Cruise Performance
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Cruise Performance
Ok peoples, got a couple of questions for you.
Whilst doing my training I was always told that MAP should always be less than RPM to prevent detonation amongst other things i.e. 22/2400 or 23/2400 settings.
Now going through my training books as I do, one states that
“Power can be produced by various combinations of MAP and RPM and these are specified in the aircrafts POH power setting graphs.
If possible, it is better to operate with a high MAP and low rpm when cruising because:”
It then goes into the reasons why this is the preferred method.
My questions are:
Why was I always taught MAP lower than RPM? Is this something that is taught everywhere or is it a misunderstood topic?
Secondly, with a turbo engine your MAP is always higher than RPM. How does this stack up with the “MAP should always be less than RPM” argument?
Over to you
Whilst doing my training I was always told that MAP should always be less than RPM to prevent detonation amongst other things i.e. 22/2400 or 23/2400 settings.
Now going through my training books as I do, one states that
“Power can be produced by various combinations of MAP and RPM and these are specified in the aircrafts POH power setting graphs.
If possible, it is better to operate with a high MAP and low rpm when cruising because:”
It then goes into the reasons why this is the preferred method.
My questions are:
Why was I always taught MAP lower than RPM? Is this something that is taught everywhere or is it a misunderstood topic?
Secondly, with a turbo engine your MAP is always higher than RPM. How does this stack up with the “MAP should always be less than RPM” argument?
Over to you
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This is one of those things you get told by well-meaning people, that once had a distant foundation in reality, but are extrapolated so far they become b@llsh@t.
It's a shame, because the underlying issue - in this case, avoiding engine detonation - is often important, but the final version becomes a lie handed down from generation to generation, believed and parrotted as an inviolable truth. This is certainly one of those lies that we as instructors have a responsibility to shed the light of truth on.
Many normally aspirated avgas engines will have "over square" settings nominated in the POH (including the venerable BE76 Duchess). I think the last diesel engine I saw for an aircraft used something like 75", and it didn't get anywhere near 7500 rpm.
High manifold settings at low rpm can be an issue. In combination, they contribute to high cylinder combustion pressures, and increase the likelihood of detonation. However, there is nothing magical about inches of mercury and rpm, that means their relationship is special for keeping cylinder pressures in the safe zone.
What constitutes a high manifold setting, or a low rpm setting is principally dependent on the design and condition of the engine, but is also affected by the fuel and atmospheric conditions.
At the end of the day, operate the engine the way the manufacturer tells you to. It's as simple as that.
It's a shame, because the underlying issue - in this case, avoiding engine detonation - is often important, but the final version becomes a lie handed down from generation to generation, believed and parrotted as an inviolable truth. This is certainly one of those lies that we as instructors have a responsibility to shed the light of truth on.
Many normally aspirated avgas engines will have "over square" settings nominated in the POH (including the venerable BE76 Duchess). I think the last diesel engine I saw for an aircraft used something like 75", and it didn't get anywhere near 7500 rpm.
High manifold settings at low rpm can be an issue. In combination, they contribute to high cylinder combustion pressures, and increase the likelihood of detonation. However, there is nothing magical about inches of mercury and rpm, that means their relationship is special for keeping cylinder pressures in the safe zone.
What constitutes a high manifold setting, or a low rpm setting is principally dependent on the design and condition of the engine, but is also affected by the fuel and atmospheric conditions.
At the end of the day, operate the engine the way the manufacturer tells you to. It's as simple as that.
Stick with what's in the POH or the operators manual for a specific engine and your butt will be covered.
Many POHs specify power settings where MP is greater than RPM/100.
I don't have one beside me but, for example, I seem to recall that one of the recommended cruise power settings for the BE55 is 24"/2200.
This is yet another area of aviation where urban myth rules! I too was told many years ago to "never operate the engine 'over-square'".
Dr
Many POHs specify power settings where MP is greater than RPM/100.
I don't have one beside me but, for example, I seem to recall that one of the recommended cruise power settings for the BE55 is 24"/2200.
This is yet another area of aviation where urban myth rules! I too was told many years ago to "never operate the engine 'over-square'".
Dr
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Thanks guys, that is what I thought.
