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kcpilot81

Member
I am trying to get a handle on running LOP and wanted to verify the ability to run LOP while over squared. I prefer to run over squared if possible (less turns of the engine and quieter) but I wasn't sure what effect this would have on LOP operations. Understanding the need to generally (some may say conservatively) run LOP below 65%, this power setting can be achieved either high-rpm/low-mp or low-rpm/high-mp. In essence, my question is does it matter, so long as I keep it under 65% power?

Thanks for the lesson.
 
My understanding of it is that low-RPM/high-MP is not good for the engine if run for extended period of time. Also, LOP is more useful in higher elevation and typically even with WOT one can not get to 75% power setting. I hope this makes sense
 
Bavafa said:
My understanding of it is that low-RPM/high-MP is not good for the engine if run for extended period of time. Also, LOP is more useful in higher elevation and typically even with WOT one can not get to 75% power setting.
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It is difficult to understand what you mean by "low RPM/high-MP" and "higher elevation" without any actual numbers. There is nothing inherentlyl wrong with oversquare operations - old myth. And LOP is very useful at all altitudes - mainly for saving fuel, your engine and your money. Read Deakin, try it, and join us converts.

erich
 
Over-square is fine, to a point.

For the most part, the taboo of over-square ops is myth. The other referals here to Deakin is good, and vital. I love his lean while climbing strategy, saves fuel and climbs faster.

If you look in the Lycoming operations manual for your engine, you can find implied limits on how far over square Lycoming is willing to run engines. Look at the charts labeled "part-throttle power curves" or something like that. (I don't have mine available right now) Anyway, look at the lines of power at constant RPM and you will see the range of manifold pressure that they run.

For example, following the 2400 rpm line, it runs up to 27 in. hg. Thats the one I remember. I think the 2500 rpm line goes all the way to 29.92 in., but I could be remembering wrong - take a look. The 2300 rpm line probably stops at 26 in. hg, something like that.

So the point is, Lycoming is implicitly saying it is fine to operate the engine there, by giving you the power curves to those points. I would not recommend running 2400 rpm and 28" hg, for example, it is off the curve.

My favorite LOP setting is 2400 and 24" - and it works out to about 8.4 - 8.6 gph on my IO-360-A1A. I will set that condition at any altitude any time (assuming I can get 24", see below)

Somewhere around 7,000 ft, I forget exactly, I can't get 24" anymore. I find that at wide open throttle, it runs a little rough if I lean to 50F LOP, so I close the throttle just a tidbit. Not enough to drop the MAP at all, or maybe just 0.1" of drop, but the angled throttle plate improves flow distribution and I can lean much more aggressively while the engine runs very smooth.

All this is with stock injectors. 3 cylinders peak at exactly the same fuel flow, the last cylinder peaks 0.1 gph less. I maybe got lucky :)
 
Rule of thumb taught to RAF students for the Bulldog (using the 200HP Lyc IO-360) was to avoid exceeding RPM/100+4" i.e. at 2400RPM, do not exceed 28". Actual limits tables quoted this margin varied with RPM and could be up to 5+".

Extra 300 I fly (IO-540) quotes cruise at 2200RPM/24" leaned to EGT gauge.

i.e. as above, running slightly oversquare is not only OK, but has benefits for economy. Running LOP I would think would further increase that margin - so safe to same rules of thumb. Just bear in mind there is little/no margin now should you either move throttle forward or descend without increasing RPM etc.

We cruise 2300RPM/24" LOP.

HTH?
Andy
 
I'm going to change all of my engine instrumentation to read rad/s and hPa. That should end any discussion of "over square" before it gets started. :)
 
Steve, this is the best contribution to this topic ever.

As long as you run your engine in the range that ist given by the manufacturers power charts, everything is fine. If you want to save fuel, reduce RPM to reduce the friction losses and increase manifold pressure to reduce the pumping losses of the engine.

