Fuel burn question

[Kalium Chloride]

On a straight oceanic stretch and all other factors being equal, do you save more fuel by:

(a) Cruising at higher speed and arriving at a destination sooner, thereby saving fuel by minimising flight time.

(b) Cruising at a lower speed and saving fuel by reducing fuel burn.

Assume your flight will be on schedule either way.

[Captain Stable]

Generally (b).

You'd check fuel burn against distance. But like a car, going faster is NOT more economical.

[411A]

Very much depends...
IF the goal is to reduce sector time (and as a result, flight time) and IF the flight is paid for by flight/block hours, then going faster makes perfect sense. Sometimes, fuel is cheap...fly fast.
Those in the charter or MAC business will certainly "know".
Scheduled carriers, much less so.

[Keith.Williams.]

If we limit the question to "what speed gives the minimum fuel burn" rather than "what speed gives the lowest sector costs", the answer is the speed which gives the best combination of engine efficiency and airframe efficiency.

If we also consider the effects of altitude, we can maximise both engine and airframe efficiencies simultaneously. Flying faster or slower than this optimum speed (or higher or lower than this optimum altitude) will increase fuel burn.

[Idunno]

If you have a strong tailwind its likely to be more fuel efficient to fly slower (LRC) and benefit from the tailwind for as long as possible.

[Draculas Teabag]

Given the fact that you have said you will be on schedule either way...

Whatever speed and altitude gives you the lowest amount of fuel used per nautical mile (or whatever units you like) covered will give you the most amount of fuel at the other end. This is known as specific ground range (SGR). You can work your SGR out by dividing your fuel flow by your groundspeed. At the speed and altitude this figure is the lowest is the one you want to fly at.

Flying at this speed (S.L.O.W) is not always operationally desirable as there are other factors such as time critical costs (crew, maintenance etc) and schedule that may dictate that the actual minimum cost of the operation is improved by flying slightly faster thereby using more fuel, as pointed out by the man most Cathay pilots would not buy a beer for, 411A.

[Intruder]

If fuel burn is the only consideration, fly at the max range cruise speed, which will give the most "miles per gallon" (or kilometers per liter). If you fly either faster or slower than the max range cruise (sometimes called "Econ") speed for the given conditions, you will burn more fuel.

[northwing]

At low speed induced drag is high because you need a lot of incidence to provide the necessary lift. Profile drag is low because the friction of the slowly passing air is low.

At high speed the opposite applies.

Total drag is the sum of the two types of drag and so the drag versus speed graph looks like a U shape, the sum of a downward sloping curve and an upward sloping curve. For longest endurance, eg holding or maritime patrol, fly at the bottom of the U where drag and hence fuel burn is a minimum. For best range it is best to fly faster, in fact at the tangent to the right hand curve of the U, because as you increase speed you are burning fuel faster but you are getting there even faster than that. If you fly even faster still then you start to lose out on range again, although some manufacturers will quote a "95% best range speed" which is higher than best range speed but gets you there a lot sooner for not too much extra fuel.

[kkbigjet]

hello
Well to answer your question first consider the stalling speed in clean config, or the en-route configuration, which for a jet are pretty high, as you would know.

The first speed we encounter is the minimum drag speed; this is typically about 1.5 times the stalling speed. And as the terminology implies - it is also the speed at which the fuel flow required or the fuel consumption is the least. At this speed the summation of induced drag and parasite drag is minimum, and hence the total drag is minimum. Minimum drag means the least thrust required, as drag equals thrust in un-accelerated flight, and therefore corresponds to a thrust setting to the minimum fuel flow required. Now any faster - the parasite drag will incr and will require more thrust and therefore incur a greater fuel burn. Any slower and u will slip up on the backside, incur an increase in induced drag, and again will require more thrust to hold speed, and so greater fuel consumption. This speed is therefore the one that gives you minimum fuel burn, that is maximum no of air hours per unit of fuel, and is therefore the holding speed of a jet, and flown when endurance is of primary importance. U will stay up in the air for maximum time if u follow this speed schedule.The next speed we encounter, and that is more appropriate for your question is the Max range speed. This is typically about 1.7 times the stalling speed, definitely higher than max endurance (minimum drag) speed mentioned above. This speed gives us the maximum TAS/drag ratio. That is a given IAS for which drag is minimum. Agreed the drag remaining same at a given IAS, as we go higher, the differential between IAS and TAS becomes greater, while the thrust, therefore the fuel flow required to push the plane remains substantially constant at the same IAS at the same weight. Again without going into the graph, this speed is not equal to min drag speed but somewhat higher which actually offsets the benefit of minimum drag, but more than makes by being a higher IAS correspondingly a higher TAS. When u fly this speed u get maximum no of air miles from a pound of fuel - and that is range. Though u will not stay in the air longer as compared to the speed in the previous paragraph. In conclusion if u want to fly farther, fly higher, that is what the combination of the swept back wing and the jet engine prefers.

