252 POH Power Settings

[milotron]

This question came from my review of power settings and LOP for some up coming higher altitude flights, at least higher then I have done in awhile.

In the 252 POH for the TSIO360-MB4 65 %  power is noted for sea level at 2500 rpm, 25.8" and 11.0 GPH and 65%

At 8000 2500 rpm, 25.8" and 11.7 GPH will give you 70% power.

 

Same RPM, MP and slightly more fuel gets more power at higher altitude.  I would have thought with the same RPM, MP and higher altitude there would be LESS power. Is this the result of ISA temperatures at work?  Pressure/air density shouldn't matter as the controller is just working the wastegate to maintain the selected MP. Can someone smarter than me explain this?

Further to this, why is it that, for a given power %, the required MP goes down as altitude goes up?  Isn't the idea of turbocharging/normalizing to provide a constant MP regardless of altitude?

I could just accept this and move on, but still would like to know more about this.

Also, does anybody use the power charts that precisely? I generally run LOP and set for 5% power above what I want ( set for 70%, lean to 65% ) and lean to %HP by fuel flow.

 

iain

[jlunseth]

Temperature.

Why would it be the case that, at exactly the same RPMs and MP, and only slight different fuel flow, power would go down as altitude goes up?  That is how NA's work because the MP is falling, but the MP does not fall in the turbo, or if it does as is the case in the more manually operated 231 that I have, I adjust it up to maintain the same number.  Same RPM, same MP, same fuel, power should be the same.  The only thing that has changed is the temperature.  

Think of it this way, and sorry if I am being very basic.  Combustion in a piston engine is a matter of how many oxygen molecules you have to react with fuel.  As temp. goes down, the number of molecules in a given volume goes up.  As the number of molecules of O goes up, the amount of fuel that can react with the O goes up, and thus more power.  In the cold temps of the high (for us) flight levels, the engine that makes 100% HP at sea level may make 100% at 32".

We are talking about operations on the rich side of peak, where there is more fuel available to be burned.  The setting you mention look to be close to peak to me, but still on the rich side.  If they were LOP settings, more O2 would in theory make no different because there is no excess fuel, no fuel not already being burned.

Edited June 13, 2017 by jlunseth

[milotron]

Interesting. I would have assumed that for a given MP the density ( amount of molecules in the air ) would be the same, but as you note this would vary with temperature. P=nRT/V or whatever that formula is from high school chemistry.

 

The power settings are from the POH and are likely peak or just ROP as you noted. I start there and lean very quickly from that point for LOP as deep as necessary.

I am a little surprised by the difference as only 10-15 degrees of temp change of outside termp on an intercooled turbo charged engine would cause that much change.

 

I looked at a turbo arrow 4 before purchasing my mooney.  See POH power settings for that aircraft with the TSIO360FB non intercooled version of our engine which seems to subscribe more to my expectations of turbo power settings. I.E. set and forget.  This must be the interplay of the intercooler into this formula?

 

[jlunseth]

Yes, think of it this way.  The pressure exerted by air in a given volume container depends on how excited the atoms/molecules are. It is the atoms running around striking the walls of the container that create pressure.  The hotter the gas is, the more excited they are.  The cooler the gas is the sleepier they are.  So if you cool them down you can stuff a whole bunch of atoms/molecules into the container and still achieve a pressure of "x," but if they are all hot and excited, it won't take nearly as many to get to "x."  The fuel only cares how many molecules there are of O2, not what pressure they are at.  So if you can stuff a lot of molecules in a given volume because they are cold molecules, there are more for the fuel to react with.

An intercooler has a marked effect most especially at cruise.  In the high teens, low flight levels, the differential temperature (the drop in temperature entering to exiting the intercooler) in my aircraft is around 100dF.  In the 231, intercoolers are an aftermarket thing, so we get a chart that tells us, with a temperature drop of "x," the MP to make a desired power is "y" and we make the adjustment.  In the 252 the affect of the intercooler is built in to the POH tables.

[milotron]

Thanks. It looks like the comparison between the two POH charts does illustrate the effects of the intercooler quite clearly.  If I felt like doing a bunch of math, this could likely be proven between the two of them, using correction factors from an intercooler installation and possibly temp correction.

 

Your help was appreciated!

 

iain