Most Economical power setting by Tach Hour.

brcase

brcase

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Brian
If you are renting an airplane (172N for example) by tach time with a fixed pitch prop is it generally more economical to run at 2500RPM or 2100RPM?

I used 6000 feet on a standard day for my example.

Brian
 
Well, that is a flight planning exercise that would require some time with the relative POH.
 
I'm with Greg -- you'd have to dig into the performance data for that aircraft. And the answer might change between wet and dry rental, too.
 
If you're being charged by the prop turn, which is what a tach rental is, you'll generally do better with slower engine speeds. But, consult the numbers from the POH. You have fuel burn and true airspeed there. Everything you need for an authoritative answer, rather than a guess.

With a CS prop, spin the prop as slow as possible within limitations.
 
brcase said:
If you are renting an airplane (172N for example) by tach time with a fixed pitch prop is it generally more economical to run at 2500RPM or 2100RPM?

I used 6000 feet on a standard day for my example.

Brian
Click to expand...

The key determinant to this is the function of the flight. Are you flying somewhere, or flying "an hour" and are you logging tach or real time? Also are you renting wet or dry? With that you have enough to go in the POH and work it out. Winds on any particular flight may also vary the outcome.
 
This thread reminds me a lot about Mexican food. There are countless ways of combining meat, tortillas, beans and rice in to various dishes but they are all still made up of meat, tortillas, beans and rice.
 
Flying slower is going to be more efficient for both distance travelled or time logged (presuming he's logging watch time and not the tach time burned).
 
Sac Arrow said:
This thread reminds me a lot about Mexican food. There are countless ways of combining meat, tortillas, beans and rice in to various dishes but they are all still made up of meat, tortillas, beans and rice.
Click to expand...

Right, but if you oder a #7 you get more meat for your $3 and if you order a #5 you get more rice and beans.
 
Henning said:
Right, but if you oder a #7 you get more meat for your $3 and if you order a #5 you get more rice and beans.
Click to expand...

Yes, much my point as well.

By the way, if you order the #5, you might want to get that prop spinning on the faster side.
 
This was one of the questions I had figured out the answer before I asked.
But i didn't get the answer I expected.
I should have specified it was a wet rental and for a cross country flight.

C-172N @ 6000Ft @ Std tep
RPM KTAS GPH MPH(tach2400)
2600
2500 116 8.1 111
2400 110 7.2 110
2300 105 6.5 110
2200 99 5.9 108
2100 93 5.5 106


If I did my math right, the last column is how many miles you will travel for 2400*60 revolutions of the prop at various RPM's. So it would seem that it is slightly more efficient to run at a higher RPM when paying per revolution of the prop aka Tach time.

wind might play a factor but I suspect it would take pretty significant winds to change the order of the results.
 
brcase said:
This was one of the questions I had figured out the answer before I asked.
But i didn't get the answer I expected.
I should have specified it was a wet rental and for a cross country flight.

C-172N @ 6000Ft @ Std tep
RPM KTAS GPH MPH(tach2400)
2600
2500 116 8.1 111
2400 110 7.2 110
2300 105 6.5 110
2200 99 5.9 108
2100 93 5.5 106


If I did my math right, the last column is how many miles you will travel for 2400*60 revolutions of the prop at various RPM's. So it would seem that it is slightly more efficient to run at a higher RPM when paying per revolution of the prop aka Tach time.

wind might play a factor but I suspect it would take pretty significant winds to change the order of the results.
Click to expand...


You may want to re figure, by reducing to 2100, you get 80% of the speed for 67% of the fuel as at 2500. Oh, never mind, wet rate, yeah, peg it to rental power.
 
flyingron said:
Flying slower is going to be more efficient for both distance travelled or time logged (presuming he's logging watch time and not the tach time burned).
Click to expand...

I'm curious, does it make any difference logging Tach vs Hobbs vs Wall Clock? I'm thinking that Tach time will be the smallest of the three numbers and so you can't get gigged for over reporting. I switched to Tach when I bought into my partnership as it's what we use for everything else and I don't need to have higher numbers then I have now.
 
The FAA will accept any of these things but in actuality the definition says

The time (that is real time) form the time the aircraft first moves under its own power for the purpose of flight until it comes to rest.

If your tach runs slow, you're cheating yourself.
Depending on how you have the hobbs wired up it could be accurate or not.

My hobbs is wired up to the gear (for maintenance issues). I use my watch or the (in motion timer on the GPS) for logging.
 
Fwiw I once figured this out. In my case it was less costly to run the higher power setting since I was renting wet. The lower power settings did cause "tach time" to go slower but the loss of cruise speed cost me more in the end. I can't remember the exact math I did but it accounted for all the variables properly. I tested this in flight at various power settings using paper, pen, stopwatch, and calculator.

Of course if you aren't trying to get somewhere you will be better off with the lower power setting.
 
Sac Arrow said:
This thread reminds me a lot about Mexican food. There are countless ways of combining meat, tortillas, beans and rice in to various dishes but they are all still made up of meat, tortillas, beans and rice.
Click to expand...
Don't forget the onions, cumin, chiles, oregano, and cilantro! Pretty dull food without them. ;)
 
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