Oversquare ops

[DC8 Flyer]

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

If I remember correctly, the seminole POH called for 25" and 2500 RPM for climb and the baron called for 2500 rpm (full throttle) for the climb. IIRC.

 

[avbug]

Most aircraft have a variety of climb schedules, just as they have cruise schedules. Climbing at full power may or may not be necessary...again, according to your mission. If you're doing pattern work, for example, carrying full power to one thousand feet or fifteen hundred feet may not be reasonable...you may be making a power reduction right after takeoff just to keep it slow and keep the rate of climb reasonable...all depends what you're doing.

The subject of operating at reduced power has been mentioned as a way of saving wear on the engine. Operating at a reduced manifold pressure does not necessarily save anything...reducing RPM does, but sometimes at other costs...and may still result in more wear. You need to look at your specific mission--what you're hoping to accomplish with the aircraft on a specific flight--to determine what schedules you want to fly.

Takeoff thrust or power is not necessarily the same as climb thrust or power. Use what you need, and what you have planned. Using maximum takeoff power when it's not needed is just as ridiculous as not using enough when you need more. Use what you need.

 

[orcfi]

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 is 100% correct

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.

This is incorrect with the respect to pressure issues. At a given manifold pressure, higher RPM's are easier on the engine. However, pressures only come into play at higher MP settings. A normal piston aircraft engine (non FADEC) has fixed timing. The cylinder usually fires about 20 to 25 degrees prior to the piston reaching top dead center. The mixture does not just explode, it burns with a flame front progressing from the plug to the center of the cylinder. Ideally, peak pressures within the cylinder happen as the piston is travelling back down, producing optimum mechanical advantage (peak power). If the piston is travelling slower (slower RPM), then the pressures are higher because the mixture is firing closer to top dead center, meaning that it has to overcome the position of the connecting rod, producing more pressure within the cylinder (bad) and less power.

 

[Fedora]

Try investigating Charles Lidburgh's efforts in the western Pacific in WWII.
He played a major part in teaching American pilots how to fly the P-38!
Oversquare was what he preached, and he increased the range of the aircraft by almost twice, without damaging the engines. Good story, really!

 

[Rally]

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.

Its a bunch of crap.....I have a Cessna 172 that is 1956, it just happens to have a manifold pressure guage. (don't know why) guess what you will be running over square until you reach about 5000 msl. Follow the POH, otherwise people are trying to reengineer or make up procedures that really have no basis. Its pretty dumb, yes most turbo a/c cruise over squared. Does'nt seem to TEAR them apart. The other thing that kills me is the BIG hurry to get the power pulled back, just relax pull it back lesiurely at 1000 agl or so, no big deal or hurry. Geez.

 

[RedGuy]

Actually running high pressures do take their toll on turbo charged engines. I fly 310's and 402's they both use identical engines, Continental IO-520's. But the 402 is turbocharged and always cruses over square. The 310's run to 2200hr TBO, and the 402's only get 1800hr TBO. On the 310 you can't tell the difference between a 200hr engine and a 2200hr engine. On the 402 you can feel her getting tired when it's getting near TBO, CHT's start running high, oil turns black quickly, uses more oil, has less power, and use more fuel. So running higher pressures definitely cause more wear.

 

[avbug]

Higher pressures don't cause more wear. More heat causes more wear, higher RPM causes more wear, inconsistant operation, heating and cooing differences...these cause more wear. The engine doesn't know, and can't tell about cylinder pressure...the fuel air mixture knows about it, and responds with detontion under very specific circumstances...which does reduce engine life. But higher pressures of their own accord do not reduce engine life.

Two entirely different airplanes. Same manufacturer of the engine, but not the same engine. Not the same fuel system, delivery system, operating parameters, cooling, induction..different engines. Apples to oranges. Poor comparison, and one cannot draw conclusions with any degree of accuracy as you've done there.

Nice try, though.

