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# Manifold pressure - Altitude effects

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# Manifold pressure - Altitude effects

6th Feb 2012, 15:07

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Maybe the question was not as well made as I first thought, but two of the constraints in the OP were constant fuel flow and constant RPM. Therefore I would suggest that pumping losses - changed though they may be - do not explain the need to adjust MAP whilst maintaining constant fuel flow.

Lycoming advise this on power setting:

"to maintain constant power, correct manifold pressure by approx 0.18"Hg for every 10F variation in induction air temperature from standard altitude temperature. Subtract manifold pressure for temperature below standard."

That sounds like a density correction to me.
6th Feb 2012, 17:19

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Well the OP wrote:

So, if at sea level, with 65% power set at 23"/2400RPM, and 20GPH -- what do I need to set the manifold to if I wanted to keep 65% power (and 20GPH) with 2400RPM at 8,000'?
The constraints don't actually make sense, as 20 GPH at 8000 ft with an equivalent mixture setting will give more power than 20 GPH at sea level. If you allow the mixture setting to be varied, to maintain 20 GPH by enriching the mixture, then there's a continuum of MP vs mixture settings that will meet those constraints.
10th Mar 2012, 18:12

Join Date: Jun 2010
Age: 32
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Apologies for taking so long to reply!

Here is a 2 page copy from the Jepp JAA book on Powerplants.

Powerplant.pdf - File Shared from Box - Free Online File Storage

It explains how the decrease in temperature (with a constant MAP) and the reduced atmospheric pressure result in more power if a constant MAP and RPM are kept when at higher altitudes.

The constraints don't actually make sense, as 20 GPH at 8000 ft with an equivalent mixture setting will give more power than 20 GPH at sea level. If you allow the mixture setting to be varied, to maintain 20 GPH by enriching the mixture, then there's a continuum of MP vs mixture settings that will meet those constraints.
bookworm... 20 GPH is the mixture setting! Creating a specific power (ie. 45% 65% 75%, etc) requires a specific fuel flow for each. If you create 65% power at 2000', 4000', 12000', etc you will be burning the same fuel at each altitude. You will have a higher TAS at altitude because of less drag due to reduced density. I'm not sure how the constraints on my original post don't make sense.
10th Mar 2012, 21:25

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bookworm... 20 GPH is the mixture setting!
No, it's not the mixture setting, it's the fuel flow. While in many circumstances the fuel flow is a reasonable surrogate, it isn't here.

Creating a specific power (ie. 45% 65% 75%, etc) requires a specific fuel flow for each. If you create 65% power at 2000', 4000', 12000', etc you will be burning the same fuel at each altitude.
Not so. The pumping losses, or exhaust backpressure as it puts it in your extract on the second page, is a real effect that changes the efficiency of the engine. At higher altitudes, you get more power for the same fuel.

I'm not sure how the constraints on my original post don't make sense.
But I now agree with you on this, I think. The constraints are OK, they're just hard to deal with.

Start with "at sea level, with 65% power set at 23"/2400RPM, and 20 GPH" and climb. At 8000 ft there are two effects that change the power to give you more than 65%: one is the lower temperature, which means that more air is going in at the same MP. The other is the lower exhaust backpressure, which means that the engine is more efficient.

So you can reduce the power in two ways: you can reduce the MP to maintain 65% power, but that will reduce the fuel flow below 20 GPH. So to restore it to 20 GPH, you creep the mixture control forward at the same time, to make it less efficient (assume we're on the rich side of peak). That will increase the power, so you keep reducing the MP and enrichening the mixture until you get that 65% power and 20 GPH. You're correct in that only one combination of mixture and throttle will give both 65% power and 20 GPH. But it's not easy to calculate what the corresponding MP is.
10th Mar 2012, 22:35

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No, it's not the mixture setting, it's the fuel flow. While in many circumstances the fuel flow is a reasonable surrogate, it isn't here.
Sorry, I'm not sure why I just wrote that! haha. Of course it isn't the mixture setting as mixture is a fuel/air ratio.

Not so. The pumping losses, or exhaust backpressure as it puts it in your extract on the second page, is a real effect that changes the efficiency of the engine. At higher altitudes, you get more power for the same fuel.
If you're feeding your engine a certain amount of fuel, you will be able to do a certain amount of work due to the energy contained in the fuel. When the engine is creating 65% power on the ground, it is putting say 50% power to the propeller and 15% for pumping losses. Those number are made up. When you go to 8000', if you're at the same fuel flow (20GPH) and mixture (0.08), you will create the same power. Now, two things to discuss:

1) Due to the decreased temperature, keeping the same manifold pressure will provide increased density air to the engine. Depending where the mixture setting currently is, that could create more power or less power. By reducing the MAP, you create the same power from the quantity of fuel.

