Running props over square?

[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]

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

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]

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]

No at 24 inches you can go to 1800 according to the engines chart here: http://www.lycoming.textron.com/support/tips-advice/key-reprints/pdfs/Key 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]

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]

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.

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. :laff: I guess I am a simple minded fool.

 

[shdw]

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]

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.

 

[derg]

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

 

[OhioStatePilot]

Man, you guys are throwing around different efficiencies here. Volumetric Efficiency is a very very different story than Thermal Efficiency. Now I could be completely wrong in this, but I'm not exactly sure that thermal efficiency is pertinent to the over square question at hand. The reason I say that is because the real concern about operating at a high MP isn't necessarily the pressure in the manifold, but instead the cylinder pressure.

The quick and dirty, in my view, is that the argument revolves around operating a high MP with low RPM which results in a higher cylinder pressure working against a greater resistance. The reason I say volumetric efficiency is pertinent is because with higher VE, more air can be introduced to the cylinder. With more air, more fuel can be added to create a larger burn, and therefore more cylinder pressure. Thermal efficiency doesn't really come much into play for building cylinder pressure, or at least in my understanding.

Oh, and by the way, automotive VEs greater than 100 can be reached N/A.

 

[SteveC]

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.

I was curious so I did a little Googling:

The first chart I looked at was for a IO-360. It shows 55% and 65% power settings at 2100 and 2400 RPMs. The MP change varied from 2.3" to 3.0" for a 300 RPM change. This equates to 0.77" to 1.0" per 100 RPM.

The second chart is for an O-320. This one shows 2450, 2500, and 2550 RPM. Searching through the chart it appears that 100 RPM changes in this engine (2450 - 2550) require 0.5" or less change in MP to hold the same horsepower.

I'm not yet convinced of your contention that 1" = 100 RPM. Do you have more charts available that support your thesis?

 

[shdw]

I was curious so I did a little Googling:

The first chart I looked at was for a IO-360. It shows 55% and 65% power settings at 2100 and 2400 RPMs. The MP change varied from 2.3" to 3.0" for a 300 RPM change. This equates to 0.77" to 1.0" per 100 RPM.

The second chart is for an O-320. This one shows 2450, 2500, and 2550 RPM. Searching through the chart it appears that 100 RPM changes in this engine (2450 - 2550) require 0.5" or less change in MP to hold the same horsepower.

I'm not yet convinced of your contention that 1" = 100 RPM. Do you have more charts available that support your thesis?

Well you are only looking at half of the problem here. If you reduce the power 1" that yields a certain decrease in power and no change in propeller effeciency. If you reduce the power by 100 rpm you reduce the power, as you pointed out by a greater margin, but you increase the propeller effeciencies leaving the resulting performance change negligable.

The only way to do this on paper that I am familiar with is to calculate for each new propeller effeciency. The charts you have will only give you the engine bhp percentage and leaves the prop out of the mix.

It will not hold 100 percent true for any given situation, it is a baseline. But go ahead and test it out, in every aircraft I have flown thus far there was no appreciable difference between the in speed/rate of descent when comparing 1" to 100 RPM.



Sorry webster not riddle, I felt I would flunk out with a beach nearby, Same thing just a different state.

 

[KevinJH7]

shdw

Did you happen to have Advanced Flight Dynamics with Teller?

 

[SteveC]

I'm not following the logic. Well, I understand how variations in prop efficiency at different RPMs affect the scenario. The problem that I still have is the variation between engines that my very small sample gives.

The IO-360 runs pretty close to your 1"/100 RPM rule of thumb, and that is based on a 300 RPM change. The O-320 runs about 1/2 that figure for a 100 change. If prop efficiency were a factor wouldn't those ratios be reversed?

Or, lets assume for the sake of argument that Prop A when put on the IO-360 and Prop B on the O-320 end up giving the results that you claim. What happens when you switch the propellers and put Prop A on the O-320 and B on the IO-360? Or switch from a two-bladed prop to a three-bladed? Are we still going to see the same 1" corresponding to 1000 RPM?

