Sorry, LeSving, but that is hugely engine dependent
The reason engines can become rough when lean of peak is simply that the mixture – power curve is much steeper on the lean side than on the rich side, so small differences in mixture between cylinders make for a larger difference in power than on the rich side. So an engine that runs perfectly smooth rich of peak might not run evenly at peak or slightly lean of it as some (leaner) cylinders drop in power quickly.
So when you lean a non-instrumented carburetted cessna that way, you will actually end up slightly rich of peak.
On the ther hand, if you have an engine with well balanced cylinders (conti 550, or one with GAMI injectors) you cannot lean properly “by ear” – for example, on a SR22 or Columbia 400 you can lean to almost engine shut-off without even a stumble, and if you then enrich you will be very lean, way leaner then best economy, and also at quite low power.
there are simply too many different engines around for a generic leaning procedure to exist – even with full instrumentation.
Cobalt wrote:
The reason engines can become rough when lean of peak is simply that the mixture – power curve is much steeper on the lean side than on the rich side, so small differences in mixture between cylinders make for a larger difference in power than on the rich side. So an engine that runs perfectly smooth rich of peak might not run evenly at peak or slightly lean of it as some (leaner) cylinders drop in power quickly.So when you lean a non-instrumented carburetted cessna that way, you will actually end up slightly rich of peak.
On the ther hand, if you have an engine with well balanced cylinders (conti 550, or one with GAMI injectors) you cannot lean properly “by ear” – for example, on a SR22 or Columbia 400 you can lean to almost engine shut-off without even a stumble, and if you then enrich you will be very lean, way leaner then best economy, and also at quite low power.
there are simply too many different engines around for a generic leaning procedure to exist – even with full instrumentation.
Lots of RVs with carb are able to run LOP, Here are a few examples 10 years ago. Just because the average C-172 does not have EGT and CHT on each cylinder does not mean you cannot run LOP, it only means you have no real clue if you do or not. The only thing you know is that you run as lean as the engine allows. You can also apply carb heat and/or partial throttle, and you can run way LOP, maybe even more so than the average Lycoming with FI.
To run as you describe, you need to have 100% perfectly tuned resonant induction system and/or 100% individually perfectly tuned nozzles. Maybe the large Continentals in a Cirrus are 100% perfectly tuned I don’t know, but an IO-550 is far away from the average aircraft piston engine used on light GA.
I chose my words quite carefully.
Quoting myself: “there are simply too many different engines around for a generic leaning procedure to exist – even with full instrumentation.”
Also: “So when you lean a non-instrumented carburetted cessna that way, you will actually end up slightly rich of peak.”
Although I might be wrong and there are actually carburetted non-instrumented cessnas out there where you end up lean of peak that way…
Given the range of engines in existence, these generic discussions about leaning are similar to the generic discussions about the feasibility of a 180 after an engine failure on after take off, which depend on whether you are flying a motor glider one-up with half empty tanks from Luxembourg ELLX, or a flying anvil such as a PA32 or C210 at 10% above MTOW on a hot day out of 800m grass.
Surely this discussion should be about the (non) development of ECUs that do all this for us? Whichever way you look at it, it’s rather embarrassing that the 21st century pilot has to rely on GAMI injectors/EDM700 series/listening for rough running.
The Aerosance ECU we have had for 8 years is anyway system which desides with help of probes the mixture and gives some benefit in consumption. The best is, that we can use high octane Mogas or Avgas and our system has not yet failed. Unfortunately Continental is not anymore supporting this product so we have to cannibalise from failed installations if somewhere available.
After being involved in any number of discussions on automated fuel management for aircraft I’ve figured out the best answer for me is that I enjoy operating machinery, using my brain to do so, and that based on ownership experience with all sorts of stuff and a lot of painful lessons learned along the way (e.g. I’ve had a car in the shop with an unsolved engine management problem since July 2016), I’ve gravitated toward enjoying stuff that I can maintain independently and indefinitely. Prior to developing that preference I developed programming for fuel injection ECUs as a hobby and sold about 1800 units of my work for about $100 each. That was enough to buy five planes, but I bought only two and invested the rest.
Not everybody sees it my way, nor should they I suppose, but regardless instead of posting on leaning techniques they post on different issues.
