Cruise at 75% power

bflynn

bflynn

Final Approach
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Brian Flynn
Reviewing an Archer POH today and saw the recommendation to cruise at 75% power. I never thought deeper about that, so I pulled out the calculator and put in tach red line 2650 * 0.75 and got 1980 RPMs. Is that a correct calculation?

Maybe my memory is bad, but in a Warrior II, 2000 RPMs gets me 80 kts, it's the pattern speed. We usually are in cruise around 2350/105 kts or so. PA28s have never been speed machines, but I'm pretty sure the cruise for an Archer ought to be around 110+/-, right?
 
bflynn said:
Reviewing an Archer POH today and saw the recommendation to cruise at 75% power. I never thought deeper about that, so I pulled out the calculator and put in tach red line 2650 * 0.75 and got 1980 RPMs. Is that a correct calculation?

Maybe my memory is bad, but in a Warrior II, 2000 RPMs gets me 80 kts, it's the pattern speed. We usually are in cruise around 2350/105 kts or so. PA28s have never been speed machines, but I'm pretty sure the cruise for an Archer ought to be around 110+/-, right?
Click to expand...

The manual should have a Engine Performance Cruise Chart. At 2000' DA it shows about 2450 rpm to be 75% power. *

(correction: This is for a PA28-180)
 
Altitude is also a factor. Cruising at 75% out here just doesn't happen. Nor 180 hp.
 
I haven't gotten down to performance yet.

Why is 2450 considered 75% of 2650?
 
Engines produce power relative to the amount of mixture they burn. That's determined by engine speed and throttle opening. It's not just a matter of RPM.
 
bflynn said:
I haven't gotten down to performance yet.

Why is 2450 considered 75% of 2650?
Click to expand...
Remember how air density decreases with altitude? 75% at MSL isn’t the same as 75% at 8500ft MSL... Percent power isn’t necessarily a constant RPM setting.
 
As you noted, its % of total engine power, not % of available rpms. To add to that, remember that power increase isn't necessarily linear with rpm increase. You can increase the throttle from 1000rpm to 1500rpm and get a smaller power increase than going from 1500rpm to 2,000rpm. Cam/spark timing, thermal efficiencies, savaging, etc. all play a part in that.
 
Horsepower is basically torque times RPM. If you reduce RPM you're also reducing torque, and the decrease compounds. DA is also a big factor, as pointed out.

The thing is easier to see in electricity. Volts times Amps = watts. 10 volts times 10 amps equals 100 watts. It takes volts to drive amps through a resistance. If we cut the voltage in half we will also reduce the amerpage by about half, depending on the type of resistance. So 5 volts times 5 amps is 25 watts, only a quarter of the original 100 watts.
 
bflynn said:
Reviewing an Archer POH today and saw the recommendation to cruise at 75% power. I never thought deeper about that, so I pulled out the calculator and put in tach red line 2650 * 0.75 and got 1980 RPMs. Is that a correct calculation?

Maybe my memory is bad, but in a Warrior II, 2000 RPMs gets me 80 kts, it's the pattern speed. We usually are in cruise around 2350/105 kts or so. PA28s have never been speed machines, but I'm pretty sure the cruise for an Archer ought to be around 110+/-, right?
Click to expand...
Those performance charts in this thread really tell the story. The performance chart shows at MGW, 129kts is the TAS you can expect from an Archer at 8K at standard temp and 75% power. Mine trues out on testing 132-133.
But those charts are based on knowing the true tach reading and flying to that parameter that corresponds to 75% at the specific altitude(DA). However, many of our tachs are off. Mine was reading 175 lower than reality causing me to initially think I was faster than book. Get your tach tested, buy a cheap digital optical checker, or get a digital tach to fly by.
 
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As others have posted, it's not percentage of max rpm, it's percentage of power, which for a fixed pitch prop is roughly proportional to the cube root of the rpm. So for 75%, the cube root of .75 is .908, so 75% power for an engine making its full rated power at 2650 (under those particular conditions) will be at 91% of 2650 or 2406 rpm.
 
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