yhmfan
Posts: 573
Joined: Tue Feb 17, 2004 2:44 pm

### Why Better Fuel Economy At Higher Altitude?

The topic says it all!
Is it something to do with less air resistance?
Can someone please explain this in simple terms.
Thanls
If at first you don't succeed, skydiving is not for you

Ikarus
Posts: 3398
Joined: Mon Jan 08, 2001 10:18 pm

### RE: Why Better Fuel Economy At Higher Altitude?

I'll be honest, I've mostly forgotten the reason.

I shall try to vaguely reconstruct it here - and I may be wrong.

As far as I know, a wing (or an entire plane) is designed towards achieving certain performance. That is measured in coefficients: Lift and drag coefficients (and the fraction between those, too).

For a wing of chord length 1 unit, these coefficients are essentially the force (lift or drag) divided by 1/2 times the air density times the velocity squared.

At high altitude, the density is lower.

This means two things:

1) in order to produce the same lift (or drag) at the same angle of attack, you have to fly faster. (the denominator of the eguation has to stay identical, so if density goes down, velocity squared has to go up to compensate)

2) if thrust = drag (steady flight), you get more speed out of the same amount of thrust, at higher altitudes.

Now the problem is: I've forgotten all of my propulsion knowledge. I believe that jet engines were the ones that produce the same thrust force, no matter which speed they're flying at (until they reach speeds close to Mach 1), and that turboprops are the ones that produce thrust that varies with airspeed. Now I suspect there's a further complication: probably the fuel consumption of an engine is not independent of air density and altitude (i.e. it may be higher or lower at cruise altitude, but I forgot which it is).

However, for the sake of an oversimplified (and therefore probably inaccurate) example:
Imagine you have an engine that produces a certain amount of force X, regardless of the airspeed. That means, your airplane can accelerate until it reaches a speed where its drag equals X. At high altitude, that speed is much higher than on the ground. So if the only factor determining fuel consumption is the output thrust force, then you are effectively using less fuel per distance flown if you fly higher. You get to travel further on the same gallon of fuel.

Now can someone clear up what the relation between fuel consumption, thrust force, and altitude is for jet engines? Because that's the bit where I'm most likely to have gone wrong in my assumptions, I think.

Sorry if I made any mistakes, or anything was not clear.

Regards

Ikarus

QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

### RE: Why Better Fuel Economy At Higher Altitude?

Basically, drag (of all types) is proportional to air density. Because air density obviously decreases with increases in altitude, drag therefore decreases at greater altitudes. For that reason, flying at higher altitudes is generally more efficient and economic.

Cheers,
QantasA332

Edit: Oh and Ikarus, jet engine thrust doesn't change very greatly with increases in speed while turboprops' and reciprocating engines' thrust does (it decreases), like you thought.

[Edited 2004-09-06 06:20:27]

Starlionblue
Posts: 17567
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Why Better Fuel Economy At Higher Altitude?

Oh and Ikarus, jet engine thrust doesn't change very greatly with increases in speed while turboprops' and reciprocating engines' thrust does (it decreases), like you thought.

The obligatory follow-up:

Isn't the fan just like a big prop in many ways? Why then is the behavior so different? Is it the ducting?
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

### RE: Why Better Fuel Economy At Higher Altitude?

The differences between thrust available (Ta) variation with airspeed for jet engines versus turboprops or reciprocating engines is really just a matter of intended use. The latter two are designed for lower speeds, and thus propulsive efficiency and Ta decreases as speed increases. The approximate opposite is true of jet engines, though the graph of jet engine Ta against airspeed is virtually a straight line (in reality Ta starts out high at low speeds, curves down slightly and then back up at higher speeds).

Cheers,
QantasA332

(Please excuse the quality and brevity of my explanations, as I just returned home from quite an ordeal following the SYD diversions last night...)

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Why Better Fuel Economy At Higher Altitude?

1) Jet engines are more efficient in colder air. I e, they gain in efficiency all the way up to the tropopause.

2) They are most efficient at high RPM.

3) The power remains largely constant with airspeed. It does, however, drop off with altitude.

4) The speed through the air (TAS) is higher for a given equivalent airspeed (same drag and lift) at high altitude.

1 is self-explanatory. 2 and 3 come together. You climb until the power at full RPM will maintain cruise speed. At low altitude, they have to have excess power for taking off and climbing etc. This means the aircraft would overspeed if you were to use the engines at optimum efficiency (full RPM) lower.

4 means you get further with the same effort higher up.

I probably missed a few factors and effects. Feel free to fill in.

Regards,
Fred

[Edited 2004-09-06 12:52:16]
I thought I was doing good trying to avoid those airport hotels... and look at me now.

L-188
Posts: 29881
Joined: Wed Jul 07, 1999 11:27 am

### RE: Why Better Fuel Economy At Higher Altitude?

The oversimplfied version of what everybody is saying.

