HiThis might sound like a weird question, but what does a fully reversed propeller look like? I've seen pics of feathered props on parked aircraft (mainly on turboprops), but never of one in full beta while the aircraft is stopped. Why are pictures never taken of a stopped propeller in beta? Is it bad for the gearbox and/or engine?

Many systems are designed that that in the event of loss of hydraulic pressure the blades return to the feathered position. This would account for the blades taking up that position when the engine is shut down.

And some have centrifical latches that prevent them from going all the way to feather when shut down. This reduces the load during start. In both cases the blades will not stay in reverse pitch after shut down.

More Info: 'Beta' is not identical with 'reverse'. Beta just defines the range of the prop blades at which they are controlled via the throttles and not the prop levers.Beta range can start already at a 'positive' blade angle like in case of the Dash 7.

And some have centrifical latches that prevent them from going all the way to feather when shut down. This reduces the load during start. In both cases the blades will not stay in reverse pitch after shut down.

In general, turboprops using variants of the Pratt & Whitney PT-6 will go to full feather on shutdown. With the PT-6, the prop is not directly connected to the engine, but is driven by a fluid clutch (similar to the torque converter in a car's automatic transmission). Thus the prop presents no significant load on the engine during start. The prop doesn't start turning until the engine is almost fully stabilized at idle. Not the case with Garrett engines, which drive the props directly - which is the reason for the stop locks which prevent the blades from going to full feather on shutdown. (If they do end up feathered, there is a requirement to unfeather them hydraulically before attempting engine start, lest there be a hot start and significant engine damage. Users of the PMDG JS-41 will be very familiar with this requirement!)It's also quite possible to accidentally feather the prop(s) on shutdown in a piston engine aircraft, if the prop lever is pulled fully back at the same time as the mixtures. Starting a piston engine with a feathered prop can be a real problem. The prop won't unfeather until the engine is actually running and has built up normal oil pressure, but because of the load presented by the feathered blades, getting the engine to turn fast enough to "catch" can be difficult. Puts a lot of strain on the starter motor and crankshaft.Jim Barrett

And there I thought the propshaft had its own turbine (after the gas generator turbine) rather than a clutch. Not so for the PT-6? (Note no engine startup loads by the propeller in this case either.)Cheers,- jahman.

In general, turboprops using variants of the Pratt & Whitney PT-6 will go to full feather on shutdown.

Yes thats true.

With the PT-6, the prop is not directly connected to the engine, but is driven by a fluid clutch (similar to the torque converter in a car's automatic transmission).

It is true that the PT-6 prop is not directly connected to the gas gennerator. But there is no fluid clutch. Instead the prop is connected to a turbine that is driven by the hot gasses from the gas gennerator. This is called the free power turbine principle.

The prop doesn't start turning until the engine is almost fully stabilized at idle.

The prop won't start turning when the starter is engaged but starts turning following light off with some lag, however, its turning long before the gas gennerator reaches idle.

Not the case with Garrett engines, which drive the props directly - which is the reason for the stop locks which prevent the blades from going to full feather on shutdown.

Yep, this is the application I had in mind when mentioning the latches.

It's also quite possible to accidentally feather the prop(s) on shutdown in a piston engine aircraft, if the prop lever is pulled fully back at the same time as the mixtures.

The key to this trick is to get the blades to start moving past the latches before the centrifical force drops low enough to engage the latches. Sometimes it is more successful to lead with the prop lever before pulling the mixtures to idle cut off.Ron

Thanks for all the informative answers, but I was just wondering exactly WHAT does a reversed prop look like. Is it just a regular prop with apparently reversed blade direction (i.e. the blades look like they were put on backward)? Or is the reverse pitch subtler than that? If possible, please include pictures and/or airflow diagrams.And about the PT6 feathering on shutdown, is this to prevent windmilling? How exactly does windmilling damage an engine?

Thanks for all the informative answers, but I was just wondering exactly WHAT does a reversed prop look like. Is it just a regular prop with apparently reversed blade direction (i.e. the blades look like they were put on backward)? Or is the reverse pitch subtler than that? If possible, please include pictures and/or airflow diagrams.And about the PT6 feathering on shutdown, is this to prevent windmilling? How exactly does windmilling damage an engine?

