Theories of fuel economy.

[stratobee]

I thought I'd start a new thread on this subject as it was getting off topic in the speed thread.

In theory all aircraft would show best fuel economy (endurance) at best glide (or Vy) speed. That's when the curves meet.

I came across this interesting paper. It talks about something called the Carson Speed, which is defined as "Least amount of fuel per knot of speed" or Fuel Flow/Knot. It's calculated as being roughly 1.3xVy (or best glide) but taking into the consideration the GS, which is very interesting. Example: My aircraft's Carson Speed would be 152kts. If you have a tailwind, you reduce the MP until your GS shows the Carson Speed and vice versa if you have a head wind.

I am unclear if this Carson method produces less total fuel consumption over a given distance compared to flying at best glide or Vy. I'm assuming it does, depending on winds, but I haven't started using my brain quite yet this morning (some would argue never).

http://pointsforpilots.blogspot.com/2013/07/flying-frugally.html

Also, again, here's a link to Klaus Savier's Vari-EZE getting 45mpg at 207mph. Which is kind of impressive.

http://www.wired.com/autopia/2009/11/45mpg-at-207mph/

For you that have fuel flow meters - have you ever reduced to Vy or best glide speed and seen what the fuel consumption is there? Be interesting to know for various types.

 

[Sac Arrow]

Vy (or best glide speed, if it happens to be different) is the most efficient aerodynamic speed, but remember that engines operate most efficiently at wide open throttle, near full power, so the best economy will be somewhat faster than Vy.

Most airplane manufacturers have figured it out and publish settings for best economy.

 

[FormerHangie]

Then, of course, you have to consider what the wind's doing to your groundspeed. Since on the average there is a headwind most of the time, you'd almost always want to fly a little faster to reduce the wind's effects.

 

[N747JB]

I've never been a fan of going slow to save fuel, if I wanted to save money, I'd drive. my guess would be that most NA piston airplanes will do best at high altitude with WOT generating 55-60% power, flying LOP. I'm sure our group of engineers can explain it better or give some scientific data.

 

[Sac Arrow]

.

http://pointsforpilots.blogspot.com/2013/07/flying-frugally.html

I kind of have a few problems with this article...

 

[RV10flyer]

I seem to get the best NMPG(16) at 12,500 and it does not change as much as I would have guessed with airspeed.

 

[Jeff K]

I try to fly near carson's speed, at WOT, LOP, at the lowest RPM I can. On my Aztec typically that will be 155-160mph indicated, 1900-2000 rpm 25" 16-18gph

http://youtu.be/g18vHgiQ3OY

 

[Ted]

I try to fly near carson's speed, at WOT, LOP, at the lowest RPM I can. On my Aztec typically that will be 155-160mph indicated, 1900-2000 rpm 25" 16-18gph

http://youtu.be/g18vHgiQ3OY

I hope you took a look at the manual limitations in the owner's manual. I'm thinking yours is a C by the panel? On my Aztec D, it prohibited operation above 24" below 2300 RPM. Torsional vibrations get really annoying when catastrophic failures occur.

I ran 2300 RPM, WOT, and got 21 GPH combined LOP doing 150-160 KTAS (summer-winter)

 

[Ted]

At high altitude, you'll end up burning less fuel. Specifics on that vary somewhat. For reasons I don't fully understand, the Aztec didn't like running leaner than 10 GPH per side, regardless of altitude. It might have had something to do with the ignition, which Klaus would surely give as the reason. The 310 shows more variation - I've gotten it down to 10 GPH per side (with 300 HP engines instead of 250) at 13k.

Klaus is quick to put down magnetos, but they actually make a very good quality spark. They lack the ability to adjust timing, and that is a negative for them. I agree that magnetos should be dumped in favor of electronic ignition, but I believe the reason is for reliability and repair cost.

 

[kyleb]

At high altitude, you'll end up burning less fuel. Specifics on that vary somewhat. For reasons I don't fully understand, the Aztec didn't like running leaner than 10 GPH per side, regardless of altitude. It might have had something to do with the ignition, which Klaus would surely give as the reason. The 310 shows more variation - I've gotten it down to 10 GPH per side (with 300 HP engines instead of 250) at 13k.

Klaus is quick to put down magnetos, but they actually make a very good quality spark. They lack the ability to adjust timing, and that is a negative for them. I agree that magnetos should be dumped in favor of electronic ignition, but I believe the reason is for reliability and repair cost.

