True or false: never set MAP higher than RPM.

[ao.frog]

Hi folks.

Up to this day, I've been under the impression that one schould never operate a Lyc on "over-square" powersettings. In other words; MAP schould never be set to more than the actual RPM.

For example 23/2300 or 21/2300 and so on.

Today, a few guys in my local aeroclub discussed this and a couple of the guys said there was no problem to operate at over-square powersettings.
On a T&G for example, one could set the RPM at 2500 on downwind and use full throttle on the T&G. That would give around 27/2500 I guess.

One mentioned that in his old days, they used 2300 RPM and 36 MAP on the Harvard.

I want to know the facts here, so this question is hereby sent out to all you engine-guru's out there....

I'm standing by...

[PCHunt]

Uh Oh!! Another NED thread! (Never Ending Debate)

[Mel]

There is no problem with operating "over-square". The old "rule" was linked to some old radial engines.
Every airplane with a fixed pitch prop operates over-square during take-off and climb out. You can operate too far over-square by hanging the airplane on the prop and lugging then engine, but normal operations over-square is not a problem.
Think of it as driving up a hill in a car in high gear. It's fine as long as you don't over do it. You wouldn't want to try to drive up that hill at 10 mph in high gear.

[Walt]

Mel is correct, but some engine/props have limits that prevent oversquare operation in certain ranges...

Propeller Model: HC-C2YR-1BF

DO NOT OPERATE ABOVE 22 INCHES
MP BELOW 2350 RPM. O-360-A1A:
OPERATION ABOVE 2600 RPM IS
LIMITED TO TAKEOFF, IOF-360-A1A:
MAXIMUM ENGINE SPEED LIMITED TO
2650 RPM.

[Ironflight]

Besides Alf - what are you doing measuring MAP in inches anyway - shouldn't you be using millibars or something?

the truth is, as was pointed out, the big old round engines had some sound reasons for making sure that you had the correct relationship between MAP and RPM. It was a pure ACCIDENT OF UNITS that the inches of mercury coincided with RPM/100. Use different units, and the relationship breaks down. Physics (and mechanics) doesn't care about the units you use, so this should raise instant suspicion.

taken to the extreme, Mel's example is excellent - don't "lug" the engine going uphill.. but within normal operating parameters (within the prop limits, as Walt points out), "over square' in a Lycoming is a myth.

BTW - I have no idea what MAP I run - I use RPM and %HP....EFI are nice!

Paul

[machineman]

Another way to think about running slightly "over-square" is it's the same as shifting to "over-drive" in a car.

Rich

[RV8R999]

you can operate anywhere within the operating curves published by the manufacturer - looking at the lyc-0360 series and you'll see operating "over-square" is within the approved limitations (pick a MP and slide down the line to see the range of RPM which are acceptable). BTW- I called Lyc directly and asked the same question and what I wrote above is what their engineer told me.

[RV6_flyer]

Originally Posted by ao.frog

Hi folks.

Up to this day, I've been under the impression that one schould never operate a Lyc on "over-square" powersettings. In other words; MAP schould never be set to more than the actual RPM.

For example 23/2300 or 21/2300 and so on.


I'm standing by...

I run over square in my O-320 RV-6 every change I get. I flew from Borger KBGD Texas to Pennsylvania KVVS over square a year ago. When I say over square, I am talking 2,300 RPM and WOT or 26" MAP. Yes I was at 3,500' once I got out of Texas. Yes I was not over square for landing.

The power setting I used is on the power chart in the Lycoming Operator's Manual for my engine. Any power setting that shows up in the Lycoming power chart for your engine should be safe.

BTW. My RV-6 will be 14-years old in September 2011. There is over 2,500 hours on it. The cylinders were replaced at 2,200 hours. There was NO Wear on the barrels of the old cylinder and they still had the "Choke" at NEW specs when they were removed. I ran over square every chance I had before replacing the cylinders.

[Peterk]

Fly a Rotax 912 and you won't be able to...problem solved!

[Mel]

Originally Posted by Peterk

Fly a Rotax 912 and you won't be able to...problem solved!

That's because with a psru, you're always running in low gear.