Effect of altitude on Fuel Burn

[Chuck Ellsworth]

One of the last big radials I flew definitely was needed oversquare for take off, 56 inches of M.P and 2800 RPM.

[Colonel Sanders]

the setting that gives the lowest rpm is the most fuel efficient?

Yes!! Yes!! Yes!!

Someone actually learned something useful here!

[Beefitarian]

the setting that gives the lowest rpm is the most fuel efficient?

Yes!! Yes!! Yes!!

Someone actually learned something useful here!

At the risk of writing something painfull to read...

Does it depend a slight bit on properly leaning the mixture at the same time?

A hundred RPM higher properly leaned might require less fuel than the lowest possible RPM setting at full rich. No?

[Colonel Sanders]

Mixture setting is incredibly important - see other rants.

Set the RPM as low as you can, then
set MP, then
lean for max airspeed.

That's a really good place to start.

If you have multi-probe CHT and EGT you can instead
spend hours messing with LOP.

[Colonel Sanders]

You will probably think I am on crack, but ...

With careful choice of altitude, manifold pressure,
RPM and mixture leaning, you can probably save
(e.g.) 2 gph. That depends on how big your engine
is, of course.

Let's say your engine runs to a TBO of 2,000 hrs
before you overhaul it. That would be 4,000
gallons that you didn't burn. At $6/gallon that's
$24,000 that you saved over the life of the engine -
enough to pay for a frikken overhaul!

Food for thought for a friday evening, as you test
your body's tolerance for C2H5OH.

[AEROBAT]

If the winds aloft are favorable, and it is a trip over 100 miles I climb as high as legaly possible and lean. Most normaly asperated engines get peak efficiency around 9000'. At 9000' you can run at WOT and be around 75% power. You get less parasitic drag up high and usually get a higher TAS. If your AC does not have a very good rate of climb, less than 1000'/minute, you would have to consider that as well.

[Colonel Sanders]

peak efficiency around 9000'

Most people don't like to fly little airplanes that
high. Back in 1997, I flew an M20J to OSH then
went up to Thunder Bay to visit family. Flew
from Thunder Bay to Ottawa almost direct in
4 hours, at 13,000 feet, on portable O2.

That poor little angle-valve Lyc IO-360 was
gasping, but at WOT and 2600 RPM and the
mixture leaned 'way back, it was hardly using
any fuel, and was really moving, even though
we had no tailwind. Had lots of fuel when I landed.

In the 421 I like to file 15/16/17,000 - nobody
is there. 11,000 feet of cabin pressurization.

[Beefitarian]

If the winds aloft are favorable, and it is a trip over 100 miles I climb as high as legaly possible and lean. Most normaly asperated engines get peak efficiency around 9000'. At 9000' you can run at WOT and be around 75% power. You get less parasitic drag up high and usually get a higher TAS. If your AC does not have a very good rate of climb, less than 1000'/minute, you would have to consider that as well.

~high five~ That's what I'm talking about! C-172 or Warrior II at 9500' going 300 miles is beautiful.

Most people don't like to fly little airplanes that
high.

Wat? Why? It's awesome!

[Taco Joe]

If your AC does not have a very good rate of climb, less than 1000'/minute, you would have to consider that as well.

That's where using the "spend no more than 10% of your trip time in the climb" rule of thumb can come in handy when determining cruise altitudes. I always found using that in conjunction with looking at the FDs helped provide a good balance between burning extra fuel in the climb and trying to get a higher TAS/groundspeed and lower fuel burn in cruise.

[AEROBAT]

A secondary advantage to flying high is if something does happen, like the engine quiting, you have lots of time to sort things out on the way down. If it is not too hazy navigation is easier as well all though I suppose that is not as much of a factor now with GPS.

[Taco Joe]

It also reduces the workload, less frequency changes as you're not going through every control zone enroute and the chances of trading paint with someone practicing airwork also goes down (not that should happen anyway as EVERYONE has a perfect lookout)

[Rookie50]

9000 is a great altitude. Worth the climb for all the reasons above, if the leg is reasonably long. 9000 east, 8000 west. The only caution I would make....is I have done some long night legs at 8000, I have not noticed any detriment in my attention, but have given it some thought.

What is safer do you all think, higher altitude at night, or lower (due to risk of lack of oxygen having a greater effect at night). So far for me, I have taken the altitude....but I notice, it is tiring after awhile. Demands more concentration.

