Leaning and Fuel Flow

[bstratt]

Learning about leaning in the Arrow and once again there is conflicting info.

POH says 10.2gph = 75% power, 9.2gph = 65% power. No adjustment in fuel flow for altitude. It also says 75% power cannot be achieved at 5,000 ft but at 4,000 is 24.4 inches.

My normal flights aren't long enough to really experiment but on the 5 hour flight to Norfolk, trimmed up at 6,000 ft, I had the time to do some.

The plane does not have an engine analyzer or even a digital EGT/CHT probe, just the old "needle", so leaning to find peak is an exercise. It does have a fuel flow meter. First discrepancy, fuel flow meter shows 10gph in the bottom of the 65% range and 14gph in the 75% range. So for 75% is it 10.2 or 14.0? The 10.2 book 75% equates to a low, 65% on the fuel flow.

Trying to find best power, I levelled off at 6,000 ft - full throttle was about 24 inches which I figured had to be close to the 75% power range. Pulled the fuel flow back to 9 gph indicated and waited 5 minutes to see where things steadied out, then 9.5, then 10, then 10.5. Best speed was at 10gph, with 10.5 slowing us down a bit. From this it appears the 10.2 book figure is accurate but this was at 6,000 ft. Would 14 be more accurate at sea level?

Without an engine analyzer, which fuel flow number would you start with?

Do you go with the book? The fuel flow? Or just lean for best speed?

 

[poadeleted20]

Well, you certainly did a good job of confirming the best power fuel flow -- best speed = best power (and also peak CHT, BTW). Given Lycoming's direction on the subject for fuel injected aircraft, leaning to book flow is the best bet if you don't have all-cylinder EGT's. Not being that familiar with Piper fuel flow indicator markings, I can't say what they mean or why they don't match up with the book flows.

 

[gismo]

Learning about leaning in the Arrow and once again there is conflicting info.

POH says 10.2gph = 75% power, 9.2gph = 65% power. No adjustment in fuel flow for altitude. It also says 75% power cannot be achieved at 5,000 ft but at 4,000 is 24.4 inches.

My normal flights aren't long enough to really experiment but on the 5 hour flight to Norfolk, trimmed up at 6,000 ft, I had the time to do some.

The plane does not have an engine analyzer or even a digital EGT/CHT probe, just the old "needle", so leaning to find peak is an exercise. It does have a fuel flow meter. First discrepancy, fuel flow meter shows 10gph in the bottom of the 65% range and 14gph in the 75% range. So for 75% is it 10.2 or 14.0? The 10.2 book 75% equates to a low, 65% on the fuel flow.

Trying to find best power, I levelled off at 6,000 ft - full throttle was about 24 inches which I figured had to be close to the 75% power range. Pulled the fuel flow back to 9 gph indicated and waited 5 minutes to see where things steadied out, then 9.5, then 10, then 10.5. Best speed was at 10gph, with 10.5 slowing us down a bit. From this it appears the 10.2 book figure is accurate but this was at 6,000 ft. Would 14 be more accurate at sea level?

Without an engine analyzer, which fuel flow number would you start with?

Do you go with the book? The fuel flow? Or just lean for best speed?

None of the above.

First, I suspect that your "fuel flow meter" is simply a pressure gauge connected to the fuel distributor which reads the pressure applied to the injectors and these are often rather imprecise WRT actual fuel flow. Typically the ranges of power shown on the gauge correspond to climb (rich end) and economy cruise (lean end). The rich side of those bands is generally a safe haven for a reasonbly shallow climb but I would use CHT is the primary indication of adequate fuel flow in a climb. The lean end is a scary place IMO unless you know you are running less than 65% power. Otherwise it's easy to be running some cylinders way too hot. Below 65% it's hard to hurt an engine with the mixture control unless you have a baffling problem and/or are flying too slowly to get good cylinder cooling.

Second, when running ROP there is a range of mixtures that will allow the engine to produce a particular power output. On the lean side this is less true (IOW when LOP power out and fuel flow have a nearly fixed relationship independent of other factors).

Third, the concept of XX% power is often confused with MP/RPM/DA combinations that result in the specified power if the mixture is exactly "correct" (vs conditions that actually result in that power).

It is true that for a specific mixture such as best power (about 100 F ROP), best economy (typically peak to 20 F LOP), you should see consistent fuel flow for a given power output, but it's unlikely you will actually get the engine to produce the same exact power under different conditions especially with a single probe EGT. One thing is certain here: If you are flowing 14 GPH at best power mixture (down low), your engine is developing a lot more horsepower (like 34% more) than it is up higher when consuming 10.5 GPH with a similar mixture.

Continental and Lycoming sell inexpensive copies of their engine operating manuals that come complete with graphs and nomograms for determining engine power under various conditions (although they generally assume best power mixture) and the appropriate fuel flow for any power output. If you had an accuarte means of measuring fuel flow you could probably use that data to set fuel flow, but I wouldn't try that with the "meter" you have.

 

[Henning]

Learning about leaning in the Arrow and once again there is conflicting info.

POH says 10.2gph = 75% power, 9.2gph = 65% power. No adjustment in fuel flow for altitude. It also says 75% power cannot be achieved at 5,000 ft but at 4,000 is 24.4 inches.

My normal flights aren't long enough to really experiment but on the 5 hour flight to Norfolk, trimmed up at 6,000 ft, I had the time to do some.

The plane does not have an engine analyzer or even a digital EGT/CHT probe, just the old "needle", so leaning to find peak is an exercise. It does have a fuel flow meter. First discrepancy, fuel flow meter shows 10gph in the bottom of the 65% range and 14gph in the 75% range. So for 75% is it 10.2 or 14.0? The 10.2 book 75% equates to a low, 65% on the fuel flow.

Trying to find best power, I levelled off at 6,000 ft - full throttle was about 24 inches which I figured had to be close to the 75% power range. Pulled the fuel flow back to 9 gph indicated and waited 5 minutes to see where things steadied out, then 9.5, then 10, then 10.5. Best speed was at 10gph, with 10.5 slowing us down a bit. From this it appears the 10.2 book figure is accurate but this was at 6,000 ft. Would 14 be more accurate at sea level?

Without an engine analyzer, which fuel flow number would you start with?

Do you go with the book? The fuel flow? Or just lean for best speed?

Well, you went through the right steps on the test flight and found your answer as to flow. As to power, remember, the book was probably written calculated to a 29.92 day, higher atmospheric barometric pressure will also lead to equally higher manifold barometric pressure base, this is a possible explantion for the altitude at which 75% power was made was and your top speed fuel flow of approx 10.2.

Since the markings on the guage dissagree with the AFM for that plane, I'd look and see if that is the correct instrument, because to me, those sound like 235hp fuel flow numbers. Someone may have replaced the instrument with a boneyard one out of a Dakota or something.