???Feathering??? Please help
Thread Starter
Joined: Jul 2005
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From: leicester
???Feathering??? Please help
As a helicopter pilot im am struggleing with the idea of feathering!
Please be symypathetic
Are these blades feathered?
Ie - they are feathered because they are in line with the aircraft, ie the chord line is in the same direction as the aircraft???
Also when would you use this position? Not in flight as it produces no thrust!?
Although on these blades in the picture, arn't they producing thrust because of the curved blade?
I know im making a mountain out of a mole hill but please help explain
Thank you
Mady
Please be symypathetic
Are these blades feathered?
Ie - they are feathered because they are in line with the aircraft, ie the chord line is in the same direction as the aircraft???
Also when would you use this position? Not in flight as it produces no thrust!?
Although on these blades in the picture, arn't they producing thrust because of the curved blade?
I know im making a mountain out of a mole hill but please help explain
Thank you
Mady
Joined: Aug 2002
Posts: 508
Likes: 16
From: Under the sea
Feathered indeed. Oil presure holds the props in the desired pitch for T/O, climb and cruise. Lose an engine and you lose oil pressure so "autofeather" should result. The curved bladeds may be adding some drag but since they are designed to spend the vast majority of their time operating (hopefully!) perhaps the small drag penalty is acceptable in a failed condition.
Joined: Aug 2003
Posts: 190
Likes: 1
From: Earth
In the case of an engine failure or shut down where the propeller is free to turn ie not thrown big chunks of metal out the back, the propeller is driven by the airflow passing through it. This causes a big increase in drag which would reduce excess power available and reduce control authority due to more rudder needed to oppose the yawing moment caused by the drag in the multi engine case and a reduced glide range in the single engine example.
If the engine fails and the propeller cannot rotate, but is not feathered you still have extra drag from the blades being flat on to the airflow. Feathering reduces the drag and should stop the blades turning. Composite blades are a little beyond the required knowledge in ATPL ground school from memory, I would imagine even with the curved blades the reduction in drag is worth feathering the prop.
If the engine fails and the propeller cannot rotate, but is not feathered you still have extra drag from the blades being flat on to the airflow. Feathering reduces the drag and should stop the blades turning. Composite blades are a little beyond the required knowledge in ATPL ground school from memory, I would imagine even with the curved blades the reduction in drag is worth feathering the prop.
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All as above, plus, with the c130-J engine as pictured above, in Hotel mode, for engine running offloads/onloads. it minimises the jetwash in such circumstances... we usually dont bother as it induces a bit of a wobble on the back end which is a bit of a norse with getting the forklift lined up... so we just "disk" (Ie engine not producing no fore/aft thrust using throttles in gorund/reverse mode) the engines, which does the same job. The Movers are not happy if they are getting buffeted around by propwash.
The J, designed by clever americans to be flown by very thick ones, does all the autofeathering automatically, your drills just put the handles in the positions that the automatics have done already... Fantastic stuff.
We have an unfeather/feather pump if things get so serious that we have to start pumping oil around to change configuration of the props or put prop in the Hotel mode above.
The J, designed by clever americans to be flown by very thick ones, does all the autofeathering automatically, your drills just put the handles in the positions that the automatics have done already... Fantastic stuff.
We have an unfeather/feather pump if things get so serious that we have to start pumping oil around to change configuration of the props or put prop in the Hotel mode above.
PPRuNe supporter
Joined: Dec 2003
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From: Planet Earth
Also
Don't know whether this will help but, think of the engine and the prop as two different systems, just like on modern turbine rotor craft, some turbo props use an engine as their APU, locking the blades from moving, no direct connection. On the PT6 I used to fly we could feather the prop on an operating engine for training, no need to shut down engine.
Joined: Feb 2005
Posts: 4,581
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From: flyover country USA
There is quite a distinction between a single-shaft turboprop (T56, TPE331, Dart, others I'm sure still active), vs. a dual-shaft or free-turbine engine (PT6, CT64, CT7, PW100 series).
You can quickly tell them apart when shut down. It the prop is feathered, it must be a 2-spool machine, because the core or gas generator can be started without rotation of the free turbine. There's no mechanical connection, but rather gas-coupled turbines. Thus the prop blades can be set for either high RPM (takeoff) or low RPM (cruise) without directly affecting the core speed.
