RE: Prop pitch cooling - the final word  
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RE: Prop pitch cooling - the final word - 7/26/2007 5:13:45 AM   
Sport_Pilot



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From: Acworth, GA, USA
Status: offline 		Greg,
A very good explaination. Better than my poor attempt.

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RE: Prop pitch cooling - the final word - 7/26/2007 1:52:41 PM   
gkamysz


 

Posts: 1588
Joined: 6/24/2003
From: Crystal Lake, IL, USA
Status: offline 		In my estimates the APC 14x6 and APC 12x8 are similar in HP. The 14x6 provides less cooling air so it runs hotter. But there is also the possibility that the lower RPM temperatures could be higher because ignition timing is too advanced, or the higher RPM is cooler because the ignition timing it too late.

In reality a temperature variation of roughly 10% is small and nothing to worry about. We should be much more concerned with knocking while running the larger props. I think even more power could be had at with the 12x8 if we increased the compression ratio slightly.

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       Post #: 152

RE: Prop pitch cooling - the final word - 7/26/2007 2:09:31 PM   
DarZeelon



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From: Rosh-Ha'Ayin, ISRAEL
Status: offline
quote:

ORIGINAL: gkamysz

In my estimates the APC 14x6 and APC 12x8 are similar in HP.



Greg,


This assumption is incorrect.

From Yallaair's post, the 12x8 APC is spun 1,000 RPM faster than the 14x6.

Most .91 engines and the OS.91FX in particular, has a rising HP curve in this RPM range (which levels off around 12,500).


Significantly more output is being put out using the 12x8.

Your assumption about 'over-compressing' the engine with the smaller prop is quite correct....
High compression is better tolerated at higher RPM levels.


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(in reply to gkamysz)
       Post #: 153

RE: Prop pitch cooling - the final word - 7/26/2007 2:19:09 PM   
gkamysz


 

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Joined: 6/24/2003
From: Crystal Lake, IL, USA
Status: offline 		Like you, I started typing my post with those words exactly, until I decided to check the numbers. I too was surprised that the two props showed similar power figures.

Dar, my "assumption" was not. It was calculated based on what I accept to be appropriate constants for said propellers. Now, if I had both props in my arsenal I would have run them like I did in our other discussion about prop power absorption.

So at this point we are both assuming that the engine should be making more HP at 10+,kRPM than 9+kRPM. But I'm also assuming that my power absorption estimates are accurate. Dar, even your trusted calculator by Pe agrees that power figures are similar. Actually, it shows the 14x6 requires 5% more HP. I assume you ran the numbers though. My estimate was about 2% more.

BTW, the engine in question is a Magnum/ASP .91 FS.

< Message edited by gkamysz -- 7/26/2007 2:20:01 PM >

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       Post #: 154

RE: Prop pitch cooling - the final word - 7/26/2007 2:42:48 PM   
DarZeelon



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quote:

ORIGINAL: gkamysz

Like you, I started typing my post with those words exactly, until I decided to check the numbers. I too was surprised that the two props showed similar power figures.

Dar, my "assumption" was not. It was calculated based on what I accept to be appropriate constants for said propellers. Now, if I had both props in my arsenal I would have run them like I did in our other discussion about prop power absorption.

So at this point we are both assuming that the engine should be making more HP at 10+,kRPM than 9+kRPM. But I'm also assuming that my power absorption estimates are accurate. Dar, even your trusted calculator by Pe agrees that power figures are similar. Actually, it shows the 14x6 requires 5% more HP. I assume you ran the numbers though. My estimate was about 2% more.

BTW, the engine in question is a Magnum/ASP .91 FS.


Greg,


Right you are...

This would explain the 'glass ceiling' limiting the RPM of this engine...


I somehow missed the fact that this was not an OS .91 two-stroke engine and I stand corrected.


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       Post #: 155

RE: Prop pitch cooling - the final word - 7/26/2007 11:02:28 PM   
Jazzy



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From: Erie, PA, USA
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quote:

Work and power are the same. Power is a means of measuring work. Load is simply the amount of force that is applied to the engine.


Sorry Sport Pilot, you are wrong and you know it. Power is a rate at which work is done. Work is force times distance. They are not interchangeable terms. At least we do agree on what Load represents!

I was unable to post yesterday so when I get home I'll copy & paste what I typed up.
Frankly, you can go on and on about work and power to you hearts content. They are a given.
How they and prop Loads relate to a hovering engine vs. an engine in level flight is what I'm talking about in an attempt to explain why some hovering engines overheat.
Enough already.

Greg, it really was not my intention to confuse you or anyone else. I hope my next post explains things to a much better extent.
While I don't care to delve so deeply into the "Hows and Whys" regarding Yallaair's figures I do find them quite interesting. I simply figured that the higher RPMs of the 12X8 were the cause of the higher level flight operation temp. Nice conclusions.

