Question about reading performance and fuel data

[CharlieDeltaUK]

When reading data about fuel burn, it looks like the data is quoted by reference to two variables:

Best economy or best power
and
% power

For example:

For best economy cruise:
8.5 GPH at 75% power
7.5 GPH at 65% power
6.6 GPH at 55% power

For best power:
10.0 GPH at 75% power
8.8 GPH at 65% power
7.8 GPH at 55% power

I'm confused because, for any given % power (ie throttle setting), I would have assumed that the net result is either an economy cruise, or a best-power setting, or somewhere between the two. But, I don't see how, for example, a 75% power setting can be applicable to both 'best economy' and 'best power'. I've doubtless got some fundamental misunderstanding here (I'm still a student) which I'd like to clear up.

Thanks as ever for your help.:confused:

[BackPacker]

The difference is in proper leaning. At anything other than best economy, you are wasting fuel as coolant for the engine.

Unfortunately under certain circumstances you will have to waste that fuel, otherwise you cook the engine, or will not be able to obtain the power required.

Most of this is in aircraft technical. Did you go through that book already?

[englishal]

75% power is....75% power so I don't understand the two tables you posted?

[dont overfil]

Wobbly prop?
Bipolar engine?
D.O.

[peterh337]

The 'best economy' and 'best power' means different things in different POHs.

It should probably mean "peak EGT', and approximately 100F ROP which delivers best power at a cost of about 10% worse fuel consumption.

But in the days when most POHs were produced these operating conditions were not as well understood as they are today, and virtually all of them would have been produced simply by reference to engine manufacturers' manuals rather than by having an accurate fuel totaliser in a test aircraft, so you need to take it all with a pinch of salt.

[CharlieDeltaUK]

I see that the table is annotated "Best Power Mixture (Mixture Leaned to 100dgrees F rich of peak EGT)

[miroc]

For best power:
10.0 GPH at 75% power
8.8 GPH at 65% power
7.8 GPH at 55% powerBest power is a term describing the highest power an engine can (or is allowed to) deliver at given conditions and is certificated to do so.

Best power is experienced at full throttle and full rich mixture at sea level (up to ca 5000 ft density altitude). Leaning not allowed, strictly full rich.

Flying high or operating at high level airports (5000+ ft dens. alt.) best power is reached by full throttle and leaning for highest RPM (regardless fixed or constant speed prop if the plane is not moving). Red knob to the wall gives a mixture too rich for best power output at this conditions.

For best economy cruise:
8.5 GPH at 75% power
7.5 GPH at 65% power
6.6 GPH at 55% powerBest economy is a term describing a condition of the engine at flight giving the lowest consumption for distance flown.

This condition is set by leaning the engine to the peak EGT for the particular power output.

The point here is: if I lean to peak EGT and I am getting 2400 RPM on fixed prop, OAT is 10 degC, then the power output is 65% according to POH (just an example), I have best economy condition. And the fuel flow should be near the lets say 7.5 GPH.

But, I don't see how, for example, a 75% power setting can be applicable to both 'best economy' and 'best power'. I've doubtless got some fundamental misunderstanding here (I'm still a student) which I'd like to clear up.If you are flying at a setting where your engine is giving 75% power at peak EGT you have best economy conditions.

Now imagine you just suddenly enrich the mixture to 80 degF rich of peak and pull the throttle a bit. The engine is working in a different regime, running richer but throttled to 75% again. Same power output, different combustion conditions. You can have 75% at both settings. However, this second case is not best power, because you throttled down to 75%. The first case was best economy.

PM me if still not clear.

Miroc

[peterh337]

When I first got my TB20 in 2002, I too was puzzled by the two tables.

I asked myself "why would anybody fly at best-power doing say 140kt when they can fly at best-economy also doing 140kt, and use less fuel".

Neither Socata nor the dealer (Air Touring) were able to answer.

I now think that Socata just pulled those figures from the Lycoming performance charts e.g. this (http://www.peter2000.co.uk/aviation/misc/io-540-fig-3.17-sea-level-and-alt-performance.jpg) or this (http://www.peter2000.co.uk/aviation/misc/io-540-fig-3.16-fuel-consumption.jpg).

I am 99% sure they never got them from actual flight testing, because I am very very sure that the original test aircraft were not fitted with precision fuel totalisers... why should they be (years before the instruments became common in GA and about 20 years before they became a TB factory option... and when they did become a factory option they stuck the transducer in the wrong place where flow turbulence caused a 20-30% error and that is how they shipped them :ugh: ) when Lycoming provide you with a nice table which you can just plug into the POH and get the whole lot certified as-is?

The upshot of this was that the POH data was probably accurate from some 1960s Lycoming engine bench tests but was almost useless for working out the real range of the aircraft in different conditions.

Fortunately, by judicious operating practices, and with an accurate flowmeter, I can now get some 20% more range out of it than the POH and the sales brochures suggest.

