Aerospace Engineer Answers Airplane Questions From Twitter

I'm Bill Crossley,

professor of aerospace engineering,

and I'm here to answer your questions from the internet.

This is Airplane Support.

[upbeat percussion music]

yaboynath asks,

Why do commercial planes fly at 35,000 feet?

So 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.

However, the higher you get,

the less density there is,

the less thrust the engine can produce.

So we try to balance the thrust the engine can produce

with the drag on the airplane

and try to find the best cruise altitude.

It's around 35,000 feet.

oniontimee says, As a plane-ologist,

I have the know how

to explain the difference between models.

For example, a 747 is 10 more plane than a 737.

That's it. That's planes.

It's kinda a humorous comment.

So Boeing started with the 707, did the 727,

then the 737, 747,

767, 777, and then 787.

Some people think the number of the model

means a bigger airplane.

That's not actually true.

It's just the order in which they came to market.

Lockstrocks asks, How does G-Force work

and what causes a pilot to pass out?

So when you're flying in an airplane

and the pilot pulls back on the stick

to make the airplane go up,

the people and the pilot sitting inside the airplane

want to keep going in the same direction,

but the plane pushes up on them.

So while the plane is pushing up,

all the blood is gonna rush down,

and that's what the g-forces are.

How much that force is

number of g's is relative to gravity.

So I'm sitting here in my chair, it's 1 g.

The pilot pulls back a little bit,

maybe it's 2 g's max you'd ever see

in a commercial airplane.

Fighter pilots sometimes pull 4, 5, even 6 g's.

That's what causes 'em to black out,

is when that blood is running out from their brain

into their feet and their legs.

So how do we keep them from passing out?

There's g-suits that actually squeeze their legs

to help keep the blood up in their head,

and some pilots even learn tricks

to grimace and scrunch themselves up

to keep from passing out when they do high-g maneuvers.

Next up, SentientSadist says,

Don't try telling me

that statistically airplanes are safer than cars.

Well, statistically they are safer than cars.

The chances of you having a fatal accident in an airplane

is something one in 120 million trips,

which is far safer than traveling by car

or any other means of transport.

Aviation has gotten safer and safer every single year.

We've gone from having a couple of fatal accidents a year

to recently having one every couple of years.

There's been a huge improvement in safety.

One of the reasons that they're safer

is because the aviation industry,

you've got two trained pilots.

They've gone through lots of training on board.

The aircraft have to be certified

and our redundant systems, multiple things.

So if one system fails, other ones back them up.

And the whole air traffic management

and air transport system is set up

so that we know where the airplane is at all times

and is staying away from other airplanes.

So from a non-statistical point of view,

all that extra stuff we do for aviation

makes airplane-flying safer than driving your car.

oofdoofoofoooof asks,

What happens when there's a bird strike?

Depends on how big the airplane is and how big the bird is

and the relative mass of the two objects.

Bird strikes have caused aircraft

or helicopter windows to break.

They sometimes get ingested into engines.

We actually design the engines to handle a bird ingestion.

One of the bird strikes that's probably most famous

is the miracle on the Hudson

where the Airbus ingested lots of geese into the engines,

so both engines went out,

and Captain Sullivan landed it on the Hudson River safely.

There was a question from Noah Zaves.

Landed at SFO.

I still don't believe that airplanes can fly!

I understand the science of it,

but how can the wings generate that much lift?

The 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.

It pushes down on the air and the airplane pushes up.

Craighton RSS asks, Can a plane fly with one engine?

Yes, it can.

Actually, 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.

There's an interesting thing you've probably noticed

in commercial aviation;

most of the airplanes operating today,

commercial aircraft, only have two engines,

even the ones flying across the ocean,

747 had four engines.

That 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.

As 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.

TangerineMaximum2976 asks,

Why has commercial aviation

not improved in the last 50 years?

It has improved quite a bit.

I think one of the things

that if you're looking at the airplanes,

the airplanes don't look a whole lot different,

but they have been improved a huge amount.

They're much safer.

The engines they use are much more reliable.

We've moved from having multiple engines

for transoceanic flights

to having two engines for transoceanic flights.

The electronics on the airplanes

are much, much smaller and much more reliable,

and so that makes the airplanes lighter.

Our 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.

Airplanes are much quieter.

