Manifold Pressure

Discussion in 'Aviation' started by GregP, Mar 19, 2015.

  1. GregP

    GregP Well-Known Member

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    Here's a good article on manifold pressure for general info only. Not for fighter engines, but helps with understanding.
     

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  2. Bad-Karma

    Bad-Karma Member

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    Great read, thanks!
     
  3. tomo pauk

    tomo pauk Creator of Interesting Threads

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    I'd thought it would be better to discuss pressure units here, rather that off-topic in another thread.

    The relation between units are linear, once the offset points are calculated in (for the units that have it, like british psig of Japanese +mm Hg).

    ...
    Japanese were using 'gauge pressure', or pressure over standard atmospheric pressure; the Soviets used 'absolute pressure'. Hence + with Japanese, and lack of it with Soviet units.

    47.25 in Hg cannot convert to 440 mm Hg, but to +440 mm Hg - small difference in writting that makes a world of difference in real world.
    Othervise i do congratulate to you for this calculator :)

    That would be nice.

    Soviets never used mm H2O ("mm bo" (Вода́ = water)), but always "mm of Hg column" ("mm Руть ct.", or "mm pт. ct."). One might check out the tables posted in the Engines sub forum.


    More later.
     
  4. tomo pauk

    tomo pauk Creator of Interesting Threads

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  5. GregP

    GregP Well-Known Member

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    Since I found out Soviet mm HG were different from Japanese mm HG, I updated the boost spreadsheet. Apparently Soviet mm HG is absolute pressure and Japanese mm HG is gauge pressure.

    The updated Excel file is below.

    View attachment Boost (2).zip
     
  6. Koopernic

    Koopernic Active Member

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  7. GregP

    GregP Well-Known Member

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    Since absolute pressure is VERY easily related to gauge pressure, how is it in any way superior? If you know one, you know the other.

    I just didn't know Soviet mm Hg were different from Japanese mm HG. Since they both had the same units, I incorrectly assumed they must have the same reference in the absence of any knowledge to the contrary.

    ANY unit is OK as long as you know how to convert it to another unit to which you want to compare it with. Meters aren't inherently any better than inches, feet, yards or barleycorns ... they are simply standard in more parts of the world and therefore more widely interpreted easily by the average person. I wonder how "accepted" the meter would be if "meter" meant different lengths in different countries? Perhaps there would be a clamor to "standardize?"
     
  8. Hiromachi

    Hiromachi Active Member

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    Hey Greg, this material was really interesting. Thank you for sharing it. However after reading it and than some other book I came up with a question in regard to emergency manifold pressure settings.
    As we all know in emergency situation pilots did not follow the restrictions and were overcoming them, increasing the RPM and Manifold Pressure to the engine limits to gain advantage or run away.
    In case of later American aircraft I understood it was achieved mostly with usage of water injections, however there was also on lower settings a so called dry WEP.

    Russians had in some of their aircraft like La-5 a button or lever to overcome engine limits.
    Same was in fact for the Japanese, let me quote the Robert C. Mikesh publication, "Japanese Aircraft Equipment 1940-1945". On page 40 there is a description of the "Emergency Boost Control" which is explained as :
    " Sometimes referred to as a Manifold Pressure Control, Emergency Power Boost, Supercharger Boost Regulator, Automatic Booster Regulator, and other combinations, depending upon the translator, this unit is used in Japanese aircraft in the high power and performance range. To avoid conflicting terms that all refer to this device, "Emergency Boost Control" will be used. This device should not be confused with the Blower or Supercharger Shift Control, which is worthy of a few words of comment.
    Engines of higher horsepower had superchargers that were geared to rotate at higher rations than the normal blower impellers attached to the crank case at the read of the engine. These air pumps, if you will, forced more fuel-air mixture into the induction system than was able to be pumped in by the pistons in the engine cylinders, often called increasing boost pressure. This greater volume of air mixed with fuel provided more power to the engine. These superchargers, or blowers, often had two speeds, with a lever or knob for manually shifting the clutch mechanism. The higher speed was beneficial at higher altitudes, where the air is thinner and more air was needed.

