What changes do modern aircraft make to lower fuel consumption?

This forum has been developed to discuss maintenance topics in Canada.

Moderators: sky's the limit, sepia, Sulako, North Shore

Post Reply
livinkingston
Rank 0
Rank 0
Posts: 1
Joined: Wed Feb 26, 2020 6:23 am

What changes do modern aircraft make to lower fuel consumption?

Post by livinkingston »

I guess I’m jumping out of my clan here… I’m a train guy. So for as long as I’ve been seriously learning about them it looks like they’ve made a march toward maximum economy. Diesel-electric, container cars, regenerative brakes, you name it. But the infrastructure of a train is a problem because money of course, but also friction on the track.

But an airplane though, it’s mostly suspended in the air! So even though it takes energy to fly, can this mean that airplanes will make less CO2 than other forms of transport?

It’s late in the day and I’m kind of bored, so I want to throw out some of my crackpot, bad ideas. Would you mind telling me if these are used at all, or if not, why?

• Induction Coils. Everybody knows that you can make electric current out of a magnet and coiled wire! So let’s say an airplane with some magnetized iron plates passes through a copper loop on the runway to slow it to a stop. Can the electricity help charge airport tugs to get the planes up again?

• Glider’s wings. Obviously you will want to move passengers quickly, but boats and automobiles are not too fast to begin with. Can a passenger plane use larger, more effective wings at the cost of speed, and still be competitive?

•Low-density chambers. Why don’t modern aircraft use a hybrid system for lift, where helium or hot air chambers supplement your usual jet engines? There must be a good reason fuel is being used for lift instead of thrust!
---------- ADS -----------
 
ahramin
Rank Moderator
Rank Moderator
Posts: 6309
Joined: Tue Feb 17, 2004 5:21 pm
Location: Vancouver

Re: What changes do modern aircraft make to lower fuel consumption?

Post by ahramin »

I was fortunate enough to attend a public forum with some very senior NASA engineers last summer and someone asked exactly the same question. Being a dumb pilot, my immediate thought was, "Stupid question, rockets aren't cars, this idiot has no clue how difficult a reentry is, move on."

The NASA engineer took the question very seriously but said that for a regenerative system to work you would have to carry both the energy capture and the energy storage system into space so it would have to be incredibly efficient to be net energy positive.

Same answer for your induction coils. The amount of energy used to stop a landing aircraft is microscopic compared to the total energy of the flight. The amount of extra fuel burned to carry extra weight on a long flight is significant. A flight to Europe from Toronto in a wide body aircraft will burn an extra 400 kg or so of fuel for every 1000 kg increase in weight. So if you put 1000 kg of iron bars in the plane you burn an extra 400 kg of fuel, which you now need to carry so you end up needing another 100 kg of fuel to carry the extra fuel to carry the extra weight.

Same for the low density chambers. They add extra weight. Plus at altitude the aircraft is in very low density air anyway, further reducing the efficiency.

As for the glider wings, you could save a massive amount of fuel by using big slow transports with long wings. But then your crew costs would be much higher and you'd need food and bunks for the passengers because YYZ-YVR is now going to take 20 hours and who is going to pay $300 for a 20 hour flight to YVR when $400 will get you there in under 5 hours?

We may someday see slow, semi-buoyant transports hauling cargo around but so far every attempt has been a disaster. Much cheaper and safer by sea and rail. Only the stuff that has to go fast goes by air, and it doesn't make sense to go slow by fast method.

If fuel was cheaper, airliners would fly slightly faster to keep the crew and maintenance costs down, but right now most are going almost as slow as possible because fuel is a disproportionately high cost compared to everything else.
---------- ADS -----------
 
goingnowherefast
Rank (9)
Rank (9)
Posts: 1947
Joined: Wed Mar 13, 2013 9:24 am

Re: What changes do modern aircraft make to lower fuel consumption?

Post by goingnowherefast »

Most airliners already have glider wings. Just look at the few instances of total engine failure on airliners. They managed to glide considerable distance, and typically were high/fast (too much kinetic and potential energy) on arrival to the runway.

If they're such good gliders, how come they fly at Mach 0.8? Cause they go quite high. The actual indicated airspeed and dynamic air pressure on the airframe is quite low. The air is thin and cold. The true air speed is high even though the indicated airspeed or "dynamic air pressure" on the aircraft is quite low. The plane's instruments will say it's going 250kts (460 km/hr) at 38000', the pressure on the front of the plane will be that of going 250kts in dense sea level air. Due to the altitude and really thin air it will actually be moving at 450kts, 840km/hr or Mach 0.79. That's a fairly simple explination and can get as complex as you'd like.

