Throttle and RPM

[Jaybird180]

I have a question that's been bugging me off/ on and it seems I may have some misunderstanding about the power selection involving constant speed props.

The common advice given for Mooneys is full throttle from takeoff to destination pattern with performance adjusted with RPM selection.

My reading and interpretation of Deakin suggests this typically yields the greatest engine efficiency as the intake tract is designed to flow best air at WOT. In this case the pilot need only adjust mixture for altitude to keep the mixture in a burnable range. It also suggests this may not be best for all airframes due to drag...that total cruise performance typically reaches max efficiency before the engine, in that it (the airframe) reaches peak of efficiency curve somewhere above 75% engine power, but this may not be conducive to engine longevity.

Planes with constant speed props typically have a power chart showing suggestions for 55%, 65%, 75%, etc at altitude ranges and different combinations of throttle MP and RPM to yield the desired power, showing the resultant cruise speed.

What is the need for all the variation? Why not just run the engine WOT from takeoff to pattern and simply go high RPM on takeoff and move the prop back to the bottom of the green arc for cruise? What's wrong with this as SOP for every CS prop airplane?

Did I answer my own question in the underlined portion? I feel like there is one critical piece of data that I am missing, which is causing me to revisit this in my mind.

Someone please show me the way to get off this carousel.

 

[poadeleted20]

Detonation. Run the MP too high for a particular RPM, and you can induce detonation in the cylinders. Look in the operator's manual for any particular engine, and you'll find a maximum MP for any given RPM. Within those limits, you can certainly fly the plane that way, but know what that limit is for your engine and do not exceed it.

 

[Capt. Geoffrey Thorpe]

Best airframe efficiency is at best L/D which is way slower than you typically cruise.

Closing the throttle causes pumping losses in the engine which reduce engine efficiency, it is better to reduce power by running lean if the engine will tolerate it for engine efficiency. (Leaning reduces the actual power output at any given RPM / manifold pressure which lets the airplane slow down.)

What works depends on what altitude you are at. If you run WOT / Lean at 10000 feet, you are running at a very low actual power output which lets you fly slow so the airframe is at a more efficient L/D

Trying the same thing at 1000 feet means that you are running at an actual, say 80 - 90% power which is too fast for airframe efficiency and engine longevity. So, you close the throttle.

 

[Jaybird180]

Detonation. Run the MP too high for a particular RPM, and you can induce detonation in the cylinders. Look in the operator's manual for any particular engine, and you'll find a maximum MP for any given RPM. Within those limits, you can certainly fly the plane that way, but know what that limit is for your engine and do not exceed it.

http://en.wikipedia.org/wiki/Engine_knocking

I don't understand how high MP would cause detonation. My concept (perhaps flawed???) is that high MP and low RPM could be akin to running my car in 6th gear at 10MPH, the connecting rod could be bent. But how would this affect when the ignition sent the spark to ignite the fuel at the improper time? Is the detonation due to Boyle's law of high heat induced by high internal pressure?

 

[Jaybird180]

Best airframe efficiency is at best L/D which is way slower than you typically cruise.

Ah, yes...IIRC it's somewhere in the 55% range for most airframes, right?

Closing the throttle causes pumping losses in the engine which reduce engine efficiency, it is better to reduce power by running lean if the engine will tolerate it for engine efficiency. (Leaning reduces the actual power output at any given RPM / manifold pressure which lets the airplane slow down.)

Got it

What works depends on what altitude you are at. If you run WOT / Lean at 10000 feet, you are running at a very low actual power output which lets you fly slow so the airframe is at a more efficient L/D

Still with you

Trying the same thing at 1000 feet means that you are running at an actual, say 80 - 90% power which is too fast for airframe efficiency and engine longevity. So, you close the throttle.

I think I lost you somewhere on how this specifically relates to my specific misunderstanding.

 

[Jaybird180]

Detonation. Run the MP too high for a particular RPM, and you can induce detonation in the cylinders. Look in the operator's manual for any particular engine, and you'll find a maximum MP for any given RPM. Within those limits, you can certainly fly the plane that way, but know what that limit is for your engine and do not exceed it.

AHA!
Are you suggesting this is something different from the airplanes POH?

