[Geico266]

I run 100% auto gas (91 Octane) and LOP in the -10. No problems at all. I run 40F LOP on the 1st cylinder to peak. All cyinders peak within .2 GPH.

Running LOP is safer (detonation wise) due to the lower fuel mixture, and lower temps.

 

[kiwipete]

Ditto

I have just gone through the nozzle tuning program with Airflow Performance and although I need one more flight to verify, I think I am within about .3 gph for all peaks. The funny thing is that when I get more than about 20 to 30 degrees LOP the temps begin to rise! Ron at Airflow explained the reason-- the cumbustion event is happening slow enough that it is still happening when the exhaust valve opens, thus higher temps--but it is unsettling to see the temps going up when it seems like they should continue going down. Maybe I should be using the absolute value away from last to peak, the combination of down and then back up? Does anyone else have this happen?

FG

FG

Mine is the same. All peak within .2 gal but I can't get much below 30 LOP

I posted this on another thread and got the same anwer as you above and I think from memory Alex Peterson said the same.

Cheers

Peter

 

[frankh]

Yup

I do the same with E10 blend as well....LOP is simply a better place to be for the engine..It will not detonate with lowish power settings.

E10 won't lean out quite as far

I have 38Deg BTDC max timing (EI)

Frank

 

[erich weaver]

UPDATE from Deakin

Previous posts on this thread quote a John Deakin article in which the statement is made that at around 8000 or more, your engine cant develope more than about 60% power and that therfore you cannot hurt your engine
at any mixture setting.

Based on Kevin Horton's previous comments and my own checking, the above statement from John Deakin didnt appear to be true. I recently sent an email to Deakin about this which I have reproduced below along with his response. I apologize in advance for the shameless sucking up I did in my email, but I did it in hopes of getting a response and for the benefit of mankind


John:

Not sure if you are in the habit of answering fan mail, but Im gonna give it a shot....

I have read and reread and reread again your series of excellent articles on LOP operations and "Where to Run Your Engine". Thanks so much for the education - I push other pilots to your articles at every opportunity to try and bring them into the 21 century with respect to engine operation. All that admiration aside, I am having a problem with a statement in one of your articles, and was hoping you could comment further on it. From "Where to Operate Your Engine, Part 3 - Cruise":

"Its important to realize that if you choose to run at full throttle in a normally aspirated engine (and you almost always should), then your only control over manifold pressure is altitude. The higher you fly, the less the manifold pressure, and the smaller that 'red box'. Its really simple if you climb above about 8,000 feet or so. The altitude will have reduced your power so that no matter what you do (normally aspirated), you cannot recover it - you cannot get more than 60% or so - and you cant hurt the engine with ANY mixture setting."

I own an RV-7A with an IO-360B1B engine - 180 HP. The Lycoming performance chart indicates that at 8000 feet, 2400 RPM, it is capable of producing about 127 HP at full throttle (standard conditions), which is 71% . Further evaluation of the chart indicates that HP does not drop off to 60% (108 HP) until a bit above 12,000 feet. This seems to be in substantial disagreement with your statement, and I can only conclude that I DO have to be concerned with mixture unless flying above 12000 feet. A quick Google search indicates that at 8000 feet, air density drops to around 74% of the density at sea level. That suggests to me, that while the curves may be different for different engines, most should be able to produce 70% power or more at 8000 feet rather than 60%.

Am I overlooking anything in my assessment? Comments?

Best regards

Erich Weaver
A big fan of yours!



Johns Response:

That was a test, and you pass. :-[

Nice catch, and to my knowledge, the first time!

You're almost entirely correct. I think the books don't take the loss of manifold pressure from the intake plumbing into account, especially from the air filter, which will bring those altitudes down a little.

Those early charts were developed off the TCM power charts, which are based on 2500 rpm. I didn't know that at the time.

Then, we developed the mantra about 60% or 65% being ok at any mixture. That's true. But then we made the leap that any mixture at 8,000 (since revised to approximately 9,000) was ok. That's only true if you're running LOP!

I'll try to be more precise. Thanks for the catch!

Best...
John Deakin
Advanced Pilot Seminars

 

[DENMACRES]

LOP

I RUN MY IO-360 ANGLE LEAN OF PEAK AT ANY PWR.SETTING BELOW 80%. I HAD HELP FROM DON AT AFP IN BAL. THE FF TO GET ALL CYL. TO PEAK WITH IN .1 OF A GAL. I CAN RUN AS LEAN AS 110 AND STILL RUN SMOOTH. TEMPS. DROP DRAMATICALLY AND A/S FALLS OFF 4-8kts. FF DIFF. IS 2gph FROM 100 RICH TO 50 LEAN.

 

[Webb]

Finally Checked it

Larry - Thanks for yours and other's help. Ran the test at 8,500 and 10,500, full throttle, 2,400 rpm.

Alt Fuel Flow at Peak EGT
8,500 8.0 7.9 7.8 8.1
10,500 7.6 7.5 7.5 7.8

I graphed it out and will fax over to Airflow Performance. They might be able to tweak it a smidge by replacing some restrictors.

When I looked at the graph, cylinders 2 & 3, and 1 & 4 are peaking together. It tells more of the real story instead of just a data point.

Looks good to me Web.

The test I referred to is simple. Give it a try and see where you are at, if nothing else you have an excuse to fly again!

Here is the test from Airflow Performance.

Nozzle Tuning Data

The basis of nozzle tuning is to get each of the cylinder EGT?s to peak at the same fuel flow. Your aircraft must be equipped with EGT information on each cylinder and fuel flow information. A digital flow meter is preferred.

