Can we build a more efficient airplane? Not really, says physics.

Update (13 October):  I emailed David MacKay to get his opinion on some of the critical comments responding to this blog post. David is a physicist at Cambridge University, author of the book ‘Sustainable Energy – Without the Hot Air’, and is the chief scientific adviser to the UK Department of Energy and Climate Change. You can read his response in the comments below. There’s also a interesting discussion of this post over at hacker news.

Boeing recently launched a new line of aircraft, the 787 Dreamliner, that they claim uses 20% less fuel than existing, similarly sized planes.

How did they pull off this sizeable bump in fuel efficiency? And can you always build a more fuel-efficient aircraft? Imagine a hypothetical news story, where a rival company came up with a new type of airplane that used half the fuel of its current day counterparts. Should you believe their claim?

More generally, do the laws of physics impose any limits on the efficiency of flight? The answer, it turns out, is yes.

Jet Man, by Ben Heine

There’s something about flying that doesn’t sit well with us. If we never saw a bird fly, it may never have occurred to us to build flying machines of our own.

Here’s where I think this sense of unease comes from. It takes stuff to support stuff. Everyday objects fall unless other things get in their way. Take the floor away, and you’ll plummet to your doom – the air below your feet isn’t going to do much for you. We move through air so effortlessly, that we barely notice it’s there. So what keeps a plane up? There doesn’t seem to be enough ‘stuff’ there to hold up a bird, let alone a Boeing aircraft weighing up to 500,000 pounds. To put that last number in context, its more than the weight of an adult blue whale!

Why is it that planes fly and whales typically don’t? The answer is easy to state, but its consequences are rather surprising. Planes fly by throwing air down. That’s basically it. It’s an important point, so I’ll say it again. Planes fly by throwing air down.

As a plane hurtles through the air, it carves out a tube of air, much of which is deflected downwards by the wings. Throw down enough air fast enough, and you can stay afloat, just as the downwards thrust of a rocket pushes it up. The key is that you have to throw down a lot of air (like a glider or an albatross), or throw it down really fast (like a helicopter or a hummingbird).

A physicist’s two-step guide to flight (it’s simple, really!)

Let’s make this idea more quantitative. Following David MacKay’s wonderful book on Sustainable Energy, I’m going to build a toy model of flight. A good model should give you a lot of bang for the buck. The means being able to predict relevant quantities about the real world while making a minimum of assumptions.

Toy models gone wrong. By Randall Munroe at XKCD.

Step 1: Sweep out a tube of air

As a plane moves, it carves out a tube of air. This air was stationary, minding its own business, until the airplane rammed into it. This costs energy, for the same reason your car’s fuel efficiency drops when you speed up on the highway. Your car has to shove air out of its way.

Exactly how much energy does this cost? You might remember from high school physics that it takes an amount of energy equal to 1/2 m v^2 to bring stuff with mass m up to a speed v.

In our case, we have

There’s still this mysterious factor of the mass of the air tube. To work this out, we can use a favorite trick in the toolbox of a physicist – unit cancellation. We can re-write the humble kilogram as a seemingly complicated product of terms.

What we’ve done here is to express an unknown mass of air in terms of other quantities that we do know. Each of these terms makes sense. Air that’s more dense will weigh more. A fatter plane (larger cross-sectional area) sweeps out more air, as does a faster plane. We’ve arrived at a meaningful result, just by playing around with units. In the words of Randall Munroe, unit cancellation is weird.

Put these two ideas together and here’s what you find:

Here’s a graph of what that looks like.

If you’re with me so far, we just found that for a plane to plow through air, it has to expend an amount of energy proportional to the speed of the plane to third power. (The extra factor of v comes from the fact that faster planes sweep out a larger mass of air.) If you want to go twice as fast, you need to work 8 times as hard to shove air out of your way.

We’ve arrived at a general rule about the physics of drag. This holds true for a car on the highway, or for a swimmer or cyclist in a race. It’s why drag racing cars get only about 0.05 miles to a gallon! If we want to reduce overall energy consumption by cars, one option is to lower the speed limits on highways.

What does this mean for our toy plane? It would seem that the slower the plane, the higher its efficiency. So are airplane speed limits also in order? Absolutely not! To see why, read on to the second half the story..

Step 2: Throw the air down

In order to fly, a plane must throw air downwards. This generates the lift that a plane needs to stay up. It turns out that slower planes have to throw air harder to stay afloat. That’s why slow moving hummingbirds and pigeons have to flap their wings frenetically. It’s also why planes extend flaps while landing – they’re not throwing the air fast enough, so they compensate by throwing more of it.

More precisely, for a plane to stay afloat, the speed of the air jettisoned downwards must be inversely proportional to the speed of the plane. (You can take my word for this, although if you want to see where it comes from, take a look at David MacKay’s book.)