    The Emergency Boost Control, on the other hand, provided a safeguard against over-boost pressure that could easily damage the engine, "blowing a jug" or cylinder being the common term. This activating lever, which was located in the vicinity of the throttle lever - often below or to the rear of it - was normally in the down or back position (This was a pull-out knob on the instrument panel of the Zero, Paul, and other examples). It functioned through a servo-motor connected to an aneroid near the carburetor. Regardless of how far the throttle was pushed forward, this aneroid would measure the over-pressure limit and prevent the carburetor throttle valve from opening beyond a rated altitude limit for respective engine (For example, the valve setting was plus 25 cm for Zero). With change in altitude, this aneroid would automatically adjust the carburetor for maintaining that limit. This allowed the pilot to concentrate on air combat where maximum power was the norm, and not have to be concerned about inadvertently over-boosting the engine.

    In emergency situation such as in combat, more power could make the difference between victory or defeat. By moving the Emergency Boost Control lever forward, up, or pulling a handle, the automatically controlled restriction limit was raised (to plus 35 cm for the Zero), and it was then up to pilot to gamble on the maximum power to apply as viewed on the manifold pressure gauge, and hope that the engine would not fail because of over-boost."

    Since in Planes of Fame you have original Zero with original engine I'm sure there is that Overboost lever in yours too :
    RoA0cp.png

    Did you ever try to use a higher settings for your Zero ? I couldn't come up with something and was wondering what is the practical difference in MP/RPM with throttle wide open and maximum rpm on your Zero with the Overboost lever pulled open and without it.
     
  9. Koopernic

    Koopernic Active Member

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    #9 Koopernic, May 17, 2015
    Last edited: May 17, 2015
    The 'original' Spitfire Merlin engine produced about 1030hp at 6psig (pounds per square inch guage with the introduction of 100 octane the 'boost' went to 12 psig and the power to 1310hp. Assuming one atmosphere is 14.2 psi the calculation that would give us power increase is

    12psig + 14.2psi / 6psig+14.2 x 1030 = 1335hp. (which is close to the actual of 1310) These figures are from the Merlins maximum power.

    For a DB605A engine the calculation would be something like this the engine was producing 1300ps at 1.30ATA but when boosted to 1.42ATA it was 1475ps. These figures are from the engines sea level performance. (At 2000m the engines power was more like 1530 metric horse power.

    The calculation is simply 1.42/1.3 x 1310 = 1430. This is simpler.

    The equation is complicated by the fact that permissible engine RPM concurrently went from 2600 to 2800 which is a 7.6% increase the same time. This was due to improvement is spark plugs, an oil deaerating centrifuge that greatly improved bearing life and the introduction of a relatively sophisticated engine RPM and pitch control " computer" that worked even in dives and seems to have given the engineers the confidence to push to the limits a little further since automatic control was gentler on the engine and stuck to the limits more reliably.
     
  10. GregP

    GregP Well-Known Member

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    Hi Hiromacci,

    Our Zero flies for a Museum. I don't even have to ask ... we never use any extra boost over and above normal settings and very rarely get to full power. Sometimes we may get to full power ... my guess is whenever Steve wants to, but mostly our planes are operated at cruise power for good economy and we takeoff at whatever boost Steve Hinton and Kevin Ekdidge determine to be best.

    The Zero's engine limits are known to Steve Hinton, Kevin Eldridge, and John Maloney. If I am not mistaken, these are the only people who currently fly it. There is only one Sakae 31 flkying in the world and it gets somewhat pampered since the supply of spare parts is limited to our second Sakae 31 engine. Our second engine is complete, but is in pieces (unassembled).

    To put that into perspective with the P-51's, our P-51's usually take off at no more than 900 - 1,100 HP and then throttle back for initial climb out. I have been in 3 private P-51's and all climbed out at about 2,500 feet per minute or slightly more, which is probably 900 - 1,100 HP. Since we aren't in a war and do not have access to even 130+ Octane fuel (much less 150 Octane fuel), NOBODY can run them at WWII power levels or even wants to. After all, WE are paying for the fuel, maintinance, and any breakage.
     
  11. Hiromachi

    Hiromachi Active Member

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    I expected that, after all this bird is preserved for airshows so the next generations can see it. This is absolutely marvelous job you do gentleman :)
    Maybe than it would be possible to ask Mr. Hinton what readings is he getting for that mentioned "above normal settings" or "full power" ?
     
  12. gumbyk

    gumbyk Well-Known Member

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    From a practical point of view, and I fly aircraft with different units of measurement, I don't care what units the gauge is calibrated in, or whether it is gauge pressure or absolute. (I couldn't even tell you whether the CJ-6 gauge or absolute) All I need to know is what figures I need to set, and what approximate numbers I should see.
    The only time it matters is when trying to compare engines, and you've got plenty of time to convert, which isn't really that hard anyway.
     