ROP is only to increase detonation margins at higher power setting. At cruise power at or below 75% you can savely run at peak EGT or LOP.
Running the engine at best power slightly ROP is only usefull, if you are at altitude with maximum throttle and RPM and you still want to go 1/2 mph faster no matter how much fuel it takes.
 
The only "rule" I follow for LOP operations is to throttle back to 75% power or less. Other than that, I an LOP at all altitudes and settings. I typically do not run real low RPMs with real high MPs. Like Erich said, come on over and join the dark side of the force.
 
Avis

Since revs/sec is Hertz, my controls prof always referred to rad/s as Avis.
That ought to confuse the panel lookers!

Flying Scotsman said:
I'm going to change all of my engine instrumentation to read rad/s and hPa. That should end any discussion of "over square" before it gets started. :)
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instructors

I can't begin to count the number of instructors that have warned me about over square ops in a constant speed airplane.

I just look at them and ask if they've ever flown a C172. Then ask them what the manifold pressure and rpm are at takeoff. BLANK STARES!
 
Radial numbers

nucleus said:
It's a holdover from radial engines.

Hans
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Really? Sorry, but I must disagree. Here are the numbers from the R-1830s that I flew on a DC3:

Takeoff 2700 RPM, 48.0 Inches MAP
Climb 2500 RPM, 42.5 Inches MAP
Cruise 2300 RPM, 31.0 Inches MAP

Note that the engine is supercharged and the prop RPM is geared to .500 (one half) of the listed crank RPM. And yes, they were run LOP.

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA
 
Good to learn new things here. Being fairly new to aviation, specially Lycoming type engine, I have been told over square for extended time is not good for the engine, this came up during my last IFR training and the instructor confirmed it. Living near sea level, I often get more of MP then RPM at take offs but that is for short period of time. Anyway, I guess I will not loose much more sleep over it from now on if I catch myself in that scenario. But I am definitely in favor of LOP and use it on trips.
 
First post here after lurking a while, so forgive my lack of local forum credibility for a moment...but:

The only "rule" I follow for LOP operations is to throttle back to 75% power
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Why not leave it WOT and use the red knob to adjust your power? Once you're LOP, your power setting is defined as (14.7)*(gallons per hour) and throttle bodies are most efficient when wide open, so why not leave it open? You wouldn't take off if you found your air filter 25% clogged, now would you? That being said, there's nothing inherently wrong with your approach, I'm just pointing out an alternative method of operation per the teachings of Deakin, Atkinson, and Braly at APS.

There is absolutely no rule that says you can't run LOP at higher than 75% power, you just need to make sure you are lean enough at higher power settings. Below 7500ft, I run my Mooney (IO360-A1A) at 50-75 LOP. Above 7500ft, I run closer to 20 LOP, and above 10,000ft, I richen up to best power to make the speed. The % of power at that altitude is well below 70% and it really doesn't matter where you put the red knob. Rich for speed or lean for economy....take your pick. Unless I have reason to be lollygaggin around the sky, I just set WOT and adjust power and speed with the propellor and mixture.

As I descend from altitude, I don't touch the mixture until pull to cutoff at the hangar, and I use throttle to control descent rates. Descending pushes your mixture further to the lean side and provided your engine runs at the right hand side of the spectrum, you're doing nothing but good things to your cylinders. (Low CHT, low ICP, and low EGT....not to mention low gph).

Walter Atkinson routinely runs his TN-A36 Bonanza at 85-90% power LOP.
 
Bavafa said:
...Living near sea level, I often get more of MP then RPM at take offs but that is for short period of time...
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As pointed out earlier - put a MP indicator in a fixed pitch Skyhawk and see what you get. I'd say most pilots have flown significantly over square for many hours while learning to fly. Every touch and go is way oversquare, and if you are renting an airplane "wet" I'm sure most people run wide open throttle going cross country - that's likely oversquare too (depending on altitude).
 