So we can now work out the second part of the puzzle. How much energy does it take to throw air down? As before, this is given by

Just as we did in the first step, let’s express things in terms of the speed of the plane.

In words, the energy spent in generating lift is inversely proportional to the speed of the plane. Here’s what this looks like on a graph.

You can see from the plot that, as far as lift is concerned, slower flight is less efficient than faster flight, because you have to work harder in throwing air downwards.

There’s a lot to chew on here. To summarize, we’ve discovered that in making a machine fly, you have to spend energy (really fuel) in two ways.

  1. Drag: You need to spend fuel to push air away. This keeps you from slowing down.
  2. Lift: You need to spend fuel to throw air down. This is what keeps the plane afloat.

The total fuel consumption is the sum of these two parts.

If you fly too fast, you’ll spend too much fuel on drag (think of a drag racer or an F-16). Fly too slow, and you’ll have to spend too much fuel on generating lift, like a hummingbird furiously flapping its wings, powered by high calorie nectar. However, at the bottom of this curve there is a happy minimum, an ideal speed that resolves this tradeoff. This is the speed at which a plane is most efficient with its fuel. Be it through the ingenuity of aircraft engineers, or the ruthless efficiency of natural selection,  airplanes and birds are often fine-tuned to be as energy efficient as possible.

Here’s a plot of experimental data of the power consumption of different birds, as their flight speed varies.

You can see that it matches the qualitative predictions of the toy model.

But we can do more than this, and actually extract quantitative predictions from the model. An undergraduate schooled in calculus should be able to work out that special optimal speed at which energy consumption is a minimum. David MacKay plugs in the numbers in  his book, and finds that the optimal speed of an albatross is about 32 mph, and for a Boeing 747 is about 540 mph. Both these numbers are remarkably close to the real values. Albatrosses fly at about 30-55 mph, and the cruise speed of a Boeing 747 is about 567 mph. 

That’s a lot of mileage from a toy model!

And so our model teaches us that flying machines should never have speed limits. Whether made of metal or meat, every plane has an ideal speed. If you stray from this value, you have to pay for it in fuel cost. Slowing a car down may improve your mileage, but for a plane, the mileage actually gets worse.

And with this physicsy interlude into the world of albatrosses, hummingbirds, and jet planes, we come back to the question of the fuel efficiency of Boeing’s new aircraft.

You can actually use the model to work out the fuel efficiency of a plane. What you find is that it really just depends on a few factors: the shape and surface of the plane, and the efficiency of its engine. And of these factors, the engine efficiency plays the biggest role. So we would predict that engine efficiency, followed by improvements in body design might drive Boeing’s fuel savings.

This agrees with Boeing’s own assessment.

New engines from General Electric and Rolls-Royce are used on the 787. Advances in engine technology are the biggest contributor to overall fuel efficiency improvements.

New technologies and processes have been developed to help Boeing and its supplier partners achieve the efficiency gains. For example, manufacturing a one-piece fuselage section has eliminated 1,500 aluminum sheets and 40,000 – 50,000 fasteners.

Try as we like, we can’t squeeze a lot of improvement out of airplanes. Engines are already remarkably efficient, and you certainly can’t shrink the size of a plane by much, as economy class passengers can well attest. New manufacturing techniques could cut the amount of drag on the plane’s surface, but these improvements would only raise fuel efficiency by about 10%.

To quote David Mackay,

The only way to make a plane consume fuel more efficiently is to put it on the ground and stop it. Planes have been fantastically optimized, and there is no prospect of significant improvements in plane efficiency.

A 10% improvement? Yes, possible. A doubling of efficiency? I’d eat my complimentary socks.

References

I based this blog post on material I learnt from David MacKay’s fantastically clear book, Sustainable Energy without the Hot Air. It’s available online for free, and is highly recommended for anybody looking to use numbers to understand energy.

David MacKay (2009). Sustainable Energy – Without the Hot Air UIT Cambridge Ltd

I used this tip to make those XKCD style plots.

  • Sms

    Wow, beautifully explained! Where can I read more of your articles?

  • http://twitter.com/daurnimator daurnimator

    Solution: go higher; where the air is thinner. Now you expend less energy counteracting drag; and can go much faster. (hello concorde)

  • http://twitter.com/diegoandrade diegoandrade

    Exellent article I enjoyed it

  • http://codeflow.org/ Florian Bösch

    There’s tons of ways to make airplanes more efficient beyond body shape and engine efficiency.

    1) Weight. The majority of the weight of an airplane is its body, not what it transports (for passenger airplanes). The heaviest part of an airplane are its engines, and then comes all the rest. As material science progresses, lighter and lighter airplanes will be built.

    2) Weight #2. You could always build a lighter than air, or slightly less heavy than its building materials aircraft body. Although that’s currently mostly a tradeoff to speed and drag. But then who says airplanes have to go fast?

