Manifold Pressure
 

I'm sure this has been posted before but :-

I am currently doing a rating on a 182 and I have the whole idea with setting the MP as per the POH for my aircraft, in the climb, cruise and being careful in the decent maintaining CHT.

What I don't understand really is the whole idea of the Manifold Pressure, why are we using it?

Sorry 'bout the dumb question.

Because with a variable pitch propeller, RPM is no longer your best indicator of engine power, since RPM can vary at constant power (or power can vary at constant RPM).

MAP is the best available indication of engine power.

G

I'll have a go at describing it in less words than the article which Bookworm has linked to!

If you leave your prop lever in the same place, the engine will always turn at the same speed. That means the same number of "suck-squeeze-bang-blow"s each second.

Therefore, the only thing which determines how much power the engine is going to generate is the amount of power in each "bang". This depends on the amount of fuel-air mixture which goes into the engine.

The manifold pressure is a way of measuring the amount of mixture going in.

Imagine an engine which isn't running. The pressure in the inlet manifold will be the same as atmospheric pressure.

Now, imagine an engine running at very low power, i.e. the throttle butterfly is almost completely closed. During the "suck" part of the cycle, the engine tries to draw air and fuel in from the inlet manifold. Because the butterfly is closed, only a very small amount of air will be able to get in. The engine sucks and sucks and sucks, and in doing so it reduces the pressure in the manifold quite a bit, as it removes air from the manifold, but the butterfly prevents the air from being replaced.

Now, open the throttle. The butterfly opens, and allows more air into the manifold, and hence more air into the engine. This means that each "bang" is bigger than it was before, and the engine can produce more power. It also means that some of the air which is being removed from the manifold can be replaced, and so the manifold pressure is a little higher than it was with the throttle closed.

As you gradually open the throttle, the manifold pressure gets closer and closer to the atmospheric pressure. It will never get as high as atmospheric pressure, because the engine will always be sucking air out of it. (Unless you have a turbo-charger or super-charger, but I assume that's not the case.) Hence, you can use this pressure as an indication of how much power the engine is generating.

Hope that makes sense!

FFF
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Genghis the Engineer

As bookwork's link says (I think), MP is actually a measure of engine torque, not power. It would be better to measure torque directly but it's awfully expensive and complicated, and measuring how hard it sucks is the next best thing.

In the end, the MP gauge is just a way to set the engine up for a particular operating point (at a given RPM, it gives you a certain % of max power, roughly speaking).

FFF,

Thanks for that. For the first time ever, I actually understand what MP is! Thank you!

dp

I am slightly concerned that someone is doing a type conversion for a complex aircraft and the instructor doesn't appear to know what one of the levers does... I know the type as I have spent £1000s with them myself, and they are a waste of time and space.

FFF

The MP isn't really a measure of engine power, not even at constant RPM, because you could lean the engine (reduce the fuel flow) while keeping the RPM constant, and the engine power would fall as you lean, until the combustion is extinguished and the engine is delivering zero power, but (for a short while at least, until the RPM starts to drop) the MP would still remain constant.

All the MP tells you is how hard the engine is sucking, and to suck it only needs to be turning; it doesn't even need to be burning any fuel!

To consider a bizzare but illuminating case, the inlet side of the engine has no knowledge of the burning/exhaust side (the inlet valves take care of that) so if you fixed the engine on a bench and rotated it with a motor at say 2400RPM, it would suck just as hard as when rotating under its own power.

Again, I am concerned that someone is being taught a plane whose engine management DOES need to be understood; you can't just fly it with the mixture all the way forward, PPL-style.

IO540,

I completely agree. And Bookworm's link goes into a lot more detail than my post, especially on the issue of how the prop speed affects the m.p.

What I've tried to do in my post is give enough information to understand the basics of how the m.p. guage works, and why it's useful. It is grossly over-simplified.

I'm not really surprised at the lack of understanding of this. I know that I wasn't taught any of it by my instructors - I've worked most of it out myself from what I already know about engines, with a bit of help from a couple of books on my bookshelf, just because I'm one of those sad geeks who likes to know how everything works. I don't really believe that it's absolutely necessary to understand it, though. As aircraft get more and more complex, flying the aircraft becomes increasingly a case of flying by numbers.