To gather correct data for nozzle tuning, set a cruise power setting. Typically 24? MAP and 2400 RPM. Set the mixture to be 0.5 GPH richer than peak on any cylinder. At this setting record all the EGT?s for each cylinder. Lean the mixture 0.2 GPH and record all the EGT?s again. Lean the mixture an additional 0.2 GPH; record all the EGT?s again. Continue leaning the mixture 0.2 GPH and record the EGT?s until all the cylinders have peaked.

An alternative method although not as accurate is to lean each cylinder to peak and record the fuel flow at that point. You will get the same data, but since the EGT reacts slower than the leaning process you may go past the peak and not know it. This is especially true if an engine monitoring lean find function is used. We get more accurate data taking the EGT data manually. If you use an automatic data acquisition function, allow 30 seconds or so at each fuel flow setting so the EGT value can stable out.

After the data is taken, we determine which nozzles to change to get all the cylinders to peak at the same time. You will notice that the EGT number at peak may not be the same for each cylinder, THIS IS NOT IMPORTANT. The cylinders that peak first (higher fuel flow) are the lean ones; the cylinders that peak last (lower fuel flow) are the rich ones.

Getting all cylinders to peak within 0.2 GPH is ideal.

 

[Steve Brown]

Use indicated airspeed to determine power setting

..... I would really like to be confident in the calculated HP figures when I operate LOP. ........

Fin
9A

I don't have a fuel flow gauge on my airplane. However, back in the early 90s I did some very careful observation of indicated airspeed verses power settings in my Mooney. I didn't use the Mooney power schedule, I used the Lycoming schedule which was very detailed in terms of RPM, MP, and air temperature.

What I found was that within very small variation, if I had the same power setting I had the same IAS. This was independent of OAT, RPM, and altitude.

How accurate is this method? I'm sure there are inaccuracies because I can think of some potential problems right up front. Nevertheless, I thinks its close enough to address this subject.

To calibrate this technique, use a well defined ROP power settings to establish an airspeeds verses power. Then, whether ROP or LOP if you have the same IAS, you have the same power output.

 

[PCHunt]

Apples to apples?

........... Then, whether ROP or LOP if you have the same IAS, you have the same power output.

Correct, but you won't have the same fuel flow. At ROP you will be using more fuel for the same power. How can this be

When ROP, some of the fuel is unburned, and acts to cool the engine, and therefore is "wasted".

It's really hard to compare "apples to apples". Sometimes those wretched "oranges" get in the way.

 

[N395V]

Correct, but you won't have the same fuel flow. At ROP you will be using more fuel for the same power. How can this be

When ROP, some of the fuel is unburned, and acts to cool the engine, and therefore is "wasted".

It's really hard to compare "apples to apples". Sometimes those wretched "oranges" get in the way.

Maybe this will help, it is a graph from a 1955 Pratt & Whitney publication and can also be found on the advanced pilot seminars website.

 

[Kevin Horton]

I don't have a fuel flow gauge on my airplane. However, back in the early 90s I did some very careful observation of indicated airspeed verses power settings in my Mooney. I didn't use the Mooney power schedule, I used the Lycoming schedule which was very detailed in terms of RPM, MP, and air temperature.

What I found was that within very small variation, if I had the same power setting I had the same IAS. This was independent of OAT, RPM, and altitude.

How accurate is this method? I'm sure there are inaccuracies because I can think of some potential problems right up front. Nevertheless, I thinks its close enough to address this subject.

Interesting observation. I'm certainly not going to say you didn't see what you saw, because you were there and I wasn't. But, it is hard to reconcile your observations against theory. Do you recall the range of altitudes you would have been at when you made these observations? Do you recall roughly how repeatable the IAS was for a given percent power?

Accepted theory says that profile drag is proportional to the square of the equivalent airspeed. In the speed and altitude range that most RVers care about, equivalent airspeed = calibrated airspeed for all practical purposes. Ignoring ASI instrument error and static system position error, calibrated airspeed = indicated airspeed. So, roughly speaking, profile drag is proportional to IAS^2.

Engine power times prop efficiency = drag times TAS. But, for a given TAS, the IAS will decrease as we climb, which causes the drag to decrease. So, for a given power, as we climb, the TAS will increase, but the IAS should decrease. To give you an idea of the rough relationships that conventional theory predicts, here are the results of some very rough cruise performance testing done on my RV-8, with old Hartzell prop. The predicted speeds at 65% power, standard day, 1600 lb are:

Altitude  TAS   CAS
 (ft)    (kt)  (kt)
 4000    167.5 158.0
 6000    170.6 156.2
 8000    173.7 154.3
10000    176.9 152.5

Between 4000 ft and 10000 ft, we see a 9.4 ft increase in TAS, and a 5.5 kt decrease in CAS. So while neither TAS nor CAS are constant with power, CAS is more constant than TAS.

But, this is theory. YMMV. As Yogi Berra supposedly said "In theory there is no difference between theory and practice. In practice there is. " Changes of prop efficiency as power, rpm and TAS change could muck things up slightly.

 

[Steve Brown]

Its drag

Interesting observation...... Do you recall the range of altitudes you would have been at when you made these observations? Do you recall roughly how repeatable the IAS was for a given percent power?

Accepted theory says that profile drag is proportional to the square of the equivalent airspeed. ...... So, roughly speaking, profile drag is proportional to IAS^2.

Engine power times prop efficiency = drag times TAS.....

Those were my observations, but the details are fuzzy. I extracted the data from lycoming graphs and did some curve fitting with excel. Then used the curve fit data to make myself a more better power schedule. Inputs were RPM, MP, and air temp in kelvins.

I did not do a documented study, I just noticed that whenever I had the same power setting, I had the same IAS.

I agree drag is roughly proportional to IAS^2. So, if you have the same IAS, you have the same drag. Therefore, the same work is being done. Thus you are at the same power setting. I think this makes the point.