    3) Speed: If you go fast enough (hypersonic) you will “fly” in a bubble of vacuum only with the frontmost part of the airplane touching the atmosphere. Getting there is expensive, but once over the bump, you’ve eliminated most drag. Material science is just about getting to the point where sustained hypersonic travel becomes possible.

    4) Height: If you go higher up, drag reduces. Getting high up is expensive. But then you can glide the rest of the way. For long-distance flights this would be an extremely efficient way to get around (quickly, too).

    5) Drag: Contrary to popular belief, we don’t know jack squat about it. Simulations of laminar flow we do today are necessarily crude. Observations can only insufficiently reveal the minutest of details, and model tests are slow to perform. Sharks for instance have a fantastic drag coefficient which puts all human built airplanes to shame. How do they do that? We’re only starting to scratch the surface of an answer to that. As commonly available computing power increases, our simulations will get better, and our ability to come up with designs that have radically reduced drag will get better.

    6) Lift: Contrary to popular belief, we don’t know jack squat about it. For instance, by the physics as we know it, bumblebees should not be able to fly. We’re only beginning to scratch the answer to that question (somehow they use vortices they generated to generate more lift later on). Same answer as above, as our ability to compute more accurate aerodynamics gets better, we’ll arrive at radically new ways to generate more lift, more efficiently with less drag.

    7) Engines: liquid fossil fuel engines might be at the end of their efficiency improvement (a few more percent isn’t gonna make the cow fat). But liquid fossil fuel isn’t the only fuel. And internal combustion is an inherently inefficient process. Good electric motors outclass any internal combustion engine by far. If we find a way to produce air-breathing batteries with fantastic energy densities, that’d be much more efficient, and that research is ongoing.

    8) Lifting bodies: It’s been repeatedly shown that lifting body aircraft can drastically improve drag to lift to internal carrying capacity. That is because such airplanes devote all their structure to generate lift. They’re difficult to handle and expensive to build, but as material science, production methods and control systems progress, it’s one avenue for good improvement.

    • http://twitter.com/campbellluke Luke Campbell

      Beat me to it, the article shows one variable but there are MANY more to deal with and can be achieved and have been. Ramjet propulsion, higher altitudes etc.

      • http://www.empiricalzeal.com Aatish

        I disagree. See my other comments here.

    • http://www.empiricalzeal.com Aatish

      Thanks for posting such a detailed comment. I’d like to quickly address some of your points about improving flight efficiency.

      1. Weight – I agree that reducing the mass would imply a greater fraction overall weight being spent on cargo, and that could increase efficiency.

      2. Weight #2 – I’m pretty sure most people would not accept ballooning as an acceptable alternative to flight.

      3. Hypersonic speed – This is definitely an interesting idea, one that I wasn’t at all aware off, but I would be extremely wary of the huge drag cost that you would incur before hitting this valley. I’d like to see some evidence that this is really feasible and more energy efficient than conventional flight.

      4. Gliding – That’s a neat idea, and one that’s used by albatrosses and sharks to cover long distances. However, I’m not entirely convinced that the gain from coasting down balances the cost of gaining height. To glide effectively you would need to reduce the lift to drag ratio (like sailplanes), which would mean bigger, heavier wings, that reduce your efficiency. So there is a tradeoff between weight and increasing your gliding efficiency. Aircraft engineers already take this into account.

      5. Drag – I’d just like to add that the overall efficiency of flight goes as the square root of the drag coefficient. So while some improvement in drag coefficients are certainly possible, I don’t think the increase in efficiency will be dramatic.

      7. Engine efficiency – This is one of the key factors in determining overall efficiency, as I said in the post. But it is also perhaps THE central and hardest problem of the transportation industry.

      8. Fuel density: I agree with you here, although David MacKay argues that the density of fuel will not have a large effect on energy efficiency. (http://www.inference.phy.cam.ac.uk/withouthotair/cC/page_277.shtml)

      9. Lifting bodies – see response to 4

      That being said, my goal in writing this piece was to get people to think about just how difficult it is to increase the efficiency of flight, and I think our discussion is showing that we would need a somewhat radical rethinking of aircraft technology or significant breakthrough in materials technologies to achieve large improvements. Planes are ridiculously well optimized – this is my point.

      • http://codeflow.org/ Florian Bösch

        2. You’re missing an obvious contraption known as a “zeppelin”, it’s a rigid body hull over lifting cells. Traditional construction of these isn’t very suitable to much more speed than your average car. However advances in material science could change the equation for lighter than air vehicles significantly (they might make it possible to construct much more rigid and airtight frames then previously possible, thereby favoring more aerodynamic body shapes for higher sustained speeds).

