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CTMI

Max rpm or?

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Ever think about a simple thing so much you get your wires crossed? 

Ok 5800 is the max RPM that you can have for 5 minutes but if you keep your rpm at 5500 you could firewall the throttle indefinitely, right????

so FULL THROTTLE can be indefinite so long as RPM is equal to or less than 5500 right? 

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My first thought is the CT climbs so well already and speed is so precious that I will optimize for speed.  That means 5,500 max at an altitude you will actually use.  I choose 5,500 at 11,000' DA but I fly at higher altitudes than most other CT pilots.

At low altitudes I throttle back anyway (5,200) to keep the noise level acceptable but above 7,000' 5,500 feels right.

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CTMI you are right.  I pitch my prop for higher than 5500 at my normal altitude (under 4000ft), because when flying cross country I will often cruise at 7500-8500ft and then my max rpm would be hindered.

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1 hour ago, CTMI said:

Ever think about a simple thing so much you get your wires crossed? 

Ok 5800 is the max RPM that you can have for 5 minutes but if you keep your rpm at 5500 you could firewall the throttle indefinitely, right????

so FULL THROTTLE can be indefinite so long as RPM is equal to or less than 5500 right? 

It runs extremely rich at that setting.

Rotax also does not recommend running at full power for extended periods of time (according to operators manual), they didn't mention rpm being a factor at the time I read it.

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21 minutes ago, Anticept said:

Rotax also does not recommend running at full power for extended periods of time

wide open throttle is not usually full power. @7,500/5,500 its a perfect 75%

 

its been said many times and places that this motor does best  at 5,500 or at least above 5

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25 minutes ago, Anticept said:

It runs extremely rich at that setting.

depends on needle setting

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11 minutes ago, Roger Lee said:

If I lived and flew at Ed's altitude I would drop my carb needle 1 notch to #2 position.

Yup, but my point was max rpm does not = extra rich.  pick the pitch that makes the most sense and if you have to adjust your needle.

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Prop pitch is different than rich for sure, but my comment goes towards when running at those rpms Rotax has the carbs richend to help stave off detonation, but for lower altitudes since they are sold all over the world. You live and fly outside of that norm so you can become less rich at your altitude. Because you live and fly at your altitudes you are more rich than me and could benefit from a little leaning.

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4 hours ago, Ed Cesnalis said:

Yup, but my point was max rpm does not = extra rich.  pick the pitch that makes the most sense and if you have to adjust your needle.

RPM no, there's no direct relationship. Throttle yes, though. It's about the air mass moving through the venturi, the air pressure under the diaphragm, the vacuum above it (dictated by air moving through the venturi), and the piston spring trying to hold the piston closed.

The needles have a taper to them. There's a point after which the piston is fully retracted, and the fuel limitation is no longer the needle, but the diameter of the jet. This is the "full rich" mode and will supply much more fuel than the engine actually needs. If you were to change the jet diameter, you could lean or enrichen the full power setting (not recommended, just saying this is how they do it in motorcycles that use these same carbs).

At high altitudes though, there might not be enough of an air pressure delta to lift the piston against the spring. So you'll never actually see full rich.

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You have changed phrasing from 'extremely rich' which sounds negative to full rich which sounds by design.

For the last 1,500 hours I have flown virtually every flight at wide open throttle, it isn't a negative but the most reasonable choice when flying normally aspirated above 10,000'. Many would compare doing otherwise to flying with a dirty intake air filter.

We have enough factors limiting our speed. A blanket caution against or aversion to WOT is limiting even at moderate altitudes.  Your caution was against full power but the discussion is about using the 5,500 limitation which is never full power.

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Technically the engine is designed to run at 5500 its entire life. My only but there is I believe there is more wear at an increased rpm its entire life. So someone that ran at 5500 its entire life may have more wear at 2000 hrs than someone that only ran 5200 its entire life. Just a personal thought.

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4 hours ago, Roger Lee said:

Technically the engine is designed to run at 5500 its entire life. My only but there is I believe there is more wear at an increased rpm its entire life. So someone that ran at 5500 its entire life may have more wear at 2000 hrs than someone that only ran 5200 its entire life. Just a personal thought.

We have always disagreed there.  I say you could be producing as much wear at 52. These are RPM and not power settings and its the motor producing the most power that will show the most wear.  You have to consider more than speed but also resistance to that speed.

When you say technically that it is designed to run 55 I think that is a sweet spot for valve train harmonics

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5 hours ago, Ed Cesnalis said:

We have always disagreed there.  I say you could be producing as much wear at 52. These are RPM and not power settings and its the motor producing the most power that will show the most wear.  You have to consider more than speed but also resistance to that speed.

When you say technically that it is designed to run 55 I think that is a sweet spot for valve train harmonics

Well, from just a mathematical point of view, one hour at 5500rpm will produce 5% more engine rotation cycles than 5250rpm.  More cycles will always mean more wear on the pistons/rings/cylinders/bearings.  That doesn't mean it's a difference that has any practical effect on engine life, but it is kind of hard to escape the math.  :)

I run my engine wherever I need to to accomplish my goals.  4000rpm sometimes when flying with my slow friends in their Cubs and Avids (I can hear Roger shuddering), 5000rpm when enjoying a flight around the patch, 5200-5300rpm in cruise when traveling (I have started to really like 5300), or 5400-5500rpm when I'm trying to get somewhere fast.

