Professor and department head for the School of Aeronautics and Astronautics at Purdue University Bill Crossley answers questions about airplanes and aerospace engineering from Twitter. How do airplane wings generate the necessary amount of lift to achieve flight? Can a plane fly with only one engine? Could electric airplanes replace fuel-burning ones? Is severe turbulence still safe to fly through? Why do commercial planes fly at 35,000 feet? Are planes safer than cars statistically? How much does it cost to build an airplane? Answers to these questions and many more await on Airplane Support.
Director: Justin Wolfson
Director of Photography: Ben Dewey
Editor: Shandor Garrison
Expert: William Crossley
Creative Producer: Lisandro Perez-Rey
Line Producer: Joseph Buscemi
Associate Producer: Brandon White
Production Manager: Peter Brunette
Production Coordinator: Rhyan Lark
Casting Producer: Nicholas Sawyer
Camera Operator: Caleb Weiss
Sound Mixer: Rebecca O'Neill
Production Assistant: Sonia Butt
Post Production Supervisor: Christian Olguin
Post Production Coordinator: Ian Bryant
Supervising Editor: Doug Larsen
Additional Editor: Jason Malizia
Assistant Editor: Billy Ward
Director: Justin Wolfson
Director of Photography: Ben Dewey
Editor: Shandor Garrison
Expert: William Crossley
Creative Producer: Lisandro Perez-Rey
Line Producer: Joseph Buscemi
Associate Producer: Brandon White
Production Manager: Peter Brunette
Production Coordinator: Rhyan Lark
Casting Producer: Nicholas Sawyer
Camera Operator: Caleb Weiss
Sound Mixer: Rebecca O'Neill
Production Assistant: Sonia Butt
Post Production Supervisor: Christian Olguin
Post Production Coordinator: Ian Bryant
Supervising Editor: Doug Larsen
Additional Editor: Jason Malizia
Assistant Editor: Billy Ward
Category
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TechTranscript
00:00I'm Bill Crossley, Professor of Aerospace Engineering, and I'm here to answer your questions from the internet.
00:04This is Airplane Support.
00:10Yeboi Nath asks,
00:11Why do commercial planes fly at 35,000 feet?
00:13So at about 35,000 feet, the air has less density, which means less drag, so you need less thrust to push the airplane through the air.
00:20However, the higher you get, the less density there is, the less thrust the engine can produce.
00:24So we try to balance the thrust the engine can produce with the drag on the airplane and try to find the best cruise altitude.
00:29It's around 35,000 feet.
00:31Onion Time E says,
00:32As a planologist, I have the know-how to explain the difference between models.
00:35For example, a 747 is 10 more planes than a 737.
00:38That's it. That's planes.
00:40It's kind of a humorous comment.
00:41So Boeing started with the 707, did the 727, then the 737, 747, 767, 777, and then 787.
00:49Some people think the number of the model means a bigger airplane.
00:52That's not actually true.
00:53It's just the order in which they came to market.
00:55Lockstrokes asks,
00:56How does g-force work and what causes a pilot to pass out?
00:59So when you're flying in an airplane and the pilot pulls back on the stick to make the airplane go up,
01:03the people and the pilot sitting inside the airplane want to keep going in the same direction.
01:07But the plane pushes up on them.
01:08So while the plane is pushing up, all the blood is going to rush down.
01:11And that's what the g-forces are.
01:12How much that force is, number of g's, is relative to gravity.
01:16So I'm sitting here in my chair.
01:17It's one g.
01:18The pilot pulls back a little bit.
01:19Maybe it's two g's max you'd ever see in a commercial airplane.
01:22Fighter pilots sometimes pull four, five, even six g's.
01:25That's what causes them to black out is when that blood is running out from their brain into their feet and their legs.
01:30So how do we keep them from passing out?
01:32There's g-suits that actually squeeze their legs to help keep the blood up in their head.
01:35And some pilots even learn tricks to grimace and scrunch themselves up to keep from passing out when they do high g maneuvers.
01:41Next up, Sentient Sadist says,
01:43Don't try telling me that statistically airplanes are safer than cars.
01:46Well, statistically they are safer than cars.
01:48The chances of you having a fatal accident in an airplane is something like one in 120 million trips,
01:53which is far safer than traveling by car or any other means of transport.
01:56Aviation has gotten safer and safer every single year.
01:59We've gone from having a couple of fatal accidents a year to recently having one every couple of years.
