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!

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.

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

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...

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.

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.

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

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

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?

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).

I was talking about a windmillling situation. :rolleyes:

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!!

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!

You're right.

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.

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 ?

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.

Absolutely correct....notwithstanding others comments to the contrary.
And yes, supercharged, not turbosupercharged.

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.

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.

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.

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

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.

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!!!!;)

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.

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:

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?

Not sure how this fits in but...
 

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

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 ?

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

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

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.

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

No

VFD

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:

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

'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:

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.

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.

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-

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.

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.

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.

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!

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