Windmilling propellers

[virgo]

I cannot convince a non-aircrew colleague of mine that on large supercharged piston engines with constant-speed-propellers, if you lost an engine during cruise due to fuel starvation, it would continue to windmill at the selected rpm with accompanying boost (manifold pressure) and oil pressure. The only indication of the engine failure -if you missed the initial hiccup of parameters - would be the falling cylinder-head or coolant temperature and yaw and decay of airspeed.
Can anyone point me in the direction of getting written confirmation of this ???

[A321COBI]

hi
this stuff is complicated, you cant simply find it on wikepedia and so on, I think you should send an email to an aviation authority and get some confirmation
I agree with you, I know its true through my training

[john_tullamarine]

Have you considered the control range of the pitch mechanism ?

Unless you have an autofeather, autocoarsen, NTS, etc system operating in cruise flight, you are more likely to end up with a significant drag load on the failed side rather than, as you appear to be suggesting, a magical low drag system ? Typical end result is yaw, roll, and a really good view of the ground below - all of which probably might alert the pilot to an impending problem.

[no-hoper]

You wont find a written confirmation because the statement is complete wrong.The prop has to go to feather position otherwise the drag will
be too high.The engine will not turn anymore-means no more indications.

[ImbracableCrunk]

Like so many ground instructors have advised: RTFQ, people.

The poster wants info to support his statement that this type of engine would continue to rotate at the selected RPM with a supercharger providing Manifold pressure, etc.

His point: the engine will spin at the same RPM, but you'll get yaw and drag so watch out for the other indications.

(I'm not a piston guy, so I'm not one to judge the veracity of his statements.)

[Micky]

Ok I read the question :)and sorry no I can not point you to source where you can find information on this. But I think you are getting things muddled up.
If an engine would fail due to fuel starvation, the engine`s power output would start to get less there for the prop governor will decrease pith from coarse pitch to fine pitch to maintain RPM.(reducing the force it takes to turn the propeller) just like when you reduce power during descent/approach. You can even hear the the propellers running to fine pitch (change in sound and vibration). Secondly no piston engine will keep on windmilling due to the internal forces of turning the engine (try turning a big six cylinder engine on the ground...) It might be different for turbo prop engine's but I have no experience with these.

And when the engine is not working no supercharger will work or turbocharger.
And Boost Indication will show 0 Boost on ground if at sea level and ISA(When engine is stopped). So inflight you would find that it would indicate the actual air pressure(when engine is stopped) there for being less then 0 Boost. Normal Cruise power setting on my Airplane being around -0,5 to +1 Boost. So yes you would notice it.
And as the engine is not working Oil pressure would be 0psi, but Oil temp would only slowly decrease as well as Cylinder heat temperature. But they would initially stay the same.
And last but not least as John has said you would notice the failure as the props being in fine pitch would mean your really creating a lot of drag. Therefore creating yaw. Hopes this helps:OK:

[Deltabravowhiskey]

The key is to follow the prop response to torque from the motor (be it turbine or piston).

Constant speed propeller systems regulate a positive torque output from the motor and convert that torque into a regulated speed. Any increase in torque results in an increase in thrust by virtue or regulating a specified rpm limit with the prop control.

In your scenerio an engine is now attempting to regulate a negative torque (not a reduction in torque). In the case of a negative torque the engines prop control responds by attempting to increase prop rpm. HOWEVER the attempt to increase rpm is in relation to a positive torque applicaiton, since you now have a negative torque deviation the actual result is a reduction in rpm since there is now a negative torque being applied and the blades are commanded to a flatter pitch rather than a higher pitch which would be required to increase rpms...

Since your scenerio has resulted in a negative torque deviation, the prop (by virute of its design to regulate positive torque) will attempt to increase rpm by commanding a reduction in blade angle in relation to the relative wind. This reduction in blade angle will therefore reduce engine rpm with a significant increase in drag. In a negative torque situation the blade angle will always be the inverse of what is required to actually increase the rotational speed again because the system is simply designed to respond to torque from the engine.

In cruise, if you were to reduce the torque or in this case the manifold pressure prop rpms will rise despite the response to a reduction in torque, however it is a REDUCTION in positive torque being applied. As the MP is reduced the blade pitch will gradually return to a flatter pitch to compensate for the reduction in available torque. Eventually and as somebody has already mentioned the blades mechanical limits will be reached and the props ability to compensate for rotational rpm will result in a gradual reduction in engine rpm. If airspeed is slow enough a windmilling prop will cease to rotate, however the extreme rise in drag as a result of the flat blade pitch will more than likely cause a loss of directional control.

Because of the requirement to regulate positive torque, multi engine prop systems have to be fitted with a feather position that permits an expanded range of movement relative to blade angle past a flat pitch detent. This allows the prop to move from the flat blade pitch detent to a pitch that follows a rotational direction that reflects a negative torque angle until the blade is positioned in a low drag position relative to the airplanes relative wind.

A feathering mechanism requires a spring that forces the blades into the negative torque position, again there exists no ability for the hydraulic forces to act negatively upon the props internal mechanisms. The springs will force the prop beyond the limits of the hydraulic system thus feathering the prop and greatly reducing the resulting drag.

[Checkboard]

It has obviously been a long time since anyone here flew pistons! :8

...as you appear to be suggesting, a magical low drag system ?
virgo isn't suggesting this at all, john - merely pointing out that a constant speed prop is just that. The control range of the pitch is relevant however.

The prop has to go to feather position otherwise the drag will be too high.The engine will not turn anymore-means no more indications.

The prop is feathered by the pilot. The question is about what happens before the prop is feathered (and the engine is thus still turning.)

Secondly no piston engine will keep on windmilling due to the internal forces of turning the engine (try turning a big six cylinder engine on the ground...)
Sorry,Micky - the air we fly through can provide a lot of force (enough to lift the entire aircraft into the air ;) ). All piston engines keep windmilling after a failure (discounting mechanical lock-up) until the pilot feathers the prop.
And when the engine is not working no supercharger will work or turbocharger.
Superchargers are directly geared to the drive train. If the engine is windmilling, the supercharger is turning, and thus doing it's job to compress the intake air. Turbochargers are powered by the extra energy in the exhaust, if the fire isn't burning, a turbocharger will stop working - but that isn't the question here.
And as the engine is not working Oil pressure would be 0psi,
After the engine has stopped (after being feathered by the pilot) yes, this is the case. While the engine is windmilling, the oil pump is still being driven, so oil pressure is still normal.

Constant speed propeller systems regulate a positive torque output from the motor and convert that torque into a regulated speed.
Sorry, absolutely incorrect. You are confusing torque sensing (for auto-feather systems in turbines) with prop governors. Prop governors sense prop RPM, and adjust the pitch to compensate. If the RPM drops, the governor commands a finer pitch - if the engine is running, this reduces prop angle of attack, and thus prop drag and the RPM increases back to the governed level. If the engine is not running, the RPM drops, the governor commands finer pitch, this increases angle of attack (relative wind is striking the front face of the prop in a windmilling situation) and the relative wind thus drives the prop faster.

So, back to the original question:

Supercharged engine, with a non-mechanical engine failure. Prop governor maintains prop RPM. Supercharger & oil pump is mechanically linked to the crankshaft so they still operate normally.

Indicated RPM, oil pressure & manifold pressure all indicate the same after the failure as they did before, and this will continue until the propeller gets to the fine pitch stop, the point of which depends on the airspeed of the aircraft.

The fire has gone out, so Cylinder Head Temp. reduces rather quickly, oil temperature reduces less quickly and (as the prop is now being driven by the airspeed) the aircraft yaws into the failed engine and the airspeed reduces. It gets a bit less noisy as well.

[anecdote: check captain checking new pilot on a Heron (http://en.wikipedia.org/wiki/De_Havilland_Heron) arrives in the cruise. Check Captain reaches over and pulls all four mixtures out to the same point - as a demonstration of "efficient" engine management. Pilot under check thinks something is wrong - so pushes mixtures back in - and with the surge of power realises that the Check Captain had shut down two engines without realising it!

[True story, that.]

