RE: Prop pitch cooling - the final word  
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RE: Prop pitch cooling - the final word - 2/16/2007 3:11:23 PM   
saramos



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quote:

ORIGINAL: DarZeelon
Wrong, Brian.


I used common sense to determine that a high-pitch prop, which provides a given load, will cool the engine better than a low pitch prop with the same load.


...And 3-D is not flying, BTW.



ROTFLMAO

That's the funniest load of ...... I've read in a loooooong time! You gotta take this act on the road man!!!

Scott


(in reply to DarZeelon)
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RE: Prop pitch cooling - the final word - 2/16/2007 3:36:53 PM   
DarZeelon



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quote:

ORIGINAL: Hobbsy

I dunno about that, common sense says a higher pitch presents a larger load and therefore more heat not less. Since heat is energy that is always trying to distribute itself evenly among any medium and air is a lousy heat conductor compared to an aluminum engine, changes in airflow velocity or volume aren't going to make big changes in engine temp, just like Brian discovered.


Dave,


You seem to have things a bit confused...

Pitch and diameter are trade-offs. For a prop with given diameter and pitch numbers, there are other props of the same basic design, with the same load; that have either a larger diameter and a smaller pitch, or a smaller diameter and a larger pitch.

If you take 10" diameter props, of course the 8" pitch prop will burden the engine more than the 6" pitch; and surely more than the 4" pitch, still with a 10" diameter.

A 9x10 APC prop has a humongous pitch, but compared to a 14x6 prop that has a flatter pitch, it offers much less load...

10" pitch is less load than 6" pitch!?... So pitch is a load only if there is a diameter attached to it.


What Brian is essentially trying to prove is that higher speed air-flow will not cool the engine better than lower speed air-flow.

This is contrary to logic, so I disagree.


As to heat conduction; air really is a lousy conductor of heat, at least when compared to aluminium.

So, only the air which passes very close to the cooling fins gets all the heat <GGG>... So, what do you do to take more heat from the engine?
Reduce the air-flow? Increase the air-flow? Stop the air-flow?

If you measure the exit temperature of the cooling air; it is going to be hotter if the air-flow is slow, than if the air-flow is fast.

But does that mean the slower air cooled the engine better?
As the Δº between the cooling air and the cooling fins becomes smaller, heat convection from the fins to the adjacent air is reduced.

The lower the relative temperature of the cooling air, the higher their heat absorption from the cooling fins.

So, higher speed cooling air will absorb more heat from the engine, despite exiting at a lower temperature.
This is because there is much more of it, compared to the slower flowing air.


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RE: Prop pitch cooling - the final word - 2/16/2007 4:38:47 PM   
proptop



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RE: Prop pitch cooling - the final word - 2/16/2007 5:12:21 PM   
Hobbsy



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Status: offline 		What Brian is essentially trying to prove is that higher speed air-flow will not cool the engine better than lower speed air-flow.

That is precisely what I said, no confusion on my part.

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RE: Prop pitch cooling - the final word - 2/16/2007 5:23:03 PM   
yallaair



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From: Floro, NORWAY
Status: offline 		Again, who has actually tested that a high pitch prop will give a higher airspeed over the engine?? It is just an assumption that the high pitch prop gives a higher airspeed through the cooling fins than a low pitch prop. The only thing that has been tested and proven, is that the pitch does not influence on engine temp.

If you fly the engine with different airspeeds at full trottle with different prop pitch and the same engine cowl shape, you may be able to get a significant higher airspeed though th cooling fins. But you wil need two different aicraft types with the same power consumption at different airspeeds with the optimum propeller for the aircraft / engine combination to perform the test. And the test must be done at the same ambient conditions.

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RE: Prop pitch cooling - the final word - 2/16/2007 5:33:34 PM   
wcmorrison



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Status: offline 		Seems to me that the test for radiant cooling would be to put the prop on backwards and measure the temperatures. That would assure the minimum effect on the "prop cooling."

Cheers,

Chip

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RE: Prop pitch cooling - the final word - 2/16/2007 6:11:50 PM   
Sport_Pilot



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Status: offline 		Dar has been saying that you keep the load about the same by using a smaller diameter as well as higher pitch. That said, extra load is not necessarily more heat. If you load up the engine to the max HP then add load to the Max torque then the max HP is the one producing the most heat because heat can be directly converted to power not force.