Thinner Air=less air for engine to mix with fuel to burn=lower fuel burn at altitude, unfortunatly also mean engine won't produce as much power at altiude

Thinner Air=less drag, which means the you don't need as much power to keep the airplane flying.
OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.

yhmfan
Posts: 573
Joined: Tue Feb 17, 2004 2:44 pm

### RE: Why Better Fuel Economy At Higher Altitude?

The oversimplfied version of what everybody is saying.

L-188;
That's exactly the version that I need!!!

Thanks everyone.
If at first you don't succeed, skydiving is not for you

timz
Posts: 6223
Joined: Fri Sep 17, 1999 7:43 am

### RE: Why Better Fuel Economy At Higher Altitude?

"...jet engine thrust doesn't change very greatly with increases in speed"

Back in the early 1960s, some airline (maybe AA) was advertising the greater power of their fanjets, and another airline (probably UA) objected, saying their non-fan jets had just as much thrust as the fans at speeds over (as I recall) 125 knots.

Just for the record, we should add that at Mach 0.8, FL 350, hi-bypass fans have a cruise thrust around a fifth of their sea-level static thrust. What they would have at M0.8 at sea level is a question-- could they even operate at all?

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Why Better Fuel Economy At Higher Altitude?

Why don't you have a look?

[Edited 2004-09-07 20:53:40]

[Edited 2004-09-07 20:53:53]
I thought I was doing good trying to avoid those airport hotels... and look at me now.

phollingsworth
Posts: 635
Joined: Sat Mar 06, 2004 6:05 am

### RE: Why Better Fuel Economy At Higher Altitude?

The "better" fuel economy at altitude for jets stems from the fact that they are thrust limited, not power limited. The most efficient way to cruise an aircraft is at a constant q (dynamic pressure). This q when multiplied by the drag coefficient and reference area = drag and, therefore, when cruising thrust. q is proportional to velocity*velocity and inversely proportional to altitude. Therefore as you go up in altitude you have to increase your speed to maintain the best q.

Since jet engines are thrust limited. Their fuel consumption is based (primarily) on thrust and time. This means that a jet is endurance limited. That is for a constant q the endurance of the jet is pretty close to constant (These are the ideal assumptions, but are a pretty good first cut). Therefore, if you can stay in the air for three hours you can get a lot further by flying higher, and thereby faster.

This trend is limited by several factors:
1. Jet engine thrust capability decreases with altitude, and decreases faster than the increase in airspeed capability
2. Drag coefficient increases substantially above a certain Mach number. This limits how fast you can fly as the speed of sound decreases with altitude until ~36,000 ft and then remains constant.
3. For commercial aircraft you have to maintain cabin altitudes and emergency descent rates rules.
4. ATC will bugger any good idea up.

Aerotech
Posts: 254
Joined: Sun Jul 09, 2000 10:44 am

### RE: Why Better Fuel Economy At Higher Altitude?

Like Starlionblue said, a high-bypass turbofan is in many ways like a ducted turboprop, so why is there such a difference in performance at altitude?

Starlionblue
Posts: 17567
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Why Better Fuel Economy At Higher Altitude?

What they would have at M0.8 at sea level is a question-- could they even operate at all?

My guess would be that Bitchin' Betty would be yelling "overspeed" at the pilots and that parts of the wing would start falling off.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

timz
Posts: 6223
Joined: Fri Sep 17, 1999 7:43 am

### RE: Engine, Not Plane

Well, yes, the airframe wouldn't like it-- but would the engine object to M0.8?

arkhem
Posts: 125
Joined: Sun Jul 18, 2004 12:23 pm

### RE: Why Better Fuel Economy At Higher Altitude?

I always wondered why step-climb is used on long hauls if the higher the altitude the more fuel efficient the aircraft becomes. I assume it is because the weight-KIAS-altitude relationship equals a lower max. altitude at a given weight? Or maybe you cruise initially at a lower altitude because you need to extra thrust provided by denser air to propel the a/c at the selected cruise speed for a given weight? Furthermore why would an aircraft be less efficient if it climbed directly to max. cruise altitude than if it did a step-climb?

pilotpip
Posts: 2828
Joined: Fri Sep 19, 2003 3:26 pm

### RE: Why Better Fuel Economy At Higher Altitude?

In most cases, the aircraft is too heavy to get up higher earlier in the flight. As they burn some fuel off, the aircraft is able to climb higher and conserve fuel as mentioned above.

DMI

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

### RE: Why Better Fuel Economy At Higher Altitude?

Arkhem, read what I posted above. They climb until the donks will be at close to full RPM to keep cruise speed. Eventually, as the aircraft gets lighter, they'll have to decrease power to maintain this speed at which point they climb a bit further.

Then it can be made immensely more complicated... even without throwing ATC into the mix.

Regards,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

### Who is online

Users browsing this forum: Branzino, meecrob, sailsman1, Starlionblue and 2 guests

### Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos