Running props over square?

[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]

Originally Posted by shdw

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]

Originally Posted by SteveC

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]

Originally Posted by SteveC

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]

Originally Posted by OhioStatePilot

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]

Originally Posted by KevinJH7

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]

Originally Posted by shdw

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]

Originally Posted by kiloalpha

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.

[Cessnaflyer]

Originally Posted by kiloalpha

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".

Yes real numbers!

I love experimentation!

[tgrayson]

Originally Posted by SteveC

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

Here are two charts that I adapted from "Theory of Flight" by Richard von Mises. I left the numbers off because they would only clutter the chart. I calculated the slope of each line. The rule of thumb that shdw is proposing is derived from the ratio of the two slopes. The slope of the left chart is the torque of the engine, which is relatively constant as you reduce RPM, keeping the manifold pressure the same.

The slope of the right chart is determined (I think) by the rate at which the mean effective pressure (MEP) in the cylinder changes with changes in manifold pressure.





The numbers support shdw's thesis, on an engine very different from the ones in the airplanes he flies. I don't consider the issue proven, though, unless you can make an argument as to why the slopes of the two graphs should always bear the same relationship to each other.

Steve, as for the "O-320" chart you posted a link to (the actual chart says "O-360"), it really doesn't have the range of data to make a comparison to the IO-360 chart you posted; rounding errors could easily throw the data into the same range as the numbers shdw and I generated.

[mshunter]

Originally Posted by shdw

Can we both be right? and both be retarded?

Hey man, I have never claimed to be a genius. But I do know that I don't know all that much.

[shdw]

Originally Posted by mshunter

Hey man, I have never claimed to be a genius. But I do know that I don't know all that much.

I think that's why we are all here, we all want to argue about what we don't know.

[shdw]

Tgray: does that graph on the left take into account engine propeller efficiency?

If so we will have to find some sort of rule or unwritten rule of thumb that designers bench off of to give the RPM graph. Meaning if they build the aircraft and equip it with a prop that will give it that similar graph. I think the RPM graph would be the only graph in question as it has more than one factor involved in performance.

The MP graph should remain fairly consistent as long as engine efficiency is the same I think, right?

[mshunter]

Originally Posted by 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.

Yes it can. But it is through such a narrow operating range, that it is negligable. You may have an engine that has a 4500rpm power band, but will only achieve better than 100% through 50-100 rpm. We are talking about extream max performance engines (think NHRA or NASCAR), and not in an everyday street driven vehicle. Most of those engines are not even streetable.

As for the greater cyl pressure debate, I have to just shake my head at that one. It takes so much pressure to bend metal in an engine, that running oversquare values of 1900rpm and 50" of MP still will not bend metal. The problem more or less lies in the direct corilation of pressure=heat. Your typical light GA engine will not be hurt by running oversquare within resonable limits. Also, as RPM changes, so does VE(lower RPM's = lower VE). The engines are desgined to be at their optimum VE at max RPM. So running an engine that has a redline of 2700rpm at 2200rpm, the BE would decrease by a large enough amount to reduce VE low enough to not hurt things. It is eaiser to bend a con rod by over speeding the engine than by running lowRPM/highMP. But it is eaiser to overheat an engine with lowRPM/highMP's. I would be more worried about detonating the thing than bending something. And properly set up, and monitered closely, you will have nothing to worry about.

[tgrayson]

Originally Posted by shdw

Tgray: does that graph on the left take into account engine propeller efficiency?

No. BHP never takes into account propeller efficiency, only THP does that. I would expect that propeller efficiency would be variable all along the x-axis on both charts. For the right one, as you reduce MP, you're reducing torque; how else can the governor maintain RPM other than by reducing the blade angle and hence changing efficiency?

[shdw]

Originally Posted by tgrayson

No. BHP never takes into account propeller efficiency, only THP does that. I would expect that propeller efficiency would be variable all along the x-axis on both charts. For the right one, as you reduce MP, you're reducing torque; how else can the governor maintain RPM other than by reducing the blade angle and hence changing efficiency?