Why don't airliners have big helium tanks? Just look at what happens to weather balloons and atmospheric research balloons. They look really limp and underinflated at launch. However when they gain altitude they expand and begin to pressurize. When this pressure increases to the density of the surrounding atmosphere, the balloon is now bouyant and remains at that altitude. The same would happen with airliners, and all that extra gear to hold the helium is just extra weight that burns extra fuel (see previous post). Blimps and other bouyancy air vehicles are great at staying in one place, but horribly inefficient at going places. The size of the gas chambers makes the design very high drag. Again, they make a great Goodyear Blimp, but not an airliner.

So what are they doing to lower fuel consumption?

Making the wings even better at being glider wings. Just look at the folding wing tips on the 777x. They're too big to fit into regular airports and terminals, so they devised a way to fold them for ground operations. Wing tips that were unheard of 20 years are now everywhere and are looking more and more exotic with split tips going upwards and downwards.

Engines
The early jet engines ran quite cool, the metallurgy wasn't sufficient to handle higher temperatures and pressures. Fast forward to today. Modern jet engines run very hot and with very high pressure ratios. The turbine blades are made of exotic alloys, single crystal, have internal cooling chanels, some are hollow. Generally stuff that would blow your mind. They are capable of operating at temperatures very close to the melting point and under extreme stress. Bypass ratios are steadily increasing, not much air is actually running through the turbines and combustion part of the engine. The big diameter is a giant fan that takes a lot of the energy out of the jet exhaust and uses it to drive that giant fan. Now instead of pushing a little air very fast, the engines are pushing a lot of air a bit slower. Added bonus to fuel efficiency is the engines are a lot quieter too.

The next technological step is geared turbo-fans. That big fan at the front that I mentioned earlier is most efficient at low RPM. It's powered by a turbine in the core of the engine (the combustion part). Turbines like to turn really quite fast to be efficient. It's quite difficult to make a small, lightweight gearbox that can handle significant amount of power over a wide RPM range. Crude, unscientific method of converting thrust to hp means that a A320 engine is about 11000hp. The 777 engine is available as a ground turbine engine, typically used for military ships. In that form it makes 54,000hp. So yes, it needs one hell of a gearbox. They're slowly figuring that out and there are some geared turbofan engines available on the Airbus A220 and new models of the A320. Rolls-Royce is working on figuring out how to make it work on some really large engines, the kind that power the Airbus A350 and Boeing 777 mentioned above. The RR Trent Ultrafan is looking into variable pitch fan as well.
---------- ADS -----------
 
photofly
Top Poster
Top Poster
Posts: 11306
Joined: Tue Jan 18, 2011 4:47 pm
Location: Hangry and crankypated

Re: What changes do modern aircraft make to lower fuel consumption?

Post by photofly »

•Low-density chambers. Why don’t modern aircraft use a hybrid system for lift, where helium or hot air chambers supplement your usual jet engines? There must be a good reason fuel is being used for lift instead of thrust!
There is an existing class of aircraft that uses helium for lift - airships. Also there's a class that uses hot air: hot air balloons. The Hindenburg class Zeppelin of airships even used hydrogen.

Given the vast sizes of these classes of aircraft relative to their payload (the Hindenburg was three times as long and twice as tall as a 747 and carried one sixth of the number of passengers) it's not feasible to make them move very fast: 45 knots for the Zeppelin vs 600 knots for the 747.

Most of the inefficiency of air travel (inded any form of transport) is to do with speed. Shipping goods on a barge pulled by a donkey is trememdously efficient, but very slow. Nobody wants to spend a week crossing the Atlantic.
---------- ADS -----------
 
DId you hear the one about the jurisprudence fetishist? He got off on a technicality.
User avatar
PilotDAR
Rank 11
Rank 11
Posts: 4053
Joined: Sun Sep 30, 2012 6:46 pm
Location: Near CNJ4 Orillia, Ontario

Re: What changes do modern aircraft make to lower fuel consumption?

Post by PilotDAR »

The Hindenburg class Zeppelin of airships even used hydrogen.
The use of hydrogen is no longer permitted:
541.7 Lifting Gas

Hydrogen is not an acceptable lifting gas for use in airships.
Helium has half the lift per volume as hydrogen, and airplanes really don't have much unused volume in which to put a lifting gas. Then you have to look at the maintenance effort required to keep it there!
---------- ADS -----------
 
photofly
Top Poster
Top Poster
Posts: 11306
Joined: Tue Jan 18, 2011 4:47 pm
Location: Hangry and crankypated

Re: What changes do modern aircraft make to lower fuel consumption?

