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Heat dissipation through prop?

deej

Well Known Member
Hi all,
I am having a discussion with an acquaintance about the role that a metal prop plays in the cooling of a traditional aircraft engine, like a Lycoming O-320/360, specifically in heat being transferred to the prop via the crankshaft and the corresponding dissipation of that heat from the surface area of the prop to the air.

One of us expresses that a metal prop plays a major role, whereas the other expresses the prop plays a very minor role, if any.

The first question for the group, does anyone have any references to actual data and/or research directly related to this topic? I will likely be sending a query to Lycoming and/or Continental asking them, but thought it worthwhile to ask here first.

The second question, has anyone changed from a metal prop to a composite or wood prop, or visa-versa, and seen any changes in the temperature measurements on the engine (CHT, EGT, oil, etc)? The thought is that since wood and composite materials do not transfer heat as readily as metal, we should see a difference in the other temps if the prop does play a role in dissipation cooling.

Thank you,

-Dj
 
Heat is indeed dissipated through the prop, though probably not like you are thinking. Heat energy from combustion is mechanically converted to thrust provided by the prop.
 
Doubt much

Next time you shut down for fuel after a flight, feel the prop blade roots near the spinner. If thet are not hot, then not much heat comes out.

I dont think the steel crankshaft conducts very well. Jmho
 
Interesting question. My guess is that the main heat path from the source (cylinder head) to the crank is via the oil. But the oil temp is controlled automatically by the vernatherm valve/oil cooler. So any difference in cooling between a metal prop and a composite one is hidden, unless you can monitor the vernatherm position.
 
No actual data, but this is what I received back from Lycoming:

?Lycoming does not factor in the propeller for cooling, so there would more than likely be no noticeable change in engine temperature between the two propellers.?
 
Avweb

I read the article on avweb today about shock cooling. It was talking about old wives tales about shock cooling and goes on to say the propeller does a lot of the cooling. Guess no one is immune.

In God we trust, all others bring data.
 
I read the article on avweb today about shock cooling. It was talking about old wives tales about shock cooling and goes on to say the propeller does a lot of the cooling.


That article was what prompted the discussion, in particular this paragraph:

"Let?s look at the numbers involved in engine cooling, starting with the small role that the cylinder fins play. Only about 12 percent of the heat generated by combustion departs from the engine via the cooling fins. The biggest proportion, 44 percent, goes out the tailpipe. Eight percent, almost as much as is handled by the cooling fins, is dissipated through the oil. Much of the rest is dissipated via the big, metal prop bolted to the crankshaft."

https://www.avweb.com/news/features/Shock-Cooling-Time-to-Kill-the-Myth-230134-1.html

I sent an inquiry to the author inquiring where he obtained these numbers, but have not received a response yet. I'll post here anything further I find out.

-Dj
 
That's 64% accounted for, with "most of the rest" being dissipated by the prop. That leaves 36% for the prop, at best. Thermal efficiencies for internal combustion engines vary 25% to the high 40's depending on design. That heat energy is converted to motion by the crankshaft and transmitted to the prop as rotational energy, not heat energy.
 
Go with Lycon.

I think if you use an Infer-Red camera you will find that the answer is as Lycon suggests. Only the hub will pick-up a small amount of heat and then through it off into the air stream. Hope this helps,
Yours, R.E.A. III # 80888
 
The wording is misleading - in context of the article, i believe what was intended is that energy in the fuel is converted to work and that is dissipated through the prop. Sure every nut and bolt attached to the engine gets warm, but no significant heat transfer. It is typical for about 10% of the fuel energy to be dissipated through general convection of all the remaining structure.

So:

exhaust 44%
Fins 12%
oil 8%
general 10%
work 26%

And these are very general and will vary with A/F ratio, general SFC, piston cooling jets and overall temperatures to mention a few. Oil temps easily can make a 2-3% shift.

One note relevant to the OP: a friend noted his metal CS prop was warm from the Reiff heater (20F OAT) so there is a noticeable heat transfer path to the hub and blades. Just not significant in terms of the quantity relative to other engine sinks. If it was outside and wind blowing at -40F, insulating the prop would help the preheat.
 
Here are my thoughts, fwiw.

Thermodynamics class was a long time ago, but I think I recall that the larger the temperature difference, the better the rate of heat transfer.

