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Oversquare is Good - "Mike Busch"

Mike Meehan

Well Known Member
Friend
I attended Mike Busch's presentation at Oshkosh on running oversquared. The long and the short of it is he shows in the (Lycoming and Continental) POH's that running oversquared is both authorized and, in his opinion, advisable in flight. I have attached the most relevant slides from his presentation. Thoughts?

https://link.shutterfly.com/AP1zZQ8glib
 

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DEAKINS TOO

Mr. Deakins also proselytized over-squared. WOT and put the prop where you want it - along with mixture - to either "Go-Fast or Go-Far."
I pretty much leave the throttle wide open from take-off to pattern, adjusting the prop for the performance desired. With EFII the mixture is programmed and pretty much a non-action. Descending to pattern altitude & landing requires pulling the throttle back to approach power configuration.
Looking at the valves, spark plugs, and pistons with a borescope show there's no bad things going on so far with over 300 hours doing it this way.
I'm sure there's others out there that have many more hours on their engines doing the same thing.
 
"Oversquare is tough to do with a fixed-pitch prop"

Yep. Although I wonder if it's easier with props that flex under load and give a little bit of "constant speed" effect over a small range? Don't the Catto props behave like that?
 
"Oversquare is tough to do with a fixed-pitch prop"

Yep. Although I wonder if it's easier with props that flex under load and give a little bit of "constant speed" effect over a small range? Don't the Catto props behave like that?

I disagree. I have an FP prop on my 6. At the beginning of the take off roll, it is 30 over 22; initial climb is 30 over 24. Way more over square than my CS prop, which is 30 over 27 in T/O and initial climb.

I agree that this is all fine and supported by the designers.

Larry
 
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One caveat here gentlemen...remember those propeller restrictions.

Example...
-
 

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I disagree. I have an FP prop on my 6. At the beginning of the take off roll, it is 30 over 22; initial climb is 30 over 24. Way more over square than my CS prop, which is 30 over 27 in T/O and initial climb.

I agree that this is all fine and supported by the designers.

Larry

Well -- 30 over 22 is pretty extreme - it is outside of the documented continuous operation envelop, at least for my engine. You get this combination with a coarse fixed pitch prop on initial take-off roll. But that is not continuous operation.

Looking at Figure 3-24 in the IO-360-A Operations Manual, it shows 27.5 " hg as the maximum (limiting) manifold pressure for continuous operation at 2200 rpm. 2400 rpm is the lowest rpm for which the manifold pressure does not have a limit - because the full-throttle manifold pressure in their test cell is 28.75" hg. Extrapolating the limit line, if their test cell could deliver 30" hg, the limiting line would have extended up to about 2600 rpm.

I believe this limiting line is based on achieving a specified detonation margin. Those boundaries are usually set with tests at worst-case CHTs but are done with 100LL (well, actually 100/130). I could imagine that on a hot day at sea level, with a hot engine, with MOGAS, opening the throttle full at initial TO roll with a coarse F/P prop could produce detonation.
 
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I disagree. I have an FP prop on my 6. At the beginning of the take off roll, it is 30 over 22; initial climb is 30 over 24. Way more over square than my CS prop, which is 30 over 27 in T/O and initial climb.

I agree that this is all fine and supported by the designers.

Larry

Fixed pitch cruise prop like the Sensenich is most always over square.
 
Well -- 30 over 22 is pretty extreme - it is outside of the documented continuous operation envelop, at least for my engine. You get this combination with a coarse fixed pitch prop on initial take-off roll. But that is not continuous operation.

Looking at Figure 3-24 in the IO-360-A Operations Manual, it shows 27.5 " hg as the maximum (limiting) manifold pressure for continuous operation at 2200 rpm. 2400 rpm is the lowest rpm for which the manifold pressure does not have a limit - because the full-throttle manifold pressure in their test cell is 28.75" hg. Extrapolating the limit line, if their test cell could deliver 30" hg, the limiting line would have extended up to about 2600 rpm.

I believe this limiting line is based on achieving a specified detonation margin. Those boundaries are usually set with tests at worst-case CHTs but are done with 100LL (well, actually 100/130). I could imagine that on a hot day at sea level, with a hot engine, with MOGAS, opening the throttle full at initial TO roll with a coarse F/P prop could produce detonation.

Agreed, In my case 30/22 is only for a few seconds and it quickly gets to 30/24, which is allowed for continuous and then the MAP starts dropping with altitude. I was only rebutting the statement made that FP props CAN'T get over square.
 
