S

Steve Sampson

Well Known Member
In the Lyco handbook performance charts there is always an area labelled 'Limiting Manifold pressure for Continuous Operation', at about 29"MP and a little lower at high revs.

Is this just relating to detonation? If so I would have thought the area would be labelled 'avoid' since you just don't want to go in there. Or is there another reason? Longevity? I realise I trespass there on every takeoff though no sign of detonation on the instrument logs.

Another question. Does anyone routinely operate down at around 2000rpm & 24"? I feel uncomfortable going there, though it is well inside the Lyco chart. I run well over square, usually 2050 and 22.5", but not that far. I would be interested in their experience. At my setting it is very economical.

I am running an O-320 160hp with P-mags.

Thanks
 
Steve,

Isn't the 29" limit the practical limit that can be achieved without a turbo/blower? It should be nearly 30", but ... I have never read anything into that end of the chart.

I was always told that as long as the manifold number is not more than 5" greater than rpm/100, then you're OK.
That is the MP limit = (rpm/100) + 5.
So at 2000 rpm the limit is 25". I don't know the basis of this "rule". I've always found operating that much over square made the engine a bit lumpy - but that was in the days before electonic ignitions. I would imagine you would get even better economy the further over square you go.

Pete
 
Hi Pete, well for example on the last takeoff the highest pressure logged was 29.4 and that was just below 2700rpm. (I have seen a little higher.) Not for long, because I pull the prop and power back. The book limits you to about 28.4 continuous at that rpm. Hence my question.

The rule of 5 is because that is how Lyco drew the graph, but the question is the same as to why.

I think I will see what 2000/24" does for economy. I find operation like that is strange, probably not because I am used to it, but between the MT prop and the P-mags, its quiet down there and smooth. Everyone I have ever discussed it with wants to operate square and does not appear to have looked at the Lyco charts.

Perhaps an engine guru will chime in.
 
Hi Steve/Pete...

Various rules of thumb about. Bulldog teaching is about +4", however, the manual gives more details - of course it varies according to RPM, but can go as high as +5" (or even +6"?).

Best economy will be oversquare. We do 2250RPM / 24" - much lower RPM and you get into the vibration band, and although it is not an avoid for us, it does feel less comfortable.

2000 + 24" sounds OK in theory... just close to the limit, so watch MP in a descent. Am sure someone will chime in if there is an issue with it...

Andy
 
I run over-square every change I get. I was even running 10:1 pistons in my O-320 for a while. My first engine had 2,200 hours on it when I did an overhaul. The valve guides failed the wobble test but compression was still good. Cylinder bores measured out to be NEW spec including the choke. Now running new ECI Titan STEEL cylinders and H2AD 9:1 pistons. Using about 1 quart of oil every 50 hours. In other words, I do not add any oil between oil changes.

Last two summers, I left Borger Texas (KBGD) for Connellsville Pennsylvania (VVS). Once I could get down to 3,500' MSL, I pulled the prop back to a power setting of 2300 RPM and WOT (Wide Open Throttle) 26" MAP. Fuel burn was just over 8 GPH and ground speed with a tail wind was over 180 Kts.

Read the power charts for the Lycoming engines. They have an over-square setting for almost every RPM. Yes there are some combinations that you do not want to run at. Look at where they are, memorize them, and set a safety factor so that you do not end up there. I can run almost 6" over square. I set my safety factor as trying not to run more than 3" over square.

If you are unsure about running over square, read the articles on AvWEB about running over square.
 
I've always said that if you don't like the oversquare numbers, use a different set of units - use Revs per Second, and none of the numbers match up, so they shouldn't scare you! ;) "Square" is an accident of the units we use...

The fight about "oversquare" operations is left over from big radials - if the engine is happy according to the power charts, it is happy, regardless of how the numbers read. Personally, I don't even know what MAP I use - I set the RPM and use % HP as shown on the EFIS. I climb at as high a %HP as I can get, cruise at the highest I get (below 75%), and pull the mixture back until it squawks. The engine is nice and smooth at 32350 RPM, so that is what I generally use.

And below 65% HP, Lycoming says you can't hurt it, no matter what you do....

Paul
 
Ironflight said:
The fight about "oversquare" operations is left over from big radials
Click to expand...
Actually, the big radials were generally highly supercharged, and ran way oversquare. I flew S-2 Trackers for a few years. Take-off power was 2800 rpm and 56" (that was if we could get 115/145 fuel - we usually burned 100/130 and then the limit was 53"). Max continuous was 2700 rpm and 51". Typical cruise power while on patrol was 1800 rpm and 29", and if fuel was tight we would decrease the rpm as low as 1500 as the aircraft got lighter.

Many of the smaller radials weren't supercharged, or were only very mildly supercharged, and perhaps some of them had restrictions against oversquare operation.
 
oversquare

29" and 1800 is also a typical power setting for the P&W R985. Most of these engines as used in the Beech 18 etc have a relatively mild supercharger ratio. The Beech runs around 35.5" for takeoff. Other 985's have higher blower ratios but these are quite rare. Early six cylinder Continentals as in the early Bonanza and Navion turned the prop very slowly by todays standards, and the Bonanza cruised in the 1900 range. These were the E185 Continentals. The Twin Commanche with the hartzell two blade prop and long extension had problems cracking the crankshaft at low r/m's. The 0 360's both parallel and angle valve are the worst of the Lycomings. Most have a restriction between 2150 and 1350 with either the two blade Sensenich metal prop or the Hartzell two blade.
 
My favorite plane, the P-51, with my favorite engine, RR Merlin, used something like 60". Max WEP was 64".

Like Paul says...why on earth something measured in oddball units like inches of pressure of mercury should "match" Rev/Min is beyond me.

What would be the rule if our instruments measured MP in hectopascals? :)

I think the "oversquare" stuff is just a rule of thumb that instructors used to keep students from doing anything really outside of the charts, nothing more.
 
jrs14855 said:
The 0 360's both parallel and angle valve are the worst of the Lycomings. Most have a restriction between 2150 and 1350 with either the two blade Sensenich metal prop or the Hartzell two blade.
Click to expand...

But isn't this more a problem of harmonic resonance than anything to do with something like detonation? From Hartzell, we see restrictions like this:

FOR UNDAMPED 200 HP IO-360:
CONTINUOUS OPERATION IS
PROHIBITED ABOVE 24 INCHES
MANIFOLD PRESSURE BETWEEN 2350
AND 2550 RPM
Click to expand...

as an example. That would clearly allow operation *below* 2350 (such as 2000) at any MP, up to and including WOT (unless otherwise limited by the performance charts)...but the key here is that it's a limitation from the *prop* manufacturer, not the engine manufacturer, right? Or am I missing something?
 
Getting back to Steve/Pete and Lycs ;)

XP-IO-360 book we have has various power curves. There is a "cutoff" from 2000RPM/25.4" to 2400RPM/29.4" that I cannot see a physics reason for... so suspect that is the "max oversquare" - in turn figures with the various rules of thumb above.

At MSL, the 180HP engine at 2000/24" is~112HP i.e. ~62%. The 2250/24" we tend to use is~68%. Of note however, the books uses 23"/2300 for "Economy Cruise", 21"/2300 for low power cruise, and 25"/2400 for "Performance Cruise".

HTH
Andy
 
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