downunder

It's taken me a while to get around to doing this final test but I've tried to meet everyone's ideas as to what's needed for a valid test. For those who missed it before, a comment was made that a high pitch prop is necessary for good engine cooling on the ground. Someone had been running an engine with an 11x4 prop and he was told to use a 10x6. I disagreed as did a few others so I did an intial test using both those size props and didn't find any noticeable difference. Of course, I was soundly rounded on because I mentioned the 11x4 had a lighter load than the 10x6 .

Obviously to meet the criteria of one person I had to not only use different pitches but also match the load. So I decided to use a fairly high pitch prop (APC 10x8) and compare it with the lowest pitch prop I could buy which was an APC 12.25x3.75, the typical 3D prop. On initial runs the 3D prop had much more load so I trimmed 1/4" from each tip. This went slightly too far because now I was getting 11,300 revs compared to 11,000 with the 10x8 with no change to mixture setting (simply swapping props and restarting). I didn't want to alter the 10x8 because it's a good size to use for comparisons between engines and with other people's engines using that size. Instead I slightly repitched the outer 2" of the 3D prop and by sheer good luck it now matched exactly the 10x8 with 11,000 revs on both.

To do the test I'd start the engine, go to full throttle and give the temps time to settle then check revs and immediately check the temperature. Stop the engine, change props and restart and repeat the checking process without touching the needle valve. The tank was set up as a uniflow to avoid any leaning out due to head change of the fuel level between runs. Engine tune was set by ear at a level that I would fly with and not peaked out.

So here are the results as best I can possibly do them.
10x8...11,000 and 270F
11.75x3.75...11,000 and 270F

I think the findings speak for themselves .

wjvail

My Feedback: (3)

Can you post a link to the original thread?

I don't find your results at all surprising but we've had discussions on the sources of cooling for alcohol, two stroke engines before.

Bill Vail
www.RCScreenProtectors.com

speedster 1919

My Feedback: (3)

Excellant Downunder---I'm not sorry I put you thur that . Now ,no one can say it was amount of fuel or one prop setting was too rich or too lean. and with an 8 pitch to a 3.75 pitch that is a big enough spread.
chaulk up 1 prop myth busted

Again good job -Who knows that trimmed prop might really scream now.

proptop

My Feedback: (8)

"Final word"...good one D-U!
Thanks [sm=thumbs_up.gif]

I wonder how many final words there are gonna be?

downunder

It took a bit of digging to find them but the rot started in post #9 in
http://www.rcuniverse.com/forum/m_5016672/tm.htm

I then started a thread at http://www.rcuniverse.com/forum/m_5025858/tm.htm which went wayyyyyyyy longer than it had any right to .

pe reivers

Chapeau DU.
The outcome surprises me to say the least! Indeed the myth seems busted, and thinking needs re-adjusting.

JNorton

My Feedback: (2)

The results surprise me too as it is contrary to what I was taught. Like the old saying goes live and learn. Thanks for doing the test.
John

EDIT I don't know where the attachments came from as I did not add any?????

DarZeelon

Brian,


Now you an try a flat prop, i.e. large diameter with just enough drag.

...No pitch.


We may find it is only radiation that cools the engine, not air-blast.

Your results are contrary to the experience of many 3-D fliers, who do suffer from over-heating, with the 3.75-4" pitch props.

pe reivers

Hey Dar, be a sport and admit defeat. I know it is not easy, but it is something you gotta do!
Who will ever fit a zero pitch prop, or 90 degree angled blades. These are extremes that will prove you right for the zero pitch. The infinity pitch however also will not cool, and just stir some air. That is why testers who use these props also use an external blower.
Heck, I am mighty amused, even if I too was wrong.

Hobbsy

My Feedback: (102)

Pe, as my Dad used to say, there ain't much in this mans creation that's set in stone, it makes life interesting.

Sport_Pilot

Was this measured with infrared? Wondering because there should have been some differance if only 5 or 10 degrees. I don't trust infrared thermometers because they measure infrared heat not temperature.

HighPlains

My Feedback: (1)

I think that the fuel is what is really cooling these engines. Excess fuel and the oil carry out much of the heat.

pe reivers

Dave, DU, Dar, and all of you.
After pondering a while, this pitch related cooling seems to follow a bathtub kind of curve with engine on test stand. There seems to be a quite broad range of pitches that cool the engine well enough, even to about the same level.
@ HighPlains :
Oil cooling is another myth. The oil contributes extremely little to cooling. There is just too little of it, and it does not evaporate. Very little heat is required to bring that amount of oil to Exhaust gas temperature. Methanol OTOH has a very high heat of evaporation, and there is lots of it. All must evaporate for the engine to reach best power, or the engine will start fourstroking. That evaporation, and air movement through the cooling fins is what keeps the engine cool.
@ sport_pilot.
IR measurement is good enough for our purposes, because it measures temperature (a value), and not heat which is a quantity. I take it the cylinder head was not shiny on that old ST DU used for the test. Shiny surfaces and IR temperature measurements do not agree so well, but in the hands of the same tester can still provide comparable results.

gkamysz

My Feedback: (19)

I did some math and it seems that the cooling from evaporation of methanol is fairly small, but not insignificant. For a .40 burning 1 liter per hour of FAI fuel 80/20 it comes out to 240 watts of heat to evaporate the methanol. Compare this to the 450W in output at the shaft and roughly 4500W of heat made when the fuel burns. A good portion of the 4500W goes right out the exhaust. When you run rich, you increase the cooling effect without increasing the potential heating beause you can only burn so much fuel with the air available.

TimC

Thanks for the test downunder. I wonder how a head button effects cooling? It would seem the heat would have a harder time passing through the lamination. Much as with a sleeved cylinder.

