erich weaver

erich weaver

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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
 
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
 
yes a few opinions

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
 
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.
 
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.
 
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
 
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
 
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!
 
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.
 
Norman CYYJ said:
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.
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Exactly. We can easily detect this point on our bicycles and in our cars by feel/sound/performance. But can we do it in our airplanes also or would I be stupid for even trying to feel where the limit lies?

erich
 
erich weaver said:
Exactly. We can easily detect this point on our bicycles and in our cars by feel/sound/performance. But can we do it in our airplanes also or would I be stupid for even trying to feel where the limit lies?

erich
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I get what you're saying...could you, let's say, set MP to some fixed setting, like say 23", whatever, and then just keep reducing RPM...2350...2300...2200...2100... all the way on down to the coarse pitch stop?

I don't know why not...I think what would happen would be the propellor would start to "stall" along some portion of its length and you'd lose airspeed, and that would get worse until you reached the stop or the whole thing was stalled (generating no thrust). I'd be surprised if it hurt the engine any, unless it was turbocharged or something. But what the heck do I know? I'm no A&P or engine guy...

In a car, the engine is lugging when you're at too low an RPM for a gear because it's trying to turn the wheels against the road, right? If you had a car up on blocks, I'll bet you get the thing running just fine at very low RPM in a high gear. Maybe the same for an airplane engine/propellor combination?

An interesting question...just sitting here over dinner pondering this a bit :)
 
There are a lot of variables

I always pondered this stuff too, so I went to the GAMI Advanced Pilot class. The GAMI guys have done a lot of work dispelling Old Wives Tales. They have sold a lot of people on lean of peak operation when that was a no-no in the past (some still don't believe it). Its really a good class and I highly recommend it.

What they teach is its all about temperature, pressure and efficiency (BSFC - a measure of power vs fuel consumption). Translated you can do anything for a while, but if you care about long term operations and maximizing engine efficiency you have to look at many variables. For example, there is a concept called Theta PP (TPP - or in laymans terms, what is the crank position in degrees at the point of Peak Pressure). They built pressure transducers and put engines on stands to replicate flight and run it in a wide variety of loads, fuels, fuel ratios, timing, etc... Pretty cool, you could see on a scope what was going on in the cylinder.

One of the things the was eye opening was that fuel at a given fuel ratio burns at the same speed (note - you can change fuel burn speed by changing mixtures - LOP mixtures burn slower than ROP mixtures) from ignition to TPP. Regardless of RPM we run the same advance (at least if you have mags), usually in the range of 20 to 25 degrees before TDC to allow reaching TPP just after TDC. Most aviation engines (not all) reach TPP at about 6 degrees after Top Dead Center or so. If you slow the engine RPM (assuming mixture remains the same), the engine will still burn the fuel in the same amount of time regardless of the fact the engine is not turning as fast. That reduced time means TPP is falling back to 2, 3 or 4 degrees. That is a bad place for the crankshaft to do work, in fact at TDC it cannot do work, it is pushing straight down on a connecting rod. That conrod is under stress and cylinder pressures go up significantly as you move TPP back toward TDC. That is hard on cylinders,valves, rods, bearings, etc...

I am not saying this is a problem per se (and many engine manuals allow some amount of it), I am saying there is an effect and a limit to it. Its not a case of some is good, more is better and too much is just enough - you can take the idea too far. This does not take into account prop effiency, which admittedly is better slower.

Long before you got to the idea below you would be putting a lot of pressure on the engine that is not necessary. Will it fail, no - within reason. Will it go to and beyond TBO if run that way all the time.... you decide. Do you have a better chance of a long cylinder life if you manage heat and pressure? Bottom line, if you believe heat and pressure is the enemy of your engine, you will take note of this.

Sorry for the book.... Its a tough subject to condense.

Flying Scotsman said:
I don't know why not...I think what would happen would be the propellor would start to "stall" along some portion of its length and you'd lose airspeed, and that would get worse until you reached the stop or the whole thing was stalled (generating no thrust). I'd be surprised if it hurt the engine any, unless it was turbocharged or something. But what the heck do I know? I'm no A&P or engine guy...
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You know, I plumb forgot about the fixed timing on aircraft engines (hey, I'm still building the fuselage...procuring the engine is still a ways off :( ). That would make a huge difference, I agree.