What I suspect is happening is that the lack of accuracy in the resultant measurement equipment (ASI, VSI) that you've used for your tests overshadow the power differences so that casual observation would fit your hypothesis. The change in knots or FPM may be too small to be noticeable maybe? Anyway, if that's true I still think it might be disingenuous to actually take those casual rule-of-thumb observations and turn them into statements of fact like "23/2300 = 24/2200 = 25/2100 = 26/2000."

I don't know, I'm just looking at a small amount of data and not coming up with the same conclusions that you are. I haven't flown a piston aircraft in years and I don't have an easy way to do field tests so I have to use what little logic and knowledge I have available to me to try to follow along.

I'd be interested if taylor or fish have any insight into this question as well.

 

[shdw]

The IO-360 runs pretty close to your 1"/100 RPM rule of thumb, and that is based on a 300 RPM change. The O-320 runs about 1/2 that figure for a 100 change. If prop efficiency were a factor wouldn't those ratios be reversed?

I have no idea, what I do know is if you changes a blades angle you change its effeciency and what you are measuring doesn't take that into account. Take aircraft A operating at 22" and 2200 RPM would be say 140 horsepower and say aircraft B is operatting at 23" and 2100 RPM. As you stated the horsepower in the second situation would be less so we have less thrust right? Well we have less drag too since the prop is now more effecient, whether or ont they go hand and hand 1 and 1 for every aircraft I am not sure. The person who taught this to our flight dynamics class was a navy engineer, he did the performance calculations for aircraft in the navy.

What I suspect is happening is that the lack of accuracy in the resultant measurement equipment (ASI, VSI) that you've used for your tests overshadow the power differences so that casual observation would fit your hypothesis. The change in knots or FPM may be too small to be noticeable maybe? Anyway, if that's true I still think it might be disingenuous to actually take those casual rule-of-thumb observations and turn them into statements of fact like "23/2300 = 24/2200 = 25/2100 = 26/2000."

As for my tests, they were an hour in the seminole and in the arrow and each included a full work up from 18"/1800 to full power. I did + 4 and - 4 for MP/RPM across the board and I gave the aircraft between 30 seconds and a minute to stabalize on each approach. This was done by the engineers at the college too, I will see if I can get the spreadsheet.

I don't know, I'm just looking at a small amount of data and not coming up with the same conclusions that you are. I haven't flown a piston aircraft in years and I don't have an easy way to do field tests so I have to use what little logic and knowledge I have available to me to try to follow along.

I'd be interested if taylor or fish have any insight into this question as well.

No big deal, we are all here to learn and if this is wrong I certainly would like to know that. For all my tests to this point (only in an arrow, seminole, and RG) the results have been I would guestimate within 20 percent of accurate.

I would like to hear from them too please! I would also like if the topic poster could test this out in his aircraft and tell us what the numbers are. Fly it at 23/23 then 24/22, 25/21 and finally 26/20 and give the numbers here for what the resultant speed was after say 5 minutes or so.


I think for the calculations side of it very few people will know what it takes to get an accurate result. We are pilots we test things, or just make it squared.

 

[tgrayson]

Thermal efficiency doesn't really come much into play for building cylinder pressure, or at least in my understanding.

Thermal efficiency relies on the biggest temperature difference between the hot sink and the cold sink, so hotter temperatures in the cylinders would imply greater pressure and greater thermal efficiency.

 

[shdw]

shdw

Did you happen to have Advanced Flight Dynamics with Teller?

Yes sir, great teacher! His best quote: A student asks, "how long do you want the paper" his reply "well think of it like a skirt, i want it long enough to cover the subject but short enough to keep it interesting."

 

[kiloalpha]

I would also like if the topic poster could test this out in his aircraft and tell us what the numbers are. Fly it at 23/23 then 24/22, 25/21 and finally 26/20 and give the numbers here for what the resultant speed was after say 5 minutes or so.

My next flight is Friday, 1/2 Part 91 so I'll give it a try. Although I'll have to keep it at 4,000 to try out the 26".

 

[shdw]

My next flight is Friday, 1/2 Part 91 so I'll give it a try. Although I'll have to keep it at 4,000 to try out the 26".

Great I am looking forward to seeing what you get out of a high performance engine as I haven't tested it there personally. Oh if you would, note the change in fuel flow also for each of the 4 settings? Thanks again.