Cruise Altitude Shortest Flight Time with Best Economy

Justin M

Justin M

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JM
Hi.

In the 1969 Cessna 182M I fly, I can cruise at about 140 MPH indicated at 5,500. I use 23 MP (full throttle) and 2350 RPM and lean rich of stumble. The plane is normally aspirated. I use rudder trim to keep the ball centered once in cruise. I don't have a useful EGT indicator. I do have a carb temp indicator that works. There is no GPS in the plane.

I want to minimize flight time and minimize fuel usage. Do you have any advice?

If I climb to 7500 or 9500, my MP reduces due to air pressure. My indicated air speed is less too.

Assuming that winds aloft have no impact on ground speed, what is the best altitude to fly at? Is it better to climb to 9500 or to stay at 5500?

I try to use flight aware to gauge speed to altitude ratios, but since the data reflects ground speed, I can't calculate tailwind/ headwind factor and eliminate it.

If there's another thread on this topic, I'd be happy to read it too. My search turned up nothing.

Justin
 
Justin,
I have useful EGT indicators, but the mixture always ends up just rich of stumble, so I don't think you're missing much there unless cylinders are out of balance.

Assuming winds aloft have no impact (which is a pretty big assumption) higher is more efficient (time and fuel) in my normally aspirated (io360) plane up to 16,000' unless on a short flight, why the airlines fly so high. If you're more efficient on fuel you'll save on fuel stops and therefore save even more on time.

I don't fly a C182, so I'm interested in hearing what other pilots say about altitudes in that plane.
-Craig
 
You can set up a profile in fltplan.com with the performance of your aircraft at each 1000 ft. Altitude, climb rates and climb speeds, descent rates and speeds and fuel burn at each altitude. Typically I can gauge my flight time for optimal altitude for flight time and for fuel burn with winds aloft provided by the fltplan Matrix. Turns out to be quite accurate as long as you enter accurate performance data. The computer will give you the Altitude for the shortest flight time, and the altitude for the lowest fuel burn, taking into account winds aloft. Often times they are not the same.
 
Justin M said:
Hi.

In the 1969 Cessna 182M I fly, I can cruise at about 140 MPH indicated at 5,500. I use 23 MP (full throttle) and 2350 RPM and lean rich of stumble. The plane is normally aspirated. I use rudder trim to keep the ball centered once in cruise. I don't have a useful EGT indicator. I do have a carb temp indicator that works. There is no GPS in the plane.

I want to minimize flight time and minimize fuel usage. Do you have any advice?

If I climb to 7500 or 9500, my MP reduces due to air pressure. My indicated air speed is less too.

Assuming that winds aloft have no impact on ground speed, what is the best altitude to fly at? Is it better to climb to 9500 or to stay at 5500?

I try to use flight aware to gauge speed to altitude ratios, but since the data reflects ground speed, I can't calculate tailwind/ headwind factor and eliminate it.

If there's another thread on this topic, I'd be happy to read it too. My search turned up nothing.

Justin
Click to expand...

Length of flight is going to be a big factor. On short flights, climbing to better TAS altitudes can be nullified by the gas used to get up there. The numbers you need to calculate total fuel burn for a flight should be in POH or whatever they called it in 69. Now on to the punchline. NO GPS!!! Is that even allowed anymore.:devil:
 
Justin M said:
Hi.

In the 1969 Cessna 182M I fly, I can cruise at about 140 MPH indicated at 5,500. I use 23 MP (full throttle) and 2350 RPM and lean rich of stumble. The plane is normally aspirated. I use rudder trim to keep the ball centered once in cruise. I don't have a useful EGT indicator. I do have a carb temp indicator that works. There is no GPS in the plane.

I want to minimize flight time and minimize fuel usage. Do you have any advice?

If I climb to 7500 or 9500, my MP reduces due to air pressure. My indicated air speed is less too.

Assuming that winds aloft have no impact on ground speed, what is the best altitude to fly at? Is it better to climb to 9500 or to stay at 5500?

I try to use flight aware to gauge speed to altitude ratios, but since the data reflects ground speed, I can't calculate tailwind/ headwind factor and eliminate it.

If there's another thread on this topic, I'd be happy to read it too. My search turned up nothing.

Justin
Click to expand...

Do you use Foreflight? The flight planning function will allow you to easily determine optimum cruise altitude for time en route or fuel consumption based on your aircraft performance and winds aloft.
 
Cogito said:
Justin,
I have useful EGT indicators, but the mixture always ends up just rich of stumble, so I don't think you're missing much there unless cylinders are out of balance.

Assuming winds aloft have no impact (which is a pretty big assumption) higher is more efficient (time and fuel) in my normally aspirated (io360) plane up to 16,000' unless on a short flight, why the airlines fly so high. If you're more efficient on fuel you'll save on fuel stops and therefore save even more on time.

