JAM747
Topic Author
Posts: 524
Joined: Thu Feb 03, 2005 12:17 am

### Does Flying Slower Saves Fuel?

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
Posts: 4249
Joined: Thu Jul 15, 2004 9:42 pm

### RE: Does Flying

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Molon Labe - Proud member of SMASH

Pihero
Posts: 4318
Joined: Mon Jan 31, 2005 5:11 am

### RE: Does Flying Slower Saves Fuel?

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
Posts: 524
Joined: Thu Feb 03, 2005 12:17 am

### RE: Does Flying Slower Saves Fuel?

Thanks for your explanation. I did not realise that the the topic was so vast and technical.
Regards.

727EMflyer
Posts: 538
Joined: Wed Mar 09, 2005 3:22 am

### RE: Does Flying Slower Saves Fuel?

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
Posts: 12213
Joined: Sat Aug 18, 2001 7:10 am

### RE: Does Flying Slower Saves Fuel?

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.
Your bone's got a little machine

HiFi
Posts: 189
Joined: Fri Apr 29, 2005 5:36 am

### RE: Does Flying Slower Saves Fuel?

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.
no commercial potential

mrocktor
Posts: 1391
Joined: Mon Jan 24, 2005 12:57 am

### RE: Does Flying Slower Saves Fuel?

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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Joined: Sat Aug 18, 2001 7:10 am

### RE: Does Flying Slower Saves Fuel?

 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).
Your bone's got a little machine

rendezvous
Posts: 538
Joined: Sun May 20, 2001 9:14 pm

### RE: Does Flying Slower Saves Fuel?

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
Posts: 12213
Joined: Sat Aug 18, 2001 7:10 am

### RE: Does Flying Slower Saves Fuel?

 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).
Your bone's got a little machine

Aviation
Posts: 1110
Joined: Mon Dec 13, 2004 9:28 pm

### RE: Does Flying Slower Saves Fuel?

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?

Cheers,
Signed, Aaron Nicoli - Trans World Airlines Collector

dc863
Posts: 1471
Joined: Fri Jun 04, 1999 10:52 am

### RE: Does Flying Slower Saves Fuel?

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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Joined: Wed Jul 07, 1999 11:27 am

### RE: Does Flying Slower Saves Fuel?

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.
OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.

PhilSquares
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Joined: Sun Mar 28, 2004 6:06 pm

### RE: Does Flying Slower Saves Fuel?

 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.
Fly fast, live slow

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