Fuel Burn as a Function of Weight

[Andy Young]

I’m wondering if anyone has ever done any calculations (or has obtained real-world numbers from experience) on how much carrying extra weight costs in fuel burn. I suppose that could also be expressed as reduced cruise speed at a given power setting as a function of weight. I imagine CG plays into it as well...

Here’s why I’m wondering: On my semi-annual trips between the Lower 48 and Alaska, I often find myself wanting to fill the aux tanks to take advantage of a lower fuel price in a particular spot, or to avoid a fuel stop, even when I don’t technically need the extra range. I’ve always heard that it “takes fuel to carry fuelâ€￾ (this is very commonly heard in the 135 world) but on this most recent trip, I got to wondering how much extra it really burns to carry a few extra hours worth. I’m sure it takes SOME extra fuel to carry more over the same distance; I’m just wondering if it’s as significant as it has often been professed to be.
There are other commonly held aviation beliefs that don’t stand up well to scientific or mathematical scrutiny, and I can’t help but wonder if this might be one of them. I haven’t noticed any difference in cruise speed between having two tanks full or all four, but I’ve not made a real study of it either. I do know that a further-aft CG increases cruise speed. The tanks are very close to the empty CG, so the extra fuel won’t shift CG much.

Anyway, what do you all think? Any math geeks out there dug into this?

[Kirk]

In the turbine world, I remember a wag of 3% additional burn each thousand pounds per hour. So an extra 1,000 pounds of fuel would increase your burn 900 pounds on a 3 hour flight.

It’s a much bigger difference for turbine aircraft though, due to how much the burn decreases with increase in altitude. So, logic tells me our difference is much less than 3% per hour. I’m guessing 1% or less.

I can’t say that I’ve noticed a difference unless I was already heavily loaded and the flight was more than 2 hours. Worth considering if it means we can’t climb as high into a nice tailwind.

Sorry I can’t give a more scientific answer. If I had to guess, I’d say it is maybe a 1% per hour penalty at high gross weight.

Kirk

[Andy Young]

Thanks Kirk,

So if I understand the math on your guess correctly, it would mean that to carry an extra 26 gallons (the volume of my aux tanks) it might cost me .26 gallons per hour to go the same speed. Although since I tend to run at the same cruise power setting all the time (22 squared), it would mean a drop in speed, not an increase in fuel burn. I wonder if the same percentage would apply to the speed. If so, it’d be about 1.2 knots. Certainly not enough to matter.

I do know that I don’t seem to have any trouble getting to the altitudes I want to get to, even if I am right at gross weight, though I rarely find need to go much over 15,000-16,000, and usually not much over 12,000.

[Kirk]

Andy,

Yep, your exact math on my inexact guess looks right. Pretty much what I think I’ve seen over the years, many times flying the same route at the same settings.

Like you, I fly the same power setting then see what altitude looks best. About the same as the altitudes you mentioned. A couple thousand lower nowadays since my engine is getting a bit tired.

For me, optimum altitude seems to be as high as you can fly with the bottom of the wing flat to the horizon at a given altitude. Flying an M5, so no reflex flaps to flatten the angle of attack out. Instead, if I am low enough and fast enough to start to see the bottom of the wing at a slightly negative angle to the horizon, I either climb or pull the power back a bit. Usually run at 65%. M5-235C with the O-540J1A5D.

Kirk

[andy]

This sounds like a linear algebra problem due to all of the variables. The solutions would vary depending on the goal: maximum range? minimum time? minimum cost? Or some optimal solution of all three. Too complicated to try and do this before each flight. When I flew to Idaho, I filled the mains and aux tanks at each stop when it was only me and all my survival gear in the airplane. When I added a 220 pound guide in the copilot seat and had to worry about takeoff and climb performance in the mountains and canyons at different density altitudes, I had to think harder about how much fuel to add to the aux tanks. I decided on full mains plus 5 gallons in each aux tank for a total of 50 gallons. That gave me an endurance of about 4 hours. Once I took off I reduced RPM during climb to 2400 and reduced throttle to climb at a leisurely 200 - 400 fpm rather than 25" and 2500 RPM. That saved a lot of fuel since we did a about 20 takeoffs and landings on back country airstrips each day.

