Running props over square?

[trafficinsight]

that makes more sense!

[mshunter]

Originally Posted by spbrian

untrue statement, at low power settings you have less pressure and at full throttle you have ambient pressure (whatever the pressure is at the altitude you are at)

Still incorrect. This would imply that you have a 100% efficent engine(I get your point though, sorry to be nit-picky). Most NA engines are around 85-90% efficent.

[dasleben]

Old wives tales are fun!

As milleR already noted, our airplanes (C402C with TSIO-520s) are run constantly oversquare. Absolutely nothing wrong with it. Consult your AFM, run the airplane within the operating limitations described therein, and you won't have a problem. Food for thought: the 402C, per the AFM, has no limitation other than temperature on running the engines at 39" MP, 2700 RPM all day long.

[ppragman]

Originally Posted by mshunter

Still incorrect. This would imply that you have a 100% efficent engine(I get your point though, sorry to be nit-picky). Most NA engines are around 85-90% efficent.

I notice about an inch less than atmospheric on the takeoff roll, anything significantly less will make me nervous.

[tgrayson]

Originally Posted by mshunter

Most NA engines are around 85-90% efficient.

Most piston engines are about 20% efficient, at best.

[trafficinsight]

There is more than one type of efficiency, so you're both right

[tgrayson]

Originally Posted by trafficinsight

There is more than one type of efficiency, so you're both right

The only other "efficiency" I'm familiar with is volumetric efficiency, and I don't gather that the loss of static pressure in the intake manifold is a measure of it. According to Wikipedia, there are some normally aspirated techniques that can get volumetric efficiencies well over 100%.

[minitour]

Originally Posted by tgrayson

According to Wikipedia...

Because that's an authoritative source.

-mini

[dasleben]

Originally Posted by minitour

Because that's an authoritative source.

-mini

Anything you read on the internet is true, Lord Captain.

[jynxyjoe]

Originally Posted by dasleben

Anything you read on the internet is true, Lord Captain.

Now THAT is an avatar

[trafficinsight]

You have thermal efficiency, which is around 20 to 30%, as you say.

You have volumetric efficiency, which varies depending on filtration, intake design, turbo or supercharging, a butterfly flapping its wings in China, and, of course, throttle position.

And you have mechanical efficiency, which is how much horsepower is lost due to friction, which is around 80 to 90% usually, but also depends on engine condition. Your average R-2800 puts out 2000 horsepower... if it's 80% mechanically efficient that means it takes 500 horsepower just to turn the engine.

[minitour]

Originally Posted by dasleben

Anything you read on the internet is true, Lord Captain.

I forgot, Captain.

-mini

[Stomp16]

retracted

[shdw]

Originally Posted by kiloalpha

Out company for years and years have told us to run at 2300 rpms and 23" on the MP. Usually trues us out at 170-172kts and burns about 30-32 gph.

Now we are being told to run at 2200 rpms and 24" of MP. We are told this has no negative effect on the engine as is better for the props since they work more efficiently now. Is this true? Are we going pop cylinders and crack cases now? or is this actually going to work?

We have Lycoming IO-540s that make 290hp. Company research shows a fuel savings of 10% with a 1-2% drop in airspeed.

No at 24 inches you can go to 1800 according to the engines chart here: http://www.lycoming.textron.com/supp...Operations.pdf on page 45. This shows the continuous operation safety levels for MP vs RPM difference for the IO-540 engine. I would advise going even further, utilize the limitations of the engine to maximize performance.

For instance 26" 2000 is within engine limitations and the cut of 200 rpms should drop you another couple gallons an hour. This is a better way to operate the engine, it is completely safe for the engine, it reduces wear, increases TBO (not actually but since tach is what deems TBO if you operate 200 rpm less all the time...), quiets the cabin, keeps the engine running cooler, and most importantly saves ALOT of gas.

This chart is available for every engine and IMO should be referenced before flying a constant speed aircraft. Then you can use the airplane like an airplane and maximize its utility. If Vfe is 150 do you wait to 140 to extend the flaps to be safe?

[shdw]

Originally Posted by mshunter

Still incorrect. This would imply that you have a 100% efficent engine(I get your point though, sorry to be nit-picky). Most NA engines are around 85-90% efficent.

I think you need to read this: The engine can now get all the air it wants, with the only restrictions being the filter, the small
area of the wide-open throttle plate (edge-on), and the turns in the ducting on the normal engine.
Usually, those factors will cost you about an inch of MP, or a bit less.

