RE: Prop pitch cooling - the final word

RE: Prop pitch cooling - the final word
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RE: Prop pitch cooling - the final word - 2/15/2007 2:13:32 PM

downunder

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quote:

ORIGINAL: pe reivers
Oil cooling is another myth. The oil contributes extremely little to cooling. There is just too little of it, and it does not evaporate.

Exactly, I've mentioned it a couple of times elsewhere that oil out the exhaust can only match oil in the carb. Not that I think it goes straight through the engine and out the exhaust but more likely slowly migrates its way through. But the volume going out can only equal the volume coming in. On a rough calculation with a .60 size engine there's on the order of .0006cc of oil going out the exhaust every cycle. In American terms that's 1/50,000th of an ounce.

I'd never try an experiment that I thought had a good chance of harming an engine, which is why it didn't concern me doing this prop test....or the ultra rich running of a new ABC

.

I like Pe's description of a bath tub type of cooling curve with regard to pitch and I suspect the curve begins to flatten out at quite a low pitch, maybe less than 3" or so? I believe for all intents and purposes any pitch that's commonly used is fine for cooling.

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Post #: 26

RE: Prop pitch cooling - the final word - 2/15/2007 2:45:34 PM

pe reivers

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boiling water has a temperature (not a heat) of 100C. It takes a lot of heat to bring it to boil.

Calibrating an IR device is not required if you use the same device in the hands of the same tester on measuring the same object. Only the comparison is of importance, not whether the measured value is exact to within a few degrees or not.

Some interesting reading on the difference between heat(energy) and temperature which combined with mass has a certain amount of transferrable heat (enthalpy)
From wikipedia:
In physics, heat, symbolized by Q, is defined as energy in transit.[1] Generally, heat is a form of energy transfer associated with the different motions of atoms, molecules and other particles that comprise matter when it is hot and when it is cold. High temperature bodies, which often result in high heat transfer, can be created by chemical reactions (such as burning), nuclear reactions (such as fusion taking place inside the Sun), electromagnetic dissipation (as in electric stoves), or mechanical dissipation (such as friction). Heat can be transferred between objects by radiation, conduction and convection. Temperature, defined as the measure of an object to spontaneously give up energy, is used as a measure of the internal energy or enthalpy, that is the level of elementary motion giving rise to heat transfer. Heat can only be transferred between objects, or areas within an object, with different temperatures (as given by the zeroth law of thermodynamics), and then, in the absence of work, only in the direction of the colder body (as per the second law of thermodynamics).

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Post #: 27

RE: Prop pitch cooling - the final word - 2/15/2007 3:09:40 PM

Sport_Pilot

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Yes Pe, and IR is radiant heat which must be calculated to obtain the proper temperature that would have created that heat. It is a form of light which carries heat. Heat creats the light, and the light will convert to heat if not refelcted by a shiny surface.

I agree that an infred thermometer can give accurate readings especially if you are more intrested in the differance in temperature. However, the main problem I have with it when used to measure air cooled engine temps is that it is more and indication of surface temp, not the actual working temp of the engine. With air cooled engines there can be a very large change in internal temperature with very little change in surface temp. That is why aircraft use a head temp with a probe buried deep in the head. They do not strap a thermocouple to the surface. They do not even recommend a reading form between the spark plug and head, except as a method to help set the fuel mixture.

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Post #: 28

RE: Prop pitch cooling - the final word - 2/15/2007 4:49:36 PM

JNorton

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From: Montague, MI, USA
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Sport Pilot
As I see it the test that Downunder performed is perfectly adequate since as Pe Reivers has pointed out only the difference of temperature was measured. The fact that the actual temperature of the cylinder might not be correct is immaterial as long as the same procedure was used, with the caveat that the engines temperature had stabilized as was not changing.
John

EDIT-> Perhaps the duration of each test should be for 10 minutes - a typical fight time?

< Message edited by JNorton -- 2/15/2007 4:52:06 PM >

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Post #: 29

RE: Prop pitch cooling - the final word - 2/15/2007 7:03:36 PM

Sport_Pilot

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quote:

ORIGINAL: JNorton

Sport Pilot
As I see it the test that Downunder performed is perfectly adequate since as Pe Reivers has pointed out only the difference of temperature was measured. The fact that the actual temperature of the cylinder might not be correct is immaterial as long as the same procedure was used, with the caveat that the engines temperature had stabilized as was not changing.
John

EDIT-> Perhaps the duration of each test should be for 10 minutes - a typical fight time?