I don't fly a C182, so I'm interested in hearing what other pilots say about altitudes in that plane.
-Craig
Click to expand...
Keep in mind that for a normally-aspirated piston airplane, the extra time and fuel to climb to altitude aren’t necessarily made up in the descent. The airlines “fly so high” in large part because jet performance and economics are different.
 
MauleSkinner said:
Keep in mind that for a normally-aspirated piston airplane, the extra time and fuel to climb to altitude aren’t necessarily made up in the descent. The airlines “fly so high” in large part because jet performance and economics are different.
Click to expand...

I had read once that the most efficient way in gas burners is to climb at a rate that gets you to, let’s call it top altitude, exactly at the half way point and then descend at the rate that gets you back down without any changes. Probably not that simple.
 
I’ve got a 73P model 182. My plane seems to like higher altitudes for sure. 10 - 12000 is my go too when traveling any real distance. I get better fuel economy and cruise speeds are around 150 knots true. It’s easier to keep the cylinders cooler up there too. Below 6000, I burn more fuel at slower speeds and I have to run fairly rich to keep the cylinders happy. I’ve never been above 12k in this plane because I don’t have Oxygen yet. I have the Pponk engine (O520) in mine but others I know with stock 470 see similar results.
 
I just looked at some of my flight logs. At 3500 - 5000 I can run 24/2400 and cruise around 145 knots burning around 20 gallons per hour. That’s keeping my hottest cylinder under 380. At 12000 I’m 17/2400 cruise 150 knots and only burning 12 gallons per hour. That’s my experience anyway.
 
luvflyin said:
I had read once that the most efficient way in gas burners is to climb at a rate that gets you to, let’s call it top altitude, exactly at the half way point and then descend at the rate that gets you back down without any changes.
Click to expand...

I has started a post with “I have some vague recollection that air racers would climb at a speed just barely below normal cruise to arrive…” and continued in the same vein as your. Benefit is you’re not losing miles covered during a slower climb, nor the more increased drag descending at a faster speed. IOW, if you cruise at 120 kts, maybe climb at 115 kts and descend at 125 kts. But really too many variables for that to be a “one size fits all” recommendation.
 
I fly a 1967 182 and find the fastest true airspeed in the 6,000-8,000 ft area, typically 150-155mph on about 13gph.
10k-12K will burn less fuel, 11-12gph, but slightly slower 140-145 mph. Stock 470 engine, wheel pants, but 3 bladed McCauley prop.
 
The Carson number might be of interest here. "So how do we find this optimum place at which we get the biggest increase in airspeed for the smallest fuel penalty? Simply multiply your airplane’s best-glide speed by 1.316—or add 32 percent—and that’s the Carson number." https://www.aopa.org/news-and-media/all-news/2010/december/01/technique-cheap-speed

This article basically says wide open throttle, minimum allowable RPM, LOP and climb until you get to Carson speed:
https://resources.savvyaviation.com...ticles_eaa/EAA_2012-10_flying-efficiently.pdf
 
@Justin M your IAS will be less higher up, it’s the TAS that you need to calculate to determine what works best for your needs/desires. You may already know/remember this, but from your initial post, it wasn’t obvious that you did, so apologies.
 
Airplanes get more efficient as they climb as there is less air to push through. NA piston engines however lose maximum HP as they climb, so at some point you'll trade speed for efficiency. Most have a sweet spot around 8k. I usually cruise at 65% power, so higher is better (still hit 65% @ 12k this afternoon), assuming you're going far enough to justify the climb. I usually use 1000'/10min flight time, assuming winds are not a factor.
 
This chart shows graphically what Jim K just posted:

51243840039_70ea9d75eb_z.jpg


So, for any given power setting you might choose, the higher the better - until that power setting can no longer be maintained.

For instance, in this plane if you wanted to cruise at 75% power, you’d do best to cruise at about 7,600’. For 65%, about 10,300’. And so on. Most normally aspirated planes should show similar numbers, albeit at different speeds.
 
Challenged said:
The Carson number might be of interest here. "So how do we find this optimum place at which we get the biggest increase in airspeed for the smallest fuel penalty? Simply multiply your airplane’s best-glide speed by 1.316—or add 32 percent—and that’s the Carson number." https://www.aopa.org/news-and-media/all-news/2010/december/01/technique-cheap-speed

This article basically says wide open throttle, minimum allowable RPM, LOP and climb until you get to Carson speed:
https://resources.savvyaviation.com...ticles_eaa/EAA_2012-10_flying-efficiently.pdf
Click to expand...
My plane wouldn’t climb past 5000 feet at that speed, or it would be at about 50 feet per minute.
 