John,

Thanks for the link to the Deakin articles. They fully explain LOP and oversquared. From the articles it really seems as though WOT LOP operations allow the most control of the engine and the most bang for the buck as far as fuel savings.

I would really suggest that anyone who is running LOP or is interested check out the articles. (Seems like my understanding of the need to be under 65% was just another old wives tale that Deakin was able to put to rest.)
 
Why not leave it WOT and use the red knob to adjust your power?

Well, my understanding is this. At 75% power and above, there is a very narrow band where detontation can occur. If I recall, it starts at about 50 ROP and goes up to some point LOP. Since my IO-540 really drinks the dinosaur juice like crazy above 75% power, I don't run there that much except for takoff and power climbs. So for me it was very easy to remember to just stay 100 ROP while in that power range. Level off, pull the RPMs back to get under 75% power, then push the LOP button on my GRT and pull the mixture back until cylinder 2 peaks and I'm done.
 
keeping it simple

f1rocket said:
Why not leave it WOT and use the red knob to adjust your power?

Well, my understanding is this. At 75% power and above, there is a very narrow band where detontation can occur. If I recall, it starts at about 50 ROP and goes up to some point LOP. Since my IO-540 really drinks the dinosaur juice like crazy above 75% power, I don't run there that much except for takoff and power climbs. So for me it was very easy to remember to just stay 100 ROP while in that power range. Level off, pull the RPMs back to get under 75% power, then push the LOP button on my GRT and pull the mixture back until cylinder 2 peaks and I'm done.
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Since my engine is carburated, I can't balance the injectors perfectly, so trying to manage power with the red knob above 75% would just end up with the thing running as rough as a cob. Now at altitudes above 8,000' or so, where wide open throttle is less than 75%, I might argue that the red knob is basically my power manager - it's the only thing I can fiddle with once I set the RPM I like. the funny thing is that whether i lean by ear/roughness, by fuel flow, or by watching the time graphs of EGT - I end up at almost exactly the same place - about .8 gph below the peak power flows shown in the Lyco manual for a given %HP, and definitely on the lean side of peak.
 
Randy:

I understand your point about detonation above 75% (from what I understand detonation is a possibility at that power setting with his CHT and high cylinder pressures) but my use of the term WOT, and I believe what Scott C might have been inferring, is using the prop control to reduce power. Hence my question about over squared. The throttle can remain wide open while the RPMs can be reduced to well below 75% power using the prop control. These prop adjustments could be used during climb if you want to reduce power (and it will be a lot quieter) and once you are cruising you won't be above 75% anyway.

I also really liked Deakin's explanation as to what people are doing by closing the throttle to reduce power in relation to using a dirty air filter. By closing the throttle you are just reducing the air coming in, why not just fly with a dirty air filter, you will have the same results (I know, we need the power for takeoff and climb but it makes sense that reducing the air into the engine doesn't seem to be the most economical power adjustment).

So yes, I agree that the red knob is not your sole power adjustment but the blue and red knobs together seem like they might just do the trick if you want to run WOT? Again, I am not really sure about how this would work, just want to see if anyone else is running their engines this way.
 
Decided I don't much care for a few methods...

kcpilot81 said:
Randy:

I understand your point about detonation above 75% (from what I understand detonation is a possibility at that power setting with his CHT and high cylinder pressures) but my use of the term WOT, and I believe what Scott C might have been inferring, is using the prop control to reduce power. Hence my question about over squared. The throttle can remain wide open while the RPMs can be reduced to well below 75% power using the prop control. These prop adjustments could be used during climb if you want to reduce power (and it will be a lot quieter) and once you are cruising you won't be above 75% anyway.

I also really liked Deakin's explanation as to what people are doing by closing the throttle to reduce power in relation to using a dirty air filter. By closing the throttle you are just reducing the air coming in, why not just fly with a dirty air filter, you will have the same results (I know, we need the power for takeoff and climb but it makes sense that reducing the air into the engine doesn't seem to be the most economical power adjustment).