[Deltabravowhiskey]

Sorry, absolutely incorrect. You are confusing torque sensing (for auto-feather systems in turbines) with prop governors. Prop governors sense prop RPM, and adjust the pitch to compensate. If the RPM drops, the governor commands a finer pitch - if the engine is running, this reduces prop angle of attack, and thus prop drag and the RPM increases back to the governed level.


When any engine (turbine or piston) are running and by definition of "running" (generating positive torque, regardless of rpm) said engine transmits that torque to the motors output shaft or in this case the prop.

Depending on the amount of torque being generated (regardless of rpm) the engine will continue to accelerate (as a result of torque being produced) until something limits that maximum rotational speed, in this case a a prop governor increases blade pitch thus providing an increase in thrust output and the resistance to rotational torque hence limiting rotational rpm.

A prop governor is ONLY designed to respond to POSITIVE TORQUE from the motor (turbine or piston) a governor RESPONDS to torque not rpm (You can't have rpm without torque). RPM is simply a byproduct of said torque and rpms are only used to determine where a governor is operating at in respect to applied torque.

If you want to debate how a governor functions, then yes internal pressures that regulate the hydraulic forces sent to a prop hub are a result of the governors intenal pumps rpm and therfore the resulting pressures generated by that hydraulic pump.

In a NEGATIVE TORQUE SITUATION the prop governor will respond counter to POSITIVE TORQUE and the prop and windmilling engine will reduce it's rotational rpm relative to the speed of the flow of air accross the props blades.

This is a critical aspect to the design of a prop because if the prop were designed to increase the blades angle in a negative torque situation, the engine and prop would overspeed and result in a catastrpphic runaway situation.

There are inertial devices that prevent a prop from going into a negative blade pitch in flight (Feather). These arm when an over-ride is activated in the prop control system thus allowing the prop to continue past a flat pitch angle to a feathered blade position and are moved their by way of a piston and spring that act against the blades internal positioning arms.

Prop governors work as a result of torque output. Torque output determines the rpm and the limits of said rpm are a result of a torque limit set by the pilot thru the governor.

RPM's are a result of torque, torque is not the result of rpms...its of them laws of physics kinda things ya know.

If the engine is not running (producing positive torque), the RPM drops due to the governor commanding finer pitch as a result of the negative torque condition, this decreases the angle of attack and the prop rpms are reduced.

Google article:
Google Image Result for http://www.hariguchi.org/flying/info/figs/csp3.jpg (http://www.google.com/imgres?imgurl=http://www.hariguchi.org/flying/info/figs/csp3.jpg&imgrefurl=http://www.hariguchi.org/flying/info/prop.html&usg=__W3yEnQ_p6tavAPLBT7k1GkIOa0A=&h=287&w=440&sz=37&hl=en&start=1&sig2=thjYSW6-O_5VOuwmO-SiQA&um=1&itbs=1&tbnid=j_qU8JH_KL2jSM:&tbnh=83&tbnw=127&prev=/images%3Fq%3DPropeller%2Bgovernor%26um%3D1%26hl%3Den%26safe% 3Doff%26sa%3DN%26rls%3Dcom.microsoft:en-us:IE-SearchBox%26tbs%3Disch:1&ei=6tIYTMGZLc_DcNOM5acM)

[Checkboard]

A prop governor is ONLY designed to respond to POSITIVE TORQUE from the motor (turbine or piston) a governor RESPONDS to torque not rpm
Perhaps you should read your own link!
What Is Governor?[sic]

The propeller governor is an RPM sensing device which operates by means of the centrifugal force working on flyweights. The governor responds to a change in system RPM by directing engine oil to or releasing engine oil from the propeller to change the blade angle and return the system RPM to the original value. The governor may be set up for a specific RPM by the cockpit propeller control.(emphasis mine)

If the engine is not running (producing positive torque), the RPM drops due to the governor commanding finer pitch as a result of the negative torque condition, this decreases the angle of attack and the prop rpms are reduced.Not correct, I'm afraid. The governor is there to govern the RPM - and that is what it does. reduce power on a fixed pitch prop, and the RPM drops. Reduce power on a constant-speed prop, and the RPM remains the same (the hint is in the name!).

[john_tullamarine]

Reduce power on a constant-speed prop, and the RPM remains the same (the hint is in the name!).

I am still concerned if folks don't include the "within the control range" caveat.

[Feather #3]

A BMEP gauge is the answer!

G'day ;)

[411A]

I cannot convince a non-aircrew colleague of mine that on large supercharged piston engines with constant-speed-propellers, if you lost an engine during cruise due to fuel starvation, it would continue to windmill at the selected rpm with accompanying boost (manifold pressure) and oil pressure.

It is not surprising that you cannot convince a non-aircrew colleague, because, your information is incorrect....in so far as the following engines are concerned:
Pratt&Whitney R2000, R2800CB16, R4360...CurtisWright R3350 turbocompound series.

Yes,the RPM remains the same (in your selected scenario), the oil pressure (usually) remains OK, however, the manifold pressure and BMEP are most definitely reduced when the fuel flow ceases due to fuel exhaustion.
And yes,I've flown all of the above types on a variety of aircraft, IE: DC4, DC6, DC7, 1649 Constellation, Stratocruiser (B377).

[virgo]

Thanks everyone for your inputs - even those who have answered questions that weren't asked !!!!!
(Checkboard obviously understands)
411A........Are those engines fitted with superchargers or turbo chargers ? (I did specify a "Supercharged" engine). Obviously if there's no exhaust the manifold pressure will drop to atmospheric pressure.
Not many British engines had BMEP indication.

[Deltabravowhiskey]

RTFQ....

Not correct, I'm afraid. The governor is there to govern the RPM - and that is what it does. reduce power on a fixed pitch prop, and the RPM drops. Reduce power on a constant-speed prop, and the RPM remains the same (the hint is in the name!)

But that is not what he was asking....

a non-aircrew colleague of mine that on large supercharged piston engines with constant-speed-propellers, if you lost an engine during cruise due to fuel starvation, it would continue to windmill at the selected rpm with accompanying boost (manifold pressure) and oil pressure

And to refer to my link...


How Is the RPM Controled?

It is done by varying the pitch of the propeller blades. The pitch is the the angle of the blades with relation to the plane of rotation. As the blade angle is reduced, the torque required to spin the propeller is reduced and the airspeed and RPM of the engine will tend to increase for any given power setting. Convesely, if the blade angle increases, the required torque increases. Then the engine and the propeller will tend to slow down Thus, we can control the RPM by varying the blade angle or pitch of the propeller.


His question is "if you lost an engine during cruise due to fuel starvation, it would continue to windmill at the selected rpm"

The answer is:

No, the engine is producing a negative torque (windmilling) by virtue of the air passing accross the blades. A Governor is only designed to respond to POSITIVE TORQUE to achieve a selected rpm, since the torque is now a negative the result is a reduced blade angle to the flattest blade pitch position (Mechanical stop).

The question is not how a governor works, but how a prop responds in respect to the available torque with an engine failure.

And again, the answer is that the rpms will decrease as a result of negative torque and the governors response to that negative torque is reducing blade pitch resulting in lower rotational rpm from the flatter pitch and reduced blade angle. And again this is why the prop will need to be feathered since the drap rise with a flatter pitch will exceed the residual thrust available from the other engine and you WILL lose altitude ( loss of airspeed then lift, remember that equilibrium thing from your private pilot days?)

[411A]

Are those engines fitted with superchargers or turbo chargers ?

All have superchargers, some two speed.
One engine I specified has both a supercharger and a General Electric turbosupercharger (R4360) and another (the CurtisWright R3350 turbocompound engine) has both a two speed supercharger and three power recovery turbines (PRT's).
IF you want to learn more about these large complicated engines, visit AEHS Home (http://www.enginehistory.org) and become a member.

[bookworm]

No, the engine is producing a negative torque (windmilling) by virtue of the air passing accross the blades. A Governor is only designed to respond to POSITIVE TORQUE to achieve a selected rpm, since the torque is now a negative the result is a reduced blade angle to the flattest blade pitch position (Mechanical stop).