The problem with the test is it had no change in the temperature. Since one had more air flow which would mean more fresh ambient air, plus an increase in the film coefficient there had to be some heat exchange. There should be some change in the temperature, but probably not at the surface. This would prove little about the exchange of heat. With no airflow the air is stagnant and there is not as much exchange of heat, but also the air temp near the engine goes up so the delta T goes down, the film coefficient also goes down so the stagnant air acts like an insulator. As the air speed goes up the temp of the air near the engine goes down, the film coefficient goes up, so with more delta T and a hugger film coefficient then the surface temp will drop, though not as much as you would think because even the stagnant air was not a great insulator. As airspeed goes up further the coefficient goes up only a little, but the average air temp near the engine goes down, thus more heat is removed but there is little change in surface temp, but the core temp will change greatly.

I alluded to this in the first test and warned that it would be invalid if he continued to measure it with an infrared thermometer. To be valid you need to measure between the sleeve and case. A thermocouple between the sleeve flange and case would work. There I suspect you would find a temp change but even there, probably not a huge one.

< Message edited by Sport_Pilot -- 2/15/2007 7:06:53 PM >

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Post #: 30

RE: Prop pitch cooling - the final word - 2/15/2007 7:41:24 PM

JNorton

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Sport Pilot,
What I was expecting to see was a large difference in temps. The fact that the temps were the same can be resolved in my mind simply because of the inadequacies of the infra red measurements, exactly what you alluded to.

The fact that we did not see a large variance is for me all the proof needed. This whole discussion is whether you can use a low pitch prop for break-in. It seems to me that you can.

I am very curious as to how long the engine ran between measurements. Perhaps then we would see the engine overheating that Dar refers to when observing 3D flying.
John

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RE: Prop pitch cooling - the final word - 2/15/2007 8:47:23 PM

Sport_Pilot

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quote:

What I was expecting to see was a large difference in temps.

A moderate change in surface temp can equate to a large temperature deep inside the engine. This is because the outside is first to be cooled. As air flows over it the air becomes warmer and as travels through the fin the temperature differance drops. With more airflow the air has the same entering temperature but the leaving temp drops. But also the coeficient drops. There is not a huge differance in temp till it becomes grossly overheated. But because of the mass flow of air there is a larger loss of heat despite the modest temperature drop. This means more heat is removed but the temp doesn't rise proportiantly at the surface.

quote:

This whole discussion is whether you can use a low pitch prop for break-in. It seems to me that you can.

I don't think that was the argument. Rather that the high pitch prop will take away more heat. People seem to take what someone says to an extreme. Obviously more airflow will remove more heat. To have no temp difference seems too erroneous to even bother to take into account.

< Message edited by Sport_Pilot -- 2/15/2007 8:51:08 PM >

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RE: Prop pitch cooling - the final word - 2/15/2007 9:58:58 PM

yallaair

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From: Floro, NORWAY
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Sport Pilot!

Running with a higher pitch prop does not necessary mean that the air over the cooling fins is faster. This can be affected by the propeller blade angle of attack, the rotation of the "jet stream", or the efficiency of the propeller.

This tests shows that different prop pitches does not influence on the engine temperature.

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Post #: 33

RE: Prop pitch cooling - the final word - 2/15/2007 10:03:03 PM

Sport_Pilot

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quote:

Running with a higher pitch prop does not necessary mean that the air over the cooling fins is faster.

Actually it does. At least if they were of the same make and type. I am an engineer, I would like to see monitoring of airflow, intake air volume, fuel intake, exhaust temp, ambiant air temp, and engine temp from a dozen locations or so. If I were doing this for a engine manufacturing company that is pretty much a minimum I would use, without it I would just be making assumptions, as we seem to be doing here.