To date, anecdotal data in the experimental world doesn't indicate a *realized* reliability or repair cost savings. I've seen (IMO) a disproportionate number of frustrating problems with Electroair and pMag systems, which have been the most popular EI's in the RV world. That doesn't mean Lightspeed or someone else doesn't have a more reliable product, but my sense is that the aircraft magneto was very important to the military through at least the 1950's, and a lot of money was spent developing the technology. There are no high volume, deep pocket buyers for today's EI's and the investment in development expenses reflects that reality.

 

[Jeff K]

I hope you took a look at the manual limitations in the owner's manual. I'm thinking yours is a C by the panel? On my Aztec D, it prohibited operation above 24" below 2300 RPM.

Hmmm..... interesting. Yes I looked before I did it, it's allowed (in fact recommended) so I wonder if they had some issues in the later aircraft?

In the Aztec C the published lower limitations are 1800-2000rpm/25" and 2000-2300rpm /27". In in the flight manual they recommend the lowest RPM for best fuel economy... specifically mentioning a lower limit of 1800rpm.

Do you still have your POH for your D? I'd like to investigate this.

At high altitude, you'll end up burning less fuel. Specifics on that vary somewhat. For reasons I don't fully understand, the Aztec didn't like running leaner than 10 GPH per side, regardless of altitude. It might have had something to do with the ignition, which Klaus would surely give as the reason.

I *rarely* cruise my Aztec above 10gph per side (20 gph total)... most typically 9gph per side (18gph total) and it's quite happy to run at 7gph per side (14gph) so I'm thinking maybe you had some issues or something intrinsic with the D aztec? I do have fine wires and had my fuel servos rebuilt though. Both sides are magnetos.


p.s.

I couldn't find a D Aztec POH online but I did find a E online:

http://www.niedra.com/pdf/PIM/MdV_Piper_PA23_250AztecE_FULL.pdf

See page 161. In the power table it lists 2200/26" as one setting. It also has the same limitation as my C listed (i.e. "DO NOT EXCEED 27" MANIFOLD PRESSURE BELOW 2300 RPM or 25" BELOW 2000 RPM")

What it doesn't have is graphs going down to 1800rpm as my C manual does. My C Aztec just republished the ones from the Lycoming manual

http://www.helium-group.com/wp-content/uploads/2010/04/Lycoming-IO-540-POH.pdf (page 47)

 

[Ted]

To date, anecdotal data in the experimental world doesn't indicate a *realized* reliability or repair cost savings. I've seen (IMO) a disproportionate number of frustrating problems with Electroair and pMag systems, which have been the most popular EI's in the RV world. That doesn't mean Lightspeed or someone else doesn't have a more reliable product, but my sense is that the aircraft magneto was very important to the military through at least the 1950's, and a lot of money was spent developing the technology. There are no high volume, deep pocket buyers for today's EI's and the investment in development expenses reflects that reality.

Keep in mind, the magneto replacements that are in the experimental world I don't believe is of OEM quality. So that doesn't make it entirely an apples to apples comparison.

To get the best apples to apples comparison, I would say retrofit a dual Ford EDIS setup and then see. Wouldn't be too hard to accomplish. Just put in a redundant power source.

Hmmm..... interesting. Yes I looked before I did it, it's allowed (in fact recommended) so I wonder if they had some issues in the later aircraft?

In the Aztec C the published lower limitations are 1800-2000rpm/25" and 2000-2300rpm /27". In in the flight manual they recommend the lowest RPM for best fuel economy... specifically mentioning a lower limit of 1800rpm.

Do you still have your POH for your D? I'd like to investigate this.

I *rarely* cruise my Aztec above 10gph per side (20 gph total)... most typically 9gph per side (18gph total) and it's quite happy to run at 7gph per side (14gph) so I'm thinking maybe you had some issues or something intrinsic with the D aztec? I do have fine wires and had my fuel servos rebuilt though. Both sides are magnetos.

p.s.