If you move between two different IAS the induced drag (due to AOA) changes so you cannot solve it that simply.

Drag related to potentially differing prop RPM are not accounted for in this simplistic approach. There may be other errors, but I think its decently correct.

you lost me in this: engine power x prop efficiency = drag x TAS

I would have thought that TAS term should be IAS. I'm not saying its wrong, I just don't understand it.

 

[Webb]

Drop in IAS

Kevin and Steve,

While I was measuring the peak EGT and fuel flow rate, I also recorded the indicated air speed.

Interesting that on 2 separate runs, then IAS starts to drop slightly just as you reach peak (2 knots). Even more interesting that you also get a slight increase (2 knots) almost exactly at 75 ROP and then drops off that slight amount as you go richer.

I did order 1 new restrictor from Airflow which should put my spread less than 0.2 gal/hr.

 

[Kevin Horton]

I agree drag is roughly proportional to IAS^2. So, if you have the same IAS, you have the same drag. Therefore, the same work is being done. Thus you are at the same power setting. I think this makes the point.

All true, except you need to replace IAS with TAS. If you have a given amount of drag, the power required is equal to TAS times drag (with appropriate unit conversions). So, power required varies with TAS times drag, with drag varying with IAS^2, so power varies with TAS * IAS^2. But, the relationship between TAS and IAS varies with altitude, so in theory, the IAS we get for any given power should also vary with altitude.

you lost me in this: engine power x prop efficiency = drag x TAS

The point was that from an aerodynamic point of view, it is thrust power that matters. Thrust power is the power that is actually transmitted by the prop to the airflow. It is equal to engine power times propeller efficiency. Prop efficiency will be relatively constant over the range of normal cruise conditions, so we can ignore it. But if we starting making big changes in altitude, power, rpm, etc, then the prop efficiency will be changing, and this will affect any predictions we attempt to make.

Anyway, I'm not arguing against your observations, on that aircraft, with that engine, prop, ASI, etc. But I doubt they point to any general conclusion that we can apply to other aircraft.

 

[Steve Brown]

Got it

........ So, power required varies with TAS times drag, with drag varying with IAS^2, so power varies with TAS * IAS^2. .........

Kevin,

Thanks for straightening me out. Put that way it's more clear:
work = force x distance

force proportional to IAS^2, distance proportional to TAS. So your assertion must be true.

That has me wondering about my early 90s observations which must be incorrect. As you say, I must have stumbled on to some conditions which roughly canceled the error. Whoops

Anyway, back to finding power when LOP

All of the information needed is in TAS, IAS, and the ROP power schedule, but drag does not exactly follow IAS^2 due to AOA. How significant is this?

The few knot IAS difference I get in my airplane going from full fuel to very low fuel on a long trip say it might be significant. The 1 mph difference between light and heavy on the vans web site say not so much. This may be another case of faulty observations.

I seems like mapping IAS^2 x TAS against the ROP power schedule should yield a solution in which given IAS^2 x TAS power is always known regardless of mixture setting. That is, at least over the relatively narrow range of normal cruise speeds. At much lower speeds the power is so low it doesn't matter how you lean and at much higher speed you should not be running LOP.

Please comment.

 

[Ron Lee]

At higher speed you can run LOP IF the manifold pressure/percentage horsepower is low enough, which it is where I fly.

 

[Kevin Horton]

Anyway, back to finding power when LOP

All of the information needed is in TAS, IAS, and the ROP power schedule, but drag does not exactly follow IAS^2 due to AOA. How significant is this?

The few knot IAS difference I get in my airplane going from full fuel to very low fuel on a long trip say it might be significant. The 1 mph difference between light and heavy on the vans web site say not so much. This may be another case of faulty observations.

As you note, there are other factors that come into play. Weight and CG for example. At a given weight and power, you'll go slightly faster at aft CG than you would at forward CG. I suspect the difference is on the order of 2 kt over the full CG range. Weight - my model suggests that at 75% power at 8000 ft the IAS would increase about 1 kt for every 100 lb reduction in weight. At lower power cruise and higher altitudes, the effect of weight will be slightly greater, as the power required vs speed curve gets flatter as you more closer to IAS for minimum power.

I seems like mapping IAS^2 x TAS against the ROP power schedule should yield a solution in which given IAS^2 x TAS power is always known regardless of mixture setting. That is, at least over the relatively narrow range of normal cruise speeds. At much lower speeds the power is so low it doesn't matter how you lean and at much higher speed you should not be running LOP.

Please comment.

We can come up with all kinds of nice theoretical relationships between speed and power, but I'm not sure they are usable in the real world in the way you want. I.e., I don't think it is practical to say "if you want 75% power, set the power to get a speed of XXX kt IAS". First, even at a given altitude, the relationship between speed and power is affected by weight, CG and temperature, with each of those possibly affecting things by a couple of knots. Second, it takes quite some time after setting power for the speed to stabilize. Third, the air is never perfectly calm. Even on the nicest of days, there will be small perturbations that cause the airspeed to wander up and down a few knots.

I think it is much more practical to simply set power with respect to rpm, MP and fuel flow.

I may regret posting this, as it'll raise more questions than it answers, but for the terminally technically inclined, several weeks ago I was playing around with some mathematical analysis software, and while I was experimenting to see what it could do, I put together a page which looked at the technical details of cruise performance analysis (if my server is down, there is another copy here). It isn't really usable in its current form, but I will eventually write a much less technical Kitplanes article, with accompanying spreadsheet. The pages have a lot of technical goblity gook about cruise performance, then several graphs showing the results of analyzing cruise performance from one flight on my RV-8. The graphs near the end show the predicted effect of weight, altitude and temperature on the relationships between power required and TAS.

 

[hevansrv7a]

Finding power while LOP

The thread raises lots of other interesting points. Some so interesting that I had to correct my "model". Thanks again, Kevin.