For example: the Avweb article in Bookworm's link descibes the commonly-taught practice of reducing to 25"/2500rpm shortly after take-off. I was taught this technique on an Arrow, and also used it, under the watchful eye of an instructor, on every other one of the small selection of complex (and the one twin) aircraft I've flown since. I agree with John Deakin that moving the prop lever from fully fine to 2500rpm will reduce the power and increase the m.p. (which I have just set very carefully). But I can't see how knowing this is going to improve my ability to fly the aircraft - I will still reduce the m.p. to 25", then reduce the rpm to 2500. I might then re-check the m.p. and reduce it further (but probably not, because it's going to drop off again very shortly as I climb). I know that this gives me a power setting which is safe from an engine management point of view, and gives me sufficient power to be able to climb. What more can I do with the extra knowledge of exactly what's going on in the inlet manifold?

Your thoughts?

FFF
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With the basics out of the way there is a very good discussion running in the dunnunda forums regarding mp, rpm and operating oversquare.

Multiple references to Deakin's work on Avweb and direct links.

Regards
rob

There's nothing magic about 25/25. They are just numbers. The Yak 52 is calibrated in percent RPM (not actual RPM) and manifold pressure is measured in fruit pastelles per tube of smarties or suchlike. No 25/25 there; you just have some numbers to work with from the POH, and move the levers so the engine is never operated at too high a power for the RPM.

SSD

Thanks guys, I also found the avweb article very interesting. It is so much easier to thinks about the controls since I know what it's all about.

One thing I don't agree with (on a separate article) was setting the mixture on the take off run. I prefer to set it at the runup (at high alt airports)

Nonetheless thanks again!!!!

FFF

I think the 25/25 is bunk, perhaps something out of some ancient POH of some plane some instructor used to fly 20 years previously... AND it can be dangerous!!

In general, one should climb with all 3 fully forward until the top of the climb. The reason for this is that the fuel metering unit, or the carb, is supposed to be adjusted to deliver the rated max engine power at sea level with all 3 forward, consistent with the engine operating in a safe region. This is 125F to 150F ROP. The other reason is that the combustion is cooler which is handy because the cooling airflow is less, especially if you climb at Vx :O

Then level off, trimming as required, until the target IAS/TAS is reached, and then set up the engine operating point (MP/RPM/GPH) which corresponds to that IAS. To make this really easy I have this table stuck to the dash (no idea how to format it to look right here):

Power
MP
RPM
Fuel lit/hr
IAS kt
Endurance hr
Range nm
MPG ukm/ukg

75% 24" 2400 52 147 6.1 901 14.8
65% 22" 2300 46 140 6.9 971 16.0
55% 20" 2200 40 130 7.9 1030 16.9
45% 21" 1800 32 120 9.9 1136 18.7

The obvious exception is altitude; once you get to about 5000ft the engine won't like running so rich. The question is when is it safe to lean. The standard Lycoming answer (IO540 engine and others I believe) is that you are authorised to lean to peak EGT anywhere below 75% power (**). So when the MP falls (due to altitude) to the value at which the power has fallen to 75% then you should lean for peak EGT, and that allows the climb to continue at the best power available. That MP value is in the curves (you have to buy the engine handbook separately, about US$50) but it is probably about 24". In other words, in a non-turbo case, pretty soon after takeoff!

As the climb continues, additional leaning is required to maintain peak EGT (unless you have an altitude compensated fuel metering system). By the time you are up there you will be grateful for any power at all :O

The stuff about pulling back the RPM from say 2600 to 2500 after takeoff is purely noise abatement; there is a significant prop noise drop. Of course you also lose a bit of power - if you have a Shadin or similar flowmeter you can see a slight fuel flow drop (less rpm -> less sucking -> less air -> less fuel) and since engine power comes only from burning the stuff....

One could make a case for a power reduction after takeoff: if the engine hasn't blown up on takeoff, it is less likely to blow up if you drop the power by say 5-10%. That's fair enough, but the climb does that for you anyway; you lose 5-10% within not many seconds of departing from sea level.

The danger of LEANING too soon during climb is that the detonation region is about 50F ROP, and is easily reached by leaning just a little when the engine is running at or close to max power.

Deakin has written lots about this stuff, and subject to the not always obvious fact that some of his writings are specific to certain engines and not others, it is well worth reading. Lycoming hate him - that's obvious from reading certain of their publications.

Nothing stops you simply setting 75% cruise settings (and peak EGT) right after takeoff (say 24"/2400RPM/14GPH) - except CHT going too high due to the low airflow. But if you climb at say 300fpm then the airflow will be just fine. Just make sure you clear the terrain :O

I suppose what I am getting at is that the region between 75% and 100% must be treated very carefully. (on other engines it could be a lot wider) That is where you are most likely to get damaging operating conditions, and really there is nothing to gain in playing there. Non-turbo, climbing at Vy, you are down to 75% power within a minute or two of takeoff anyway, so why risk it? The extra fuel used is negligible.

