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Ilan Kroo: Better ways to build an airplane

An expert in aeronautics discusses how recent developments in fuels, engines, materials, and computer controls are leading to an era of airplanes unlike any that have gone before.
3D architectural sketch of a jet plane.
The confluence of changes in fuels, materials, and computer controls are creating a new generation of airplanes. | iStock/hh580

Our guest on this episode of Stanford Engineering’s The Future of Everything podcast, Ilan Kroo, is an expert in aircraft design. But when Kroo talks of aircraft, he means a new generation of flying vehicles that could transform our very concept of transportation—like personal flying cars that take off and land vertically or commercial airliners fueled by clean-burning hydrogen.

Kroo says the rapid changes he’s seeing in the industry could lead to safer, less expensive, more efficient—and quieter—air travel. Listen in as he shares more with host Russ Altman on this episode of The Future of Everything podcast.

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Professor Ilan Kroo (00:00):

There's been quite a bit of progress made in developing vertical takeoff and landing electric airplanes. They will not be certified by the FAA in the same way that commercial airplanes are, but they will indeed be certified and various companies are now working quite hard to put these together so that they can fly. Some of them carry one person, some of them carry eight or nine people. And so there's a wide range of possibilities for these devices.

Russ Altman (00:40):

This is Stanford Engineering's The Future of Everything, and I'm your host Russ Altman. Today, Professor Ilan Kroo will tell us about exciting changes in how airplanes are conceived, designed, built and fueled. This includes both personal flying cars as well as commercial airplanes. It's the future of airplanes.


Those of us who are old enough can remember that we were promised jet packs by now where we would put them on our back and fly around, and those never really materialized. But flying cars were also promised, and it looks like those may actually come to pass. There's been great changes in how airplanes can be fueled, how they can be controlled, how they can be built, the materials that are used and the fuels that are used.


Ilan Kroo is a professor of aeronautics and astronautics at Stanford University. He's an expert at airplane design, especially of unconventional aircraft. He'll tell us how the confluence of changes in fuels, materials, computer controls, are creating a whole new generation of airplanes that will change what we see when we look up in the sky.


Ilan, you're an expert at airplane design, and I believe you're talking now about a change in how airplanes are fundamentally designed and built, even how they're fueled. Where did these changes come from? Because for the last 50 years, planes have looked the same to me when I look up into the sky. So what's happened and what's changing?

Professor Ilan Kroo (02:05):

Yeah, it's not so much in the way we design airplanes, although many advances in computational methods and optimization methods have allowed us to do things in much more detail. But I'm particularly excited about the change in the technology that can be applied and assumed to work for new aircraft. This is new engine technology, new active control systems, new structures, new engine concepts, new fuels even now. And this allows us to do many things that we just couldn't do in the past and to explore things digitally and in simulation without having to build them.

Russ Altman (02:56):

That's fantastic. And there's so many great topics you just brought up obviously, and you know this very well. So let's kind of start, maybe you should tell us about the basics of how a plane is designed, because that would give us a nice background for understanding where these revolutions are occurring.

Professor Ilan Kroo (03:12):

Yeah. Well, in the past, airplanes have often been designed by looking at previous airplanes, trying to understand the basic physics involved, and then doing a lot of testing and iteration that took a very long time. This was iteration using simple models of the aerodynamics and structures, et cetera, getting more complex as those tools developed. But now we can do multidisciplinary simulations, so combining the structures and the aerodynamics and the controls and only at the end do wind tunnel testing and flight testing. So this allows us to go much faster and explore many possibilities that we wouldn't have explored in the past.

Russ Altman (04:04):

One of the things you mentioned in your previous comment was active controls. And I think a lot of us can imagine what you mean because we are all seeing these drones flying around and we see they have these very independently controllable little flat things with rotors in them. Is that the kind of technology that we might see in aircraft that are actually carrying humans around?

Professor Ilan Kroo (04:25):

We're seeing that now actually. There are some airplanes flying with a form of active control that makes it easier for the pilots to fly the airplanes, but this is extending into cars. Many electric cars and cars that have a lot of sensors in them can do at the very least, maintain your speed with cruise control, but also with steering. And these are just simple implementations of the more general field of active control, which can use a combination of sensors and actuators to make the airplane easier to fly, safer to fly and react to things that the pilots might not be able to react to.

Russ Altman (05:16):

So I know that you work in both the design and creation of personalized cars, the dream we all have, we never got our jet packs, we're still very bitter about that, but we might actually get flying cars. But you also think about the big commercial aircraft. So I really want to cover both. But what would you like to say about the status of the personalized car or the small flying apparatus? Is that something we should be looking out for soon or are there fundamental problems where we should not get so excited so we don't get burned again like we did with the jet packs?

