Effects of changing propeller rpm on propeller pitch

[sam92]

During a glide with idle power and constant IAS, if the RPM lever of a constant speed propeller is pulled back from its normal cruise position, the propeller pitch will:
decrease and the rate of descent will decrease.
increase and the rate of descent will increase.
decrease and the rate of descent will increase.
increase and the rate of descent will decrease.

If anybody could explain this i would be grateful...just can't get my head around it!!!

[aerobat77]

increase and the rate of descend will decrease.

pulling back the prop lever means reducing prop speed so prop pitch has to increase. a reduced prop speed with higher prop pitch will reduce the drag and so the descend rate.

[MarkerInbound]

In effect you're moving the prop towards "feather." While most single engine piston planes don't have feathering props, you can reduce the drag caused by an idling engine by going to the low rpm setting.

[Old Smokey]

sam92,

The previous 2 posters have passed on very good information to you. It's extremely valuable to remember and understand, as, following engine failure in a single, the propeller RPM control can be used to vary drag to adjust descent profile during the glide to the forced landing. (High RPM = More Drag, Low RPM = Less Drag).

The rate of descent (with idle power) change between High and Low pitch is VERY noticeable.

I hope that you don't need to use it in anger, a possible life-saver if you do:ok:

Regards,

Old Smokey

[barit1]

The rate of descent (with idle power) change between High and Low pitch is VERY noticeable.

Yes - it takes more energy to spin props to a higher RPM, and instead of it coming from the engine, it comes out of the slipstream. And it shows up as parasitic drag, hence a steeper glide angle.

[john_tullamarine]

Those of us with parachute flying time will have used this techique to do the usual defacto forced landing with constant low power setting from drop height.

As OS suggests, the effect on glide angle is very significant and can be used, in effect, like a speed brake in reverse - providing that all relevant AFM limitations etc are observed.

[QJB]

Would having the throttle wide open help reduce the drag also since it reduces the additional energy required to turn the prop due to the closed throttle plate??

Referring to an engine failure situation of course..

[markkal]

Hi guys..
How can you increase or decrease prop pitch on a dead engine....If there is no oil pressure !!!!!!!

[Old Smokey]

There are many reasons why an engine might fail markkal, loss of oil pressure is just one of them. In other failures, e.g. ignition, fuel malfunction, the prop would still be windmilling, and thus extracting energy from the air stream to drive the dead engine. In these cases, the propeller pitch is still controllable.:ok:

In the event of an engine failure caused by loss of all oil pressure, the engine would quickly sieze, leading to a stationary propeller, thus eliminating extraction of energy from the air stream, a lower drag condition than having the propeller windmilling at either high or low pitch. (This too, is very noticeable when you do it). Most CSUs allow the propeller to go to fine pitch following engine failure, some (the best) allow the propeller to go to the full feather position ..... the best of all.

An interesting scenario posed by QJB, at first glance a fully open throttle butterfly seems to pose less drag, but would provide a much larger mass of air to be compressed within the cylinders ..... more energy extraction required! With the butterfly fully closed (closed throttle), most of the incoming air would stagnate in the intake, posing minimal drag. (This paragraph is my opinion, not a 'proven' situation, awaiting more informed opinion than my own).

Regards,

Old Smokey

[aerobat77]

well, initially the question was regarding a descent with idle not with a dead engine.

when i remember right this is a question from atpl theory cataloque. right sam ?

[markkal]

Hi Old Smokey....Did you ever have a dead engine with a windmilling prop ?????

I can assure you that that pitch control becomes totally useless, experienced myself back in 1993 in my Cap21.....And believe me the engine will not seize, unless there is no oil.

But right the thread is about windmilling props, with an aerobatic plane three blade prop pulling back throttle, and pitch to fine will throw you forward like an airbrake; The rate of descent if you do not reapply power will go to the variometer bottom stop.

[barit1]

Although oil pressure may be the most common way of controlling the prop, there are alternative systems: Electric (e.g. Curtiss), and manual (Roby). Neither is affected by oil pressure.

And I'll assure you that an R-985 w/ Ham Std counterweight prop will still be controllable as long as there's oil pressure.

[Escape Path]

There are many reasons why an engine might fail markkal, loss of oil pressure is just one of them. In other failures, e.g. ignition, fuel malfunction, the prop would still be windmilling, and thus extracting energy from the air stream to drive the dead engine. In these cases, the propeller pitch is still controllable.

Am I right thinking that the statement above would be true for single-shaft turboprop engines but not for free turbine engines?

In a free turbine engine both turbine sections are not mechanically linked, so windmilling of the propeller would not drive the gas generator turbine which usually drives the gearbox to which the oil pump is geared to. :confused:

[barit1]

In a free turbine engine both turbine sections are not mechanically linked, so windmilling of the propeller would not drive the gas generator turbine which usually drives the gearbox to which the oil pump is geared to.

The prop control has its own oil pump, at least in the props I'm familiar with, thus not dependent on the engine oil pump. The prop system may even have its own independent reservoir.

[aerobat77]

on a free turbine the prop will with feather when all oil presure is lost .

[barit1]

on a free turbine the prop will with feather when all oil presure is lost .

Not on all types. If counterweights and/or springs are involved, the above is true.

But if it's solely a double-acting hydraulic system (H-S Hydromatic e.g.) then centrifugal force on the blades proper will tend to move them to low pitch/high rpm, constrained only by pitchlocks.

[aerobat77]

hey barit !

that maybe, i do not know all possible designs and so cannot say for sure at all applications. do you have an example where pitch locks are installed in a free turbine design ?

the pt6a definitly does not has any pitch locks, you see it nicely on a normal shutdown- you can leave prop lever full foward and the propeller with nevertheless fully feather when the engine spools down .

best regards !