2) Due to decreased back pressure at altitude, there are less pumping losses, resulting in more power available for the propeller. So now, it would be say 55% power to the propeller and 10% for pumping losses.

The following two reasons contribute to a higher TAS at altitude in this situation. 1) You are pushing more power to the propeller which is able to take a bigger bite of the air. 2) The decreased atmospheric density at altitude will create less drag for a given TAS.

The manufacturer could have chosen to relate the charts to what the actual power going to the propeller is but relating it to the quantity of fuel the pilot/hard-working-owner is pumping into the engine is probably easier to calculate and simpler for the pilot to understand.

So you can reduce the power in two ways: you can reduce the MP to maintain 65% power, but that will reduce the fuel flow below 20 GPH.
I don't believe so. Say, at SL, 20GPH, 0.08 fuel/air and 20" MAP you create 65% power. If at 8000', 0.08 fuel/air and 20" MAP you will be creating more power and to do that, you need more fuel flow. Both the power and fuel flow will be higher in that case. You could lean it back to 20GPH or lower, but that would change the fuel/air mixture. To get back to 65% power, you adjust the throttle (decreasing it to lower the MAP) and adjust the mixture so that when the mixture is equal to the one at SL, the fuel flow also equals 20GPH. At 8000' it would look something like this: 20GPH, 0.08 fuel/air, 19.1" MAP and 65% power.

But it's not easy to calculate what the corresponding MP is.
That's why the manufacturer's do it for us and put it on nice charts!

Last edited by italia458; 11th Mar 2012 at 00:42. Reason: Wording
11th Mar 2012, 11:16

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As pointed out earlier, lower ambient temperature does not necessarily mean the temperature in the manifold (which is what matters) is any lower if the MAP is the same. It all comes down to the atmospheric lapse rate.

Fuel used does not equal fuel used in combustion.
11th Mar 2012, 12:26

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If you're feeding your engine a certain amount of fuel, you will be able to do a certain amount of work due to the energy contained in the fuel. When the engine is creating 65% power on the ground, it is putting say 50% power to the propeller and 15% for pumping losses. Those number are made up. When you go to 8000', if you're at the same fuel flow (20GPH) and mixture (0.08), you will create the same power....So now, it would be say 55% power to the propeller and 10% for pumping losses.
So I think this comes down to interpretation of what we mean by the "same power". If we're talking about "brake horse power", the pumping losses must be taken into account before measuring the power -- it's power at the brake, after all. But you're saying that the constraint is that the nominal power, before taking into account such losses, is the same at altitude as at sea level.

If you look at a typical engine nomogram (as Brian Abraham posted earlier in the thread), the process of finding the (brake horse) power consists of 4 steps:

1) Find the power at full-throttle altitude
2) Find the power at sea-level
3) Interpolate between the two
4) Then correct for non-standard temperature as a density correction formula

That implies to me that steps 1 to 3 include both a correction for temperature in the ISA and for pumping losses. If the latter were not included, why bother with the complex nomogram? You'd just apply the density correction formula as in step 4.

What is not clear to me is how to work out fuel consumption as altitude changes. Lycoming, for example, publishes a chart of fuel consumption as a function of "actual brake horsepower", RPM and "mixture strength" (i.e. a line for best power and a line for best economy). I assume that this is measured at sea level. No guidance seems to be given as to how that varies with altitude. The temperature will not affect the fuel consumption for a given BHP, since denser air just means more fuel for the same mixture setting. But the pumping efficiency will mean that more BHP are produced for the same fuel at altitude compared to sea level.
11th Mar 2012, 22:10

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As pointed out earlier, lower ambient temperature does not necessarily mean the temperature in the manifold (which is what matters) is any lower if the MAP is the same. It all comes down to the atmospheric lapse rate.
Fred, I read over the thread again and I understand what you're saying. It makes sense that if you bring it back to the same MAP, then the temperature should be the same. And like you said, there could be a possibility of a difference in lapse rate but I agree, if there is a difference, it would be small. Not sure why I didn't see it before. If we're all on the same page, so far it seems that a reduced back pressure at altitude is the only reason for the increased power or lower MAP required?

It seems then that the JAA book is wrong on that matter?

