Optimal cruise questions

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turbofan

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turbofan
I recently started flying an airplane that is basically minimum fuel from the moment you are gear up, and I had some questions that I've never had to think about that I would greatly appreciate help with.

In order to achieve maximum range performance, do you want to get as high as possible up to optimum cruising altitude (360-370), as fast as possible, followed by max range AOA, followed by an idle descent at max range AOA?

Practically speaking, a leg that you wouldn't have the time/distance to get to your optimum altitude would ideally be you climbing until a point where you need to begin a descent. No cruise segment.

I am looking at recommended altitude for distance charts, and I have altitudes above what aircraft performance charts are saying are optimum max range altitudes.

Now, for maximum speed would I want to be at the same altitudes that give me the lowest fuel burn for max range, but operating at higher power settings? My performance charts say I get a higher max range TAS above 370 but at the cost of higher fuel / distance.

The charts I'm getting the data from are generalized, so for real world application if I wanted to find the actual optimum altitude for that day, I would need to find the altitude at which temperature ceases to decrease(tropopause)... correct?

Also, is there any practical truth that the increase in fuel consumption by flying a route as fast as possible is acceptable because you are actually decreasing the total operating cost by decreasing the time on the engine / airframe, and therefore the need for inspections? It would be easy to figure this out if I knew the specifics of what we pay for maintenance, but I don't.

Basically:

How do you fly a max range profile?

How do you fly a max speed profile (or minimum time) (in an airplane that cannot achieve Vne in straight and level flight)?

And is the second less expensive?

Thanks!
 
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Max range on a normally aspirated piston engine airplane is independent of altitude.

Go for the highest tail wind altitude, within reason.
 
I was never in your situation and don't have the expertise to answer your question. My observation is that you can get the same climb rate with the nose pretty high in the air and going slower or lower the nose and pick up some more lift via more airspeed and thereby get to the same altitude in the same time but a little farther down the road.

You'll have to run the numbers to see if getting there faster is cheaper than slowing down. I know that my limited experience was that it didn't seem to pay to slow down but I can't give you any figures on that.

The maintenance numbers are probably based on some typical operations - you might have to inspect all the assumptions before you apply them to profiles that are markedly different.

Sorry I'm not of much help.
 
Might I ask what kind of airplane carries little fuel but cruises at 360? For many jets, max economy requires climbing 'til descending...I've climbed to 390 between DFW and Little Rock, but it was worth it because we saw a GS of 635kts! Was only level for about 30 seconds before I slowly pushed the nose over and reduced the thrust to idle and just coasted down. didn't touch the throttles again until under 1000ft. Climb steeply with wind on the tail and shallow/high speed with wind in the face.
 
j1b3h0 said:
Might I ask what kind of airplane carries little fuel but cruises at 360? For many jets, max economy requires climbing 'til descending...I've climbed to 390 between DFW and Little Rock, but it was worth it because we saw a GS of 635kts! Was only level for about 30 seconds before I slowly pushed the nose over and reduced the thrust to idle and just coasted down. didn't touch the throttles again until under 1000ft. Climb steeply with wind on the tail and shallow/high speed with wind in the face.
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L-39s, Hawks, Alpha Jets, T-1, T-2, T-38, etc, i.e. the sort of planes that weren't built for cross countries, so no one ever thinks of questions like these.
 
turbofan said:
In order to achieve maximum range performance, do you want to get as high as possible up to optimum cruising altitude (360-370), as fast as possible, followed by max range AOA, followed by an idle descent at max range AOA?
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That's it, for jets.

Practically speaking, a leg that you wouldn't have the time/distance to get to your optimum altitude would ideally be you climbing until a point where you need to begin a descent. No cruise segment.
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Correct. That's how the "bingo" profiles in our books for the tactical jets I flew were built.
 
Optimum isn't the highest you can go. Optimum for most turbojets is a function of weight and outside air temperature vs. the power required for a particular desired trimmed speed; simply going as high as you can isn't optimal.

Generally you're better off cruising below optimum, rather than above it.

Optimum will be the place where you can achieve about 90% of your engine speed (for most turbojets) at the desired mach number for your optimum cruise.