[barit1]

Sorry aerobat77 - there are two possible pitchlock systems, serving different purposes.

The one I described above is a safety device, protecting against destructive overspeed in case of a governor failure on a purely hydraulic prop such as the HSD or Aeroproducts design. It works when oil pressure is lost and the blades are threatening to go full fine (low) pitch. Makes no difference if the engine is one or two shafts, or a piston engine.

The pitch lock you are asking about is used (I believe) only on props on a single-shaft engine. Its function is to secure the blades to full low pitch during a normal ground shutdown; this makes it possible to crank the engine to a safe speed for the next start. Without it, the drag of the blades would make it difficult/impossible to restart.

And as you note, a free-turbine 2-shaft engine has no need for the latter system, since the core engine can always be started with or without the drag of the prop. (In fact, in a helicopter with a rotor brake, the core can be started with the power turbine locked tight.)

[JammedStab]

the pt6a definitly does not has any pitch locks, you see it nicely on a normal shutdown- you can leave prop lever full foward and the propeller with nevertheless fully feather when the engine spools down .



In terms of always feathering on shutdown, this is not always the case. For ease of explanation, I will quote an earlier thread on this subject...

"many turbine floatplanes fitted with PT6 engines are fitted with blade latches to allow the propeller to remain in a zero thrust position at zero airspeed after the engine is shutdown in preparation for the next start. To engage the blade latches you have to shut the engine down in beta or reverse - if you miss the latches the blades will feather.

The reason why blade latches are fitted is to increase control options on start. When we're on the water we don't have brakes - if we're starting up in a tight location the forward speed generated whilst we wait for the blades to come out of feather onto the fine stop can make life very difficult. It is only when the blades reach the fine stop you have a beta or reverse option available, and by this time in a C208 we're traveling at about 3 to 4 knots.

When we start with the pitch locks engaged a touch of reverse after startup is required to disengage the latches and give us instant control over the propeller - forward, zero thrust and reverse. The biggest drawback with shutting down on the pitch locks is it takes ages for the propeller to slow down enough to get past the propeller - it is very disconcerting attaching a mooring line to the bow cleats with your head only feet away from a propeller that is still spinning at high speed with no engine noise. Additionally, if you're facing directly into a significant breeze with the blades latched in zero thrust they will not stop spinning at all in zero pitch - and with the gas generator shutdown there is no oil pressure available to lubricate the reduction gearbox. Stopping them requires the use of lots of care and a towell for padding.

There are two advantages with shutting down in feather in the C208. The first is the propeller stops quicker. The second is the scavenge pumps return the oil from the reduction gearbox to the accessory case to allow the oil level to be accurately determined. There is a slight difference in resistance to gas flow depending on whether the blades are feathered or not - the ITT at feather is very slightly lower than when the blades are in fine pitch."

[JammedStab]

Do you have an example where pitch locks are installed in a free turbine design ?



The PW 100 series engine is a free turbine engine with pitchlock capability on the Hamilton-Standard propeller.

To quote from my notes...

"Aerodynamic loads on the propeller tend to drive the blades toward low blade angles. Should the supply oil pressure be lost, a pitchlock feature engages to prevent aerodynamic loads from reducing the Blade Angle more than 1 degree and the propeller speed increasing more than 2% from the point of oil pressure loss. It then acts like a fixed pitch prop, increasing and decreasing speed with PL movement. The pilots could still feather the prop because the electrical feathering pump would pump oil from a cavity in the case (like a stand pipe). This is activated when the CL is placed in the feather position."

While the method to achieve pitchlock may be different, the effect is the same on the Allison engine that has a very different type of Ham Standard prop. Like the PW 100-Hamilton-Standard combination, a pitchlock leads to the equivalent of a fixed pitch propeller operation. There just seems to be more undesirable postential consequences on the Allison if rpm is not managed properly.

[aerobat77]

@ barit : ah ok, we talked two ifferent things- my fault. i referred to pitch loks which prevent the prop goeing into feather at shutdown - like described such installation exists at singleshaft installation to reduce drag at startup and is not necessary at a free turbine since the core engine does not care about drag from the prop.

you talked about locking the pitch inflight when the governor wents out to avoid a destructive prop overspeed.

@ jammed stab: i think you also refer with the pw100 series to pitch locks which prevent an prop overspeed when the governor fails- i have no experience on lets say an atr- but seeing them shutting down the prop always feathers there. beyond that- interesting statement on your floatplanes , never heard this but it maybe true that such special installations are made there. what i can say for sure is that when you shut down a land plane with the pt6a the prop will fully feather, you do not even have a possibility to prevent it from feathering.

barit may be right that the situation of a prop going into feather with loss of oil pressure may be valid only for an installation where springs force the prop to high pitch /feather and oil pressure forces it to low pitch ( pt6a e.g) . with double acting oil systems things may be different.

thanks for the interesting details gents !

[barit1]

JammedStab:"Aerodynamic loads on the propeller tend to drive the blades toward low blade angles... "

Were I editing this class note, it would say: "Aerodynamic and centrifugal loads on the propeller..." -- and -- were I a betting man, I'd say the centrifugal load creates a greater pitch torque than the aero. There are a couple accident reports I could quote to support this statement.

[JammedStab]

JammedStab:

Were I editing this class note, it would say: "Aerodynamic and centrifugal loads on the propeller..." -- and -- were I a betting man, I'd say the centrifugal load creates a greater pitch torque than the aero. There are a couple accident reports I could quote to support this statement.

That portion of my note is a direct quote from a Dash-8 manual. Of course, that doesn't mean that you are incorrect.

Thanks.