When the airplane flies over you on approach to the airport,

you 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,

and now we get the big fat turbofans,

which are much quieter.

So commercial aviation's improved

a huge amount in the last 50 years.

The part about the air passengers' experience;

that's not the aerospace engineer,

that's up to the airlines.

That might not have improved much in the last 50 years.

Next up, RGBSays,

What if airplanes were made of

the same material blackboxes are?

Since black boxes generally survive plane crashes,

the plane might be intact too.

The black boxes are made of really strong material,

but the really strong material is really heavy.

And 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

and 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.

Aircraft designers have settled on aluminum

and composite fiber-reinforced plastic

are the two main things that aircraft

are made of these days.

hsn407 asks, Can I ask a flight attendant

to change to an empty seat?

You always can ask the flight attendant.

There's all kinds of reasons

why you might not be allowed to.

Airlines often use the different seats to generate revenue,

so you might not be able

to move to seat you didn't pay for.

On 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

and the pilots want you to be in a certain spot

so that aircraft is easy to fly and stay safe.

Vyyyper says, Hey aviation Twitter!

I have a question for you:

If both the pilots of a commercial airplane are compromised,

can't air traffic control

take remote control of the aircraft?

Answer today is no, they can't,

because they don't have the equipment on board.

But actually aerospace engineers

have been working for quite a while to fly airplanes

that have a lot of autonomous operations

remotely piloted.

The military regularly flies

the Predator and Global Hawk without a pilot on board.

The pilot or operator sits well off and they can do that.

To 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.

Some 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.

That pilot would be remote.

So this is coming.

naominox asks, Is severe turbulence really safe?

At least from an airframe point of view,

from the aircraft point of view,

most of the turbulence will be

rapid up and down accelerations,

and so the airplane may drop

and may climb during the turbulence,

but it's usually so high in altitude

it's not gonna throw you into the ground

from a cruising altitude.

There is a concern about something called wind shear.

So if you encounter that

as you're coming close to an airport,

that can actually push the airplane down.

And there was a really well-known accident

at Dallas-Fort Worth when an airplane hit this wind shear

and it actually crashed short the runway.

We as an industry have put in sensors

and better meteorological devices

to make sure we're measuring,

so that if that's going on around the airport,

we don't fly into that.

Next up, Eggcedrin asks, Do planes have an MPG gauge?

Which I think it means miles per gallon.

They sort of do.

Especially modern aircraft today.

Fuel is such an expensive part of flying airplanes,

but a flight from Indianapolis to New York

will require several thousand pounds of fuel

for that flight.

Airlines or anybody who operates an aircraft

wants to use as little fuel as necessary,

and 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,

and that gives them the highest, quote,

miles per gallon for the airplane.

@thetradingchick 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?

If so, what would be the pros and cons?

It is possible to make an electric airplane,

and actually there are a handful of electric airplanes

actually out there already.

There's some small trainer airplanes,

two seats for people learning how to fly.

Some of the pros are electric airplane

cost a lot less to fly.

Electricity is a lot cheaper

than using jet fuel to fly an airplane,

or using aviation gas for a smaller airplane.

Another advantage is electric motors

have a lot fewer moving parts,

so the maintenance requirement's much lower

and the reliability is much higher.

The biggest con, batteries are heavy.

We've gotten to the point where batteries are pretty good

and we can use 'em for airplanes for short flights,

but for long flights,

the physics just doesn't work.

We can't store enough energy in the battery.

And 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,

so the airplane gets lighter as it flies.

So if you're flying across the ocean,

the airplane's a lot lighter when it lands

than when it takes off.

And because it's burning that fuel,

it means it's using less energy

as you go along the distance.

ClydeinLimbo asks, How do plane wings not snap

or break more often?

Well, it's because we do a good job designing them.

When we design an airplane,

we know about how many g's,

how strong a maneuver that airplane

is ever gonna be able to perform,

and then we make sure that the wings

are almost twice as strong

as that maximum load that the airplane can put in.

Just the aerodynamics of the airplane

limits how many g's you can pull,

and then we design the wings

to handle a load that's higher than that,

and that's why they don't snap or break.

Next one is from katzeye_1.

Can a science person please tell me

how the driver of a plane

can survive a noise vibration like a sonic boom?

