What is F.T. an acronym for?

Pretty sure that is full throttle

Sounds right and it makes sense. Thank you.

Any idea where I can obtain inHG values for 75% Power for Best Economy Mixture?  Also, can you confirm what the term power in "75% Power" is in reference to  -- is it 75% of full throttle, rpm, mixture, or all three?

28 minutes ago, bofhlusr said:

Any idea where I can obtain inHG values for 75% Power for Best Economy Mixture?  Also, can you confirm what the term power in "75% Power" is in reference to  -- is it 75% of full throttle, rpm, mixture, or all three?

It's 75% of max rated power.  In the case of the non-turbo Arrow III, that means 75% of 200HP, or 150HP.  Power ratings are always going to be given by some combination of MP and RPM at an appropriate mixture and altitude (and temperature and pressure - you get the idea).  Keep in mind that most older (non-modified) engines are not really designed to run lean of peak.  Running lean of peak became popular with the advent of more accurate fuel injectors like GAMI. 

Those book values shown are for peak EGT and that is as lean as you should run that particular engine and shows you basically the best economy you'll get at that power rating.  Want better economy and increased range?   Run at 65% or even 55%.  Yep, you'll go slower but... TANSTAAFL!  🙂

 

Scott

Edited November 6, 2025Nov 6 by tttocs

"75% Power" refers to 75% of the engine's maximum rated horsepower, not 75% throttle, RPM, or mixture. To obtain the corresponding inHg (manifold pressure) values for best economy mixture at 75% power, you'll typically need to consult the engine or aircraft performance charts.

In your chart for instance, 65% power will be achieved at ISA conditions at 2200rpm and 27.5 inHg

11 minutes ago, jcomm said:

"75% Power" refers to 75% of the engine's maximum rated horsepower, not 75% throttle, RPM, or mixture. To obtain the corresponding inHg (manifold pressure) values for best economy mixture at 75% power, you'll typically need to consult the engine or aircraft performance charts.

In your chart for instance, 65% power will be achieved at ISA conditions at 2200rpm and 27.5 inHg

I asked because I couldn't locate the manifold pressure values for best economy at 75% power in the POH which I assume is all I need for checking aircraft performance.

 

9 minutes ago, bofhlusr said:

I asked because I couldn't locate the manifold pressure values for best economy at 75% power in the POH which I assume is all I need for checking aircraft performance.

It looks as if the manual defines "best economy" as running at peak EGT (which implies the engine can't be run lean of peak).

I imagine the manual doesn't give a "best economy" setting for 75% power because the engine can't be run at peak EGT at that power without exceeding the CHT limit. You have to use a richer mixture to keep the engine cool enough. See this article for more context:

https://www.aopa.org/news-and-media/all-news/2023/january/pilot/savvy-maintenance-egt

26 minutes ago, tttocs said:

It's 75% of max rated power.  In the case of the non-turbo Arrow III, that means 75% of 200HP, or 150HP.  Power ratings are always going to be given by some combination of MP and RPM at an appropriate mixture and altitude (and temperature and pressure - you get the idea).  Keep in mind that most older (non-modified) engines are not really designed to run lean of peak.  Running lean of peak became popular with the advent of more accurate fuel injectors like GAMI. 

Those book values shown are for peak EGT and that is as lean as you should run that particular engine and shows you basically the best economy you'll get at that power rating.  Want better economy and increased range?   Run at 65% or even 55%.  Yep, you'll go slower but... TANSTAAFL!  🙂

 

Scott

Thanks. How is 75% max rated power checked. Is there a gauge(s) similar to the NG gauge found in turbines that can quickly check % of rated power?

9 minutes ago, bofhlusr said:

I asked because I couldn't locate the manifold pressure values for best economy at 75% power in the POH which I assume is all I need for checking aircraft performance.

 

Looking at the PoH for the Arrow 75% power isn't achievable when the engine is leaned for Best Economy.
Instead according to the book you should lean for Maximum Performance (100F Rich of Peak) in order to reach 75%

 

Thank you.

9 minutes ago, bofhlusr said:

Thanks. How is 75% max rated power checked. Is there a gauge(s) similar to the NG gauge found in turbines that can quickly check % of rated power?

No. The manufacturer determines which combinations of engine settings (throttle, prop rpm, mixture) and ambient conditions result in 75% power and publishes these in the performance tables. As the pilot, you can only know indirectly that you're operating at 75% power because you're using settings from the manufacturer's performance tables.

1 hour ago, martinboehme said:

No. The manufacturer determines which combinations of engine settings (throttle, prop rpm, mixture) and ambient conditions result in 75% power and publishes these in the performance tables. As the pilot, you can only know indirectly that you're operating at 75% power because you're using settings from the manufacturer's performance tables.

You do sometimes see an an engine power scale on the fuel pressure gauge but those seem to be a rough guide rather than something precise to rely on.

Edited November 6, 2025Nov 6 by Matchstick