02:04There's been a huge improvement in safety.
02:05One of the reasons that they're safer is because the aviation industry, you've got two trained pilots.
02:10They've gone through lots of training on board.
02:11The aircraft have to be certified and are redundant systems, multiple things.
02:15So if one system fails, other ones back them up.
02:17And the whole air traffic management and inter-transport system is set up so that we know where the airplane is at all times and is staying away from other airplanes.
02:24So from a non-statistical point of view, all that extra stuff we do for aviation makes airplane flying safer than driving your car.
02:30Oof Doof Oof Oof asks,
02:32What happens when there's a bird strike?
02:33Depends on how big the airplane is and how big the bird is and the relative mass of the two objects.
02:37Bird strikes have caused aircraft or helicopter windows to break.
02:40They sometimes get ingested into engines.
02:42We actually design the engines to handle a bird ingestion.
02:45One of the bird strikes is probably most famous is the miracle on the Hudson where the Airbus ingested lots of geese into the engine.
02:51So both engines went out and Captain Sullivan landed on the Hudson River safely.
02:55It was a question from Noah Zobs.
02:56Landed at SFO.
02:57I still don't believe that airplanes can fly.
02:59I understand the science of it, but how can the wings generate that much lift?
03:02The simple answer is the shape of the wing, the speed of the airplane, and the angle of attack of the wing change the momentum of the air going past the airplane.
03:09It pushes down on the air and the airplane pushes up.
03:11Creighton RSS asks,
03:14Yes, it can.
03:15Actually, the way aircraft are designed, if you have more than one engine to be certified, they all have to be able to operate with one engine out.
03:22There's an interesting thing you've probably noticed in commercial aviation.
03:25Most of the airplanes operating today, commercial aircraft, only have two engines, even the ones flying across the ocean.
03:30747 had four engines.
03:31That was because if one engine went out, you needed the three engines to get all the way to your destination or to come back to somewhere safe.
03:37As engines have gotten more reliable and more efficient and bigger, we're able to make aircraft with just two engines that can operate with one engine if one goes out to make it to a safe destination if one of the engines goes out.
03:48TangerineMaximum2976 asks,
03:50Why is commercial aviation not improved in the last 50 years?
03:54It has improved quite a bit.
03:55I think one of the things that if you're looking at the airplanes, the airplanes don't look a whole lot different.
03:59But they have been improved a huge amount.
04:01They're much safer.
04:02The engines they use are much more reliable.
04:04We've moved from having multiple engines for trans-oceanic flights to having two engines for trans-oceanic flights.
04:09The electronics on the airplanes are much, much smaller and much more reliable.
04:12And so that makes the airplanes lighter.
04:14Our impact on the environment, because the airplanes are lighter and the engines are more efficient, has gone down a huge amount in the last 50 years.
04:20Airplanes are much quieter when the airplane flies over you on approach to the airport.
04:23You hear it, but it's not the roar you used to hear from the older turbojet engines that look like skinny cigarettes underneath the wings.
04:30And now we get the big fat turbo fans, which are much quieter.
04:32So commercially, aviation has improved a huge amount in the last 50 years.
04:35The part about the air passengers experience, that's not the aerospace engineer.
04:39That's up to the airlines.
04:40That might not have improved much in the last 50 years.
04:42Next up, RGB says, what if airplanes were made of the same material black boxes are?
04:47Since black boxes generally survive plane crashes, the plane might be intact too.
04:50The black boxes are made of really strong material.
04:53But the really strong material is really heavy.
04:55And as aerospace engineers, we have to figure out what's the right material to be strong enough to do everything we need it to do
05:00and be reliable enough to make many, many flights over its lifetime, but also be light enough for us to pick it up off the ground.
05:06Aircraft designers have settled on aluminum and composite fiber reinforced plastic are the two main things that aircraft are made of these days.
05:13HSN407 asks, can I ask a flight attendant to change to an empty seat?
05:17You always can ask the flight attendant.
05:19There's all kinds of reasons why you might not be allowed to.
05:21Airlines often use the different seats to generate revenue.
05:24So you might not be able to move to a seat you didn't pay for.
05:26On some airplanes, if there's not a whole lot of passengers on them, if you move to a different seat, you change how the weight is distributed in that airplane.
05:32And the pilots want you to be in a certain spot so that aircraft is easy to fly and stay safe.
05:37Viper says, hey, Aviation Twitter, I have a question for you.
