How a Plane Flies

When I would take a group of IT people on a gemba walk or tour of the Everett factory, invariably as we would walk under a 747 in the final assembly area, someone in the group would say they didn't understand how such a massive thing could get off the ground.

The standard explanation is illustrated here.

I've never seen the standard explanation actually work for someone who has to ask the question or make the remark in the first place.  Something is missing.  That something is quite simple, so let's walk through it.

Hold out your hand.  What do your feel?  Maybe a little gravity that will tire your arm eventually?  But that's about it.  What you don't feel is important.  At sea level there are approximately 14.7 pounds of air per square inch pressing on your hand.  Depending on the size of your hand, that's about 100 pounds on the back of your hand alone.  And yet, you can't feel it.

The total surface area of your hand, back, palm, sides, and around the fingers, adds up to over 200 pounds of air pressure on your hand.  That's a lot of pressure.  If we extend this exercise to cover the entire average human body, which is just under 2,900 square inches, we end up with 42,203 pounds, or just over 21 tons of pressure!  Let's walk through the calculation again, because trusting these numbers is critical to the rest of the story. 

Atmospheric pressure at se level = 14.7 (14.696) pounds per square inch.  Check it here.

Average surface area of the human body = ...  Well check the data here and I'll walk through how I used.

Frist, the average surface area of the human body for an 18 year old male is 21.313 square feet.  For an 18 year old female it is 18.579 square feet.  Averaging those together, and in this case averaging averages is not statistically that bad, since the female population is only slightly higher, we get 19.946 square feet.  We should round that down a bit to correct for the methodological issue here, and use a value of 19.8.  Then if we multiply that by 144, which is the number of square inches in a square foot (12x12) we get 2,871.  Multiplying that by our 14.7 pounds per square inch, we get 42,203.7 pounds.  Dividing that by 2000, which is the number of pounds in one ton, we get a little over 21 tons.  That's our average.  For bigger people it is more.

Now let's talk about the gas laws.  One of the most basic one is usually skipped in a chemistry class when listing the gas laws.  Most such lists includes the three big ratio and change observations by Avogadro, Boyle, and Charles.  That's because Pascal's observation that gasses and liquids exert a uniform pressure in all directions just seems so obvious.  But, it's really important, especially when we start putting some numbers on it, as we have just done for air pressure at sea level.  Everyone knows that unless there are some funny thick or thin spots in the rubber, that a balloon stretches pretty much uniformly as it is inflated.  

Balloons that have thick or thin spots can take on funny shapes.  Sometimes this is accidental due to poor quality in the manufacturing process, and sometimes it is deliberate.  A good party store will have a stock of funny shaped balloons.  So, messing with the properties of the material can mess with the shape of a balloon.

Well, the same is true for the opposite.  If we mess with the pressure and move it around a bit so it isn't uniform, then we can also change the shape of a balloon.  We can do this by squeezing our balloons or tying them in knots.

This is important to help us understand how a plane flies.  Messing with the shape can move the pressure around on it.

You can feel the effect of doing this by doing something very unsafe while riding in a car.  Open the window and briefly stick your hand out.  The pressure is greater on the surface of your hand that is forward relative to the direction the car is moving.

If you move your hand around a bit, you can feel the air pressure changing what it wants to do.  Some positions will maximize the force to the rear, others will press down on your hand, and others will push it up.

This is what we do in the design of the wing of an airplane.  We put a relatively blunt surface on the leading edge.  When the plane is moving forward, the pressure increases on that leading edge.  As a result, just like our funny shaped balloons, the pressure has to go down someplace else.  And remember those 21 tons of pressure on your body - well just think about how many tons of pressure are on something really big like a 747.

There is an even more dangerous way to extend the experiment of holding your hand out the window of a moving car.  If you hold it stationary with the palm of your hand forward long enough.  The back side of your hand will start to feel cold.  Pressure is a form of energy, and energy can be expressed in several ways.  Heat is one of them.  If you take away pressure, you also take away heat.  If you have ever used a spray can of paint for a long session, say using at least half of a can at one time, you will have felt the can get cold.

This relationship between heat, cold, air pressure and what feels like wind will come up in another page on this site when I dig into what is usually called global warming, which might better be called something like rising atmospheric energy levels, because not everything we experience from it feels hot.  Sometimes it can feel cold, or even like a very fierce wind - but back to why airplanes fly ...

It turns out that if we deflect as much air upward over the top of the wing as we can, that as it scrambles to recover from being so forcibly displaced, it isn't pressing much on the wing.  This is a little bit like being pushed by your buddy instead of gently tapped in a game of tag.  Before you can push back, you have to recover your balance.  Well, the air is the same way, it needs to sort itself out and get back to equilibrium before it can start pushing on things uniformly again.  So that's what they are going on about when they say that air moving faster over the top of the wing exerts less pressure.  However, the total pressure on the wing isn't less, it stays the same.  But, I think people are quicker to understand this and have an intuitive feel for it if we just say hey, because we are going fast there is a whole bunch of pressure on the leading edge, a normal amount on the bottom, and a whole lot less on the top, because the sum of the three has to remain constant.  That's what Pascal observed, and it works darned well.

So we can say that the bottom surface of the wing is pressing upward because the top isn't being pressed down as much.  Or, we can think of it like a vacuum cleaner and describe things from the point of view of the low pressure area.  Thus, it is equally valid to say that the wing is being sucked up into the air.  Lift or suction?  It's the same thing - just a pressure differential the point of view one chooses.  And in the case of a big airplane with all of those acres of wing surface (I exaggerate), that's a heck of a lot of pressure or suction.  Remember those 21 tons on your body?  Well, gee now we see that a plane just has to fly and get off the ground.  There's a gazillion tons of pressure differential sucking or pushing it into the air (your preference).

All of this reminds me of one of Dave Boyd's favorite jokes he would roll out in his accounting classes. 

Q:  How do angels fly?

A:  They take themselves lightly.

OK, one more thing while we are explaining how wings work, and just to be clear, we are barely scratching the surface of all that is going on with a wing.  You may have noticed that there are these funny flappy pieces of metal on the top surface of a wing toward the back edge where the flaps and ailerons are mounted.  Sometimes they pop up a little in flight like part of the wing surface is loose.  When you land, they get propped way up in the exact same instant that the wheels touch the runway.  Obviously, those little bits of metal are not air brakes of some sort.  They aren't even close to being big enough for that.  So what are they?

They are spoilers.

When they pop up, they are "spoiling" the flow of air and causing it to slow down.  That kills lift on the wing.  In flight, computers do this in relatively small increments to help smooth out the ride.  But when landing and those wheels hit the runway, the spoilers are pushed way up and lift is really killed in a hurry.  This keeps the plane from bouncing and plants it firmly on its wheels so you can roll up to the terminal instead of flying or bouncing along on your way there.