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