Change Forum... Civil Aviation Travel, Polls & Prefs Tech/Ops Aviation Hobby Aviation Photography Photography Feedback Trip Reports Military Av & Space Non-Aviation Site Related LIVE Chat My Starred Topics | Profile | New Topic | Forum Index | Help | Search
 Why Better Fuel Economy At Higher Altitude?
 Yhmfan From Canada, joined Feb 2004, 608 posts, RR: 0Posted Mon Sep 6 2004 03:59:29 UTC (11 years 9 months 1 hour ago) and read 20270 times:

 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 From United Kingdom, joined Jan 2001, 3524 posts, RR: 2 Reply 1, posted Mon Sep 6 2004 04:39:24 UTC (11 years 9 months ago) and read 20251 times:

 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 From Australia, joined Dec 2003, 1500 posts, RR: 22 Reply 2, posted Mon Sep 6 2004 06:16:28 UTC (11 years 8 months 4 weeks 1 day 23 hours ago) and read 20178 times:

 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 From Greenland, joined Feb 2004, 17668 posts, RR: 65 Reply 3, posted Mon Sep 6 2004 06:29:59 UTC (11 years 8 months 4 weeks 1 day 23 hours ago) and read 20160 times:

 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."
 QantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 22 Reply 4, posted Mon Sep 6 2004 06:51:48 UTC (11 years 8 months 4 weeks 1 day 22 hours ago) and read 20140 times:

 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 From United Kingdom, joined Feb 2002, 2185 posts, RR: 26 Reply 5, posted Mon Sep 6 2004 12:51:29 UTC (11 years 8 months 4 weeks 1 day 16 hours ago) and read 20084 times:

 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 From United States of America, joined Jul 1999, 30410 posts, RR: 57 Reply 6, posted Mon Sep 6 2004 13:28:53 UTC (11 years 8 months 4 weeks 1 day 16 hours ago) and read 20069 times:

 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 From Canada, joined Feb 2004, 608 posts, RR: 0 Reply 7, posted Mon Sep 6 2004 15:08:29 UTC (11 years 8 months 4 weeks 1 day 14 hours ago) and read 20043 times:

 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 From United States of America, joined Sep 1999, 7228 posts, RR: 7 Reply 8, posted Tue Sep 7 2004 19:29:56 UTC (11 years 8 months 4 weeks 10 hours ago) and read 19938 times:

 "...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 From United Kingdom, joined Feb 2002, 2185 posts, RR: 26 Reply 9, posted Tue Sep 7 2004 20:51:23 UTC (11 years 8 months 4 weeks 8 hours ago) and read 19914 times:

 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 From United Kingdom, joined Mar 2004, 825 posts, RR: 4 Reply 10, posted Tue Sep 7 2004 22:11:06 UTC (11 years 8 months 4 weeks 7 hours ago) and read 19894 times:

 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 From United States of America, joined Jul 2000, 259 posts, RR: 1 Reply 11, posted Wed Sep 8 2004 00:47:55 UTC (11 years 8 months 4 weeks 4 hours ago) and read 19863 times:

 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 From Greenland, joined Feb 2004, 17668 posts, RR: 65 Reply 12, posted Wed Sep 8 2004 01:56:35 UTC (11 years 8 months 4 weeks 3 hours ago) and read 19843 times:

 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 From United States of America, joined Sep 1999, 7228 posts, RR: 7 Reply 13, posted Wed Sep 8 2004 02:34:52 UTC (11 years 8 months 4 weeks 3 hours ago) and read 19831 times:

 Well, yes, the airframe wouldn't like it-- but would the engine object to M0.8?
 Arkhem From Ghana, joined Jul 2004, 128 posts, RR: 0 Reply 14, posted Sat Sep 11 2004 08:01:40 UTC (11 years 8 months 3 weeks 3 days 21 hours ago) and read 19617 times:

 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 From United States of America, joined Sep 2003, 3176 posts, RR: 9 Reply 15, posted Sat Sep 11 2004 14:22:42 UTC (11 years 8 months 3 weeks 3 days 15 hours ago) and read 19576 times:

 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 From United Kingdom, joined Feb 2002, 2185 posts, RR: 26 Reply 16, posted Sun Sep 12 2004 15:52:59 UTC (11 years 8 months 3 weeks 2 days 13 hours ago) and read 19518 times:

 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.
 Top Of Page Change Forum... Civil Aviation Travel, Polls & Prefs Tech/Ops Aviation Hobby Aviation Photography Photography Feedback Trip Reports Military Av & Space Non-Aviation Site Related LIVE Chat Forum Index

Reply To This Topic Why Better Fuel Economy At Higher Altitude?
• Tech/Ops related posts only!
• Not Tech/Ops related? Use the other forums
• No adverts of any kind. This includes web pages.
• No hostile language or criticizing of others.
• Do not post copyright protected material.
• Use relevant and describing topics.
• DETAILED RULES

 Similar topics: More similar topics...
Why The Little Hole At The Bottom Of The Window? posted Tue Oct 17 2006 03:19:07 by Gh123
Vref At Higher Field Elevation posted Wed Jul 19 2006 23:23:04 by XXXX10
Fuel Useage At Major Airports? posted Sat Jan 21 2006 04:33:40 by Jourdan747
Why Is Fuel Coming Out The Wing? posted Sun Oct 23 2005 13:49:51 by Wrighbrothers
PW2040 Fuel Burn At Max Thurst At Takeoff posted Wed Sep 21 2005 21:41:27 by 9V-SPJ
777 Take-off - At What Altitude Can I 'relax'? posted Tue Apr 26 2005 02:56:47 by Visionfusion
No Turbulence In Higher Altitude? posted Fri Dec 17 2004 11:59:00 by Aidan
What Restrictions Exist At High Altitude Airports? posted Tue Sep 28 2004 14:19:01 by Mozart
737 JT8 Vs. CFM56 Fuel Economy posted Tue Jun 24 2003 09:05:30 by Miles_mechanic
Airliners Cruising At Low Altitude posted Tue Mar 25 2003 13:18:37 by Il75
Landing Lights On At Cruise Altitude posted Sun Sep 16 2007 09:14:10 by Thai744
Cabin Pressurisation At High Altitude Airports posted Mon Feb 26 2007 19:13:05 by Mozart
Wing-to-Fuselage Fairings: Why No Fuel Tanks? posted Tue Nov 13 2007 04:22:40 by Faro
Do Cleaner Airplanes Get Better Fuel Mileage posted Wed Sep 19 2007 20:24:55 by Warowl40
Landing Lights On At Cruise Altitude posted Sun Sep 16 2007 09:14:10 by Thai744
Cabin Pressurisation At High Altitude Airports posted Mon Feb 26 2007 19:13:05 by Mozart