How much power is a motor producing?

[Chris Gilbert IRL-1638]

Guys,

Is there an easy way of figuring out how much power a motor is producing, based on the flying setup?

I recently replaced the motors in 2 of my models with smaller capacity units and the performance has made me wonder about how much power I'm actually using to fly my aircraft. What I did discover is that with the smaller units the setup is more critical (tank height), so it seems to me I'm using a greater percentage of the available power with the smaller capacity unit. What I mean by this is that with the bigger motor I thought there might have been a problem in one model with tank height, but at my level the improvement by changing the level of the tank would not have been significant, but with the smaller motor the loss in performance caused by incorrect tank height just had to be corrected.

The information I have is

Prop:             11 x 6
Takeoff RPM:  8200
Model Weight: 52 oz
Lines   :         60 ft
Laptime:         ~5 secs

OK, so I know the line length and lap time will give me an approximate speed/velocity, that the air RPM is maybe takeoff+600
so then speed = (slipage factor) * (8200 + 600) * 6" (pitch).

So, can anyone tell me how I can relate all of this information to the power my motors are generating?

I can see what the manufacturer is saying, but I'm not using anywhere near what the peak HP output of the motors is claimed as (I hope).
Of course I've no idea where to find a HP curve for any of my motors.

Chris

[Alan Hahn]

I can tell you I am using about 300 watts~0.4 hp (averaged over the flight) on my electrified Nobler. Cruise is ~260 watts with peaks in the vertical maneuvers are at about 450 watts.

That's with an APC E 12-6 prop turning at 8000 rpm, with ~62 foot handle to plane center lines and ~5.0 to 5.2 s laps.

Larger electrified planes use more power, I am not sure if people have posted such info on their setups in the Electric forum. I know there are plots. I'll include one of my data recorder plots from last year. Watts are shown in the orange trace.

[Chris Gilbert IRL-1638]

Hi Alan,

Thats rather interesting, as one of the two models I refitted was a Nobler, your numbers appear to be close enough to what I'm using.
I had 2 models powered by 45s, and after re-engining one to make it eligible for a P40 competition I decided to replace the motor in my Nobler

I still haven't taken the plunge and gone electric, so just to be clear both my models are glow powered.

As a rough starting point then I guess my cruise power would be similar, but I'm pretty sure I'm not doubling my power output during the verticals (which might explain a few things too).

Think I have a few interesting hours ahead with Excel (and a stop watch).

Chris

[Alan Hahn]

For grins I have thought I might put my data recorder on my Brodak 40 powered Nobler and record the rpm's during a flight, just to see what goes on during a maneuver. I do have an optical rpm sensor that could monitor the rpm.

[Chris Gilbert IRL-1638]

That sounds like a rather cool experiment.

[Pinecone]

You could run the same prop on an electric motor to develop the power vs RPM curves for that prop.  Then RPM from a logger will give you power.

Some guys and RC Gruops did curves on most popular electric props, so the only testing needed for a different motor is RPM.

[Alan Hahn]

One detail that I left out is that my numbers and my plot are the power input from the battery to the motor. The power the motor is developing (giving to the prop) is less, somewhere between 70-80% of the numbers I gave.

So actually you need to calibrate the electric motor output first, and then the prop method would give you an idea of what the glow engine is developing at some rpm.

Now what the plane needs is still another issue---that you need to measure in the air and not on the bench. And that will depend on the prop efficiency too. A lot of details!

But for a ballpark value, just derate my numbers by ~25% to get what the prop was getting from the motor. Probably another 20-30% for what the plane is getting from the prop.

[rustler]

Many years ago the Aeromodeller tested the FP 40, possibly Mike Billinton did the test. First they got rather a low figure anyway. Then for interest he tested it on a typical stunt prop in typical 4-stroke mode. It produced something like 0.3bhp. This did get me thinking, and I planned to make a 35/40 size stunter and power it with e.g. a Cox TD 15, after all, this makes clearly more than 0.3, and run it at the rpm where it poduced around 0.3+, Just for interest. Like so many ideas, I haven't got round to it yet!

[Brett Buck]

For grins I have thought I might put my data recorder on my Brodak 40 powered Nobler and record the rpm's during a flight, just to see what goes on during a maneuver. I do have an optical rpm sensor that could monitor the rpm.

