JAM747
Topic Author
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Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 12:36 am

I know that there are certain speeds an aircraft cruise depending on its type. However if a aircraft was to cruise slower would it help overall fuel economy. For example if a 747 normally cruise at mach .85 and was to cruise at .83 or .84 would it increase its fuel economy.

[Edited 2005-10-17 17:36:36]
 
miamiair
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RE: Does Flying

Tue Oct 18, 2005 12:37 am

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Pihero
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RE: Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 1:49 am

JAM747,

Vast subject !
I'll start with a few pointers and the rest of us will elaborate, I'm sure.
1/- If by fuel economy, you refer to the trip burn-off, yes , a lower speed wiil improve your consumption.Up to a point. The main parameter is the aircraft incidence/AOA.You will then fly an incidence that would maximize your fuel flow.That incidence, identified on the "polar" graph means thatr your speed will decrease along with your weight. The catch is the bottom of that graph is a hard limit, flying slower means increasing your AOA,thus increasing the drag,thus your fuel burn-off.
On a 744, that speed normally gives you a .86/87 Mach at max weight, going down to .81/82 at the end of your flight.
2/-Economics are a balance of costs....Direct operating cost takes in all the expenses incurred for an average hour of flight.And very often, flying too slow will in fact increase your expenses (maintenance,crew,....). That's the reason why we'd input a cost index into our flight computers (its a formula that balances fuel price with DOC -and that statement is very simplistic).

That's the bulk of it, without going too far into flight aerodynamics and economy.

Regards.
Contrail designer
 
JAM747
Topic Author
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RE: Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 2:25 am

Quoting Pihero (Reply 2):
Vast subject !
I'll start with a few pointers and the rest of us will elaborate, I'm sure.
1/- If by fuel economy, you refer to the trip burn-off, yes , a lower speed wiil improve your consumption.Up to a point. The main parameter is the aircraft incidence/AOA.You will then fly an incidence that would maximize your fuel flow.That incidence, identified on the "polar" graph means thatr your speed will decrease along with your weight. The catch is the bottom of that graph is a hard limit, flying slower means increasing your AOA,thus increasing the drag,thus your fuel burn-off.
On a 744, that speed normally gives you a .86/87 Mach at max weight, going down to .81/82 at the end of your flight.
2/-Economics are a balance of costs....Direct operating cost takes in all the expenses incurred for an average hour of flight.And very often, flying too slow will in fact increase your expenses (maintenance,crew,....). That's the reason why we'd input a cost index into our flight computers (its a formula that balances fuel price with DOC -and that statement is very simplistic).

That's the bulk of it, without going too far into flight aerodynamics and economy.

Regards.

Thanks for your explanation. I did not realise that the the topic was so vast and technical.
Regards.
 
727EMflyer
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RE: Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 3:15 am

Even a NW DC-9 thread could get vast and technical in this forum!

The aircraft manufacturers are nice enough to work out the math, and verify with test flights, what speeds will offer the best fuel economy. Even a little ole' C172's POH offers charts and graphs to help the pilot choose a speed. While working on my PPL my instructor advised me to fly at 2300 rpm, claiming it was best economy, and like a buffoon I took his word. However on my checkride, the DPE took me throught the tables and we worked out some math determined that operating at 2400 RPM gave a negligible increase in fuel consumption while providing a noteworthy increase in airspeed. Surely the airlines will care about a few hundred pounds of jet fuel, but like Pihero said, that fuel may be less expensive than the salaries that are being paid so long as the plane is moving. In the case of my wet rental rate though... hmm, which do I want more, a few tenths of an hour more in my log book or a couple ten dollars left in my pocket?????
 
777236ER
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RE: Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 3:34 am



The total drag is shown in red, as a product of parasitic and induced drag. The maximum endurance (and hence least fuel use for a given unit of time) is at the turning point of the total drag curve. This is the maximum L/D airspeed point.

As fuel is burnt and the aircraft becomes lighter, the induced drag decreases and the maximum L/D occurs at a lower airspeed, and at a lower value of total drag. Bear in mind though that the actual value of L/D is only a funtion of angle of attack, not aircraft weight.