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  13. cherry blossom

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    Clearly it might not be a good idea to run our only working Sakae at full power. However, hesitantly as I am not an engineer, would it be a good idea to fix accelerometers to the motor and obtain vibrational spectra under the various safe conditions for running it on the ground? The spectra might not tell you anything that a skilled mechanic could not hear or feel but it would be computer readable and it would be a permanent record in case a part were to fail in future. Vibrations might at least show how evenly the charge is distributed amongst the cylinders. A good computer model of the Sakae would enable the power under all conditions to be calculated and prediction of the vibrational spectra would be one test that the model was accurate. We might even be able to see if a particular vibrational mode prevented Nakajima from pushing the Sakae's output any further.

    Any comments from anyone who understands piston engines, especially modern design practises?
     
  14. GregP

    GregP Well-Known Member

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    The Sakae has a certin way it runs. Only 3 pilots fly it and all agree to flag anything out of the ordinary. We're not talking about low-time warbird pilots, we're talking about Steve Hinton, Kevin Eldridge, and John Maloney, three of the best warbird pilots in the world.

    They have a "feel" for planes that most test pilots would kill for. If they feel a vibration that wasn't there last time, they check the prop for nicks, dress it is necessary and get to the bottom of it if it isn't the prop. None of them are given to accepting changes in feel without investigation of same. If the source isn't dangerous, it may or may not get fixed immediately, but anything that affects airworthiness is addressed.
     
  15. GregP

    GregP Well-Known Member

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    #15 GregP, May 17, 2015
    Last edited: May 17, 2015
    Hi gunbyk,

    The CJ is a nice warbird. I love tghe feel of heim joint in the alerons and elevator.

    If I were betting, I'd bet it was absolute just because we found out Russian mm HG were absoulte in WWII. Countries typically don't tend to change neasurement references without some pretty big shakeup.

    I know the M-14 likes to be shut down in a very specific sequence, but it is a wonderful engine. I have a friend who flies a Yak-52 and loves it. I think the systems in the Yak are a bit easier to access than in a CJ, but neither is exactly easy to work on, is it? and troubleshooting the ignition system is "interesting."
     
  16. gumbyk

    gumbyk Well-Known Member

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    We've got the original engine in ours, the Housai 6a. the more I think about it, the more I'm inclined to say its absolute too. Take-off is about 810mmHg.
    Luckily, I've never had to work on it (besides helping to paint strip it for a repaint), but I've never heard anyone complain about working on one, and from what I've heard they are better than a Yak.
    I've never had any qualms about flying behind the Chinese engine. Once they are set up right and you have an engineer who knows their quirks, then they are a good reliable engine.
    Shut-down is 30 seconds @ 800RPM, and then shut off the ignition and advance the throttle, as it doesn't have an idle cut-off. Varies a bit from most modern aircraft, but the other machine I fly is a Tiger Moth, which has the same shut-down sequence. I'm not sure how this compares with the M-14 shutdown sequence.
     
  17. GregP

    GregP Well-Known Member

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    I thought the originals were mostly Ivchenkos or Zhouzhou HS6A's of about 285 HP US. My friend Robin knows the procedure for the M-14, but suffice to say, following the book usually give best results.

    810 mm HG in absolute boost is 32 inches of Mercury. If your engine is not supercharged or has very mild boost, that sounds about right. 810 mm HG in gauge boost or about 62 inches of Mercury and is not all that far from what they run at Reno.

    I'd bet on absolute boost for your CJ-6. The engine you name above is a version of the 14 family, so Robin's preocedure are likely about the same. His is a nominal 360 HP, but the Russians usually quote the low end of the power range, and his makes abit more.

    Perhaps the difference between his engine and yours is a supercharger ... I'm not too sure. Not being an owner or flying one very often, I haven't really looked into it.
     
  18. gumbyk

    gumbyk Well-Known Member

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    Yeah, that's the same engine, HS stand for Housai AFAIK.
    There is quite a bit of difference between the two engines. 9 cylinder radial and physical size is about where the similarity ends. Gear ratios are different, and I don't think there are any interchangeable parts. A bit like the airframe, which started out as a license built Yak 18T, but then developed into a different machine.

    The HS6a is more expensive too! Last quote I heard was about $115,000. Pretty expensive when they have an official TBO of only 600 hours!
     
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