   I vote for that one!  We have some data points, but an entire logged flight with an *engine* would be exceptionally useful. And, what data logger do you use? I would love to have a way to do that, if the data logging is fast enough.

    If all you care about is the power, the logger should get you in the ballpark even using the electric motor. It's not like they are a lot different- if the airplane flies about the same way, the engine is putting out about the same as an electric motor.

    You do have to consider that the loggers log the *input* power. Figure around 90% for the efficiency of the motor to get the shaft power. What you need to use for the efficiency of the *prop* is a far more interesting question. Whether it seems counter-intuitive or not, the more efficient the propeller, the lower the shaft power will be for a given level of performance. Given that you need a fixed power to fly the airplane at a constant rate, the more efficient the propeller, the less power you will be running.

      The prop efficiency is probably going to be, absolute max, no more than 70%, and likely much less.

    Brett

[Howard Rush]

Be sure to record air density.  Power is a strong function of density.  You can calculate it from temperature and pressure.   

[Alan Hahn]

   I vote for that one!  We have some data points, but an entire logged flight with an *engine* would be exceptionally useful. And, what data logger do you use? I would love to have a way to do that, if the data logging is fast enough.

    If all you care about is the power, the logger should get you in the ballpark even using the electric motor. It's not like they are a lot different- if the airplane flies about the same way, the engine is putting out about the same as an electric motor.

    You do have to consider that the loggers log the *input* power. Figure around 90% for the efficiency of the motor to get the shaft power. What you need to use for the efficiency of the *prop* is a far more interesting question. Whether it seems counter-intuitive or not, the more efficient the propeller, the lower the shaft power will be for a given level of performance. Given that you need a fixed power to fly the airplane at a constant rate, the more efficient the propeller, the less power you will be running.

   The prop efficiency is probably going to be, absolute max, no more than 70%, and likely much less.

    Brett

Here's a link to the datalogger.
http://www.eagletreesystems.com/MicroPower/micro.htm

They actually are reasonably priced ($70 for basic recorder+ extra for the sensors). For electric, where you can blow a $200 battery if you run it outside of its specs, they are a reasonable expense.

The raw datalogger will log the average amps and volts from the battery at a 10Hz rate. You can add rpm sensors, airspeed, and altitude, and if you really get into it, a GPS unit.

Unfortunately the GPS resolution isn't quite Milspec, so we can't write a judging routine yet!

[Brett Buck]

Here's a link to the datalogger.
http://www.eagletreesystems.com/MicroPower/micro.htm

They actually are reasonably priced ($70 for basic recorder+ extra for the sensors).

   Agreed, that's about 1/12 of my gas costs for the two 38-hour drives to Muncie this year. But it doesn't leap out at me how that would work as an RPM sensor without a tap into the power controller.

    Brett

[Pinecone]

Also, temp sensors are nice on nitro engines.

But the airspeed might be interesting.  Fly the pattern with various engine tunes and see which one actually holds the airspeed better through the manuevers.

[Alan Hahn]

   Agreed, that's about 1/12 of my gas costs for the two 38-hour drives to Muncie this year. But it doesn't leap out at me how that would work as an RPM sensor without a tap into the power controller.

    Brett

The power measurements aren't useful for a glow setup (they are meant to monitor the current and power going to the electric motor), but the optical rpm sensor (and other sensors like temperature and airspeed and altimeter plugs into the connector block. You would have to carry an onboard battery (a small 4.8V Nicad would work) to supply the data recorder.

I have some issues with my altimeter and airspeed indicators---haven't really spent much time getting them dialed in. In addition I wonder if they can handle our abrupt maneuvers like a square corner---at least during the time interval it is happening. But I do find it interesting to get an idea what the plane is doing.

[Howard Rush]

Considering how much effort it takes to calibrate pressure systems on full-scale airplanes, I'd be surprised if that system works very accurately. 

[Paul Smith]

In old ENGINE REVIEWS in magazines, guys like Peter Chinn always included HP and torqure curves.  They did an engine every month and came up with RPM/HP/torque graphs.  Back in the vacuum tube and slide rule days.

How did they do it?

[Brett Buck]

The power measurements aren't useful for a glow setup (they are meant to monitor the current and power going to the electric motor), but the optical rpm sensor (and other sensors like temperature and airspeed and altimeter plugs into the connector block. You would have to carry an onboard battery (a small 4.8V Nicad would work) to supply the data recorder.