Fuel used does not equal fuel used in combustion.
Can you expand on that. I think under most conditions the fuel would be completely burnt, however, not always inside the cylinder and not always providing an increase in power.
14th Mar 2012, 12:08

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italia:

Fred, I read over the thread again and I understand what you're saying. It makes sense that if you bring it back to the same MAP, then the temperature should be the same.
I wouldn't be so quick to back-track. The question is; if you take an MAP at sea level, and then climb to 'x' grand and set the same MAP, will the air density in the manifold be the same as before? It won’t be unless by chance the inducted air is heated by just the right amount.

The environmental lapse rate is definitely not adiabatic, whilst the process of inducting air is not strictly adiabatic either: they are different processes that exchange energy with their surroundings at a different rate per unit pressure change, are they not? ft decided “the ideal gas law seems to be valid to use for the lapse rate” by using your figures that were already derived from the standard lapse rate! I thought you'd spotted that little error.

Merely altering the throttle to set the same manifold pressure as before is unlikely to result in the same density as before. To my mind that is a very uncontroversial concept that either someone or something has to account for if our performance data is to be valid.

If we're all on the same page, so far it seems that a reduced back pressure at altitude is the only reason for the increased power or lower MAP required? It seems then that the JAA book is wrong on that matter?

It doesn’t seem that way to me. That JAA book is certainly correct if taken in context. I’ve got the same thing in two different sets of official study material. Variation of exhaust back pressure and change of density in the manifold are both going on at the same time along with variation in throttle losses - the real variation in pumping losses.

I would certainly be hesitant to question credible texts on the basis of various ramblings in this thread.

Last edited by oggers; 14th Mar 2012 at 18:56.
14th Mar 2012, 21:34

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I'm not sure any of us really have a good idea of what is happening or else we would have this solved by now! I am entertaining the thought that it could be different from what I initially thought.

I understand ELR to be the actual lapse rate that you would measure. I do agree that it varies with altitude but if we assume that the lapse rate is actually constant all the way up to the tropopause, what else would change? Dewpoint is also changing - usually dropping, which means that the quantity of water per cubic metre in the air is dropping. But is the quantity of water per cubic metre dropping because of the expansion - or is it something else? If it was exactly because of expansion, then the water quantity per cubic metre should remain constant at the same absolute pressure, thus not affecting the density.

to maintain constant power, correct manifold pressure by approx 0.18"Hg for every 10F variation in induction air temperature from standard altitude temperature. Subtract manifold pressure for temperature below standard.
The charts would be based on a standard atmosphere so this would correct for a non-standard atmosphere. Assuming the lapse rate remains constant (and all other factors constant), wouldn’t ft be correct in saying that if you keep the same absolute pressure, the temperature should be the same?

ft decided “the ideal gas law seems to be valid to use for the lapse rate” by using your figures that were already derived from the standard lapse rate! I thought you'd spotted that little error.
I didn't really show he was wrong, I just said that it didn't necessarily prove something. The standard lapse rate is based on that ideal gas law and that's what ft showed.

I'm no expert with this stuff, but I don't believe air moves 100% adiabatically in the atmosphere and I don't believe air will go from the dry adiabatic lapse rate, immediately to the saturated (moist/wet) adiabatic lapse rate as soon as any moisture is present. If you want examples, look at a METAR and then calculate the cloud base based on the dry adiabatic lapse rate and dewpoint lapse rate (2.5/1000') and you'll find that most of the time the cloud bases will be different than calculated. I’m currently studying meteorology in more depth now so I’m hoping to find more answers that might help answer this MAP scenario. There are articles out there that will explain why the SALR is not actually constant.

I would certainly be hesitant to question credible texts on the basis of various ramblings in this thread.
I'm not hesitant to question texts - they're only credible if they can stand up to questioning and prove they are correct. However, I am hesitant to discredit what a 'credible' text says. I do think the JAA books are credible from what I've seen so far...
15th Mar 2012, 12:29

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italia:

I'm not sure any of us really have a good idea of what is happening or else we would have this solved by now!
Engineers and meteorologists have solved this. There is a variation in density (even for a given MAP), back pressure and throttle losses all going on. The principles are well described. You had the density bit right to begin with, it's just that you hadn't considered the other two things at that stage.

The standard lapse rate is based on that ideal gas law and that's what ft showed.
No. What ft showed was that if you take pressure and density from sea level and apply the ISA derived correction for a given altitude (which accounts for gravity, radiation, convection, humidity etc), when you put them back into the gas law, lo and behold you can get the ISA correction for temperature at that altitude!

As I said before: the environmental lapse rate and the inlet tract are different processes that exchange energy with their surroundings at a different rate. It doesn't matter whether you use standard conditions or real time conditions.
15th Mar 2012, 19:43

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Engineers and meteorologists have solved this.
I meant the people in this thread!

