear, which also incorporates the engine torquemeter. The idler gear then drives a power output gear. The power take-off for the tail and main rotors is split at this power output gear.
So based on your example given, the following can be deduced:
1) There are at least 3 gear meshes and 1 shaft that must transfer 100% of the torque produced by the power turbine, minus any efficiency losses in those 3 gearshafts and minus the parasitic losses incurred by driving the engine accessories.
2) The torquemeter measures 100% of the torque produced by the power turbine, minus the mechanical losses in the pinion/idler gear mesh, its bearings and the bearings on the power turbine shaft (these losses likely total less than 1.5%). Since the torquemeter is ahead of the tail/main rotor power take-off point, the torquemeter should be indicating the total torque transmitted to both tail and main rotors.
3) The overrun clutch is not shown, but is likely part of the main rotor gearbox proper. Since the overrun clutch is located after the tail/main rotor drive split, it is only subjected to the main rotor torque loads.
riff_raff
Joined: Nov 2006
Posts: 1,951
Likes: 90
From: After all, what’s more important than proving to someone on the internet that they’re wrong? - Manson
Since the overrun clutch is located after the tail/main rotor drive split, it is only subjected to the main rotor torque loads.
fly911's diagram.
Item 1 Shaft Inner Race and item 28 Adapter are connected. Item 28 drives the TR. The OR clutch is BEFORE the shaft inner race at item 11 and is subject to ALL the torque.
From the Shaft Inner Race forward to the main rotor is only subject to main rotor torque and aft is subject to TR torque. The torquemeter measures ~ TOTAL torque applied.
Explain in your scenario what would drive the TR in autorotation?
That skinny bit of Item 1 Shaft Inner Race leading to item 28 Adapter is what is driving the TR. It is probably about as thick as your little finger.
