Loss of oil pressure to the hub, coarse or fine pitch, that is the question?

[Flying tigers]

This is for all the engineering guru's out there, we have had a lengthy discussion with our chief engineer this morning. For all the pilots that have been told that we know nothing of engines, props and a/c systems, let us unite and prove these engineers wrong. Below is an extract from the AFM on the Cessna 402B

Extract: Oil pressure from the propeller governor drives the blades towards the low pitch/fine (increasing RPM) while the springs and counterweights drive the blades towards high pitch/coarse (decreasing RPM)

So.... in the event of a loss of oil pressure centrifugal force assisted by counterweights and springs drive the propeller into high pitch/coarse (decreasing RPM) position. This also will stop the engine from exploding due to excessive overspeed of the propeller.

However we have been told by our engineers that all of the above is incorrect and that the good people at Cessna are also incorrect of their understanding of this rather important subject. The engineers believe that in the event of loss of oil pressure to the hub, the propeller counterweights and springs will drive the prop to a low pitch/fine (increasing RPM) i.e. upon shutting the engine down there is a subsequent loss of oil pressure and the propeller will remain in the low pitch/fine position.

Help us PPrune your our only hope....

[Douche]

Fine Pitch Locks; these *LOCK* the propeller in fine pitch at idle (or close to it) power settings, preventing the prop from running to coarse (feather) pitch after shutdown. The prop should still run to coarse if oil pressure is lost in flight at normal power settings.

It can occasionally occur that a prop will feather after shutdown if these locks do not engage correctly.

For example i once had a 404 feather after shutdown (to my suprise!!) because these locks had in fact been broken. The plane was grounded then until the unit was repaired

I'm not a guru by any means so maybe someone else can explain it in more detail

Douche

[Frank Arouet]

The term "fail safe" comes to mind. Does this help?

[AerocatS2A]

Never mind, it's been too long since I looked at this stuff.

[Ejector]

Yep, dare I say it, Cessna is right, Your Engineer should be fired and his or her name made public so he or she can work and McDonald where he or she belongs. The weights and springs are there for that purpose.

[propblast]

Yep, what Frank said. It's a fail safe, so the engine can be feathered in the event of a loss of oil pressure so protect it (remembering that in theory it can fly OEI).

He may be a little confused (or alot), single engine aircraft can go to fine to keep the engine running so there is some power available to make a powered approach and not a glide approach. Its better to F&*k the engine than not be able to make a field just beyond engine out gliding range. IMHO anyways.

[AerocatS2A]

Right, I've done a bit of research now.

I don't know if you have McCauley props, but they have this to say about pitch changing mechanisms.

Pitch is changed hydraulically in a single-acting system, using engine oil controlled by the propeller governor to change the pitch of the propeller blades. In constant-speed systems, the pitch is increased with oil pressure. In full-feathering systems, the pitch is decreased with oil pressure. To prevent accidentally moving the propellers to the feathered position during powered flight, which would overload and damage an engine that is still running, the controls have detents at the low RPM (high pitch) end.

I've bolded the last bit to confirm that by "decreasing pitch" they mean high RPM, so the oil pressure on a fully feathering prop is used to decrease the pitch or, in other words, increase the RPM.

I'm assuming that a C402 has a full feathering system like most twins do.

Further more, on the subject of feathering,

Feathering is achieved through a mechanical linkage that overrides the flyweights and speeder spring. When the cockpit control is moved to ³feather,² the governor lever and shaft are turned beyond normal low-RPM operating limits. As the threaded shaft backs out, the shaft lift rod engages the pilot valve spindle and lifts the pilot valve. This causes oil to flow out of the propeller, and it moves to feather pitch position. (Fig. 12)

So when there is no oil in the prop, it feathers.

More here. (http://www.mccauley.textron.com/pro-sup/prosupframeset.html) Click on "Tech Guide" and "Variable Pitch Propellers".

[currawong]

For what it is worth, a Cessna single that I flew that lost oil pressure went to full fine.
The same if the pitch cable breaks.
Default position is full fine - kind of a limp home option.
Although they do not limp far with no oil pressure...

[AerocatS2A]

Singles don't normally have full feathering props and are generally set up the other way where a loss of oil from the hub will cause the prop to go fine. The reason for the difference being that twins don't limp home with the prop on the failed engine at max rpm.

[MakeItHappenCaptain]

Exceptions to the rules being;

Single-engine turboprops such as Caravans;
Fully feathering and fully reversible.

Aerobatic aircraft;
The prop is counterweighted and loss of oil pressure will send the prop coarse (eg during inverted maneouvres with a temporary oil pressure fluctuation until the inverted system kicks in) to prevent overspeed.
Check out Decathlons or Yaks.:ok:

[Roxy_Chick_1989]

For what its worth in most of the light GA Cessna singles, the CTM (Centrifugal twisting moment) is far greater than ATM so the prop has a natural tendency to move to the 'fine' position. Interesting to note that there are some singles out there that actually use counterweights too bypass this tendency and actually move the pitch toward 'coarse' too reduce drag in event of engine failure.

[Flying tigers]

Thankyou to all the people that replied to this thread and for the effort in research that was clearly undertaken, this was very much appreciated, we now have some real ammo to throw at the engineers on this particular subject. We as pilots can again hold our heads high on the hanger floor.

Cheers.:ok:

[Captain Dart]

The Winjeel (and presumably the Beaver, and anything else with an R985 radial on it) has a prop that will go to coarse pitch, due to the counterweights on the hub, when oil pressure is lost. In fact the Gin Wheel is often started with the prop in coarse so it doesn't rob the engine of oil during the critical start phase.

[AerocatS2A]

The Harvard is like that too. Move it to coarse prior to shut down and leave it there for the start. Put it to fine once it's warmed up a bit.

[MakeItHappenCaptain]

For what its worth in most of the light GA Cessna singles, the CTM (Centrifugal twisting moment) is far greater than ATM

Try every propellor blade ever made, not just little cessnas.:rolleyes:

some singles out there that actually use counterweights too bypass this tendency and actually move the pitch toward 'coarse' too reduce drag in event of engine failure

Um...turboprops, fully feathering, already mentioned.
Pistons, aerobatic only to prevent overspeed in the event of an oil pressure interruption during negative g loadings, again already mentioned. When counterweights are used, the oil system will also be reversed using pressure to send the prop to fine instead of coarse.
It has nothing to do with increasing glide range at all.:=


Singles will generally tend fine in the event of a governor failure because the ability to climb (if the engine is still operating) is more important.

If you want to increase range, the windmilling engine may provide enough pressure (if you still have oil) to send the prop to coarse if commanded. You will still be limited by the coarse pitch stops (about 30 degrees for a non-feathering unit), but it's better than full fine. Returning the pitch lever to fine will increase the drag to slow the aircraft prior to touchdown.
Best results, however, will be achieved by (with enough altitude), slowing the aircraft enough to stop the engine windmilling altogether, the returning to Vg.

[werbil]

The direction on the Beaver depends on which prop is fitted. The three bladed conversion will go fine, the standard two blade will go coarse.

[43Inches]

There are also some props which have locks which engage if oil pressure is lost creating a fixed pitch prop at whatever angle it lost pressure, no counter weights, no spring, oil actuates in both directions. Have encountered this on a turbo-prop which has two very different prop options. The other option is the single direction oil actuated with springs and weights etc...

This may be to do with maintenance facillitation but not sure. The non-coarsening prop you could exchange individual blades without re-balancing, the other required replacement of opposing blades.

What the prop will do is dependent on the installation, not the aircraft type.