The governor doesn't care where the torque comes from, it cares only about the RPM. In the situation of a windmilling prop, the net torque remains "positive" in the sense that you mean it, with a negative torque from the engine offset by a positive torque from the air forces on the prop.

The mechanism of the governor is the same: the blade pitch adjusts until the net torque manages to drive the prop at the set-point RPM. If the engine starts ingesting pure air, the torque from it will reduce, and (in principle) the pitch will become finer and finer until it is being driven at the set-point RPM by the air forces, or it hits the fine stop, in which case the prop has to slow down.

I find it hard to believe that the other engine on a twin will continue to produce enough power that sufficient airspeed will be maintained so as to windmill the prop on the dead one at a cruise RPM. But if you had 6 or 8 engines, I can quite believe that one could fail and the aircraft would settle at a lower speed with one prop, in effect, in reverse.

What I don't get is the significance of the supercharger in the original Q. Doesn't the same effect occur on a normally aspirated engine with a wide open throttle?

[Basil]

Largest supercharged piston I've flown is the Hercules 14 cyl. sleeve valve radial.
IIRC, to re-start in the air: RPM lever out of feather and press feathering button had it windmilling merrily away. Lot of push in that Q formula.
(There were some other things to actually get it to run - fuel and ignition would probably help :) )

[virgo]

Thanks bookworm. You're absolutely right about the operation of the propeller control unit (CSU)The significance of the supercharger is that if the engine rpm is maintained and the supercharger is engine driven (as compared to a being turbo-charged), the boost (manifold pressure) and charge temperature will be maintained.
So you've got a dead engine - causing drag - that maintains the same RPM, Boost, Charge temperature and oil pressure as the functioning engines.

If you were totally distracted at the time of the failure - setting up an attack on a submarine or dealing with another emergency (an electrical fire in the fuselage), the failure could be missed until the aircraft has slowed down.......which I agree on a twin would be pretty quickly but with another three engines it takes a bit longer.

411A..............my information is NOT incorrect, I've had it demonstrated to me, seen it and done it. Have you any experience of multi-engined British piston engined aircraft ?

[411A]

my information is NOT incorrect,

Sorry, it is incorrect, in so far as the American piston engines that I described.
I've flown 'em all...not just seen 'demonstrated'.:rolleyes:

Have you any experience of multi-engined British piston engined aircraft ?
Negative...the Brits were very good with turbopropellor designs.
Pistons?
Second fiddle, by far, compared to the American designs.

[stevef]

Pistons?
Second fiddle, by far, compared to the American designs.

Hmmm... I guess that's why the P51 was eventually fitted with licence-built Merlins, then... :}

[FlugMonkey]

Ok, let's use the KISS method

1. The RPM will drop noticeably
2. every other engine parameter will drop with the RPM

Why? no fuel=no combustion=no turn

The propeller will now be driving the engine which will be acting like a brake due to compression in the cylinders.

The RPM will vary depending on airspeed and blade angle and engine type.

What won't vary is that you'll be trying to feather the prop/restart engine and hold the a/c level(twin), might not be as noticeable on a fourbanger but you'll know pretty darn quick.

Hope this helps.

FM

[ImbracableCrunk]

Funny, when I was doing multi-engine training, in order, it was:

Identify
Verify
Feather

So many of these posts are something along the lines of, "You shouldn't see these confusing indications because you should feather the engine!"

See the list above. :ugh:

[Tankengine]

and that is:

Identify - with the leg.
confirm - with the throttle
feather - with the prop lever.:rolleyes:


the instruments CAN be confusing under pressure!:=

[ImbracableCrunk]

and that is:

Identify - with the leg.
confirm - with the throttle
feather - with the prop lever.:rolleyes:


the instruments CAN be confusing under pressure!:=

"dead" leg - "dead" side. . . Check.

Now which throttle do you pull on a 4-engine prop? Or do you guess on the first pull?

[411A]

Now which throttle do you pull on a 4-engine prop? Or do you guess on the first pull?
No guesswork involved with the large American radial piston engines...you look at the torquemeter (BMEP gauge) and it will tell you straight away, if the engine is developing BHP...or not.
feather - with the prop lever.
Not on the abovementioned types.
A propellor feathering pump is used to drive the prop blades to the feathered position.
Just....push the feather button.

[ImbracableCrunk]

No guesswork involved with the large American radial piston engines...you look at the torquemeter (BMEP gauge) and it will tell you straight away, if the engine is developing BHP...or not.

Exactly my point. http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif (i.e. It's not as simple as dead foot = dead engine.)

[Mach E Avelli]

So much theory. Scenario originally given was running a tank dry, not initially changing throttle. With a DC3 THIS is what happens. At 150 knots TAS, there is no change in RPM or MAP. At 90 knots, there is some decrease in RPM and because the supercharger RPM drops with it, some drop in MAP. All of course accompanied by (expletive deleted) as throttle is hastily reduced and fuel tanks changed over.
John Tullamarine is usually right on these matters. It's all about the point at which the prop hits the fine pitch stops when there is no power coming out of the engine. At low airspeeds it must go full fine then it behaves as a fixed pitch prop if speed is further reduced. Also less RPM = less MAP. At higher airspeeds, because the engine is turning and producing oil pressure, the governor/prop combo MAY be capable of constant speed control, depending on TAS.
As an aside, I once had a prop run away to just over 2700 RPM on approach but we got slowed down and found that at 90 knots the RPM was quite controllable with throttle, so we kept it running rather than feather it.
Can't speak for other supercharged types.
Heron with the old Gypsy Queen engines, which were not supercharged (as someone recently tried to tell me - he was confused with the DH Dove): Similar reaction if you switched off the ignition in the cruise. Nothing to see except a gentle yaw. When airspeed reduced the RPM decayed, but now (because it was not supercharged), there was no visible change on the manifold gauge. From memory manifold indication was meaningless on those engines anyway as they had an ingenious single lever control of throttle and RPM - for once the Poms built something that was simple for pilots, if not the engineers. Basically if you could hear noise, the engines were working, and if one failed you knew because it got noisier as they went out of synch.

[dasher1973]

simple thing is the engine will windmill a little until feathered

any reciprocating internal combustion engine is only around 30% efficient, therefore the other 70% goes in heat...... and the requirement to turn the engine and keep it going.

so if fuel runs out..... no bang no power, just drag......
and a supercharger takes 50% of the power it creates to run itself... its the law of conservation of energy, you don't get something for nothing!!!! - a turbocharger uses wasted energy (heat out of exhaust) to drive a compressor, so it is grabbing some of that 70% waste and using it to increase volumetric efficiency, that is the ability to induct the same air volume as the capacity of the engine

If you want to increase volumetric efficiency over 1, you need forced induction...

anyhow.... a windmilling piston engine that is supercharged may give some MAP but not much, as turbocharged engine wont give any MAP as there is no heat coming to run it..

Cheers

Dash

[oxenos]

Virgo and Checkboard have got it right, as applied to a supercharged (not turbocharged) engine, and provided the fine pitch limit is not reached. Virgo's reference to submarines suggests that he is an ex Shackleton man, as I am.
If the fuel supply to an engine was cut, there would be a momentary dip in the RPM as the CSU compensated for the power loss, but it would return to its original RPM. The MAP would also dip and return, since the supercharger speed was related to engine RPM. The oil pump being engine driven, the oil pressure would be maintained.
If you missed seeing this brief dip, you were then faced with yaw and reducing airspeed. It would be some time before coolant temperature would show, as the radiator flaps would normally be in auto, and would close to maintain the temperature.
Having decided which side the failed engine was (i.e. dead leg), and after increasing power all round, the procedure was to close the throttle of the
inboard engine on that side, while watching the RPM gauge.
If the RPM dipped, that was the good engine, going from thrusting to windmilling. Restore the power on it promptly, as you are now double assymetric.
If the RPM did not dip, it was because that engine was already windmilling, i.e. it was the failed engine, so go ahead and feather it.

