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Old 06-29-2020, 10:37 AM   #1
Pikey
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Post Easy reading guide on Manifold pressure

A great thing about the WW2 forums is the passion, enthusiasm and collective knowledge of the readers here.

This can make things quite intimidating to ask basic questions in, after all, its not a mechanics forum or an aeronatutical forum, but a Flight sim forum. It's just that these things get wrapped up together and the knowledge begins to overlap.

Back in the day my Dad enthused about engines and mechanics and I ignored him and played with computers...the generation gap split right there. About all I know about the internal workings of the combustion engine is that pistons go up and down moving a drive shaft with a propellor on the end. After that I'm lost, I don't understand manifold pressure or its relationship with prop pitch/RPM, very well. Not confidently at least. And it's always an interesting thing that, whilst a car has a throttle, why do planes have all these extra "go-faster" levers? . Some of you are laughing, others are nodding, right? OK you are all laughing... Penny form the Big Bang theory is finally in the ED forums.

But I found a dummies guide for normally aspirated engines and thought I'd share it. I found it helpful. I'm sure im not the only person who would find it useful.

If you have related "dummies" guides that work on Penny's level, please share below, ask below, argue intelligently below etc.

https://www.avweb.com/features_old/p...ressure-sucks/
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Old 06-30-2020, 05:35 PM   #2
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Thank you for the link.
Yor are not the only Penny.
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Old 07-02-2020, 04:21 AM   #3
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Excellent, Pikey! This is a great place to start the discussion.

Supercharged engines are a little different, which is how you end up with 52" of MP at takeoff with the P-47.

The Spitfire's boost gauge works on PSI rather than inches of mercury. Zero boost there at sea level is 14.7 PSI. When you push the throttle to +8 boost for takeoff, the manifold pressure is 8 PSI above ambient pressure, or 22.7 PSI. Measured in inches of mercury, that would be 46.2" (PSI x 2.036 = inches HG).
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Old 07-02-2020, 05:38 AM   #4
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Quote:
Originally Posted by Yoda967 View Post
Excellent, Pikey! This is a great place to start the discussion.

Supercharged engines are a little different, which is how you end up with 52" of MP at takeoff with the P-47.

The Spitfire's boost gauge works on PSI rather than inches of mercury. Zero boost there at sea level is 14.7 PSI. When you push the throttle to +8 boost for takeoff, the manifold pressure is 8 PSI above ambient pressure, or 22.7 PSI. Measured in inches of mercury, that would be 46.2" (PSI x 2.036 = inches HG).
I don't know but p-51 and spitfire has supercharged engines too.
I use +18 boost for take off in spitfire which is 67 inHg MAP. Im curious why Spitfire is allowed to take off at 67" and P-51 with pretty same engine only 61".
Spitfire boost gauge is absolute pressure sensor just like in P-51.
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Old 07-02-2020, 09:35 AM   #5
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Quote:
Originally Posted by Yoda967 View Post
Excellent, Pikey! This is a great place to start the discussion.

Supercharged engines are a little different, which is how you end up with 52" of MP at takeoff with the P-47.

The Spitfire's boost gauge works on PSI rather than inches of mercury. Zero boost there at sea level is 14.7 PSI. When you push the throttle to +8 boost for takeoff, the manifold pressure is 8 PSI above ambient pressure, or 22.7 PSI. Measured in inches of mercury, that would be 46.2" (PSI x 2.036 = inches HG).
Oh man, I always wanted to know about this and the Spitfire but was too embarrassed to ask!! thanks!
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Old 07-02-2020, 12:02 PM   #6
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Quote:
Originally Posted by grafspee View Post
I don't know but p-51 and spitfire has supercharged engines too.
I use +18 boost for take off in spitfire which is 67 inHg MAP. Im curious why Spitfire is allowed to take off at 67" and P-51 with pretty same engine only 61".
Spitfire boost gauge is absolute pressure sensor just like in P-51.
Didn't say they don't have supercharged engines. As per Pikey's linked article, a normally aspirated engine won't produce MP above ambient air pressure, and without a supercharger on the Spitfire's Merlin, max boost will be 0 at sea level. Any engine giving you more than 29.9" Hg/0 PSI has to have some device which raises the air pressure in the intake manifold.

The Spitfire is NOT allowed to take off at +18 boost -- it's placarded for +12 boost max takeoff power up to 1000 feet, and standard takeoff power is +8 boost. MP in PSI decreases by .5 PSI per thousand feet elevation, so if field elevation is 2,000 feet, your max power takeoff would be +11.5 boost.

My guess is that unless you're using 100% takeoff assist, you've been having some trouble holding a straight line during takeoff. At +18 boost, you're bound to have torque swings.
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Old 07-02-2020, 12:12 PM   #7
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The Spitfire is NOT allowed to take off at +18 boost -- it's placarded for +12 boost max takeoff power, and standard takeoff power is +8 boost.

My guess is that you've been having some trouble holding a straight line during takeoff. At +18 boost, you're bound to have torque swings.
It is allowed to take off with 18 lbs. Absolutely no problems with holding straight line. I can do it all day
Merlin 66 was allowed 18 lbs take off.
Pls no take off assist. I will record and post if you don't believe
Older engines were limited to 12lbs for take off and 15lbs for emergency but merlin 66 70 and 266 got 18 for take off and emergency power.
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Old 07-02-2020, 01:10 PM   #8
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Great link sir, thank you!

Fortunately I grew up with Grandparents that let me disassemble their lawnmowers, and my neighbor had a nasty little 67 Camaro with a 327 Small Block that he would let me tinker on. At 13 years old, I bought a Chevy 327 block from a junk yard, built a stand for it, and tore it apart and "rebuilt" it many times over.

However, when the P-47 quadrant first dropped, the first thing I did was get out my old vacuum gauges, take some readings, put them on a hand held pump and think things through. The P-47 has at least 4 things (outside of altitude) that will affect MP, and it is a real thought experiment to think through various settings and what will happen to MP.

This article is a great refresher, and keeps the concepts simple before layering up with all the things that can cause a P-47 to hit 70 inches or more!
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Old 07-02-2020, 01:51 PM   #9
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. MP in PSI decreases by .5 PSI per thousand feet elevation, so if field elevation is 2,000 feet, your max power takeoff would be +11.5 boost.
It does not matter what elevation of airfield is. Spitfire boost gauge is AP sensor so take off boost will be always the same is it 18 or (12 for early merlins) it wont change. Limits stay the same no matter of airfield elevation.
Only limitation is, the supercharger wont be able to provide MAP level if airfield is very high.
Scale of this boost gauge is calibrated for SL, so at SL it will show actual boost, but at higher alt it will not show the truth. Higher plane go, higher boost is provided to keep power constant. So manifold pressure gauges are AP sensors,
Of if we understand Boost the same way, for me it is difference between MAP and ambient pressure.
Just to not force pilot to calculate allowable boost for take off, that would be so stupid.
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Old 07-02-2020, 02:56 PM   #10
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Grafspee,

I'm more concerned about contemporary info than what can be done in the sim.

This link to the testing done in 1943 on the LF Mk IX specifically says the limitation that applied was +12 boost at 3000 RPM.

http://www.spitfireperformance.com/bs543.html

The oft-maligned Pilot's Notes for the Mk IX (from 1946) also state that at training and normal loads, takeoff power is set to between +7 and +9 boost, with power increased after takeoff to +12.

I admit I may be wrong about the +11.5 boost, though I would question why the need to placard the engine limitation as applying up to 1000 feet if there's no expected difference above that.
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