FastEddieB said:
This chart shows graphically what Jim K just posted:

51243840039_70ea9d75eb_z.jpg


So, for any given power setting you might choose, the higher the better - until that power setting can no longer be maintained.

For instance, in this plane if you wanted to cruise at 75% power, you’d do best to cruise at about 7,600’. For 65%, about 10,300’. And so on. Most normally aspirated planes should show similar numbers, albeit at different speeds.
Click to expand...

The chart you want to see is altitude vs range, but that chart usually doesn't include time and fuel used to climb and descend. The Cherokee 140 listed is under powered above 7500 feet, so while you can get as high as 12-14000 feet it will take a long time to get there. Time to climb vs total length of flight plays into overall efficiency. On a three hour or longer flight 45 minutes in climb is acceptable, but not on a one hour flight. It is a judgement call, and you and your airplane will differ.
 
William Pete Hodges said:
The chart you want to see is altitude vs range, but that chart usually doesn't include time and fuel used to climb and descend. The Cherokee 140 listed is under powered above 7500 feet, so while you can get as high as 12-14000 feet it will take a long time to get there. Time to climb vs total length of flight plays into overall efficiency. On a three hour or longer flight 45 minutes in climb is acceptable, but not on a one hour flight. It is a judgement call, and you and your airplane will differ.
Click to expand...
This is where efb apps are awesome. Most have the ability to calculate time enroute & fuel used at various altitudes. I use garmin pilot, and it gives the ability to select different power settings depending on whether you're in a hurry or more interested in saving fuel. My plane gets the best mpg at 65% power leaned to peak egt, but I assume that varies by airframe. "Just rich of stumbling" is going to be very close to peak egt, maybe a bit lean of peak. That's fine at 65% power, but could be in the danger zone at higher power levels. I would want a simple egt & cht on a 182 at a minimum, and preferably a full engine monitor.
 
Justin M said:
I want to minimize flight time and minimize fuel usage. Do you have any advice?
Click to expand...
Read chapters 19 and 20 in Stick and Rudder, open Excel and get to work. :)
(In fact, I'm gonna review them myself right now! It's been a long time.)

Oh, and by the way, aren't maximum range speed and what you're asking about exactly the same, at least in the cruising section of the flight? Also, wouldn't the cost of fuel vs. the direct costs of operation affect your calculus?

Got time for a "war story"? A friend bought a Cessna 421 and asked that I fly 25 hours with him for insurance purposes. We needed to fly our last leg from Ft. Meyers to Mansfield, Ohio late in the evening. The weather enroute was so bad he asked me to check weather and make the go-no-go decision myself. I was on the phone with FSS for nearly an hour. Zero-zero conditions throughout the Carolinas, Thunderstorms in northern Florida, wind, rain, low ceiling forecasts and a moonless night from Tri-Cities to Mansfield. Considering that Charley West, IIRC, was going to be the alternate it looked like a fuel stop at Tri-Cities was inevitable. BUT, as we got further into the flight I could see that by throttling back and "making" fuel, we should be able to make it non-stop rather than put ourselves at risk by making an approach and IFR departure at KTRI. My friend was real nervous, since he always wants lots of fuel (so do I, usually, but this case was different). We made it non-stop in one leg, with less risk, less stress and arrived more rested and alert with still enough legal fuel reserves. But it was hard on my friend when we throttled back and plodded along at such a relatively slow pace. The upshot was there were so many aircraft accidents along that route that day it made news in Flying Magazine the next month. It was a Thanksgiving holiday weekend and a lot of pilots met their Waterloo that day.
 
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The above discussion is great with no significant winds aloft, but that is rarely the case. The situation with max range and efficiency is obviously better with a tailwind, but with a headwind your range can really be affected negatively. You must consider those winds aloft in your calculations. Having fuel flow coupled to your GPS is probably, IMO, the gold standard way to maximize your efficiency. I’m not as familiar with using EFBs, but the fuel burns are estimates without the actual fuel flow, even though I don’t doubt they would get close, as did FLTPLAN.com in my earlier days.

Despite the headwind penalties at higher altitude headwinds, I generally prefer to fly up high in my NA single. Consider also the benefit of smoother air generally up high, which is always desirable from a pilot fatigue perspective during longer cross countries, and also flying high enough to avoid slowing down to maneuvering speeds that could be a consideration down low. From a practical perspective in the wind, I generally felt that flying higher into the higher altitude winds at 65% with the smooth and less dense air at 8k, even with the definite but slight ground speed penalty, was preferable than flying lower at say at 4k at the same power setting, excepting of course extreme headwind gradients at high altitude. Also, my gliding distance is obviously better up high.
 
One thing you could do is use a flight plan and aircraft profile and run a few iterations at different altitudes and different winds. You will need accurate climb times and fuel burns to altitude if possible, and run a few tests with different variables. It is free, it is enlightening, and it could be a fun exercise.
 