The question was about finding power while LOP, right? As said above:
Finding power while LOP is best done not from MAP and RPM per the Lyc charts but from fuel flow. Walter Atkinson and the GAMI guys have shown that when there is excess air (that defines LOP) your SFC is about 0.40. Said another way, if you multiply your fuel flow in gallons per hour by 14.9 you will get your BHP. (Your THP is your BHP times your prop efficiency but you really don't care about that in this application.) You cannot do it with MAP and RPM and altitude if you are LOP.

0.40 pounds per horsepower per hour
times
6.0 pounds per gallon (actually varies with temp)
means
6.0 / .40 = 15 horsepower per gallon per hour.
Example: 8 gph times 15 = 120 HP which is 67% for a 180 HP engine.

Acccording to Walter, RPM, MAP and altitude may vary but this relationship will be "pretty doggone accurate".

 

[rwhittier]

It works pretty well on my 6 Cyl TCM...

As others have posted, I am not sure why you are seeing anything different. This formula is not cylinder specific - its piston engine physics. Air and Fuel mixtures burn well in a range that is well understood in piston engines and it doesn't change. The actual formula is 14.9 X FF = HP when LOP. I use 15 because it gets me within 1 or 2 HP on my IO-550 - which is close enough for me. Plus I can do it in my head

The original research on this stuff dates back many decades. The APS course relies heavily on two books written in the 50's from work done back into the 30's. As you can imagine at that time it mostly was done on radials - some quite large radials with many more than 6 cylinders. One book, published in 1957 is collaborative work between American Airlines (at the time operating DC-7's) and Curtis-Wright. APS says "It is a remarkable little book that very clearly and concisely explains the principles behind mixture control in ALL internal combustion, spark-fired, gasoline engines." They sell the book on their website along with another one published by Pratt and Whitney. Anyone really interested in this stuff ought to read them.

This may work for a 4 cylinder, but it doesn't work for a 6.

 

[Kevin Horton]

As others have posted, I am not sure why you are seeing anything different. This formula is not cylinder specific - its piston engine physics. Air and Fuel mixtures burn well in a range that is well understood in piston engines and it doesn't change. The actual formula is 14.9 X FF = HP when LOP. I use 15 because it gets me within 1 or 2 HP on my IO-550 - which is close enough for me.

The old Lycoming info I have certainly suggests that 14.9 X FF = HP when LOP isn't too far off the mark. The exact answer you get from this old Lycoming analysis technique varies a bit depending on rpm, compression ratio, power setting, and how far LOP you are, but usual results seem to be in the range of 14.5 X FF to 14.7 X FF. But, given typical accuracy of FF indications, there is nothing wrong with rounding up to 15.

Here is an example of the relationship between fuel flow, power, power/FF and bSFC for an IO-360 at 2400 rpm, with 8.7 compression ratio, and a power setting that resulted in 9 GPH at peak EGT:

 Fuel   Pwr    Pwr      bSFC
 Flow          per          
               GPH          
(GPH)  (hp) (hp/GPH) (lb/h/hp)
 6.0   38.6    6.43     0.934                        
 6.1   44.7    7.33     0.820                        
 6.2   50.6    8.16     0.737                        
 6.3   56.2    8.92     0.674                        
 6.4   61.5    9.61     0.625                        
 6.5   66.6   10.24     0.587                        
 6.6   71.4   10.82     0.556                        
 6.7   76.0   11.34     0.530                        
 6.8   80.3   11.81     0.509                        
 6.9   84.4   12.23     0.491                        
 7.0   88.3   12.61     0.476                        
 7.1   92.0   12.95     0.464                        
 7.2   95.4   13.26     0.453                        
 7.3   98.7   13.52     0.445                        
 7.4  101.8   13.75     0.437                        
 7.5  104.6   13.95     0.431                        
 7.6  107.3   14.12     0.426                        
 7.7  109.8   14.26     0.421                        
 7.8  112.2   14.38     0.418                        
 7.9  114.4   14.48     0.415                        
 8.0  116.4   14.55     0.413                        
 8.1  118.3   14.60     0.412                        
 8.2  120.0   14.63     0.411                        
 8.3  121.6   14.65     0.410  Fuel flow for best efficiency
 8.4  123.0   14.65     0.410                        
 8.5  124.4   14.63     0.411                        
 8.6  125.6   14.61     0.411                        
 8.7  126.7   14.57     0.413                        
 8.8  127.7   14.51     0.414                        
 8.9  128.6   14.45     0.416                        
 9.0  129.5   14.38     0.418  Fuel flow for peak EGT
 9.1  130.2   14.31     0.420                        
 9.2  130.8   14.22     0.423                        
 9.3  131.4   14.13     0.425                        
 9.4  131.9   14.04     0.428                        
 9.5  132.4   13.94     0.431                        
 9.6  132.8   13.84     0.434                        
 9.7  133.2   13.73     0.438                        
 9.8  133.5   13.62     0.441                        
 9.9  133.8   13.52     0.445                        
10.0  134.1   13.41     0.448                        
10.1  134.3   13.30     0.452                        
10.2  134.6   13.19     0.456                        
10.3  134.8   13.09     0.459                        
10.4  135.0   12.99     0.463                        
10.5  135.3   12.88     0.466                        
10.6  135.4   12.77     0.471  Fuel flow for best power
10.7  135.4   12.65     0.475                        
10.8  135.4   12.54     0.479                        
10.9  135.4   12.42     0.484                        
11.0  135.4   12.31     0.488                        
11.1  135.4   12.20     0.493                        
11.2  135.4   12.09     0.497                        
11.3  135.4   11.98     0.502                        
11.4  135.4   11.88     0.506                        
11.5  135.4   11.77     0.511                        
11.6  135.3   11.66     0.515                        
11.7  135.2   11.56     0.520                        
11.8  135.1   11.45     0.525                        
11.9  135.0   11.34     0.530

Points of interest:

The best efficiency is with a fuel flow that is about 0.6 to 0.7 GPH lean of peak EGT. If you go leaner than that, the hp per fuel flow starts to drop. At this condition, the power is about 9% less than peak power, but the fuel flow has dropped about 22% from the fuel flow for peak power. The efficiency, in terms of fuel required to produce one hp, is about 13% better than at mixture for best power.