It just rotates to about 20° back from flat pitch. This is from an aviation dictionary. The diagram on the right shows what the blade does.Windmilling on a PT6 isn't really bad. It is dangerous in high winds you could get wacked pretty good by a blade if it was spinning.

In general, turboprops using variants of the Pratt & Whitney PT-6 will go to full feather on shutdown. With the PT-6, the prop is not directly connected to the engine, but is driven by a fluid clutch (similar to the torque converter in a car's automatic transmission). Thus the prop presents no significant load on the engine during start. The prop doesn't start turning until the engine is almost fully stabilized at idle.

You sure about that? The prop on most (all?) PT6 derivatives have their own turbines (connected to a reduction gear) basically spun by exhaust gases.EDIT: Ron covered that. Sorry Ron.

And about the PT6 feathering on shutdown, is this to prevent windmilling? How exactly does windmilling damage an engine?

The prop feathers at shut down because it's spring loaded to go there in the event of engine failure. Feathered prop=less drag (You know this, of course).Windmilling can damage an engine in time as there is no oil circulating through the engine, thus no lubrication. But as Chris mentioned, I've yet to see a King Air's props get any momentum in stronger winds. I've been around a LOT of King Airs... :(

Operating a PT-6 in feather does not damage the Gearbox-engine,the damage that is done,is to the acrylic window panes being melted and warped from prolonged operations in feather.The Beech 1900C-D has a ground fine solenoid on each engine, grounded thru the R/H main landing gear squat switch. Upon landing the low pitch stop is adjusted from,(if memory serves me correctly),13 degrees to 9 measured at the 43 inch blade station. There are also micro switches on each power lever, if the levers were lifted in flight the props would go to 9 degrees.Not a good thing,as we lost one of our best aircraft on short final to KHYA. on occasion one solenoid would stick on touchdown,causing a surprising yaw. http://aircrashed.com/cause/cNYC99LA052.shtmlThis engine prop system was very difficult to rig correctly as a mechanic, because the flight crew did not like mismatched power lever knobs,(Split more than 1/2 knob)As they set power more with feel at first,and would get a torque mismatch,with a corresponding yaw. Picture trying to land at Bar Harbor Maine in the winter, with icy runway with a crosswind.

It just rotates to about 20° back from flat pitch. This is from an aviation dictionary. The diagram on the right shows what the blade does.Windmilling on a PT6 isn't really bad. It is dangerous in high winds you could get wacked pretty good by a blade if it was spinning.

On that subject, how exactly does a reversed prop reverse airflow? Doees it disturb airflow? Does anybody have an airflow diagram that shows how a reversed prop works?

On that subject, how exactly does a reversed prop reverse airflow? Doees it disturb airflow? Does anybody have an airflow diagram that shows how a reversed prop works?

Airflow is not reversed. Thrust is reversed from positive thrust to negative thrust. Thrust is to a prop blade what lift is to a wing. Negative thrust is generated via a negative angle of attack on the prop blade.In an aircraft you can get the same effect by diving upright at negative G.An easier way is to (carefully!) stick your hand out of a car window (while somebody else is driving) and shape it like an airfoil to generate lift. Then gradually rotate your hand to decrease lift, pass through zero lift and keep rotating your hand until you generate negative lift, i.e. the force of the airflow on your hand-y airfoil is acting downwards.Now change your frame of reference and instead of a wing think of your hand as a prop blade: Positive lift is now thrust while negative lift corresponds to reverse thrust. The prop governor changes the blade's angle of attack by turning it at the root just like you did turned your hand with your forearm.Cheers,- jahman.

Keep it simple!Normal prop angle blows...Reverse prop angle sucks... :( Seriously, many box fans have a "reverse switch," so simply trying that out will give one a real world example.

Seriously, many box fans have a "reverse switch," so simply trying that out will give one a real world example.

The blades twist there angle to change the thrust. They don't stop and go the other direction like a box fan does.