Over Square?

[mvivion]

If you're going to muddy the water with supercharged engine MP you may as well add geared engine RPM to add to the confusion.

Stewart,

My point wasn't to "muddy the waters", it was related to radial engines, many if not most of which are supercharged.

I've run O-360s over square regularly ever since and it works great.

MTV

 

[WindOnHisNose]

Mr. Busch, from the Cessna Pilots Assn, strongly encourages running over squared. He feels that having that pressure is important, and suggests that when we descend we lower the rpm to maintain over squared. His reasoning seems sound.

Randy

 

[n40ff]

Not only were the P38's able to extend their range by a wide margin but the Japanese were able to extend the range of their A6M Zeros using similar methods. I recall that after one raid on OZ the allies were sure the Japanese had a new base somewhere closer and spent time looking for it.....I also recall the Japanese developed special spark plugs for such operations. Their cruise speed was just over 100 Kts as I recall, it took a long time to get there but they went a long ways.
RPM was so low you could count the prop blades going by....

Jack

 

[hotrod180]

... He feels that having that pressure is important, and suggests that when we descend we lower the rpm to maintain over squared. ...

When I descend in my 180, the MP goes up if I leave the throttle alone.
Maybe you're thinking lower the rpm as the MP goes down as you climb?

 

[Brandsman]

I run a GO-480 in Piaggio 149D. Here’s something wierd - best cruise is 2 750 at 21” (I think, the book is metric) - but I tried taking it down to 2400 and the oil consumption goes up - suddenly it starts throwing oil out the exhaust! Thump it for aerobatics (full throttle and 3000 rpm) and it uses very little oil and the belly stays clean. I understand this engine is not getting over square just closer to it than recommended, but could the oil issue be the reason it is not recommended? No doubt I am displaying serious ignorance here but - having read this thread - I tried an oversquare setting in my Cherokee 235 this last weekend and found an oily belly from the O-540 which usually runs clean


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[GeeBee]

Increasing oil consumption running running over square is due to increase in BMEP (brake mean effective pressure) While theoretical, when you develop the same horsepower at lower prop RPM, the duration of the combustion stroke is longer creating more time for high pressure to act on your rings. Oil expelling from the breather pipe indicates your rings are worn or ring end gaps are too large or incorrectly installed. You are not developing higher pressure so much as you are giving the pressure more time to act on the rings. At higher rpms there is less time to act and the oil control ring wipes up excess more quickly. On a high time engine or one with worn rings you may see more blow by into the case pushing oil out the breather. Given that most aviation engine oil sumps are over serviced for most missions "breather topping" is not uncommon and can be aggravated by more blow by. Simply put there is nothing free. Run over square and you get better range as more heat value of the fuel is directed to the propeller and not out the exhaust pipe and heating your engine heads. The flip side of that is your rings are under combustion stroke pressure longer and your oil control ring does not "swing the mop" as fast.

 

[Brandsman]

Increasing oil consumption running running over square is due to increase in BMEP (brake mean effective pressure) While theoretical, when you develop the same horsepower at lower prop RPM, the duration of the combustion stroke is longer creating more time for high pressure to act on your rings. Oil expelling from the breather pipe indicates your rings are worn or ring end gaps are too large or incorrectly installed. You are not developing higher pressure so much as you are giving the pressure more time to act on the rings. At higher rpms there is less time to act and the oil control ring wipes up excess more quickly. On a high time engine or one with worn rings you may see more blow by into the case pushing oil out the breather. Given that most aviation engine oil sumps are over serviced for most missions "breather topping" is not uncommon and can be aggravated by more blow by. Simply put there is nothing free. Run over square and you get better range as more heat value of the fuel is directed to the propeller and not out the exhaust pipe and heating your engine heads. The flip side of that is your rings are under combustion stroke pressure longer and your oil control ring does not "swing the mop" as fast.

Thanks very much for that easy to understand explanation - it makes sense. As both of these are low time engines (less than 300 hours TSMOH) its worrying, but it does make sense!