[Mach E Avelli]

In the cruise or higher speed range that I describe with the DC3 and Heron ( and I expect most other types) of course, when the engine quits due to fuel starvation or because some mug has hit the magneto master, there will be a MOMENTARY fluctuation downwards in RPM while the CSU makes a correction, but then the indications will be as I describe. Also, although the theory may say otherwise, the throttle needs to be retarded before re-introducing fuel/ignition. Because the pitch has fined off during the power loss, a gob-full of power would cause a momentary overspeed until the CSU gets ahold of it and coarsens the pitch to restore RPM to where it was at the time of the power loss. No theory - simple fact.

[virgo]

Thank you SO much Oxenos and Mach E Avelli.

I knew that eventually someone who knew what they were talking about would confirm my statement.

[barit1]

Just to add $0.02 to the prop governor discussion:

The governor is designed to this paradigm: To increase RPM, reduce blade pitch angle (i.e. reduce AOA); to decrease RPM, increase pitch (incr. AOA).

As long as the engine is producing positive torque, this paradigm applies nicely.

But if the engine ISN'T producing torque but is instead absorbing torque from the windmilling prop, the above paradigm creates bad results. As the RPM initially drops, the governor moves blades to a lower pitch angle, which now means a NEGATIVE AOA.

The prop now speeds up, and probably passes into a mild overspeed. The governor senses this, and reduces pitch angle some more. This of course means a MORE NEGATIVE AOA, windmilling the prop to an increased overspeed. This cycle repeats until the blade pitch reaches some mechanical stop. It's supposed to be a pitch-lock mechanism, which hopefully contains the problem before something breaks.

[Checkboard]

Absolutely incorrect, barit1.

Repeating my post above:

Prop governors sense prop RPM, and adjust the pitch to compensate. If the RPM drops, the governor commands a finer pitch - if the engine is running, this reduces prop angle of attack, and thus prop drag and the RPM increases back to the governed level. If the engine is not running, the RPM drops, the governor commands finer pitch, this increases angle of attack (relative wind is striking the front face of the prop in a windmilling situation) and the relative wind thus drives the prop faster.

Finer pitch when the engine is producing power increases RPM.

Finer pitch when the prop is windmilling also increases RPM.

In the first case the engine is driving the prop, the airflow is striking the prop on the "back" of the blade (the side the pilot sees) so a finer pitch reduces the blade angle of attack, decreasing the drag and so allowing the engine to spin faster.

in the second case the airflow is striking the prop on the "front" of the blade, so a finer pitch increases the angle of attack from the relative wind, which increases the force that wind is applying to spin the prop, which increases the RPM.

The prop governor thus controls the pitch in the correct sense in both cases.

[Mach E Avelli]

More modern systems do have all sorts of exotic pitch locks, auto-coarsening, beta backups etc. Not applicable to the original question.

[27/09]

A bit off the topic but to reply to an earlier statement.

stevefHmmm... I guess that's why the P51 was eventually fitted with licence-built Merlins, then

Correct to a point.

It was the supercharger technology that made the Merlin superior to the Allison orginally fitted to the P51. It took the Yanks to refine the Merlin for mass production.

As much as Rolls Royce has everyone believe that they produced the finest engines, they were effectively hand made. Each piston individually fitted to each bore and each crankshaft individually fitted to each journal. You couldn't take a piston out of one cylinder and fit it to another. The Roll Royce produced ones are/were a nightmare to own.

The "Parkard" Merlins were a much better engine. True, a British design at the end of the day.

Back to the original question.

I would expect a supercharged engine to act in a similar manner to a normally aspirated one in the situation described in the question.

That is RPM and Manifold Pressure would not change or if so only momentarily, oil pressure would stay the same also. Temps would drop slowly and of course there would be some yaw and speed would decay.

Lack of fuel flow, assuming a flow gauge is fitted, would be the major indicator.

[Yamagata ken]

It was the supercharger technology that made the Merlin superior to the Allison orginally fitted to the P51. It took the Yanks to refine the Merlin for mass production.

As much as Rolls Royce has everyone believe that they produced the finest engines, they were effectively hand made. Each pistoin individually fitted to each bore and each cranckshaft individually fitted to each journal. You couldn't take a piston out of one cylinder and fit it to another. The Roll Royce produced ones are/were a nightmare to own.What is the basis for this assertion?

Wikipedia (fwiw) gives the following:

" By the end of its production run in 1950, almost 150,000 Merlin engines had been built; over 112,000 in Britain and more than 37,000 under license in the U.S"

Wikipedia breaks down the numbers:

" Factory production numbers:
Rolls-Royce: Derby = 32,377
Rolls-Royce: Crewe = 26,065
Rolls-Royce: Glasgow =23,675
Ford Manchester= 30,428
Packard Motor Corp = 55,523 (37,143 Merlins, 18,380 V-1650s)
Overall: 168,068"Ref Gunson 1995

[barit1]

My Apologies. I was clearly not thinking clearly! Checkboard's right.

[ImbracableCrunk]

Yamagata Ken, check the wikipedia (fwiw) for the Packard V-1650 entry.

I'm no historian in this regard but I'm guessing it went something like this:

Great design+improved production technique=155k engines.

UK/US synergy at its best.

[ZQA297/30]

One dot right, you are on centreline.

In long range cruise on R2800 there were 2 leaning techniques, "2 bmep drop" and "11 bmep drop". Couple of times a dozey F/E would go past 11 bmep on the lean side, leading to a sudden drop in bmep to almost zero. Prop kept turning same RPM, MAP stayed same unless throttle moved. Capt blood pressure rose. Engineer bought many beers.

I suspect if airspeed low enough, prop would not have enough "fine pitch" available, and RPM would start to decay with decaying airspeed. But that area of flight is academic and of interest only to the very brave.


Use of feathering pump became very apparent when I had a runaway prop on DC-6B. Prop governor(which normally controls RPM ) mechanically broke (governor control spring failure) resulting in loud howl from prop and Capt simultaneously.
Luckily, feathering pump pressurises governor control piston to port oil to "increase pitch" side of prop servo piston. It seemed like an age, but prop eventually slowed a bit, and then feathered normally. Ten more grey hairs.

[Flash2001]

An aside to this discussion is how to practice/demonstrate forced approaches in a high perf single. As one doesn't actually stop the prop in a practice forced approach shouldn't one move the pitch control to lowest RPM to most accurately simulate a "dead stick" condition? I know I did and when questioned by the check pilot I gave the reason mentioned above. He said "Hmm, I hadn't thought of that."

After an excellent landing you can use the airplane again!

[Checkboard]

In a single (no feather) you move the pitch control to full coarse in order to reduce drag and give you the best glide distance.

It used to be a demonstration as part of the Constant Speed Unit endorsement (it's a separate endorsement in Australia) to set up a glide, and then pull the pitch control back to point out the surge of extra speed.

Lack of fuel flow, assuming a flow gauge is fitted, would be the major indictaor.

Only if the engine has failed because you have run out of fuel. ;)

[Flash2001]

Thanks CB, that piece of knowledge wasn't current when I checked out. Constant speed wasn't an endorsement then here and I don't know if it is now!

After an excellent landing etc...

[john_tullamarine]

that piece of knowledge wasn't current when I checked out

Worth playing with the CSU on a single. On parachute ops years ago, we did the whole descent control on the pitch lever through to flare - with a fairly (low) constant throttle setting so that we didn't have to worry too much about engine temps.

[SNS3Guppy]

I cannot convince a non-aircrew colleague of mine that on large supercharged piston engines with constant-speed-propellers, if you lost an engine during cruise due to fuel starvation, it would continue to windmill at the selected rpm with accompanying boost (manifold pressure) and oil pressure. The only indication of the engine failure -if you missed the initial hiccup of parameters - would be the falling cylinder-head or coolant temperature and yaw and decay of airspeed.

Having flown large supercharged piston engines professionally, and having worked on the same for a number of years as a mechanic and inspector, as well as having overhauled, installed, rebuilt, and maintained both the propellers and governor assemblies for those engines, I'll allow that you're substantially correct in general.

Several posters have touched on correct principles, and some have been fairly misguided by partial facts.

