leaning a PA-38
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how much economy would you gain by leaning at 2000ft - half a gallon per hour?
Peak EGT operation is authorised by Lyco at below 75%.
How does one know where 75% lies? It is in the POH. For a fixed pitch prop it will be a specific RPM figure.
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Alternative view?
As far as I remember, and I will have to get into the Lycoming books on Monday to get you words from the approved publications, Lycoming are happy for the leaning lean of peak. The problem is that most non injected engines run a little rough LOP. so lets look at the numbers for a rich of peak operation with an O-360 Lycoming.
My O-360 burns 48 lts/hr at 75% full rich or 40 Lts/hr when slightly Rich of peak, as soon as I get into the cruise I always set the mixture at RoP.
I except that the cylinders will run a little hotter than if I had the setting at full rich but I save about 20% on the fuel bill, this saving over 1000 hours based on an Avgas price of £1,65 ltr is IRO £ 13,200.
Now using the assumption that I might be doing some damage to the cylinders by running hotter than if I used full rich mixture (remember I am using the Lycoming approved RoP drill) and have to replace all four cylinders at half engine life the cost of this would only be IRO £6000. So over the life of the engine (TBO 2000) always running the engine leaned RoP will save £ 20,400.
Running LoP would save another 2-3 Lts/hour but the risk to the cylinders is much higher on a non-injected engine so I don't think that extra risk of LoP leaning unless you have a well instrumented injected engine.
My O-360 burns 48 lts/hr at 75% full rich or 40 Lts/hr when slightly Rich of peak, as soon as I get into the cruise I always set the mixture at RoP.
I except that the cylinders will run a little hotter than if I had the setting at full rich but I save about 20% on the fuel bill, this saving over 1000 hours based on an Avgas price of £1,65 ltr is IRO £ 13,200.
Now using the assumption that I might be doing some damage to the cylinders by running hotter than if I used full rich mixture (remember I am using the Lycoming approved RoP drill) and have to replace all four cylinders at half engine life the cost of this would only be IRO £6000. So over the life of the engine (TBO 2000) always running the engine leaned RoP will save £ 20,400.
Running LoP would save another 2-3 Lts/hour but the risk to the cylinders is much higher on a non-injected engine so I don't think that extra risk of LoP leaning unless you have a well instrumented injected engine.
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For a fixed pitch prop it will be a specific RPM figure.
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Backpacker is totaly correct, that is why I got into the flight manual and extracted the numbers putting them on a graph that gives the RPM at 60% & 75% for all Density altitudes up to FL120.
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Now using the assumption that I might be doing some damage to the cylinders by running hotter than if I used full rich mixture (remember I am using the Lycoming approved RoP drill) and have to replace all four cylinders at half engine life the cost of this would only be IRO £6000. So over the life of the engine (TBO 2000) always running the engine leaned RoP will save £ 20,400.
With injected/GAMI engines it is certainly possible to control power over a very wide range with just mixture (from about 100%-60%) And if you wind back the RPMs you can get down to about 45% - which for me results in a slow sink. (Acknowledge none of that is relevant to a stock carb PA38 with minimal engine instrumentation).
