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  #1  
Old 06-28-2012, 08:59 AM
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Default oversquaring to really low rpms

Normally I like to run a constant speed RV at 2300/25" in cruise. Lower RPMs help eliminate tip losses and increase efficiency, and also reduce wear on the engine.

I'm wondering if anyone's done any testing at lower RPM's for cruise settings at 1800 RPM or so and 25"-26"-whatever.

If not I'm going to do some experimentation.
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  #2  
Old 06-28-2012, 09:21 AM
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Default

I seem to remember Lindbergh doing something like that in WW2 to stretch the range of our planes. I think he was teaching the concept to the pilots.

Details have faded, but the concept still resides in my poor tired old noggin.
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  #3  
Old 06-28-2012, 09:55 AM
DEWATSON DEWATSON is offline
 
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Default rpm

Rocketbob,

Do the experimenting and post your results. I have thought of this often but have never practiced it because of the old wives tales. I'll run a two to three increment difference every now and then for a short flight, but I've been reluctant to do it for a long three hour flight. I never have had anyone explain to me why running "oversquare" would damage the engine.
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  #4  
Old 06-28-2012, 10:33 AM
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Lindbergh invented the concept of LOP operations at low RPM's.

What I am interested in finding is at what RPM the peak volumemetric efficiency occurs which should coincide with the peak torque RPM.
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N9187P PA-24-260B Comanche, flying
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  #5  
Old 06-28-2012, 10:55 AM
gpiney gpiney is offline
 
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Default

Check out the following link, especially Page 6.

http://44rf.com/11hc/manuals/engine-..._3b-charts.pdf
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  #6  
Old 06-28-2012, 12:17 PM
scsmith scsmith is offline
 
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Default How far over square is OK?

Looking at the Lycoming manuals, they provide operating curves for part-power operation. The chart I'm referring to is unfortunately not in the linked document. For each manifold pressure, they show the range of RPM that they think the engine can be run at. I don't have it in front of me so I am just going off the top of my head, but for example, the WOT curve goes down to something like 2200 RPM. The 25" MAP curve goes down to something like 2000 RPM, etc. (Again, those are not the real numbers, I'm just making them up as notional examples of what the curves show).

Anyway, point 1 is, Lycoming is fine with running over square.

point 2 is that there is a lower bound of how far over square, as a function of MAP. This is essentially a boundary on what the crankshaft and bearings can take -- in cars it is referred to as "lugging the engine". In extreme cases you can get pretty nasty crankshaft vibration from putting too much load at too low an RPM.

point 3. The benefit of going to very low RPM depends a lot on the particular propeller design. Running below the design RPM the efficiency will drop. Not as precipitiously as running above the design RPM, but it does drop off.

point 4. The full-throttle specific fuel consumption curve (which is in the linked document, first chart) shows that it levels off around 2200 RPM, so there is no compelling reason to go below that anyway.
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  #7  
Old 06-28-2012, 12:38 PM
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Default It's possible

Quote:
Originally Posted by rocketbob View Post
Normally I like to run a constant speed RV at 2300/25" in cruise. Lower RPMs help eliminate tip losses and increase efficiency, and also reduce wear on the engine.

I'm wondering if anyone's done any testing at lower RPM's for cruise settings at 1800 RPM or so and 25"-26"-whatever.

If not I'm going to do some experimentation.
Hey Bob:

The data you seek is in the 540 operators' manual. 1800/25" is the lowest setting, with 2000 turns allowing 27". Looks like the lower setting is about 137HP, but I can't see a fuel flow number there. 2000/27" gives about 172HP. The chart notes say the mixture is a max power setting, so it's gonna be thirsty there! Another chart sez 2000/27"/170HP is 12.5GPH @ max economy, and 1800/25"/138HP is about 9.5GPH.

BFSC of .44 on the 2000 setting, and about .41 on the 1800RPM setting.

These look like low altitude numbers, considering the MP numbers.

At altitude, it's gonna be way different as you will need to spin 'er up to get enough HP to get anywhere. Look at 55% & LOP as a good target, and run the lowest RPM/WOT/LOP (if possible) to get the 55% number. At 17500, you won't be running LOP as the power would be too low, with an IAS to match.

Make sure your prop is up to the job at the lower RPMS - Hartzell can help there. As for the prop question: will it work efficiently at low RPMs, you will be the first to know!

Carry on!
Mark

Last edited by F1Boss : 06-28-2012 at 12:41 PM. Reason: More math added
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  #8  
Old 06-28-2012, 12:55 PM
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Default low rpms

About 30 years ago, flying mag had a road test of an observation aircraft built in Europe. Cockpit was a bubble similar to a helicopter. It had a Lycoming engine and used low rpm/high mp for efficient loiter speeds. Seemed like it was 1200/25. Maybe someone can dig up more info.
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  #9  
Old 06-28-2012, 01:32 PM
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As a kid playing with 2 stroke motorcycles, we used to design our power band around the "mission". Drad racing, road racing or dirt bikes all used a different band peak and width. We would use port sizing, port timing, carb. venturi velocity and then custom build a set of exhaust chambers for our desired power band. We could build an engine that would be a dog to 6,000 rpm and then fall off the planet at 8,500 rpm.
My point is that I would assume that the Lycoming is cam'ed, carbed, timed and exhausted for our standard rpm's. Anything more or less is probably less efficient. Easy to see from the power and torque curves.
It will be interesting to see how these Lycomings do at the lower rpm's.
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  #10  
Old 06-28-2012, 01:35 PM
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Default

Quote:
Originally Posted by dlloyd3 View Post
About 30 years ago, flying mag had a road test of an observation aircraft built in Europe. Cockpit was a bubble similar to a helicopter. It had a Lycoming engine and used low rpm/high mp for efficient loiter speeds. Seemed like it was 1200/25. Maybe someone can dig up more info.
http://en.wikipedia.org/wiki/Edgley_Optica
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