downunder

Ok, so maybe it's not so much a myth as a disagreement. There's been some discussion in another thread about the advisability of using a fine pitch prop when running in as a means of reducing the load instead of/as well as reducing the diameter. One school of thought says that the fine pitch won't give enough airflow to cool the engine sufficiently and the other (OK, my) school of thought says it won't make any difference. The first school of thought says to use at least a 6" pitch while I've been saying a 4" pitch is more than sufficient. Both have actually been just guessing, one based on things that have been read and the other (me ) based mainly on having felt the airflow from a fine pitch prop.

So which is correct? Well, after much thought and many sleepless nights I figured why not just try it and get some actual real world figures? So I did.

I chose to use my old ST GS45 (because it was handy) and I had a couple of props that suited both it and the test. First up I fitted an APC 10x6, fired it up and got it close to peak at about 13,800 then backed it off to 12,000 to more closely simulate where an engine might be run in (actually a lot faster than I'd do it myself but I had to pick a figure that was well off peak). So 12K revs gave a head temp of 220F. I pulled the fuel line and shut it down.

Then I fitted an MA 11x4 and without touching the needle setting I started it back up and went to full throttle. I didn't touch the needle because I was interested in seeing how much difference in load there was. This gave right on 13,000 revs and the head temp was.....wait for it....220F!

But because it was now running 1000 revs faster I decided on one final check by reducing the revs to the original 12,000 on the APC. That gave a head temp of 200F but was right on the verge of 4 stroking rich. Which would have been perfect for running in anyway.

So a fine pitch prop cools just as well as any other.

the pope

Oh no wheres dar the pope

Flyboy Dave

My Feedback: (21)

Good job, Downunder....but you gotta admit....the prop pitch thing does sound good.

I've always subscribed to the idea to break-in the engine on the same prop that your going
to run....I generally run smaller, higher pitched props that allow maximum RPM's from the
engine, so overheating has never been a problem, on the ground or in the air.

I don't own a prop under an 6" pitch anyway.

FBD.

Hobbsy

My Feedback: (102)

Dave, those popsicle sticks just won't move an airplane will they?

surf

Don't know if this is relevant, but years ago I put a cylinder head temp probe on my motor cycle and it would run the same temp in 'stop and go' as wide open. This surprised me.

My conclusion was that once an air cooled engine reaches operating temperature, it stays there regardless of air flow. I think this applies to most large air cooled engines. The example above does not surprise me as long as the engine has available cooling means (no restrictions).

BUT I do know that effective cooling on model engines is important, especially with cowling.

DarZeelon

Brian,


This is not very typical of you, to reduce the RPM by running the engine as rich as a hog...

You were supposed to do it by using a larger diameter prop, with a 4" pitch, so the RPM would be reduced back to 12K...

AND, break-in as mostly done even by you, on an ABx engine (when you are not too busy trying to break one of my 'myths'...), the engine is richened to 500-800 RPM below peak; not 1,800 RPM.
So on the 10x6, the engine should have been spun at about 13,000-13,300 RPM.

The Xx4 prop that you were to use should also have peaked at 13,800... Possibly an 11.5x4 APC and it should also have been spun at about 13,000-13,300 RPM, for your "test".

AND, I hope you were using a thermo-couple, glow-plug-gasket sender type thermometer; not one of those totally worthless, infra-red, visual thermometers.


I am really flattered by you regarding things that I write as 'mythical'.
I just wonder why you try to break them all... Just regard them as axioms. They work; don't they?

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

The Pope,


Don't call me 'the pope'. I am too Jewish to be one...

Hobbsy

My Feedback: (102)

Brian, it just proves once again that every issue in life cannot be put in a pigeon hole or a slot that agrees with the way a person thinks. I break AB? engines in with a big prop because that's what it's going to do in real life. As the other Dave said.

j.duncker

My Feedback: (2)

Just to throw a grenade over the parapet.

What would be wrong with fitting a smaller prop. Say a 9x4 then regulating the rpm with the THROTTLE. LESS FUEL TO ACHIEVE THE SAME RPM SHOULD RESULT IN A LOWER HEAT INPUT FROM THE FUEL BURN.