So yes, I agree that the red knob is not your sole power adjustment but the blue and red knobs together seem like they might just do the trick if you want to run WOT? Again, I am not really sure about how this would work, just want to see if anyone else is running their engines this way.
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After flying and experimenting with WOT yesterday............I've about decided "bla" to it all. First off, since my Hartzell C/S has a prop restriction between 2000 & 2250 rpms, I usually run at 2300 to 2400 rpm for cruise. I don't care for going down to 2000 rpms at all. I was flying in the 8500 - 9500' bracket, and I know just where I like the red knob to be in regards smoothness & potential plug fowling. I have a carb and have no intention of using the red mixture knob as some sort of power control. I've pulled the throttle back when flying with some slower aircraft, and have always done better fuel wise for the same miles traveled as compared to WOT; but not a whopping difference. At fillups between throttleing back & WOT, it's something like saving 3/4's of a gallon out of 12. And of course WOT just gets me there faster.

L.Adamson --- RV6A
 
Larry,

I'll just say that LOP has helped me to extend my range on a couple of cross country's (with 1/2 hr reserve). I was able to make it all the way back to Indy from Austin, TX in 4 hours and a slight tail wind.

I do it more to keep the engine buring clean rather than using fuel to cool the engine. Depending upon how much you fly, it "can" make a dent in your fuel bill.
 
****...had a long reply typed up and the forum lost my login cookie and my reply. Cliff notes:

Carb guys - if you have trouble running LOP at WOT, try closing throttle just a smidgen to turbulate the intake air and better atomize the fuel.

The detonation risk above 75% power is greatly exaggerated. Once you're safely LOP, CHT and ICP goes way down and detonation margins go way up. If you are worried about leaning slowly through the red zone, try doing the Big Mixture Pull instead. Level off and trim for high cruise at your climb power. Then smoothly but quickly pull the mixture until you feel a slight deceleration, like you just took your foot off the gas pedal in your car at highway speed. Voila!, you're now LOP and safely through the red zone in minimal time. You can now set LOP cruise by finding peak from the lean side of the curve.
 
Scott_C said:
The detonation risk above 75% power is greatly exaggerated.
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By whom? State facts please.

I'm a big believer in LOP operations, but don't just discount the real dangers of detonation from running within 50 ROP without backing up your claims with facts. I choose to run LOP but in doing so, I am aware of the few risks. I'm not overblowing the detonation risks. It is very manageable and easy to avoid, but I also don't ignore it either. Because I tested exensively at various power settings and MAP settings, I had a chart that provided me with exactly the fuel flow for peak LOP operations at all these various settings. For me, running LOP was very easy and I could dial in the FF in seconds and cruise along safely and confidently.
 
I'll have to get back to you with actual facts (I'm at work, away from materials at home), but for now...by the thousands of hours of test data produced by GAMI. They run a variant of the Lyc TIO-540 (sorry, don't remember specifics) on a test stand. This engine is reputed to be one of the most detonation prone engines in the GA fleet, and they have an incredibly well instrumented test stand. Basically, under normal conditions, detonation is actually quite difficult to produce. I'm not saying it doesn't or can't happen, just that the margins are better than the old wives tales indicate.

One could argue, and they do at GAMI, that LOP is considerably safer than ROP when it comes to detonation margins. Assume for a moment that you're running the same CHT for ROP and LOP, your internal cylinder pressures are much lower LOP and the speed of the flame front at ignition is slower.

I'll try to dig up the actual facts in the next day or two, but in the mean time, please accept this anecdotal info as pretty close to the real thing.
 
I can't recommend enough that any pilot operating a piston engine take the Advanced Pilot Seminar. It is truly eye opening and you will be armed with a great deal of knowledge to operate your aircraft more safely....whether that be ROP or LOP. They obviously believe that LOP is the best way, but they do give a fair amount of attention to operating safely on the ROP side should you choose to do so.
 
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