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5 minutes ago, Ed Cesnalis said:

You are talking like Roger hear.  Thought experiment:  turn your engine by hand, now take out the plugs and do it again, do you really think the wear was equivalent?  Your math is missing a variable.

How many flights have you made without plugs installed?  :ive_got_it-1379:

 

My point is that a revolution of the engine puts roughly the same wear on the engine from friction on cylinders, rings, pistons, and bearings.  And yes, those items listed will have about the same wear with or without plugs.  Compressive force on the pistons and rods and other forces were not addressed by my post.  Which is why I referenced and agreed with Roger's "all other things equal" comment.  If you rub two bits of metal together separated by a tiny bit of oil 5500 times, they will have more wear than if you do it 5200 times.  Or do you think if you turned your engine by hand without plugs a billion times it would have zero wear?

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6 minutes ago, FlyingMonkey said:

I agree, there are only a few knots from 5300 to 5500, and fuel burn goes up a LOT.  I flew from Georgia to Oshkosh at 5500rpm the whole way keeping up with two faster airplanes (a Vari-EZ and a Sonex) and it was fine, but burned a lot of fuel.  Luckily the CT has bigger tanks than those other two airplanes.

I used to use either 5200rpm or 5400rpm, but lately I like 5300rpm.  That seems to be the sweet spot for my airplane between fuel burn and speed.  Above that I gain a couple of knots and make more noise.  

at 14,000' the difference between 53 and 55 is pretty huge, like 10kts

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Just now, Ed Cesnalis said:

at 14,000' the difference between 53 and 55 is pretty huge, like 10kts

That can certainly be true.  I'm talking general experience here, I think we were at 6500-8500ft.  I definitely allow that you basically need WOT in your environment.

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3 minutes ago, FlyingMonkey said:

Or do you think if you turned your engine by hand without plugs a billion times it would have zero wear?

Its not wear with high resistance and zero wear with low its less wear with low.

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So perhaps it's being boiled down a little too much.

A generic engine running at 1000 rpm under light load will see piston, ring, connecting rod pin, journal, and bushing wear all more equally than say, an engine running at 500 RPM under very heavy load, which will see scraping along the cylinder walls, near TDC, higher wear along the journal and bearing interface on the piston side towards the piston, higher wear along the connecting rod pin and piston interface towards the top of the piston, and a few other areas such as chattering gears from the heavy impulses.

Just simply looking at RPM isn't really enough information to make anything but rough assumptions for the sake of argument. And there's nothing wrong with that! It helps us to digest the workings of complex devices more easily!  From a super rough viewpoint, you are correct, usually higher RPM means more wear, but it's a rule of thumb, not a hard and fast rule.

But, remember, when it comes to mechanical devices, load determines the wear patterns and rate a lot, especually as we approach the film strength limits of the libricating oil. A heavy load does often mean more wear than a light and fast one, but again, not in all cases!

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4 minutes ago, FlyingMonkey said:

That can certainly be true.  I'm talking general experience here, I think we were at 6500-8500ft.  I definitely allow that you basically need WOT in your environment.

just like the wear issue it isn't on or off its a sliding scale

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Wear is wear. Rub something together with heat 5200 times per minute vs 5500 times per minute for 2000 hrs and there is more wear.

At 5200 rpm for 2000 hrs that's 624,000,000 revolutions with metal against metal.

At 5500 rpm for 2000 hrs that's 660,000,000 revolutions.

That's 36,000,000  more revolutions in it's 2000 hr. time. Rubbing metal to metal that many more times causes more wear.

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22 minutes ago, Roger Lee said:

Wear is wear. Rub something together with heat 5200 times per minute vs 5500 times per minute for 2000 hrs and there is more wear.

At 5200 rpm for 2000 hrs that's 624,000,000 revolutions with metal against metal.

At 5500 rpm for 2000 hrs that's 660,000,000 revolutions.

That's 36,000,000  more revolutions in it's 2000 hr. time. Rubbing metal to metal that many more times causes more wear.

try this,

compare an 80% power setting that uses 5,200 to a 70% power setting that uses 5,500.  

now you would be saying that 70% power produces same wear as 80%, that's just not right.

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Rotax numbers assumes you use 5800 rpm WOT settings for their calculations.

You don't get to change the numbers.

Rub two pieces of metal together 1000 times then rub another at only 750 times. Which has more wear?

 

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The Archard equation

Q=(KWL)/H


    Q is the total volume of wear debris produced
    K is a dimensionless constant
    W is the total normal load
    L is the sliding distance
    H is the hardness of the softest contacting surfaces

Given similar loads as previously stated by Roger, he is correct. It's more complicated than that, but it's an alright rule of thumb.

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35 minutes ago, Roger Lee said:

Rub two pieces of metal together 1000 times then rub another at only 750 times. Which has more wear?

Rotax assumptions change nothing.  Rub 1,000 times lightly or 750 times with a lot of pressure 750 has more wear.

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14 minutes ago, Anticept said:

Given similar loads as previously stated by Roger, he is correct.