If you get too lean, the power and efficiency fall off very quickly. In my experience you can sense this, as the engine starts to run rough, and the IAS falls off quick significantly.

The Lycoming data suggests that there is wide band of fuel flows around peak power mixture where the power is essentially constant.

 

[rwhittier]

Here is a good chart...

The guys at APS have put together a chart that very much describes what your saying about efficiency. I got this at the APS class, probably the best money I have ever spent on a class before. I knew engines pretty well when I went, but it just brought it all together. They go into the physics of whats happening, including engine pressures at degrees of rotation of the engine. They call it Theta PP (crank angle at peak cylinder pressure). Without a long drawn out description, you get peak pressure later after TDC which is less pressure on Con rods, cylinders, valves, etc...

The gist of it is pretty straight forward and matches what you posted. There is a new term on there called ICP (Internal Cylinder Pressure). They have a cool pressure transducer they designed that can monitor the engine internal pressures to get that. The best measure of efficiency is BSFC, which basically is power produced per unit of fuel. You can see the most efficient setting (as measured by BSFC) for the engine is about 25 degrees LOP and drops only slightly out to 50 degrees LOP. After that the engine will start to lose power pretty quickly. I run LOP not because it saves fuel, but because I believe that heat and pressure is the enemy of my engine and these settings are better for the engine. Provided of course your Air Fuel mixtures are tight enough to do it, typically only on FI engines.

The best efficiency is with a fuel flow that is about 0.6 to 0.7 GPH lean of peak EGT. If you go leaner than that, the hp per fuel flow starts to drop. At this condition, the power is about 9% less than peak power, but the fuel flow has dropped about 22% from the fuel flow for peak power. The efficiency, in terms of fuel required to produce one hp, is about 13% better than at mixture for best power.

 

[hevansrv7a]

Question about the APS chart

On the chart, what do the numbers 1.66 and 2.63 mean? I don't think they are BSFC or HP. They could be the inverse of BSFC for charting purposes. (1/2.63=.38) Is the chart copyright or may it be freely distributed? Thanks.

The guys at APS have put together a chart that very much describes what your saying about efficiency. I got this at the APS class, probably the best money I have ever spent on a class before. I knew engines pretty well when I went, but it just brought it all together. They go into the physics of whats happening, including engine pressures at degrees of rotation of the engine. They call it Theta PP (crank angle at peak cylinder pressure). Without a long drawn out description, you get peak pressure later after TDC which is less pressure on Con rods, cylinders, valves, etc...

The gist of it is pretty straight forward and matches what you posted. There is a new term on there called ICP (Internal Cylinder Pressure). They have a cool pressure transducer they designed that can monitor the engine internal pressures to get that. The best measure of efficiency is BSFC, which basically is power produced per unit of fuel. You can see the most efficient setting (as measured by BSFC) for the engine is about 25 degrees LOP and drops only slightly out to 50 degrees LOP. After that the engine will start to lose power pretty quickly. I run LOP not because it saves fuel, but because I believe that heat and pressure is the enemy of my engine and these settings are better for the engine. Provided of course your Air Fuel mixtures are tight enough to do it, typically only on FI engines.

 

[rwhittier]

I don't remember at the moment

But I think you are correct about the inverse for charting.

I got a hard copy at the APS class (that particular soft copy I got from another board I participate in). They openly encouraged us to go out and talk up LOP with our fellow pilots and to use that chart/data to convince them that LOP was actually better for the engine - properly done. They are on a mission to turn around the ROP/LOP debate. So I think its ok to use but certainly you should credit APS. After all, they are the ones trying to bring LOP back into aviation after a 50 year hiatus.

On the chart, what do the numbers 1.66 and 2.63 mean? I don't think they are BSFC or HP. They could be the inverse of BSFC for charting purposes. (1/2.63=.38) Is the chart copyright or may it be freely distributed? Thanks.

 

[Webb]

Best Power??

I'm not sure I understand the chart completely. It was my understanding that the rich side of peak produces more HP.

When I was measuring IAS during fuel flow testing, it was greatest at 75F ROP. It also dropped off slightly just as I reached peak. According to the chart, most HP is developed on the LOP side of peak. According to my airspeed indicator, it doesn't agree with the chart.

What am I missing here?

The guys at APS have put together a chart that very much describes what your saying about efficiency. I got this at the APS class, probably the best money I have ever spent on a class before. I knew engines pretty well when I went, but it just brought it all together. They go into the physics of whats happening, including engine pressures at degrees of rotation of the engine. They call it Theta PP (crank angle at peak cylinder pressure). Without a long drawn out description, you get peak pressure later after TDC which is less pressure on Con rods, cylinders, valves, etc...

The gist of it is pretty straight forward and matches what you posted. There is a new term on there called ICP (Internal Cylinder Pressure). They have a cool pressure transducer they designed that can monitor the engine internal pressures to get that. The best measure of efficiency is BSFC, which basically is power produced per unit of fuel. You can see the most efficient setting (as measured by BSFC) for the engine is about 25 degrees LOP and drops only slightly out to 50 degrees LOP. After that the engine will start to lose power pretty quickly. I run LOP not because it saves fuel, but because I believe that heat and pressure is the enemy of my engine and these settings are better for the engine. Provided of course your Air Fuel mixtures are tight enough to do it, typically only on FI engines.