 

[Capt. Geoffrey Thorpe]

http://en.wikipedia.org/wiki/Engine_knocking

I don't understand how high MP would cause detonation. My concept (perhaps flawed???) is that high MP and low RPM could be akin to running my car in 6th gear at 10MPH, the connecting rod could be bent. But how would this affect when the ignition sent the spark to ignite the fuel at the improper time? Is the detonation due to Boyle's law of high heat induced by high internal pressure?

Knock / Detonation happens in the end gas regions (the last part to burn) well after the spark has fired. At high manifold pressures and low engine speeds you have more compression heating (from geometric compression and from the compression in the unburned regions due to combustion / heating) which causes the long HC molecules to break down and spontaneously ignite. Solutions are to retard timing (which delays the increases in pressure due to combustion), reduce manifold pressure or increase engine speed.

 

[Capt. Geoffrey Thorpe]

Ah, yes...IIRC it's somewhere in the 55% range for most airframes, right?

Probably well below 55% in most aircraft. Best airframe efficiency will be very close in speed to best glide.

I think I lost you somewhere on how this specifically relates to my specific misunderstanding.

You wrote...

What is the need for all the variation? Why not just run the engine WOT from takeoff to pattern and simply go high RPM on takeoff and move the prop back to the bottom of the green arc for cruise? What's wrong with this as SOP for every CS prop airplane?

Running WOT at low altitude beats up your engine and is too fast for efficiency - even if you lean the snot out of it.

 

[Ted]

At high manifold pressures and low engine speeds you have more compression heating (from geometric compression and from the compression in the unburned regions due to combustion / heating) which causes the long HC molecules to break down and spontaneously ignite.

This is true. However because of the small operating range of piston aircraft engines and cylinder pressures, I have observed the worst detonation to be at high RPM and high MP. Basically by the time you get to low RPM, your cylinder pressures end up decreasing because of engine dynamics. Since on a traditional piston engine the maximum RPM you see is 2700 or so, that's still quite low.

The summary of why there are so many combinations is that the old theory was one should give the pilot options. Some people liked to run engnes at high RPM, some low. This is part of why you see so much variation in operating from pilot to pilot. Each pilot somehow decided along the way "this is the best way to run my engine." In many cases, they were wrong. Limitations exist for both detonation concerns and also also torsional vibrations.

So here's what to do. If your POH says it's ok, it is. Highest MP that's allowable will be most efficient because of pumping losses.

 

[poadeleted20]

AHA!
Are you suggesting this is something different from the airplanes POH?

Yes, I am. The Lycoming Operator's Manual is shipped with the plane from the factory, but is often separated later. If you have a Lycoming engine, you can order a new one from Lycoming.
http://www.lycoming.com/support/publications/service-letters/pdfs/SL114AU.pdf

 

[Dr. O]

Taking care of an engine with a constant speed prop is not rocket science.
But reading the tortured descriptions of how to do it will have you doing equations rivaling Einstein's.

Jaybird, your proposed solution of always being at WOT and loading the engine down with the prop so it does not overspeed is analogous to operating your car at WOT and standing on the brakes to control the speed. Do you see a problem there?
In your plane the problem will be detonation when you load the engine down with the prop during WOT (while below critical altitude)

Every engine has a critical altitude. This is the altitude at/above which you can run WOT at maximum cruise RPM and not exceed the magic 75% of maximum horsepower for continuous operation.
At any altitude below that, you must reduce the % of HP or you will overload/overheat the engine and reduce lifespan.
You basically have two solutions (assuming we are at a given RPM all the time)
1. You can starve the engine for fuel so that it is producing less horsepower (and less heat) and stays below the magic percentage of HP for continuous operation. This is how diesels operate. And the purveyors of expensive, but very good, fuel injectors.
2. Or you can partially close the throttle to be at your chosen percentage of HP, and then lean the engine for efficient operation. (98% of the world does it this way)
Above the critical altitude you can pretty much do whatever you wish.

What I generally see is that folks who have not actually flown with a constant speed prop find this to be very confusing and go searching the literature for the holy grail of that magic solution that seems to have escaped us mere mortals who routinely fly with a constant speed prop.