It literally scared the crap out of me.

The pilot of the airplane

won't actually feel the sonic boom.

When an airplane starts to go at the speed of sound,

the air starts to behave differently.

I'm talking here in the studio.

My voice, my vocal chords are making a pressure disturbance

that's getting picked up by the microphones.

That's traveling at the speed of sound.

The airplane has enough thrust

it tries to go faster than that,

and essentially the molecules of air

can't get out of the way.

So instead, they pack together

and the properties of those molecules as a whole

change almost instantly.

So right across the sonic boom,

you have a change in pressure and temperature and density.

Now, what happens is that gets pushed

out in front of the airplane.

So if I take this airplane,

which is a supersonic trainer,

a Mach cone will form in front of the airplane.

So the airplane won't actually be in the sonic boom.

But that sonic boom will radiate down toward the ground,

and that's probably what this person heard,

and it does sound a lot like thunder.

John Gunderman asks, Can we bring back

supersonic commercial flight?

Pretty-please?

And the answer is yes, if you're willing to pay for it.

It's very expensive to fly faster than the speed of sound.

It's gonna require a lot of extra thrust

to push you that far.

Extra thrust means extra fuel and bigger engines,

more expensive airplane.

Current commercial aircraft

fly about 80% of the speed of sound

or 85% of the speed of sound,

while supersonic airplanes like the Concorde

when almost two times as speed of sound.

So you can see there's a huge difference in that speed.

You're not allowed to fly faster than the speed of sound

over land currently in the US or in Europe.

If we can make that sonic boom a lot lower,

we can actually fly supersonic over land

and it would make it much more profitable for the operator.

Nervous_Driver334 asks,

I'm not an expert, and then asks,

but why can't airports

have a ramp at the end of the runway

like some aircraft carriers do?

Well, the simple answer is aircraft carriers,

people operating on those are Navy pilots,

and if their airplane doesn't make it

off the end of the runway,

they know they're going into the drink.

We can't do that with commercial aviation.

We need the airplane to have enough runway

that if the pilot's trying to take off

and something goes wrong,

he or she can step on the brakes, turn the engines off,

put on the thrust reversers, and stop in time.

The other reason is if an airplane's coming into land

and something goes wrong on the landing,

we need a lot of extra margin

to allow everything possible to stop that airplane in time.

Ralphie_008 asks, Why don't planes have parachutes?

I've always been curious.

Well, some airplanes do.

This is a picture of a Cirrus SR20.

The Cirrus is a small single-engine airplane

that was certified with a ballistic recovery system.

If something goes wrong on the airplane,

the pilot or somebody else with the pilot

can actually pull a handle.

It'll have a very small rocket that launches a parachute out

and that lets the airplane

sink to the ground at a controlled rate

and people can get out of the accident.

So small airplanes can have parachutes.

The problem is the parachute needs to grow in size

with the size of the airplane.

So airplanes much bigger

than something like that single-engine airplane,

the parachute would be too big.

It would take up too much room and be too heavy

so the airplane couldn't carry passengers or other goods.

primalpoly says, 50 years ago, the X-15 flew at Mach 6.7.

No aircraft since then has ever gone as fast.

But why?

So first off, Mach 6.7 means

that's 6.7 times the speed of sound.

We would think of that as hypersonic flight,

much, much faster than the speed of sound.

This is the X-15.

The X-15 was a research aircraft,

and at the time we flew this,

we needed a pilot on board

to make sure it could do all of its operations.

We used it to learn about what it takes to fly that fast

and what happens to the aircraft

and the flow around it going that quickly.

Since then, we've actually flown aircraft

or hypersonic vehicles that don't have pilots on board.

The flight control system

is good enough to do all those things

and we don't put a pilot in any jeopardy

flying that fast.

smi74499326 says, It's true those jet engines

produce a lot of thrust,

and you will get sucked into them with no real problem.

Unfortunately, that is true.

There's been a few incidents where that's happened.

Also, cargo containers have also been sucked into engines

when they're turned on on the ground.

When you're at the airport,

there's an area marked out usually

where the airplane's parked at the gate,

and there's usually a spot

that's telling people on the ground stay out of this area.

Joey-tv-show-season2 asks, Generally speaking,

how much does it cost to build a plane?

