Manifold Pressure

[photofly]

Here's a really good page on Manifold Pressure:
http://www.avweb.com/news/pelican/Pelic ... 081-1.html

[danishroy]

i would be interested to hear from the OP if they got, as my kids used to say...to much information....

I think I do understand it but i'm still trying to understand what happens in different scenarios. Like at full throttle the manifold pressure should be 1 inch below static pressure. So lets say static is 26 inches so at full throttle for take off it should read around 25 inches. But what would the pressure be at idle? would it be higher than 26 or lower? and what about during cruise? I got a question wrong which said: During cruise flight you increase the RPM to redline what would happen? The answer was it would drop sharply? I do not understand why it drops sharply?

[photofly]

Like at full throttle the manifold pressure should be 1 inch below static pressure. So lets say static is 26 inches so at full throttle for take off it should read around 25 inches. But what would the pressure be at idle?

Very low, because at idle the engine is sucking hard against a nearly-closed throttle plate.

I got a question wrong which said: During cruise flight you increase the RPM to redline what would happen? The answer was it would drop sharply? I do not understand why it drops sharply?

Because the engine is an air pump, trying to suck air through the throttle. If you increase the RPM without moving the throttle plate the engine merely sucks harder, so the pressure in the manifold drops.

Read carefully the page by John Deakin to which I posted a link in the previous post. All your questions are answered very clearly there, with diagrams.

[eh3fifty]

I'm back! haha

Well that was an interesting read.

I think PilotDAR and photofly are both correct. Except I really gotta side with photofly here. He's spot on and it's important to understand that the power of the engine is affected by manifold pressure, RPM, and fuel flow (plus temperature but we don't really get to have much control of that with a fuel injected engine).

With the mixture at the recommended setting, and the RPM at a specified value, a limit on the power of the engine can be set by stating a maximum value of the manifold pressure. Also, at that particular mixture setting and RPM, power will vary with manifold pressure in a consistent manner. All of this is good to know, PilotDAR.

You're both correct - except, I think PilotDAR is wrong to say that photofly is wrong. On the other hand, it's very easy to argue that PilotDAR is wrong from a technical standpoint.

Edit: I highly recommend all of John Deakin's articles on piston engines. Lots of great information!

[photofly]

it's important to understand that the power of the engine is affected by manifold pressure, RPM, and fuel flow (plus temperature but we don't really get to have much control of that with a fuel injected engine).

With the mixture at the recommended setting, and the RPM at a specified value, a limit on the power of the engine can be set by stating a maximum value of the manifold pressure.

We don't have direct control over the temperature but it makes a huge difference to the amount of air, and therefore to the power. So it's a bit useless trying to limit the power of a normally aspirated engine by fixing a manifold pressure limit, even with a specified RPM and mixture.

I guess you could be conservative and set a limit for a manifold pressure under winter conditions, but you'd be robbing yourself of 20% of the rated power you could have in the summer.

[Strega]

So it's a bit useless trying to limit the power of a normally aspirated engine by fixing a manifold pressure limit, even with a specified RPM and mixture.

I just flew a helicopter yesterday, that has a "Manifold pressure limit" to limit its power... You better write Frank Robinson and let him know they are doing it wrong.

S

[PilotDAR]

I just flew a helicopter yesterday, that has a "Manifold pressure limit" to limit its power.

Though it was not recently, the helicopters I flew (Schweizers) also had such manifold pressure and RPM limitations:

Engine Limits HP RPM MP IN HG Altitude Feet
HO-360-B1A, -B1B
Max. Continuous 160 2900 26.0 S.L.
Max. Continuous 160 2900 24.8 4000
Takeoff 160 2900 25.0 To 300 above terrain
Max. power rating (5 min.) 180 2900 Full Throttle More than 300 above terrain

With no reference to mixture, as we were trained to never lean in flight.

Being an ungoverned engine, the pilot set continually manifold pressure manually....

(sorry, the cut 'n paste from the TCDS buggered up the columns of limitations)

[eh3fifty]

photofly, you're correct from a technical point of view but in operation it isn't practical to also include air temperature. It's worked just fine to limit the engine using a MP gauge. I don't design airplanes but I'd imagine they'd just be conservative plus have limits on CHT. That's really all you need.

[photofly]

20%

Frank Robinson probably thinks pilots are too dumb to read the air temperature. Who knows; perhaps he's right.

[eh3fifty]

What should I do about that 20%?

What should a pilot do then to ensure they don't exceed an engine limit if you're telling them not to follow the POH/AFM?

You haven't mentioned humidity yet either. Water vapour is about 64% the weight of dry air. That's gotta affect power a bit eh!

[PilotDAR]

I have issued STC's which included limitations of manifold pressure, not so much to regulate power, but more to assure detonation margins were maintained. "Margins" being the operative word. Similarly, I placed RPM limits on the modification. The limits I placed were approximate, based with conservatism upon the observations I made during the testing. A manifold or RPM limit might not always be for reasons of "power".

[photofly]

What should I do about that 20%?

I don't know. You have to obey a posted limit, regardless.

What should a pilot do then to ensure they don't exceed an engine limit if you're telling them not to follow the POH/AFM?

I'm not telling anyone not to follow anything. I'm saying that trying to impose a power limit by limiting RPM/MP without reference to temperature is accurate to no better than 1 part in 5. That's not very accurate.

You haven't mentioned humidity yet either. Water vapour is about 64% the weight of dry air. That's gotta affect power a bit eh!

Fully saturated air at 35°C holds about 3.5% water by number of molecules (which is the figure that counts). That decreases the oxygen available to burn fuel (and therefore the power) by the same 3.5%. Cold saturated air holds almost no water, by comparison, so the relative humidity at winter temperatures is irrelevant to power output.

I have issued STC's which included limitations of manifold pressure, not so much to regulate power, but more to assure detonation margins were maintained.

That's very interesting. Do you know how detonation margins vary with inlet air temperature? I imagine you'd be more interested in the cylinder internal temperatures during the compression cycle, which will depend on the heating of the air from the hot cylinders and the adiabatic compression, so be less dependent on the atmospheric conditions.

[eh3fifty]

I'm not telling anyone not to follow anything. I'm saying that trying to impose a power limit by limiting RPM/MP without reference to temperature is accurate to no better than 1 part in 5. That's not very accurate.

photofly, you're telling me that the POH/AFM has set limits that aren't accurate to your standard. What should I do? Am I to not follow the limits? Unless you can tell me what I should be doing instead of following the limits, I feel your information is useless and fear mongering.

If you feel the way the limits have been set isn't accurate enough then contact the manufacturer of the aircraft involved.

I'm sure they've accounted for the less than perfect accuracy with which they've set the limits but you clearly feel otherwise.

Edit: I fully support you from a technical standpoint. But from a practical standpoint you're completely out to lunch!

[photofly]

This thread began with a ground school question from someone who didn't understand the explanation of manifold pressure and how it varies. If you feel my information is useless to the OP, you're welcome to say so. But really that's his or her call, not yours.

If you don't understand that the power available from a given MP and RPM will vary by more than 20% from winter temperatures to summer, then you are out to lunch.

As for how you personally fly your engines or which limits you choose to respect or disrespect: I have no advice for you. None at all. it's your business and not something I care about in the slightest.

[PilotDAR]

the power available from a given MP and RPM will vary by more than 20% from winter temperatures to summer,

This is accounted for in the aircraft performance tables, so how is it a problem? Yes, you may not achieve full rated power on a warm summer day, or at high altitude in a normally aspirated aircraft, that's not a mystery. We deal with it, or buy a turbo model... The aircraft is certified to meet performance requirements with less than full power.

[photofly]

Happily my airplane has a POH with settings for a variety of air temperatures, from 20 below that of the standard atmosphere at a variety of pressure altitudes, to 20 above. And while it has a red-line RPM, there's no restriction on MP. I'm well set up to account for the effect of temperature on rpm and MP settings.

Since I don't have to deal with a brain-dead power limit based on MP, I happily leave it to you to deal with a 50% longer takeoff roll and/or a 20% lower rate of climb than you could have, during these very hot summer days, just when you'd appreciate the extra power the most.

[PilotDAR]

....during these very hot summer days, just when you'd appreciate the extra power the most.

I'm well set up to account for the effect of temperature on rpm and MP settings.

So with a normally aspirated engine, which does not have a limitation on the use of full open throttle, how could you operate the engine differently than any other pilot flying that type, to account for the effects of ambient air temperature?

deal with a 50% longer takeoff roll and/or a 20% lower rate of climb than you could have

Dealing with decreased performance (which could include "it's too hot to attempt this takeoff now") is a part of disciplined piloting. Failing to accept decreased performance and engine power limitations can lead to melted pistons - I know pilots who have done it. They limit takeoff and climb performance a whole lot more, and are much more difficult to rectify while the plane is floating on a northern lake.

Since I don't have to deal with a brain-dead power limit based on MP

And that is often aircraft or engine type specific, so certainly you can choose what you fly. But, if you chose to fly a type which had power limitations based upon MP, you'd adhere to them? A limitation imposed on operation would have been done after considerable testing and thought....

[photofly]

So with a normally aspirated engine, which does not have a limitation on the use of full open throttle, how could you operate the engine differently than any other pilot flying that type, to account for the effects of ambient air temperature?

I don't really understand the question. I think you're looking to find out what, if anything, I do differently to other pilots? I have no advice for you, or anyone, on that subject. Nor is that what this thread is about. This thread is about understanding manifold pressure, and more recently, about power limits based on manifold pressure.

Dealing with decreased performance (which could include "it's too hot to attempt this takeoff now") is a part of disciplined piloting.

Having a bogus power limitation that prevents you from making a flight when in fact it would be perfectly safe to do so - and the thermometer is the instrument that provides the knowledge that it's safe to do so - is brain-dead. Brain dead engineering - if the limit is imposed by the person documenting the aircraft. The point of limitations is to allow maximum performance while remaining safe. If a limitation achieves the second part of that sentence but doesn't do the first part of the sentence, it's brain dead. You might as well say "never fly - it's safer."

Failing to accept decreased performance and engine power limitations can lead to melted pistons - I know pilots who have done it. They limit takeoff and climb performance a whole lot more, and are much more difficult to rectify while the plane is floating on a northern lake.

Nice try at putting words into my mouth. I'm not denying that. The real point is that a limit that based on a fixed MP and RPM combination without thought of the air temperature is not, in fact, a power limit. It doesn't limit the power to a fixed amount.

A limitation imposed on operation would have been done after considerable testing and thought....

You'd hope so. You'd also hope it was based on correct thought. But who knows?

Suppose you take this plane with this bogus power limit to the far north, and operate it mid-winter at -40°C, at sea level. Was the terrific air density available then taken into account when setting an MP limit? Perhaps now your brain-dead power limit, strictly adhered to by the scrupulous pilot, is leading him towards melted pistons. What a pity!

Anyway, I don't have advice for you about how to fly. Neither a cavalier "ignore the limitations!" nor a pompous "obey the limitations!". Personally I don't have such a limitation, and I'm content that I understand the effects of temperature, RPM, manifold pressure and mixture on the power output of my engine. I advocate only that others try to understand those factors too.

[PilotDAR]

I understand the effects of temperature, RPM, manifold pressure and mixture on the power output of my engine. I advocate only that others try to understand those factors too.

Absolutely! Pilots should understand at least well enough to operate safely, and certainly more, if their personal curiously takes them there. In the mean time, following prescribed operating limitations will keep the plane out of the danger zone, like them or not. I do respect and acknowledge the effects of temperature, altitude, and mixture on engine performance, I just don't worry about them too much, beyond the flight manual references to their affects upon the flight I intend.

Melted pistons and other damage to the engine are not necessarily the result of the engine developing too much power, but rather how the power is being developed. During several detonation testes I have conducted on large Continental engines, it was required that I cause detonation to calibrate my equipment. In doing so, full power, within the stated limitations for the engine protected it from detonation. It was reduced MP which took the engine into a condition in which detonation did occured.

For this reason, when applying my brain to creating engine operating limits, sometimes those limits will specify minimum MP for some conditions. Interestingly, (alarmingly) once I had caused detonation, closing the throttle did not immediately stop it, it continued much longer than I hoped, and I feared damage to the test engine. It is certain that the detonation could not be detected without the test equipment I had installed. I was operating the engine from inside the aircraft, and had no indication it was detonating, other than my instruments. I did this testing both on the ground, and in flight.

