saramos

ROTFLMAO

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

Scott

DarZeelon

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.

proptop

My Feedback: (8)

Hobbsy

My Feedback: (102)

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.

yallaair

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.

wcmorrison

My Feedback: (2)

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

Sport_Pilot

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.

asmund

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

Sport_Pilot

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.

yallaair

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.

Sport_Pilot

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.

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.

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.

Sport_Pilot

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.

DarZeelon

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.

JNorton

My Feedback: (2)

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

Sport_Pilot

I am not a field techie if thats what you mean. I have designed some complex HVAC control systems to include differantial and intregal proportional control. I know of no differantial that would accomplish this. This is a term associated with electronics, I don't know how it could be applied to a thermal measurement, but I don't assume that you can't.
I am a registered engineer, however its been years since I have done design, I now manage projects instead of design. In fact I picked up a book to brush on heat transfer, but had to put it down, will get back to it later. However, one thing I did notice, I am used to heat tranfer through building materials and the air film is a very minor barrier to heat loss, however, with the very huge heat conductance of aluminum it becomes the major barrier. More later, probably next week as I will be busy this weekend.

MJD

My Feedback: (1)

Those that are mentioning they are engineers would identify with the credo that if the results do not match the model, go back and check your assumptions. This cooling scenario contains myriad variables, and to me, despite the test being done in a passably scientific fashion, it is just one pair of numbers, and while I do not disagree with the observations whatsoever to me it is not enough information to make a blanket statement.

Heat transfer from the engine to the air is not linearly proportional to the air velocity - increasing air speed by a factor of 1.5 does not mean that 1.5 times as much heat is being removed from the surface. I see three basic steps in the process - the rate at which heat is transferred from the combustion chamber to the walls [which at a given rpm and power output, i.e. fuel/air mass flow rate, will stay the same], the rate at which heat flows through the cylinder walls [proportional to the product of the specific heat and the thermal conductivity, and again which will stay constant in a fixed rpm/power scenario], and the rate of heat transfer to the gas medium. Increasing the air speed does not increase the thermal conductivity of the cylinder, the heat can only flow so fast. It is logical (to me) at first to conclude from this that a smaller prop of higher pitch, i.e. constant load, will produce a faster air stream across the cylinder, but because the heat output of the engine is constant, but the rate of heat transfer is not linearly proportional to the air velocity, the smaller prop/higher air speed scenario would result in higher cylinder temperatures than with the larger prop and lower pitch scenario. How much? Can't say.

Dar says the 3D models suffer from overheating - this is an observation, and observations are tough to challenge unless they are the result of faulty methods of measurement, so I feel comfortable agreeing that they have their problems, but I don't claim to know all the factors involved. The test mentioned here says the engine temps stay the same, and that is another observation, although one might be picky and say there is room for error, and perhaps if the difference between the two scenarios is not that great (if it exists) then the difference might be less than the measurement process can resolve.

It is a good exercise to look at extremes, and not something to ridicule. Often by thinking about an extreme example one can see where initial assumption may be in error, or at least begin to see how a situation cannot be described in simple terms. I cannot think of how many problems I have solved, or at least understood better, in this manner. One such example is to repeat the test with a load beam that also results in 11,000 rpm. Now what is the cylinder temperature? I think we can safely assume they will be higher, as not all engine cooling is radiative. So what does this tell me? It tells me (meaning it changes my assumptions) that despite the same temps observed in the test, it is not safe to assume that no matter what prop you put on the engine, as long as it runs at 11,000 rpm at WOT it will have the same cylinder temperature. Another possible conclusion is that the cooling is 100% radiative, and I do not believe that at this time.

So all I can conclude from this so far is that nobody has really run a scientific experiment that runs the gamut of possibilities and that considers all the variables, and therefore there is insufficent information so far to make any blanket statements whatsoever. So therefore, from that I would have to say that the most useful information we have at hand is our observations, not blanket claims that "large prop low pitch" and "small prop high pitch" are the same, or that one is an improvment over the other in terms of cooling etc.

If one observation is that on the test stand the engine runs at the same temp with different props running at the same rpm, great, those observations are "correct" within the terms of the experiment. Same that if one observes 3D models with low pitch large dia. props have a tendency to overheat, that too is "correct". I don't think the two need to be at loggerheads with the other, even if there are arguable or debatable points in the logic.

Oh s---, it's 5pm - time to go home!!! That I know for sure..