So do we fly this speed in airliners - actually no. If u maintain this speed, if the airplane wud slow down, it will continually slip back the curve and require more thrust and so greater fuel flow. What the manufacturers have done is that the manuals are provided with what is called a LRC - long range cruize chart. This speed is slightly higher than the max range speed - uses slightly more fuel, but the range penalty is just about 1%. That is to say - for a slightly higher speed, assuring u speed stability u will still achieve 99% of max range. And this is the speed u wud like to fly to get maximum mileage out of your tanks.

Again so the airlines use it, depends on company to company. One thing for sure no one flies slower that this. But many use higher speeds to get shorter block times, and on very long haul legs it may shave off 30 to 45 minutes of flying time, therefore the requirement of an additional crew is eliminated due flight duty time limitations

Again to make it sound simpler..Consider the following examples at atypical weight and crz level
Stall speed- 160 knots.
Max endurance (minimum drag or best L/D speed or normally hold speed) 240 knots.
Max range speed 280 knots
LRC speed 285 knots ( to assure speed stability)
Airline may use 295 or 300 or whatever suits their requirement.
Just a note here. Except the hold speed, all others are normally dealt with in Mach no. figures.

In the end - if u r flying in headwind increase mach no ,and in a tail wind decrease mach no. to achieve max mileage from ur tanks.

bi bi
kunal

[SuperRanger]

And I thought it's all in the Cost Index?!?!

[Intruder]

If Cost Index = 0, then ECON speed will be max range speed for current conditions (including wind), resulting in minimum fuel burn.

If Cost Index > 0, factors for crew costs, etc, are introduced, raising the ECON speed and fuel burn.

[Keith.Williams.]

Although (most of) the replies in this string deal with the matter of airframe efficiency, they have all ignored the matter of engine efficiency.

Greatest range occurs at the speed and altitude giving most miles per hour per pound of thrust, and lowest fuel consumption per hour per pound of thrust. For the airframe this means flying at about 1.35 VMD at the altitude at which the ratio of TAS : Drag is greatest. For jet engines it means operating at between about 85% and 95% RPM.

At low altitudes this RPM range produces too much thrust to fly at 1.35 VMD, so the aircraft must climb. Optimum range occurs at the altitude where the drag at 1.35 VMD is equal to the thrust at between 85% and 95 RPM.

The above applies only in still air, but can be adapted to headwinds or tailwinds by using the speed giving the best Ground speed : Drag ratio rather TAS : Drag ratio.

Fly any faster, slower, higher or lower and range will decrease.
Unless of course the atmospheric conditions are very unusual. To fly half way around the world in a 600 Kt headwind for example you would be better to turn through 180 degrees (asuming this were possible) and fly very slowly!

[quid]

Of course in the real world, you'd attempt to climb or descend to take advantage of winds. But from the theroretical point of KC's question, and once again, speaking of only fuel burn (and not the other many variable costs), speed adjustments into a headwind gives diminishing returns in specific range very quickly.

Depending on your a/c, anything more than 1-2 kts increased indicated a/s per 20 kts. of headwind wil decrease your NAM.

kkbigjet-

I think he's talking about any given altitude. In that case, flying best specific range speed would cause you to decrease the optimum airspeed as you burn off fuel.

and... your speed adjustment into a headwind should be very minor.

[MasterGreen]

Here is a link that seems rather relevant.

Wind Trade and stuff (http://www.iasa-intl.com/wtt.html)

MG

[GlueBall]

It's not always just about "saving fuel." There also is some economic relationship between time en route and airframe hours. Recurring maintenance C-checks and D-checks cost millions of dollars.

If, for example, an airplane is flown at M.86 instead of at LRC M.84 (average) for 3600 hours (typical annual utilization), then the difference could be 18,000 miles. That translates into 37 hours of airframe utilization, equivalent to 4 "extra" Atlantic crossings (and additional revenue) between expensive maintenance checks.

Depending upon the cost of fuel, increased utilization by flying "faster" can be more economical!