A propeller governor, especially for a Hamilton Standard Hydromatic and other types of propeller systems found on large radial piston engines, doesn't work on torque, but strictly on RPM. The amount of torque produced by the engine will determine the pitch commanded by the propeller governor, in it's attempt to maintain a particular RPM...but it won't determine the RPM.

Until the propeller reaches it's high or low pitch stops, the propeller and engine will maintain a constant RPM (assuming no governor or stephead malfunctions). Once the propeller comes to rest on the low pitch stops, it's in an underspeed condition and it's RPM is a function of airspeed (assuming no engine power, or in other words, assuming no torque). If the propeller is resting on the high pitch stops, then it's reached an overspeed condition and it's RPM is a function of both airspeed and torque...engine power.

Whether the RPM gauge will indicate an engine failure is largely a function of the engine RPM at the time of the engine failure. It may, or may not show a loss of RPM. At a low airspeed, a loss of engine power may result in an RPM decay, because if the propeller is already resting on the low pitch stops or reaches them in an effort to maintain RPM via the governor, nothing can be done to keep RPM up once airspeed is no longer sufficient to drive the propeller at it's scheduled RPM. Therefore, at lower airspeeds, one may see a loss of RPM following an engine failure or power loss. At higher airspeeds, quite likely no loss will be seen.

Oil pressure often remains constant following power loss, particularly if the only change has been cessation of fuel flow. If the engine continues to turn at the same RPM, then oil pressure remains constant.

Whereas the original poster said nothing about a BMEP (Brake Manifold Effective Pressure) gauge, it need not be addressed here. It will show a power loss, but as it hasn't been included in the instrumentation stipulated in the original post, it's irrelevant.

Manifold pressure is a more complex subject, and may or may not show a decrease. The subject powerplant in the original post was equipped with a supercharger, which is geared to the engine and has an output based on throttle position and engine RPM. Whether one sees a manifold pressure drop following a power loss depends on where the manifold pressure was to begin with. One may or may not see a drop. I've experienced engine failures involving disintegration of the supercharger clutch, for example, in a R2600, in which the manifold pressure merely reduced to ambient, or barometric. I've experienced other types of power loss or failures, especially at lower power settings, in which the manifold pressure indication remained the same.

Someone suggested that because the supercharger is geared to the engine, and because the supercharger is theoretically turning at the same speed as the engine, there will be no manifold pressure loss. This isn't true. Bear in mind that because of the use of a constant speed propeller, one can increase manifold pressure substantially above barometric, with the use of a supercharger, with no increase in RPM. RPM, therefore, is not the only determining factor regarding manifold pressure, when a supercharger is used.

I recall a P2V crash (using the R3850 motor) some years ago, involving not a power loss, but a governor failure. The governor couldn't regulate speed within limits. The crew could push the power up, but the propeller would overspeed, and power would have to be retarded. The airplane would descend. Power would be pushed up again, the airplane would accelerate, and the propeller would again overspeed. The power would be retarded, the airplane would sink...and this dance continued over the course of an hour or so until the airplane finally crashed on the high desert floor.

Had the crew realized what was happening, they could have flown slowly and still used engine power. The engine RPM was exceeding limits largely because it was being driven not only by engine torque, but by airspeed. Slow down, and while full power wouldn't be available, partial power would be...and the crew could have carried power on that engine, and used it to return to the airport and land.

EGT is an important indication, or other temperature indications to include CHT (not in all circumstances). Other indications such as an ignition analyzer may also prove useful, though on a large radial engine one usually is fully aware of the power loss by simple fact of assymetrical thrust, yaw, and the heavy rudder required to keep the airplane flying straight...as well as the change in sound, and fuel flow.

Fuel flow sometimes proves to be an excellent indication of engine health, especially in conjunction with EGT, but is particularly so in the case of the scenario presented by the original poster: fuel exhaustion.

Being able to look at the full picture and make a decision before attempting to mash a feather button is important, as it's easy to rush and feather the wrong engine...particularly on a piston-powered airplane.

In a single (no feather) you move the pitch control to full coarse in order to reduce drag and give you the best glide distance.

That really depends on the engine and propeller combination, and is not true for all aircraft, by any means.

[27/09]

Only if the engine has failed because you have run out of fuel

Well that's what Virgo said right at the start of this thread.

if you lost an engine during cruise due to fuel starvation

[Checkboard]

Someone suggested that because the supercharger is geared to the engine, and because the supercharger is theoretically turning at the same speed as the engine, there will be no manifold pressure loss. This isn't true. Bear in mind that because of the use of a constant speed propeller, one can increase manifold pressure substantially above barometric, with the use of a supercharger, with no increase in RPM. RPM, therefore, is not the only determining factor regarding manifold pressure, when a supercharger is used.

Yes, the manifold pressure can be changed by opening or closing the throttle, with the same RPM - but that manifold pressure isn't changed by spinning the supercharger any faster or slower. As long as the prop can provide the same torque to turn the engine at the same speed, the supercharger will rotate at the same speed, and if the throttle hasn't been changed, the boost will be the same. N'est pas?

[SNS3Guppy]

As long as the prop can provide the same torque to turn the engine at the same speed, the supercharger will rotate at the same speed, and if the throttle hasn't been changed, the boost will be the same. N'est pas?

I don't speak french.

The propeller doesn't provide torque to the engine. The engine provides torque to the propeller.

So long as as the engine continues to rotate at the same speed, the supercharger will continue to rotate at the same speed, as I previously stated. This does not mean that manifold pressure will remain constant, even if the throttle position remains untouched...because manifold pressure is not merely a function of throttle position and engine RPM.

At high boosted settings, such as takeoff power, without touching the throttle position, during a power loss one will see a manifold pressure loss. The supercharger, despite turning the same RPM (assuming airspeed is adequate to drive the engine via the propeller to maintain that RPM) and despite the throttle position remaining unchanged, will show a loss in manifold pressure (trending back toward barometric).

[Checkboard]

The propeller doesn't provide torque to the engine. The engine provides torque to the propeller.

I was talking about a windmillling situation. :rolleyes:

[one dot right]

Whereas the original poster said nothing about a BMEP (Brake Manifold Effective Pressure) gauge, it need not be addressed here. It will show a power loss, but as it hasn't been included in the instrumentation stipulated in the original post, it's irrelevant.


Pedant mode on-

BMEP = Brake MEAN Effective Pressure - Is a theoretical pressure developed inside the combustion chamber that if maintained equally throughout the two or four stroke cycle will develop a set horsepower. It has ABSOLUTELY NOTHING to do with MANIFOLD PRESSURE!!

Someone suggested that because the supercharger is geared to the engine, and because the supercharger is theoretically turning at the same speed as the engine, there will be no manifold pressure loss. This isn't true. Bear in mind that because of the use of a constant speed propeller, one can increase manifold pressure substantially above barometric, with the use of a supercharger, with no increase in RPM. RPM, therefore, is not the only determining factor regarding manifold pressure, when a supercharger is used.



Nope, RPM is not the only factor governing manifold pressure but it is the only factor that governs supercharger output and with a constant throttle position, which is supposedly what we're dealing with here, would be the only thing that would affect manifold pressure!

And boosting manifold pressure above barometric is the whole point of the thing!!!!!!!

Pedant mode off!

[SNS3Guppy]

BMEP = Brake MEAN Effective Pressure - Is a theoretical pressure developed inside the combustion chamber that if maintained equally throughout the two or four stroke cycle will develop a set horsepower. It has ABSOLUTELY NOTHING to do with MANIFOLD PRESSURE!!

You're right.

Nope, RPM is not the only factor governing manifold pressure but it is the only factor that governs supercharger output and with a constant throttle position, which is supposedly what we're dealing with here, would be the only thing that would affect manifold pressure!


RPM is the primary factor determining supercharger output at a constant throttle setting when the engine is developing power. It is not longer the determining factor after a power loss.

A supercharged engine developing 49" of manifold pressure during takeoff, for example, will not maintain 49" of manifold pressure, or the same supercharger output, after a power loss, even though the same propeller RPM remains.

[virgo]

Guppy, a supercharger is a simple air pump ? Surely it doesn't matter whether it's being driven by an engine under power or windmilling..... if the supercharger rpm is maintained, assuming the throttle position is constant, why will the output (manifold pressure) change ?

