P337 Intercooler Flight Test Comparison

[Walter Atkinson]

HP calculation ROP (mass airflow):

(100+(((RPM-maxRPM)/100*2.5)+((MP-maxMP)*3.5)))/100*maxHP.


HP calculation LOP (FF):

NA engines = 14.9 * FF = HP

TC engines = 13.75 * FF = HP

[CaptainRod]

Excellent posts, Walter. Thank you.

[JamesC]

Walter I really appreciate your input - LOP ops certainly become easier given the simpler relationship between fuel flow and HP in that regime. Much safer than choosing CHT targets particularly at cold altitudes.
For ROP your formula leads to 100 % power at max RPM and MP.
However AA placards 35.3 " MP for takeoff power - was this based on HP dyno measurements or math ?
Even if there is exhaust back pressure, could the peak cylinder PSI's still not be too high at 37 "MP full rich with the extra O2 from the colder denser intercooler air?
Thanks very much.

[Walter Atkinson]

Originally Posted by JamesC

For ROP your formula leads to 100 % power at max RPM and MP.
However AA placards 35.3 " MP for takeoff power - was this based on HP dyno measurements or math ?

The engine test stand data confirmed the math! This is a very good "rule of thumb" calculation that can be off a very small amount as other factors do affect the mass airflow. Those differences are insignificant operationally.

Originally Posted by JamesC

Even if there is exhaust back pressure, could the peak cylinder PSI's still not be too high at 37 "MP full rich with the extra O2 from the colder denser intercooler air?
Thanks very much.

Very good question. The answer is "no." Peak ICPs are controlled nicely. The additional FF provided by the fuel controller covers that problem in conjunction with the decreased volumetric efficiency. Mass airflow-type systems like Lycoming's make it a non-issue. The TCM fuel controller handles the increased O2 a different manner, but it works just as well.

[JamesC]

So then if AA placards t/o power MP = 35.3" then that is what we use in the equation for MaxMP when calculating cruise HPs, correct?
The AA data from their table however correspond more accurately to the formula when you plug in 37" as MaxMP.

[Walter Atkinson]

Originally Posted by JamesC

So then if AA placards t/o power MP = 35.3" then that is what we use in the equation for MaxMP when calculating cruise HPs, correct?

Yes.

Originally Posted by JamesC

SThe AA data from their table however correspond more accurately to the formula when you plug in 37" as MaxMP.

Sounds like they may have forgotten to include the exhaust back pressure effects in their calculations. Not sure how they got their numbers.