(**) this is what I believe is generally regarded as an indirect authorisation by Lyco to operate LOP. LOP is clearly not as bad a case, thermally, as peak EGT, and since peak EGT is allowed anywhere below 75%.... unfortunately lots of people, including nearly all instructors, think that LOP runs the engine hotter than peak, as if the fallacy of that wasn't self evident from the terminology :O

Here's another thread that discusses this question.

Manifold Pressure - What is it?

Love this thread: I remember finding the whole subject confusing at first too...I think its also to reduce engine wear and tear, at least that is what I have heard said at one club I fly from. Not sure though if this is b*llocks and would be interested in others views.

Andy

I totally agree, you don't want to be messing with the mixture control on take-off from a high altitude airport.

EA

Pulling the power back after t/o in something as underpowered as your average single is not the most sensible thing.
Many schools go under the mistaken belief that running at lower power settings will help engine life. Nonsense to be honest. Less fuel consumption, yes, but an engine will be knackered sooner.

Lycomings etc. were meant to be run at around 75%, but in most schools, the larger engined machines are usually used at lower settings- 21" and 23-2400rpm. You really can tell the difference between an a/c that has been used properly and one that has been run around at lower power settings when the engine gets a bit older.

All this guff about 'oversquaring' an engine is also a load of crap unless you have something like a Wright Cyclone or a Griffon/Merlin running at 50"+. Does your car blow itself because you used a large throttle opening at low-ish RPM? What relationship does RPM have to "Hg. anyway? As SSD has mentioned, would we have this discussion if we measured the MP on a differing scale? Don't think so.

SSD

I think it's difficult to draw conclusions from flying school planes and power settings, because f.s. planes are used regularly relative to all other classes. There is no real control group, i.e. planes that fly regularly but fly at say 45% power, unsuprisingly because engine/prop hours are a significant part of the direct operating cost and whether you are an owner or are renting, it actually costs more to do a given flight at 45% power than at say 65% power.

I don't think there is evidence that Lycos are "meant" to run at 75% power. It just happens to be a historically accepted cruise setting. The vast majority of f.s. planes are set to something else anyway; the only way you know you are at 75% is to have a fuel flowmeter. I have hard from people with plenty of knowledge that 65% is the best setting for long engine life. But none of this makes sense unless you have a fuel flowmeter; without it one is really only guessing.

Englishal

Yes, taking off at high altitudes (not UK) one leans for peak EGT on the ground. But, going by Lyco recommendations, that is only if taking off from an altitude where the engine would not develop 75% even on the ground.

Aussie Andy

I don't think it's obvious that wear is related simply to RPM. There are plenty of parts which are torque dependent.

IO540 said

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SSD

I think it's difficult to draw conclusions from flying school planes and power settings, because f.s. planes are used regularly relative to all other classes. There is no real control group, i.e. planes that fly ....... etc.
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IO540

It was 'Say Again Slowly' that said that, not me.

SSD

IO540 and SaS,

As far as I know, both of you are far more experienced than me, so please take my comments as requests for further discussion, and not as criticism.

In general, I agree with the comments which both of you have made regarding reducing power/rpm shortly after take-off. The bit which I don't agree with is when you say that it's "wrong" to reduce to 25/2500. There is always more than one way to skin a cat. To use any power setting which is given in the POH is fine by me, and that includes take-off power (assuming there's no restriction on it in the POH), or any lower setting which gives adequate performance, such as (in all the cases I've seen) 25/25. To push the point a little further... do either of you ever use a "cruise climb" technique? What settings would you use for this? Why can't I "cruise climb" as soon as I'm at a safe height after take-off?

FFF
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(Also, to touch very briefly on two other subjects which a couple of people have raised. Over-square is something which has been discussed before, and I think we all agreed that it is "bunk", to use IO540's word. And mixture, although a very interesting subject in itself, is only indirectly related to manifold pressure - and I think it's probably best discussed in another thread so as not to cloud the issue of manifold pressure.)

Its common as far as I'm concerned to switch to "cruise climb" at about 500' AGL (terrain dependant of course), even in a single such as a TB20 or Dakota, and especially in a twin. Normal en-route climbs would then be cruise climbs, rather than everything forward.

Typical values I use are 2500/25" in a Seminol or TB20 or in the case of a turbo charged A/C like the Seneca 2500/31", obviously easing the throttles forward as MP drops with altitude.

Agree, there is no where in the UK I'd lean before takeoff, I'm thinking along the lines of 6-7000'+ airports, like Big Bear in the US.

Cheers
EA