< Message edited by Sport_Pilot -- 2/15/2007 10:05:39 PM >

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RE: Prop pitch cooling - the final word - 2/15/2007 11:11:36 PM

JNorton

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Sport Pilot

quote:

A moderate change in surface temp can equate to a large temperature deep inside the engine. This is because the outside is first to be cooled. As air flows over it the air becomes warmer and as travels through the fin the temperature difference drops. With more airflow the air has the same entering temperature but the leaving temp drops. But also the coefficient drops. There is not a huge difference in temp till it becomes grossly overheated. But because of the mass flow of air there is a larger loss of heat despite the modest temperature drop. This means more heat is removed but the temp doesn't rise proportionately at the surface.

Last post as I'm not going to continue to beat a dead horse as we disagree. The mass in a single cylinder model engine is small with the cylinder being adjacent to the cooling fins so the heat transfer is very good. With respect your argument that you could see a large temperature difference internally without it showing on the outside is one I don't buy. For a larger multi-cylinder motor I'd completely agree with you. By the way just for the two cents it's worth I'm also an engineer which doesn't mean a hill of beans in this circumstance because my expertise is not engine related although I have a lot of experience designing and utilizing proper heat sink design for solid state devices monitoring those with both infra-red and thermocouples.
John

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RE: Prop pitch cooling - the final word - 2/16/2007 3:02:20 AM

downunder

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To avoid what seems to be a bit of confusion here, I used a contact type thermocouple probe which plugs into my tacho/multimeter so it was just a matter of flicking one switch to go from revs to temp.

On the first run with the control 10x8 prop I ran at full throttle and fiddled with the mixture until I had a steady 11,000 revs where it sounded just nice if I were to go flying, not necessarily to run in an engine. I then shut it down and waited for a minute or so to simulate the time it would take me to change a prop. I restarted and checked the revs again to be certain it would then still have the same 11,000 revs which it did. I then took the temp reading with the probe touching the cylinder head and up against the plug washer. The thermocouple readings varied a little (around 10F or so) so I used the steadiest reading as the base figure.

I then shut down, changed props and took the next set of readings. I checked revs first to be assured they were still the same and then the temp. I must admit I was surprised the temp was the same but I'd have been quite happy even if they'd been 10 or 15 higher seeing it was now running with less than half the original pitch.

This is such a simple experiment to do so I can't figure why no one takes 10 minutes at the field to try it themselves. Almost everyone has a tacho and temp guns are starting to turn up now but I don't think it's necessary to go to the extent I did of matching prop loads because I only did that to satisfy one of Dar's many objections when the results didn't match what he expected. A few more results passed on would be better than just my one set.

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Post #: 36

RE: Prop pitch cooling - the final word - 2/16/2007 6:17:25 AM

DarZeelon

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Brian,

In the previous thread you began with a 10x6 prop.
If anything, going to the 8" pitch, should have brought the diameter to 9"; i.e. a 9x8 prop.

A 10x8, at least compared to what most sport fliers use on a .46 engine, is overloading it to begin with.

With the 10x8 and the comparison 12.25x3.75W, I am surprised the later had to be chopped down at all!

The issue is break-in, where a light load is desired and the 10x8 on a .46 engine, is not...

Also, is there any way you can monitor the fuel flow?

I believe the air-blast produced by the prop at the carburettor's intake, can influence the amount of air the engine takes in.
If air mass intake is smaller, so would fuel intake, despite the needle being open the same amount and the RPM being the same.

Less fuel burnt is less heat to get rid of.
I guess you understand my point. Both points, actually.

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Post #: 37

RE: Prop pitch cooling - the final word - 2/16/2007 10:51:58 AM

JNorton

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Dar
Could you can weigh the fuel tank on a gram scale and plot the weight over time to determine fuel flow?

There are a lot of fairly inexpensive DVM with recording capabilities on the market so doing a time / weight graph should be fairly simple. I've got the stuff do do it with but with outside temperatures of 12 degrees F, -11 C - now is not the time. I think I'll try it in the spring simply because it is such an interesting experiment.
John

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RE: Prop pitch cooling - the final word - 2/16/2007 12:22:56 PM

DarZeelon

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This is possible, J., but it is Brian who is out to prove that what is correct by any common sense, i.e. that higher air-flow speed provides better cooling, is a myth....

So, it is Brian who must purchase that sensitive scale, or some kind of a flow-meter...