yhmfan
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Why Better Fuel Economy At Higher Altitude?

Mon Sep 06, 2004 10:59 am

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
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RE: Why Better Fuel Economy At Higher Altitude?

Mon Sep 06, 2004 11:39 am

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
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RE: Why Better Fuel Economy At Higher Altitude?

Mon Sep 06, 2004 1:16 pm

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]
 
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Starlionblue
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RE: Why Better Fuel Economy At Higher Altitude?

Mon Sep 06, 2004 1:29 pm

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:  Big grin

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."
 
QantasA332
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RE: Why Better Fuel Economy At Higher Altitude?

Mon Sep 06, 2004 1:51 pm

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
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RE: Why Better Fuel Economy At Higher Altitude?

Mon Sep 06, 2004 7:51 pm

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]
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L-188
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RE: Why Better Fuel Economy At Higher Altitude?

Mon Sep 06, 2004 8:28 pm

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.
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yhmfan
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RE: Why Better Fuel Economy At Higher Altitude?

Mon Sep 06, 2004 10:08 pm

The oversimplfied version of what everybody is saying.

L-188;
That's exactly the version that I need!!!  Smile/happy/getting dizzy

Thanks everyone.
If at first you don't succeed, skydiving is not for you
 
timz
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RE: Why Better Fuel Economy At Higher Altitude?

Wed Sep 08, 2004 2:29 am

"...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
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RE: Why Better Fuel Economy At Higher Altitude?

Wed Sep 08, 2004 3:51 am

Why don't you have a look?

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

[Edited 2004-09-07 20:53:53]
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phollingsworth
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RE: Why Better Fuel Economy At Higher Altitude?

Wed Sep 08, 2004 5:11 am

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
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RE: Why Better Fuel Economy At Higher Altitude?

Wed Sep 08, 2004 7:47 am

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

Wed Sep 08, 2004 8:56 am

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."
 
timz
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RE: Engine, Not Plane

Wed Sep 08, 2004 9:34 am

Well, yes, the airframe wouldn't like it-- but would the engine object to M0.8?
 
arkhem
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RE: Why Better Fuel Economy At Higher Altitude?

Sat Sep 11, 2004 3:01 pm

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
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RE: Why Better Fuel Economy At Higher Altitude?

Sat Sep 11, 2004 9:22 pm

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
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RE: Why Better Fuel Economy At Higher Altitude?

Sun Sep 12, 2004 10:52 pm

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.  Big grin

Regards,
Fred
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