So with my experience, and knowledge of the background of how and why the testing is conducted, I'm content to enjoy flying planes within their stated limitations, and accepting inevitable reduced performance in some cases.

All of this goes well beyond the OP's initial question, and I hope somewhere in amongst this the OP found the answer they were looking for. But the additional thing which readers might learn, is that for engines, as is the certain case for whole airplanes, there are often limitations, or operational techniques prescribed, the background of which may be other than it appears. Testing I have done has been illuminating, and caused me to understand that massive amounts of operational wisdom are not taught during flight training - they are summarized in a oneliner in the flight manual limitations or normal procedures section. Failing to regard limitations and prescribed techniques without a very broad understanding, holds the risk of being unsafe, or at least expensive. We saw these [soon to be expensive] engines come into the engine shop I used to work for in Orillia. I have long since learned to be content with "normal" performance, and pay the "normal" operating costs.

[eh3fifty]

This ridiculous banter is why I end up leaving AvCanada for months at a time. It's hard to handle!

photofly, I think all of us that are currently discussing this matter understand the effects of temperature on aircraft performance. All I'm saying, and I think PilotDAR is saying, is to follow your AOM/POH/AFM. If they have a limit on something then follow it. If you choose not to follow it then that's up to you - you may not be covered under warranty or something like that. I don't care.

As PilotDAR said, temperature is taken into account with performance charts. Limits on power are then set and usually indicated by MP. If you have a set temperature, ambient pressure, RPM, and mixture, then you can set power limits using MP. It'll probably translate to, "if you have these certain conditions, do not exceed this MP."

Can we be done with this? It doesn't look good when experienced pilots are arguing about this simple topic.

[photofly]

If you don't like the banter, you can exercise an important online skill: letting someone else have the last word. PilotDAR gave you one opportunity, and you blew it. Here's another chance.

[Strega]

In doing so, full power, within the stated limitations for the engine protected it from detonation. It was reduced MP which took the engine into a condition in which detonation did occured.

Can you elaborate?

In my short time working on engines, typically reducing the mp (all else being equal) will reduce the likelihood of detonation, not increase it.

[PilotDAR]

I found during testing that once I had caused detonation, which was high MP, and low RPM, reduction of MP seemed to actually worsen it before it got better, it did not stop in proportion to the MP. Increasing RPM really did help. This was very difficult testing to accomplish, and potentially very damaging to the engine, so I did not prolong it, beyond proving detection as intended. The ultimate goal was to prove detonation margins on Mogas in the carburetted IO-470 and IO-520, which I did accomplish for both projects. My talks with the Cessna Engineer of the day (this was in the late '80's) confirmed that the relationship of MP, RPM and detonation was not linear, and CHT and oil temps were also important factors. After understanding the in depth testing Cessna had done, and the melted engines, I resigned myself to be very pleased to waste a bit of fuel, and operate with conservatism, to maintain wider margins against detonation.

Don't get me wrong, I'm not ringing alarm bells for anything near normal operations. I'm just describing old experience I have, which has taught me that the work accomplished on this a long time ago was not "brainless", and in my opinion, is worthy of respect as a "middle of the road" safe way to operate the engine. I no longer have the reports I wrote all those years ago, they were the property of my client at the time....

[trey kule]

To the OP

I sense that the doctoral level answers you are reading here are not really the answer to the question you asked.

Are you trying to understand the relationship between RPM and MP from a practical standpoint?
I sense you want to know what will happen as you increase or decrease either one.
At least that question makes sense to me.

Unfortunately, sometimes you get answers to questions you did not ask..a bit scary when you realize some of the posters are instructors.

Maybe try and make you question very clear and simple, and hopefully you will get an answer that is both clear and simple in response.

[pelmet]

Feel free to keep posting guys. The multiple responses creating long threads can frequently be extremely useful for furthering ones knowledge(as long as accurate info is posted) as people post their experiences and knowledge on various angles of a subject. Much better than a quick single response and that is it.