My favorite strategy to optimize range on the way there and back was: fill all 4 tanks at each fuel stop; fly as high as possible to find favorable winds (also to find cooler temps and less turbulence), increase true airspeed and reduce drag; lean the engine 50 degrees rich of peak with my JPI EDM800; set RPM and throttle to 94 mph indicated airspeed.

[Andy Young]

Hmmm....that made me have to think about what my goal really was. I realized I wasn’t really clear on that. For the most part, I put in what I need to feel comfortable about getting the range I want, given the terrain and weather, up to and including filling all four tanks. Where the question came from was just musing on the fact that most 135 operators/owners preach carrying as little fuel as possible, as “It takes fuel to carry fuelâ€￾, the theory being that you’re wasting gas carrying extra around. Got me to wondering if the extra you’re burning to carry an extra, say, 25 or 30 gallons around is really a significant number.

On another note, interesting that you run at 50* rich of peak. My own research (reading Mike Busch’s stuff, talking to Lycoming and Continental tech reps, etc.) indicates that 50* ROP is the absolute worst place to run, as it gives the highest combustion pressures (thus highest temps). Of course, you indicate that you’re doing this at cruise, likely less than 75% power, so you can’t hurt anything at that power setting no matter where you set the mixture. Not criticizing at all here, by the way; still plenty to be learned on my part, which is why I like having these conversations. Given your experience, I’m sure what you’re doing has been working well for you over many, many hours.

[andy]

I like Mike Busch's articles, too. I re-read the article that you referred to at https://www.avweb.com/news/savvyaviator ... 162-1.html. I think Mike was mostly talking about higher performance fuel-injected and turbocharged engines with modern engine analyzers where you can and should run lean of peak to reduce internal cylinder pressures.

Mine is a carbureted, normally aspirated engine. Per the Lycoming Operator's Manual page 3-9 I lean until the engine runs rough then enrich until it runs smoothly. The EDM800 usually shows peak EGT around 1510F so I'm enriching from there. I watch the CHTs at that point since they tend to run hot with the close cowling and slower airspeed. My goal is to richen the mixture until I get CHTs down around 400F, which both Lycoming and Mike agree with. That usually works out to the hottest CHT around 1470F. When it's hot out I sometimes have to enrich the mixture more to cool the CHTs. That might work out to be 50 degrees rich of peak but I'm not really looking for that number so much as watching the CHTs.

I almost always cruise around 60% power so there's little danger of over-stressing the engine. I've been operating the engine this way since the 2004 major overhaul and haven't had any problems with it. Compressions are still 72 or higher. I'm not saying that there isn't a better way to operate an O-360-C1F, but everyone seems to agree that keeping CHTs at 400F or below in economy cruise is important for engine longevity. Once that goal is achieved, then it's a matter of leaning to get the best economy.

[andy]

Oops! I meant the hottest EGT around 1470F.

[Andy Young]

Ah, that all makes sense to me (except the “CHTâ€￾ at 1470 ). Mine is a fuel injected 540. I have an engine analyzer; I’ve found that doing the “Lean until it’s rough, richen until its smoothâ€￾ puts me right at peak EGT, which is where the Lycoming guys and MB said is best. That’s about 1410 on my hottest cylinder. The 540 is a different beast heat-wise than the 360; I rarely see CHTs over 310*, and most of them run in the high 200s. So no need to use fuel to cool on my engine. I do note that running at peak EGT drops my power slightly from when it is at 50* rich, but I get a more significant drop in fuel burn than I do in power. This is all at 75% power or less (Lycoming says you can lean however you want below 75% and you can’t hurt it). At higher power settings, I run much richer.

At the end of the day, it’s all about having happy CHTs, as you indicated. Sounds like the 360 requires different strategies to do that than the 540 does.