    I knew the power thing wouldn't work, but I am surprised the RPM is directly sensed, not just taken off the motor or controller.

    I sent Eagletree an email about it (how it would work on a glow setup, and, do they have a Mac version of the software (or can i just get the data as a text file so I can manipulate it myself)).

     Brett

[Alan Hahn]

    I knew the power thing wouldn't work, but I am surprised the RPM is directly sensed, not just taken off the motor or controller.

    I sent Eagletree an email about it (how it would work on a glow setup, and, do they have a Mac version of the software (or can i just get the data as a text file so I can manipulate it myself)).

     Brett

Ok I haven't really explained everything.

For rpm I have two sensors---the one I use all the time for the electric motors simply plugs onto one of the motor supply lines and it can sense the commutations, and from that gets the rpm. The second sensor is the optical one that I mentioned above -- I bought it explicitly check the rpm readings from a glow engine.  Just haven't tried it out yet.

They don't have a native Mac application program. I think the data comes out of the recorder in a compressed mode, and EagleTree seems to treat it as proprietary information. I have used the VM Ware Fusion to run an XP virtual machine on my Mac--I don't like it, but I also need it for my speed control software.

Once the data is readout of the recorder, the application can export it in text mode to use in other applications (which I do because I can control the plotting a bit more).

[bfrog]

Alan, I think it would be very interesting to hook up the Eagle Tree to a glow model. At least the RPM data would be a start. I don't fly stunt (mainly carrier) but data is data and could help with other electric/glow conversions.

How does the airspeed sensor work? If it's a pressure based system it could be pretty tricky to get consistent data from one plane to another. If anyone can make it work I would think you can!!

Bob

[Alan Hahn]

Bob,

The airspeed sensor is a standard pressure sensing setup with a pitot tube. I have the original sensor, the newer one EagleTree offers looks a lot better. On mine the static pressure port is right on the sensor. What is tough is to try and get atmospheric pressure. I mounted it (and the altimeter) in the belly pan of my Nobler, and then ran the pitot flex tube out to the outboard wing--roughly at half span. It is out of the prop wash. The problem is to make sure the belly pan stays at outside pressure, and isn't sensitive to fluctuations coming from some of our abrupt maneuvers. Another detail is that the average level flight airspeed claims 45mph, but from my lap times I can see that I'm flying about 54mph. I don't know if that is a calibration error, or something related with flying a circle, or maybe I am flying with an angle to the circle tangent (need 33 degrees for 45/54 type of discrepancy) or a combination of several factors (including ram pressurization of the belly pan. The Nobler ARF doesn't have adjustable leadouts, so I can't play with leadout position to see if I can change it.

I posted a plot onetime (from last year) of the whole setup, here is a link to that plot. I think it is actually the same plot (but blown up) of the flight I showed above.  http://stunthanger.com/smf/index.php?topic=7574.0

The problem with the altimeter is that it has some type of drift which apparently is temperature related. So after takeoff it indicates my plane is more or a submarine--the base altitude drifts negative. You can still see the maneuvers, but it gets hard to analyze. It also is pressure sensitive.

The truth is I haven't spent a lot of time trying to get it figured out. Maybe I'll spend some time this year. One thing I have been intrigues with is a dual axis g-meter. That would certainly give a good marking point for the square maneuvers.

[Pinecone]

The new airspeed sensor has a combined pitot static tube.  Similar to the probes you see in test aircraft.  Gets the whole thing out in front of the airframe.

There are three RPM sensors. 

The brushless sensor hooks to one or two of the motor wires, and directly reads the RPM off the ESC/RPM.

The optical sensor "sees" light/dark transitions.  So you could paint the spinner backplate white/black and read that.

The magnetic sensor normally uses a magnet attached to some rotating part. But Spektrum makes a sensor that mounts in the backplate depression that senses the crank pin. I know it works with many governors on helis, but haven't tried one with an Eagle Three.

[john e. holliday]

Shouldn't this be down in the electric forum?  Motor = electric.    Engine = fuel or gas power.  At least that is what I was alway taught.  All I know about either is what I have read or experienced.  DOC Holliday

[Alan Hahn]

Doc,
Last time I looked, glow motors made power too. That's what we are discussing here---how to figure out how much power a motor and/or an engine is producing.