 

[rwhittier]

Perhaps your reading the labels incorrectly

The way I read the HP curve it peaks just about 75 degree ROP (the black line). The brown colored (bottom line) is an efficiency measure (BSFC) which basically says you reduce FF more quickly than you reduce power production when LOP. I think you might be seeing the labeling, which is a bit weird. The BSFC label applies to the bottom line and is on the left side of the graph. Its easy to see the black HP label and apply it to BSFC.

I'm not sure I understand the chart completely. It was my understanding that the rich side of peak produces more HP.

When I was measuring IAS during fuel flow testing, it was greatest at 75F ROP. It also dropped off slightly just as I reached peak. According to the chart, most HP is developed on the LOP side of peak. According to my airspeed indicator, it doesn't agree with the chart.

What am I missing here?

 

[Webb]

Your right, it's the labeling. When I look at the "correct" curve, it is dead on.

The way I read the HP curve it peaks just about 75 degree ROP (the black line). The brown colored (bottom line) is an efficiency measure (BSFC) which basically says you reduce FF more quickly than you reduce power production when LOP. I think you might be seeing the labeling, which is a bit weird. The BSFC label applies to the bottom line and is on the left side of the graph. Its easy to see the black HP label and apply it to BSFC.

 

[Webb]

Graphed results

Here are the graphed results that Kevin is using some of the other data in another post regarding fuel flow and power. Best IAS was at 9.2 gal/hr when #4 was 75 LOP but the others were approximately at 90 ROP. I'm hoping to see the flow rate drop to around 8.8 when all cylinders will be close to 75 LOP.

I'll post again after replacing the #4 restrictor to verify the change.

 

[logansc]

I apologize as this is not quite over the plate in this conversation, but it's still in the same ballpark, I think.

It appears from the graph provided by Kevin, that useful bSFC's run from approximately .410 to .471 but these only apply at LOP. I will fly my Rocket this summer and was wondering what I should expect to see for fuel flow on takeoff at full rich? I'll be at sea level, turning 2,700 rpm on an IO-540 with AFP fuel injection (FM-200) and 9.5 to 1 pistons. I have been told to expect something on the order of 280 hp. A quick check shows a pretty big range of fuel flows if you plug in bSFC values from say, .40 to .50 of from 18.7 gph to 23.3 gph. It might be nice to know on initial takeoff if I am seriously lean at full power or not.

Any ideas on what I should see, or even more to the point, what the bSFC should be under these conditions? Assuming the bSFC in these conditions is well established, shouldn't I be able to legitimately "back out" my hp at full power from my fuel flow? I imagine many RV-10 guys have this same or a similar engine configuration.

Regards,


Lee...

 

[rv7charlie]

IIRC, you should see significantly more than .50 lb/hp/hr. I've seen numbers as high as .55.

The Lyc operator's manual #60297-12 shows a carb, 8.5/1 compression O-360 180 hp engine full throttle Bsfc between .52 & .54 as rpm increases from 2200 to 2700 rpm, power from 160 to 180 hp.

A lower compression -B series engine at 168hp (max) is ~.58.

Charlie

 

[Kevin Horton]

I apologize as this is not quite over the plate in this conversation, but it's still in the same ballpark, I think.

It appears from the graph provided by Kevin, that useful bSFC's run from approximately .410 to .471 but these only apply at LOP. I will fly my Rocket this summer and was wondering what I should expect to see for fuel flow on takeoff at full rich? I'll be at sea level, turning 2,700 rpm on an IO-540 with AFP fuel injection (FM-200) and 9.5 to 1 pistons. I have been told to expect something on the order of 280 hp. A quick check shows a pretty big range of fuel flows if you plug in bSFC values from say, .40 to .50 of from 18.7 gph to 23.3 gph. It might be nice to know on initial takeoff if I am seriously lean at full power or not.

Any ideas on what I should see, or even more to the point, what the bSFC should be under these conditions? Assuming the bSFC in these conditions is well established, shouldn't I be able to legitimately "back out" my hp at full power from my fuel flow? I imagine many RV-10 guys have this same or a similar engine configuration.

This ancient Lycoming power from fuel flow data I have seems to suggest a bSFC for your engine in the range of 0.46 at best power. But at full rich the bSFC should be quite a bit higher than that. I'll venture something on the order of 0.55 as a SWAG. Assuming you really have a 280 hp engine, I think I'd want to see at least 23 gph. I suspect you'll actually see 25 or 26 gph.

But, keep in mind that I have no experience with 540s, and this Lycoming data is a long ways from having been validated. These comments are worth what you paid for them.

 

[logansc]

Charlie/Kevin: Thanks for taking the time to respond, that is certainly useful "ballpark" information. Normally aspirated piston engine BSFC's don't change much with engine size, but do change with compression ratio. Being lean on takeoff with an experimental engine is a possibility worth monitoring, I think. Wouldn't expect it with a factory new engine, but with an experimental engine presumably it is possible to be lean at takeoff power due to a too small main jet or inappropriate installed injector sizes for the demands of the engine. That is what flight test is all about, of course, but I am just looking for a rough number that I shouldn't be too far away from on my first takeoff.

I think figures you two are suggesting are probably about right, which equates to takeoff full power fuel flows in the neighborhood of 24-26 gph. If I see significantly less than that passing through 100 knots (full rpm), I will likely terminate full power and perhaps shorten my initial flight. You can fix too rich in the air, but not too lean!

Again, thanks for the help!

Regards,


Lee...

 

[f1rocket]

Lee,

I typically saw 24-28 GPH at full rich, WOT during takeoff.