Professor Ilan Kroo (05:47):

Yeah. Well actually there's been quite a bit of progress made in developing vertical takeoff and landing electric airplanes. They will not be certified by the FAA in the same way that commercial airplanes are, but they will indeed be certified and various companies are now working quite hard to put these together so that they can fly. Some of them carry one person, some of them carry eight or nine people. And so there's a wide range of possibilities for these devices, but they are relatively quiet because they use electric motors, most of them fly on battery energy. And so while the range is much more limited than a passenger airplane, a commercial airplane, there are many advantages and some of them may fly autonomously without a pilot or with a pilot on the ground. Many of them will be flown with pilots in the airplane. So wide variety of possibilities, and they're very exciting to see because the first one was just flying a few years ago. So, very rapid development.

Russ Altman (07:06):

Just a little bit more on these because I think it captures people's imaginations. Should we think that this will be kind of regulated the way personal aircraft are regulated at the local small airport, or is this going to be a whole new way of getting them in the air and doing basically air traffic control? Because I could imagine that if these become too popular, it could be a nightmare. So how is the frontier looking in terms of actually scaling this and having it be safe and effective?

Professor Ilan Kroo (07:32):

People have been looking at this even before some of these airplanes were flying. So people have devised a variety of ways in which these airplanes can be used. One of the early groups that was thinking about this was a company associated with Uber, and that branch of Uber was called Uber Elevate, and they put out a number of ideas on this, including approaches to taking off and landing at what they called vertiports. So you would drive your car to this parking structure or whatever it was, drive it up to the roof, and all of these airplanes would be waiting, not like Ubers maybe, but more like ride-share vehicles of some sort.

Russ Altman (08:28):

Like the scooters that we see all over LA.

Professor Ilan Kroo (08:31):

But these might come with pilots and people to make sure they were all ready to go. So hopefully not like all of the little scooters, which might be broken or might be fine. So many people or many companies have adopted different approaches to this, but the idea of you are driving either to a local airport, which would, for example, where I live, even the little Palo Alto Airport might be a good place to drive to quickly rather than having to drive to San Francisco International Airport. And that little airplane could take you right to the airport in San Francisco or San Jose, and from there you would get on an airplane to go to Washington DC or London or wherever.

Russ Altman (09:25):

And the air traffic control challenge, is that manageable or does that look like a big headache or both?

Professor Ilan Kroo (09:31):

So it remains to be seen because there are not hundreds of these, let alone thousands or tens of thousands of them flying, but many people whose job it is to really think about this carefully, not just those who are going to manufacture and operate these what are sometimes called eVTOL, which is a strange acronym, which means electric vertical takeoff and landing aircraft. But many of the people who are not associated with building and operating them, people who are working at NASA for example, are looking at this as a challenge and trying to develop methods to make that more appropriate for lots of small airplanes that might be autonomous as opposed to just talking to pilots. The FAA is very much involved along with all the companies in setting up standards that will make this safe.

Russ Altman (10:33):

Great. So that is an exciting future, but I do want to move to commercial aircraft because I know you think about those as well. And in fact, I think you've written that this is where we could make a lot of impact on many more lives, much more impact on climate and on fuel usage. So I guess the first thing is about fuel. I was very intrigued that the smaller aircraft can run on electricity and get useful ranges. What are the challenges and prospects for jet fuel and how we actually fly our airplanes in terms of sources of energy?

Professor Ilan Kroo (11:03):

So this is something I'm particularly interested in right now, as are many people all over the world and many companies that are working on that's very hard. The field of sustainable aviation is very interesting and many things are happening right now. This is important because air transport actually has a relatively small impact on climate change. It is something that's very obvious and something that we could do something about, but it's something like 3 to 5% of the human impact on climate.

Russ Altman (11:48):


Professor Ilan Kroo (11:50):

So it's a small effect, but if we reduce that substantially, people will be more comfortable flying, and I think this would also make the public in general more comfortable in booking a flight to England or Sweden or wherever one might be flying [inaudible 00:12:14]-

Russ Altman (12:13):

Yes. And it just makes sense that even if the industry is not contributing a ton, the emerging public opinion will be, every industry should do what it can and the net result will be better. Even if it's not percentage wise a huge impact, they would like to see the aircraft industry make whatever the appropriate contribution is.

