Will this idea really help cut down on fuel for airlines?

Question

If they fly slower they say they save fuel....but if they go slower it will take more time to get there=more fuel. If you go fast you use more power (n1)=more fuel.....

Maybe this might work when flying east/ you have a good tail wind, but what if you have a head wind? Pointless i think.

http://www.msnbc.msn.com/id/24410809

Answer 1

Its actually very simple actually, yes it does, and greatly.

The energy required to propel any given aircraft increases exponentially, by its square. That means if you go twice as fast you use 4 times as much fuel.

If you slow from 500mph to 400mph, even though your sfc goes down a little (about .03-.05 increase), you save over 40% of your fuel per hour, for only a 20% increase in flight time. Since they use nearly 2500 gallons per hour per engine, 20% fuel savings will save hundreds of millions of dollars over the 60-100k hour life of the aircraft.

It can also be used to allow an aircraft to fly extreme distances. A person once flew a piper Aztec from North Africa all the way to California, a 6000 mile trip with an aircraft that can only fly about 1100 at normal cruising speed.

All he did was fly at the best glide speed the entire way, which is about 1/3 the top speed.

Answer 2

Have you ever compared long range cruise speeds to high speed cruise? In general, long range cruise is slower. One of the jets I fly has an official long range of Mach .72. (I've never cruised it that slow.) Its high speed cruise is .77 to .80, depending on altitude. And John B is correct that the FMS becomes a primary reference source for fuel/range issues once you are underway.

At least some of the airlines have been flying at reduced power for a while now. For maybe two years now, I've been hearing Mach seven four and seven six in response to ATC inquiries, from airline aircraft which ordinarily would cruise faster.

Answer 3

You would be surprised. Since my company has started we have been saving quite a bit. Most jets are not very efficient at their maximum cruise speed. Basic aerodynamics.

Answer 4

My apologies to dragon, you made a sincere effort, but your answer is incorrect.

The wind does not affect your fuel efficiency. Flying into wind will require more TOTAL fuel for the trip, all other things being equal, but the RATE of fuel burn will not change. The wind only changes your motion relative to the ground. It does not change your motion relative to the air. We relate fuel efficiency and power required to AIRspeed, whereas we relate time and total fuel used to GROUNDspeed.

Let me give an example. Let's say your aircraft cruises at 100 kts. That is, it travels through the AIR at 100 kts. Now, if we put that aircraft in air that is also moving, at 10 kts say a tailwind, then the aircraft will still flow through the AIR at 100 kts, but since the air is also moving, the aircraft will move past the GROUND at 110 kts.

So, if I fly one leg of a trip with wind, and then return into the wind, the return trip will always take longer and eat more fuel AT ANY SPEED. However, the RATE of fuel use will not change because of wind, but only because of airspeed. Sometimes you have to go against the wind. You'll use more fuel in that direction no matter what you do.

Now, you need to understand that the rate of fuel burn does not have a 1 to 1 relationship with speed. For simplicity, let us only talk about the forces of thrust and drag, and let us limit the discussion of drag to parasite drag because this is the one that applies at the speeds where airliners would operate during cruise. Finally, let us exclude takeoff, climb, descent, and landing, as they complicate things and have a negligible effect on the question. Let us just pretend the whole flight happens in the en route phase at a constant altitude, etc, etc.

Thrust is the force that drives our aircraft through the air. Drag opposes that force. Thrust MUST equal drag to maintain any airspeed during cruise flight. It's not like airplanes cut their engines once they reach cruise. If they did, their speed would decrease because drag would exceed thrust. Therefore, engine thrust is used throughout the flight to equal drag.

Parasite drag is basically the air resistance of the fuselage and any non lift producing surfaces. It is proportional to the square of the airspeed. If you double your speed, you quadruple parasite drag. Flying at 600 knots requires 4 times the thrust of 300 kts. So, any reduction in speed will have a greater impact on thrust required, and therefore fuel used.

At high speeds, parasite drag is high. See the following chart:

http://en.wikipedia.org/wiki/Image:Drag....

There is a low point where fuel efficiency will be greatest. Therefore, if airlines fly at a speed that is closer to that minimum point, but not below it, they will save fuel.

In a car, the same is true. You may get to your destination SOONER at a higher speed, but your MILES PER GALLON will be lower, and fuel economy will be worse. I live in Texas and a few years ago they reduced speed limits on many major highways for this reason so that vehicles would produce less pollution by operating at lower speeds. The airlines are doing the same by reducing their speeds. They will save fuel at the expense of a few extra minutes of flight. The trips from NY to LA and back will BOTH be longer, but they will BOTH require less fuel.

For Further Reading:

http://en.wikipedia.org/wiki/Ground_spee...

http://en.wikipedia.org/wiki/True_airspe...

http://www.dynamicflight.com/aerodynamic...

http://en.wikipedia.org/wiki/Drag_%28for...

http://en.wikipedia.org/wiki/Drag_equati...

http://en.wikipedia.org/wiki/Induced_dra...

http://en.wikipedia.org/wiki/Parasitic_d...

Note in this one where it says the following:

"The combined overall drag curve therefore shows a minimum at some airspeed - an aircraft flying at this speed will be at or close to its optimal efficiency. Pilots will use this speed to maximize endurance (minimum fuel consumption), or maximise gliding range in the event of an engine failure."

http://en.wikipedia.org/wiki/Flight_plan...