From this article: http://www.advancedpilot.com/downloads/prep.pdf

Manifold pressure can be best thought of as a spectrum where the center point would be ambient pressure, any lower values would be incurred from suction of air into the intake lowering pressure and any increased values would be from air being forced in such as a turbo.

When you full throttle the engine you have basically nothing restricting the engine from sucking air inside except for the filters and such described above. If you took out these filters and had an open intake with no filters bends and a straight shot into the engine you would always see MP matching ambient pressure at full power.

Just so it is said, we typically cause this suction by closing the throttle and forcing the engine to suck in harder on the intake which lowers the pressure.

[bLizZuE]

I chose to watch this one from the sidelines for the most part.

I hear it a lot on stage checks, "Never more MP than RPM". I open up the POH and point to a power setting that clearly shows more MP than RPM.

Their jaw drops open, and they just can't understand it. (I'm a personal fan of these little AHA moments)

Then I tell 'em about Lindbergh.

Not over squaring the engine is a rule of thumb to keep you out of potential danger zones. Nothing will substitute plain 'ol working knowledge of the engines and manufactures recommended power settings.

[mshunter]

Originally Posted by tgrayson

Most piston engines are about 20% efficient, at best.

Originally Posted by shdw

I think you need to read this: The engine can now get all the air it wants, with the only restrictions being the filter, the small
area of the wide-open throttle plate (edge-on), and the turns in the ducting on the normal engine.
Usually, those factors will cost you about an inch of MP, or a bit less.

From this article: http://www.advancedpilot.com/downloads/prep.pdf

Manifold pressure can be best thought of as a spectrum where the center point would be ambient pressure, any lower values would be incurred from suction of air into the intake lowering pressure and any increased values would be from air being forced in such as a turbo.

When you full throttle the engine you have basically nothing restricting the engine from sucking air inside except for the filters and such described above. If you took out these filters and had an open intake with no filters bends and a straight shot into the engine you would always see MP matching ambient pressure at full power.

Just so it is said, we typically cause this suction by closing the throttle and forcing the engine to suck in harder on the intake which lowers the pressure.

Look guys, I spent 9 years as an auto tech for the then Chrysler Corp. and was 4 tests away from being Gold certified (think ASE Master Tech). A 4 stoke engine can, and usually does achieve 85-90% volumetric efficiency regardless of cam timing/intake port design/valve angle/carb-throttle body size, etc. All engines really are (every engine) by design is an air pump. Manufactures try their best to design 85-90% efficiency into the engine because achieving 100% efficiency would require forced induction. If an cylinder is, say, 30 ci (cubic inches), than usually it is able to draw in around 24-26 ci of air for a given cumbustion cycle. Hence the 85-90% efficiency. If you want to get all 30ci of air moving through the engine, then you need to force it in.


Or, if you want to use shdw's article, if you loose one inch of MP at 30 inches, you get around 92-95% (pretty good actually)efficiency. MP is a directly connected to how much power the engine is making, and it's efficiency. The lower the MP, the lower the power output.

There are numerous ways to increase an engines efficiency. Limit it's operating RPM range, change port design/cam timing/valve angle for different "power band", etc.

shdw, as far as removing the intake system and seeing ambient air in the cyl, sorry bud, but restrictions from valve angle, valve design, and intake port design would dictate otherwise. I was in the business (albeit side business) of speed for a while, building SBC's that would run with BBC's, and am still involved with tuning a high 6 second, 215mph quatermile machine. I think I know a bit engines, and how to make them work at their optimum. If you tried to spin a O-540 at 8000rpm, just as a guess, you'd probably see a MP (if it could hang together) of around 24-25"MP, it just isn't designed to run at that kind of RPM.

tgray, are you talking about the power that is created from the fuel being burned?

[SteveC]

Originally Posted by mshunter

tgray, are you talking about the power that is created from the fuel being burned?

That's exactly want comes to my mind when talking about efficiency. Energy released from the fuel divided into the amount of energy actually imparted to the vehicle.

[ppragman]

Originally Posted by shdw

No at 24 inches you can go to 1800 according to the engines chart here: http://www.lycoming.textron.com/supp...Operations.pdf on page 45. This shows the continuous operation safety levels for MP vs RPM difference for the IO-540 engine. I would advise going even further, utilize the limitations of the engine to maximize performance.

