Oversquare ops

[unreal]

I've noticed that the old rule of "never operate with a higher MP than RPM divided by a hundred" is still alive and well, both at my current school, and apparently at others as evidenced by the thread about power reductions after takeoff.

This never made a whole lot of sense to me to begin with, simply because turbocharged engines are almost always operated oversquare, and normally-aspirated engines often takeoff oversquare, and even have cruise power settings that allow for oversquare operations. Now, I have two questions:

1) Are turbocharged engines typically built to handle a higher loading compared to normally-aspirated engines, or is the oversquare fear on NA engines just a kickback to that "old rule"?

2) On a typical NA engine found a light aircraft (we'll say anything up to an IO-550), does operating the engine oversquare even really do anything that'll cause a reduction in service life? My understanding is that the old oversquare rule had more to do with old radials than anything else.

Just curious about all of this. I've found that my school is chock full of "old rules" that someone came up with years ago and nobody ever questioned. Unfortunately a lot of that has seeped into my own training, so it'd be nice to get to the bottom of this as to get out some of those bugs.

 

[CA1900]

I've noticed that the old rule of "never operate with a higher MP than RPM divided by a hundred" is still alive and well, both at my current school, and apparently at others as evidenced by the thread about power reductions after takeoff.

It amazes me, too. You have two entirely separate readings, in different units, measuring different things. Why in the world would they be related? What if you measured centimeters of mercury instead of inches? What if the prop tach were in "Thousands of revs per hour" instead of "Hundreds of revs per minute"? 2400 would all of a sudden be 144, but would be the same speed. Would that mean I could then go to 144 centimeters of mercury? What about 144 inches?

There's only one "guideline" for operating an engine, and it's found in the operating handbook for it. I'm not sure where that "oversquare" idea originated, but I'd venture that very few engine models are actually restricted that way.

This never made a whole lot of sense to me to begin with...

Go with your gut! Because you're absolutely right.

1) Are turbocharged engines typically built to handle a higher loading compared to normally-aspirated engines, or is the oversquare fear on NA engines just a kickback to that "old rule"?

Depends entirely on the specific engine. Some simply use turbocharging to maintain sea-level manifold pressure to a higher altitude. Others allow pressures higher than sea-level pressure, and of course that would require a sturdier engine (or shorter engine life).

2) On a typical NA engine found a light aircraft (we'll say anything up to an IO-550), does operating the engine oversquare even really do anything that'll cause a reduction in service life?

Quite the opposite, actually. Assuming you stay within the pressure limits of the engine, let's say you're presented with two possible power settings for a given percent of power: One is a higher-RPM, lower-MP setting, and one is lower-RPM, higher-MP. I'd contend that the more revolutions you put on that engine, the more you're wearing it out. Every time it goes around, each valve is getting cycled, each bearing is wearing, etc. I'd go for the lower-RPM setting for the same reason I do it in my car by going to the highest gear possible: a quieter airplane (inside and out), less engine wear, and better fuel economy because of reduced engine friction.

My understanding is that the old oversquare rule had more to do with old radials than anything else.

It's another bit of "knowledge" that was never correct. Take the F4U Corsair's R2800 engine: Rated-power climb was 42.5" of manifold pressure on 2600RPM.

Just curious about all of this. I've found that my school is chock full of "old rules" that someone came up with years ago and nobody ever questioned.

That's the beauty of teaching people to fly: You're motivated to find real answers, not conjecture, and you'll often find that the things you thought were true--like the "oversquare" myth--really aren't. If someone refuses to believe that it's a myth, ask them to show you in the aircraft manual, a flight training manual, or any kind of definitive source. I'll bet you they can't.

 

[USMCmech]

1) Are turbocharged engines typically built to handle a higher loading compared to normally-aspirated engines, or is the oversquare fear on NA engines just a kickback to that "old rule"?

First off, there are two kinds of turbocharged engines on airplanes.

Turbonormalized engines with automatic wastegate turbos should maintain no more than 31" no mater what you do with the throttle. In this instlation the turbo does nothing for you at sea level. It gradually adds in boost to offset the lower pressure as you gain altitude.