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"Oversquare is tough to do with a fixed-pitch prop"

Yep. Although I wonder if it's easier with props that flex under load and give a little bit of "constant speed" effect over a small range? Don't the Catto props behave like that?

Bernie Warnke Almost Constant Speed wood prop was designed to perform as you described. No knowledge of Catto in this manner.
 
Personally I think 2400 rpm is the sweet spot for a 4 banger and that's where I operate in cruise (2500 for climb).

I can get close to 23" at 8000' so with 2400rpm that's approx. 75% power, which is recommended max continuous cruise power.

So all this over square talks is relative down low anyway.

I feel like I'm lugging the engine when I get into the 2300 and lower RPMs and the vibs increase.

Full throttle all the time, no thanks, to hard on the engine.

Last time I checked Mr. Bush didn't help pay for my engine :eek:
 
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Too hard on engine

Full throttle (WOT) is too hard on the engine?

I would be interested in knowing what data you have to support that statement...

I am honestly curious, that's all...no flame bait...
 
Full throttle (WOT) is too hard on the engine?

I would be interested in knowing what data you have to support that statement...

I am honestly curious, that's all...no flame bait...

I think it's pretty common knowledge/practice that for maximum longevity engines should be operated in the 65% economy cruise to 75% Perfomance cruise power ranges.

From Lyc Tech Tips
9. For maximum service life, maintain the following recom- mended limits for continuous operation.
a. Engine power setting — 65% of rated or less.
b. Cylinder head temperatures — 400 ̊ F. or below.
c. Oil temperature — 165 ̊ F. — 220 ̊ F.
 
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I think it's pretty common knowledge/practice that for maximum longevity engines should be operated in the 65% economy cruise to 75% Perfomance cruise power ranges.

I would agree with what you are saying, in cruise. However, if you are cruising at your example altitude of 8000' and 23"/2400rpm, you are likely running WOT, and unlikely to be able to exceed 75% power. My question, then, concerns only the climb portion. It is pretty common knowledge/practice that in GA aircraft, we use WOT for takeoff. During the climb, some will reduce to 25/25, and others will climb WOT and let the MAP decrease with altitude. Do you know of any data that proves this is "harder" on the engine?
 
I think it's pretty common knowledge/practice that for maximum longevity engines should be operated in the 65% economy cruise to 75% Perfomance cruise power ranges.

Sure, and it's easy to get there WOT if you limit power with mixture and with prop. Need tuned injectors to do so down low for sure. Percent power doesn't have to be percent throttle. There is nothing magical about 25 squared or other "normal" percent power settings. As Dan pointed out, avoid prop RPM restrictions, and also regard CHTs as primary instruments for avoiding damaging conditions.

Ed Holyoke
 
The bottom line is higher power settings are logically harder on the engine, sorry but I'm really not interested in providing the data you require to believe that simple fact.
 
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In radial engines, I absolutely have used over-square power settings, as they are advised in the NATOPS / -1. In the RV-8, however, every time I try and go over-square in cruise the vibration increases to an uncomfortable level.

My engine was dynamically balanced (by Walt) at my preferred cruise setting of 2400 RPM, and it is very smooth there. I suspect that a 6-cylinder, or a radial would be smoother at lower RPM's, but my 4-cylinder is not.

During climb I absolutely go over-square, as I reduce RPM to 2500 IAW Hartzell's limitation with PMAGs, then climb at WOT / 2500 RPM to cruise when I pull the RPM back further to 2400. I typically cruise around 9500'-11500', so the throttle doesn't move until it's time to descend.
 
The bottom line is higher power settings are logically harder on the engine, sorry but I'm really not interested in wasting my time trying to convince you otherwise or providing the data you require to believe that simple fact.

Wow.

I deleted the body of my reply.

Sorry I wasted your precious time asking an honest question…have a good evening.
 
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I Do you know of any data that proves this is "harder" on the engine?

Wow.

I deleted the body of my reply.

Sorry I wasted your precious time asking an honest question…have a good evening.

I tried to answer your question but I’m sorry I can’t supply the data you require, if you won’t believe Lycoming then I give up. We are all capable of reading the manuals and making our own choices. I just find it frustrating when “data” must be provided when a little common sense should be enough.

And yes we climb with higher powers for safety, not because it’s easy on the engine.

