What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

True or false: never set MAP higher than RPM.

ao.frog

Well Known Member
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... :)
 
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.
 
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.
 
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
 
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.
 
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.
 
Old Wives Tales.........aint they great. Glad to see some are gradually being eroded. Except some of the CPL's coming out of flying schools around here :rolleyes:

I generally push the throttle forward and leave it there until I reduce power in the circuit or for any ATC reason.

RPM is set at TOC and mixture does the rest. :)
 
I had not heard of a prohibition on running over square in radial engines aside from power limits or severe over square. There are lots of caution, notes and limitations on running under square (more RPM than MAP) found throughout the old operating handbooks I have read. Having the prop drive the engine in a radial is a really good way to chew up the master rod bearing in a big hurry.

None of this translates to opposed engines.
 
Thanks guys...

.... I'm learning something new everyday in this business!
Seems like I have a whole new line of powersettings to try out now. (I'll check the engine and prop-manual first ofcourse)
It'll be interesting to try out WOT and lower RPM when flying at cruise at my favorite cruise-powersetting and altitudes (55% and 5-10.000')

Paul: even though most European countries use metric by now, here's how it is when it comes to planes in the majority of the Euoropean countries:

Altitude: feet
Speed: knots (mph is for cars and NOT planes :D )
Altimeter: millibars
MAP: inches
Temp: (OAT,EGT/CHT etc) Celcius
Lenghts (wingspan etc) meter and centimeter
Lengths of a flown distance: NM (SM is for cars and NOT planes :D )
Quantity: litres
FF: litres per hour
Weight: kilos and tons
Pressure: (oil/fuel) PSI
Pressure: (tires) PSI or BAR
ATC language: english and the local language

These are just some units I remember right now, so there might be others that I've forgotten.
There are also some variations to these units in some countries.

Confusing...? :confused: naah... it's just something to get used to...
 
Last edited:
Good question Alf..

You are/were mostly a jet guy but my old Pratt and Whitney 1340 cu. in. radial was placarded for 36"/2200 RPM at take-off, in my Air Tractor crop-dusting airplane.

Once level and during work in the field, it stayed at 30" and 2,000 RPM...ALL DAY, for several years, until overhaul!

My RV-10 with its IO-540, climbs at WOT 28"+ and 2500 RPM.

Best,
 
I had not heard of a prohibition on running over square in radial engines aside from power limits or severe over square. There are lots of caution, notes and limitations on running under square (more RPM than MAP) found throughout the old operating handbooks I have read. Having the prop drive the engine in a radial is a really good way to chew up the master rod bearing in a big hurry.

None of this translates to opposed engines.

Heck if I were to fly undersquare in the Beaver, I wouldn't be able to maintain altitude!

36.5"/2300rpm max 1 minute
33"/2200rpm max continuous
30"/2000rpm normal climb
28"/1850rpm cruise
(R-985)
 
Oversquare is fine, a good rule of thumb in a normally aspirated is to try to keep it within 4 over during cruise (example-- 27" and 2300rpm).
Consider that many fixed pitch setups will exceed even this on takeoff roll.
Lindbergh used to teach oversquare operations to military pilots for fuel efficiency.
 
I run my Duke on take off at 41", and 2800 rpm. My guess is that this would be over square. TIO 541, 380 HP. Throttle back to 38"/2750, and eventually to 30"/2700.
 
Last edited:
You are/were mostly a jet guy but my old Pratt and Whitney 1340 cu. in. radial was placarded for 36"/2200 RPM at take-off, in my Air Tractor crop-dusting airplane.

Once level and during work in the field, it stayed at 30" and 2,000 RPM...ALL DAY, for several years, until overhaul!

My RV-10 with its IO-540, climbs at WOT 28"+ and 2500 RPM.

Best,

Pierre, the 1830s in the DC3s I flew were 48" and 2700 on takeoff. Cruise was similar at 30" and 2050. They limited them to 48" because of the unavailability of higher octane fuel. I never flew one with 115/130 but I believe it was something like 54" allowed.
 
The tech guy...

.... in our local aeroclub found this on the Lyc website yesterday:


?Is it a fact, or is it fiction, that engines with constant speed props should not use power settings where inches of mer*cury exceed RPM in hundreds? I am referring, of course, to non-turbocharged engines in general .?

The answer to this question is easily found in cruise power charts of the airframe Pilot?s Operating Handbook. Whatever the combinations of RPM and MP listed in the charts ? they have been flight tested and approved by the airframe and power plant engineers. Therefore, if there are power settings such as 2100 RPM and 24" MP in the power chart, they are approved for use.

The confusion over so-called ?squared? power settings (i.e., 2400 x 24" MP), appears to have been a carry-over from some models of the old radial engines which were vulnerable to excessive bearing wear where an MP higher than ?squared? was used. More pressure on the bearings with the higher than ?squared? MP was the cause of their problem. However, changes in design, metals and lubricants permit changes in operation in the more modern flat-opposed power plants.

Let?s look at the power charts in a couple of the Pilot?s Operating Handbooks of two different aircraft manufacturers, but where both are using the four-cylinder 200 HP Lycoming engine.

Cessna?s Model 177 RG, using the Lycoming IO-360-A1B6D, in the cruise range at 6,000 feet, lists a cruise power-setting range at that altitude of anywhere from 2100 RPM to 2500 RPM with variations all the way from 18" MP to 24" MP. They list a recom*mended power-setting for 66% power at 2100 RPM at 24" MP.

The Piper Arrow, powered by the Lycoming IO-360-C series engine, lists the following cruise power settings at 6,000 feet in their chart at 65% power at full throttle (about 23" MP) x 2100 RPM.



Now, I better be done with the annual, because I have a whole lot of new cruise-settings to try out... :)
 
Now, I better be done with the annual, because I have a whole lot of new cruise-settings to try out... :)

Alf, should you not move your annual to the winter? Assuming you have a heated workshop/hangar.

PS, how are you finding being retired?
 
There's .....

.... no heating in my hangar, so instead of doing the annual under these conditions.... :mad:





I prefer these conditions instead.... :)






And because of the conditions on the pics below, the annual takes two weeks instead of two days... :D








My hangar is in the same area as the local aeroclub and a RV-4, so there's always alot of hangar-talk going on, and ALOT of coffee-breaks!

Would't miss it for the world!

And yes; I LOVE the retirement-life! I can't imagine life can be any better now... and on the 1st of July, the wifey starts a new turnus at her work with ALOT of more off time, so we feel soooo blessed these days.....
 
Last edited:
MP RPM etc.

Alf, thanks for helping me out, after reading thru two dozen posts, you finally answered my question.....HOW the heck can I operate oversquare when I have only one knob to adjust?......for us guys that have been around airplane engines for years, it's still disconcerting to find I don't know what the heck you other guys are talking about .....until I realized I need to go buy a C/S prop to join in the discussion!

nice hangar....you've evidently discovered the 'social' part of aviating often intereferes with the 'airborne' part.
 
Back
Top