Squared RPM's?

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francesco.doenz
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Squared RPM's?

Post by francesco.doenz »

Sorry about a most likely basic question, but not being of english native language, what does "squared" RPM mean? it's most likely a precise value in relationship to some other value or number?
Thanks in advance once more!
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AKar
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Re: Squared RPM's?

Post by AKar »

In short, it refers to setting an RPM/100 and manifold pressure (in inches of Hg) to an equal value. As in 2400 RPM and 24'' of MP resulting in 24-24, or "24 squared" as they say.

-Esa

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Jacques
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Squared RPM's?

Post by Jacques »

"Over" squared means running your engine at a manifold pressure over your RPM divided by 100. For example an engine running with an MP of 30" and an RPM of 2300 is being run "over square", with the idea that this was bad and would wreck the engine. I assume running "square" in this case would be 23" at 2300 RPM.

Perhaps this was taught to ensure that you never advanced the throttle ahead of the propellers?

--apologies, I doubled up what Esa just wrote.

William Hughes
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Re: Squared RPM's?

Post by William Hughes »

The relationship between manifold pressure and rpm and fuel mixture is a bit complex. Some operating conditions at some power levels could damage or degrade the engine. Running over square is fine under some circumstances with the appropriate power settings and mixture. It was more critical for the larger radial engines.

As a general rule of thumb, you probably can't hurt the engine if you run it at or under square power (manifold pressure equal to or less than rpm / 100) with a full rich mixture. So if a person was in a hurry and wasn't familiar with the engine or settings that is an ok place to go.

In the long run that isn't a very economical way to run these types of engines.

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AKar
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Re: Squared RPM's?

Post by AKar »

Yeah.....quite complex indeed! But when it comes to the over-square operation, one can in practice always operate an engine over-square with no issues.

The "oversquare" thing appears to be mostly...well, horseshit. :mrgreen: I've went quite far on attempt to find anything to support the often-quoted fact the oversquare operation was harmful to any engine, and so far have found none. Of course, forcing an engine to excessively low speed when attempting to output some high torque is not a smart thing to do. But the relationship has nothing to with being at squared power setting. Actually, the data seems to suggest that for almost any given goal, accomplishing it with under-square power setting is way more demanding to the engine in overall than utilizing a reasonable over-square setting. Of note is that the takeoff settings are exclusively over-square, and in most naturally aspirated engines, they also represent the maximum continuous power setting.

But this is not to say there isn't a limit on how much throttle one wants to give to an engine at restricted RPM. For the O-540-A, as in Piper Comanche, the manufacturer's plot would go like this:

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The red dots I added represent the "squared" settings. Noteworthy is, how the line that would go through them, would diminish off-scale to the left from the performance envelope when we go towards higher power settings, due to elementary physics of a naturally aspirated engine, and in scale of inches of MP, would also go out of meaningful existence as we lower the power settings. In that sense, there is a critical area of high power settings, where the RPM is limited from the below. But with this specific engine, the margin is some six inches of MP at MP settings that come RPM-limited.

Of course, this is not to say that running at the limiting line would be a smart thing to do, as it would tend towards high mean effective pressures to provide a given output. But neither it would be smart to keep up the RPM, and associated unbalanced reciprocating loads, as happen with operation at under-squared setting. :)

The radials, as it turns out at least for the specific ones I've studied, were not any more critical in this respect per se - actually, vice versa! With those, it is often discouraged to operate the engines under-square unnecessarily. This is to avoid negative torque situation - the crankshaft arrangement in radial engines apparently necessitates a lubrication scheme in which the situation is somewhat prone to lack of lubrication if exposed to negative torque. Against another misconception, this is not directly applicable back to the flat piston engines in GA fleet, however, most of them appear to dislike pronounced high RPM, low MP running due to counterweight issues.

Edit: A worthy addition would be that with naturally aspirated engines, I can't think of any credible source or data suggesting that running at squared precisely would be bad thing to do on its own, that's of course not what I'm saying. However, much of hangar discussion out there quote the squared settings as "high limit" for MP...that's the thing I've got a beef with, for it actually seems to be near the bottom limit of sensible operation. :) Forcing one under-square with these engines is rather similar to young (and certain very old! :mrgreen:) drivers who are shy towards gear shifts, driving on first or second the engine screaming at 4000 RPM.

-Esa
Last edited by AKar on 19 May 2016, 00:00, edited 2 times in total.

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AKar
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Re: Squared RPM's?

Post by AKar »

...and when we get to the high performance, turbocharged engines, the gap in between just expands. With TIO-540-J2B, the squared settings go off-scale low, not even seen in the plots. In itself, this graph "allows" for some far more dangerous operating regimes than being over-square. :)

Image

-Esa

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