What if we assume variable timing, though, perhaps via some EI system?
 
You could

Yes, if you retarded the timing properly you would would offset the RPM effect on TPP. Everything is a trade off. You will be producing less power, but cruise is not a place where you generally need a ton of HP or you wouldn't be pulling back the prop anyway :D :D

Sophisticated engine management systems in today's cars do that very effectively. They run LOP and adjust fuel/timing - even shutting down unnecessary cylinders as appropriate for the conditions.


Flying Scotsman said:
You know, I plumb forgot about the fixed timing on aircraft engines (hey, I'm still building the fuselage...procuring the engine is still a ways off :( ). That would make a huge difference, I agree.

What if we assume variable timing, though, perhaps via some EI system?
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PROP RPM RESTRICTIONS

Up to this point, no one has mentioned the effect of solid-state ignitions on propeller blade resonance. Certain engine/prop combinations with solid-state ignitions have RPM ranges where you must not operate due to damage to the prop.

I have a 160HP O-320 with a Harzell constant speed prop, 1 Slick mag, and one solid-state ignition. With the O-320, the power pulse is not strong enough to damage the prop. But with a similar setup on a 180HP or 200HP Lycoming, there is a range of RPM where you don't dwell. I seem to remember it is somewhere in the 2100-2350 RPM range. I am sure some the users of these engines will chime in now that I have raised the issue.
 
Started studying the lycoming performance curves in operator's manual. Man, these things give me major mental fatiuge. One thing for certain - there are LOTS of variables that come into play. Based on info presented in the chart, RPM, MP, altitude, temperature, and mixture all have a part in this. The charts ASSUME you are at best power mixture, so right there you are introducing error. Also, if you opted for high compression pistons, theres no curves provided - the best you can do is use the curves for the standard (8.5-1 in my case) pistons.

Well, if you can get over all those limitations, theres still no real way to be able to follow these charts in flight to calculate power, but it looks like using only RPM and MP, and perhaps assuming some "normal" cruise altitude will get you in the ball park.

The manual provides performance curves for many different engines. Interestingly, only some of them indicate a limit on MP during continuous operation for various RPM settings - not sure why. For my IO-360B1B, (using chart for IO-360B1A - there is no chart specific to -B1B), I wrote down the limiting MP for each 100-RPM setting from 1800 to 2700. It is apparent from this that over-square operation is permitted throughout that range of RPM settings, but more so at the higher RPM settings. For instance, at 1800 RPM they want you to stay at or below approixmately 21 inches MP, while at 2400 RPM you can be well above 28 inches MP. Flight at typical cruise altitudes would tend to automatically limit the MP anyway, so realistically you probably wont be going much more than a few inches over-square anyway.

Not sure, but Im guessing the MP limitiations built into the curves must be related to the spark timing issue - at the lower RPM settings, the piston will start getting too close to TDC during peak pressure, and you start losing power and risk detonation etc.

erich
 
erich weaver said:
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
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Erich,

Try this read: http://www.avweb.com/news/pelican/182081-1.html

It's one of the Deakin's articles and should give you a good understanding of MP vs RPM. Not just the theory, but in practice as well. He addresses exactly what you are asking.

Here's what would happen if you ran at WOT and course pitch (from the link)

From full throttle at 4,000 feet, 25 inches MP and 2,700 RPM, we reduce the RPM back to 1,200, changing nothing else. (Again, please don't do this!) The pressure in the intake is already at ambient pressure, so it can't go any higher, nor can it go any lower. The pistons are still pumping, but at less than half-speed, so they're not pulling nearly as much air through. The MP will not change appreciably. (A few pilots report a very slight rise in MP, which is probably because there is less resistance to the slower-moving air at the filter screen, but essentially there is no change.) Because of the dramatic drop in RPM and airflow, the fuel flow drops too, meaning much less power is being developed, with no change in MP.

Here's a link to a whole bunch of Deakin articles - they should answer many if not all the quetions posed in this thread:

http://www.avweb.com/news/pelican/182544-1.html
 
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Yup, good stuff John. Had read that Deakin article on MP some time ago, but needed a refresher.

thanks

erich
 
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