Sometimes I wonder why I bother posting at all.
 
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Imagine if manifold pressure was measured in hPa or mm.Hg instead of in.Hg, we’d never have heard the made-up term “oversquare” in our entire lives, and nobody would have ever tried to argue about whether it was good or bad for an engine.

- mark
 
Imagine if manifold pressure was measured in hPa or mm.Hg instead of in.Hg, we’d never have heard the made-up term “oversquare” in our entire lives, and nobody would have ever tried to argue about whether it was good or bad for an engine.

- mark

Exactly. And Rotations per Minute? Maybe rad/s (although seconds are *also* a fabrication of humans).

Over-square = OWT.
 
Exactly. And Rotations per Minute? Maybe rad/s (although seconds are *also* a fabrication of humans).

Over-square = OWT.

How about micro radians per fortnight? You would likely never have to worry about going over square...unless you used nano-torr for manifold pressure!

Skylor
 
How about micro radians per fortnight? You would likely never have to worry about going over square...unless you used nano-torr for manifold pressure!

Skylor

I prefer to do all my physics computations in the FFS system...Furlongs/Fortnights/Slugs. (Alternatively, the FFF system...furlong/fortnight/firkin...). :)
 
Exactly. And Rotations per Minute? Maybe rad/s (although seconds are *also* a fabrication of humans).

Over-square = OWT.
I think like many OWTs there may be some nuggets of truth in them. The root of this one could be a simple as this possible discussion between the engine designer to the test pilot:


Engine Designer: "Don't lug the engine."
Test Pilot: "What do you mean by 'lug the engine'?"
Engine Designer: "Don't give too much power at too low of an RPM."
Test Pilot: "I need numbers, my good sir."
Engine Designer: "Don't make the number on this gauge higher than the number on this other gauge."
Test Pilot: "Ah, that I can do. Thank you, sir!"

I think the question for me is what is the definition of "lugging" our 4 or 6 cylinder aircraft engines?
 
In the 2004 turbo 182 I flew up until a few months ago, my normal cruise power setting was 2000/28.0". CHT's ran 340 at cruise. The only difference internally is compression ratio.
 
OWT’s

Percent power is percent power. How you get there is up to you and your maker. OWT’s are nothing more than stories gynecologists tell.
 
In the 2004 turbo 182 I flew up until a few months ago, my normal cruise power setting was 2000/28.0". CHT's ran 340 at cruise. The only difference internally is compression ratio.

The Navajo's I flew were 2500/36 climb. Cruise was 2200/31. Did that for about 1000hrs, my coworkers did the same as it was SOP. Engines all healthy and made TBO.

We are both cruising 8 or 9 over-square in Lycoming 6 cylinder engines with no ill effects.

It really boils down to follow the prop restrictions (such as Hartzell mentioned earlier). And yes, going easier on the engine will help it last longer within reason. Maybe 55% to 75% is better than cruising around at 90% all day.
 
I attended Mike Busch's presentation at Oshkosh on running oversquared. The long and the short of it is he shows in the (Lycoming and Continental) POH's that running oversquared is both authorized and, in his opinion, advisable in flight. I have attached the most relevant slides from his presentation. Thoughts?

https://link.shutterfly.com/AP1zZQ8glib

Well if Mike Busch said it then it must be so...
 
Imagine if manifold pressure was measured in hPa or mm.Hg instead of in.Hg, we’d never have heard the made-up term “oversquare” in our entire lives, and nobody would have ever tried to argue about whether it was good or bad for an engine.

- mark

In the auto world, it's called "lugging" and has been considered hard on engines for a long time. This is no different than starting to pedal your bicycle in too high of a gear. It produces a lot of stress on the rotating parts, as well as the ring/wall interface. Not saying it shouldn't be done, but running low vacuum/high MAP (a measure of load) is harder on the engine than higher vacuum. I agree that the whole square thing is too simplified and a poor way to manage things, but you can't escape the fact that high load operations are harder on an engine than lower loads.

There is a reason that aviation and marine engines don't last as long as an auto engines. They run continuously at higher loads, due to lack of gears. If your plane was a car, it would likely be turning 4000 RPM at a much lower load in cruise. Most cars are cruising around at 16-18" of vacuum during steady state, which is around 14" MAP.

It seems logical that wear is proportional to MAP and seems to be supported by the longevity difference between marine engines and auto engines, which for decades came from the same GM plant. In the marine world, a small block Chevy rarely ran well after 2000 hours. Admittedly there were other issues, such as head warping, due to the poor cooling control.