I was told to do this sort of thing by Mr Super Tigre, Senor Garofoli, himself many many moons ago.

Hobbsy

My Feedback: (102)

You need the heat to expand the sleeve and piston to their normal operating tolerances.

da Rock

And since lower pitch props have been proven time and again to provide more thrust in static running, who would assume that they don't blow as much cooling air as a higher pitched prop? Someone who didn't understand all the important details?


And who had actually passed final say on how much cooling air is enough? Or even included it in the original myth?

Looks like it's been proven once again, that it's fairly easy to test whatever we want to find out and all we gotta do is test it.

But heck, this engine forum proves over and over again that it's lots more fun to contentiously argue and argue and argue.

What this thread needs now is for some fullscale pilots to start in with how it works in the real world and then get into a spitting match over who's right because they got the most hours.

Hobbsy

My Feedback: (102)

They have a Full Scale Forum for that, it would be blown away by a Moderator here and for good reason.

downunder

OK Dar, although I didn't name names in that first post it's obvious I was aiming at one of your statements in that OS 55AX thread where you stated "Don't do any break-in with less than 6" pitch. Cooling is impaired and reducing the diameter lessens the load without hampering the cooling. In this case, a 10x6 would load the engine like an 11x4." I disagreed with you but had nothing to back up my disagreement so I did this experiment precisely according to the prop figures you mentioned. Now, because the results I got showed it had no affect, you're shifting the goal posts and making silly arguments about what I should have done (in your opinion).

As for inferring that I might be using a totally worthless type of thermocouple to fault my readings, I use a contact type thermocouple which plugs into my digital tacho/multimeter and has been calibrated using the phase transition points of dihydrogen monoxide.

Hobbsy

My Feedback: (102)

Brian, you are pi$$ing into the wind, so to speak trying to reason with him on this.

wcmorrison

My Feedback: (2)

What fun. Amazing what good correspondence we have between continents. The fuel is the same (well maybe); what about local conditions - - temperature, elevation, humidity. Standard day anybody? Rigid control conditions do not apply, eh.

Still it is imformative and good give and take. What a great forum for discussion.

Cheers,

Chip

DarZeelon

You did not do the experiment precisely enough!

The fact that you needed to run the engine richer with the 11x4 prop, since it obviously unloaded the engine less than did the 10x6 prop, resulted in making the engine run cooler, or is that also a myth, according to your opinion?

This is one major imprecision.

I precisely wrote 'not one of those totally worthless, infra-red, visual thermometer'. Reread my post!
Thank you for clearing that up.

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If by writing 'trying to reason with him' Dave meant 'to make me accept a common misconception', then you really are pi$$ing against the wind... Misconceptions are! I prefer facts. I analyze, so I understand; instead of just following in the footsteps of others.

The only reason those 'big names' are all telling you to use a prop of the same pitch and a 1" smaller diameter, or the normal 6"+ pitch, chopped 0.5" from each blade, is that they all fear cooling could be impaired by using a flatter prop.
Or, do you think there is another concrete reason for this? What is it?

You may want to do the break-in in Antarctica in August, just to prove an engine doesn't need cooling...

asmund

This is great! It would be a boring forum if everyone had the exact same opinions about everything, woldn`t it?? I have run in engines on both 6 and 4 in pitch and never noticed any difference, but then again we are not burdened with too hot and nice weather here in Norway[:'(]

rmh

does anyone actually fly the models?
Some theory guys apparantly just jerk off and read bench results .

The in air cooling has NOTHING to do with prop pitch- -
It has to do with air flow thru the cooling fins - and the engine -if fully cowled --will cool-or- get hot depending on the setup used
all the engine cares about is load and cooling- prop size means squat.

daveopam

My Feedback: (9)

Downunder, I would like to see the following experiment. Set your 45 up on a test stand. Directly behind the test stand set up a prop balancer with a 13x6 or whatever you have on hand. Run each of your two props up to 12,000. Then get a tach reading from the prop on the balancer.The tach reading should tell which of the two is pushing more air.

David

NM2K

My Feedback: (14)

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This makes sense with a ringed engine (conventional rings), but does not make sense with an ABX system where the engine is allegedly perfectly fit from the factory as far as piston/liner fit are concerned.