Professor Ilan Kroo (12:31):

Absolutely. And if all of these industries make a small contribution, even if it's 5%, it doesn't take that many to make a big difference. So you also asked about fuels and what people are doing to make this a reality. Yes. Many things are actually happening right now with increasing the efficiency of aircraft. This is done not only to reduce climate impact, but also to reduce costs and to make these things have less noise and many other things that aren't associated with climate, but still are important. One of those things is new engine concepts. So we are seeing the efficiency of the engines themselves dramatically improved over the last few decades, but also new fuels and new ways of operating the airplanes, composite structures which make the airplanes lighter and computational fluid dynamics, which allows us to fly the airplanes on the computer and tweak them a bit so that they are more efficient in ways that we could not do in the past.

Russ Altman (13:48):

So are we stuck with fossil fuels? I mean, I know that the energy density in those fuels is very high, that's why they're so good. But when you're talking new fuels, how new are these fuels?

Professor Ilan Kroo (13:58):

So they're really interesting and they're basically three classes of fuels that we may see in the future. We might see versions of kerosene fuels, which is what Jet-A is basically, but they might produce less impact on the environment. But what the new fuels that people are looking at now are sometimes called sustainable aviation fuels, or SAF. And these are made in a number of different ways and people have many different approaches for producing these, but one could produce them from bio sources, whether it be algae or wheat grass or-

Russ Altman (14:45):

Corn. We seem to make everything else out of corn. Why not jet fuel?

Professor Ilan Kroo (14:49):

Yeah, well, because we can eat corn and we probably can't eat wheat grass. But actually some of the airlines are actually experimenting with vegetable oil, particularly in Europe. So we'll see where that goes. But also people talk about SAF as maybe being really synthetic. That is combining various materials that might include hydrogen, might include refining hydrogen and other elements of the fuels into fuels that can be used and can be used by aircraft without actually using carbon dioxide that hasn't been in the environment before. So petroleum companies like British Petroleum are looking into how to manufacture SAF fuels from hydrogen and other atmosphere-

Russ Altman (15:54):

So we really are seeing a potential move away from fossil fuels and fuels that have kind of a similar energy profile so that they'll be able to literally fuel the airplanes going across these big oceans.


This is The Future of Everything with Russ Altman, more with Ilan Kroo next.


Welcome back to The Future of Everything. This is Russ Altman and I'm speaking with Professor Ilan Kroo from Stanford University. In the last segment, Ilan told us about changes in airplane design, materials and fuel that are fueling both personal cars as well as commercial airplanes. In this segment, he'll tell us about hydrogen as a promising fuel for airplanes. He'll tell us how climate change interacts with airplane design, and finally he'll tell us about some of the unconventional designs that we might see in the sky soon.


So Ilan, you mentioned in passing that these new fuels include hydrogen, and I couldn't help but think of the Hindenburg when I thought about the hydrogen. So tell me what is the opportunities with hydrogen and are they safe and are we really going to see them perhaps in commercial airplanes?

Professor Ilan Kroo (17:07):

Yeah. One of the big advantages of hydrogen is that it doesn't require carbon at all, so it doesn't produce carbon dioxide in the atmosphere, and the output of a hydrogen fueled airplane, which combines the hydrogen with oxygen in the atmosphere is water. And so that is an interesting feature and one that could be very helpful for making truly sustainable commercial airplanes. Hydrogen can also be used in fuel cells for the smaller airplanes in cars and things, and so that is an interesting possibility as well for small airplanes in a variety of applications. But hydrogen can also be burned directly and doesn't require large scale conversion to electricity. It is a very good fuel. In fact, with the weight of hydrogen compared to the energy in the hydrogen being very low a factor, much better than conventional jet fuel.


On the other hand, engines for airplanes, for jet airplanes are designed to burn Jet-A and other petroleum based fuels. And so a lot of work has to be done on the engines. And one of the advantages of hydrogen is its large energy to weight ratio, but one of the disadvantages of hydrogen is its very small energy to volume ratio. So it would take bigger fuel tanks to have this hydrogen, even though its weight is less. So it makes a little bit less sense for cars than it would for airplanes where weight is very important, but it also means that the airplane may have to be designed a bit differently to accommodate larger volumes of fuel.

Russ Altman (19:16):

And forgive me for not knowing this, but do we have a good supply of hydrogen or can we create all the hydrogen we need?

Professor Ilan Kroo (19:22):

Well, it's a very good question, it's an interesting question that many people are working on because if we can figure out a sustainable way of making hydrogen from water, so for example, having a zero emissions power plant that just electrolyzers water, so breaks it up from H2O into H two and oxygen, then we can collect the hydrogen and compressing liquid hydrogen, which would work in a future airplane.

