Manifold Pressure

[Boom]

Have always been slightly confused by manifold pressure. It is mentioned constantly in bio's by American pilots when describing flying procedures, eg I set 42 inches of manifold pressure ready for take off.

Yet I have never come across it being mentioned in bio's by RAF/German pilots.

Why is that?

[SYN_Vander]

1 hour ago, Boom said:

Have always been slightly confused by manifold pressure. It is mentioned constantly in bio's by American pilots when describing flying procedures, eg I set 42 inches of manifold pressure ready for take off.

Yet I have never come across it being mentioned in bio's by RAF/German pilots.

Why is that?

British call it 'boost' and the Germans use 'ATA' (technical atmospheres absolut) - from the top of my head.

Found this video, so it's not all the same: 

 

Edited May 31, 2023 by SYN_Vander

[Lusekofte]

One of the things I do love with DCS is their startup training missions. 
They take you around the cockpit and teach you the real procedure and why. I did a repetition on P 47. Just love that plane. 
They do teach you on how and why you should be kind to the engine. The complexity of engine damage and small parts in undercarriages and other structures really give you a lesson on keeping an eye on the gauges. I start up the plane within half a minute. Procedure calls for several minutes. 
simplified engine parameters and dm make you less aware on what happening, and more aware of time limits

[Boom]

Thanks for the replies.

Boost I have heard mentioned, but only in the context of pilots advancing the throttle. Was boost and power all done via the throttle? Or was there another leaver that RAF pilots operated to manage boost? I understand that there was a separate leaver in US aircraft for manifold.

[BlitzPig_EL]

The throttle is what sets "boost" or "manifold pressure", they are the same thing.  The Brits measured it in PSI and the US in inches of mercury.

[Boom]

Oh ok. So when I read that 'he set manifold pressure at 42 inches for take-off', the pilot was simply advancing the throttle to that point. Correct?

[Art-J]

Correct for majority of mass-produced WWII planes.

P-47 was a peculiar exception from the rule with one of its two stages of forced induction, a turbocharger, being controlled by the pilot with yet another, separate lever on the quadrant, installed next to the throttle one. Both levers could be linked together, and the whole contraption was dropped eventually in N version in favour of a single combined throttle lever for easier operation.

But, as I said, the original setup was very unusual and most (if not all) of other planes had just one throttle lever for MAP/boost + one RPM lever for engine speed, or, in case of some German planes, one "go fast/go slow" lever for everything combined (like in modern, FADEC equipped light planes of today).

Edited June 1, 2023 by Art-J

[Bremspropeller]

TBH I've never read that many technical particularities (ATA or RPM) in most german accounts whatsoever. Might be a cultural thing (anglosaxons love their numbers) or just bourne out of the fact that the "one lever does it all" is much less critical in engine parameters, so they didn't drum the specific numbers all too much into their pilots' heads.

Also, this might be an indication for the different flight-training systems (standardisation, focus on checklists, etc.) in different countries.

[DavviZ]

The brits used "Boost" which was the amount of manifold pressure in Lbs (Pounds per square inch) above or below the atmospheric pressure at sea level. The US planes used manifold pressure in inches of mercury where ~30 inches of mercury would correspond to 0 lbs of boost on the British gauge. The Germans used ATA which is as previously pointed out the "amount of atmospheric pressure" in the manifold. So ~1 ATA would correspond to ~30 inches of mercury in a US plane or 0 lbs of boost on the British gauge.

[Spidey002]

2 hours ago, DavviZ said:

The brits used "Boost" which was the amount of manifold pressure in Lbs (Pounds per square inch) above or below the atmospheric pressure at sea level. The US planes used manifold pressure in inches of mercury where ~30 inches of mercury would correspond to 0 lbs of boost on the British gauge. The Germans used ATA which is as previously pointed out the "amount of atmospheric pressure" in the manifold. So ~1 ATA would correspond to ~30 inches of mercury in a US plane or 0 lbs of boost on the British gauge.

Thanks for the explanation! I always wondered why I could fly my Mossie with "negative" boost, and it would still stay in the air!