 

[RedGuy]

Higher pressures don't cause more wear. More heat causes more wear, higher RPM causes more wear, inconsistant operation, heating and cooing differences...these cause more wear. The engine doesn't know, and can't tell about cylinder pressure...the fuel air mixture knows about it, and responds with detontion under very specific circumstances...which does reduce engine life. But higher pressures of their own accord do not reduce engine life.

Two entirely different airplanes. Same manufacturer of the engine, but not the same engine. Not the same fuel system, delivery system, operating parameters, cooling, induction..different engines. Apples to oranges. Poor comparison, and one cannot draw conclusions with any degree of accuracy as you've done there.

Nice try, though.

Your right higher heat does cause more wear. With higher cylinder pressures you will get higher temps, you'll also get more blowby which breakes down the oil faster. The planes while they may look entirely differant, they're not, they share many systems and from the wing root out they are identical to a turbo 310. And actually they are the same engine, same fuel system, and same fuel delivery, induction and cooling are the only differences. The 402 is turbo charged, and has additional cooling via cowl flaps. The 310 is generally operated at 2300rpm and 23 inches MP, the 402 is run at 2300rpm and 30 inches of MP. This isn't apples to oranges, it doesn't get any closer than this.

 

[avbug]

Again, nice try. Two entirely different airplanes. Make very small changes, such as small holes or positioning of baffles, or sizes or shapes of inlets, you have completely changed the thermodynamic cooling process of the engine, and you can make zero comparisons between the two...mount even identical engines (these are not, despite what you may think) on two different airplanes, swinging different propellers, at different speeds under different conditions in different cowlings with different baffles with different inlets, different inductions, different cowl openings...you have different engines and most certainly you do have apples to oranges comparisons.

Same spark plugs and timing? Same cooling inlet area, outlet area, and shape? Same baffling, same fuel flow, same airflow? No.

Moreover, operating at 30" isn't much of a boosted pressure...certainly not enough to cause excess wear and damage. Think about it...the airplane at sea level when normally aspirated runs at that value, paricularly with ram rise in flight. That's not highly turbocharged, that's turbonormalized...and yes, there is a difference. You're boosting to sea level pressure only...no different than what the normaly aspirated airpalne is capable of seeing. Blowby isn't increased, nor is it really an issue. I believe you mentioned darkening of the oil before, which is a ridiculous measure by which to assess wear in an engine...color is irrelevant and meaningless. You appear to believe this is a result of blowby and evidence that one engine is experiencing more wear, which puts your observations squarely in the realm of those of a housewife understanding her car. You can do better than that.

During a standard compression check I flow 80 psi or greater to the cylinder, and some of it may or may not leak past the valves or the piston. An engine close to TBO may have nearly perfect compression, or it may not. TBO is a reference number, and other than some legal implications, it's largely imaginary. Further, an overhaul may be nothing more than verifying that parts are within tolerance and putting them back...laying some lack of creditiblity to the idea that engines wear out at TBO...TBO doesn't mean much, and overhaul means even less.

W(h)eather you believe it or not, you have two different engines there, in two different aircraft, under two different circumstances and two different operating parameters. Apples to oranges. Examine two identical 402's, operating one at the lower RPM and manifold pressuree setting, and the other at the higher power setting, and if operated properly, you won't see much difference, if any. Especially at those minimal power settings.

Of course, when you're running 30 inches, you're running the same induction pressure as you get with the normally aspirated engine at sea level, when the normally aspirated engine is SHUT OFF...that's barometric pressure, and that's what your manifold pressure gauge will read when the airplane isn't even running...think about it. What you have are thermal differences which are not a consequence of the manifold pressure, but the combustion process and your mixture and power settings, being operated by different pilots at different times.

I ran a fleet of nearly 30 airplanes that all had the 0-520's, some turbocharged, some not. Most everything made it through TBO without worries, and most could have gone far beyond, very few problems, because we looked after them. They were regularly used, regularly inspected, some coming in for 100 hours two times a month. When I flew the boosted airplanes, I seldom used much boost because I didn't need it, but operationally, we had better value with the normally aspirated because they weighed less and were a little less complex and did the same job.