[Pinecone]

Shouldn't this be down in the electric forum?  Motor = electric.    Engine = fuel or gas power.  At least that is what I was alway taught.  All I know about either is what I have read or experienced.  DOC Holliday

Actually engines are motors, but not vice versa.

Engine - converts fuel into kinetic energy, directly or in directly.

Motor - converts a given energy source (electric, hydraulic, fuel) into kinetic energy

Think motorcycle, Ford Motor Company, General Motors.

Interestingly the term motor was first used for Motor Cars to distinguish them from Steam Engine automobiles.

[Chris Gilbert IRL-1638]

In old ENGINE REVIEWS in magazines, guys like Peter Chinn always included HP and torqure curves.  They did an engine every month and came up with RPM/HP/torque graphs.  Back in the vacuum tube and slide rule days.

How did they do it?

I'm not really fussed about how they did it, what I'd really like is a set of those curves for all my motors!

That'd give me something to doodle about when I can't be in my balsa dust factory turning large pieces of balsa into smaller pieces.

[Alan Hahn]

At least a few years ago you could buy a test stand setup that would measure engine torque. Put on a prop, the engine would rotate and lift up a weight. The generated torque was just r x weight (r the radius to the weight). The power produced (in watts) is just the torque (in Newton-meters) x rpm/10   (10 is approximation or 60/2pi).

I think most of the engine guys had stuff like this (I think Clarence Lee mentions it in his old reviews).

Of course, once you measured the needed torque to turn a prop at some rpm (depends on air density), you can use those props to get the torque, once you make the air density corrections.

There is no magic.

edited--of course the magic is to know what it is in the air! And that has changed since you can easily measure rpm now. If you know the torque curve as a function of rpm (on the bench), then simply knowing the rpm as you fly lets you know what power you are supplying to the prop in the air. In the past I have measured the rpm in the air by analyzing the frequency of the sound the engine makes as it flies (not doppler corrected though).

Is this all necessary--no, but some of us like to do it, just like some of us like to make 20 point mirror finishes. It's what floats your boat!

[Brett Buck]

At least a few years ago you could buy a test stand setup that would measure engine torque. Put on a prop, the engine would rotate and lift up a weight. The generated torque was just r x weight (r the radius to the weight). The power produced (in watts) is just the torque (in Newton-meters) x rpm/10   (10 is approximation or 60/2pi).

I think most of the engine guys had stuff like this (I think Clarence Lee mentions it in his old reviews).

Of course, once you measured the needed torque to turn a prop at some rpm (depends on air density), you can use those props to get the torque, once you make the air density corrections.

There is no magic.

edited--of course the magic is to know what it is in the air! And that has changed since you can easily measure rpm now. If you know the torque curve as a function of rpm (on the bench), then simply knowing the rpm as you fly lets you know what power you are supplying to the prop in the air. In the past I have measured the rpm in the air by analyzing the frequency of the sound the engine makes as it flies (not doppler corrected though).

Is this all necessary--no, but some of us like to do it, just like some of us like to make 20 point mirror finishes. It's what floats your boat!

        Ted has a dyno, and we used it for various purposes. And a thrust stand. It got some interesting confirmation of the basic assumptions but I don't think it revealed anything that was very surprising (except to the "Static Thrust Experts").  I am very confident that the analysis for the basic engine performance and the basics of the prop performance at static is well in hand and well-confirmed.  Thats why I sometimes get frustrated with the majority of engine analysis on SSW and Stunthangar, 90% of it is utter nonsense. Even when I post actual data, people don't "believe" it. So we still have nonsense about "torque flies airplanes", people hanging larger and larger props on airplanes and leaving the pitch at 6", guys talking about flywheels and heavy pistons increasing torque, engines "going lean" to make the 4-2 break happen, etc.

    What we really need is a dyno in a wind tunnel - or some way of logging data in the air. I think the in-flight analysis is also pretty good as far as it goes, but there are clearly areas of prop performance that we still don't understand.

   BTW, the doppler shift can be eliminated by doing the "sound measurement"  method from the center of the circle.