I have been flying a 206 and have been using the manual for settings, TAS and fuel flow for all my flight planning and using it a lot more than I ever did when learning to fly at the aero club.
Never stop learning
I have been flying a 206 and have been using the manual for settings, TAS and fuel flow for all my flight planning and using it a lot more than I ever did when learning to fly at the aero club.
Never stop learning
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A lower RPM keeps the noise down for the passengers on those long cross country legs...
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Yep, I've had to sit there while some guy flying a Baron with the engines howling away at 23" and 2500 until I could stand it no more and politely asked why.
Told the usual bull about blowing up engines if run osquare. I thought that went out with the Wright Bros!
After calmly suggesting that the POH has MOSTLY osquare settings if the best range is to be realised, the reaction was that I thought that I had just said something REALLY bad about his mother.
Well its his aeroplane I guess and if he wants to wear out all the auxillaries, use a whole bunch more fuel, not to mention glazing bores, I guess that's his problem. I just wouldn't want to pay for his overhauls.
I didn't think it was a good time to suggest running Lean of peak
Told the usual bull about blowing up engines if run osquare. I thought that went out with the Wright Bros!
After calmly suggesting that the POH has MOSTLY osquare settings if the best range is to be realised, the reaction was that I thought that I had just said something REALLY bad about his mother.
Well its his aeroplane I guess and if he wants to wear out all the auxillaries, use a whole bunch more fuel, not to mention glazing bores, I guess that's his problem. I just wouldn't want to pay for his overhauls.
I didn't think it was a good time to suggest running Lean of peak
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Thanks guys, that is what I thought.
I have been flying a 206 and have been using the manual for settings,
I have been flying a 206 and have been using the manual for settings,
Some people seem to think the manual is for ballast...
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Why?
I dont know that is why i was asking. As I said throughout my training I was always taught not to go over square.
I think what they were getting at was that if the MAP is too high, excessive pressures in the cylinders could lead to detonation and engine damage.
I suppose though that if you stick to the POH this shouldnt happen.
As I said I dont know, it was just what I was taught "Dont have MAP higher than RPM"
In fact the latest advice I have received is that:
If possible, it is better to operate with a high MAP and low RPM when cruising because:
I dont know that is why i was asking. As I said throughout my training I was always taught not to go over square.
I think what they were getting at was that if the MAP is too high, excessive pressures in the cylinders could lead to detonation and engine damage.
I suppose though that if you stick to the POH this shouldnt happen.
As I said I dont know, it was just what I was taught "Dont have MAP higher than RPM"
In fact the latest advice I have received is that:
If possible, it is better to operate with a high MAP and low RPM when cruising because:
- low rpm means - increased volumetric efficiency, less frictional loss and lower mechanical loads on the reciprocating parts of the engine, less noise and vibration in the engine
- high MAP, achieved with a wider throttle opening means less restriction to the airflow throughthe throttle butterfly valve in the carby, which contributes to better volumetric efficiency. There is less frictional loss of energy in the airflow during its passage through the inlet manifold.
Last edited by av8trflying; 25th Jul 2008 at 02:07.
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Any Turbo Charged or Supercharged engine will ALWAYS have MAP higher than RPM in fact most round engines have MAP significantly higher than RPM at all time, R1820 cruise MAP 30 inches RPM 2000.
Take Off MAP 52.5 inches RPM2700 for 5 minutes.
Take Off MAP 52.5 inches RPM2700 for 5 minutes.
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It is fine - just as long as you remember to never lean below 5000'. 
Could you explain this to me more please. My POH tells me to lean at all times for best fuel economy no matter what the MAP/RPM is.
Cheers
Could you explain this to me more please. My POH tells me to lean at all times for best fuel economy no matter what the MAP/RPM is.