Turbocharged engines with fixed wastegate turbos will have a maximum allowable MAP, useually about 40". If you add full throttle at sea level you will overboost it and poossibly cause a seal to blow on the intake manifold. The engines are built to handle the added pressure, therefore as long as you don't exceed the limits, you're OK.

2) On a typical NA engine found a light aircraft (we'll say anything up to an IO-550), does operating the engine oversquare even really do anything that'll cause a reduction in service life? My understanding is that the old oversquare rule had more to do with old radials than anything else.

Operate your engine by the figures in the POH and you'll be fine. 2100/23" is no problem on most engines. The slower speed may actually reduce vibrations which wear on your accesories. All the old radials ran oversquare, some racers at Reno run about 2500 RPM and 70" MAP!

Now, don't get stupid. It's apropeller not a paddelwheel! 1800/29" on most engines is not published in the operating handbook, and will proabaly break soemthing eventually.



I too learned most of these false myths about piston engines as I was training at a large flight school with glossy ads in Flying. My time at A&P school cured quite a few of them.

Read the articles from Avweb by John Deakin.

http://www.avweb.com/news/pelican/list.html

Best tutorial on engine operation I have seen.

 

[pilotyip]

undersquare bad

On round engines, R-1820's etc. Oversquare ops are not a big deal if done within the limits specified for the RPM and MAP. Used to launch off the boat in the C1A, no cat 59.5" 2800 RPM. But on round engines undersquare is to avoided if at all possible. Undersqaure causes the prop to drive the engine and reverses the loads throughout the drive gears and increases the likely hood of premature failure.

 

[midlifeflyer]

Open a piston POH. Look at the power setting table. Then look at the engine manufacturer's operating tips on its web site (Lycoming generally recommends the highest MP and lowest RPM combination in the table for smooth operation).

 

[avbug]

Correct responses on all accounts. Oversquare operation is a myth.

You asked if there's a difference between a boosted engine and one that isn't, with respect to construction...in most cases, no. The same engine that's normally aspirated vs. one that's boosted (turbocharged, supercharged, turbonormalized, turbocompounded, whatever) usually involves the same part numbers. For example, an O-520, IO-520, and TSIO-520 may use the same cylinder part numbers. Same construction, same material, same workmanship...but for different engine designations. This isn't always the case, and you need to refer to the specific engine...but the truth is that blown engines (boosted) often use the same parts that normally aspirated engines do. The increase in pressure isn't so much stress on parts due to pressure, but due to head, and decreases the margin approaching detonation in cylinders...which tends to do more internal damage than external...which is part of the reason that in many cases the same cylinders are used for both engines.

If you think about it, a normally aspirated engine operating at sea level will be showing nearly thirty inches of manifold pressure before any ram air rise at high power settings. Your RPM is typically 2500 to 2700 rpm at the prop, which means your takeoff manifold pressure is already "oversquare."

As was previously stated, you need to look at your manufacturers power tables and limitations to determine what's applicable to your aircraft, and you should always stay within those limitations. Many more issues apply to a power setting than oversquare...oversquare is a non-existant theory without any backing, which has been passed down from instructor to student in what I like to call the heritage of inexperience...one statement of ignorance passed from an inexperienced instructor who was told it by another inexperienced instructor who was told it by another inexperienced instructor, ad nauseum.

Other issues affecting takeoff power setting are the compression ratio, propeller limitations, type of drive and gearing, crank shaft design, harmonic issues, and a host of other reasons that a given limitation or power combination is provided by the manufacturer. The manufacturer has subjected the engine to very expensive and extensive testing...the manufacturer does not explain why the limitations apply in most cases, but you should always abide by the power schedules, fuel flows, temperatures, RPM's, and other criteria established by the manufacturer.

Pilotyip touched on the issue of undersquare operations...operating undersquare, per se, won't necessarily cause harm to your engine. But operating undertorque can...the potential for damage is greater in some engines than others. As a general policy, operating when the propeller is driving the engine isn't a good policy and it's a matter of poor airmanship. You'll find some in aviation who disagree with this...Avweb publishes a list of "Aviation Myths" which include this among others. I disagree, and I've had discussions about it with the author and many others...the particular type of engine is a big consideration, but even simple direct drive engines as a rule shouldn't be descended for prolonged periods of time at idle at high airspeeds, when the prop ends up driving the engine.

