horsepower vs throttle/prop settings

Tandem46 · #1 01-04-2006, 07:44 PM

I'm trying to better understand my engine. I usually pull my throttle back to 24" and prop back to 2400rpm soon after takeoff to save wear and tear. I was wondering what if I just pulled the prop back to 2400 and left the throttle full in......or leave the prop at 2700 and pull the throttle back to 24". Which would produce more power or which would produce less stress? Also, would the hp be roughly the same for these two settings, 2700rpm/24" and 2400rpm/full throttle (30"mp). Just wondering how it all ties together. I know that at sea level, 2700rpm and full throttle gives 180hp but full throttle and 2400rpm....how much less hp than 180. I'm really only concerned about this down low. Up high my mp is always below 24 anyway. Hope some one can chime in.

Tobin

osxuser · #2 01-04-2006, 08:07 PM

I just run the RPM back to 2500 and leave the MP alone. 2400 is a bit low, you'd be making less than 75% power at 24 squared.

deene · #3 01-04-2006, 08:29 PM

Hey, guys, this is not hard...take a look at your engine manual and the curves. You can quickly (well, maybe not so quickly) find out what is allowed and the power.

In general higher MP and lower RPM will get you better efficiency and lower fuel flows.

Old flight intructor tales about over square operation should not play here...

Deene Ogden
RV8 QB due in Mar/Apr
CFII, MEI, CFIG, EAA Tech Counselor, EAA Flight Advisor

deene · #4 01-04-2006, 08:53 PM

Hey, guys, this is not hard...take a look at your engine manual and the curves. You can quickly (well, maybe not so quickly) find out what is allowed and the power.

In general higher MP and lower RPM will get you better efficiency and lower fuel flows.

Old flight intructor tales about over square operation should not play here...

Deene Ogden
RV8 QB due in Mar/Apr
CFII, MEI, CFIG, EAA Tech Counselor, EAA Flight Advisor

mahlon_r · #5 01-04-2006, 08:59 PM

2400 and full throttle= approx 160HP
2700 and 24 inches = approx 154HP
Don't know which power setting delivers less stress. I would guess that the less power would, but who knows for sure?
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at you own risk."

joe gremlin · #6 01-04-2006, 11:15 PM

I used to fly a 182 with an IO540 (or was it a 520?). The POH said the engine was rated for full power for a maximum of something like 2 minutes and dictated that power be set to 24 or 25 square as soon as practical after takeoff.

I asked the mechanic we were using what 'as soon as practical' meant and he said if you're staying down low, square up the power. If you're climbing, just dial the prop back a bit and keep the throttle to the firewall. You'll climb faster and MP will fall back on its own as you climb so you'll end up with the same result but you'll get there a bit sooner. It made sense to me so I've done it that was ever since.