I couldn't find a D Aztec POH online but I did find a E online:

http://www.niedra.com/pdf/PIM/MdV_Piper_PA23_250AztecE_FULL.pdf

See page 161. In the power table it lists 2200/26" as one setting. It also has the same limitation as my C listed (i.e. "DO NOT EXCEED 27" MANIFOLD PRESSURE BELOW 2300 RPM or 25" BELOW 2000 RPM")

What it doesn't have is graphs going down to 1800rpm as my C manual does. My C Aztec just republished the ones from the Lycoming manual

http://www.helium-group.com/wp-content/uploads/2010/04/Lycoming-IO-540-POH.pdf (page 47)

I don't have the manual for my D anymore (it went with the plane), but if your C manual allows it, then it's fine. Those limitations are typically a function of the engine/propeller combination, so maybe the Ds had a different propeller.

2300 RPM was the best for what I did with the plane, so that's the point at which the engine wasn't happy leaner than 10 GPH. To be honest, I was in enough of a hurry that to run it down at 1900 RPM or so with the speeds you're advertising would have been much too slow. The plane did have massive plugs, but I never experimented with running at lower RPM.

 

[Jeff K]

2300 RPM was the best for what I did with the plane, so that's the point at which the engine wasn't happy leaner than 10 GPH. To be honest, I was in enough of a hurry that to run it down at 1900 RPM or so with the speeds you're advertising would have been much too slow.

Don't forget Carson's speed is indicated... not true airspeed. At altitude my true airspeed was about the same as you listed.... at about 3+ gph less fuel burn. 160mph indicated = 155kts TAS @6500 ft. The advantage of lower RPM's (if you can do it) is there is less swept friction for the engine to overcome. The other advantage of lower RPM's is when you are running LOP since the charge is burning longer... you'll contain it longer. For a given power setting at LOP... two RPM's, the lower RPM will give you a higher CHT and a lower EGT... this isn't a bad thing since it indicates more energy is being extracted from the charge burn.

 

[Jeff K]

For you that have fuel flow meters - have you ever reduced to Vy or best glide speed and seen what the fuel consumption is there? Be interesting to know for various types.

Missed this part of the question... haven't tried it yet on the Aztec (I suspect it will be below 6gph per side...) but I did try it on my Cherokee Six at a bit faster then Vy (which as I recall was 105mph.... I was doing some cruise climb experiments here... 115mph)

http://youtu.be/cotVB864sIc

I got between 9.7-10.0 gph at a IAS of 115mph. Now this was at a bit faster RPM then I generally run the Aztec + it was a carborated engine. So I might be able to get it down a bit lower on an injected PA32 + if I did it at Vy.

I'll make a point to try it on the Aztec next time I'm up and video it.

 

[Skylane81E]

Stratobee, wind, engine and airframe efficiencies all take part. I have found my plane to get about the same MPGs from 90-125 indicated. Now obviously going higher I can improve my TAS up to a point. I start to loose TAS starting around 7k depending on the conditions, remember I'm naturally aspirated. My engine likes to go fast and the plane likes to go a bit slower and in my case they more or less cancel each other out. I suppose my most efficient would be right at my absolute ceiling where WOT would only net me VY, but that would be north of 23,000ft as I've done over 21k density altitude and was still climbing at over 100fpm. That might be fine going down wind but how often is the wind at that altitude slower than single engine cessna VY?

On the topic of wind the rule of thumb is to fly fast into headwinds to minimize their effect and slow with tail winds to ride them longer. However in my case any. Reasonable cruise speed is about as efficient as any other so I'll just go for the faster ones.

 

[Ted]

Don't forget Carson's speed is indicated... not true airspeed. At altitude my true airspeed was about the same as you listed.... at about 3+ gph less fuel burn. 160mph indicated = 155kts TAS @6500 ft. The advantage of lower RPM's (if you can do it) is there is less swept friction for the engine to overcome. The other advantage of lower RPM's is when you are running LOP since the charge is burning longer... you'll contain it longer. For a given power setting at LOP... two RPM's, the lower RPM will give you a higher CHT and a lower EGT... this isn't a bad thing since it indicates more energy is being extracted from the charge burn.

There's also the OAT aspect of it, and your particular airframe. Mine had boots (hurts speed), lots of dents and dings (hurts speed), etc. What's your speed at 2300 RPM and 6500? Time of year matters. This time of year it'd be closer to 160.

And either way, a moot point for me. I wasn't going to operate in a MP/RPM range that was unapproved per the manual.

 

[Jeff K]

There's also the OAT aspect of it, and your particular airframe. Mine had boots (hurts speed), lots of dents and dings (hurts speed), etc. What's your speed at 2300 RPM and 6500?