05:40If both the pilots of a commercial airplane are compromised, can't air traffic control take remote control of the aircraft?
05:45The answer today is no, they can't because they don't have the equipment on board.
05:48But actually, aerospace engineers have been working for quite a while to fly airplanes that have a lot of autonomous operations and are remotely piloted.
05:55The military regularly flies the Predator and Global Hawk without a pilot on board.
05:59The pilot or operator sits well off, and they can do that.
06:02To put this into commercial airlines would require a huge amount of upgrade to the equipment, and it'd be just an expensive endeavor at this point, not to mention the certification issues.
06:09Some new entrants in urban air mobility are pushing really hard at having more autonomous or perhaps even autonomous flights around a city where you don't have a pilot on board.
06:18That pilot would be remote.
06:19So this is coming.
06:21NowMinox asks, is severe turbulence really safe?
06:24At least from an airframe point of view, from the aircraft point of view, most of the turbulence will be rapid up and down accelerations.
06:30And so the airplane may drop and may climb during the turbulence, but it's usually you're so high in altitude, it's not going to throw you into the ground from a cruising altitude.
06:37There is a concern about something called wind shear.
06:39If you encounter that as you're coming close to an airport, that can actually push the airplane down.
06:43And there was a really well-known accident at Dallas-Fort Worth when an airplane hit this wind shear, and it actually crashed short of the runway.
06:50We as an industry have put in sensors and better meteorological devices to make sure we're measuring.
06:54So if that's going on around the airport, we don't fly into that.
06:57Next up, Excedrin asks, do planes have an MPG gauge, which I think it means miles per gallon.
07:02They sort of do.
07:03Especially modern aircraft today, fuel is such an expensive part of flying airplanes.
07:07But a flight from Indianapolis to New York will require several thousand pounds of fuel for that flight.
07:12Airlines or anybody who operates an aircraft wants to use as little fuel as necessary.
07:16And so the computers on board can track how much fuel is being burned, what's the relative speed to the ground, and can suggest what's the speed the pilot should fly at to be flying at the best range possible.
07:26And that gives them the highest, quote, miles per gallon for the airplane.
07:29AtTheTradingChick says, I'm not the best at science, but can one of my nerds let me know, would it be possible to make an electric airplane?
07:36If so, what would be the pros and cons?
07:38It is possible to make an electric airplane.
07:39And actually, there are a handful of electric airplanes actually out there already.
07:42There's some small trainer airplanes, two seats for people learning how to fly.
07:46Some of the pros are electric airplane cost a lot less to fly.
07:49Electricity is a lot cheaper than using jet fuel to fly an airplane or using aviation gas for a smaller airplane.
07:55Another advantage is electric motors have a lot fewer moving parts.
07:58So the maintenance requirements are much lower.
08:00The reliability is much higher.
08:02The biggest con, batteries are heavy.
08:04We've gotten to the point where batteries are pretty good, and we can use them for airplanes for short flights.
08:08But for long flights, the physics just doesn't work.
08:10We can't store enough energy in the battery.
08:12In an airplane that's using jet A, jet fuel, which is petroleum-based, not only is there a lot of energy in that, you burn it during the flight.
08:19So the airplane gets lighter as it flies.
08:21So if you're flying across the ocean, the airplane's a lot lighter when it lands than when it takes off.
08:25And because it's burning that fuel, it means it's using less energy as you go along the distance.
08:31Clyde in Limbo asks, how do plane wings not snap or break more often?
08:35Well, it's because we do a good job designing them.
08:37When we design an airplane, we know about how many Gs, how strong a maneuver that airplane is ever going to be able to perform.
08:43And then we make sure that the wings are almost twice as strong as that maximum load that the airplane can put in.
08:49Just the aerodynamics of the airplane limits how many Gs you can pull.
08:52And then we design the wings to handle a load that's higher than that.
08:54And that's why they don't snap or break.
08:56Next one is from CatsEye1.
08:58Next person, please tell me how the driver of a plane can survive a noise vibration like a sonic boom.
09:03It literally scared the crap out of me.
09:05The pilot of the airplane won't actually feel the sonic boom.
09:08When an airplane starts to go at the speed of sound, the air starts to behave differently.
09:12I'm talking here in the studio.
09:14My voice, my vocal cords are making a pressure disturbance.
09:16That's getting picked up by the microphones.
09:18That's traveling at the speed of sound.
09:20The airplane has enough thrust.
09:21It tries to go faster than that.
