Single Engine CSUs

[High Wing Drifter]

All,

Without access to solid info at the moment, I think I've living under the misaprehention that Piper Arrow's CSU would default the prop to course pitch in the event of loss of oil pressure (as per an ME a/c). I now believe from the Hartzell Site (http://www.hartzellprop.com/engineering/engineering_constant_speed.htm), that this is common for single engine types but not SE aerobatic types and turboprops.

It seems sensible to me to put the prop in a low drag configuration in the event of an engine failure. My assumption would be that if you loose oil pressure then you will loose the engine. Why are singles designed to fine the blades? Is it to safe-guard against a possible fault with the CSU or oil blockage in the system? If so why would SE turboprops then default to feather?

[Say again s l o w l y]

Power produced by an engine is directly linked to RPM. So in the case of a CSU failure in an SEP a/c then it should go to fine pitch to give you maximum power.

In the event of an engine failure rather than a CSU problem, then having the blades as coarse as possible (feathered if you can) would be sensible due to a reduction in drag.

The manufacturers work on the principle that a CSU problem is more likely than a total engine failure. In the case of a multi engine a/c, then as you have a second engine to hopefully stop you have to make an immediate forced landing, reducing the drag from the unpowered prop is critical for performance reasons.

There is an old "trick" that if you are in a CSU equipped single and have an engine failure, then pulling the pitch back (coarser) will help your glide performance, but this only works if you still have oil pressure in the engine.

Hope this helps!

[dublinpilot]

There is an old "trick" that if you are in a CSU equipped single and have an engine failure, then pulling the pitch back (coarser) will help your glide performance, but this only works if you still have oil pressure in the engine.

I too was taught to pull the prop leaver in the case of an engine failure, to reduce the drag. However as time has gone on, I've began to wonder about this. I can certainly see the benefit in reducing the drag. I've no problem with that.

But when the prop is only wind milling, does pulling the blue leaver actually make any difference? If your engine is still running, and you have rpm set at 1,000rpm, you can do what you like with the blue leaver, but it won't change the pitch of the blade. Will it really make a difference when the prop is just wind milling?

By the way, for the sake of clarity, I'm asking the question, rather than pretending to know the answer! ;)

dp

[High Wing Drifter]

Cheers SAS,

Yes it does help. My remaining uncertainty (in addition to Dublin's query) is why do, as Hartzell describe, SE turboprops default to feather instead of fine pitch?

[eharding]

We had a CSU failure in the Yak - the CSU valve piston controlling the
flow of oil broke free, initially allowing a large impulse of oil into the
hub, causing the prop to slam fully fine and briefly going to 100% RPM -
as the oil pressure in the hub fell away, the prop swiftly reverted
to fully coarse (so swiftly in fact the hub, when removed, was actually
seized in that position).

[Say again s l o w l y]

The "normal" engine in most turbo prop singles is the PT6, a free turbine, which means that the power unit is not directly connected to the blades via a shaft.

The mechanisms that drive the prop are different to your average piston powered unit, in fact if you were strong (or daft) enough, you could hold onto the blades as the engine was started without them turning around. (Only for a very short time though!)

You also have two different ranges for the prop. Ground and Flight (the same for the fuel) and also you have the ability to go in beta pitch, i.e reverse. so the CSU on a turbo prop is alot more complex than the ones used on an average piston machine.

I'll dig out my PT6 manuals when I get home tomorrow and I'll see if I can give you an answer from the horses mouth as it's a while since I've looked at them and nearly a year since I've flown a turbo prop. So I wouldn't want to give you any duff gen!

[ShyTorque]

On a windmilling engine on a piston engine type, as long as you have oil presuure you can control the CSU as normal. The oil pump doesn't know the ignition or fuel failed. ;)

However, it's vitally important to know the Flight Manual - because they don't ALL go to fine pitch!