[one dot right]

Please enlighten us all!

Or are you perhaps confusing Supercharging with Turbocharging which will indeed lose boost pressure after a failure due to no longer being driven by the exhaust gasses.

[411A]

A supercharged engine developing 49" of manifold pressure during takeoff, for example, will not maintain 49" of manifold pressure, or the same supercharger output, after a power loss, even though the same propeller RPM remains.
Absolutely correct....notwithstanding others comments to the contrary.
And yes, supercharged, not turbosupercharged.

The rest who think otherwise are sadly misinformed.

[one dot right]

The rest who think otherwise are sadly misinformed

And still the legends in their own lunch breaks will not explain to us how a functioning compressor at the same RPM as it was before the failure is somehow not compressing now!!!

Just because you say it loud enough doesn't make it so.

[oxenos]

A lot of people spouting on this thread are failing to RTFQ.
In his OP, Virgo specifically referred to a SUPERCHARGED engine in the CRUISE. Others have made the point that what he describes will indeed happen PROVIDED the pitch stop limits are not reached, and yet we are still being told what happens to turbocharged engines, and what happens on take off.
I only did 2,500 odd hours on Shackletons, which is not a lot compared to some people, but I recall only too well that on every check ride, an engine would be failed in the cruise (by the engineer cutting the fuel flow, on a signal from the checking Captain ) so that you had to go through the procedure I described in post #29.
May I suggest that before anyone else sounds off, they RTFQ, read the thread, and stick to answering the FQ.

[411A]

...will not explain to us how a functioning compressor at the same RPM as it was before the failure is somehow not compressing now!!!

No need for you to know why, those of us whom have flown the engine types I specified previously, would know for sure, IF they had operated same....which clearly is not the case, with the above quoted 'compressing now!' diatribe.

No fuel, MP decreases, and depending on the throttle blade opening a little, or a lot.

[werbil]

I'm guessing less exhaust back pressure = lower pressure in cylinder whilst intake valve open = more airflow into engine = lower manifold pressure.

[Neptunus Rex]

411A
oxenos is correct. The Rolls Royce Griffon was an exceptional engine, producing some 2,500 horsepower from a V12. Any more power would have resulted in torsional flexing of the crankshaft. Instead of the traditional butterfly, it had a 'Rolls Royce Coreless Valve' in the fuel injector. This is best described as a cylinder within a cylinder, such that at full throttle, there was absolutely no restriction to the flow in the inlet manifold.
The propellors were contra-rotating; the engine drove the front prop which was geared via a rack bolt to the rear prop. The CSU was so efficient that it took just one second to fully restore the RPM following a fuel cut.

[one dot right]

No need for you to know why, those of us whom have flown the engine types I specified previously, would know for sure,

I can only assume from that response that you don't know why yourself.

We have examples cited from people who have flown supercharged Griffons on Shackletons that categorically state that once the status quo has been resumed,ie the prop is now driving the engine, there is no loss of manifold pressure.

In the absence of any information from your supposed font of knowledge I can only assume that the American engines that you rave about suffered excessive back pressure such as Werbil describes, and there lies the problem!

Usually when people puff and blow without any real substance to their argument It's due to lack of knowledge!!!!;)

[barit1]

There is a scenario in which SNS3Guppy is right: If the intake manifold is compromised by a large leak - e.g. loss of a cylinder head - then there's less downstream load on the blower, and thus less static pressure is developed at a given RPM. In other words, less back pressure.

However, simple fuel cutoff would not give the same result.

[Checkboard]

From the bottom of every PPRuNe page:

*"sciolist"... Noun, archaic. "a person who pretends to be knowledgeable and well informed".

411A and SNS3Guppy both tend to fall into that category - they'll tell you they have flown the serum to Mercury, and advised everyone from Glenn Curtiss to Kelly Johnson.

... but never any verifiable details, like company, or type ... :hmm:

[ImbracableCrunk]

Is it possible that there are two possibilities? Maybe the RR engines have one behavior and the Pratts and Curtisses have a different?

Can we agree that the supercharger doesn't care where the rotation comes from?

[GarageYears]

One of the problems with the mechanical superchargers was that they also consume considerable engine horsepower – as much as 300 bhp in FS [fast speed] gear.

You don't get something for nothing. It seems to me this factor simply contributes to the effective drag of the 'failed' engine and would not explain a drop in boost pressure.

- GY

[virgo]

Crunk, maybe you're right ?

I am absolutely correct in saying that on a Rolls-Royce Griffon engine the boost (manifold pressure) will be maintained if the engine rpm is unchanged. I

411A and a few others insist that on Pratt and Whitney supercharged engines the manifold pressure will REDUCE even if the rpm is unchanged.
(Both cases assume the is no change to throttle (power lever) position)

To summarise,
1. Aircraft straight and level in cruise
2. Propeller in the constant-speeding range (On speed)
3. Throttle (Thrust lever) set to cruise boost - say +6 lbs (42 " Hg)
4. Power is lost due to either total fuel starvation or both magnetos being switched off

Most people agree that engine rpm will be maintained by the CSU (propeller governor) moving the propeller to a finer (lower) pitch to maintain the selected engine rpm.

Question...........What happens to engine boost (manifold pressure ) and WHY

Is there anyone out there, who understands the question and knows what they're talking about, who can give an explanation for the alleged differences between British and USA engines ?

[VFD]

I am going to take this in reverse.

Can you take a feathered stopped engine unfeather the prop to fine pitch and bring it up to full rpm and turn back on the fuel or mags (which ever was off) I am thinking not.

So back to the original question which is will the prop in fine pitch be able to put enough torque back into the engine to overcome friction loss, compress the gases in the cylinders, and drive the supercharger, and still maintain rpm. I think we are talking a decaying situation as far a rpms go and manifold pressure would drop accordingly.

VFD

[virgo]

VFD................do you know 411A ?

[oxenos]

VDF
The normal way (indeed the only way) of unfeathering a Griffon in flight was exactly what you describe ( But you think it would not work )
Move the RPM lever out of the feathering gate, press the feathering button to get the pitch change going, ignition on, fuel on and the engine starts up. You certainly did not start it with the starter motor.
We are getting lots of people on this thread, " sciolists", telling us what they think, and not listening to people who know.

[VFD]

Move the RPM lever out of the feathering gate, press the feathering button to get the pitch change going, ignition on, fuel on and the engine starts up. You certainly did not start it with the starter motor

Correct you are about enough rpms for a start vs bringing the engine up to operational rpms.

VFD................do you know 411A ?

No

VFD

[411A]

... but never any verifiable details, like company, or type ...

Oh dear, Checkboard, you must really pay attention.:rolleyes:
I already mentioned 'types'...specifically Pratt&Whitney R2800CB16 and R4360, as well as Curtis Wright R3350 turbocompound series...about British types, I would have no idea, as I have not (nor do I desire) to operate these.
Pay attention now, least you become confused...:bored:

[one dot right]

Hmmm. Still no mention of companies operating these types.

Oh, and it's "lest you become confused" not least. Still, we wouldn't expect Americans to get the nuances of the English language, after all it's only their mother tongue!:p

[411A]

Still no mention of companies operating these types.

'Companies' operating these specific types don't make a difference, one dot right, as the engine specifics don't change with operating companies.
Of course, if you had operated the types I specified, you would know this.
That you don't only leads to the conclusion that you (specifically) are misinformed.
No surprise there.:rolleyes:

Now, if I were to mention 'Willair International', and you had been around in 1970, when this company was operating 1649 Constellation aircraft transPacific
(been there, done that) it simply wouldn't help your learning process, such as it is...:ugh:

[Lancman]

Children, children! Let's all play nicely together.
To split another hair, the two speeds for RR superchargers were MS (moderately supercharged) and FS (fully supercharged). They were generally referred to as M gear or S gear in order to differentiate more clearly on the intercomm.

[one dot right]

411A

You could help my learning process enormously,and everybody elses, if you would give a reason why there is a drop in manifold pressure on a supercharged engine with a supercharger doing the same RPM as before the failure.

