Running props over square?

[shdw]

Originally Posted by LatitudeDancer

They're not doing it right if that's the outcome.

I'm not too smart so I'll just insert this excerpt from a DA40 POH. The first time I went for about a 2000/25" combo on an XC with an ATP instructor, he about pooped. I just wanted to see what the engine sounded like.

So thats the culprit?!?!? Those ATP guys teach you that you must be squared?

[mshunter]

Originally Posted by tgrayson

Detonation limits, I believe. Reducing RPM increases BMEP (brake mean effective pressure) in the cylinders. As for why it varies from engine to engine, displacement does appear in the formulas, with larger engines tending to reduce the increase in BMEP produced by RPM decreases. Also, compression ratios probably have a role, with higher compression engines having a greater propensity to detonate.

Bingo! The higher the compression ratio, the more likely it is to detonate. That is one reason why so many are afraid to get away from 100LL, because the lead can act like a cushion for detonation. I know this is talking cars, but the therotical limit of premium mogas compression is around 11to1 for street use(and most consider that a ragged edge). The car I pit crew on (my brothers car) runs something like 130 (10to1 but 32lbs boost, HUGE CYL PRESSURE). More pressure (think compression ratio) equals more likelyhood of exploding the fuel rather than burning it. More pressure also equals more heat. Basically, there is less space in the cyl when the piston is at TDC on a higher compression engine. And lower compression also equals less power (as a general rule). The fuel burnt is still capible of makeing the same pressure, even if burnt in a smaller area. But, as RPM's decrease, so do VE, because the engine is desgined to operate at certian RPM's (think power band). So running an engine at a lower RPM is going to generate less HP. Do you think a 172 SP makes 180hp at 2300rpm. Nope. Mabey it makes about 145-150 at WOT and 2300rpm. 180hp is only made at WOT/sea level and 2700RPM. At 6500'/2700rpm/WOT it's still not makeing 180hp. And just because you have a constant speed prop, doesn't mean that because you are getting max rpm, does not mean you are getting full power at WOT either.

Originally Posted by shdw

So thats the culprit?!?!? Those ATP guys teach you that you must be squared?

I went to ATP, and they teach a standardized procedure to get you to the airlines. Thats why they are the way they are. I did well there, and their porgram works. But most of their instructors just regergitate what they have been tought, and it's not always right. Running a Seminole at 25/25 in a climb is not the best way to do things, so when it was my turn to be PF on x-countries, I would set the airplane up as I saw fit (full power in a climb, and whatever I could get MP/RPM wise per perf. charts). It almost got me the boot because one person I flew with told dispatch, and I got a talking to. My argument was why would Piper put it in the perf. charts if it was going to hurt the airplane. Never the less, I lost, but didn't get the boot. They basically had no leg to stand on other than "thats how we want it." So I never flew with her again.

[kiloalpha]

Okay here is the info that you all been waiting for.

4,000 feet
MP / RPM TAS FUEL
22 / 24 154 92 lb/hr
23 / 23 156 89
24 / 22 158 83
25 / 21 155 80
26 / 20 145 75

6,000 feet
MP / RPM TAS FUEL
22 / 24 166 90 lb/hr
23 / 23 167 87
24 / 22 161 81
25 / 21 156 77
26 / 20 --- --

[trafficinsight]

Originally Posted by kiloalpha

Okay here is the info that you all been waiting for.

4,000 feet
MP / RPM TAS FUEL
22 / 24 154 92 lb/hr
23 / 23 156 89
24 / 22 158 83
25 / 21 155 80
26 / 20 145 75

6,000 feet
MP / RPM TAS FUEL
22 / 24 166 90 lb/hr
23 / 23 167 87
24 / 22 161 81
25 / 21 156 77
26 / 20 --- --

remind me again, but those are book power settings too right?

nice data

[shdw]

Originally Posted by trafficinsight

remind me again, but those are book power settings too right?

nice data

I think it was all taken off of page 45/46 from http://www.lycoming.textron.com/supp...Operations.pdf which is the engine manual for his aircrafts engines. I don't know if his book gives the same information or not.


Great data, unfortunately it disproves the units of power at least as they are described. I guess still ok for a baseline, not perfect calculations, but probably still fine to make benchmark power settings at altitude.

Hey you might want to play around with some other settings, maybe even explore the 75 power range and see if you can get the fuel burn lower while keeping that speed up. I don't know if you noticed, but that link from that I linked again here has the chart for the engine as well to help you.

23 / 23 156 89
24 / 22 158 83
25 / 21 155 80

They got 6 gallons an hour and 2 knots speed increase, you can save them another 3 and loose 1 knot from the original for a total of 9 ghp saved and 1 knot lost. IMO that is a number maybe worth mentioning, good luck.

[alloyicon]

Originally Posted by Orange Anchor

For a little perspective of how long this debate has been going on, Lindberg flew lean of peak AND over-square. When he went into the Pacific in WWII he demonstrated the techniques in the P-38 and almost doubled the range. Later (1943) this increased range was used to fly a low level intercept of Japanese Admiral Yamamoto's airplane. One way the intercept was over 400 miles, beyond the range of other US fighters.

Later, the airlines used lean of peak and oversquare to increase the life of their big radials and reduce fuel burn.

I recall reading an article in an old 1970's plane magazine which discussed this very issue. It talked about Charles Lindberg and how he demonstrated the LOP & over-square just as mentioned above.

I still have the magazine in my "library" of technical books & publications over at my dad's house. I'll try to did it up and perhaps even scan and post it sometime soon.

[moxiepilot]

Originally Posted by dasleben

Damn, when you all say "Technical Talk," you ain't kiddin'.

I might have said something in page 1, but then I got to page 3 and decided I'm stupid. I do however like the scroll wheel on my mouse. It made me feel better and my head hurt less.