Jeff

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       Post #: 156

RE: Prop pitch cooling - the final word - 7/27/2007 1:31:12 PM   
longdan



Posts: 495
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From: Auckland, NEW ZEALAND
Status: offline 		Well, it's taken a while but I've finally read through to the end of this thread. I think some (I won't name any names) have taken it way too far.
As Hobbsy said in a much earlier post: The armchair mechanic is the loser. The guy who actually did the test wins.
To me the conclusions seem to be:

1. A propellor with some pitch will provide enough airflow to cool an engine.

2. Fine pitch props provide engine cooling airflow.

3. The terminal velocity of a skydiver or a bowling ball has no effect on engine temperature.

4. Until someone actually posts some experimental evidence of coarse pitch props providing better cooling than fine pitch props, the results posted by Downunder are the final word on prop pitch cooling.

Regardless of all the extraneous drivel posted on this thread, I will continue to break in engines with a relatively fine pitch prop so as not to put too much load on said engine but still provide a good flow of air, and not worry about whether it is overheating or not. If anyone thinks that a good flow of air is not enough, then please reply with your experimental data that shows once and for all that a good flow or air is ineffective for cooling an engine that is being broken in.



Edited to correct my poor spelling

< Message edited by longdan -- 7/27/2007 1:33:50 PM >

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       Post #: 157

RE: Prop pitch cooling - the final word - 7/27/2007 9:39:07 PM   
Jazzy



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From: Erie, PA, USA
Status: offline 		Decisively put Longdan.

This is my final attemp at explaining why hovering engines overheat at a given RPM while they can fly around normally at the same RPM and not overheat.

Form two days ago I wrote:

"Greg,
Propellers are basically (and very generically speaking) air screws. (Hence the name "MasterAirscrew" ) At a particular RPM a propeller will "pull" until the air flow coming into the prop matches the propwash speed (the speed of the air behind the prop).
Ok, here is an example: An 11X6 prop is spinning at 10000 RPM. The prop is trying to screw through the air six inches for each revolution. In an hour the prop will try to move 3,600,000 inches forward (6in *10000*60). The equivalent speed would be ~56.8 MPH. (Notice I said "try". The airframe, in level flight and still air, will likely never reach that speed.)
Do you think the engine has to work hard to maintain 10000 RPM if it is traveling at 56.8 MPH? Ideally, the prop load would diminish and asymptotically approach zero since the prop and airframe are 100% efficient. In reality there will always be some load since 100% efficiency is not possible. Ok, so the prop is this case is as efficient as it can get. The load of the prop is very small. The engine only requires enough fuel to spin and therefore does not work very hard at all.

Now hold the prop/engine/airframe stationary and run the engine at 10000 RPM. The prop is now at its least efficiency. The engine is going to have to work pretty hard to maintain 10000 RPM. It is going to burn more fuel than the 56.8 MPH engine because the load of the prop is at or near its greatest. More strain will be put on the bearings too. Burnning more fuel=more heat. Greater force on bearings=more heat (by way of friction). Put another way, the engine in a hover will require more fuel to do the same amount of WORK that it would do in level flight at the same RPM.

The engine temps in Yallaair's post verify what I'm trying to say. Hovering is just about the same (ok, similar at best) as tying your plane to a post and letting the engine sing along. It is harder on your engine than normal flight and heat build-up is a result.

I hope this helps explain what I'm driving at regarding RPM, prop loads, and heat build-up.


Yallaair,
Thank you for taking the time to compile and share your data! Please post more!

I'd be willing to bet that if the throttle was adjusted to maintain the static RPM while in level flight the cylinder temps would be even less those posted."

Yes, I'm sure detonation and or other engine operations that differ from those occurring at level flight or a lightly loaded prop contribute to the heat. They are included in overall operation.

There. That is it. That's all.

< Message edited by Jazzy -- 7/27/2007 9:46:18 PM >

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RE: Prop pitch cooling - the final word - 7/27/2007 11:20:11 PM   
gkamysz


 

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Joined: 6/24/2003
From: Crystal Lake, IL, USA
Status: offline 		From post #138 by Jazzy:

quote:

I think that the main issue here is the load on the engine. Do you all think that the RPMs will be higher in WOT level flight as opposed to WOT hover? If the RPMs are higher in level flight that would indicate less load on the engine... right? Less load - less work - less heat?



I guess I thought we were talking about two engines at full throttle, but different orientations. If you mean same RPM, I agree with what you said but it should be noted that the engine in level flight will have a lower throttle setting.

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       Post #: 159

RE: Prop pitch cooling - the final word - 7/28/2007 5:17:48 PM   
Jazzy



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Joined: 1/8/2002
From: Erie, PA, USA
Status: offline 		Yes. It was implied.
quote:

Put another way, the engine in a hover will require more fuel to do the same amount of WORK that it would do in level flight at the same RPM.


And yes, the statement you quoted is a little misleading. Look how far back you went to find it. There were revisions and adendums after it.

As I mentioned again and again, the WOT example was only to show that the unloading of a prop when an aircraft is at speed will allow the RPMs to increase over a stationary engines Rs. Same amount of fuel but less load. Instead of keeping the amount of fuel constant I kept the RPMs constant which in turn will require different amounts of fuel depending on the orientation/speed of the engine/airframe.

Wow, with all due respect, for a group of engineers, armchair, self-proclaimed or degreed, you sure don't read very carefully and insist on making everything so difficult.

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RE: Prop pitch cooling - the final word - 7/28/2007 5:44:42 PM   
gkamysz


 

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From: Crystal Lake, IL, USA
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quote:

ORIGINAL: Jazzy

Yes. It was implied.
quote:

Put another way, the engine in a hover will require more fuel to do the same amount of WORK that it would do in level flight at the same RPM.


And yes, the statement you quoted is a little misleading. Look how far back you went to find it. There were revisions and adendums after it.

As I mentioned again and again, the WOT example was only to show that the unloading of a prop when an aircraft is at speed will allow the RPMs to increase over a stationary engines Rs. Same amount of fuel but less load. Instead of keeping the amount of fuel constant I kept the RPMs constant which in turn will require different amounts of fuel depending on the orientation/speed of the engine/airframe.

Wow, with all due respect, for a group of engineers, armchair, self-proclaimed or degreed, you sure don't read very carefully and insist on making everything so difficult.



I guess I don't understand. My reply was the first one after your post #138. My post was a direct reply to what your wrote. There was no confusion. I read carefully and the quote I posted is the statement I responded to.

My post about the engine making more power at WOT at speed was a direct reply to your assumption that load had decreased. For most engines, especially two stroke engines this would be invalid. An engine by nature if at WOT is on a rising portion of it's HP curve is making more power as the plane accelerates, even if the prop "unloads". Unloading is a poor choice in words as power absorbed is probably increasing in most cases. Many variables are involved.

Say an engine WOT makes 1HP at 10K and 1.2HP at 12k. If it turns 10k on the ground static and turns 12k in the air at top speed it is still making that 1.2HP in the air. The engine is making 1.2HP so it is also probably burning more fuel than it is at 10k, because it would be unusual for engine efficiency to increase that much, but a tuned pipe could have this effect on a two stroke. If you look read about competiton engines they are always setup to run at peak HP in the air, even if it means that on the ground they are well below peak HP and RPM.

This is opposite of Electric airplanes where most of my experience is. Most electric airplanes make less power as the prop RPM increases in the air. This is by nature of the electric motor, as RPM increases current drops and it makes less power. So if an electric motor makes 1HP at 10k on the ground static it probably won't wind up to 12k at top speed. In fact RPM will increase slightly, but power output will go down.

My main point in all of this is that there are a lot of variables. You have to include all of them and clearly define the argument to come to a realistic conclusion.

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RE: Prop pitch cooling - the final word - 7/28/2007 6:03:33 PM   
DarZeelon



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From: Rosh-Ha'Ayin, ISRAEL
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quote:

ORIGINAL: Jazzy

.....Same amount of fuel but less load. Instead of keeping the amount of fuel constant...



Jazzy,


Fuel flow is NOT kept constant...


The main needle in the same position and RPM increased, as a result of a lesser prop load at speed, will cause a definite increase in fuel flow... The increased RPM will have more air flowing through the venturi, which will increase suction and fuel flow...

If the engine is not under-propped, even its HP output will be increased, as it unloads!

This is one of the reasons the mixture setting must be kept a bit rich of the nose-up maximum RPM.


The mixture will become proportionately leaner at speed; not because no more fuel is passed through the orifice between the needle and the nozzle.
It becomes somewhat proportionately leaner because fuel, being a more viscous liquid, will increase its flow at a lower rate than the thinner air.


While people did give some strange example of terminal speed, I believe most participants in this thread did regard this discussion seriously.

The terminal speed of a bowling ball is not material to this discussion, but empirical results are...
I believe determining if higher air-flow speed will cool the engine better, is best done in a wind-tunnel with the engine spinning load beams...


After all, it was my small diameter - high pitch break-in prop recommendation, that brought Brian (downunder) to begin this thread...


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RE: Prop pitch cooling - the final word - 7/28/2007 7:45:33 PM   
Sport_Pilot



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From: Acworth, GA, USA
Status: offline 		Jazzy,
You are correct that power is the rate or speed that work is done, I do make mistakes from time to time. But my point was that the engine will make more power and heat at full speed. You latest discussion of better cooling at speed vs when hovering is essentially correct, however I don't think this in itself is enough to make the engine overheat. After all it is similar to running an engine on the bench and they don't generally overheat even when peaked out, though they do run a bit hotter than when running flat out in the air. I think the reason for planes to tend to overheat is a combination of the effects you mention and the fact that the tank is now under the engine and this leans the engine out. Mostly the latter.

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       Post #: 163

RE: Prop pitch cooling - the final word - 7/28/2007 9:10:09 PM