I generally don't run it at the higher RPM's in cruise. Redline on the the IO-540-C4B5 engine is 2575 rpm. But at a given power setting the speed would be the same... just full consumption higher. However, you can operate at a higher power setting in the higher RPM range. What I'm saying however, for a given power setting, the lower RPM version of it was more efficient.

I'll however try it next time I'm up.

I also had boots.

And either way, a moot point for me. I wasn't going to operate in a MP/RPM range that was unapproved per the manual.

In the range I'm discussing, both the C and E models are approved. The engine is also approved. And I've never seen an issue of any vibration... if anything the lower RPM is smoother. You've got me very curious.... I've seen identical airframes (for example the 1970 PA32 vs. the 1974) have entirely different warnings in the POH. Often for very good reason yet it never results in a revision for the earlier manual. If you ever run across a D POH I'd like to confirm this and in the mean time I'll keep looking myself.

Thanks for the thoughts.

 

[Ted]

I generally don't run it at the higher RPM's in cruise. Redline on the the IO-540-C4B5 engine is 2575 rpm. But at a given power setting the speed would be the same... just full consumption higher. However, you can operate at a higher power setting in the higher RPM range. What I'm saying however, for a given power setting, the lower RPM version of it was more efficient.

I'll however try it next time I'm up.

I also had boots.

I'm talking about running at a higher power setting allowed by the higher RPM. Yes, lower RPMs will be more efficient for a given power setting.

In the range I'm discussing, both the C and E models are approved. The engine is also approved. And I've never seen an issue of any vibration... if anything the lower RPM is smoother. You've got me very curious.... I've seen identical airframes (for example the 1970 PA32 vs. the 1974) have entirely different warnings in the POH. Often for very good reason yet it never results in a revision for the earlier manual. If you ever run across a D POH I'd like to confirm this and in the mean time I'll keep looking myself.

Thanks for the thoughts.

Singles vs. Twins can have some dynamics there as far as vibration waves interacting between the engines that may have been considered.

As far as the torsional vibrations that are of concern, you won't feel those typically in the same manner you feel an imbalance vibration. They have to do with basically exciting the right frequencies and causing excessive stresses in the propeller or in the crankshaft, which can lead to failure of either one. When tested, the props are instrumented with a number of strain gauges to measure forces at any particular location.

I go by the manual for these things because I've been involved in the tests before and understand where it comes from. If the manual says a combination is good, I go with it.

 

[Jim Burns]

Hey Jeff,
I could fly my C in nearly any power/rpm combination, including all the way down to 1900 rpms as you do, however, my F absolutley hated an area around 2100. It felt like the engines would come right out of the mounts. 2000 was fine, but the vibration came back down at 1900.

 

[stratobee]

My plane is in for engine overhaul, or else I would have tried it myself. But I've asked a fellow club member to pull back to Vy (best glide), which is 117kts in the Aerostar, and see what the fuel burn is. My guess the fuel flow will be somewhere around 8-10gph, maybe even a little bit less at LOP. That means, theoretically, with the aux tanks at 225gal, you're range goes to a whopping 3290nm (you'll spend 28hrs doing it, but...:wink2. That's transatlantic range. Even with the standard 165gal tanks the range becomes 2400nm. That's Ny to LA non stop easily (if you don't mind spending 20hrs getting there).

 

[Jeff K]

My plane is in for engine overhaul, or else I would have tried it myself. But I've asked a fellow club member to pull back to Vy (best glide), which is 117kts in the Aerostar, and see what the fuel burn is. My guess the fuel flow will be somewhere around 8-10gph, maybe even a little bit less at LOP. That means, theoretically, with the aux tanks at 225gal, you're range goes to a whopping 3290nm (you'll spend 28hrs doing it, but...:wink2. That's transatlantic range. Even with the standard 165gal tanks the range becomes 2400nm. That's Ny to LA non stop easily (if you don't mind spending 20hrs getting there).

Set the autopilot and go to sleep?

It's a good baseline to have (full consumption at Vy) and as I'm planning on flying to Denver this summer, maybe I'll see if I can use it to come back non-stop to Michigan (west to east) with a tailwind. They invented adult diapers for a reason.

 

[Ted]

It's a good baseline to have (full consumption at Vy) and as I'm planning on flying to Denver this summer, maybe I'll see if I can use it to come back non-stop to Michigan (west to east) with a tailwind. They invented adult diapers for a reason.

Gatorade bottles are far nicer.