When breaking-in ABX engines, we are concerned with breaking-in the lower end and wrist pin areas, NOT the piston/liner. Yes, I know there are those that will disagree. However, I'm not saying that this is a universal truth. Life isn't like that. The only universal truth is that there are no universal truths.


Ed Cregger

NM2K

My Feedback: (14)

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


Real modellers know that very little full-scale info is applicable to our R/C models. Which totally ticks off a full-scale pilot when they take up R/C and then discover that just about everything they thought they knew was wrong and now they are back to square one again.

Some of them really get ticked when they find out that now they are their own mechanic and they actually have to learn something about engines and airframes. I've seen droves of them quit the hobby over the years when they realized they couldn't call someone to maintain their models. <G>

Fortunately, even among full scale pilots, there are some very mechanically inclined folks that enjoy working on their models - just not enough of them.

Don't mind me. As an ex R/C flight instructor for nearly forty years, I've formed opinions about particular groups of folks seeking instruction. My best student pilots have been airline pilots. My least favorite students have been Army helicopter pilots and general aviation pilots. Not all of them were bad, mind you, but they are a more difficult group(s) to instruct than the general population. It is difficult to teach someone to fly when they are convinced that they know more than you, the instructor, and then won't keep their mouths shut long enough to absorb anything.

Sorry for going so far off topic. It's a two pill morning, if you know what I mean...


Ed Cregger

JPMacG

My Feedback: (2)

I thought a lower pitch prop provided more thrust at low airspeed (airspeed of the plane) because it moved more air at slow airspeed than a higher pitch prop. Thust is created by the action of accelerating air after all.

I thought high pitch props were not effective at low airframe airspeeds because the prop airfoil became ineffective due to the high angle of attac of the blade (stalled?). At higher airspeed the higher pitch prop starts to work better because the angle of attack decreases as airspeed increases.

So if the goal is to cool the engine on a test stand I would think you would want a low pitch prop.... or am I wrong?

edited for clarity.

Flyboy Dave

My Feedback: (21)

I think that low pitch prop business is another myth. I think the 3D guys are circulating
this myth, because they use low pitch props.

It is not possible for a lower pitch prop to give more thrust than a higher pitch prop,
unless the high pitch prop is stalled...or as a water prop...cavatating.

If you were to draw that out to it's logical conclusion....a 1" pitch prop would deliver
more static thrust than an 10" pitch prop.

I don't think so. The more pitch (thrust angle) you have in a propeller, the more thrust
you will have at any RPM.

Besides that, what good is static thrust ?....a measurement of how hard you cal pull on a
fishing scale ? That measurement, and a two dollar bill will get you a cup of coffee
almost anywhere.

FBD.

JPMacG

My Feedback: (2)

Static thrust would be of interest to someone breaking in a new engine on an engine test stand. The greater the static thrust the greater the airflow past the cooling fins of the engine.

DarZeelon

JP,


Any prop spinning in air creates a vortex, in which air moves through it at nearly pitch speed.
The prop blades may initially be partially stalled, but after the vortex has formed, they no longer are.


A lower pitch, larger diameter prop provides more thrust, not because it does move more air, but because it has a lower disk loading.

Use a smaller diameter, higher pitch prop, spun by the same engine at the same RPM and you will get less thrust, because the disk loading is higher.


Cooling an engine doesn't require a large amount of air moving relatively slowly. It require a smaller amount of air moving at higher speed in closer vicinity to the engine... Air moving 2" above the head fins doesn't cool the head at all. Neither does air moving 3", or 4" above it...

A small, higher pitch prop causes air to move quickly and close to the engine, while loading the engine the same as does a larger, low pitch prop. The effect - better cooling, even if Brian insists the head temperature (which is controlled, among other things, by the mixture strength) is the same.


A piston engine powered helicopter has the most thrust (lift, actually) and moves the largest amount of air...
But does its engine get good cooling? Not from the rotor, it doesn't.
Instead, there is a separate, small fan blowing air directly on the engine cooling fins.

Cooling is the only other function of the break-in prop; besides providing a load for the engine.


So the answer is yes, you are wrong.

TimC

It would seem a prop that over-cools an ABC/ABN engine would be detrimental to the run-in process.