On some turbojets where you can practically see the fuel gauge move, it's a matter of getting to altitude as soon as possible, achieving your desired cruise profile (max endurance, max range, etc), and descending as late as you can. An idle descent all the way isn't always possible, so don't plan your fuel that thin. Inevitably you'll end up getting held at altitudes on the way down, and in some locations such as the northeast corridor, you'll end up getting run down to low altitudes far before your destination; this increases burn and should be planned, depending on where it is you're going. Weather and other delays will also increase your necessary fuel above whatever the profile you draw for yourself calls out.
 
turbofan said:
I recently started flying an airplane that is basically minimum fuel from the moment you are gear up, and I had some questions that I've never had to think about that I would greatly appreciate help with.

In order to achieve maximum range performance, do you want to get as high as possible up to optimum cruising altitude (360-370), as fast as possible, followed by max range AOA, followed by an idle descent at max range AOA?

Practically speaking, a leg that you wouldn't have the time/distance to get to your optimum altitude would ideally be you climbing until a point where you need to begin a descent. No cruise segment.

I am looking at recommended altitude for distance charts, and I have altitudes above what aircraft performance charts are saying are optimum max range altitudes.

Now, for maximum speed would I want to be at the same altitudes that give me the lowest fuel burn for max range, but operating at higher power settings? My performance charts say I get a higher max range TAS above 370 but at the cost of higher fuel / distance.

The charts I'm getting the data from are generalized, so for real world application if I wanted to find the actual optimum altitude for that day, I would need to find the altitude at which temperature ceases to decrease(tropopause)... correct?

Also, is there any practical truth that the increase in fuel consumption by flying a route as fast as possible is acceptable because you are actually decreasing the total operating cost by decreasing the time on the engine / airframe, and therefore the need for inspections? It would be easy to figure this out if I knew the specifics of what we pay for maintenance, but I don't.

Basically:

How do you fly a max range profile?

How do you fly a max speed profile (or minimum time) (in an airplane that cannot achieve Vne in straight and level flight)?

And is the second less expensive?

Thanks!
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Sounds like you're referring to a T-38. If you look at the diversion charts, it proposes a flight profile to minimize fuel consumption per mile flown. The propulsion method matters in this discussion. Due to differing specific fuel consumption and efficiency variances between a turbofan/turbojet and unducted fans (what a propeller is) such in the case of a turboprop, these profiles can vary.

Generally speaking yes, these engines are designed to run optimally at certain pressure altitudes. As such, getting to those altitudes as quickly as possible mitigates the fuel burn. No partial power climb configuration would save you gas versus mil power and max rate climb airspeed. Afterburner is a no-no, that's for acceleration (of tactical value), not efficiency. Idle descents are efficient for turbojets/fans, not so for turboprop.

The reality is that the fuel capacity is so limited on these samples that any effort to extend range in a non-emergency situation more than 10% is an exercise in poor airmanship.

As to mainteance costs. LOL Are you kidding? A T-38 trainer has 2nd generation jet technology, analog fuel metering and the like. They'd be better off putting a high aspect ratio wing and losing both engines for a commercial grade small bypass turbofan. Nobody is saving jack by trying to fly a 38 cross-country. Cool as f$ck though, and that's as good as money to some :D

Find a 100kt tialwind tailwind and pull them throttles back to best endurance AOA. Oh and file great circle route. lol :rofl:

As an aero engineering dude I can tell you the book answer is that for jet engined aircraft the maximization of range is attained by maximizing the product of speed (V) and aerodynamic efficiencies (call it E), latter which could be generalized as maximizing L/D for instance. This generally means that you will need to gain as much altitude as you can. As you pointed out, certain distances are not efficient to attain the optimal altitude, so that affects the profile. There are also compressibility effects that stump the V curve (i.e. it's not just as simple as going as fast as possible). Mach drag effects in the transonic regime kill aerodynamic efficiency and these effects are compounded at altitude by the inability of the powerplant to produce the thrust required for level flight at high altitudes. So both altitude and velocity encounter non-linear detrimental effects to aircraft range as both variables increase in value. Some literature puts the maximum range target L/D ratio as 86.6% of it's maximum possible value based on iteration-based flight testing. For the T-38 Mach 0.765 (I love these Dash one postulations...given that the dial only shows 2 decimals) on the dial is generally regarded as your best bet, recognizing these efficiencies change with weight as well and the medium's own velocity (headwind/tailwind). Clear as mud?:idea:

Stop trying to squeeze range out of a dart. Just hit the burners and grin.
 
