what happens if I....

[erich weaver]

Ok, need some education here.

For better or worse, my simple mind thinks of a constant speed prop like the gears on my bicycle. Start off in lower gear (high RPM) for relatively low speeds and shift to higher gears (lower RPM) for cruise. For me, "high gear", or lower RPM, means 2400 - just going off what my transition trainer suggested here. After that, I generally dont touch it until shortly before landing, when I go to the high RPM setting - of course at that point the RPMs wont increase any because my throttle setting is so low.

I seem to remember reading somewhere - maybe one of the John Deakin articles - that for cruise, the best efficiency will be with the prop set at the highest possible pitch (lowest RPM) without overburdening the engine. Kind of makes sense using my bicycle gear analogy. But when I ride my bicycle, that point is intuitively obvious - where does it occur for my RV - how do you KNOW when to stop decreasing RPM? What happens if I progressively decrease my RPM below my standard 2400 using the prop control while keeping my manifold pressure constant? What changes can I expect in power/speed and CHTs? Can I damage the engine at some point, or will I somehow be warned before this occurs from the engine parameters or perhaps ugly engine sounds? Are there circumstances where I might WANT to set the prop at less than 2400 RPM in cruise?

Hoping to move beyond "doing what I was told", and instead do what makes sense - perhaps those will end up being the same thing

thanks

erich

[yakdriver]

Boy, did you open a can of worms. You will find many, many opinions on this subject. Lycomings like RPM and I like to run mine between 2400 and 2550 in cruise with full throttle at altitude. Just a FYI. In WWII they would run full throttle, prop lever all the way back and 50 -100 degrees lean of peak in the Pacific for max range. Charles Lindberg developed this technique when he did the NY to Paris flight. Don

[frankh]

Wide open throttle at altitude (MP at 24" or less) 2350 to 2400RPM LOP.

Thats what i run for cruise...If I need a little more altitude I'll add some more RPM if the rate of climb is not enough.

When running LOP I would only do that and keep below 75% however

Frank

[William Slaughter]

The fundamental physical processes at work here are that WOT results in the lowest pumping losses for the engine (imagine that with the throttle partially closed, the engine is trying to "breathe thru a straw"); lower RPM results in lower frictional losses in the engine; and lower prop rpm results in improved propeller efficiency, largely, but not entirely due to the reduced mach number out at the tip. This last effect can be observed quite easily when one observes a throttled up AT-6 Texan that seems to be making more noise than thrust. So, running WOT whenever possible, and adjusting power output via rpm is undoutedly an efficient way to go. How far "oversquare" one is willing to operate is open to lots of debate, and depends on lots of factors including your compression ratio, octane rating, temperature, etc. etc. etc. Most normally aspirated GA engines have very wide detonation margins, however. I'd certainly urge anyone interested in this topic to google for John Deakin's numerous articles. There is a ton of excellent historical data that John makes very accessible. There was also a great article in Sport Aviation many years ago titled something like "Horses That Eat but don't Pull". With the CD archives of the back issues this could probably be found pretty easily. Highly recommended reading.

[asav8tor]

People get way too engrossed in "pumping losses". As the altitude increases the air gets thinner so the "pumping losses" become less of an issue. In the RV there is generally so much excess power if you want max range (no wind, or with a tailwind) you will need to run the prop at at slower RPM, pull the throttle back and lean, slowing down near L/D max. EI is a big plus.

The increased frictional losses and airframe drag of going fast trump "pumping losses" because the RV cruises so far above L/D max at higher power settings. Not the case with spam cans.

[Steve Sampson]

Erich, can I suggest you take a look at the Lyco Operators Manual. There is a chart on p3-16 for an O-320-A for example. It shows the limiting manifold pressure for various rpm and I would suggest that it provides a lot of leeway. If you don't have one you really should get one!

I typically cruise at 2050 and 22.5, so 2" oversquare. That is well inside the chart and everything feels 'right'. The RV4 also slides along at a good speed on very little fuel at that setting. (165-170mph and about 23lph.)

My view is if you are inside Lyco's spec you should be fine

[Finley Atherton]

Erich,

Have a read of pages 43 and 44.
http://www.lycoming.textron.com/supp...Operations.pdf

Fin
9A

[erich weaver]

Gonna take me awhile to digest the info in the lycoming document - a quick read gave me the impression they are kinda speaking out of both sides of their mouth, but I need to look at it more closely.

I found this "why oversquare is good" article by Mike Busch on AvWeb:

http://www.avweb.com/news/airman/184483-1.html

Im a believer, but I still havent seen anything articulating what one would see/hear/feel if taken to the to extreme - i.e., too much oversquare. Will CHTs or some other engine parameter warn me that I am exceeding limits before detonation and engine damage occurs, or can I fall off the cliff before any red lights flash at me from my engine monitor?

Dont get me wrong - Im not aching to go out and test the limits of my engine - just want an understanding of what happens under these circumstances

erich

[Flying Scotsman]

I have never understood the "rule" against running "oversquare". As long as you're on the performance charts, you should be good, right?

I think it came about as just an easy way keep students from doing something really harmful.

Hey, they ran Merlins in Mustangs during WWII at 60", and during Reno at something like 150", right? LOL!

[Norman CYYJ]

I guess it can best be answered this way. How much gas do you give your car and how slow do you drive it before selecting a lower gear. Same thing with the prop. It will get to the point where the engine is just lugging and trying to work and it can't.