Can you coarsen a SEP propeller after the engine has failed?

[Contacttower]

I am slightly embarrassed that I don't know the answer to this...

Some CSUs are 'fail coarse'...ie the prop will coarsen if the engine stops...but with a CSU that fails to fine when the engine fails would there still enough pressure in the unit to coarsen the propeller (to reduce drag and increase the glide range)?

I would have thought not (unlike a unit on a multi that should feather provided its still spinning above about 1000RPM).

Thanks in advance...

[IO540]

AFAIK the answer is implementation dependent.

On a typical single, the prop should go to fully fine if oil pressure is lost. It is up to you to move it to fully coarse if you want to improve the glide ratio upon engine failure, but you have to do it before the oil pressure fails totally.

On twins, the props should go to fully coarse upon loss of oil pressure, I believe, to minimise the drag of the failed engine.

[Contacttower]

I thought so...

But what is the logic behind having it fail to high RPM?

On the Bulldog for example weights in the CSU ensure it fails to coarse so that if you happen to be in a steep dive (during an aeros sequence or whatever) the prop won't spin out of control. But I'd have thought that it would be a good idea to fail to coarse anyway because it causes less drag.

On a twin, unless you fly an all singing, all dancing, auto feathering DA42 or similar the prop will tend towards coarse after the engine fails but to actually feather it the pitch lever is moved to the 'Feather' detent.

[Islander2]

Contacttower

Not sure this helps, but on all the CS-prop SEPs I've flown, oil pressure is used to counteract the blade centrifugal twisting moment (as well as sometimes a spring) to achieve high pitch / low RPM.

If the engine fails whilst retaining sufficient oil pressure, the prop is still controllable. If oil pressure is lost, however, the centrifugal twisting (possibly aided by spring force) takes the propeller automatically to low pitch / high RPM regardless of the prop control position. So just when you want maximum range, you cannot get the optimum configuration.

I think all popular GA SEPs are probably configured like this, but doubtless someone will be along in a moment to set me straight.

Islander2

Edited to add: as you say, the Bulldog is not like that (and since I used to have a share in one, that's something I should have remembered :hmm:)

[Contacttower]

:hmm:....

I suppose the question I'm asking is how long would one have before oil pressure is lost?

The question arose in my mind after the engine stopped during aerobatics in the Firefly I was flying...I was amazed at how quickly the prop became stationary, due to the aircraft's 'fail coarse' propeller. Later on in the flight my instructor and I compared glide performance with the prop idling at fine and with it at coarse...the difference was quite a lot.

So why don't all CSUs fail to coarse? It would surely make sense if they did.

[Tony Hirst]

CT,
I suppose the question I'm asking is how long would one have before oil pressure is lost?I don't know if a windmilling engine would generate enough pressure to coursen the blades, even a little. If so then I suspect that pulling the lever back will reduces the windmill effect to some degree. I think that selecting full course upon engine failure is in the Arrow PoH (Arrow 2 if mem serves me well).
So why don't all CSUs fail to coarse? It would surely make sense if they did.Because the CSU is assumed as more likely to fail than the engine. If it fails and the blades revert to course, then the blades will be stalled and produce virtually no thrust.

[Contacttower]

Thanks Tony, that's exactly the sort of answer I was looking for.

I suppose I could go out and try it...:eek:

[Zulu Alpha]

Aerobatic CS props fail to coarse. This is opposite to normal and is done to prevent overspeeding when the oil pressure drops, which it does in some manouvres.

ZA

[A and C]

The prop on an SEP is a constant speed prop and in the case of loss of engine power it will try to maintain the speed set by the blue lever. The only way this can be done if the engine fails is to go towards fine pitch.

Twins and aerobatic aircraft are different, above you have a very good explanation of the aerobatic reasoning.
With a twin you can feather the prop and so in the case of engine failure the prop needs to go past fully coarse and into feather, the controling logic is the same as an SEP so it has to be selected to feather to drive the prop via coarse to feather.

In the case of lost oil pressure it is VITAL that the prop is featherd before the centrifugal latches engage ( at about 800 RPM ) otherwise the prop can't be feathered. The loss of oil pressure will drive the prop towards coarse but feather must be selected quickly, if you do not the latches will engage and you will be faced with increasing power to an engine with no oil pressure to enable the latches to dis-engage long enough to get the prop into feather....... not a very atractive proposition!

