Dear friends and followers, welcome back to my channel and to another fantastic little video about the question
I am very happy to answer for you guys, "Joe, Why is there white stuff in the engine upon takeoff roll?"
I'll try to explain this via a little experiment as you've seen in the intro with a plastic water bottle,
so get out your physics books and let's get started!
This video is brought to you by brilliant.org; the best online learning platform for maths and science classes.
More information within the video and check out the link in the description box below. Now though these videos of departing
aircraft look ever so cool as they hurtle down the runway
but why is there white smoke, fog or whatever you want to call it within the engines? What you see here is actually
a miniature cloud
generating in front of the engine compressor. To explain this a little easier
we have to get a better understanding of relative humidity. Now important facts first:
warmer air can hold
more water vapor,
humid air or moisture than colder air. Now
let's say we have one cubic meter of air mass at 20 degrees and a relative humidity at 25 percent.
That equals about 4 grams of water the air is holding within. Now if we add
more water vapor we can increase the relative humidity up to 100% which would give us
15 grams of water per cubic meter at 20 degrees. Now as that is the
maximum of water the air can hold at that temperature,
that temperature is then called the dewpoint. Now this can easily be felt during Monsoon season
in Asia for example
compared to the Middle East where it's much more of a dry heat with only little
relative humidity. Now if we cool down the cubic of air by a few degrees, so
below the dew point the air can't hold that water anymore
which will then start to condensate forming a cloud.
So how does that relate to our engine? Now if we take the Boeing 777
fitted with the General Electric GE90 engines as an example, each engine sucks
5000 cubic meters of air through its engine upon takeoff roll and wait for it, per
second! So the engine acts like a giant vacuum cleaner.
Still, air is being sucked into the engine and wherever there is a suction
there is a drop in pressure and in this case a rapid acceleration of air. Now we all know
accelerated air is cooler otherwise a fan would lose all its purpose. Now
let's say the outside air temperature in this video is 20 degrees with 100% relative humidity
because by the looks of it, it just rained a few minutes ago
so only a minor decrease in air temperature as it would happen whilst the air is
accelerated into the engine that temperature drops below the dew point and the water vapor in the air
condensates and forms the cloud within the intake. Now to show you this a little better my dad and I came up with a little
experiment. We used a plastic water bottle and put a few drops of water in it. Now the water
obviously to act as water vapor in the air or relative humidity.
We drilled a hole through the cap giving enough space to fit the hose of our air
compressor and as I added air to the bottle the air pressure within the bottle increased literally
energizing the water in the bottle
so it got relatively hot. Now to get a picture of what's gonna happen the air pressure surrounding
the bottle is much lower than the one in the bottle. So
that relates to the engine intake pressure and the higher pressure within the bottle is
the still air with high humidity sitting on the runway. Now as soon as I remove the compressor hose
the air in the bottle
rapidly loses pressure so a pressure drop as it gets sucked into the engine and
accelerates the air through the hole which cools down and immediately forms a cloud. So let's do that.
Three! Two! One!
And one more time in slow motion. Three! Two! One!
Does that make sense? Please
comment below if something is misleading. Now you can see a similar effect if you use your Soda Stream machine.
Give it a few pumps into a bottle with only a little bit of water and as soon as you open to release the pressure
equalizes high pressure in the bottle low pressure surrounding and a cloud is generated within the bottle. A similar effect if you
open a can of soda.
I admit there is CO2 involved and it has other characteristics
but the principle is fairly much the same the pressure drops and the air is accelerated. A cloud! Now
the question immediately arises. Does that in any way affect the engines performance?
Yes it does, but it's so marginal it's not even worth mentioning *chuckles*
because after the air has passed through the engine's compressor the air is
approximately at 450 degrees hot before being mixed with fuel and any water will have
evaporated by then for sure. That's like trying to extinguish a massive wildfire with one drop of water.
I'm not seeing that happening. That's more or less it for today!
But if today's video was a little too much physics for you and you had trouble following my experiment,
Brilliant.org is a fantastic learning web site to help you get better understanding from math and physics.
They offer a great range of courses in maths like basic algebra, linear equations, logical reasoning,
statistics etc. and the same goes for physics.
So if you go to everyday physics in the house and
refrigerators you get a great and fun explanation about what I've just talked about in the last few minutes.
So if you want to get better at maths and physics
which I highly recommend if you want to become a pilot, go to
brilliant.org/captainjoe
and sign up for free. And also the first
200 people that go to the link in the description box below will get 20% off on the annual premium subscription.
So that's it for today. Thank you very much for your time!
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See you next week, all the best, your Captain Joe!