During first flight, it is possible to have an instrumention error when everything is working just fine. Test your fuel pump on the ground per the plans and my website to make sure you can flow enough fuel. If that checks out, you should be good to go.

 

[BillOrcutt]

The "big pull" is very important in turbo applications, where a 6-cyl ROP fuel flow of 25-27 gph is reduced to 17 gph, LOP. If done too slowly, TIT's get way too hot, and take some time to recover......hence, the value of the quicker "big pull" which minimizes time at the high TIT. For nomally aspirated LOP ops, the fuel savings are generally about 2 gph (from 9 GPH down to 7 GPH on our 4 cyl engines). Balanced fuel injectors are a MUST, either GAMI's or AirFlow Performance. Do the "GAMI lean test" to check yours, to be sure the "delta" between first and last cylinder to peak is .5 gph or less. This will insure smooth LOP ops, which can be done at any altitude. This will also help you insure that all cylinders are at least 50 LOP, just outside the "red box." The speed drops off rapidly as we go LOP, but a speed loss of 5 KIAS is normal at 50 LOP. Go further lean, and the speed loss is much greater. Hence, 50 LOP is ideal - - - as long as all cyls are matched. Other benefits of LOP ops besides fuel savings, include low lead build-up in plugs, longer cyl life due to reduced internal cylinder pressures, and no carbon monoxide production when LOP.

 

[Webb]

Here are the results after changing the restrictor on cylinder #4. Note how the cylinders are all peaking much closer. I'll get a chance to verify the flow rate at best power/speed and at 75 rop this weekend. Also going to look at # 3 and may put put a smaller restrictor in it.

Here are the graphed results that Kevin is using some of the other data in another post regarding fuel flow and power. Best IAS was at 9.2 gal/hr when #4 was 75 LOP but the others were approximately at 90 ROP. I'm hoping to see the flow rate drop to around 8.8 when all cylinders will be close to 75 LOP.

I'll post again after replacing the #4 restrictor to verify the change.

 

[vjdslk]

LOP Ops - Deakin's "big pull"

A friend of mine just forwarded me this article from ECI regarding rapid leaning. I would like to hear other's thoughts and practices.

http://www.eci.aero/pdf/93-6-7.pdf

Thanks!

Jon D.

 

[Russ McCutcheon]

A friend of mine just forwarded me this article from ECI regarding rapid leaning. I would like to hear other's thoughts and practices.

http://www.eci.aero/pdf/93-6-7.pdf

Thanks!

Jon D.

Interesting read, I’m not sure I buy there theory, just how fast is rapid? They don’t say. I cruse at about 55% power and after establishing 55% cruse above 3500 feet I slowly pull the mixture tell the engine starts to run ruff then slowly push it back in tell its smooth, at this point I am LOP on all four cylinders, I boar scope them every year and have none of the problems they are describing. My “Big pull” takes about 5 or 6 seconds.

 

[erich weaver]

The article provides so little information it is essentially useless for anything except inspiring fear. There are many many people going LOP using "the big pull" with no I'll effects and lots of good effects. If it makes you squirm, use the other method of leaning gradually by adjusting every 1000 to keep EGTs at takeoff temps.

Erich

 

[airguy]

The article provides so little information it is essentially useless for anything except inspiring fear. There are many many people going LOP using "the big pull" with no I'll effects and lots of good effects. If it makes you squirm, use the other method of leaning gradually by adjusting every 1000 to keep EGTs at takeoff temps.

Erich

Agreed. While I like ECI in general, this article (with absolutely zero test datapoints) is nothing short of useless.

I'm an engineer, I want data - not fearmongering. Gimme RPM, MAP, CHT/EGT, fuel flow, oil temp, airflow conditions since it was a teststand (velocity, temperature, pressure), fuel type, ignition type, ignition timing, and then put that on a graph versus time for the event that occured - THEN we can proceed to properly analyze what happened. This article doesn't even say if the engine was carburated or injected, for goodness sake! Completely worthless!

 

[John Clark]

Note the date...

..on ECI's letter. The "information" is 18 years old. Also very weak science, an issue with one engine on a test stand that could have been caused by a number of conditions.

John Clark ATP, CFI
FAA FAAST Team Member
EAA Flight Advisor
RV8 N18U "Sunshine"
KSBA

 

[bignose]

Where is the Red Box ?

Now, most important to know, where is really the so tamed Red Box to be avoided ?

To know the right thing to do, we should first know what NOT to do under no sircumstances, right ?

Lycoming recomends Peak under 75% and that's it ! So, where is the Edge?

That's the question... Where is RED BOX ?

 

[airguy]

Now, most important to know, where is really the so tamed Red Box to be avoided ?

To know the right thing to do, we should first know what NOT to do under no sircumstances, right ?

Lycoming recomends Peak under 75% and that's it ! So, where is the Edge?

That's the question... Where is RED BOX ?

Here you go - read these columns by John Deakin. He does a much better job of explaining it than I do.

http://www.avweb.com/news/pelican/182544-1.html

 

[bignose]

So, now it's explained !

Here you go - read these columns by John Deakin. He does a much better job of explaining it than I do.

http://www.avweb.com/news/pelican/182544-1.html

Good reading, thanks a lot !

 

[bignose]

Be careful up there...

But still there are some substantial diferences.

Lets face it from this side:

http://www.lycoming.textron.com/support/troubleshooting/resources/SSP700A.pdf

Who is right ?

 

[DanH]

Lets face it from this side: http://www.lycoming.textron.com/support/troubleshooting/resources/SSP700A.pdf

The article seems to focus on LOP operation of turbocharged models.

I'm not advocating one view or the other. Personally I would like more data. I have written the Thunderbolt folks at Lycoming and asked when they will have a complete engine power chart available for my IO-390. Unlike power charts for other models, the current IO-390 chart has no detonation limit line.