[Rod1]

Those of us with electric CS props can of course play with the pitch whenever we like…

Rod1

[Oktas8]

I always thought that fail-fine propellers were lighter (no counterweights) and cheaper than fail-coarse. Very important on light & cheap SEP aircraft.

I've never tried to coarsen the propeller of a genuinely failed SEP engine. But an idling engine (when gliding at typical speeds) can be coarsened just fine in the aircraft I've flown. I suspect there's not much difference in oil pressure between idling and windmilling - if that helps to answer the question!

In the case of lost oil pressure it is VITAL that the prop is featherd before the centrifugal latches engage ( at about 800 RPM ) otherwise the prop can't be feathered.

True. But this is unlikely to happen if you maintain at least Vyse in a light twin with oil in the crankcase. The windmilling speed of a failed engine is typically 1300rpm+ in asymmetric cruising flight. I refer to aircraft such as the Seminole and Duchess, naturally assuming the engines haven't seized completely... Therefore, when feathering following an engine failure, I advocate taking care and not rushing.

[Tony Hirst]

Zulu Alpha,

Thanks for that, no aeros experience and I was wondering what the inevitible exceptions were. Tell me, with the blade fully coarse on an aeros setup, how coarse is coarse? Would there be thrust?

[Contacttower]

True. But this is unlikely to happen if you maintain at least Vyse in a light twin with oil in the crankcase. The windmilling speed of a failed engine is typically 1300rpm+ in asymmetric cruising flight. I refer to aircraft such as the Seminole and Duchess, naturally assuming the engines haven't seized completely... Therefore, when feathering following an engine failure, I advocate taking care and not rushing.

That's always a good policy, I think often people are taught to rush through the 'engine shutdown/failure' checks on a twin...which could mean something obvious is missed or a bad mistake is made...

[SNS3Guppy]

But what is the logic behind having it fail to high RPM?

Where does your propeller on your typical light airplane go when you pull the power to idle? Think about it. ou go to the fine setting, or the high RPM setting. In cruise flight, pull your propller RPM back to about 2200. Now retard the throttle to idle. Note the RPM. Push the propeller lever up. Do you get a change? No, because the propeller is riding on the low pitch stops...which is what happens when you retard the power.

With a propeller governor failure, when you no longer have governing action, the propeller essentially becomes a fixed pitch prop, riding on the low pitch stops...the fine position.

You have to specify which propeller you're talking about...differnet props do differrent things. Some feather automatically. Some go to low pitch, so ride high pitch.

[ShyTorque]

Thanks for that, no aeros experience and I was wondering what the inevitible exceptions were. Tell me, with the blade fully coarse on an aeros setup, how coarse is coarse? Would there be thrust?

Tony, I used to instruct on the Bulldog, on a UAS. There was an accident at RAF Cosford in the early 90s where a student bounced and PIO'd on landing, then tried to go around. However, the nose oleo had been disrupted upwards during his "heavy landings".

Unfortunately, on the Bulldog, the oil pump for the CSU is just above the top of the oleo. It got knocked and was damaged so the prop went to fully coarse, as per the design.

Fully coarse isn’t meant to be a “powered” configuration, it’s to minimise prop drag with a dead engine.

The student applied full throttle but the engine couldn't produce much rpm at fully coarse pitch. He managed to overfly the full length of the runway without climbing and crashed upwind, outside the airfield. I think he was quite badly injured but he survived.

I used to teach my students to put the aircraft down if possible, rather than try to climb away if there was any possibility of nosewheel damage due to a landing bounce, for this reason (plus they might have bent the prop tips, which might put them in a similar position).

[Zulu Alpha]

Tell me, with the blade fully coarse on an aeros setup, how coarse is coarse? Would there be thrust?

My prop has the coarse/fine as a number of degrees and I don't know what this means for thrust. However during engine runs at 2000 rpm the prop control can take the rpm down to 1000 rpm so I think there would be little thrust.

Thus I expect the performance to be very marginal with a prop stuck in coarse.

ZA

( before you ask, I don't normally take it this far in an engine run, but do occasionally to check its working fully)