Note the following from this article. There is no mention of wind:

"The rate of fuel consumption for aircraft engines depends on: air temperature, height as measured by air pressure, aircraft weight, aircraft speed relative to the air, and any increased consumption as compared with brand-new engines due to engine age and/or poor maintenance (an airline can estimate this degradation by comparing actual and predicted fuel burn). Note that a large aircraft such as a jumbo jet may burn up to 80 tons of fuel on a 10 hour flight, so there is a substantial weight change during the flight."

John B is right about too many variables.

To dragon

You're right, you will use more fuel flying against the wind. You won't have a choice, though. You can't control the wind. If you're flying from east to west, you'll probably be going against it. If you go into that wind at a slower speed, you will use less fuel than if you went into that wind at a higher speed (the many variables aside). When you return going from LA to NY, you will use less fuel going with the wind at a lower speed than going with the wind at a higher speed, (all other things being equal, complicated variables aside).

Notice from the article that they are only making subtle changes, adding a few minutes to any given flight by slowing a few mph, but still saving millions. If you reduce speed too much you reach Cessna Man's case where nobody wants to go on your longer flights. You also reach a point where reducing speed actually starts to decrease efficiency. I'm not sure which would happen first.

Answer 5

Not only pointless to slow down with a headwind, it's harmful. Without getting into numbers, in a calm wind there is a power setting for an airplane that will yield the minimum specific fuel consumption per nautical mile. In headwinds the idea is to reduce the time that the negative effect of the headwind works against you so you would increase power to obtain the new MSG/NM. In other words fly faster but only up to a point. Now you see why I ain't gonna talk numbers here. Too many variables but a good FMS will figure it for you. Similarly, reducing power and slowing, to a degree, will increase the time that the tailwind can help you. Which ever way the wind blows though, it's a delicate balancing act but the answer to your question is yes, speed management is crucial in saving fuel.

Answer 6

You may think you're wasting more gas flying faster with higher power but you actually aren't.

Like say a car. You're still wasting the same amount of gas going 20 MPH as you are 50 MPH. The car consumes gas to stay powered. If you were to go faster you would consume less gas because you get from point A to point B faster. A higher speed may consume a slight amount more gas to get the car going but the car will adjust to the speed and even out the gas consumption. It'll wind up taking nearly the same amount of gas, you get there faster, and you actually save gas.

Now with a plane. If you were to fly the same direction as the wind and ride the air currents you won't hardly need any power to get going and you'll use less gas than if you were to go head on with the current. The current adds drag and force to oppose the thrust. You'll consume more gas, but you'll still consume less going faster speeds because you are still going faster. It's worse if you were to go against a current at a lower speed because you're going even lower than the speed you are trying to go, with the force opposition.

Planes of course have large engines and consume gas in bulk amounts, but they also have special fuel that allows them to clock such high speeds with a low effort.

If you were to fly with a current at fast speeds you will consume little gas. If you were to fly against it at fast speeds you will consume a decent amount of gas but less than if you were to go lesser speeds at the current.

Flying faster gets you to the place faster, and you're not consuming power bursts of gas when you are trying to speed up to get to a speed. You really only consume LARGE amounts of gas when getting up to a speed. Once at the speed the plane or car will adjust to the speed and won't have to consume gas to power up to that speed. So now it's balanced out. Which will consume less gas? Getting there in 4 hours or getting there in 2?

The only real consumption of gas is getting up to the speed. That's were it's consumed a lot, but slowing down on higher speeds also require fuel. So it's sort of a win/lose situation.

You consume less fuel at high speeds because it doesn't take as much time or effort when at these speeds to get anywhere.

And you wouldn't have to worry about slowing down because the only time the plane slows down is when it's getting ready to land. So riding the air current and not opposing it at a high speed will consume the least gas, but riding against it at slow speeds will consume the most.

You're talking about large airplanes not jets right?

Answer 7

I don't know if it will save fuel but according to an article up at yahoo they are all doing it, also they are flying a very old fleet (US) and they are staying plugged into the terminal before start up.  Virgin Atlantic's Richard Branson did some experimenting at Gatwick in London with pushing aircraft all the way out to the runway to save fuel.  I never read anywhere if the experiment had any merit.  

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Our air travel industry is in terrible shape.  That article mentioned that each increasing full plane has seats that were sold for 200 different prices.  It's time to put and end to all this mess.  

The price to fly coach should be one price.  All tickets must be paid for in advance and there should be no refunds for missed flights unless the carrier is able to sell your seat before the flight departs less a cancellation fee.

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If you miss a flight and the airline is able to sell you seat before departure you would still incur a 30% ccancellation fee  This would eliminate overbooking.  Use it or loose it.

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I get a lot of negative feedback when I mention it but the time may be right for the re-regulation of the airlines and prices the same way the U.S. government subsidizes the railroads.

A country with a strong economy needs an efficient airline system that does not leave the traveler at the mercy of company economics and runaway fuel prices.

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Answer 8

No its not to good of an idea. "Hello ladies and gentelmen , welcome to Northwest blah...blah ....sorry I mean Delta Airlines , Your flight from Newark liberty to LAX , inflight time will be 9 hours. lol