 

[airguy]

The article seems to focus on LOP operation of turbocharged models.

I'm not advocating one view or the other. Personally I would like more data. I have written the Thunderbolt folks at Lycoming and asked when they will have a complete engine power chart available for my IO-390. Unlike power charts for other models, the current IO-390 chart has no detonation limit line.

Agreed - I'm a data kind of guy. Deakin has it, Lycoming does too - but Deakin shares it and Lycoming won't. Secrecy doesn't buy much faith.

 

[F1Boss]

Wottsa matta U?

A balanced fuel delivery system is an absolute requirement for LOP ops, and I will assume you have made the adjustments to balance your system. The rest is easy.

Simply put, the Red Box contains the conditions that will generate 400F+ CHTs. 400F+ CHTs are part of the requirement to generate pre-ignition, which is a requirement to generate thermal runaway. Thermal runaway generates a lot of repair bills...especially when 14 cylinders are involved.

So, keep your CHTs under 400F and you are good to go. This may be an over-simplification, but it is also pretty easy to remember.

MP/PM combo is not really something you have to pay close attention to - LOP @ max available power will work fine - just stay out of the Red Box, either ROP or LOP. Turbo'ed engines (their Red Box indicator is TIT) run along happily @30"+ all day - your NA engine will do the same.

The Big Mixture Pull involves going thru the Red Box, but the idea is to do it quick enough to keep the CHT below 400F. Once you get a feel for the fuel flow required to be LOP at the power setting in place when you make the BMP, it's easy. Get 'er close, and tweak it from there.

It was once explained to me like this: "Sonny boy, you can cool your engine with fuel, or air. Air is cheaper...."

Carry on!
Mark

 

[pierre smith]

Can you clarify some, Mark...

I read about the "Big pull" but it's difficult to know when to stop pulling, or do you keep on 'til the engine stumbles, the way we used to do Skyhawks and 150's?

Since Don at Airflow and I balanced the injectors pretty closely, I screw the mixture out and allow enough time to see a change and keep going until the first cylinder peaks and keep on leaning with the Dynon in "Lean" mode. So apparently, I reach peak and it stays there for a few seconds but my CHT's never go over 400F, while I continue on to LOP on all cylinders.

I don't think that I stay at peak long enough to do any damage because I'm usually a little under 23" anyway...around 22.8"

Thanks,

 

[F1Boss]

it takes a litel practice

Hey Pierre:

No worries - if your FF indicator is quick enough, you simply pull until it gets close to the number you know is correct. I was so familiar with Ol' 84 that I did it by volume (listened for the slight decrease in the exhaust note); not quite there with the 550 beast yet...so I watch the FF reading. I don't use the leaning feature anymore, as I move the mix lever too quickly, and the EGT doesn't get to peak. Could be that the system (Dynon), or the sensors, are too slow? This wouldn't be the case if the EGT never gets to peak....

BTW if your injectors are VERY closely matched, the engine never gets rough - it simply quits. Try it once -- your passengers will think it's very funny! NOT.

Carry on!
Mark

 

[airguy]

In my C-172 with IO360 and matched Gami-jectors, it never gets rough. I simply grab the mixture and pull it until the RPM begins to sag, then fine-tune it. From ROP to LOP is maybe 2 seconds, not even enough time for the EGT's to react. With a FP prop, I level out at my desired cruise altitude, and as the engine RPM reachs and exceeds 2550 with increasing airspeed I do the big pull.

With a good mixture distribution from well-matched injectors, you never feel the roughness from leaning - you just lose power. I can make my engine run smoothly at WOT and 2000 rpm just by leaning it, and it runs way cool. Standard cruise for me is 6k-8k, WOTLOP and control power by mixture to maintain the desired RPM. The throttle stays wide open from takeoff until descent, at about 5k' I go back to ROP and control power with the throttle. I normally true out 118knots on 7.7-8.0 gph.

 

[F1Boss]

In my C-172 with IO360 and matched Gami-jectors, it never gets rough. I simply grab the mixture and pull it until the RPM begins to sag, then fine-tune it. From ROP to LOP is maybe 2 seconds, not even enough time for the EGT's to react. With a FP prop, I level out at my desired cruise altitude, and as the engine RPM reachs and exceeds 2550 with increasing airspeed I do the big pull.

With a good mixture distribution from well-matched injectors, you never feel the roughness from leaning - you just lose power. I can make my engine run smoothly at WOT and 2000 rpm just by leaning it, and it runs way cool. Standard cruise for me is 6k-8k, WOTLOP and control power by mixture to maintain the desired RPM. The throttle stays wide open from takeoff until descent, at about 5k' I go back to ROP and control power with the throttle. I normally true out 118knots on 7.7-8.0 gph.

I gotta ask - is that the Lyc 360 or the TCM 6 cyl unit? With your methods, you'll never wear out that throttle cable!

Carry on!
Mark

 

[bignose]

The Truth is:

Ok, after so long time searching, and thinking about the importance of the issue, I came to this conclusion: Here is the Holly Bible, writen by Textron Lycoming itself for smal, normaly aspirated, four cylinder, flat opposed engines, like those found on most RVs.

There is no such thing as a Red Box, as long as you respect four very simple rules of thumb: 000 - 100 - 200 - 400 !!!

000: Peak EGT for best economy cruise under 75% Power

100: Degrees Fahrenheit ROP for max power cruise under 75% Power

200: Never exceed Oil Temperature

400: Never exceed CHT

Take off and climb (till 5000 feet): Full power and full rich...

Lycoming does NOT recomend LOP under no circumstances !!!

As simple as that !!! I will follow what the factory recomends ... Certainly the mother knows her Baby better than anyone else...

Here it is:http://www.lycoming.com/support/tips-advice/key-reprints/pdfs/Key Operations.pdf