[Shiny Side Up]

Most people don't like to fly little airplanes that
high.

I'll admit I'm one of those people, but that usually due to preference rather than a thought out process. I have this bad tendancy when I'm on my own to want to go check stuff out and plan trips in a grossly inefficient leap frog type way to check out any secret places I can find. On the rare occasion I want to go somewhere fast, we'll wind the P210 up around 13 to 15,000 feet. Theoretically it can cruise more efficiently at the 18,000 foot range but my personal comfort is impacted, the pressuization doesn't do enough to offset my bum knees.

[dr.aero]

Hmm... a lot of 'stuff' being discussed here.

I think breaking this up into two segments to help comprehension would be best. There is the segment that involves the 1) performance of the airplane and the segment that involves the 2) performance of the engine. Let's say you want to produce 65% power for your flight. There are a range of altitudes that you can do that at - the highest altitude being where you're at max RPM with full throttle and the appropriate max power mixture setting. For a specific power setting, you'll be consuming a fixed amount of fuel - sort of. As CS said, you can vary the mixture, throttle, and RPM to create 65% power but you'll get varying fuel flows. Generally, the higher RPM will require a higher fuel flow, when creating 65% power due to the frictional losses. Consider an airplane flying with 65% power set on both engines. If the left engine MAP is increased, the power increases above 65% power. Next, reduce the mixture so that 65% power is now being created - you'll end up with 65% power on both engines but the left engine is consuming less fuel with lower CHTs. John Deakin did exactly that and has written extensively about piston engines - http://www.avweb.com/news/pelican/182084-1.html. I'd at least read that and two more of his articles: 1) Manifold Pressure Sucks!, 2) Those Marvellous Props.

Essentially, if you're creating the same pressures (power.. kinda) inside the cylinder, you'll require the same amount of fuel. The reason, in the above example, that the airplane was creating 65% power on both engines but the left had less fuel flow is due to how efficiently that engine transferred that power to the propeller. If you have the exact same torque, measured at both propellers at equal RPM, you'll have the same power at the propeller, and if the propellers are exactly the same then you'll have equal thrust. So in the process of transmitting the power created inside the cylinder, to the propeller, there are certain losses and there are ways to reduce those losses. Without repeating John Deakin, I'll stop there.

Then there are issues of getting the most efficient propeller performance. David Rogers writes some good things about it here: http://www.nar-associates.com/technical ... screen.pdf

Now on to airplane performance. Flying at the speed for maximum L/D is ideally what you want. David Rogers writes good stuff on this too!

Now let's say we want to find the ultimate max range possible - we need to combine the factors of the engine and the airplane. Starting with the airplane part, we know the speed we want to target. Then the complicated part is matching the engine to the airplane. There will be a certain power required (to be transmitted to the propeller) so that you get your max L/D speed (EAS). Changing the MAP, RPM, and throttle you can adjust how efficiently the engine makes that power. But I doubt you'd get the RPM and TAS to exactly match the most efficient advance ratio! So let's say you sorted the engine out to get you to fly at your max L/D speed. What happens when you decrease the power or increase the power? Does the increase or decrease in speed outweigh the increase or decrease in fuel flow? Maybe you're not the most efficient (specific air range) to fly at max L/D. How does the size of the engine fitted to the airplane affect this? Have you seen the Top Gear episode where there is a BMW M3 that races, I believe, a Prius around the track? The M3's engine is significantly bigger but if it just keeps up with the Prius (going at its max speed around the track) it burns less fuel. Same goes for an airplane engine!

Don't forget wind too!

Oh... I almost forgot about the thrust required and power required charts! By the way, when you read "power" on these charts, it's referring to thrust horsepower (THP) and is not the same as BHP that the engine creates. THP required = thrust required * TAS.

This is not even close to being an exhaustive look at performance analysis for maximum efficiency. Has anyone considered opening up their performance pages in the POH/AFM for their airplane and seeing where it'd be best to cruise for their flight?

[Colonel Sanders]

As a rule of thumb, all of these are more efficient

- higher (except for truly epic headwinds)
- slower airspeed (except for back side of power curve)
- slower RPM
- increased MP
- leaner mixture (eg LOP)

However, realities often intrude. For example, there
may be a relatively low cloud ceiling and the aircraft
only has VFR equipment.

I would be happy if pilots were just aware of the above.

[dr.aero]

CS...

Completely agree!