Wow I just want to smack myself for not thinking that, and I will shoot an e-mail off to my flight dynamics professor if we don't come up with anything for this. I am about tapped out, this is the limit of my knowledge in this area and as you can see I'm still working out the kinks on this level. I wouldn't even know where to start looking for how that graph was created, my dole book might have it I will see tomorrow.

[shdw]

Originally Posted by mshunter

As for the greater cyl pressure debate, I have to just shake my head at that one. It takes so much pressure to bend metal in an engine, that running oversquare values of 1900rpm and 50" of MP still will not bend metal. The problem more or less lies in the direct corilation of pressure=heat. Your typical light GA engine will not be hurt by running oversquare within resonable limits. Also, as RPM changes, so does VE(lower RPM's = lower VE). The engines are desgined to be at their optimum VE at max RPM. So running an engine that has a redline of 2700rpm at 2200rpm, the BE would decrease by a large enough amount to reduce VE low enough to not hurt things. It is eaiser to bend a con rod by over speeding the engine than by running lowRPM/highMP. But it is eaiser to overheat an engine with lowRPM/highMP's. I would be more worried about detonating the thing than bending something. And properly set up, and monitered closely, you will have nothing to worry about.

I have been under the impression that the intake manifold is what limits the difference in MP/RPM because it isn't made as strong as things like the cylinders. Is this wrong, if so what is the limiting factor for that chart I have been posting if it isn't the intake, is it really cylinder pressure?

For the bold I assume you mean follow the engine limitations and just like any other limitation if you stick to it you wont' break things?

[tgrayson]

Originally Posted by shdw

Wow I just want to smack myself for not thinking that

Propeller, engine, and aerodynamic interaction is, in my view, the most complicated subject matter in basic flight theory, and I certainly haven't mastered the subject. You need to hold about a half dozen charts in your head at the same time and that's at the limit of the human short-term memory capacity. Any conclusions I draw are tentative.

[USMCmech]

Originally Posted by shdw

I have been under the impression that the intake manifold is what limits the difference in MP/RPM because it isn't made as strong as things like the cylinders.

The stress on the intake manifold is highest a IDLE, not full power.

Think of your intake manifold like a straw. Sucking in with nothing blocking the end prdouces no stress, block the end with your finger (or a throttle vlave) causes it to collapse.

[shdw]

Originally Posted by USMCmech

The stress on the intake manifold is highest a IDLE, not full power.

Think of your intake manifold like a straw. Sucking in with nothing blocking the end prdouces no stress, block the end with your finger (or a throttle vlave) causes it to collapse.

Ok I see where my confusion was there, I was taking that thought from the manifold pressure sucks article and just applying it where ever the hell I saw fit.

Can anyone explain the limiting factor in the sea level and altitude pressure chart for continuous operation that I linked earlier. What are they concerned with keeping safe as MP/RPM spread increases and how does this link with different engine powers having different MP/RPM spread limits?

Thanks.

[tgrayson]

Originally Posted by shdw

What are they concerned with keeping safe as MP/RPM spread increases and how does this link with different engine powers having different MP/RPM spread limits?

Detonation limits, I believe. Reducing RPM increases BMEP (brake mean effective pressure) in the cylinders. As for why it varies from engine to engine, displacement does appear in the formulas, with larger engines tending to reduce the increase in BMEP produced by RPM decreases. Also, compression ratios probably have a role, with higher compression engines having a greater propensity to detonate.

[LatitudeDancer]

Originally Posted by kiloalpha

I've seen people with GamiInjectors on their BE35 lean it way out and blow out rings all the time.

They're not doing it right if that's the outcome.

I'm not too smart so I'll just insert this excerpt from a DA40 POH. The first time I went for about a 2000/25" combo on an XC with an ATP instructor, he about pooped. I just wanted to see what the engine sounded like.

[dasleben]

Damn, when you all say "Technical Talk," you ain't kiddin'.