Post by photofly »

PilotDAR wrote: Thu Feb 27, 2020 5:37 am Helium has half the lift per volume as hydrogen
Your point is good, but this detail isn't correct: hydrogen gas weighs 0.09g/l at standard temperature and pressure, and helium under the same conditions weighs 0.179g/l. But the lift arises from the weight of air displaced (a.k.a. Archimedes principle), and the density of (dry) air at STP is 1.2g/l.

So a direct comparison of the two shows that helium when used as a lifting gas has (1.2-0.179)/(1.2-0.09) = 92% as much lift per volume as hydrogen.
---------- ADS -----------
 
DId you hear the one about the jurisprudence fetishist? He got off on a technicality.
User avatar
PilotDAR
Rank 11
Rank 11
Posts: 4053
Joined: Sun Sep 30, 2012 6:46 pm
Location: Near CNJ4 Orillia, Ontario

Re: What changes do modern aircraft make to lower fuel consumption?

Post by PilotDAR »

92% as much lift per volume as hydrogen.
Okay (he says in an Alvin the chipmunk voice....)
---------- ADS -----------
 
digits_
Top Poster
Top Poster
Posts: 5930
Joined: Mon Feb 14, 2011 2:26 am

Re: What changes do modern aircraft make to lower fuel consumption?

Post by digits_ »

photofly wrote: Thu Feb 27, 2020 6:10 am
PilotDAR wrote: Thu Feb 27, 2020 5:37 am Helium has half the lift per volume as hydrogen
Your point is good, but this detail isn't correct: hydrogen gas weighs 0.09g/l at standard temperature and pressure, and helium under the same conditions weighs 0.179g/l. But the lift arises from the weight of air displaced (a.k.a. Archimedes principle), and the density of (dry) air at STP is 1.2g/l.

So a direct comparison of the two shows that helium when used as a lifting gas has (1.2-0.179)/(1.2-0.09) = 92% as much lift per volume as hydrogen.
Using those numbers for helium, and a wikipedia 737 with a cabin volume of 44 cubic meter and fuel tank volume of 25941 liters, you could say that, theoretically on take off (dense air), a helium filled cabin could lift 44 kilograms. If burned fuel was replaced by helium, you could lift an additional 26.48 kg.

Doesn't really seem worth it in the grand scheme of things...
---------- ADS -----------
 
As an AvCanada discussion grows longer:
-the probability of 'entitlement' being mentioned, approaches 1
-one will be accused of using bad airmanship
photofly
Top Poster
Top Poster
Posts: 11306
Joined: Tue Jan 18, 2011 4:47 pm
Location: Hangry and crankypated

Re: What changes do modern aircraft make to lower fuel consumption?

Post by photofly »

digits_ wrote: Thu Feb 27, 2020 9:31 am
photofly wrote: Thu Feb 27, 2020 6:10 am
PilotDAR wrote: Thu Feb 27, 2020 5:37 am Helium has half the lift per volume as hydrogen
Your point is good, but this detail isn't correct: hydrogen gas weighs 0.09g/l at standard temperature and pressure, and helium under the same conditions weighs 0.179g/l. But the lift arises from the weight of air displaced (a.k.a. Archimedes principle), and the density of (dry) air at STP is 1.2g/l.

So a direct comparison of the two shows that helium when used as a lifting gas has (1.2-0.179)/(1.2-0.09) = 92% as much lift per volume as hydrogen.
Using those numbers for helium, and a wikipedia 737 with a cabin volume of 44 cubic meter and fuel tank volume of 25941 liters, you could say that, theoretically on take off (dense air), a helium filled cabin could lift 44 kilograms. If burned fuel was replaced by helium, you could lift an additional 26.48 kg.

Doesn't really seem worth it in the grand scheme of things...
It would be more efficient if you just remove all the air from the cabin and let the passengers sit in a vacuum. Then you don't even have to carry the helium, which (by the way) is quite expensive. Or you could just fill the cabin with hot bleed air, which is less dense than cool air, as though the fuselage were the envelope of a hot air baloon.
---------- ADS -----------
 
DId you hear the one about the jurisprudence fetishist? He got off on a technicality.
digits_
Top Poster
Top Poster
Posts: 5930
Joined: Mon Feb 14, 2011 2:26 am

Re: What changes do modern aircraft make to lower fuel consumption?

Post by digits_ »

photofly wrote: Thu Feb 27, 2020 10:37 am
digits_ wrote: Thu Feb 27, 2020 9:31 am
photofly wrote: Thu Feb 27, 2020 6:10 am
Your point is good, but this detail isn't correct: hydrogen gas weighs 0.09g/l at standard temperature and pressure, and helium under the same conditions weighs 0.179g/l. But the lift arises from the weight of air displaced (a.k.a. Archimedes principle), and the density of (dry) air at STP is 1.2g/l.