MJD

JNorton

My Feedback: (2)

MJD
I get so tired of that same old saw. I spent 15 years working on the floor. The last 15 as an electric controls engineer. No engineer worth his salt would make the results match the model. PERIOD END OF STORY.
John

JNorton

My Feedback: (2)

Sport Pilot
Cold junction offset helps eliminates errors introduced by ambient temperature.
http://www.ecnmag.com/article/CA508469.html?ref=nbra
Commercial controller using differential input to implement a cold junction offset.
http://www.omega.com/ppt/pptsc.asp?ref=DP26&Nav=temp04
John

yallaair

Dar.
I believe that a low pitch prop is producing more or less the same cooling in the centre of the hub as a high pitch prop. If you look at a high pitch prop, it's more "bulky" than a "slimmer" low pitch prop around the centre. This may be a factor in the cooling and how the air is flowing though the cooling ribbs.

When a 3D airblane tends to overheat, most likely this can have many other reasons than the prop selection. Most likely are those engines set quite lean and the vertical acceleration will lean the engines even more.

Sport Pilot:
This is not correct. The cooling changes as a natural logarithm, as a function of the airspeed (a).
(cooling = Xln(a), where a>0)

downunder

Dar, your common sense allowed you to reach one conclusion but my common sense allowed me to reach a different conclusion. Why should my "common sense" necessarily have to agree with yours seeing we've both based them on different observations? Your observations have come from some 3D flyers. My observations have come from CL stunt where very low pitch props are the norm (piped engines running in a fast 2 stroke and RC engines adapted to stunt) and there's never a problem with overheating.

So who was more correct? I didn't know because I'd never tried to find out. If I'd found a 30 or 40 degree difference then I could have said "Hey, Dar was right, fair enough" but that wasn't what I found. I didn't set out to prove anything at all, I set out to test two different assumptions.

Now put your ego aside for a moment and try to understand the difference between "proving" and "testing". We're not doing rocket science here, we're doing the equivalent of shoving a thermometer into a chicken in the oven to see if it's cooked.

DarZeelon

Brian,


You know that even though we often have our discussions, I respect what you have to say.
I usually agree to change my original assumption, when proven wrong (as was the case with the tuned-pipe's boost in that racing engine).

But in this case we have a problem...

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.

---------------------------

It is either this, or the engine is cooled ONLY by the evaporation of the methanol and the nitro (as may have been observed by someone in your original thread...), with air-flow contributing nothing.

So, as long as our engines are manufactured with ample cooling fins, there are many who think the speed of the cooling air-flow is an issue that could maintain an engine in good shape, or destroy it; depending on its magnitude.

It would be much simpler and cheaper to cast an engine without any cooling fins, but no manufacturer does so.


Your latest observation did not produce the results I expected, so I could only conclude something was overlooked.

...Or the cooling fins (and cooling air-flow) really are of absolutely no importance...


I liked that chicken-thermometer comment...

speedster 1919

My Feedback: (3)

Guys this is the JERK WAD --Sport Pilot I would have figuired you had a super duper thermo coupler up your sleeve!!!!!!!!!!!I did start out life as an Industrial Engineer-2 years- Here's the facts -These are the dumbest simple 2 stroke engines on the face of the planet. The simple fact is without spending tons of money the test by Downunder was fair. 270 -270 was enough for me. The simple fact is the simlplest way to test load is static RPM and when an engine turns 11.000 to 11,000 is close enough for me. We all know deep down if the HS needle was richened a few clicks the temp would be affected. This was not about prop diameter. It started out the 4" pitch was the problem. Now the fact is the 4 pitch produces less airflow accross engine fins and slower I am sure is true but it is enough As for the 2/3 of the prop provides all the flow and the hub does the cooling. If you ever stand behind a plane the prop blast is not a small colum but a larger radius. As mention in a previous post . The air could pass over the fins at 1000 MPH but the simple fact is The heat transfer rate is a set factor between cylinder,crankcase and fins and will transferr heat just so fast and at a certain air flow the heat can be taken away no more faster. Yes I'm sure that the expansion of liner and case changes a little at different temps , but both props had the same load and heat. If you think about it 11,000 RPM for that Super Tigre is bogging it down a little which is good as to produce a little more heat that would show a wider gap in temp if pitch was a factor.

Hobbsy

My Feedback: (102)

It's as simple as , the arm chair mechanic loses and the hands on mechanic who actually did a test wins.

JNorton

My Feedback: (2)

YIPPEE Finally someone whose elocution in the written language might finally put a stop to this very interesting and rather pointless discussion that I've enjoyed immensely. I'm still going to run the test this spring when the temperature warms up. Since I have the equipment I'll do it both with thermocouple and infrared at 10 minute run intervals and utilize a data logger to dump it into an Excel spreadsheet. Just for giggles I'll see if my postal scale has an output and I'll graph fuel consumption too. Why? Just for fun.

Hobbsy -> good point.
John