For instance 26" 2000 is within engine limitations and the cut of 200 rpms should drop you another couple gallons an hour. This is a better way to operate the engine, it is completely safe for the engine, it reduces wear, increases TBO (not actually but since tach is what deems TBO if you operate 200 rpm less all the time...), quiets the cabin, keeps the engine running cooler, and most importantly saves ALOT of gas.

This chart is available for every engine and IMO should be referenced before flying a constant speed aircraft. Then you can use the airplane like an airplane and maximize its utility. If Vfe is 150 do you wait to 140 to extend the flaps to be safe?

I know several people and companies that do that, and frankly it irritates me.

[tgrayson]

Originally Posted by mshunter

tgray, are you talking about the power that is created from the fuel being burned?

Sure. Unqualified, that's probably what most people think of when the word efficiency comes up. The theoretical maximum thermal efficiency of, say, an automobile engine is around 56 percent, but actually achieves about half that.

[kiloalpha]

Thanks for the post. But we are going to be running at 2200rpm, no lower since it will start sacraficing speed, which is what the customer wants.

And actually, we've been using company recommended V speeds (140 instead of 150 for flaps, and 160 instead of 180 for gear) just because the MX side of the company says it will extend the life of the pushrods, pullies, actuators, ect. Not to say that when I'm in a hurry I'll use the higher speed, but in places like BFE where no one is there, I'll relax and use the lower speeds. Besides, it's more time in the logbook.

And as far as doing 26"/2000, we fly at 6/7,000 feet when possible. Higher for longer routes. The most we can get is 24".

[mshunter]

Originally Posted by SteveC

That's exactly want comes to my mind when talking about efficiency. Energy released from the fuel divided into the amount of energy actually imparted to the vehicle.

Originally Posted by tgrayson

Sure. Unqualified, that's probably what most people think of when the word efficiency comes up. The theoretical maximum thermal efficiency of, say, an automobile engine is around 56 percent, but actually achieves about half that.

You non-mechanic types, too funny. I guess I am a simple minded fool.

[shdw]

Originally Posted by mshunter

Or, if you want to use shdw's article, if you loose one inch of MP at 30 inches, you get around 92-95% (pretty good actually)efficiency. MP is a directly connected to how much power the engine is making, and it's efficiency. The lower the MP, the lower the power output.

Can we both be right? and both be retarded?

The engine sitting on the ramp running full throttle will yield the numbers you gave because they are right for an engine sitting still. Well an airplane engine does a funny thing, it moves and when it does that the natural ram air...yada yada yada. In the second case with ram air the numbers ppragman and I gave of about 1 inch would make sense. Would you agree the ram air would add about 2 to 3 inches? I will definitely test this next weekend.

Can't believe I missed that.

Oh thanks for the info on engines beyond that you should write some more posts on it I would be interested in learning more. Between you and roger I am sure there are some good engine discussions.

[shdw]

Originally Posted by kiloalpha

Thanks for the post. But we are going to be running at 2200rpm, no lower since it will start sacraficing speed, which is what the customer wants.

And as far as doing 26"/2000, we fly at 6/7,000 feet when possible. Higher for longer routes. The most we can get is 24".

You do realize that 23/2300 = 24/2200 = 25/2100 = 26/2000, in fact because of the reduced drag from the increased blade angle at 26/2000 you will likely gain a few knots cruise. Ask your mechanics if it is safe and try it out, I bet your managers s**t their pants.

Units of power: 1 unit = 100 RPM = 1 inch MP = about 100 FPM = 3-5 knots
Gear = about 5 units
Flaps: 10 degrees = 2-3 units | 20 = 3-5 units | 30 = 5-7 units

The point is for this if you reduce power by 100 RPM if you increase MP by 1" you will yield exactly the same BHP with reduced propeller drag.

Take those above numbers from the first line and apply it to units of power:

23+23 = 46 units
24+22 = 46 units
25+21 = 46 units
26+20 = 46 units

Pretty basic, I am sure many of you won't believe it. Please don't argue until you have spent at least an hour testing it. The 1 inch = 100 rpm you will find is true in mostly any piston you fly, the performance yielded for 1 unit = 100 fpm may vary especially with larger more powerful aircraft.

[Doug Taylor]

Holy cow, were you one of my classmates at Riddle?