We know that a fin arrangement is far more efficient at transferring heat to the air than a solid block of metal, thus why we use fins on the cylinders as well as on heat sinks in electronic components, etc.

The cross section of the crankshaft is fairly small compared to the engine and prop, so the rate of heat transfer from the engine to the prop is likely also relatively small.

As we know, the main source of heat is from the combustion process within the cylinders. If we look at the relative temperatures of the various parts of the engine, the higher the temp, the more likely that part plays a higher role in the overall cooling process, along with the amount of metal surface area that is exposed to the air.

At 1400-1500F, and considering the rate of air flow within it, the exhaust is very likely the main component to removing combustion heat from the engine.

At 350-400F, and considering the amount of air flowing over the cylinders, they are very likely the second main component to removing heat from the engine.

At 180-200 F, and with the lesser amount of air flowing through it, and the lesser amount of metal exposed to the air (the surface area of the fins of the oil cooler) the oil cooler is likely the third main component.

All of this matches logically with the paragraph in the article if we ignore the comment regarding the prop.

I?m not (yet) convinced that the prop plays any major role in cooling the aircraft in terms of dissipating heat (it obviously plays a role in moving air over the engine, especially on the ground). Given the small cross section of the crankshaft, the corresponding rate of heat transfer through that small cross section, along with the prop being a solid hunk of metal, my guess would be that the prop plays a far less role in dissipation cooling than the cylinder fins do.

I fully agree that some heat will go to the prop, and if preheating over a few hours of time, the prop will get warm. However, I suspect that the rate of heat transfer from the engine to the prop is far too slow for the prop to be a significant part of the dissipation cooling when flying the airplane.

I would be very interested in knowing where the numbers in the article came from, ie, 44% exhaust, 12% cylinders, 8% oil cooler.
 
I know the Avweb article is not accurate with regard to shock cooling and cracking cylinders when glider towing, so why should it be accurate concerning heat lost through the prop? Avweb is normally very good ...

Pete
 
how about NON-traditional? ( d r i f t i n g h e r e )

Hi all,
.... heat being transferred to the prop via the crankshaft and the corresponding dissipation of that heat from the surface area of the prop to the air.
One of us expresses that a metal prop plays a major role, whereas the other expresses the prop plays a very minor role, if any.
....... has anyone changed from a metal prop to a composite or wood prop, .........

-Dj

...So.....I'm thinking about the CHT's and oil temp in this scenario;

heavily laden AirBoat, Pusher config of course, going 20-30 mph in ambient temps of 90+ degrees. The hot cylinder fin air goes INTO the prop, heating it up! ( maybe a little exhaust as well!)

ok, it's spinning even at idle, so dissipates well. Most airboats have composite or wooden props anyway, for cost and ease of repair, (plus it saves the crank when the steering fails and you blast headlong into a mangrove jungle!)
...any deep southerners care to comment? ;-) our airboats here have de-icing equipment! :D
 
I would be very interested in knowing where the numbers in the article came from, ie, 44% exhaust, 12% cylinders, 8% oil cooler.

Having had the job of providing heat rejection for spec sheets in the past, I can say it is difficult to accurately measure the various sinks. Exhaust is pretty directly calculated from mass flow and temps. But the temp has to be far enough down the pipe to have complete combustion. Collected exhaust is best.

I might guess that the factory numbers are in the ball park, but know for a fact that some numbers can be way off individually as the balance of heat flow to different areas can change. Factory numbers might be the basis for the fractions in the article. A parallel valve fin/oil balance will be different than an angle valve. Restricting barrel & head air can drive up heat to oil.

There are many factors that will affect the flow chart of heat.

Now that the prop "heat rejection" has been addressed what is your goal?
 
FWIW, Continental replied with "The propeller doesn't heat up during engine operation, unless you have de-icing boots installed."
 
Now that the prop "heat rejection" has been addressed what is your goal?

Hi Bill,
The goal is simply to satisfy curiousity and education. The Avweb article started us thinking, and led to asking questions.

-Dj
 
Are you saying shock coolong IS a concern when towing?

Yes, most definitely. Easy for a single tow pilot to wreck an O-360 in a tank full of gas. Chop the throttle suddenly at the top of the tow and dive quickly to Va +10 or 20kt. Cracks will form between the exhaust valve seat and the nearest spark plug hole. 4 new cylinders required - no point in welding.

Pete
 
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