Silvaire wrote:
That works great as long as you (1) always cruise at 65% power, and (2) have a table to hand showing MP and RPM for 65% for every altitude at which you cruise.
Actually, MP and RPM have nothing to do with the power when operating LOP over a very wide range of settings of MP and RPM. I do cruise at 65% power as I am interested in a specific performance not with arbitrary RPM or MP or EGT. If I want to cruise at 60% or 55% power, I will set the appropriate fuel flow to achieve that.
At higher altitudes, roughly above 9000 feet, 12.5 GPH is not LOP, so I would set a lower fuel flow to match the maximum power achievable or run at peak EGT.
When operating ROP, fuel is in abundance of that needed for combustion. IOW the number of molecules of oxygen delivered to the engine determine power. It would be nice to have an instrument that measured the O2 flow to the engine per unit time, as it could be calibrated in terms of % power. Alas, we don’t have such a O2 flow indicator, so we must use tables provided in the POH to determine power that include all of the parameters that affect the flow of O2, in particular RPM, MP, pressure altitude, and temperature.
When operating LOP, things are much simpler, since O2 is in abundance of that needed for combustion. IOW the amount of the fuel delivered to the engine per unit time determines power. That makes life simple, because I have a digital fuel flow indicator. Setting the fuel flow sets the power. My engine develops HP as a function of fuel flow by the equation of FF X 14.9 = HP. My engine develops 285 HP, so % Power = FF X 14.9 X100 / 285. So 12.5 GPH = 65%, 12.0 GPH = 60%, 11.5 GPH = 55% and so on. You can’t get any simpler than that.
2700 RPM, 27 in MP, fuel flow set to 12.5 = 65 % power. 2600 RPM, 24 in MP, fuel flow set to 12.5 = 65% power, 2500 RPM 22 in MP, fuel flow set to 12.5 = 65% power. 2400 RPM, 21 in MP, fuel flow set to 12.5 = 65% power. 2300 RPM, 22 in MP, fuel flow set to 12.5 = 65% power. What is important is that when the engine is operating LOP, fuel flow sets power. If you are operating at an altitude or combination of MP and RPM where 12.5 GPH is not LOP or peak, then 12.5 GPH will be ROP and the engine will be producing less than 65% power.
I flew about a hundred hours in a SR22TN. It allowed LOP operation up to 85% power. Climb was at full power and mixture full rich, about 36 GPH. At cruise, one retards the power lever to an RPM of 2500 and sets the fuel flow to set power by aligning the fuel flow with a blue reference line (LOP). The TIT is monitored for not exceeding the maximum value. There is a placard that says the following:
Avoid continuous operation with the fuel flow set between 18 GPH and 30 GPH with the MP above 26 in Hg.
Normal High-Power cruise 2500 RPM 29 to 29.5 in MP at 16 to 17.6 GPH.
These are all LOP operations. Power is displayed as % power.
NCYankee wrote:
Actually, MP and RPM have nothing to do with the power when operating LOP over a very wide range of settings of MP and RPM.
Note below the variation of BSFC (efficiency) lean of peak EGT, depending on how much air you add to a given fuel flow, i.e. depending on how lean of peak you go. A change in BSFC by definition means that engine output power varies with a given fuel flow. It appears to me based on that data that you need to get within a fairly close range of airflow at a given fuel flow to achieve a predicted power output when lean of peak, although obviously different combinations of MP and RPM will achieve the same air flow.
Silvaire wrote:
A change in BSFC by definition means that engine output power varies with a given fuel flow.
The graph is given for a specific MP and RPM combination. The abscisse is fuel flow. So the BHP curve shows power output as a function of fuel flow. Actually you do see that the lowest BSFC is obtained 50 LOP and the BHP curve is very linear on the lean side of 70 LOP – you get a 20 HP gain for 5 pounds per hour of fuel. The gradient of the BHP curve diminishes north of 85 pounds per hour which is located about 70 LOP.
The 14.9 HP/gph figure (for 8.5:1 compression) is an approximation and will not provide the exact same performance for all RPM values, but it works well enough. One can convince oneself of that in-flight by observing the resulting TAS and compare it to the power required to obtain that speed at that altitude in the POH.