Jim Thomerson

It seems to me that the amount of air moving past the engine and cooling same would be proportional to static thrust.

Turk1

This can be a new experiment subject,what do you say Downunder?Some reduce of oil percentage and a short run only to be able to have temp reading without risking the rod bushings.

JNorton

My Feedback: (2)

I'm sure not speaking for Downunder but there is just no way I'm going to risk my engines reducing oil content. What I feel was proved was that there are many more props we can use to break in an engine without ill effects. It also proved there was nothing wrong with the traditional method.

Infrared isn't going to be able to measure something it can't see as the connecting rod is behind the back plate. I would think it would be more on the order of dissassembly to check for visible signs of wear.

John

speedster 1919

My Feedback: (3)

Some of you guys forget that the original test was done with a thermal coupling and not infared temp. Ruling out that variable. Dar in a good 3D session think of the fuel constantly sloshing around making bubbles and away from clunk pickup part of the time. Then on vertical drive the g-forces drive the fuel to the back of the tank and the engine has to overcome this plus push the fuel totally uphill to feed engine, A whole lot of leaning going on. It is the flying style and not the prop causing their problems.

An oil cooling test to keep it scientiffic would be as follows----Take a Morgan Fuel oil percentage of 17% and a middle prop size. Run WOT and take temp and RPM ,now bump up oil content 3% to about 20% total oil. Refill tank and do not touch needles (start first on low oil so no damage occurs going down on oil) Now start and run WOT and take temp and RPM . RPM should be a little lower , but how about temp?

DarZeelon

Randy,


I do not think so.

The 3D fliers that I know are accomplished ones.

They:

1. Adjust the engine to run consistently and a bit rich in 'nose-up' attitude, so as not to ever encounter a lean condition; even during launch from a hover.

2. Mount the fuel tank with sufficient latex-foam isolation, to minimize the vibration level there.


In any case, fuel will not become foam as a whole and as long as the clunk is where vibration forces pull it, it will be where fuel in liquid state is situated. With the vibration quelled sufficiently, so the clunk doesn't bounce and deflect around much, there is no chance the clunk will be anywhere within foam, or air, above the fuel level.

These 3-D engines over-heat during prolonged hovering, because the flat 3D props don't seem to cool them sufficiently.


I cannot say what temperature they are running at, when the symptoms set in, but they do.

gkamysz

My Feedback: (19)

To make a scientific test this way would be incorrect. By not adjusting the needles the engine would be running leaner. Part of the methanol in the oilier mix would be displaced by the lubricant, leading to a leaner air/fuel ratio. The mix would also have a slightly higher viscosity and flow slightly less easily through the fuel system. To do it scientifically, one would need a way to measure actual mixture strength.

speedster 1919

My Feedback: (3)

Dar You are a stubburn lot----------Have you ever watch a nylon tank water or fertilizer truck go down the road hitting bumps and turning corners. The liquid in the tank performs tidal waves splashing and sloshing around. Your clunk is not always in the fuel and you can prove it to yourself.
Take your plane up really high up and then do a dive straight down. At first the fuel will slush to the back of the tank because of G-force . Most clunks will not fall forward in tank. After a bit old gravity will take over and fuel will fall to front of tank leaving the clunk high and dry. Your engine will now run out of fuel and die. No matter how rich your settings are. Your example of holding your nose up for a few seconds and a upwards of 1 minute hover are 2 different things........

pe reivers

which can be done using EGT measurements. Here we go again!
@ Dar:
overheating engines in a hover situation needs scrutinizing the engine cooling air flow. In most cases it can be traced back to a lousy cowl design with very little applied understanding of how air flows through a cowl.

Sport_Pilot

Have you calibrated an IR thermometer? It must be calibrated to standard color and surface. It will have differant readings for a white or black surface. IR by definition is heat, not temperature. The IR heat is measured and the temperature is calculated with the assumption that the surface is the same as the standard surface. Still for most surfaces they will be close, however if an engine is putting out the same amount of heat the thermometer will give close to the same reading despite the actual temperature.

If the reading is from the fins there will be another effect cause by the film effect of the cooling air on the fins. The air near the fins will be stagnant and maintain a fairly constant temperature. That is the temperature of this film and the metal near the surface will not vary much from low flow to high flow but the larger exchange of mass will cause more heat to be extrated and the temperature of the metal below the surface will change greatly. This is because the air is acting like an insulator. When any heated object is insulated and there is no film effect the surface will always be the same, or very near the same as the ambiant air, however it is the film effect of air that makes the surface much hotter than the ambiant air. If you were to break up the film with turbulant air you reduce the film effect, however fins tend to have turbulant air. This is why it sometimes takes a second for somthing hot to burn. If there is little film effect as with say and under water object or a smooth surface in turbulant air, then the surface is actually not that hot. When you put you hand on it your hand acts as an insulator and the surface quickly rises to the temperature of the metal under it. This only takes a fraction of a second, and there is also a time lag for your nerves to send the pain to your brain, but it is a significant part of the time lag.

DarZeelon

Randy,


Yes, I am!

In 3-D, I am talking precisely about over-heating during the hover.
It is needless to say that in a vertical dive, the clunk will not follow the fuel to the front of the tank, unless it is a very large tank, with a very flexible clunk-line...

...But the situation I am referring to is again, the hover.

In this situation, the clunk is in the fuel, at the back of the tank; exactly where most of the fuel is.
No air bubbles to be seen in the fuel line, yet the engine still over-heats... not from being run too lean.

And the hover is very similar to the model being held nose-up for a long time.
In fact, over-heating is observed even if this hand hold is for extended periods...

Opening the needle further just makes the engine run with lower power. So much so, that the model cannot launch with authority.