This is basically a calculus problem with variables different for each flight.
 
Thanks for the great feedback everyone. I've put some of your suggestions into play during my most recent flight of 4 hours from Brookfield Wisconsin (02C) to Franklin Pennsylvania (KFKL). I tracked data on my knee board and once I'm caught up with work, I'll give an update.
 
You mention in your original post that "assuming winds aloft have no impact on ground speed", but that is often the overriding factor for any of these decisions. While the other information is good to know, I have rarely found it to have more impact on overall ground speed than winds aloft.
 
Kevin Eggert said:
You mention in your original post that "assuming winds aloft have no impact on ground speed", but that is often the overriding factor for any of these decisions. While the other information is good to know, I have rarely found it to have more impact on overall ground speed than winds aloft.
Click to expand...

You got that right. When I have a 60kt tailwind, I want to get up there as quick as possible, and come down as late as possible.
 
luvflyin said:
Length of flight is going to be a big factor. On short flights, climbing to better TAS altitudes can be nullified by the gas used to get up there. The numbers you need to calculate total fuel burn for a flight should be in POH or whatever they called it in 69. Now on to the punchline. NO GPS!!! Is that even allowed anymore.:devil:
Click to expand...
I thought this too, but now I almost always climb to altitude for any flight > 0.5hr.
The reason? I climb at a minimum of 1000fpm up to around 8000MSL.
That means in 6 to 8 minutes I'm at altitude.
For the extra 1/2 gallon max of fuel spent getting to altitude, it's not a big expense.
It's a heck of a lot cooler in the summer up there!
 
Adam Weiss said:
I thought this too, but now I almost always climb to altitude for any flight > 0.5hr.
The reason? I climb at a minimum of 1000fpm up to around 8000MSL.
That means in 6 to 8 minutes I'm at altitude.
For the extra 1/2 gallon max of fuel spent getting to altitude, it's not a big expense.
It's a heck of a lot cooler in the summer up there!
Click to expand...

Yeah. The comfort factor is a thing.
 
And altitude is super important if you have any sort of emergency. My only emergency found me at only 3500 MSL (2500 AGL). Partial power got me back to an airport 10 miles away, but it was only trees otherwise with a wife and 3 year old child aboard a Cessna 150. Not a great scenario for tree branches to be spearing the compartment or find ourselves hanging 40 feet above the ground. I would much rather have been at 5500, or even 7500 which would have meant a much more relaxed glide back to the class D.
 
You stated in the original post that you wanted to minimize flight time and minimize fuel... well, which is it? You kind of have to pick. Truly minimizing fuel is going to mean flying waaaay slower than anyone wants to travel - I’d bet somewhere down around 100mph to truly maximize efficiency (experts here can chime in). To minimize flight time you’ll want to fly at best power ROP, which is going to be something like 50 degrees rich of peak and flown around 8500’ in general, which is where you achieve the optimal combination of maximum amount of fuel effectively burned through the engine at the maximum allowable continuous power at the minimum drag (due to thinner air - but not so thin that your NA engine starts to lose power over and above it’s max continuous rating).

this is all in perfectly calm air at all altitudes, but that’s kind of a silly assumption - winds aloft are often the *only* variable that matters when picking cruise altitudes and associated engine settings. Good tailwind? Go higher. Strong headwind? Go lower.
 
I fly a naturally aspirated 182T. My best speed is around 8-11k.

keep in mind you said your airspeed will decrease, indicated yes, true, not necessarily so, it may increase until you have too big a loss in power then it’ll go the other way. The cruise chart will give better indication on this. True airspeed in cruise is what we really care about so long as the indicated is in a safe range for the plane

the EFB apps like ForeFlight will allow you to set a profile up for climb cruise and descent rate, speed, and fuel burn. Sometimes going higher will bet you better fuel other times not at much. Most of my flights are sub 300nm for reference
 
I have the model that precedes the OP's: a 1968 (or 1967) 182L. FWIW, I usually cruise at 22"/2300, which nets about 130-140 mph IAS depending on how smooth the air is (and if I've remembered to close my cowl flaps). Most of my flights are around 3,000-4,500 because the distances involved aren't too far. This past fall I made a trip from the Boston area to New Orleans, and found myself flying low (~3,000') southbound due to headwinds, but on the way back I found 7,500 to be pretty comfortable. Looking a picture I took early on the return leg(s), my IAS was 130 mph, which translates to about 113 knots IAS, and my E6B says that's about 130 knots TAS (which comports with my GPS showing a 118 knot ground speed with an 11 knot headwind -- I had picked 7,500 because that headwind dissipated and turned into a tailwind a little further along the leg). My pic of the engine monitor showed me leaned to 10.9 gph while at 7,500/22"/2300.
 
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