        3. NASA is experimenting with hypersonic aircraft. So far they’re still wrestling with problems like sustaining oxydizing airflow inside the combustion chamber and with the whole thing disintegrating altogether. So the only thing we really know about them is pretty much theoretical with sparse real-world data inbetween. That will change however with time as more experiments are conducted.

        4. You perhaps missunderstood this point. I didn’t mean gliding as much as “coasting”. I.e. suborbital flight at altitudes with neglible atmospheric density. You have a huge energy expenditure at the start, but once you go suborbital, you can cover a huge distance with no fuel at all. You’ve eliminted drag altogether from “flying” and at the destination you’re first using the air to break, so no fuel expenditure there, and then glide down to the airport.

        5. This view of drag is somewhat flawed. Let’s look at an example where it breaks down entirely. Superfluidity. A fairly excotic branch of physics research. Now the interesting thing is that plasmas under certain conditions exhibit behaviors somewhat similar to superfluidity. While it may not be practical at the moment to encase an airplane in a plasma sheat to bring its drag to near zero. That may not be the case in the future. And that’s just scratching the surface of the things. Laminar flow is a much more complex field of enquiry than a few simple formulas about displacement and drag might suggest.

        Planes might be ridiculously efficient by our current technological standards. Radical re-thinking of airplane shapes and modes isn’t that unlikely. And the radical material science breaktroughs are mostly made, but not yet incorporated into mainstream commercial airplane construction. Over the last 20 or so years the last “radical breaktrough” (composit fibre materials) has been slowly seeping into airplane construction and its fruits are still ripening with every new airliner built. Things are not as radical or unlikely as you think, they just happen sufficiently slowly so you don’t know in what pot your frog is boiling, yet not so slowly that you couldn’t see that change is all about.

        Lastly, a vanishing supply of fossil fuels will either make “traditional” air traffic impossible, or we think of something radically new. It’s possible we just give up flying at a large scale, but who knows. Maybe not.

        • http://www.empiricalzeal.com Aatish

          Please try to honestly engage with the material before ruling it out as overly simplistic. Anyone can wave their hands and say ‘we don’t even know how a bumblebee flies’ but this is not how science works and both wrong and counter-productive.

          Here is what you suggested: ” If you go higher up, drag reduces. Getting high up is expensive. But then you can glide the rest of the way.”

          1. If I assume you mean what you said, then you are talking about flying in high altitude to reduce drag.

          Yes, it is true you will reduce drag. But that is assuming that you are flying at the same speed as before. Of course, this will NOT be true as the air is much less dense. So you will have to increase your speed to reach the new optimum speed (if you do not, you won’t get enough lift). And this makes the drag go back up again. If you read the details of this simple calculation (I have linked to it in my post), you will see that in the end the energy per unit distance does not depend on the density of air! It cancels out.

          Also, as my friend Deepak points out, at a higher altitude there is less oxygen making
          the engine combustion less efficient, and carrying oxygen will add to your overall weight.

          To everyone here who says you can fly higher to reduce energy consumption, you are wrong. It may intuitively sound like a good idea, but it’s not.

          2. In your response, you indicate that you are talking about coasting and not gliding. Whatever you call it, you can not fly in negligible atmospheric density. Flight requires lift, which requires air to be dense enough so that you get sufficient upwards momentum from throwing it downwards.

          What you seem to be suggesting, then, is not conventional flight, but suborbital spaceflight, where you essentially FALL to your destination (and glide at low altitude). I do not agree that this will be a more fuel efficient method of transport, let alone the obvious concerns with safety, reliability, and cost effectiveness.

          You can always recourse to future technologies and a better future understanding of physics, but this is entirely missing the point. Neither you nor I know what future physical principles may be discovered that change the equation, and until then we should make our best attempt to address questions of sustainability with physics as we know it and technology as we can reasonably expect it to be.

          • http://codeflow.org/ Florian Bösch

            I did just clarify that I meant suborbital flight. I don’t know what you can misconstrue about that. Carrying an oxidizer is of course not really efficient. However that is the whole point about hypersonic flight. Accelerate while the oxydizer is cheap. It’s really not much of a brainer to recognize that suborbital trajectories minus oxydizer mass are more energy efficient than pushing thousands of miles trough dense air if your goal is to hop around half the globe. The calculation has been done countless times and history is littered with attempts to make it happen. Sooner or later it will.

            Your article is also missleading. You’re laying flight out in your article as if that was the only way to do it. Yet your headline claims to have relevance for all of flight. Yet in your comment you crawl back to mean you’re talking just about the local maxima of currently used technology. Talking about a tripple standard here…

            And In case I’ve not made myself sufficently, abundantly clear. There are *many* ways that flight can be more efficient. And they’ve got nothing to do with your reasoned thought scribbled on a paper napkin and put on your “homepage”. They’ve got to do with the fact that flight will have to change, and that neither me, nor you, can forsee where things will go. But unlike you, I don’t assume we’ll stay in some local maximum. The only constant in history and technology is change. And if not change because we can. Then change because we must. If fossil fuel prices go trough 1000 per barrel, then trough 5000 per barrel, and then hit heights we haven’t even dreamed of. What then? Will we give up flight, referring to your inane scribblings? Will we come up with alternatives we can still afford? Will we finally make those radical changes because we’re forced to? I don’t know, but more importantly, you don’t know either. So don’t pretend your high-school algebra is holding any relevance whatsoever over the doability of flight.