09:22Essentially, the molecules of air can't get out of the way.
09:24So instead, they pack together, and the properties of those molecules as a whole change almost instantly.
09:29So right across the sonic boom, you have a change in pressure and temperature and density.
09:33Now what happens is that gets pushed out in front of the airplane.
09:35So if I take this airplane, which is a supersonic trainer, a mach cone will form in front of the airplane.
09:40So the airplane won't actually be in the sonic boom.
09:42But that sonic boom will radiate down toward the ground, and that's probably what this person heard.
09:46And it does sound a lot like thunder.
09:47John Gunderman asks, can we bring back supersonic commercial flight?
09:50Pretty please?
09:51And the answer is yes, if you're willing to pay for it.
09:53It's very expensive to fly faster than the speed of sound.
09:56It's going to require a lot of extra thrust to push you that far.
09:59Extra thrust means extra fuel and bigger engines, more expensive airplane.
10:02Current commercial aircraft fly about 80% of the speed of sound or 85% of the speed of sound,
10:07while supersonic airplanes like the Concorde went almost two times the speed of sound.
10:11So you can see there's a huge difference in that speed.
10:13You're not allowed to fly faster than the speed of sound over land currently in the U.S. or in Europe.
10:18If we can make that sonic boom a lot lower, we can actually fly supersonic over land,
10:22and make it much more profitable for the operator.
10:24NervousDriver334 asks, I'm not an expert, and then asks,
10:27but why can't airports have a ramp at the end of the runway like some aircraft carriers do?
10:30Well, the simple answer is aircraft carriers, people operating on those are Navy pilots.
10:34And if their airplane doesn't make it off the end of the runway, they know they're going into the drink.
10:38We can't do that with commercial aviation.
10:39We need the airplane to have enough runway that if the pilot's trying to take off and something goes wrong,
10:44he or she can step on the brakes, turn the engines off, put on the thrust reversers and stop in time.
10:48The other reason is if an airplane's coming into land and something goes wrong on the landing,
10:52we need a lot of extra margin to allow everything possible to stop that airplane in time.
10:55Ralphie008 asks, why don't planes have parachutes? I've always been curious.
10:59Well, some airplanes do. This is a picture of a Cirrus SR-20.
11:02The Cirrus is a small single-engine airplane that was certified with a ballistic recovery system.
11:06If something goes wrong on the airplane, the pilot or somebody else with the pilot can actually pull a handle.
11:11It'll have a very small rocket that launches a parachute out,
11:13and that lets the airplane sink to the ground at a controlled rate, and people can get out of the accident.
11:17So small airplanes can have parachutes.
11:19The problem is the parachute needs to grow in size with the size of the airplane.
11:23So airplanes much bigger than something like that single-engine airplane, the parachute would be too big.
11:27It would take up too much room and be too heavy so the airplane couldn't carry passengers or other goods.
11:31Primal Polly says, 50 years ago, the X-15 flew at Mach 6.7.
11:35No aircraft since then has ever gone as fast. But why?
11:38So first off, Mach 6.7 means that's 6.7 times the speed of sound.
11:42We would think of that as hypersonic flight, much, much faster than the speed of sound.
11:46This is an X-15. The X-15 was a research aircraft.
11:49And at the time we flew this, we needed a pilot on board to make sure it could do all of its operations.
11:53We used it to learn about what it takes to fly that fast,
11:56and what happens to the aircraft and the flow around it going that quickly.
11:59Since then, we've actually flown aircraft or hypersonic vehicles that don't have pilots on board.
12:03The flight control system is good enough to do all those things,
12:06and we don't put a pilot in any jeopardy flying that fast.
12:09SMI 74499326 says, it's true those jet engines produce a lot of thrust,
12:15and you will get sucked into them with no real problem.
12:17Unfortunately, that is true. There's been a few incidents where that's happened.
12:20Also, cargo containers have also been sucked into engines when they're turned on on the ground.
12:24When you're at the airport, there's an area marked out usually where the airplanes parked at the gate,
12:28and there's usually a spot that's telling people on the ground to stay out of this area.
12:31Joey TV Show Season 2 asks,
12:34Generally speaking, how much does it cost to build a plane?
12:37I don't actually know how much it costs to build a plane,
12:39because the manufacturers usually keep that as a company secret.
12:42But I do know what the book price or the listed price of an airplane is.
12:46The 787 costs about $300 million if you look on Boeing's website.
