RE: Prop pitch cooling - the final word  
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RE: Prop pitch cooling - the final word - 2/17/2007 1:26:00 PM   
speedster 1919



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quote:

I cannot accept that in subsonic flow conditions (no compressibility), a slower moving mass of air will cool an engine as well as a faster moving mass of air.

Investigating this further; the flow speed produced by an 8" pitch prop @ 11K RPM is 37.25 m/s and even if it has to flow between the cooling fins at twice the speed (supposing the fins and the 'channels' share the space 50/50), it is Mach 0.23. Absolutely no chance of compressibility, or slowing down of the air-flow.

The 3.75" pitch prop would produce about 17.5 m/s cooling flow speed.

The high-pitch prop producing over twice the speed MUST cool the engine better.

I want to put this in the simplest terms . This is all about maximum velocity or at least maximum. Take a 2 round bowling balls one weighs 10LB and the other weighs 20LB , Dropped from the tallest building in the world they will both fall at the same speed and reach terminal velocity in a small amout of feet and will not fall any faster the farther they fall (even the heavier one). Take water that freezes at 32 degrees. If I keep a cube at 30 degrees and then keep it at 15 degrees it will get no harder or larger and will stay frozen and in a sense has reached terminal velocity. Now since the heat transfer rate is fixed like gravity and freezing at a certain airflow amount and speed it will reach terminal velocity . Hence the 4 pitch prop is like the lighter 10lb bowling ball and the warmer 30 degree ice cube. It is enough flowwwwwwwww!!!!
Now if the wrong diameter of a 4 or 8 pitch prop for break in is used and the load is inapropiate this could be a problem.

< Message edited by speedster 1919 -- 2/17/2007 1:34:56 PM >

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RE: Prop pitch cooling - the final word - 2/17/2007 1:36:50 PM   
Hobbsy



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Status: offline 		By the same token, if you hold a rifle barrel perfectly parallel to the ground and hold a penny at the same heighth, then fire the rifle and drop the penny at the same instant, the bullet and the penny will hit the ground at the same instant.

The faster the air goes the less time it has to soak up heat.

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RE: Prop pitch cooling - the final word - 2/17/2007 2:56:06 PM   
JNorton



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Status: offline 		Whose got the bowl? Pass the popcorn please.

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RE: Prop pitch cooling - the final word - 2/17/2007 2:57:38 PM   
speedster 1919



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Status: offline 		Hobbsy the same amount of heat has soaked up , but just spread out over a greater volume of air. I like your rifle analogy -I 've heard it before and I beleave it.

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RE: Prop pitch cooling - the final word - 2/17/2007 4:32:50 PM   
DarZeelon



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quote:

ORIGINAL: Hobbsy

...the arm chair mechanic loses and the hands on mechanic who actually did a test wins.



Who you callin' a 'armchair mechanic'?

Besides, I have not lost.
Empirical data did show no difference, but it still doesn't add up to convince logic.
There is some reason for the lack of temperature difference, despite one instance that should be a superior cooling setup.

I just don't know what it is yet.

quote:



The faster the air goes, the less time it has to soak up heat.




Ditto (see a previous post)! This is why faster moving cooling air comes out colder than slower moving cooling air...
But since the Δº is greater, the total amount of heat taken from the engine must be greater (is this an axiom?).

But where do you stop, Dave? Let's ask Brian to just run the engine, as suggested, with load beams and no cooling at all...

The stagnant cooling air will then have plenty of time to absorb the heat from the engine... And then what?
Do you think it will stabilize at 270ºF and prove a glow engine doesn't need any air cooling?

...But no. This doesn't work for all those R/Cers who burnt up their engines from lack of cooling air-flow, does it?








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RE: Prop pitch cooling - the final word - 2/17/2007 4:33:07 PM   
gkamysz


 

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quote:

Take a 2 round bowling balls one weighs 10LB and the other weighs 20LB , Dropped from the tallest building in the world they will both fall at the same speed and reach terminal velocity in a small amout of feet and will not fall any faster the farther they fall (even the heavier one).


See the problem with this is the details. The truth to the above is that you are only correct when there is no atmosphere. Air drag is real and quantifiable. The 20LB bowling ball WILL have a higher terminal velocity. Depending on the height from which it's dropped and the timing equipment, the results might be within the error of the experiment and not obvious to the observer. If you take it to the other extreme and have a 20lb ball and a 1/2lb ball the assumption becomes obviously invalid, for the same reasons I just mentioned.