[TxAgfisher]

This is interesting, I just cut up a perfectly good egress scoop trying to conquer CHT’s on the IO-360. I usually set for highest EGT to be around 1350 and have found since moving up in the world (in elevation only lol) that CHT’s have been a struggle. My last trip over the divide ambient was in the 50’s at altitude and CHT’s were tough to keep under 420. I’ve heard 360-380 is the place to be so you saying 400 makes me feel better. I did a test run Saturday and at 50 degrees and 7500ft my hottest cylinder was 382.

I mocked up a larger egress scoop after talking to Maule which I will fly on Saturday but I am thinking the angle it is at will cause turbulence and not help my cause. Trial and error...

[andy]

One of the things that I've noticed with the EDM800 engine analyzer is that the location of the EGT probes and how new they are makes a difference in EGT reading. I had a probe go bad years ago and replaced it with a new one. The EGT reading was about 20 degrees different. JPI's installation manual said to install the probes 2" - 4" from the exhaust flange so I installed mine exactly 3" from the flange. Just a few millimeters difference can change the temperature reading. Since then my exhaust system has been replaced a couple of times by mechanics who probably re-installed the probes in slightly different places. We all probably have them in slightly different places so we should expect to have differences in EGT readings.

[Andy Young]

One of the things that I've noticed with the EDM800 engine analyzer is that the location of the EGT probes and how new they are makes a difference in EGT reading. I had a probe go bad years ago and replaced it with a new one. The EGT reading was about 20 degrees different. JPI's installation manual said to install the probes 2" - 4" from the exhaust flange so I installed mine exactly 3" from the flange. Just a few millimeters difference can change the temperature reading. Since then my exhaust system has been replaced a couple of times by mechanics who probably re-installed the probes in slightly different places. We all probably have them in slightly different places so we should expect to have differences in EGT readings.

That’s just one of the many reasons that EGT numbers themselves are meaningless, and there’s is no max for EGT (unless you have a turbo, in which case it’s TIT) like there is for CHT. What matters is the DIFFERENCE between EGT readings in that particular engine, and the difference between max EGT for THAT engine and where you run it.

[Andy Young]

This is interesting, I just cut up a perfectly good egress scoop trying to conquer CHT’s on the IO-360. I usually set for highest EGT to be around 1350 and have found since moving up in the world (in elevation only lol) that CHT’s have been a struggle. My last trip over the divide ambient was in the 50’s at altitude and CHT’s were tough to keep under 420. I’ve heard 360-380 is the place to be so you saying 400 makes me feel better. I did a test run Saturday and at 50 degrees and 7500ft my hottest cylinder was 382.

I mocked up a larger egress scoop after talking to Maule which I will fly on Saturday but I am thinking the angle it is at will cause turbulence and not help my cause. Trial and error...

Where in Colorado are you? I’ll be back in Boulder soon; if you’re going to be in that area, I’d be happy help if I can. We can put our heads together, compare scoop shapes on the planes, toss ideas around etc. There are several Maules at Boulder, but they are all 540s so no direct comparisons to yours, unfortunately.

[TxAgfisher]

Where in Colorado are you? I’ll be back in Boulder soon; if you’re going to be in that area, I’d be happy help if I can. We can put our heads together, compare scoop shapes on the planes, toss ideas around etc. There are several Maules at Boulder, but they are all 540s so no direct comparisons to yours, unfortunately.

Yeah, I don't know of any other than Patrick. He said they had a similar issue with their M4-180V but it'd be good to get to know some more local folks. The airplane is at Platte Valley and I live over in Commerce City.

[Andy Young]

Where in Colorado are you? I’ll be back in Boulder soon; if you’re going to be in that area, I’d be happy help if I can. We can put our heads together, compare scoop shapes on the planes, toss ideas around etc. There are several Maules at Boulder, but they are all 540s so no direct comparisons to yours, unfortunately.

Yeah, I don't know of any other than Patrick. He said they had a similar issue with their M4-180V but it'd be good to get to know some more local folks. The airplane is at Platte Valley and I live over in Commerce City.

Yeah, and the M4-180 is gone; I think that’s the one I delivered to Florida a few years ago. Still, we might be able to learn something by looking at our lower cowl shapes on the 235s vs yours.