Your have to negate my argument with similar loads for Roger to be correct.

The long standing argument here never included similar load. Roger like Andy now always argued more RPM means more wear period. I always argued that the power setting was the right metric not RPM. 

More power = more wear. (anomalies excluded) RPM is multiplied by MAP before it has much meaning.

 

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Ok so theoretically, though you wouldn’t do it, you can have WOT indefinitely as long as RPM is 5500 or less 

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1 hour ago, Ed Cesnalis said:

Your have to negate my argument with similar loads for Roger to be correct.

The long standing argument here never included similar load. Roger like Andy now always argued more RPM means more wear period. I always argued that the power setting was the right metric not RPM. 

More power = more wear. (anomalies excluded) RPM is multiplied by MAP before it has much meaning.

 

 

You're not wrong either! It's complicated!

If this were constant speed props, you'd win that argument hands down. Except in one case that I know of!

In IO-360 engines with constant speed props, running them below 1850 rpm for extended periods does weird things to the compression rings. It's enough that lycoming says that you should restrict operating below that to short periods of time.

Since we're dealing with an unchanging pitch... RPM and power are so closely related that for our purposes, they are the same.... until you try to do weird tricks with the prop pitch and load the engine down...

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20 minutes ago, Anticept said:

Since we're dealing with an unchanging pitch... RPM and power are so closely related that for our purposes, they are the same.

our props are ground adjustable and my side of this long standing argument has always been that RPM alone doesn't determine power/wear.  

In this thread and prior the other side argues that RPM alone determines wear no matter what the pitch.

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

granted in recent posts 'similar loads' has crept in but that makes me right not them

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3 hours ago, Ed Cesnalis said:

Your have to negate my argument with similar loads for Roger to be correct.

The long standing argument here never included similar load. Roger like Andy now always argued more RPM means more wear period. I always argued that the power setting was the right metric not RPM. 

More power = more wear. (anomalies excluded) RPM is multiplied by MAP before it has much meaning.

 

I guess you missed where Roger and I said ALL OTHER THINGS BEING EQUAL.  At the same engine load, for example.  You are arguing against something we never said.

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"ALL OTHER THINGS BEING EQUAL."

See I told you he was smart. He used to have me worried with a 19 page spark plug thread, but he's got it now. :giggle-3307:

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3 hours ago, Anticept said:

The Archard equation

Q=(KWL)/H


    Q is the total volume of wear debris produced
    K is a dimensionless constant
    W is the total normal load
    L is the sliding distance
    H is the hardness of the softest contacting surfaces

Given similar loads as previously stated by Roger, he is correct. It's more complicated than that, but it's an alright rule of thumb.

Well done.

I think some of the disagreement is due generalizing “wear” in the discussion. The term W above is normal load, meaning perpendicular to the sliding surface. For some parts this component is relatively small, so additional load doesn’t increase wear much. For parts like bearings, the normal component is the majority, so increased load definitely increases wear. The discussion started with generalities, and “rpm = wear” is a good rule of thumb. “rpm x MAP = wear” is probably more accurate.

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My take from this is you are trying to make an empirical decision without the actual data needed. (In computer vernacular GIGO, or your guess is as good as mine.) 😎

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12 hours ago, Warmi said:

So 5200-5300 it is then ..:D

Yes.  If you are really trying to get somewhere in a hurry or catch a faster airplane you can bump it up higher, but the speed gains will be low and the fuel consumption high.

If I get a chance before my shoulder surgery next week, I will try to get some speed numbers at various RPM for comparison purposes.  I don't have fuel flow calculations in my airplane, so you'll have to use Rotax or somebody else's numbers for that. 

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9 minutes ago, FlyingMonkey said:

Yes.  If you are really trying to get somewhere in a hurry or catch a faster airplane you can bump it up higher, but the speed gains will be low and the fuel consumption high.

If I get a chance before my shoulder surgery next week, I will try to get some speed numbers at various RPM for comparison purposes.  I don't have fuel flow calculations in my airplane, so you'll have to use Rotax or somebody else's numbers for that. 

Be advised folks, Andy's CT is a miniture rocket ship. He gets spectacular cruise performance out of it. He leaves me in the dust.

YMMV.

Good luck with the surgery, Andy!

 

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16 minutes ago, FlyingMonkey said:

...

If I get a chance before my shoulder surgery next week, I will try to get some speed numbers at various RPM for comparison purposes.  I don't have fuel flow calculations in my airplane, so you'll have to use Rotax or somebody else's numbers for that. 

That would be lovely - I am actually flying Sting S4 so it won't be apples to apples but I think Sting should be able to get similar numbers as CTSW ... 

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8 minutes ago, Warmi said:

That would be lovely - I am actually flying Sting S4 so it won't be apples to apples but I think Sting should be able to get similar numbers as CTSW ... 

You could fly your airplane and get the speed numbers at various rpm, and then look up the fuel consumption online if you don't have a fuel flow instrument.  I'm happy to do the tests, but they may or may not apply well to your airplane with different drag characteristics.

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