Russ Altman (19:54):

So we will keep our eyes open for hydrogen fuels, maybe even in airplanes. I wanted to briefly touch upon climate modeling and climate because I was surprised that this plays a pretty big role in a lot of your work, and it's not just about worrying about the fossil fuel emissions. I think the modeling of the future climate actually will affect how you think about building airplanes. So just tell me a little bit about that.

Professor Ilan Kroo (20:17):

So this is a very hot topic both in airplane design and in atmospheric sciences in general because we really need to understand it regardless of whether we're building airplanes or not. So a lot has happened in this field over the last 10, 20 years, and one of the things that motivates this is being able to predict what it is that on the ground or what happens at 30,000 feet or at 40,000 feet affects what is sometimes called radiated forcing, well, you could think of it as the significance of the greenhouse effect. And the different gases in the atmosphere have a big effect on this, whether those gases are carbon dioxide or even water vapor or nitrous oxide or nitrogen oxides in general, understanding how it is that the things that we produce interact with the atmospheric system are very important.


And this is particularly important, well, when people are looking at future in airplanes, because the altitude makes quite a big difference, the altitude at which those emissions are introduced. So at low altitudes, carbon dioxide has almost the same effect as at high altitudes, but water vapor at low altitudes is not a problem. Water vapor at high altitudes might be a problem, and people have seen pictures of contrails and seeing contrails in the sky, and it's quite possible that the interaction with contrails with solar radiation is very important to climate.

Russ Altman (22:13):

Wow. Okay, great. So there's a placeholder for the future. I wanted to spend the last three or four minutes talking about new airplane concepts that are emerging. I know this is another area of your work, and I think people will be excited to learn what we might be seeing in the skies over the next decades.

Professor Ilan Kroo (22:29):

People have worked on unconventional airplanes for a very long time, but I think some of this focus on sustainability provides real motivation to work on airplanes that are quite different from what we've seen before. One of these that I'm currently working with, along with a number of other people and some startup companies and NASA and other places, is an airplane that is basically a flying wing. And the center section of the airplane has a lot more volume than a conventional airplane wing does, and the performance of the airplane is greater as well. And so it allows us to think about things like hydrogen or fuels that require more volume.

Russ Altman (23:21):

So this all interacts?

Professor Ilan Kroo (23:23):

It does. Right. And so these airplanes could also accommodate people at different sizes in much more interesting cabin configurations than we see in the so-called tube and wing designs.

Russ Altman (23:40):

So just so I can be very clear, we're talking that big long cylinder that we all usually sit in that might go away, and also the whole tail wing, the tail that might go away. And instead just the kind of V-shaped wing that we associate with, that might be the whole aircraft.

Professor Ilan Kroo (23:57):

Yeah. It would also blend into a center part of the wing, which is a little bit bigger.

Russ Altman (24:03):

Right, little bulge in the middle.

Professor Ilan Kroo (24:05):

That's right. Or a big bulge in the middle. [inaudible 00:24:08] bulge, it's a very aerodynamic-

Russ Altman (24:10):

Of course, of course. We have to make sure it can fly.

Professor Ilan Kroo (24:13):

That's right. So that also enables us to carry different fuels. It also because it doesn't have all those pieces of the airplane, like the tail and the fuselage, it can be more efficient.

Russ Altman (24:29):

Have there been prototypes of these that are actually flying around?

Professor Ilan Kroo (24:33):

There have been subscale airplanes sponsored by NASA, worked on by Boeing and McDonnell Douglas for a while when they existed. And in fact, our lab built some small models that allowed us to experiment with different control methodologies, and we flew that a few years ago.

Russ Altman (24:59):

What other things do you want to tell us about potential new concepts? I know you had mentioned or written about speeds and altitudes, where are we looking at there?

Professor Ilan Kroo (25:09):

Yeah, so other very different looking airplanes besides the advanced air mobility things that we talked about, flying car-like things include people looking at supersonic concepts, which are in a sense the opposite of sustainable aviation to some people, but at the same time might be made much more environmentally compatible than the old very large style concord and other things that large airplane companies have been looking at. There are other concepts as well, including airplanes with much longer wings and larger span wings, which Boeing has been working on recently, and this also allows much more efficient light.

Russ Altman (26:03):

I see. Well, there you have it. We have the amazing visual impact imagining these new airplanes that we might see in the sky even in our lifetime.


Thanks to Ilan Kroo, that was the future of airplanes. You have been listening to The Future of Everything with Russ Altman. You can follow me on Twitter at RBAltman, and you can follow Stanford Engineering at StanfordENG.

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