  • Eric

    “The only way to make a plane consume fuel more efficiently is to put it on the ground and stop it. Planes have been fantastically optimized, and there is no prospect of significant improvements in plane efficiency.”

    Just like breaking the sound barrier was an impossibility. As other commenters have pointed out, there are plenty of technologies and aerodynamic theories that we will be able to take advantage of in the future. Blended-wing-body designs, lifting bodies, new propulsion techniques, blown surfaces, wingtip devices, planforms (http://en.wikipedia.org/wiki/Annular_wing, for example). All of these, if put into use, could cause significant efficiency gains from both the aerodynamic and fuel sides of the equation.

    Saying planes are tapped out and there’s no more efficiency to wring from them is foolish and demonstrates a pretty conclusive lack of knowledge about the field.

    • J Sterling

      The denial, it burns.

      I opened the comments to quibble that the author had ignored wing span in the part where you couldn’t go slower. It turns out increasing wing span lets you encounter more air to throw down, allowing you to slow the optimal speed.

      But there are mechanical limits on the wing span you can achieve. The military would love bombers with longer wings that could fly further than a B52, and airlines would love to save money by lengthening the wing of a 777 or an A380. It’s not like they’re truncating their wings at the current length because they don’t care.

      Of course you can get a few more percent out. The straw man cries of “hey, he thinks there’s absolutely no improvements to be made!” are just straw men. But halving the fuel use as the price doubles? Halving it again as it doubles again? Not going to happen. Two things are absolutely, law of physically certain: we will never be able to offer the five billion or so poorest people on Earth the flying experience we rich people take for granted, unless they make way more money than they do now; and flying will cost more money as fuel becomes more expensive. “Efficiency” isn’t going to fix that.

      • Eric

        The Russians have been developing cryo fuels since the 80s, if not earlier. Research into non-fossil fuels has been happening as well. It’s not as if we will only ever power airliners with Jet A, and I feel like that’s much of the logic behind claims that we cannot increase efficiency.

        Wing spans often have physical (but not physics) limitations. The 777 was originally designed with a longer span but gate size dictated they go with the current span. This is going to be a limitation into the future. Hangars are likewise too small for larger spans. They could build them, but the infrastructure costs are high.

        You’re right about costs, though. Low cost air travel is an anomaly brought about by low fuel prices and airlines’ ability to cut costs. When those both disappear, so will low cost air travel – where by low cost I mean relatively affordable to the masses.

        • J Sterling

          All those other fuels have a lower energy return on energy investment (EROEI) than fossil fuels, or they’d already be used in preference to fossil fuels. When fossil oil gets sufficiently expensive, we may reluctantly switch over to them, but that will be a less-expensive-than-oil-now sort of saving, not a less-expensive-than-oil-back-in-2012 sort.

          The logic behind claims that we cannot increase efficiency by very large amounts (again that straw man that the claim is we cannot increase efficiency at all) is based on physics of flight, not on the chemistry of any particular fuel. All the inputs are in Newtonian units of length, mass, and time, and show that if you want to raise the mass of a human into the air and drill him through the atmosphere across the Atlantic, using wings, it takes mega joules. Mega in the colloquial as well as literal sense. Where you get the joules from is irrelevant to this point.

          Air travel has never been affordable for the global masses, i.e. any large fraction of seven billion people. The “masses” we usually speak of when we say “air travel for the masses” are quite a minority. Physics says that unless the masses get much richer, it never will be. Air travel can’t be reduced down to a few dollars, the only way the global median traveler will ever be able to afford it is if his income rises above a few dollars.

  • GeorgeLocke

    “The only way to make a plane consume fuel more efficiently is to put it on the ground and stop it.” heh. great article, Aatish.

  • http://twitter.com/frankus Frank

    One quibble: The energy required varies with the cube of speed only if you’re comparing two flights of equal time.

    Normally you’d want to compare two flights of equal distance. A flight that’s twice as fast takes half as long, so you end up with the energy required varying with the square of speed.

    (The power required still goes up with the cube of speed, which is why it’s so much harder to make a car go 200mph than 100mph).

    • http://www.empiricalzeal.com Aatish

      Thanks for pointing that out. You’re right, I should really have labelled those plots as Power, not Energy. As you said, this is only energy if you are comparing flights of equal time. If you want energy/distance (a more reasonable unit of energy efficiency), each term gets divided by a factor of speed (Power/speed = Energy/distance), but the argument doesn’t change. You get the same optimum speed.