Why don't you you go get checked out in a plane with a CS prop and come back and tell us how to do it?

cheers

 

[poweroff360]

I have a question that's been bugging me off/ on and it seems I may have some misunderstanding about the power selection involving constant speed props.

The common advice given for Mooneys is full throttle from takeoff to destination pattern with performance adjusted with RPM selection.

My reading and interpretation of Deakin suggests this typically yields the greatest engine efficiency as the intake tract is designed to flow best air at WOT. In this case the pilot need only adjust mixture for altitude to keep the mixture in a burnable range. It also suggests this may not be best for all airframes due to drag...that total cruise performance typically reaches max efficiency before the engine, in that it (the airframe) reaches peak of efficiency curve somewhere above 75% engine power, but this may not be conducive to engine longevity.

Planes with constant speed props typically have a power chart showing suggestions for 55%, 65%, 75%, etc at altitude ranges and different combinations of throttle MP and RPM to yield the desired power, showing the resultant cruise speed.

What is the need for all the variation? Why not just run the engine WOT from takeoff to pattern and simply go high RPM on takeoff and move the prop back to the bottom of the green arc for cruise? What's wrong with this as SOP for every CS prop airplane?

Did I answer my own question in the underlined portion? I feel like there is one critical piece of data that I am missing, which is causing me to revisit this in my mind.

Someone please show me the way to get off this carousel.

Manifold pressure doesn't mean much. Cylinder pressure does.

Operate your Mooney within the bounds of the manuals and you won't have a problem.

If you don't understand how to trade RPM and manifold pressure, spend more time with a fixed-pitch aircraft, not a Mooney.

 

[MAKG1]

Manifold pressure doesn't mean much. Cylinder pressure does.

Operate your Mooney within the bounds of the manuals and you won't have a problem.

If you don't understand how to trade RPM and manifold pressure, spend more time with a fixed-pitch aircraft, not a Mooney.

Huh? Manifold pressure is directly related to engine air consumption. That means quite a bit.

 

[PilotAlan]

Huh? Manifold pressure is directly related to engine air consumption. That means quite a bit.

True, but air consumption is only half of the equation. Fuel is the other.
Manifold pressure is a proxy for fuel flow, which is only valid within a narrow range of mixture settings.
Running lean of peak, there is no correlation between MP and power.

An extreme example would be pulling the mixture with the throttle full forward. You have RPM from the windmilling prop, and manifold pressure. But no power.

 

[oldShar]

What I generally see is that folks ... go searching ... for the holy grail of that magic solution that seems to have escaped us mere mortals who routinely fly with a constant speed prop.

... the latest being the purveyors of --
FADEC

 

[Skylane81E]

Best airframe efficiency is at best L/D which is way slower than you typically cruise.

Closing the throttle causes pumping losses in the engine which reduce engine efficiency, it is better to reduce power by running lean if the engine will tolerate it for engine efficiency. (Leaning reduces the actual power output at any given RPM / manifold pressure which lets the airplane slow down.)

What works depends on what altitude you are at. If you run WOT / Lean at 10000 feet, you are running at a very low actual power output which lets you fly slow so the airframe is at a more efficient L/D

Trying the same thing at 1000 feet means that you are running at an actual, say 80 - 90% power which is too fast for airframe efficiency and engine longevity. So, you close the throttle.

Bingo,

As an example let look at my plane.

From about 90-125kias my fuel economy is nearly constant, roughly 10nm/gal. The engine is happier at higher power but the airframe would rather be down around best glide. It just so happens that in my case the reduction in airframe efficiency as we speed up cancels out the increase in engine efficiency. For this reason I like to cruise at what ever altitude give me WOT power conducive to running at about 125 indicated, so I end up in the 5-7k range with 22-23in MAP and the prop pulled to "square" so that I burn about 12.5gph (or a little less, using lean to rough, enrich to smooth) go about 125 indicated and true out in the 130s depending on the air I'm flying through.

A mooney is "slicker" but the same basic ideas hold

 

[zaitcev]

Are you suggesting this is something different from the airplanes POH?

If your Mooney's POH has all the answers, by all means follow it (if the engine was not upgraded by STC or other means). But in my Arrow manual, same 200 hp engine as in M20E, it says "refer to Lycoming manual".
Edit: See a more complete answer by Ron above. Good thing Lycoming still exists as a company.