I suspect that despite your attempts to be recognised as an old sage of aviation with the wisdom of Soloman, and I have no doubt your operational knowledge is second to none, your technical knowledge is rather lacking.

I have always found that people who put their hand up and admit to not knowing something are generally held in a higher regard than those who pretend to know.

[Lancman]

It may be that 411A is exactly right in what he says. What would be far more constructive would be to discuss why he is right.

[one dot right]

It may be that 411A is exactly right in what he says. What would be far more constructive would be to discuss why he is right.


I'm trying to, look-

You could help my learning process enormously,and everybody elses, if you would give a reason why there is a drop in manifold pressure on a supercharged engine with a supercharger doing the same RPM as before the failure.


To my mind,assuming the original scenario where fuel starvation is the cause of the engine failure, the only thing that has changed is the fire in the combustion chamber has gone out.

With no hot gasses being forced out of the combustion chamber and with a bit of valve overlap, it is feasible that the back pressure on the inlet manifold is reduced causing a drop in MAP as Werbil said in an earlier post.

[Basil]

Can you take a feathered stopped engine unfeather the prop to fine pitch and bring it up to full rpm and turn back on the fuel or mags (which ever was off) I am thinking not.
As I said in a previous post: yes, you can. :ok:

Even heard of attempts on the ground by turbo-prop and jet - understand the jet attempt ended in tears in fleet manager's office :{

Re the manifold pressure Q: forty something years ago I could have tried it for real to see exactly what the change was but no Varsities left flying.

[Lancman]

Now here's an idea. I have little experience of flying U.S. engines, the only ones that I flew were the JT3D, so I'm very willing to be corrected. But as I understand it some of the later American piston engines had power recovery turbines in their exhaust system which mechanically fed energy back from the gasses into the main engine. If the engine was windmilling and no fuel was being burned these turbines would become pumps sucking air out of the combustion chambers and the intake manifold as previously suggested by werbil. Comments please.

[one dot right]

Lancman

If that is the case then it would make MAP drop entirely engine model dependent.

Funnily enough that seems to be exactly what we have here, with ex Griffon operators (presumably without power recovery turbines) saying no MAP drop, and 411A et al saying MAP drop with yank engines!

[virgo]

One Dot.
I don't think Power recovery turbines can be the answer. I understand they were only fitted to the Wright 3350s. (Plus a couple of British engines). Certainly SOME large Pratt and Whitney engines had superchargers AND turbochargers (the 4360 ?) but I think the 2000 and 2800 only had superchargers ?

If I've got that wrong I'm sure someone will correct me.

So we're no nearer an explanation as why at constant rpm SOME supercharged engines will maintain manifold pressure and others won't. Come on 411........... give us the answer

[411A]

Come on 411........... give us the answer



Use your brain...the answer has already been suggested.
Twice, I believe.

Suggested reading....AEHS Home (http://www.enginehistory.org)

If you read carefully,you will know the score, just like us old timers.
IE: do your homework.

[virgo]

411A............your answer is WRONG !

I've been there, I've seen it and I've done it - probably before you !

I've explained what actually happens, which has been agreed and supported by a number of similarly experienced and knowledgeable airmen.

Our experience makes sense............if you rotate a pump, the output varies according to rpm - remember physics at school ? RPM constant = Pressure Constant if nothing else changes.

411A, Re-read your Boyles and Charles Laws and fluid dynamics.

[ft]

I have to chime in with SNS3 (love reading your posts on technical matters) and 411A, and especially with the seemingly much overlooked one line post by werbil a way back.

No combustion -> No pressure increase in the cylinder -> less exhaust backpressure -> less remaining pressure in the cylinder as the exhaust valve closes -> less pressure in the cylinder as the intake valve opens -> more air into the cylinder -> drop in MAP, even with everything else remaining constant.

The effect would be there. I have no expertise or experience which enables me to tell if it would ever be noticeable, so there I'll have to trust those who claim it will be.

Exhaust backpressure is often overlooked but can still have some very interesting effects on the performance and behaviour of piston engines.

[one dot right]

I have no expertise or experience which enables me to tell if it would ever be noticeable, so there I'll have to trust those who claim it will be.


Why would you take the opinion of one 'expert' over another? Simply because they are louder and more objectionable than others?

[411A]

No combustion -> No pressure increase in the cylinder -> less exhaust backpressure -> less remaining pressure in the cylinder as the exhaust valve closes -> less pressure in the cylinder as the intake valve opens -> more air into the cylinder -> drop in MAP, even with everything else remaining constant.


That is quite correct for US manufactured engines that I have referenced previously.

411A............your answer is WRONG !
I've been there, I've seen it and I've done it - probably before you !


Quite likely not, and as we can see from the above from ft, virgo must have been sleeping during his claimed ops....such as they might have been.:rolleyes:

[ft]

one dot right,
the effect is there. The question is on what engines it will be noticeable. All of them? I think not. But on some engines? Weeeeeell... I wouldn't know.

If someone who claims to have extensive experience of operating the Twadoddle PXF38 radially indisposed contraindicating aircraft powerplant says it will show a MAP drop, I'll check on other references to see if that individual is someone I'd generally trust to be correct - until I get the chance to run a Twadoddle PXF38 radially indisposed contraindicating aircraft powerplant in a test bench, or find hard evidence from such bench tests.

In short, I'm only convinced that it will be noticeable as far as my trust in those saying it will goes. I will however not say they are wrong as it can't be - as some people are doing.

If we had several people having operated the Twadoddle, some saying it will show, some saying it won't, then it gets tricky for real - but that does not seem to be the case here. Unless I missed something we have Twadoddle PXF38 flyers saying it will show, and we have people saying it won't based on academical merit (wrong!) or based upon experience of the PXF33.5.

[blackhand]

A very interesting thread.

Still working the original question through in my head.

I was taught that "supercharging" is an effect of valve timing, rather than just the blower per se. Blowers or superchargers are also used on two stroke diesel engines for more effective scavanging with no increase in MAP.
"Supercharging" is caused by allowing the inlet valve to stay open slightly longer than normal during intake stroke - with the exhaust valve closed, hence not valve overlap as some one has suggested.

Can anyone tell me where the MAP sense line is tapped from on these "big" engines?
My thoughts are that the MAP gauge should be sensing the Blower output.

No combustion -> No pressure increase in the cylinder -> less exhaust backpressure -> less remaining pressure in the cylinder as the exhaust valve closes -> less pressure in the cylinder as the intake valve opens -> more air into the cylinder -> drop in MAP, even with everything else remaining constant.

I find this confusing. Back pressure assists to hold the exhaust valve closed, so I can't see why less back pressure on a dead engine would affect cylinder pressure.
I imagine there would still be some back pressure even with a dead engine.
As the piston swings past the top of exhaust stroke it will induce a slight negative pressure in the cylinder, my thoughts are that this would be the same with a dead engine so there is a nil net affect.

All ways learning.

Cheers
BH

[one dot right]

In short, I'm only convinced that it will be noticeable as far as my trust in those saying it will goes. I will however not say they are wrong as it can't be - as some people are doing.


You will notice I never said they were wrong. All I asked for was an explanation,which, via a roundabout route and other peoples posts, we finally have.

Whether it is the right explanation remains unclear as all 411A has done is to agree with other posts (mine included) and claim he already knew that.

It may well be the right explanation, but I would rather hear an engineering experts point of view, rather than that of an abrasive, bigoted, self appointed authority on all things aviation. But that's just my opinion.:O

[MoodyBlue]

Can anyone tell me where the MAP sense line is tapped from on these "big" engines?
My thoughts are that the MAP gauge should be sensing the Blower output.From the Pratt & Whitney "Manual of Engine Operation" (1949, relates to all Pratt radial engines): "In practice this pressure is not measured at an intake port, but at the rim of the supercharger collector. It is known as manifold pressure, or absolute blower rim pressure (abrp), or even manifold absolute pressure (map), and is expressed in inches of mercury (in. Hg.).*"

So yes, it senses blower output, one would think.