"Undersquare" operation is okay, to a point...only when you hit a point of zero torque when the engine isn't driving the prop, can you run into potential problems. In piston engines, and reduced power settings, thermal issues can become a concern, which is another reason to avoid prolonged operations at reduced power, too.

Operation "oversquare" isn't so much about overboosting anything or lifting heads, but more about detonation...but again, the "oversquare" issue is a mute point...nonexistant becuse it presumes a relationship or rule which simply does not exist. As always, refer to the manufacturer recommendations, proceedures, schedules, and limitations.

 

[Huggyu2]

"Undersquare" operation is okay, to a point...only when you hit a point of zero torque when the engine isn't driving the prop, can you run into potential problems. In piston engines, and reduced power settings, thermal issues can become a concern, which is another reason to avoid prolonged operations at reduced power, too.

I'm trying to understand, since I'm very new to all of this. Is the zero torque issue a problem because everything is sort of "floating", e.g. the prop isn't driving the engine and vice versa?
And in your second sentence, are you referring to "shock cooling", or something along those lines?

 

[avbug]

This depends on the engine. Geared engines vs. non geared engines, size, drive, make up etc. Floating isn't really a correct analogy, though backlash, especially in geartrains, can be an issue. Torque is reduced or reversed, with a reverse loading on bearing journals, crankshafts, etc. Pressure, wear, and fit is brought to bear opposite and out of sync in gear teeth, and on bearing surfaces. (Some argue this, saying the engine is still turning in the same direction and therefore this cannot be...but they're incorrect).

Thermal issues can include shock cooling, though that discussion is beyond the scope of this thread. Shock cooling is a controversial subject. A primary means of temperature control and cooling in an aircooled engine is airflow, as well as power management (to include power settings, engine RPM [and subsequently propeller RPM], mixture setting, etc).

Suffice it to say that one should treat one's engine as though one's life depends upon it, becasue surely it can do. The ignorance with which engines are often treated, as a function of the heritage of inexperience (particularly in the training arena) means that many pilots are scarcely aware of the proper operation of the powerplants under their control...a lot of abuse takes place. One can get away with it in some powerplants, but not in others. Fostering good habits is best done when both taught properly, and practiced properly, and good airmanship dictates that one properly treat one's equipment, even when the equipment is forgiving enough that one may get away with poor behaviors.

 

[Big Dog]

I have done a little research on the topic. As it turns out, 25" - 2500RPM is about 75% on most IO type engines, which is recommended for climb in many A/C. 23" - 2300rpm is about 65%, which is good for cruise in many A/C. I would imagine many operators told there students to operate the engines at those settings to 1. Give them a starting point & 2. Help control wear and abuse.

In short, operating over square is a myth. One should always consult the POH to find the proper power setting. In general, pick the percent power that is appropriate for the operation (high speed cruise/climb power - 75%, reduced wear/normal cruise - 65%, extended endurance - 55%, holding power - 45%, ect.) I can't stress how general that all is. The C402B (large bore, turbo charged) recommends no more than 70% power where the C172R can be run at 80%. Then use the combination of RPM/MP that gives that percent power. In general, higher RPMs will give you higher CHTs (increased friction heating) and higher fuel burns. I tend to use the higher MPs with 2300rpm w/ most normally asperated engines to get the best combination of KTAS/engine wear/fuel burn as opposed to incresing the RPMs to 2400-2500 rpm, although according to most POHs either is allowed. Of course this will all vary slightly with specific atmospheric conditions and altitudes.

Again, I can't stress how general that all is and how important it is to read both the A/C and engine POHs and to read, understand and follow the power charts and recommended procedures. Also remember that many older A/C have STC's and the books may not reflect what is actually in the A/C, for example, older C172s w/ O-360 180hp Lycomings or BE58s w/ IO-550s. But that's another kettle of fish...

 

[Wiggums]

What aircraft POH recommends you climb at 75%? Always climb at max continuous at the recommended airspeed from the POH.