So a direct comparison of the two shows that helium when used as a lifting gas has (1.2-0.179)/(1.2-0.09) = 92% as much lift per volume as hydrogen.
Using those numbers for helium, and a wikipedia 737 with a cabin volume of 44 cubic meter and fuel tank volume of 25941 liters, you could say that, theoretically on take off (dense air), a helium filled cabin could lift 44 kilograms. If burned fuel was replaced by helium, you could lift an additional 26.48 kg.

Doesn't really seem worth it in the grand scheme of things...
It would be more efficient if you just remove all the air from the cabin and let the passengers sit in a vacuum. Then you don't even have to carry the helium, which (by the way) is quite expensive. Or you could just fill the cabin with hot bleed air, which is less dense than cool air, as though the fuselage were the envelope of a hot air baloon.
Maintaining a vacuum that size would require a much stronger fuselage though and at ground level it would still only lift 52.8 kg.
---------- ADS -----------
 
As an AvCanada discussion grows longer:
-the probability of 'entitlement' being mentioned, approaches 1
-one will be accused of using bad airmanship
photofly
Top Poster
Top Poster
Posts: 11306
Joined: Tue Jan 18, 2011 4:47 pm
Location: Hangry and crankypated

Re: What changes do modern aircraft make to lower fuel consumption?

Post by photofly »

digits_ wrote: Thu Feb 27, 2020 10:46 am
photofly wrote: Thu Feb 27, 2020 10:37 am
digits_ wrote: Thu Feb 27, 2020 9:31 am

Using those numbers for helium, and a wikipedia 737 with a cabin volume of 44 cubic meter and fuel tank volume of 25941 liters, you could say that, theoretically on take off (dense air), a helium filled cabin could lift 44 kilograms. If burned fuel was replaced by helium, you could lift an additional 26.48 kg.

Doesn't really seem worth it in the grand scheme of things...
It would be more efficient if you just remove all the air from the cabin and let the passengers sit in a vacuum. Then you don't even have to carry the helium, which (by the way) is quite expensive. Or you could just fill the cabin with hot bleed air, which is less dense than cool air, as though the fuselage were the envelope of a hot air baloon.
Maintaining a vacuum that size would require a much stronger fuselage though and at ground level it would still only lift 52.8 kg.
Ok. As a compromise, how about we just don't pressurize it at all. Then at altitude it only has what, 1/3 of the air inside, so you get 2/3 of the benefits of pumping it down to vacuum, and you can just ditch all that messy and heavy pressurization equipment; and the hull actually gets lighter since it doesn't need to be strong enough to be pressurized.

The passengers will sleep better, too.
---------- ADS -----------
 
DId you hear the one about the jurisprudence fetishist? He got off on a technicality.
digits_
Top Poster
Top Poster
Posts: 5930
Joined: Mon Feb 14, 2011 2:26 am

Re: What changes do modern aircraft make to lower fuel consumption?

Post by digits_ »

photofly wrote: Thu Feb 27, 2020 11:18 am
digits_ wrote: Thu Feb 27, 2020 10:46 am
photofly wrote: Thu Feb 27, 2020 10:37 am
It would be more efficient if you just remove all the air from the cabin and let the passengers sit in a vacuum. Then you don't even have to carry the helium, which (by the way) is quite expensive. Or you could just fill the cabin with hot bleed air, which is less dense than cool air, as though the fuselage were the envelope of a hot air baloon.
Maintaining a vacuum that size would require a much stronger fuselage though and at ground level it would still only lift 52.8 kg.
Ok. As a compromise, how about we just don't pressurize it at all. Then at altitude it only has what, 1/3 of the air inside, so you get 2/3 of the benefits of pumping it down to vacuum, and you can just ditch all that messy and heavy pressurization equipment; and the hull actually gets lighter since it doesn't need to be strong enough to be pressurized.

The passengers will sleep better, too.
And that would be an actual weight saving. Look at that, improving aviation one step at a time!

Seriously though, there must be a market for an unpressurized freighter that can take a heavier load....
---------- ADS -----------
 
As an AvCanada discussion grows longer:
-the probability of 'entitlement' being mentioned, approaches 1
-one will be accused of using bad airmanship
User avatar
PilotDAR
Rank 11
Rank 11
Posts: 4053
Joined: Sun Sep 30, 2012 6:46 pm
Location: Near CNJ4 Orillia, Ontario

Re: What changes do modern aircraft make to lower fuel consumption?

Post by PilotDAR »

Seriously though, there must be a market for an unpressurized freighter that can take a heavier load....
Image

I watch them come and go when I'm working in Germany. 2 in the same day last week! They are awesome!
---------- ADS -----------
 
Post Reply

Return to “Maintenance”