12:50Caveat to that is when Boeing sells their airplanes to customers,
12:53the airline might buy lots of airplanes and might work out a deal.
12:56So maybe like when you go to buy a car and you end up paying less than the sticker price,
12:59airlines often pay a lot less than the sticker price.
13:02But needless to say, commercial airliners cost hundreds of millions of dollars.
13:05Calvin Meter asks, how many hours of maintenance does his aircraft need for every flight hour?
13:10Actually, the number of maintenance hours per flight hour is an important metric
13:13for people who operate airplanes like airline pilots.
13:16Or even if you own your own airplane, how long can you fly it before you need to go do some maintenance?
13:19Commercial airplanes go through all kinds of levels of maintenance
13:22depending on how many hours they've been flown.
13:24There's some simple checks where they just look through major systems
13:26and make sure there's nothing they see on a visual inspection,
13:28all the way up to a major overhaul where they almost take the airplane completely apart,
13:32look for any kind of problems, and put it back together
13:34to make sure that that airplane can last for 30 or 40 years.
13:37If you're waiting on a tarmac or you're waiting to board the airplane
13:39and you hear there's a maintenance issue, that's usually unscheduled maintenance.
13:42Those are in most cases very minor.
13:44Something like a light in the fasten seatbelt signs is out.
13:47They're not allowed to fly without that light working.
13:49AtTryingToDoRight says, I wonder why we have an air traffic controller shortage.
13:54Do you have any theories?
13:55The airline industry continues to grow after the pandemic.
13:58We're at flight levels that were way above what we saw before the pandemic in 2020.
14:03There's just a need for aviation and there aren't quite enough people to fill those positions.
14:06Another thing is a lot of people have been attracted recently to other jobs.
14:10Aerospace is really cool, and so I'm looking forward to teaching lots of students in my classes
14:14to get them in the aerospace industry.
14:16SSBMniffen asks, do planes need co-pilots or is it purely a safety mechanism
14:21if something should happen to the first pilot?
14:23In modern aircraft, that's true. It really is a backup. It's being redundant.
14:26We still have pilots on the aircraft to make decisions about flying the airplane,
14:29but for most of the airplane flights these days, the pilots get to their cruise altitude
14:33and they allow the airplane to fly itself.
14:35And the pilots are on board to react to something that's unexpected
14:38or take control of air traffic control, ask them to go to a different altitude, etc.
14:42Atflylakeland asks, what's your favorite airplane?
14:45My favorite airplane is the Lockheed Constellation.
14:47It's a World War II-era aircraft that was designed by Lockheed.
14:51It became the first airplane that could successfully and reliably make it across the Atlantic Ocean.
14:56Not only do I think it's a cool-looking airplane,
14:58I think it's a really neat airplane because it made the world smaller.
15:01And one of the reasons I like being an aerospace engineer,
15:03flying around the world makes the world smaller,
15:05and I think making the world smaller makes the world a better place.
15:07ThickCactus85 asks, how do jet engines work?
15:10We like to have sort of a joke statement here in aerospace engineering.
15:13The jet engines suck, squeeze, bang, and blow.
15:16So this is a picture of a jet engine with the cowling or the nacelle taken off of it.
15:19The front end has a compressor. Well, at first it has an inlet.
15:22The inlet is the suck. That entrains the air into the engine.
15:25Then the compressor increases the pressure of the air. That adds more energy to it.
15:29Then we add fuel to that air and we combust it.
15:32When we combust air, that adds a lot of heat, so we've added a lot of energy to the flow.
15:35It passes past a turbine, which takes a little bit of the energy out to run the compressor,
15:39and then the shape of the nozzle at the back end of the jet engine
15:42makes sure we get the most push out of that exhaust.
15:45So we're basically increasing the momentum of the flow from what's in front of the engine
15:48to what's out that back, and that difference in momentum is what gives us the thrust.
15:52What I explained describes a turbojet engine.
15:54This picture is actually a turbofan engine, and this big part at the front is a fan.
15:58So in an engine like this, the turbine part of the engine,
16:01some of the energy goes to the compressor, some of it goes to turn this big fan.
16:04This big fan accelerates a lot of air that doesn't go through the combustor.
16:07That's more efficient than sending all the air through the combustor.
16:10You just can't fly as fast.
16:11Propellers are at the extreme end of that.
16:12Propellers can't go very fast, but they're even more efficient.
16:15That's it. That's all the questions I have.
16:17I hope you learned something.
16:18Until next time.