[busdriver]

No disrespect towards anyone but this is going down the rabbit hole. @Spidey002 that ~30 inches of manifold pressure represents what you would see sitting in the chocks with the engine off. The manifold pressure gauge is reading the nominal barometric pressure which on a “standard day” is 29.92 inches. While that same situation would show 0 boost for a British instrumented engine. For the record, if you check the Pilot Notes for American airplanes flown by the RAF or FAA, you’ll find they refer to Boost in Inches of Hg (since that’s how the manifold pressure gauge reads).
 

In normal cruise flight you wouldn’t see negative boost. I’m scratching my cranium trying to figure that out.

 

[edit] Pulling out the Pilot Notes and thinking about this some more. I suppose you were doing some kind of max endurance profile, like holding. 

Under typical combat planning, pilots would favor maximum range airspeeds and power settings. Why? Save gas and spend less time over enemy territory. If on the other hand, I'm overhead a suitable airfield waiting for the weather to improve so I can land, then I'd be thinking max endurance. 

Edited June 1, 2023 by busdriver

[Boom]

Oh, my head is starting to hurt! 

If the throttle and boost or manifold is all controlled by the same lever, why have the separate gauges for Boost or Manifold? Why not simply operate the throttle based on what's needed eg 1/3 throttle for cruise, 2/3 climb, full for takeoff etc. An keep an eye on the rpm gauge to not over-rev the engine?

[Bremspropeller]

Boost and Manifold Pressure are the same thing - it's just a differently labelled gauge. One islabelled in psi (Boost) and one is labelled in inches of mercury (Manifold Pressure). It's basicly two countries (Britain + Empire and the USA) divided by a common language.

RPM should normally behandelled by the RPM governour, but before constant speed props (= they'll hold the RPM automatically which you're telling them to maintain constant, e.g. 2700RPM) you only had variable pitch propellers, where overspeeding could become a normal issue to look out for. You'd just control the pitch of the propeller with a switch or lever, but there was no governor to maintain a set RPM. The same is true for fixed pitch propellers, even though those were very heavily on the way out, when the war began.

For setting a specific power (e.g. economic cruise, fast cruise, combat, climb) you'd set a certain Manifold Pressure (Boost) and a certain RPM and possibly even a mixture setting (like Auto Rich or Auto Lean) and blower (supercharger) gear.

It sounds all terribly complex - and to a certain degree it is - but after all, it's just setting a value on a gauge and it becomes second nature over time.

[busdriver]

10 hours ago, Boom said:

Why not simply operate the throttle based on what's needed eg 1/3 throttle for cruise, 2/3 climb, full for takeoff etc. An keep an eye on the rpm gauge to not over-rev the engine?

That is an intelligent question. It goes back to what my mate @Bremspropeller wrote at the end of his post, "It sounds all terribly complex - and to a certain degree it is..."

Without posting an epistle (TL:DR) pilots would in fact set their throttles and airscrew/propeller controls to ballpark (rule-of-thumb) numbers for different phases of the mission. The formation leader would strive to set his parameters and leave them alone to ease the burden of wingmen flying formation. But there are several things to consider regarding which rule-of-thumb numbers to use. For example the notion of maximum range versus maximum endurance. Ballpark numbers tell us that if your clean configuration stall speed is for example 100 mph, then your max endurance is ~1.2 times 100 (120 mph) and your max range is ~1.8 times 100 (180 mph). Your stall speed varies with your gross weight and external stores (drop tanks, bombs, rockets) so your max endurance/range speed will tend to decrease as the mission progresses.

As 1GCCFPs [*tm] we don't really need to worry too terribly much about flying by the book (Pilot Notes/Flight Manual).

***1GCCFP is short for 1 g comfy chair fighter pilot, a term that I hold the trademark for from the old days over at Brand X***

Edited June 2, 2023 by busdriver

[Boom]

10 hours ago, busdriver said:

As 1GCCFPs [*tm] we don't really need to worry too terribly much about flying by the book (Pilot Notes/Flight Manual).

***1GCCFP is short for 1 g comfy chair fighter pilot, a term that I hold the trademark for from the old days over at Brand X***

Luv it busdriver. Now that makes more sense to me.