Best climb rate airspeed, which normally starts out pretty fast and then intercepts a mach number that you sustain until you hit cruise altitude. Throttle back and slow to max range AoA if you don't have some sort of on-board best cruise calculation figures to reference. This will correspond to slower and slower indicated airspeeds as you burn down fuel and get lighter. Idle descent, or if you have afterburning motors with VENs/variable nozzles, then a puckered nozzle descent. Do your fuel planning for a worst case scenario with some wind in the face, and you will not hate your life when that forecast 100 kt tailwind suddenly vanishes somewhere over the midwest :)

As a reference point, our bingo profiles (same same as "divert" charts mentioned previously) basically show that you burn most of the gas in the climb to altitude, and while the numbers for sea level bingo (ie no climb to altitude) are higher in fuel burn for each of the respective distances, they are not higher by a whole lot. In other words, higher is not always better, as you reach a point of diminishing returns eventually. Somewhere in the 20's-low 30's is probably a good starting point for most high performance/tactical jets. You just don't save enough gas cruising at FL400 compared to 250 to really get anything out of burning the extra gas to get up there, all other things being equal. Favorable winds could make a difference, but that would be my starting point. Also, if you are talking about old jets with fighter style canopies and pressurization systems, going real high is also comparatively risky. Blowing out a canopy seal at 45k is no joke, and if you survive, I've heard DCS is pretty uncool.
 
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35 AoA said:
As a reference point, our bingo profiles (same same as "divert" charts mentioned previously) basically show that you burn most of the gas in the climb to altitude, and while the numbers for sea level bingo (ie no climb to altitude) are higher in fuel burn for each of the respective distances, they are not higher by a whole lot. In other words, higher is not always better, as you reach a point of diminishing returns eventually. Somewhere in the 20's-low 30's is probably a good starting point for most high performance/tactical jets. You just don't save enough gas cruising at FL400 compared to 250 to really get anything out of burning the extra gas to get up there, all other things being equal. Favorable winds could make a difference, but that would be my starting point. Also, if you are talking about old jets with fighter style canopies and pressurization systems, going real high is also comparatively risky. Blowing out a canopy seal at 45k is no joke, and if you survive, I've heard DCS is pretty uncool.
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Depends on the jet. In the Eagle it saves a ****load of gas by climbing over the RVSM airspace. (long cross country flights are usually filed at FL430) the savings is quite high compared to staying down at FL280. All airframe/wing dependent. In a '38, do whatever you have to do to get direct routing. It's been over 10 years since I flew one of those little things, I wouldn't want to go too high in it anyway. (not that it stopped me when I was a student)

Another good rule of thumb we use for divert numbers (i.e. you have less than full tanks and have to go somewhere unintended to land) is climb to FLxxx (depends on airframe) or until you burn 1/2 the gas you have left. Play the power to level off at max range aoa, then select idle to hit your max idle range descent numbers at descent point. (FL440, 14CPU AOA, idle 220kias for the Eagle)
 
You just need to find the right tailwind...

IMG_1698.jpg
 
EvilEagle said:
Depends on the jet. In the Eagle it saves a ****load of gas by climbing over the RVSM airspace. (long cross country flights are usually filed at FL430) the savings is quite high compared to staying down at FL280. All airframe/wing dependent. In a '38, do whatever you have to do to get direct routing. It's been over 10 years since I flew one of those little things, I wouldn't want to go too high in it anyway. (not that it stopped me when I was a student)
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Fair enough. We typically get into RVSM when we are allowed (legacy Hornet is non-RVSM) but normally not much higher than the mid-high 30's. But again, your wing is very different than ours, and the Hornet is just a more draggy airplane. Like you said, it depends on the aircraft.
 
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