Helos use free-turbine turboshaft engines - they can be started with the rotor braked.
On the other hand a single-shaft (single-spool) engine will always be in flat or fine pitch when shut down, because minimum drag for the prop load is necessary to get it started. The prop speed and the core speed are directly linked by a gearbox.
You can quickly tell them apart when shut down. It the prop is feathered, it must be a 2-spool machine, because the core or gas generator can be started without rotation of the free turbine. There's no mechanical connection, but rather gas-coupled turbines. Thus the prop blades can be set for either high RPM (takeoff) or low RPM (cruise) without directly affecting the core speed.
Helos use free-turbine turboshaft engines - they can be started with the rotor braked.
On the other hand a single-shaft (single-spool) engine will always be in flat or fine pitch when shut down, because minimum drag for the prop load is necessary to get it started. The prop speed and the core speed are directly linked by a gearbox.
Joined: Oct 2005
Posts: 3,218
Likes: 2
From: USA
Gmady,
If you imagine your propeller disc the same as your rotor disc, you can think of a windmilling propeller the same as the rotor disc in autofeather. When a propeller is windmilling, it is being driven by the airflow through it, and as such, is absorbing energy...drag. Same in auto rotation. You descen with the collective lowered, encouraging airflow to maintain rotor RPM...the rotor is absorbing energy, and creating an enormous amount of drag in the process. In a helicopter in a descent, this is desirable, it's the lift and associated induced drag that's created which prevents you from hitting the ground.
In the case of the propeller if the engine has failed, then the drag is of no benefit. In a multi engine airplane, it can produce substantial yawing as it creates more drag on one side of the airplane than the other, and the result is that a lot of drag is created by either the airplane being yawed (more of the side of the airplane exposed to the slipstream), or control deflection used to compensate for the assymetric drag, or both. In any event, less excess thrust is available from the remaining engines because much or all of it is being used to account for the extra drag of the failed engine.
The propeller is feathered by aligning the blades with the slipstream to minimize drag. Some engines will autofeather...go to feather on their own when oil pressure is lost, others won't. Some use autofeather systems that not only feather the engine, but ensure the others won't follow or feather, for takeoff or sometimes landing. Some engines, as others have mentioned, feather on shutdown on the ground. The free turbine engines such as the PT6A do this. Other engines such as the T76/TPE331 will feather if you let them, but are required to be placed into reverse and locked out of feather before being shut down.
The reason for the latter type of engine not being feathered on shutdown is to make engine starts easier. A feathered blade, if being driven directly by the engine, produces a lot of drag, and therefore resistance, which means the engine doesn't turn as fast as quickly, and therefore experiences less airflow during the start...and starts hotter. by ensuring the propeller isn't feathered when it's shut down, it's not feathered during start-up...the engine can spin up faster because there is less drag from the propeller, and it starts cooler.
An engine such as the PT6 doesn't actually turn the propeller. It's just a gas generator which uses it's end product exhaust gasses to blow across the power turbine attached to the propeller gear box. The engine isn't turning the propeller, only it's exhaust gasses are, and there is no need to prevent the airplane from going into feather on shutdown. It goes into feather, and comes back out, with changes in oil pressure...which is a good thing. In the case of these types of engines, attempting to move the propeller controls to the reverse position will damage the linkages...they can't be operated in the same manner as the turboshaft engines mentioned above (TPE331,etc).
The C-130 T-56 and later engines and propellers are a slightly different breed in their mechanical operation...they're some of the most complex engines built, mechanically. Operationally they're fairly straight forward, but a slightly different approach and mindset.
Another function similiar to feather on these engines is NTS, which is negative torque sensing. This moves a propeller toward feather, but doesn't actually feather it, when the engine senses that the propeller isn't under a load. If a propeller attmpts to overspeed under normal circumstances, a propeller governor will increase blade angle, which increases drag and load, and prevents the propeller from going faster. This is similiar to what the negative torque sensing unit does, except that it's not responding to engine speed, but to the actual shaft torque to the propeller. If torque drops, such as a time when the engine isn't giving enough power or the slipstream is driving the engine instead of the prop, the NTS system increases blade angle toward feather to load the propeller and keep positive torque on the system.