Cheers
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av8trflying, John Deakin is some thing of an educator and guru when it comes to engine management and dispelling myths that abound. The question you ask may have your answer here
Detonation Myths
Pelican's Perch #43:<br>Detonation Myths
Other engine management articles
Props Driving Engines
Pelican's Perch #78: Props Driving Engines
Start and Run Up
Pelican's Perch #77: Startups & Runups
Dreadful POHs Part 1
Pelican's Perch #75:<br>Those Dreadful POHs (Part 1)
Dreadful POHs Part 2
Pelican's Perch #76: Those Dreadful POHs (Part 2)
Where to Run the Engine Part 1 Take Off
Pelican's Perch #63:<br>Where Should I Run My Engine? (Part 1)
Where to Run the Engine Part 2 Climb
Pelican's Perch #64<br>Where Should I Run My Engine?<br>(Part 2 — The Climb)
Where to Run the Engine Part 3 Cruise
Pelican's Perch #65<br>Where Should I Run My Engine?<br>(Part 3 -- Cruise)
Where to Run the Engine Part 4 Descent
Pelican's Perch #66<br>Where Should I Run My Engine?<br>(Part 4 -- Descent)
Fried Valves
Pelican's Perch #59:<br>Fried Valves
Operation of Turbo Charged Engines (6 Parts)
Pelican's Perch #31:<br>Those Fire-Breathing Turbos (Part 1)
Pelican's Perch #32:<br>Those Fire-Breathing Turbos (Part 2)
Pelican's Perch #33:<br>Those Fire-Breathing Turbos (Part 3)
Pelican's Perch #34:<br>Those Fire-Breathing Turbos (Part 4)
Pelican's Perch #35:<br>Those Fire-Breathing Turbos (Part 5)
Pelican's Perch #36:<br>Those Fire-Breathing Turbos (Part 6 — and FINAL!)
Index to all his articles
"Pelican's Perch" Index
One of those terrible myths that abound
Detonation Myths
Pelican's Perch #43:<br>Detonation Myths
Other engine management articles
Props Driving Engines
Pelican's Perch #78: Props Driving Engines
Start and Run Up
Pelican's Perch #77: Startups & Runups
Dreadful POHs Part 1
Pelican's Perch #75:<br>Those Dreadful POHs (Part 1)
Dreadful POHs Part 2
Pelican's Perch #76: Those Dreadful POHs (Part 2)
Where to Run the Engine Part 1 Take Off
Pelican's Perch #63:<br>Where Should I Run My Engine? (Part 1)
Where to Run the Engine Part 2 Climb
Pelican's Perch #64<br>Where Should I Run My Engine?<br>(Part 2 — The Climb)
Where to Run the Engine Part 3 Cruise
Pelican's Perch #65<br>Where Should I Run My Engine?<br>(Part 3 -- Cruise)
Where to Run the Engine Part 4 Descent
Pelican's Perch #66<br>Where Should I Run My Engine?<br>(Part 4 -- Descent)
Fried Valves
Pelican's Perch #59:<br>Fried Valves
Operation of Turbo Charged Engines (6 Parts)
Pelican's Perch #31:<br>Those Fire-Breathing Turbos (Part 1)
Pelican's Perch #32:<br>Those Fire-Breathing Turbos (Part 2)
Pelican's Perch #33:<br>Those Fire-Breathing Turbos (Part 3)
Pelican's Perch #34:<br>Those Fire-Breathing Turbos (Part 4)
Pelican's Perch #35:<br>Those Fire-Breathing Turbos (Part 5)
Pelican's Perch #36:<br>Those Fire-Breathing Turbos (Part 6 — and FINAL!)
Index to all his articles
"Pelican's Perch" Index
Never lean below 5000
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Thanks Brian and Archer
Brian, yeah i was aware of those articles and have read the ones on lean of peak operations (it seems a lot in the industry still dont want to hear it though). Thats why i questioned the never lean below 5000'. I thought it was odd.
I will have a read of some of those other links though, cheers for that.
Brian, yeah i was aware of those articles and have read the ones on lean of peak operations (it seems a lot in the industry still dont want to hear it though). Thats why i questioned the never lean below 5000'. I thought it was odd.
I will have a read of some of those other links though, cheers for that.
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There's another article about too... entitled "Operating Tips for big-bore Continentals" by Mike Busch (a former AvWeb editor) and is a damn good starting point, especially in light of the other articles addressed above. You'll find it here. No need to let the title or content prevent Lycoming drivers learning either... every word is equally as applicable to Lycoming big-bores as Continentals.