The MAP is used as a (but not THE) engine power measuring value, hence measuring the blower output will be largely useless to the pilot as variations in throttle setting will not alter the blower output. Only RPM change or (on some) vane angle change will do that.The throttle valve is before the supercharger, so variations in throttle setting will change the amount of air allowed into the blower and as a consequence change the output pressure of the blower (= MAP, see above). Obviously RPM changes will also change blower output (MAP).


http://img210.imageshack.us/img210/7213/mapld.jpg


And PS, this is not just paper wisdom. They let me fly DC-3's every now and then (Pratt & Whitney R-1830's), and every time I move the throttles the MAP (= blower output, see above) definitely changes - a lot. When I move the propeller pitch levers, the MAP also changes - a little.

And before anyone asks, no, I'm not gonna try to switch off the fuel to one engine next time to see what happens :=

http://www.pprune.org/c:%5Cdata%5Cdc-3%5Cmap.jpg

[one dot right]

Quite right. Post removed. Senior moment.:ugh:

[enicalyth]

Stanley Hooker et al's treatise "The Performance of a Supercharged Aero Engine" is available price £6 from the Rolls-Royce Heritage Trust. It is a facsimile of the original with handwritten emendations.

Oh and cheers Checkboard, see you soon!

[ft]

I find this confusing. Back pressure assists to hold the exhaust valve closed, so I can't see why less back pressure on a dead engine would affect cylinder pressure.

The valve spring holds the valve closed. Back pressure in fact acts against the spring, trying to push the valve into the cylinder (i e open).

Less back pressure means the piston will have to work less as it empties the compression chamber on the exhaust stroke. This translates to lower pressure in the combustion chamber all through the exhaust stroke as it vents easier into the exhaust manifold.

I imagine there would still be some back pressure even with a dead engine.

Yes. But less than had the engine been running.

[Flash2001]

Let me add that the average piston aircraft engine turns rather slowly in relation to the speed of sound in hot exhaust gas and the length of the exhaust system. You shouldn't have much back pressure unless there is a turbo, a PRT or a silencer.

After an excellent landing you can use the airplane again!

[blackhand]

Back pressure in fact acts against the springHi FT
Thanks for the correction. Back pressure does in fact slow the scavenging of exhaust gasses.

In practice this pressure is not measured at an intake port, but at the rim of the supercharger collector. It is known as manifold pressure,Thanks MoodyBlue for the info.
So this would infer that regardless to what is happening during the Otto Cycle(modified) the output of the blower would stay the same as long as RPM and throttle position remained constant.


Cheers
BH


A good landing is one we can walk away from

[virgo]

BH..............Thanks. This is exactly what I have been saying since page 1 !

[ZQA297/30]

I have never been involved in the operation of UK piston engines, but I was always told that the main difference was that UK engines were "boost limited" whereas US engines were not. What I understood that to mean was that the pilot was in charge of max boost on a US engine, and it was possible to overboost the engine by advancing the throttle. The UK engines reached max boost and were automatically limited, permitting no overboost. How all that was achieved is a mystery to me, but maybe some UK engineer will remember.

On the MAP issue, my only experience of windmilling was at cruise ( around 33" MAP on a P&W R 2800 cb3/4 (single speed supercharger) or cb15/16 (two speed supercharger), and scavenge back pressure would have been close to atmospheric so no noticeable change in MAP took place.
Possibly at high altitude in "high blower" (CB 15/16) the low outside pressure would affect scavenge back pressures and thereby indicated MAP.

I am trying to remember what happened on DC-6 when RPM switches were toggled up for approach. If my memory is correct MAP decreased with increasing rpm and vice versa. (Throttle not moved)
Thing is, it is all back in early 70s, so it is a bit hazy.

[johns7022]

My gawd 95 posts arguing about what happens when you shut off an engine in flight?

How about a raise of hands...everyone in here with an MEI that has actually taken up a student, shut down an engine on a muti-engine turbocharged aircraft, feathered a prop, then started it back up....

(my hand is up)

[Brian Abraham]

You shouldn't have much back pressure unless there is a turbo, a PRT or a silencer.
No additional back pressure with a PRT, as it uses the blow down turbine principle.

[Neptunus Rex]

By way of comparison, IIRC the boost figures (in inches of mercury) were:

Neptune, Wright Cyclone - 61"
Shackleton, RR Griffon - Low Gear 58" High Gear 81"

[Lancman]

Could it be that when an RR engine is deprived of fuel in flight that there is a drop in boost pressure that is detected by the boost regulator which immediately restores it by opening the throttle (a Corliss valve on the Griffon, by the way)?

[Checkboard]

How about a raise of hands...everyone in here with an MEI that has actually taken up a student, shut down an engine on a muti-engine turbocharged aircraft, feathered a prop, then started it back up....

The point is, have you done the same in a supercharged engine?

(my hand is up)

I think you can put your hand down now. :hmm:

[one dot right]

My gawd 95 posts arguing about what happens when you shut off an engine in flight?

Nice that you noticed how many there were.

Shame you didn't read and digest the content before wading in.:hmm:

[MoodyBlue]

It shouldn't come as a big surprise that we are not the first to contemplate this matter.

This text, "Gauge Indications on a Failed Engine", Warbird Notes #6 (http://www.douglasdc3.com/sohn/6.htm) was written by Mr. Randy Sohn in 1994.

I have no way of knowing whether Mr. Sohn is correct in everything he writes, but he does appear to carry a lot more credibility than most if not all of us here: Randy Sohn (http://www.avweb.com/news/profiles/182142-1.html) .

An index to all of his "Warbird Notes" can be found here Warbird Notes INDEX (http://www.douglasdc3.com/sohn/warbird.htm) . Highly recommended!

[Pugilistic Animus]

I think one person here knows of burnt 'flatnose' with Big Ernie

:)

[oxenos]

Thank you Moody Blue. At last a reference, as opposed to someone telling us what he thinks will happen.
"Cut a mixture and just leave the throttle alone where it was set at cruise, lets say, for example in this case, 27". If you can visualize a big air pump, that is exactly what the engine is, with the throttle located at the intake. After seeing for yourself that the MP stays unmoving at 27" ....."
"The engine keeps turning and as long as it does the oil pump keeps pumping, right? So the oil pressure stays up and doesn't give you a clue about which one failed. Over the short period we're discussing here the temperature stays up too."
"When we cut the mixture at cruise airspeed the RPM sagged about a hundred and then went right back to where it had been. Let's analyze that for a minute. The engine is still turning so the oil pump is still putting out normal pressure. Where does the prop governor get its supply? That's right, from engine oil pressure. So it keeps doing its thing according to your request through the prop control. You had the control set to cruise RPM so (after a second or two of decrease followed by an immediate increase as the blades assume a new angle) that's what it delivers. It will as long as it has oil pressure and, with this proviso, that the cruise speed stays high enough so that the blades don't reach the pitch stops while trying to maintain the requested RPM."
This all exactly what virgo wanted confirmed, and exactly what a few of us who have actually flown supercharged (NOT turbocharged) multi's have been saying

[411A]

but he does appear to carry a lot more credibility than most if not all of us here: Randy Sohn .

Unfortunately, Sohn is just slightly wrong with a couple of the previously mentioned engines I referenced earlier (both supercharged)...namely, the Pratt&Whitney R4360 and the CurtisWright R3350-EA4 trubocompound...a slight MP reduction will ne observed....two to three inches, all due to the effects I mentioned previously.
Perhaps Sohn hasn't flown these types..IE, only concerned himself with 'warbird' types...:rolleyes:

[MoodyBlue]

Quote:
but he does appear to carry a lot more credibility than most if not all of us here: Randy Sohn .I knew I should have stuck with "most of us here" i.s.o. "most if not all of us here"... ;)

I only know the R4360 and R3350 from museums, they certainly are different beasts than the R-1830 (the one radial I know a bit about) so I wouldn't have a clue as to who's "right" here. It could indeed very well be that there's a difference in behaviour between these types of engines, and you are both right... now wouldn't that be nice...

Looks to me like we're not going to get the definitive answer - but the good thing is that we did get to learn something maybe (I sure did) :ok:

[blackhand]

Ah is most satisfying when theory and the reality coincide.

BH