For example; those flying the ex-military Bulldog - if you lose engine oil pressure the CSU automatically goes fully COARSE.

I know of one ex-Birmingham UAS student who is lucky to be alive - he made a heavy landing on the nose oleo, dislodging the assembly upwards into the engine bay. Above the top of the oleo assembly, at the rear of the Lycoming crankcase, sits the oil pump for the CSU. He damaged the pump and the prop went fully coarse. He attempted to go around but even at full throttle the otherwise fully serviceable engine could not produce much rpm and therefore gave very little power. He drifted over the full length of RAF Cosford runway and crashed in a field upwind.

[High Wing Drifter]

Thanks SAS.

eHarding and ShyTorque,

I guess those types fall under the aerobatic category which, my Hartzell link above, says generally default to course pitch.

[dublinpilot]

On a windmilling engine on a piston engine type, as long as you have oil presuure you can control the CSU as normal. The oil pump doesn't know the ignition or fuel failed.

Shy,

That much I understand. But the csp seems to "ignore" any movement of the prop control, when your rpm is below a certain speed (somewhere around the 2000rpm mark in anything I've flown). When your engine fails, the prop will be windmilling at a speed significantly below that. Will moving the prop leaver really change the prop pitch in this condition?

dp

[B2N2]

Different aircraft, different systems.
The Piper Arrow CSU uses oil pressure to move the blades from fine to coarse,
therefore if you lose engine oil pressure the blades will go to full fine.
If you have an engine failure as a result of loss of oil pressure you're stuck with a windmilling prop that''s full fine. (max drag)
All you can do then is to slow down to get the prop to stop windmilling.

On the Piper Aztec ( Twin) if you lose oil pressure it will feather..
http://www.fergworld.com/instructors_corner/feathered_prop.jpg

[A and C]

The thinking on the prop position after CSU failure is one of safety on a twin you want to reduce the drag from the prop ASAP so driving the prop towards feather (course) will enable the light twin to continue to fly. . . Just ! !.

In the case of a normal SE aircraft a failure of the engine will result in a forced landing, if small amounts of engine power are still avalable then the only way to make this power avalable is if the prop is in full fine, this case is also good for the CSU failuer case a prop in full fine is a much safer way to land an aircraft than one stuck in full course.

Aerobatic aircraft have a problem with engine oil supply this is OK if the aircraft is flying normaly OR inverted but the transition between the two gives the inverted oil system problems hence the limmitation on knife edge flght is normaly around 30 sec. If the oil supply to the CSU is interupted during aerobatic flight the last thing that is wanted is the prop going fine and overspeeding an engine that is lightly to be at a high power setting so in aerobatic engines loss of CSU control (oil px) drives the prop towards course pitch preventing engine overspeed.

During an engine faiure in a SE aircraft in theory moving the pitch lever to full course should decrease the prop drag, BUT THIS WILL ONLY WORK IF THE ENGINE RPM IS WITHIN THE CONTROLING RANGE.
the controlling range is well above 1600 RPM and if you have this sort of RPM with a windmilling engine you are going to be well above best glide speed so I cant see the point of trying to reduce prop drag by vastly increasing airframe drag !!.

[ShyTorque]

A & C,

Thanks for that - I spent an hour last night looking for my CSU and prop theory notes; instead of posting here I got distracted and ended up cleaning off the top of my wardrobe, as one does...

I agree with your point about the "controlling range"; we taught this as the "constant speed" range. Once the engine rpm goes below this bottom setting, the CSU will try to fine off the pitch again, to increase the rpm, provided oil pressure is available, irrespective of the position of the rpm lever.

It's different on twin aircraft where the prop can be physically motored to a desired position (feathered, either by a pressure reservoir or electrically), where it will stay to minimise propellor drag and help keep the aircraft flying.

[Zulu Alpha]

A and C,

Many thanks for the detailed reply. I had wondered why an aerobatic prop worked the other way to 'normal' ones.