There is a variation in density (even for a given MAP), back pressure and throttle losses all going on. The principles are well described.
Where are these principles 'well described'? If you know, please share!

You had the density bit right to begin with, it's just that you hadn't considered the other two things at that stage.
What are the two other things I had to consider, other than the density?

No. What ft showed was that if you take pressure and density from sea level and apply the ISA derived correction for a given altitude (which accounts for gravity, radiation, convection, humidity etc), when you put them back into the gas law, lo and behold you can get the ISA correction for temperature at that altitude!
Do you realize that's what I said? The 'ISA derived correction for a given altitude (which accounts for gravity, radiation, convection, humidity etc)', as you stated, would be the pressure and density at that altitude. If you plug that pressure and density change from SL into the ideal gas law, you'll get the temperature change at that altitude. You just expanded what I had said.
16th Mar 2012, 13:34

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italia:

Do you realize that's what I said?
Er, that'll be why I wrote "I thought you'd spotted that little error" and "I wouldn't be so quick to back-track". The whole point is that since then you have changed position and said that ft's post made sense after all, and you are now questioning your own textbooks.

I meant the people in this thread!
Yes I realise that. The point I was making is that the understanding is already out there, including the reference you yourself gave. Meanwhile, two people in this thread - and you are one - are attempting to reinvent the wheel on this density topic to the extent that you are now questioning your own sources of knowledge.

The ref you linked to explains succinctly the change of density at constant MAP as one climbs. I'm at a loss to understand why you can't accept the explanation as it makes perfect sense. When one goes to altitude, for a given ambient pressure the density is higher. Ergo, when one goes to altitude, for a given MAP the density is higher.

What doesn't make sense is ft's hypothesis that if you set an MAP the density will also be set at any altitude.

So before we go any further can you just confirm which of those two makes sense to you? Because so far you have swapped between both.
18th Mar 2012, 04:56

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The ref you linked to explains succinctly the change of density at constant MAP as one climbs. I'm at a loss to understand why you can't accept the explanation as it makes perfect sense. When one goes to altitude, for a given ambient pressure the density is higher. Ergo, when one goes to altitude, for a given MAP the density is higher.
Succinctly, yes but in exhaustive detail, no. just because something makes sense, does not mean that it is correct. I have seen countless times, things that are explained and 'make sense' but are either misleading or outright wrong.

I can take either stance. If I make assumptions, I can 'prove' either side. That should be obvious after reading this thread. However, it's also obvious on here that none of us really know what the answer is. You might actually have the right answer, oggers, but there isn't enough info presented here to definitely say which answer on here, if any, is correct.

My goal is to find the correct answer. If it seems that my initial 'guess' was wrong, I don't care. I'd rather find the correct answer than hold on relentlessly to my initial 'guess' and know I might not be correct.

You still didn't answer my question about where the principles are 'well described'. Or that other question I asked. Can you answer those two questions before we go on?
26th Mar 2012, 12:19

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italia:

The reference you gave is correct on this. It's very uncontroversial. It requires only that you read it and understand it.

OTOH the opposing view being pushed by yourself and ft whereby the air density in the manifold will be the same at any altitude as long as you set the same MAP, is nonsense that violates one or both of the following ground truths:

a) From basic meteorology; that the atmosphere exchanges energy with its surroundings - ie ground and space - and the standard ELR is therefore not adiabatic.

b) From basic common sense; that air passing into a manifold will not (except by coincidence) exchange energy with the hot engine parts at the same rate as either the standard, or actual, environmental lapse rate - no matter where you put the throttle.

-----

I can take either stance. If I make assumptions, I can 'prove' either side. That should be obvious after reading this thread
Well, you certainly haven't proved this one:

so far it seems that a reduced back pressure at altitude is the only reason for the increased power or lower MAP required? It seems then that the JAA book is wrong on that matter?
...
26th Mar 2012, 16:04

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I don't think this conversation is helping answer any questions.

From basic common sense
Sometimes 'basic common sense' can lead you in the wrong direction. I've seen it many times. I don't disagree with what you're saying, however, I don't see definitive evidence either way. I think your definition of 'definitive evidence' is different than mine. Try and find a scientific report that has 'basic common sense' written somewhere in it.

Well, you certainly haven't proved this one:

so far it seems that a reduced back pressure at altitude is the only reason for the increased power or lower MAP required? It seems then that the JAA book is wrong on that matter?
I put question marks there for a reason.
26th Mar 2012, 16:23

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What a lot of waffle.

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