Commercial aircraft fly at a point on the positive gradient part of the total drag curve (the maximum range speed is the point at which a line drawn from the origon of that graph intercepts the total drag curve. So yes, commercial aircraft will use less fuel if they fly slower, up to a point, when the induced drag increases.
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HiFi
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RE: Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 4:39 am

L/D is not the best way to rate comercial flight efficiency.. It does not take the time variable into account!

A measure commonly used in the industry is ML/D (Mach*Lift/Drag). Just like Pihero explained, "time is money", and for the vast majority of airliners, the optimum cruise efficiency is at a speed slightly higher than minimum drag.. It represents a very small drag increase for a significant Mach increase.

Refer to some aircraft design bibliography if you want some more details and the maths of it all, it's very well documented.  Wink
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mrocktor
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RE: Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 4:44 am

Good answers already, basically:

1- Certain costs are related directly to flight duration, such as pilot and cabin crew wages.

2- Revenue per aircraft is also tied to flight duration, as additional sectors may be possible in one day by flying faster.

3- The drag curve has a global minimum, so there is a speed that minimizes fuel burn rate, however the minimum fuel burn rate does not mean minimum fuel burn for the trip (if you burn 4% less fuel per hour and fly 5% slower, you are actually spending more fuel).

A frequent parameter used to evaluate performance is ML/D, or mach*lift/drag. This is sometimes called "productivity". Of course it does not bring into the equation the costs I mentioned in (1) and (2), but it describes (3) pretty well.

mrocktor
 
777236ER
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RE: Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 7:02 am

Quoting Mrocktor (Reply 7):
however the minimum fuel burn rate does not mean minimum fuel burn for the trip (if you burn 4% less fuel per hour and fly 5% slower, you are actually spending more fuel).

Which is why for minimum fuel burn per unit distance, you fly at the maximum range speed, which is marginally higher than the maximum range speed (max L/D speed).
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rendezvous
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RE: Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 7:26 am

I believe the FMC on Boeing aircraft has two options: LRC (long range cruise) and ECON (economy cruise). The LRC burns the minimum fuel per distance I believe, whereas the ECON burns slightly more furl but takes less time. Is that correct?
 
777236ER
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RE: Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 8:28 am

Quoting 777236ER (Reply 8):
the maximum range speed, which is marginally higher than the maximum range speed (max L/D speed).

Lies! I meant, 'the maximum range speed, which is marginally higher than the maximum endurance speed (max L/D speed).
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Aviation
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RE: Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 8:28 am

Id say it would, but the greater benifit would be high altitude.

Also you have to think in a car you save fuel by going slower, however on a plane this is less effected because of air density at altitude, Drag in other words is less at high altitude. So, the effect of velocity vs. drag is reduced.

But, heres the big one does an airline what 5 flights a day or 4 flights a day and save a little on fuel costs?
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dc863
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RE: Does Flying Slower Saves Fuel?

Tue Oct 18, 2005 1:24 pm

During the fuel crisis in 1973/4 airlines flying longhaul often cruised at lower Mach speeds to save fuel. Modern Air was able to fly nonstop trans Atlantic charters using their thirsty CV990s flying at Mach .80.
 
L-188
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RE: Does Flying Slower Saves Fuel?

Wed Oct 19, 2005 11:05 am

All other things being equal you want to be at the bottom of the lift/drag cure that 777236ER put up.

That is all other things.

There really are way too many factors to narrow down fuel economy to a single factor.
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PhilSquares
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RE: Does Flying Slower Saves Fuel?

Wed Oct 19, 2005 3:42 pm

Quoting Rendezvous (Reply 9):
believe the FMC on Boeing aircraft has two options: LRC (long range cruise) and ECON (economy cruise). The LRC burns the minimum fuel per distance I believe, whereas the ECON burns slightly more furl but takes less time. Is that correct?

You are correct. However, at higher gross weights, LRC will be a higher speed with slightly lower fuel flow and at lighter gross weights, LRC is slower with again, a lower fuel flow.

ECON is more a constant M cruise and varies less with large gross weight chagnes than LRC does.
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