     Brett

p.s. on the topic of in-the-air performance, we also have a perfectly good idea what we are doing in level flight as far as power goes. That can be done with a dyno and a "known good" engine setup. What we did was take the PA 61 out of my airplane and put it on the stand with no changes. Fired it up, set it for normal launch rpm with the normal prop. Then we started changing the props without changing anything else, bigger and smaller, and thus got a "constant setting" torque and HP curve for the engine. Since we knew the in-flight RPM in level flight, that gave us the shaft HP in level flight. We did that with several different engines with settings for the same or very similar airplanes, and wonder of wonders, the HP at level flight RPM was very close to the same. The deviations even followed the expectations for prop efficiency (higher RPM = less efficient). And it was all as predicted from theory. Of course no one "believed" it, it didn't actually follow the preconceived notions of some people, so it was a "thread for the ages" on SSW.

      So far, the minimal amount of in-flight data from electric loggers is consistent with *my* expectations, with a few surprises. The missing link is the resolution to tell what is going on in the middle of the corners.

[RandySmith]

Hi Chris

These are close numbers for you

you are making very close to 50 in ounces of torque, 
and  right at 1\4 HP ,
this assuming level flight and the normal RPM gain from the motor unloading


HP and torque  will vary some what (assuming you numbers stay the same) depending on which 11 inch brand of prop you have on, what the pitch really is, what nitro percent, day condition, needle setting , etc.

Regards
Randy

[RandySmith]

""So far, the minimal amount of in-flight data from electric loggers is consistent with *my* expectations, with a few surprises. The missing link is the resolution to tell what is going on in the middle of the corners.""


Hi Brett

You can record the flight on a video recorder, run the video back to a monitor and the sound to an oscilloscope, you can then watch and record RPMs in the tricks,you will get a very accurate  RPM change reading in flight , in the maneuvers, and plot and very close Torque-HP curve of what you getting in the tricks , using a paper graph.

Regards
Randy

[Pinecone]

Or use a logger, like the Eagle Tree with RPM sensor, and get a nice graph of the RPM at every 0.1 seconds.

[Chris Gilbert IRL-1638]

Hi Chris

These are close numbers for you

you are making very close to 50 in ounces of torque, 
and  right at 1\4 HP ,
this assuming level flight and the normal RPM gain from the motor unloading


HP and torque  will vary some what (assuming you numbers stay the same) depending on which 11 inch brand of prop you have on, what the pitch really is, what nitro percent, day condition, needle setting , etc.

Regards
Randy

Randy,

Thanks for that. I knew my question was going to head into deep water, just wasn't sure how deep.

What brought this on is the introduction of a P40 class at one of the local competitions I regularly enter. I've been flying a Tom Warden Trophy Trainer for some time now, but with a 45 in the nose. My dilemma was whether the model would fly OK with a 40 as my model production rate has hit fractional models per annum for now and there was no way I would complete a model in time.

Peak output of the 45 is rated at 1.2 hp.

My replacement 40 has a peak output of 1.1 HP, but of course I've no idea where this power occurs in the torque/rpm curve of either motor.

What stumped me at first was that with the same prop, the take off RPM was lower with the smaller motor, but my lap times were the same, and the only difference I could discern in the performance was that the smaller motor was more critical on tank alignment, what was OK for the 45 was clearly not good enough for the 40.

From this comparison I can only assume the smaller motor is leaning out more in the air as tank/prop/lap time are the same and ergo both motors are producing the same amount of power, as the lap time seems to me to be determined by rpm and pitch, given that we're talking about the same propeller/airframe here.

I also have a Nobler with the same power plant and did a 1:1 swap of 45 for 40. Identical behaviour, so my experience with Trophy Trainer was no fluke.

I'm unlikely to ever be world champion, but this little experiment has made me re-evaluate what I thought I knew about CL stunt.

More to the point, it has made me question the choice of power plant for my current build and planned next build, I'm thinking I've been going well overboard and what I was planning to fit 60s to might fly just as well or even better (since they'd be lighter) with, say, my 45s.

Of course, the other side of the coin here is that my recent flying has been in stunt heaven conditions, whereas at least 2 of the competitions I plan to attend will be in much less favourable conditions, if only just because of their locations.

Chris

[Vincent Corwell]

Hi Chris

Wasnt there also an issue with longer lines

having little or no difference, in calm air ? ?

 

Vicente

[Chris Gilbert IRL-1638]

I suspect strongly that this behaviour is independent of either motor in this situation.