I don't worry so much about what rpm or throttle setting I'm running at any given moment, I just try to operate using best practices in general.  If I need 5600rpm to catch up to another airplane for a minute or two I'm not going to sweat it, nor am I going to get wound around the shaft about operating my airplane for an hour at 4000rpm.  Rotax has not come out against either one in any strong way, so I assume those are acceptable, considering my other 90% of the time within the normal 5000-5500rpm range.

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7 minutes ago, FlyingMonkey said:

Well, from just a mathematical point of view, one hour at 5500rpm will produce 5% more engine rotation cycles than 5250rpm.  More cycles will always mean more wear on the pistons/rings/cylinders/bearings.  That doesn't mean it's a difference that has any practical effect on engine life, but it is kind of hard to escape the math.  :)

I run my engine wherever I need to to accomplish my goals.  4000rpm sometimes when flying with my slow friends in their Cubs and Avids (I can hear Roger shuddering), 5000rpm when enjoying a flight around the patch, 5200-5300rpm in cruise when traveling (I have started to really like 5300), or 5400-5500rpm when I'm trying to get somewhere fast.

I don't worry so much about what rpm or throttle setting I'm running at any given moment, I just try to operate using best practices in general.  If I need 5600rpm to catch up to another airplane for a minute or two I'm not going to sweat it, nor am I going to get wound around the shaft about operating my airplane for an hour at 4000rpm.  Rotax has not come out against either one in any strong way, so I assume those are acceptable, considering my other 90% of the time within the normal 5000-5500rpm range.

It is not just RPM's that lead to wear, it is also load on the engine. A freely running engine at 5500 RPM will likely have less wear than a engine running at 5250 under a heavy load. That is the reason why prop pitch is so important.

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Andy quote:

"Well, from just a mathematical point of view, one hour at 5500rpm will produce 5% more engine rotation cycles than 5250rpm.  More cycles will always mean more wear on the pistons/rings/cylinders/bearings.  That doesn't mean it's a difference that has any practical effect on engine life, but it is kind of hard to escape the math."

 

Tom quote:

"It is not just RPM's that lead to wear, it is also load on the engine. A freely running engine at 5500 RPM will likely have less wear than a engine running at 5250 under a heavy load. That is the reason why prop pitch is so important."

 

I agree with both statements, but everything being equal more revolutions per minute will cause more wear over the life of the engine.

 

Engine load can even be an engine killer if too high and cause cracks on the top of the crankcase. Engine load plays a bigger part by far than just rpm alone.

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17 hours ago, Ed Cesnalis said:

You have changed phrasing from 'extremely rich' which sounds negative to full rich which sounds by design.

 

For the purposes of the point I am trying to make, they are one and the same. Full rich is extremely rich. It's very excessive. It's to help prevent detonation in areas with a high air density, but as a consequence, wastes a lot of fuel and leaves a lot of carbon.

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3 hours ago, Roger Lee said:

 

 

 

but everything being equal more revolutions per minute will cause more wear over the life of the engine.

 

That's all I was getting at, thanks.

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3 hours ago, Tom Baker said:

It is not just RPM's that lead to wear, it is also load on the engine. A freely running engine at 5500 RPM will likely have less wear than a engine running at 5250 under a heavy load. That is the reason why prop pitch is so important.

Agreed.

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Hmmm ..so what is the sweet spot for cruise RPM ? 

Is it 5000 since we want to stay above or at 5000 while at the same maintaining lowest revs per minute ?

 

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With a prop pitch set get around 5600-5650 rpm at WOT at your average altitude most would usually cruise between 5200-5400. The will be some lower and some higher. usually anything above 5300 doesn't give enough speed gain vs fuel use.

 

But it's a personal choice. Where ever it makes you happy.

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1 hour ago, Warmi said:

Hmmm ..so what is the sweet spot for cruise RPM ? 

Is it 5000 since we want to stay above or at 5000 while at the same maintaining lowest revs per minute ?

 

I wouldn't worry about minimizing revs.  Anywhere between 5000 and 5500 is a good place.  As Roger said, the engine is designed to go to TBO at 5500rpm.  I'd say cruise at the number that gives you a balance you like of fuel economy and speed for your airplane and prop combo within 5000-5500rpm.  Note that can change flight to flight based on your "need for speed" and willingness to burn fuel to get it.

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5 hours ago, FlyingMonkey said:

Well, from just a mathematical point of view, one hour at 5500rpm will produce 5% more engine rotation cycles than 5250rpm.  More cycles will always mean more wear on the pistons/rings/cylinders/bearings.  That doesn't mean it's a difference that has any practical effect on engine life, but it is kind of hard to escape the math. 

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.

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Just now, Roger Lee said:

usually anything above 5300 doesn't give enough speed gain vs fuel use.

 

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.  

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