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  #1  
Old 05-03-2022, 01:33 PM
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Av8rRob Av8rRob is offline
 
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Default Oversquared Lycoming chart for -390?

Just curious if anyone has been able to find any Lycoming data on oversquared operations in the IO-390?

Thanks
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  #2  
Old 05-03-2022, 01:46 PM
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Default

Here is some interesting reading.

https://resources.savvyaviation.com/...Oversquare.pdf
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  #3  
Old 05-03-2022, 03:42 PM
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Av8rRob Av8rRob is offline
 
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Quote:
Originally Posted by rv8ch View Post
Exactly, he shows the chart for the -360 so where is the -390 chart?
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  #4  
Old 05-03-2022, 06:14 PM
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It is on page 57 of this manual.

Click image for larger version

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ID:	25399

Thank you DuckDuckGo for helping me find this.
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  #5  
Old 05-03-2022, 09:27 PM
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Av8rRob Av8rRob is offline
 
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Default

thanks, as I read that chart you can run essentially any amount oversquared to a RPM as low as 2,000. Anybody else read this differently?
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  #6  
Old 05-03-2022, 10:50 PM
Untainted123 Untainted123 is offline
 
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Location: Azle, TX
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Default Use the app

Using the free app is way easier, correlates directly to the book, handles all the calculations and chart tracing for you.

AircraftPower by James Petty
https://apps.apple.com/us/app/aircraftpower/id718023187

iOS only, but thatís probably not a limitation for most.

Basically, you can choose any MAP/RPM combo you want (that your prop restrictions will allow) that gets you the percent power you want.

Combined with the Savvy info above (assuming no prop restriction), letís say 22/2400 gave you 75% power at whatever altitude. Using the chart/app, you see that 25/2000 also gives you 75% power. You should choose the over square, low rpm setting vs the under square, high rpm, for the host of reasons in the Savvy presentation.
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  #7  
Old 05-03-2022, 11:55 PM
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Av8rRob Av8rRob is offline
 
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Quote:
Originally Posted by Untainted123 View Post
Using the free app is way easier, correlates directly to the book, handles all the calculations and chart tracing for you.

AircraftPower by James Petty
https://apps.apple.com/us/app/aircraftpower/id718023187

iOS only, but thatís probably not a limitation for most.

Basically, you can choose any MAP/RPM combo you want (that your prop restrictions will allow) that gets you the percent power you want.

Combined with the Savvy info above (assuming no prop restriction), letís say 22/2400 gave you 75% power at whatever altitude. Using the chart/app, you see that 25/2000 also gives you 75% power. You should choose the over square, low rpm setting vs the under square, high rpm, for the host of reasons in the Savvy presentation.

Very cool app, thanks
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  #8  
Old 05-04-2022, 04:59 AM
Skysailor Skysailor is offline
 
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Default Normally aspirated is the key

Light Plane Maintenance was a magazine I used to enjoy when it was published. Engine operation recommendations were much akin to information people are getting from Mike Busch these days. They said there should be no term "over squared operations" in a normally aspirated engine in the sense they must be avoided. Every takeoff is that way given the manifold pressure goes up to the 29-30 inch range while all our Lycomings are limited to 2700 RPM. It's more so in a fixed pitch prop aircraft as initial RPM is more likely 2200 or so.
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Last edited by Skysailor : 05-05-2022 at 05:38 AM. Reason: Updated information
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  #9  
Old 05-04-2022, 06:19 AM
Freemasm Freemasm is offline
 
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Default Look at the roots, a different perspective

If pressure had been in PSI, inH2O, millibars, pascals....

If rotational speed basis had been radians/x, hertz, ....

Would this myth have even been started since the associated numbers wouldn't conveniently line up? The validity of the over square myth was disproved a long time ago.

Side question; did "prop forward/throttle back" come from trying to avoid the over-square mythical danger zone? Avoid potential overspeed from sloppy governors?

I still do it mostly from muscle memory. It also keeps any related questions away.
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  #10  
Old 05-04-2022, 07:04 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Freemasm View Post
If pressure had been in PSI, inH2O, millibars, pascals....

If rotational speed basis had been radians/x, hertz, ....

Would this myth have even been started since the associated numbers wouldn't conveniently line up? The validity of the over square myth was disproved a long time ago.

Side question; did "prop forward/throttle back" come from trying to avoid the over-square mythical danger zone? Avoid potential overspeed from sloppy governors?

I still do it mostly from muscle memory. It also keeps any related questions away.
It is my belief that the old radial engines suffered problems when lugging or heavily loaded (think trying to accelerate in a car without downshifting) and this is where the over square stuff came from. Pushing the prop in, increases RPM and reduces the load on the engine. It is akin to down shifting before accelerating in a car. No problems loading the modern boxer engines, therefore no need to do this, though it hurts nothing to do it.

turbos are ALWAYS over square in a direct drive engine, so this type of mnemonic would have made no sense. Those guys MUST understand their charts and follow the MAP gauge religiously, excluding the more benign turbo normalized set ups.

Larry
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Last edited by lr172 : 05-04-2022 at 07:09 AM.
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