I don't actually know how much it costs to build a plane

because the manufacturers

usually keep that as a company secret,

but I do know what the book price

or listed price of an airplane is.

A 787 costs about $300 million

if you look on Boeing's website.

The caveat to that is when Boeing

sells their airplanes to customers,

the airline might buy lots of airplanes

and might work out a deal.

So maybe like when you go to buy a car

and you end up paying less than the sticker price,

airlines often pay a lot less than the sticker price.

But needless to say, commercial airliners

cost hundreds of millions of dollars.

Godwin Meter asks, How many hours of maintenance

does his aircraft need for every flight hour?

Actually, the number of maintenance hours per flight hour

is an important metric for people

who operate airplanes like airline pilots,

or even if you own your own airplane.

How long you can you fly it

before you need to go do some maintenance?

Commercial airplanes go through

all kinds of levels of maintenance

depending on how many hours they've been flown.

There's some simple checks

where they just look through major systems

and make sure there's nothing they see

on a visual inspection,

all the way up to a major overhaul

where they almost take the airplane completely apart,

look for any kind of problems,

then put it back together

to make sure that that airplane can last for 30 or 40 years.

If you're waiting on the tarmac

or you're waiting to board the airplane

and you hear there's a maintenance issue,

that's usually unscheduled maintenance.

Those are in most cases very minor.

Something like a light

in the fastened seatbelt signs is out.

They're not allowed to fly without that light working.

@tryna_do_rite says, I wonder why we have

an air traffic controller shortage.

Do you have any theories?

The airline industry continues to grow after the pandemic,

we're at flight levels that were way above

what we saw before the pandemic in 2020.

There's just a need for aviation

and there aren't quite enough people

to fill those positions.

Another thing is a lot of people

have been attracted recently to other jobs.

Aerospace is really cool,

and so I'm looking forward

to teaching lots of students in my classes

to get them in the aerospace industry.

SSBMniffin asks, Do planes need co-pilots

or is it purely a safety mechanism

if something should happen to the first pilot?

In modern aircraft, that's true.

It really is a backup.

It's being redundant.

We still have pilots on the aircraft

to make decisions about flying the airplane,

but for most of the airplane flights these days,

the pilots get to their cruise altitude

and they allow the airplane to fly itself,

and the pilots are on board

to react to something that's unexpected

or take control of air traffic control,

ask them to go to a different altitude, et cetera.

@FlyLakeland asks, What's your favorite airplane?

My favorite airplane is the Lockheed Constellation.

It's a World War II-era aircraft

that was designed by Lockheed.

It became the first airplane

that could successfully and reliably

make it across the Atlantic Ocean.

Not only do I think it's a cool-looking airplane,

I think it's a really neat airplane

because it made the world smaller.

And one of the reasons I like being an aerospace engineer

is flying around the world makes the world smaller,

and I think making the world smaller

makes the world a better place.

ThickCactus85 asks, How do jet engines work?

We like to have a sort of a joke statement

here in aerospace engineering.

The jet engines suck, squeeze, bang, and blow.

So this is a picture of a jet engine

with the cowling or the nacelle taken off of it.

The front end has a compressor.

Well, at first it has an inlet.

The inlet is the suck.

That entrains the air into the engine.

Then the compressor increases the pressure of the air.

That adds more energy to it.

Then we add fuel to that air

and we combust it in a combustor that adds a lot of heat.

So we've added a lot of energy to the flow.

It passes past the turbine,

which takes a little bit of the energy out

to run the compressor.

And then the shape of the nozzle

at the back end of the jet engine

makes sure we get the most push out of that exhaust.

So we're basically increasing the momentum of the flow

from what's in front of the engine to what's out that back,

and that difference in momentum

is what gives us the thrust.

What I explained describes a turbojet engine.

This picture is actually a turbofan engine

and this big part at the front is a fan.

So in an engine like this,

the turbine part of the engine,

some of the energy goes to the compressor,

some of it goes to turn this big fan.

This big fan accelerates a lot of air

that doesn't go through the combustor.

That's more efficient

than sending all the air through the combustor.

You just can't fly as fast.

Propellers are at the extreme end of that.

Propellers can't go very fast,

but they're even more efficient.

That's it, that's all the questions I have.

I hope you learned something.

Until next time.

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