  • io

    could we just trow air on the plane instead of plane on the air ? :)

  • http://www.empiricalzeal.com Aatish

    I am posting here, in entirety, an email reply by David MacKay, the author of the book mentioned in this blog post. David is a physicist at Cambridge University and is the chief scientific adviser to the UK Department of Energy and Climate Change. Here is the link to the book he recommends. http://www.amazon.com/The-Simple-Science-Flight-Insects/dp/0262513137/ref=sr_1_1?ie=UTF8&qid=1350166607&sr=8-1&keywords=tennekes

    Dear Aatish,

    the people on your blog who say “There’s tons of ways to make airplanes more efficient beyond body shape and engine efficiency” really ought to engage their brains and read the friendly book before they allege that I have got it wrong.

    For example when they say “go high because drag is much less when you are high up”, they have simply not read the book. The energy consumption of a plane per unit distance (in the back of envelope model) has no dependence on the air density. This is a pretty cool result. The point is, yes, there is less drag AT A GIVEN SPEED, but the optimal speed of the plane also increases when the air density goes down, so the plane HAS to SPEED UP, so the drag goes back up again and it all cancels, amazingly, and the energy per distance is exactly the same. Flying higher makes no difference at all. Except that it allows you to travel at a faster optimal speed, so what I mean is it makes no difference _to fuel burn_.

    As for “Lift: Contrary to popular belief, we don’t know jack squat about it. ” – this is just stupid. To that person: Why not read a book first, then open mouth later when you have seen what is known? I recommend the lovely book by Tennekes (1997).

    all the best

    David

  • http://twitter.com/AkshatRathi Akshat Rathi

    Aatish: I think you are right about airplane efficiency with current take off routines. But if that were changed planes could become more efficient. For instance a 2 hour flight can can take up as much as 20% fuel while taking off.

    Currently there are two ways to make take off more effective. Both ideas are to catapult the plane into the air. One is crude and already in use: Steam-powered catapults. The more sophisticated one, that I think Boeing or Airbus is working on, is the to use a linear motor drive to accelerate the plane at predictable rates and then let it go.

    There are number of benefits. First if this was possible planes will need smaller engines than now, making planes lighter. Second the runways around the world can be smaller than they are today.

    • http://www.empiricalzeal.com Aatish

      Hey Akshat, that’s really interesting. I know this was used in aircraft carriers, but didn’t realize it was more effective. I wonder where the source of the improvement lies. The amount of energy you need to output is presumably the same in either case (catapult powered or self propelled), so I guess the catapult mechanism must have a higher engine efficiency?

      • http://twitter.com/AkshatRathi Akshat Rathi

        I think the energy efficiency is higher for the catapult. But combined with lighter engines and shorter runways there seems to be big enough energy saving for active research in this area. The Economist plans to write about this so I will let you know when it is published.

    • Haakon Dahl

      Hang on there, Bub. (feigns offense) There’s nothing crude about a steam catapult unless you’ve been assigned to clean its track. They are used on carriers to very rapidly accelerate lightweight aircraft to high speed (small wings after all) in a short distance. Hence the brutality of the motion. Meanwhile, nothing is being consumed (sacrificial uh railgun armature thing) and there are no massive current or fields sweeping the aircraft .
      It should certainly be possible to design a catapult with a gentle throw for a big heavy passenger jet given the permissible length of the device.
      But it will be more expensive and complicated than the simple and predictable system of runways and jet engines we have now. And I definitely want ALL of the currently available power to be retained for when the plane I’m on gets in trouble. Save all the jet fuel you wish on take-off; don’t fly me about without plenty of excess power in reserve.

  • revetahw

    Your logic about energy for lift increasing with slower speeds is only true for a given wing span, the reality is that the energy per unit distance for lift is roughly constant at different speeds if the wing size is chosen for the given speed.
    The energy consumption of a plane is determined by its weight divided by its lift to drag ratio. The best current gliders have a lift to drag ratio of over 70/1 while current commercial jets are under 20/1, if you would combine that improvement with a significant reduction in weight you can see that we are nowhere close to achieving optimal efficiency. Of course there are huge practical obstacles to making such planes possible but it is not true that physics prevents more efficient flight.

    • revetahw

      One way to increase the lift to drag ratio of airplanes is to increase the aspect ratio of the wings to reduce the induced drag by throwing more air down at a slower speed which means less kinetic energy is put into the air.

      Another way to increase the lift to drag ratio is to increase the wing size which lowers the speed required for the wing to achieve its optimal L/D ratio meaning the drag on the fuselage is reduced and the total L/D of the plane gets closer to the ideal L/D of a pure flying wing.