[RandySmith]

Hi Chris

The majority of people flying stunt only use a fraction of the power ,in total, of the IC motors they are using.
To get the max power you would normally use a smaller prop higher nitro and peak the engine for max RPMs.
I can get much more power from , say a N-40, than used in most any stuntship now, problems would be the screaming noise, no cycling 1 speed, and judges would hate it. 
As far as that goes  you could power one with an RC Car nitro motor, a Hi Output 12  or 15  would fly a stuntship.
I saw some number on a FOX 35 that were much higher than my measured ones, the differance was 15% nitro, a 9x6 prop, and much higher peaked RPMs
Most of the numbers you will see for  HP specs for typical 40 45 engines will be in the 16,000 to 19,000 RPMs, the higher RPMs will help get an increased HP and Torque numbers. This is not an RPM range you will see  on any typical stuntship

Randy

[frank williams]

This plot is for data for a TopFlite 11-6 paddleblade prop.  Data is from wind tunnel tests as reported in a 1978 NFFS Sympo. article.  The papers data is in coefficients vs advance ration.  I have "dimensionalized" the data and plotted as four plots together.  This "quad plot" format allows you to see what happens from takeoff to level flight where thrust = drag.

[RandySmith]

"What stumped me at first was that with the same prop, the take off RPM was lower with the smaller motor, but my lap times were the same, and the only difference I could discern in the performance was that the smaller motor was more critical on tank alignment, what was OK for the 45 was clearly not good enough for the 40.

From this comparison I can only assume the smaller motor is leaning out more in the air as tank/prop/lap time are the same and ergo both motors are producing the same amount of power, as the lap time seems to me to be determined by rpm and pitch, given that we're talking about the same propeller/airframe here."


Hi Chris

Not So much tank location but load.The same airframe with differant motors will act sometime much differant, Some motors unload much more than others. You can get it either way, but normally small motors will unload more than larger ones will.
Example is a plane I helped a friend with had an OS VF 46, we had to lean it out in almost a dead 2 cycle on the ground, it would unload and drop to a 4 cycle once in level flight, We swapped for a 61 PA and use same plane prop fuel etc. 12 inch 3 blade. The 61 was set in a dead 4 cycle on the ground and didn't unload nearly as much.

Of course your HP will increase in the maneuvers if the engine leans out and increases power-RPMs

I had much the same with my Dreadnought, when using the 40 I had it just into a 2 cycle on the ground, it would drop to a solid 4 cycle within 1\2 lap, even going richer it would gain RPMs. With the 61-65 it would set in a dead 4 stroke at launch, after you could hear the larger motor unloading and gaining 4 cycle RPMs.

The same prop you are using, and launching in the 8s, would be turned, all out, at close to 11,000 RPMs by a  good ST 46 even more by a good loop charged 40, that would be approx 3\4 HP and about 68 in oz  of torque. Of course no one would peak a 46 for a stuntship....well almost no one.
There is a lot going on in flight.
It will be  very interesting to see  data loggers data from a 2 stroke motor when this becomes available

Randy

Randy

[Alan Hahn]

This plot is for data for a TopFlite 11-6 paddleblade prop.  Data is from wind tunnel tests as reported in a 1978 NFFS Sympo. article.  The papers data is in coefficients vs advance ration.  I have "dimensionalized" the data and plotted as four plots together.  This "quad plot" format allows you to see what happens from takeoff to level flight where thrust = drag.

Cool!

[Brett Buck]

What stumped me at first was that with the same prop, the take off RPM was lower with the smaller motor, but my lap times were the same, and the only difference I could discern in the performance was that the smaller motor was more critical on tank alignment, what was OK for the 45 was clearly not good enough for the 40.

From this comparison I can only assume the smaller motor is leaning out more in the air as tank/prop/lap time are the same and ergo both motors are producing the same amount of power, as the lap time seems to me to be determined by rpm and pitch, given that we're talking about the same propeller/airframe here.

     It's not "leaning out" more, it's unloading more because it's bogged down more on the ground.  The lap time is determined by the RPM, pitch, prop airfoil,  and *diameter*. The speed you get is about 60% of the "experimental" pitch x in-flight RPM, but a larger prop might get 65% of the pitch x rpm.

    The lowering of launch revs was one of the criteria we used to use with the ST46 to determine when it was worn out - with my system I was setting a brand-new strong ST46 at maybe 8300-8400 on the ground. If it started having to be set below 8000 that was an indication that it was tired, and time to change rings and maybe rough up the cylinder walls.

    Brett