When you shut down your turbocharged models, are you running them for five minutes to cool the turbo and help eliminate coking in the turbo bearings? your black oil is the result of residual oil in the case following a change, and thermal differrences. Blowby does contribute to darkening of oil, but may be considered inconsequential, becuase to some degree you're always getting it, and if it's enought to be causing breakdown of the oil, you're already low enough on your compressions (and cylinder pressure isn't the cause) that you need to be doing something about it. Regular oil changes are the order of the day. If you're not operating the engine properly, you're going to see more wear. Adequate warm up and cooling periods, as well as preoiling, will make a lot of those problems and the wear you're seeing go away.

As far as running at 30" being the cause of excessive wear...it's good for a laugh, but not very realistic.

How do you suppose we could run 45" or more inches of manifold pressure and still have engines left? How do you suppose the normally aspirated engines run all day long at the same 30" hg. of turbonormalized induction pressure as your 402, without a hiccup? Truth is that you're not providing anything to that engine that it doesn't see sitting on the ramp cold and quiet...other than heat and pilot abuse.

Again, nice try.

 

[RedGuy]

Look I know I could go on an on and you would always have some sort of counter because you know everything. But comparing there 2 are not apples and oranges. Are these two exactly identical circumstances, no, but relevant to this conversation since no one else has given any other comparisons, including you, it's the best that we've got. Comparing these to are like comparing green apples to red apples. Also the fact that you say the 402 is turbo normalized leads me to believe you don't know a dang thing about the 402. They are not turbo normalized, 30 inches is just the cruise setting that we use, you can use 32inches and 2300rpm and they go up to 34.5 inches and 2700rpm for max power in the B models and 39.5/ 2700 in the C, BUT the C DOES have a different engine, still a 520 but a different model, where as the B's use the same model 520. The inlets are the same, the baffling is different and the outflow are different to allow MORE cooling on the 402, which should make the engine last longer. Also they are swinging the same propeller and both A/C go the same speed. So again for the purposes of this discussion this is as close as it gets, since no one has come up with a better example.

Also I never said it causes excessive wear, it increases wear. Blowby IS increased, anytime you put more pressure into a cylinder you will inevitably get more gases that pass by the cylinder rings (more blowby), they teach you that in basic auto shop! Also color of oil is not irrelevant, if your oil is dark you have more particulates in it, which will break the oil down chemically or cause it to form sludge blocking oil passages, again basic auto shop here, nothing earth shattering. If blowby and oil color was meaningless and irrelevant then we wouldn't have systems to remove blowby or change our oil. Why do you think you can drive 100,000 miles on your transmission fluid (which is just oil) but only 3,000 on your engine oil? It's because blowby breaks the oil down, since there's no blowby gases in the transmission the oil doesn't break down.

You also keep referring to the fact that when you cruise in the 402 at 30 inches it's the same as a N/A engine at sea level. This is true, but a N/A engine is only going to be at that pressure for a few seconds because even at full throttle the instant you break ground the pressure will drop. No N/A engines cruise anywhere near 30inches and in most cases you don't even get 30inces on takeoff since the air is restricted slightly through the induction system and filter. Also I still don't understand your rationalization that an engine not running on the ramp showing 30in is the same as a running engine at any MP, that just doesn't make any sense.

We run or engines far past TBO through approval from the FAA, the 402 goes from 1400hrs to 1800hrs and the 310 goes from 1600hrs to 2200hrs. The 402 still gets a lower TBO because of the simple fact that they do wear out faster. Why because we're running higher pressure through them, which produces more power, which produces more heat, more blowby and just generally causes things to wear out alittle faster over the long term. Is it excessive wear no, but it is increased wear.

And yes, we use adequate warm up, cool down and cool the turbo's before shut down.

The end.