As the propeller moves toward feather, the drag that would be absorbed by a windmilling pressure is relieved, and drag decreases. NTS helps relieve drag and load the propeller shaft normally...restore positive torque. It is filling a feathering function, though it isnt' feathering the propeller. Think of it as a device that automatically raises the collective for you when you have an engine failure in a helicopter...the difference is that in the airplane you want the blade mroe aligned with the slipstream, and in the helicopter you're going for just the opposite. Both enhance performance in their respective platforms, just from different sides of the fence. Hope that wasn't too confusing.
If you imagine your propeller disc the same as your rotor disc, you can think of a windmilling propeller the same as the rotor disc in autofeather. When a propeller is windmilling, it is being driven by the airflow through it, and as such, is absorbing energy...drag. Same in auto rotation. You descen with the collective lowered, encouraging airflow to maintain rotor RPM...the rotor is absorbing energy, and creating an enormous amount of drag in the process. In a helicopter in a descent, this is desirable, it's the lift and associated induced drag that's created which prevents you from hitting the ground.
In the case of the propeller if the engine has failed, then the drag is of no benefit. In a multi engine airplane, it can produce substantial yawing as it creates more drag on one side of the airplane than the other, and the result is that a lot of drag is created by either the airplane being yawed (more of the side of the airplane exposed to the slipstream), or control deflection used to compensate for the assymetric drag, or both. In any event, less excess thrust is available from the remaining engines because much or all of it is being used to account for the extra drag of the failed engine.
The propeller is feathered by aligning the blades with the slipstream to minimize drag. Some engines will autofeather...go to feather on their own when oil pressure is lost, others won't. Some use autofeather systems that not only feather the engine, but ensure the others won't follow or feather, for takeoff or sometimes landing. Some engines, as others have mentioned, feather on shutdown on the ground. The free turbine engines such as the PT6A do this. Other engines such as the T76/TPE331 will feather if you let them, but are required to be placed into reverse and locked out of feather before being shut down.
The reason for the latter type of engine not being feathered on shutdown is to make engine starts easier. A feathered blade, if being driven directly by the engine, produces a lot of drag, and therefore resistance, which means the engine doesn't turn as fast as quickly, and therefore experiences less airflow during the start...and starts hotter. by ensuring the propeller isn't feathered when it's shut down, it's not feathered during start-up...the engine can spin up faster because there is less drag from the propeller, and it starts cooler.
An engine such as the PT6 doesn't actually turn the propeller. It's just a gas generator which uses it's end product exhaust gasses to blow across the power turbine attached to the propeller gear box. The engine isn't turning the propeller, only it's exhaust gasses are, and there is no need to prevent the airplane from going into feather on shutdown. It goes into feather, and comes back out, with changes in oil pressure...which is a good thing. In the case of these types of engines, attempting to move the propeller controls to the reverse position will damage the linkages...they can't be operated in the same manner as the turboshaft engines mentioned above (TPE331,etc).
The C-130 T-56 and later engines and propellers are a slightly different breed in their mechanical operation...they're some of the most complex engines built, mechanically. Operationally they're fairly straight forward, but a slightly different approach and mindset.
Another function similiar to feather on these engines is NTS, which is negative torque sensing. This moves a propeller toward feather, but doesn't actually feather it, when the engine senses that the propeller isn't under a load. If a propeller attmpts to overspeed under normal circumstances, a propeller governor will increase blade angle, which increases drag and load, and prevents the propeller from going faster. This is similiar to what the negative torque sensing unit does, except that it's not responding to engine speed, but to the actual shaft torque to the propeller. If torque drops, such as a time when the engine isn't giving enough power or the slipstream is driving the engine instead of the prop, the NTS system increases blade angle toward feather to load the propeller and keep positive torque on the system.
As the propeller moves toward feather, the drag that would be absorbed by a windmilling pressure is relieved, and drag decreases. NTS helps relieve drag and load the propeller shaft normally...restore positive torque. It is filling a feathering function, though it isnt' feathering the propeller. Think of it as a device that automatically raises the collective for you when you have an engine failure in a helicopter...the difference is that in the airplane you want the blade mroe aligned with the slipstream, and in the helicopter you're going for just the opposite. Both enhance performance in their respective platforms, just from different sides of the fence. Hope that wasn't too confusing.