      To see what this would mean for an airplane equivalent to the boeing 737 just imagine increasing the aspect ratio of the wing from about 10:1 to 40:1. This would increase the wingspan from 117ft to 234ft while still having less efficient wings than the best gliders which have aspect ratio’s of over 50:1.

      Then consider increasing the wing area by a factor of 4 for the total lift to drag ratio of the plane to approach the ideal of the wing and you would end up with a wingspan of 470ft. Clearly there are some practical issues with airports. But the biggest obstacle is the strength of the materials, increasing the aspect ratio of the wings by a factor of 4 for a given wing area would require materials with a strength to weight ratio that is 8 times higher to be able to build the wings with the same weight as the original lower aspect ratio wings. Increasing the wing area by a factor of four would further double the required strength to weight ratio of the materials.

      If a plane like this could be built with a 4 times bigger wing area for a given weight it would have to fly 5-6 miles higher to achieve the much higher efficiency for a given speed. If it could be built at half the weight of current planes then the altitude would be increased by another 2.5-3miles, the end result would be a 6x increase in fuel efficiency and very different looking planes.

      http://2.bp.blogspot.com/-5GHOyUWmRQo/UCcNwe5U1oI/AAAAAAAABGg/Uxgn7g1Z6ao/s1600/229093_338088959611038_1802256166_n.jpg

    • revetahw

      I do not actually expect planes like this the size of current commercial jets will be built but i do think very small auto piloted planes for 1 to 4 passengers flying at altitudes of 100000 feet or higher could happen someday and they could achieve this kind of efficiency.

  • Emily Torem

    Hi Aatish,

    Thank you for this post, it is so engaging and informative! I cited it as a resource in a blog post I just wrote on the future of flight for Strawville, a blog on all things architecture, environmental and food. http://moss-design.com/2013/03/01/friday-favorites-the-future-of-flight/ If you get a chance, I’m curious as to what you think about changing the shape and/or fuel source as in the examples I listed.

    Thanks so much!

    Emily

  • http://twitter.com/pageeasy1 pageeasy.com

    I disagree with a lot here. Wings provide lift because of their cross section, which causes the air below the wing to be denser than the air above. This difference in pressure pushes the bottom of the wing upward. When a wing uses flaps to “throw down more air”, it does so at the cost of increased drag, which is a good thing when an aircraft is slowing down for a landing.

    • http://www.empiricalzeal.com Aatish

      I’m not sure what exactly you’re disagreeing with. I agree that wings provide lift and lead to an increase in drag, and by making your wings larger you’re also adding weight. So there’s an optimization process that goes into working out the most energy efficiency wing length that strikes a balance between lift, drag, and weight.

  • Mike Adli

    Dear Sirs,

    It is obvious that behind every car travelling down the road there is a colume of hot air left behind. That is also true about airplanes. There is a much more effiecient way to use energy.

    I am patent owner of the new invention: “Hot Compressed Gas Vehicle”, Patent Number: US 7,926,610 B2. A revolutionary vehicle that is completely environmentally friendly unlike any other vehicle to date. The patent was recently issued on April 19, 2011. The vehicle uses a combination of free or inexpensive compressed air, clean burning natural gas and closed loop steam as its power source and it is applicable automobiles as well as to aircraft.

    Briefly, compressed air is produced using solar or wind energy or produced by air compressors powered by the vehicle or braking or downhill energy which is stored in portable cylinders and is then regulated to approximately 400 psi similar to combustion pressures inside conventional engines. It is then heated in a combustion chamber to increase its volume and pressure (theoretically up to 150 times) by burning a clean fuel such as natural gas or alcohol or similar fuel entering the chamber at the same pressure of 400 psi. The temperature and pressure is controllable (preferable by a microprocessor) by adjusting the amount of fuel or by adding unheated compressed air into the chamber.

    Water flows inside the combustion chamber through a small tubular coil which is heated inside the combustion chamber to several hundred degrees as needed to immediately turn the water into steam. The temperature of the flame inside is up to 3000 degrees and the tubular coil (preferably stainless steel or similar metal with a higher melting temperatures) water entering with a similar pressure of 400 psi (flow amount is controlled by the microprocessor) flowing through the tube will turn into flash steam and up to 1700 times its volume and flows out of the tubular coil and combines with hot compressed exhaust gases also flowing out of the combustion chamber. The water with a much greater expansion values would exit with greater volume. The water coil inside the chamber will cool the hot air inside the chamber to an ideal temperature controlled by the microprocessor.

    As long as all pressures of entering air and gas and water are maintained at 400 (or any ideal pressure) then exiting steam and hot air combination will also have the same pressure but with substantial increase in volumes. Exiting flow rate is controlled by means of a throttle type valve leading to a double acting cylinders or air motor or to a turbine or similar type device resulting in a rotary power to propel an automobile or an aircraft.