The same is true of our discussion here. Each individual has there own set of details they are thinking provides the answer to the question. Yet we have only one test, and everyone except the experimentor thinks it's invalid.

Edit- It would seem quite obvious why car engines have such huge heatsink heads on their engines. But then again a .21 makes something like 3HP today, so there is much more heat to dissipate. But compare the fin area on a CL Speed .15 to that of the typical sport .15 and you'll find significantly less area on the speed engine. Maybe velocity is a factor, no?/Edit


I feel the test is valid. The results are real, unless downunder made a serious error in his methods, but that doesn't sound likely.

I remember seeing the older MAN Engine Talk articles where the tests were done with load beams and fan cooling. I don't remember who the author was at the time, but certainly they could easily answer our futile discussion. Where is AJC? He has plenty of static and in-flight data of exactly what is being discussed.


< Message edited by gkamysz -- 2/17/2007 4:45:50 PM >

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RE: Prop pitch cooling - the final word - 2/17/2007 8:38:13 PM   
speedster 1919



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Status: offline 		Greg you are full of crap. period. As long as they are both round for no resistance and made of the same material ---They will fall dead even -----even outside a vaccum............

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RE: Prop pitch cooling - the final word - 2/17/2007 8:40:35 PM   
gkamysz


 

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Status: offline 		Prove it.

Here's a simple explanation of what I said before.

http://dev.physicslab.org/Document.aspx?doctype=5&filename=Compilations_NextTime_FallingSpheres.xml

< Message edited by gkamysz -- 2/17/2007 8:49:10 PM >

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RE: Prop pitch cooling - the final word - 2/17/2007 9:03:22 PM   
JPMacG


 

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Status: offline 		Actually, both of Speedster's analogies were incorrect. The heavier bowling ball will have a higher terminal velocity, and ice does shrink and expand with temperature. Not trying to get on your case Speedster, I do understand your point.

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RE: Prop pitch cooling - the final word - 2/17/2007 10:26:41 PM   
speedster 1919



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Status: offline 		Like I said Greg your full of it. I said bowling balls and the same material and you give me wood and iron balls of the same size. I am sure the 20lb bowling ball will be a larger diameter but it makes no difference if the shape and made of material is the same.

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Hey Buddy -That was the most spectacular crash I've ever seen , That rolling cart wheel with parts slinging out was cool...

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RE: Prop pitch cooling - the final word - 2/17/2007 10:35:39 PM   
speedster 1919



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Status: offline 		CRAP Now I get faulted for a couple of thousands on an ice cube for a 15 degree temp change.
I give up ---I should have left you slide rule boys to duke it out. Downunder was smart to back out of his own forum----but then sometimes I'm not a smart man. I think I change my name to GUMP..............

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RE: Prop pitch cooling - the final word - 2/17/2007 10:39:09 PM   
gkamysz


 

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Status: offline 		I'm sorry I assumed you meant say they were the same diameter with different densities and the same surface finish. Please open your Fundamentals of Fluid Dynamics text book and do your math, present it here and prove me wrong. You could also research how a falling sphere viscometer works and try to explain why it shouldn't work according to your incorrect assumption about drag and gravity. Or you can prove me wrong by using math to prove that both spheres will have the same terminal velocity in an atmosphere.

I could go as far as saying you never studied physics, but I wil stop short of that by saying you are full of crap.

"Anything is possible if you don't know what you are talking about."

Also take the time to look up the coefficient of thermal expansion for water and ice. Then tell me that ice's properties won't change with temperature.

Our little discussion here is far more complicated than the falling spheres math problem. There are many more variables.

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RE: Prop pitch cooling - the final word - 2/17/2007 11:33:31 PM   
Jim Thomerson



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Status: offline 		I pay a heck of a lot more attention to the results someone gets from actually doing an experiment and making measurements than I do to specious comments from the peanut gallery. And, as you would might deduce from the previous statement, No, I am not an engineer.

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RE: Prop pitch cooling - the final word - 2/17/2007 11:33:49 PM   
speedster 1919



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Status: offline 		OK GREG and JPmacG your both full of crap and this will prove it.
http://www.sciencebuddies.org/mentoring/project_ideas/Phys_p015.shtml

PS Greg make sure you take back that I am full of crap

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Hey Buddy -That was the most spectacular crash I've ever seen , That rolling cart wheel with parts slinging out was cool...

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