Larry
 
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RVs

I think it is safe to say that most of the RVs we fly are NOT turbocharged. So this whole discussion is really moot.

At sea level, the best we can do is around 30" MAP, and that will be considered "full power". MAP will decrease by approximately 1" per 1000' as we climb, and we will ALWAYS be running less than "full power". Depending on a myriad of factors, we will reach approximately 23" of MAP around 7000-8000 feet. At this point, the throttle will be wide open, and the engine will not be able to produce more than ~75% power. Any higher altitude will reduce the power available even further. So, in a normally aspirated engine, at a typical RV cruise altitude, you will never be able to exceed the manufacturers recommendation.

If, however, you choose to fly low, it is possible to exceed the 75% recommendation...and it will always be exceeded in the climb. Reducing to 25/2500 is still above the recommended range.

Turbocharging is a whole different discussion, and as has been previously posted, we fly those engines "over square" all the time. I had a C-421 and cruised it at 29"/1900 ALL the time. Those engines went 850 hours past TBO and were still fine when I sold it. The A&P that bought it flew it several hundred hours more, before rebuilding...

I choose to climb WOT and 2550 in my -10. Typically cruise at or above 8000', WOT and 2400 rpm. Cruise power, according to the G3X, is usually around 65-70%. It will be interesting to see the result of the 12 minutes per flight that I am above the recommended range...time will tell.
 
42 years driving planes (currently own 3) and (apart from Turbo engines I've driven) I've never run over square other than briefly at T/off, each to their own but I don't like stress and neither do my engines:) I'm Happy to run ROP & matched or under square for hopefully a few thousand more hours -)
 
I would not be concerned about "lugging", usually characterized in automobiles by resonant torsional vibration of the long, limber powertrain. Although the prop restrictions and engine shake are for very similar reasons, the issue here is probably a concern for detonation margin. Not all engines have the same compression ratio, or the same ignition timing, so an "oversquare is good" generalization may be edgy for the user with 10:1 and a bunch of advance.

You can see the trends in detonation data. I've attached two charts for an IO360 angle valve at 20 degrees advance, running under hot conditions. Yes, the conditions are pro-detonation per the rules, but also note this is CR in the 8's and very conservative timing.

The "normal" power setting of 2700-28 only detonates if the operator allows very high CHT and pulls mixture to near peak EGT, something which no sane operator would do at this power level.

The oversquare setting of 2400-28.6 starts nibbling into detonation around 100 ROP, near best power. It nicely illustrates the why the power charts have a line around 4 to 5 inches oversquare.
 

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Lugging an engine is a lot easier to do if you have drive wheels hooked up on pavement. Sure, you can raise the MAP by pulling prop, but you can't pull the RPM low enough to really lug the thing like you could an old 6cyl Rambler station wagon on a hill. Goes straight to detonation. If you can't keep it from cackling, you have to downshift. If you don't downshift or let up, that's when the real lugging begins. You could drag it right down to where it stalls. And yeah it might throw a rod. Hard to drag the RPM down that low with a prop.

The governor downshifts for us. It changes pitch to shed load so the prop will speed up. If you push throttle, pull the prop to the stop, (go around maybe? should be pushing the prop for climb) and manage somehow to load the prop so much that the blades are on the stops and hold it there, you'll either overheat (and the more than probable onset of detonation), or the prop will stall and the engine will speed up, or the wing will stall. The pilot already has.

We, at least I, don't normally operate anything like that. Climb at higher RPM, more horsepower there = better ROC, and cruise it lower = better efficiency. Lean, when and to the extent, appropriate. I cruise well lean of peak and WOT, even down low, as my injectors are tuned for it and I regard CHTs as a primary instrument for setting power. If my cylinders start trending upwards towards my 400* personal redline, I'll lean more (or get well ROP), push nose over for better cooling, open cowl flaps, and/or pull some throttle, that being my last move.

Dan is right as usual - avoid detonation. Always monitor CHTs at higher power settings, ROP or LOP, and set a conservative alarm temp. If it isn't getting hot it isn't detonating is a pretty good rule of thumb, and always start your experimenting at much higher altitudes than where you can make 75%.

If you don't have CHTs for all cylinders don't try to operate lean of peak at high MAP.

YMMV
Ed Holyoke
 
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