    Exhaust gases exiting the rotary device are cooled as a result of expansion process within the device and steam will turn into water and will be gathered and re-circulated back into the combustion chamber. Other exhaust gases contain minimal amount of pollution similar to a natural gas stove used in our homes.

    In comparison of this concept with other systems it becomes apparent that it works similar to an internal combustion engine but without all the moving parts, wasted heat, friction, and excessive weights. It also works like a jet engine where compressed air is heated to produce trust but without the wasted 1600 degree hot column of exhaust flowing out a typical jet engine. It also works like a steam engine but without a boiler that can blow up and without all the resulting heat loss and wasted steam discarded into the atmosphere. We can say it works similar to all above three systems combined minus all the waste.

    I believe the efficiency of this concept can reach maximum obtainable. No other system to date has been able to achieve this level of efficiency. First, not a lot of compressed air or fuel is needed to produce power. Only enough compressed air and fuel is needed to produce a flame inside the combustion chamber in order to turn the water into steam. Water turning into steam is the main source of power and it is supplemented by hot combustion air. Second, this vehicle or aircraft would be very light weight. It will not have a heavy engine, a heavy transmission, differential (which is replaced by a simple tee connection), exhaust system, radiator, and all the supporting structures to carry all these heavy machinery. Substantial amount of the energy used will be converted into motion and none of it is wasted through a radiator or though a high friction engine and transmission. There is no place in this system where any heat is escaped into the atmosphere unlike other systems to date. The combustion chamber is insulated (photo 1a). Even if any heat is escaped, it would still be contained within the box (photo 3) which is also insulated inside and even outside of the box can be insulated as needed. The heat inside the box is needed to keep the valves and cylinders warm. A hot steam cylinder is much more desirable than a cold one.

    The box I have made containing cylinders, crankshaft, valves, and the combustion chamber weighs about 30 lbs. Compressed air tank is about 30 pounds. Natural gas tank is less than 5 lbs. The entire system is less than a 100 lbs. Weight of a filled 15 gallon gas tank in a typical car including all of the gas tank supporting structures are probably more than 200 lbs. alone.

    There is also a lot less friction in this system. In a typical internal combustion engine during the four stages of piston travel power is generated only in a fraction of a second during combustion. therefore, thousands of rpm and a heavy crankshaft with counter weights are needed to counter the momentary combustion forces. In this system power is applied to the piston every second at all times and up to 80% of the piston travel both ways. Only a very small rpm and a very light weight crank shaft without the need for counter weights is needed. Therefore not much heat is generated due to friction because there is not much friction at such low rpm.

    Overall efficiency is also increased by saving braking and downhill energy by means of powering air compressors to store compressed air back into the system.

    Considering all the above, I believe if it is correctly engineered and produced by a qualified organization it may achieve as much as 80% efficiency. An unheard of value up to now. I believe this invention is a technological breakthrough that could have an enormous positive impact on the environment. It has the potential to make gasoline-powered automobiles obsolete and thereby change the world as we know it.

    This is a novel concept that can be also applied to modify of existing automobiles. I am interested in discussing a possible cooperation to help with development of the first vehicle prototype or the first aircraft prototype.
    So there you have it. Thanks

  • Mariano Chouza

    See the D8.5 concept by Mark Drela in this presentation: http://web.mit.edu/drela/Public/N+3/tasaa.pdf
    It would require substantial modifications to existing airport infrastructure, but it offers a 70% reduction in fuel consumption.

  • http://www.facebook.com/people/David-Kay/100000559247894 David Kay

    Beyond what has been already said here in comments about increasing wingspan, etc in conventional airplane, there is also option of unconventional ideas.

    For example at slow speed, lighter than air craft can go vastly more efficient even then a glider especially if they follow the normal air stream.

    A ground effect an aircraft can achieve much more lift and thus more efficiency, and could get electric power from the ground to power electric motors for big weight savings, as a alternative to high speed train, would still have to be much slower than regular aircraft as air density resists high speeds.

    A network of gliders and elevators could travel with only fuel required for elevators.

    If the ground was like a checkerboard, half black and half white, a skilled glider could use the thermal updrafts to travel with no fuel.

  • Scotdoc

    What about Ground Effect Vehicles- a clever way to achieve higher efficiency whilst flying which the Russians were especially interested in researching. Maybe a commercial application is possible?

  • Ed

    You seem to lack a complete understanding of the differences between aerodynamics and efficiency. It takes only a three percent reduction in drag to increase fuel economy by fifty percent.

    I see it as not only feasible, but likely that technology and creativity will help us reduce drag and increase efficiency to unimaginable levels. The question is, when will we as consumers see this change? When will there be FAA certified aircraft we can fly on? One needs only to look at experimental aircraft to realize that there are much more efficient designs in use.