My advice is to take POH data with a huge pinch of salt. And if you want to fly real distances, get a fuel totaliser installed. It will be the best 1-2k ever spent (alongside TKS prop de-ice :) ).

[BackPacker]

I asked myself "why would anybody fly at best-power doing say 140kt when they can fly at best-economy also doing 140kt, and use less fuel".

Neither Socata nor the dealer (Air Touring) were able to answer.

Prolonged full-power climb with a normally aspirated engine? After all, you'd lean for best power in those circumstances. (And at some altitude, the best you could achieve might be 75% engine power when leaned for best power, but only 65% or 70% when leaned for best economy.)

I think there's an example in the PA28 POH that details the fuel burn calculation for a fictitional flight, taking into account taxi, climb, cruise and descent with all the different fuel burns from the various tables in the book. Climbing is (obviously) done at best power mixture, while the cruise is done at best economy.

On the other hand, the R2160 POH only has instructions to lean for best power, and tables for the associated fuel burn. Due to the incredible crudeness of the mixture knob, resulting in almost immediate rough running if you lean beyond best power, leaning for best economy is virtually impossible and not covered in the POH whatsoever.

[englishal]

I still don't understand how "75% best power setting" can be better power than "75% best economy setting" when they are both at 75% rated HP of the engine......Unless someone if flying like a numpty and leaving the red knob full forward all the time that is. In which case the tables should be labeled:

For Moron Pilot (leave the red knob alone)
&
For proper Pilot (adjust the knob as required)

:}

[mm_flynn]

englishal (I am sure you know this already but just answering your rhetorical question)

there are typically three power settings a pilot (of a carbureted basically instrumented aircraft ) can use.

1 - leave the red knob fully forward (this will typically achieve less than book power except at near sea level density altitudes - particularly unfortunate from a back country airport during the summer in the Rockies)
2 - pull the red knob back to achieve best power and the black one to set the desired rpm
3 - pull the red knob back some more to achieve best economy and don't pull the black one quite as far.

2 and 3 can achieve the same power output, however,
3 will result in a lower ceiling as for each power setting you need more air (I.e. are less throttled)
3 will also typically result in higher CHT and internal pressures. At the high end of power output, combined with a hot day and low cooling airflow can result in pre-ignition.

obviously those who fly injected and instrumented aircraft have a 4th option which is to generate the power LOP, which has the benefit of low CHT and internal pressures coupled with low fuel consumption (of option 3)

[peterh337]

It is very true that to reach the aircraft's operating ceiling you need to go to the best power mixture i.e. about 80-100 F ROP.

In the TB20 (IO540-C4) I have to do this to get above about FL170 in ISA conditions. Then one can reach FL200.

Otherwise Englishal is right - only a moron is going to fly at a few thousand feet at 100F ROP at say 65% power when they can fly at peak EGT and be burning some 10% less juice. But imagine yourself flying some old heap with no instruments... that is still the bulk of today's GA.

BTW, I don't believe LOP gives you more MPG, compared to peak EGT, other than as a result of flying slower :) I did some tests, at a constant IAS, RPM, etc and any difference was below the measurement resolution which was about 1%.

[BackPacker]

Yep. That was what I was trying to say, and Englishal perhaps too. There is absolutely no reason to fly with the throttle partly closed and the mixture leaned for best power. Because you can achieve the same %power (thrust, RPM) with the throttle opened slightly wider and the mixture pulled back just a bit further, leading to a lower fuel burn (best economy mixture).

The only time you would be leaning for best power is when you want to have (duh) the best power available. That means with a wide open throttle. And that generally limits it to a climb (above 5000'), or the (rare) situation where you want to reach maximum level airspeed, regardless of fuel burn. (Take care not to overrev the engine then though.)

In all other cases you would be leaning for best economy. Or fly full rich of course (if maneuvering, or if the engine is delivering more than 75% power).

[mm_flynn]

BTW, I don't believe LOP gives you more MPG, compared to peak EGT, other than as a result of flying slower :) I did some tests, at a constant IAS, RPM, etc and any difference was below the measurement resolution which was about 1%.
Correct. Ignoring changes in RPM setting, 75% power achieved LOP vs. peak both achieve very similar Specific Fuel Consumption, that is the same power per gallon of fuel and hence the same MPG. This is fortunately true both in theory and practice.


As a note, prop rpm seems to make a substantial difference to MPG when LOP (I.e. generating 75% power with a lower prop rpm is more efficient than a higher prop rpm)

[peterh337]

Yes, though lower rpm gives better MPG at all the lower power settings too - at least all those at which the plane will still stay airborne :)

My favourite long range setting is FL100, full throttle, 2200rpm, peak EGT. TAS is about 140kt and you get almost 10hrs endurance.

[bookworm]

As a note, prop rpm seems to make a substantial difference to MPG when LOP (I.e. generating 75% power with a lower prop rpm is more efficient than a higher prop rpm)

I wonder if this is predominantly about the timing: LOP is slower burn, so the best matched (pressure peak vs mechanical advantage in the stroke) RPM is lower?