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RE: Prop pitch cooling - the final word - 2/16/2007 6:16:14 PM   
asmund


 

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Status: offline 		I have started getting steady at hovering. Right now I am doing alot of hovering on my Harrier 46, sc 91 fs on a 14-6 .No problems with temps. Later this season I will bring out my new OMP Katana profile and hopefully be able to hover that one steady for extended periods also. This one have ASP XLS 52 and APC 12-4 in the nose. I guess I will have no problems with temps on this one also, but then again we are freezing our a.s off here in Norway most of the year, summer too

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RE: Prop pitch cooling - the final word - 2/16/2007 6:35:46 PM   
Sport_Pilot



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quote:

It is just an assumption that the high pitch prop gives a higher airspeed through the cooling fins than a low pitch prop.


It's not an assumption, static testing has been done before and a higher pitch does give higher static airflow. However, it will not be completely proportional, a higher pitch may have lower efficiency and more slip so double the pitch probably is not double the airflow.

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RE: Prop pitch cooling - the final word - 2/16/2007 8:11:58 PM   
yallaair



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quote:

It's not an assumption, static testing has been done before and a higher pitch does give higher static airflow. However, it will not be completely proportional, a higher pitch may have lower efficiency and more slip so double the pitch probably is not double the airflow.


I belive that a higher pitch can give a higher airspeed on some parts of the "propellers rotating disc", but not necesary over the cooling fins. Due to the fact that most of the effect produced by the propeller is on the outer 2/3 of the radius, and the inner 1/3 of a high pitch prop is less effective due to the very high angle of attack and low relative airspeed.

As I sad, "It is just an assumption that the high pitch prop gives a higher airspeed through the cooling fins"

It's mentioned before, the cooling effect is more dependent of the engine istallation design, engine needle setting, ambient conditions and fuel type than prop selection within makers spsifications.

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RE: Prop pitch cooling - the final word - 2/16/2007 8:19:21 PM   
Sport_Pilot



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quote:

Due to the fact that most of the effect produced by the propeller is on the outer 2/3 of the radius, and the inner 1/3 of a high pitch prop is less effective due to the very high angle of attack and low relative airspeed.


The inner part may not be as effective but the same principal applies. However, if you are using a larger prop cut down to match a smaller prop, then the inner less effective part is just as large as before. Thus the smaller prop may move more air or the same air near the fins as the larger prop, regardless of pitch. So I see what you mean, without taking an airflow measurement you may indeed be making an assumption.

quote:

It's mentioned before, the cooling effect is more dependent of the engine istallation design, engine needle setting, ambient conditions and fuel type than prop selection within makers spsifications.


Yes, I agree with that. As I said before the surface temp decrease goes down exponentially as airspeed increases. The amount of heat removed also decreases but not as dramatically as surface temp. Still there should be a small differance. I have just been reminded that one of the problems with taking a surface temp is that the thermometer is not insulated and thus is taking an average temp of the air and the surface. That is the main reason temp probes are buried in a well in a head, or pipe, or whatever. If you don't have a well then the probe should be wrapped with insulation with full contact to the medium bening measured. This is basically how the thermostat to a water heater is installed.

< Message edited by Sport_Pilot -- 2/16/2007 8:53:17 PM >

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RE: Prop pitch cooling - the final word - 2/16/2007 8:28:24 PM   
asmund


 

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Status: offline 		The 3-d guys must have overcome the overheating problems because no-one flyes 3-d with 10" pich props, not in small glow planes anyway.

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RE: Prop pitch cooling - the final word - 2/16/2007 8:52:05 PM   
Sport_Pilot



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quote:

ORIGINAL: asmund

The 3-d guys must have overcome the overheating problems because no-one flyes 3-d with 10" pich props, not in small glow planes anyway.



I don't think anyone is saying that the airflow is inadequate for cooling. Just that the smaller higher pitched prop does not significantly improve cooling. Which may be true, but I think it would have to improve cooling at least a little.

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RE: Prop pitch cooling - the final word - 2/16/2007 9:04:13 PM   
DarZeelon



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Status: offline 		Yallaair,


Assuming most props have something close to a true helical pitch, once the vortex is formed by the prop, the airspeed throughout the span is nearly identical. No part of the blade is stalled within the vortex.

This means a higher pitch prop would produce a higher speed flow between the cooling fins.

It is the blade outer 1/3 which produces most of the thrust, but that doesn't mean the inner part produces less cooling.

It is the middle 1/3 of the blades, which produce the cooling air-flow.
The inner 20% of the prop, on its own, may not even build a vortex.

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RE: Prop pitch cooling - the final word - 2/16/2007 9:11:08 PM   
JNorton



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quote:

I have just been reminded that one of the problems with taking a surface temp is that the thermometer is not insulated and thus is taking an average temp of the air and the surface. That is the main reason temp probes are buried in a well in a head, or pipe, or whatever. If you don't have a well then the probe should be wrapped with insulation with full contact to the medium bening measured.
You can always use a differential measurement if that's a problem no biggy. I'm beginning to wonder how much of this stuff you've actually done or how much you're just parroting.
John

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