Cruising Oversquare

ZRotheohv · Post by **ZRotheohv** » Tue Dec 15, 2015 5:22 pm

On my M-5-235C the only official propeller limitation is to keep more than 2050RPM if over 23" MP. I've been reading about best economy cruise, and apparently Lindbergh was a proponent of running oversquare.

As long as I set something below 23", I should be able to pull the RPM back as much as I want (to maintain speed/altitude) right? Or would something like 22" and 1800RPM be bad for the engine?

Mountain Doctor · Post by **Mountain Doctor** » Tue Dec 15, 2015 8:52 pm

Answer is it is usually fine. You will want to pay attention to keeping below 75% power in cruise, monitor your temperatures etc. I believe it is John Deacon who did a series of articles on this, with carefully documented engine management data to prove that it is what he refers to as a " Old wives tale "

Kirk · Post by **Kirk** » Wed Dec 16, 2015 2:02 am

Sounds like you have the O540-J1A5D with a Hartzell prop, same combination I have.
The limitation is for a vibratory stress. It is mentioned in the AFM for the airplane. I've found that you can feel the vibration when you approach that range. It's a bit of a shake and gets fairly pronounced. I wouldn't ignore it.

You can pull back to a little over 2100 RPM and cruise over square, but I have always gotten better economy with 2000 RPM. The Lycoming manual can be found online and has power tables for Climb, 75%,65% and 55%. I've gotten best range at 65%.

Going to a 3 blade or whatever other prop is approved probably does away with the limit, but I am not familiar enough to say what prop is best for cruise on the O540-J1A5D

Jeremy? You probably know.....

Kirk

Rezrider · Post by **Rezrider** » Wed Dec 16, 2015 9:45 am

I have a J-series O540 and cruise at full throttle (usually 20" MP at my altitudes) and 2200 RPM if I want to get there and 2100 RPM if I want to save fuel and/or have a tailwind.
I think the over square thing is largely a myth, since if it were a big deal we would be seeing placards and limitations all over the place trying to stop us from doing it.

Jayson v Schalkwyk · Post by **Jayson v Schalkwyk** » Wed Dec 16, 2015 1:54 pm

Kirk wrote:
You can pull back to a little over 2100 RPM and cruise over square, but I have always gotten better economy with 2000 RPM. The Lycoming manual can be found online and has power tables for Climb, 75%,65% and 55%. I've gotten best range at 65%.

Jeremy? You probably know.....

Kirk

I am new to Maules and trying to soak it all in. I have downloaded the Lycoming IO-540 manuals as I have the W1A5D but dont seem so see the tables as you mention above Kirk.

Would you mind pointing me to a link perhaps?

Many thanks

ZRotheohv · Post by **ZRotheohv** » Wed Dec 16, 2015 4:24 pm

I found the manual here:

http://www.7ts0.com/manuals/lycoming/54 ... 540_OM.pdf

That has most O-540 and IO-540 models I believe, but it doesn't mention whether cruising oversquare is bad for it. The reason I asked was because of this article:

http://www.aopa.org/News-and-Video/All- ... ient-Pilot

It says engine reps say you're good at least until the MP exceeds RPM by 4 (24"@2000RPM). I may have to do some experimentation!

Kirk · Post by **Kirk** » Wed Dec 16, 2015 4:41 pm

My manuals are paper copies. The power setting charts are graphs rather than tables. I made my own table for inflight reference. Most of the 75 % settings are oversquare and so is 65 % below I think 5,000'.

Sorry I am not where the documents are. Just remember on The J1A5D to respect the vibratory limit and not exceed 23" at or below 2000 RPM.

KIRK

Kirk · Post by **Kirk** » Sat Jan 16, 2016 10:14 am

Got the power table I mentioned posted on a new thread. Have a look and you will note quite a lot of the power settings per Lycoming are "over square".

Heard a comment many years ago by an old timer that the source of the don't oversquare belief goes back to radial engines....

Hope the table helps,

Kirk

Jayson v Schalkwyk · Post by **Jayson v Schalkwyk** » Sun Feb 14, 2016 1:34 am

Hi Kirk, any chance of a link to the table you created?

I have an IO-540-W1A5D

Kirk · Post by **Kirk** » Mon Feb 15, 2016 7:13 am

Jayson,
Just the photobucket link on that post.
http://maulepilots.org/forums/viewtopic.php?t=4388
. You can save and print from that post.
Right click on it if using Windows, tap and hold if usin an iPad.

I'm not greatly computer savvy but it would be nice to put it out as a spreadsheet.

Brenton C · Post by **Brenton C** » Thu Feb 18, 2016 9:50 pm

Where in the sequence to you lean it out? Before or after setting the prop in cruise?

Kirk · Post by **Kirk** » Fri Feb 19, 2016 3:08 am

Do your leaning after you have it all set to cruise power, including prop. On my airplane at least, a half turn of the vernier = 10 degrees EGT.

Higher altitudes, particularly hot weather you may have to lean slightly in climb to get best power. Personally, I've never seen much need for that. Maybe just slightly in Density Altitudes above say about 7-8,000.

Kirk

Brenton C · Post by **Brenton C** » Sat Feb 20, 2016 2:16 pm

Thanks Kirk!

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