Take Flight

I flew to a girl’s weekend last week and since then the kids have been asking a lot of questions about flight.  In fact they’ve been begging for a “flying experiment”.  Today we’re “finally” going to talk about how things fly in the air.

When the kids get up from their naps they refuse their usual snack and half hour of tv to get straight to the experiment.  I love the enthusiasm.  Since I know that they have been thinking about flying for a while now, I asked them why they think a plane stays up in the air.  “Because that’s what flying means” Briana tries.  I hand them foam airplanes (courtesy of Arby’s happy meals from some time ago) to get them thinking about the parts of the planes.  Then I ask again the question in a more specific way – What PART of the plane keeps it up in the air?  Briana looks down at the plane in her hands and says “The Wings!”

Abby is playing with her plane already and saying, “flap flap” as she flies her plane.  I tell Abby that birds have to flap their wings to fly, but airplanes don’t flap their wings.  Before I can elaborate, Briana actually explains this further to Abby.  “Abby do you remember on Dinosaur Train when the pteranodons had to flap their wings but then that big big one just glided around.  That’s like what an airplane does – it glides.”  I vaguely remember that episode, since Dinosaur Train is what the kids usually watch in our bed on Sunday mornings while my husband and I try to sneak in an extra half hour of sleep before church.  But she is right, an airplane glides through the air and I’m going to show them how.

First, the science behind it:

I ask the kids if they can say Bernoulli’s principle.  Briana says it just fine. Abby gets it right after a couple of tries.  As an aside, this morning when I dropped Abby off at school she tried to tell her teacher that she knew about flying and said “I know how airplanes fly.  Bernoulli pal holds the wings up.”  That’s pretty good since we did the experiment 2 days ago…sometimes I’m slow to blog.  Sorry ;)  So don’t get scared off by the science talk.  I’ll explain the basics of the science first and then tell you the way that I simplified it for the kids.

Here’s the full scientific explanation-

There are two principles that are thought to explain why airplanes stay up in the air.  The general consensus is that both are correct and saying the same thing in two different ways.  Most people therefore simply use Bernoulli’s principle to explain how airplanes remain in the air.

The second theory describing flight is a combination of Newton’s laws and the conservation of momentum. Basically it says that airplanes fly because the wings deflect air downward so that in reaction the plane is forced upward.  For every action there is an equal but opposite reaction.  I’ll teach them that as well at another time (There are more straightforward ways to demonstrate Newtonian physics so I chose to focus on Bernoulli since that’s what you usually hear as it relates to flight anyway – plus Bernoulli’s principle is more in line, to me at least, with the way that you learn about the balancing of forces on an object in high school physics).

Back to Bernoulli - In Bernoulli’s principle the shape of the airplane’s wings (specifically the upwards bump in the front that tapers down in the back) allows air to flow faster over the top of the wing and slower underneath.  Since fast moving air yields low air pressure while slow moving air yields high air pressure, the high air pressure under the wings pushes the plane up.  Said another way, the plane is held up because there is more air pressure pushing it upwards than the combination of gravity and air pressure pushing it down.

I found a good illustration of why faster air equals lower air pressure from www.aviation-for-kids.com that I had planned to share with the kids, but we ended up not getting as detailed as I had planned.  I’ll share it here. 

“Take a room full of children, and ask each child to start running at top speed. Children will start bouncing off each other, and the walls, with impressive collisions (ouch!).Now take those same children out of the room, and ask them to run down the hall at top speed. Now they are all running together, and all collisions between children are much more gentle than before since they are all running in the same direction.

The children in both cases represent the atoms in the fluid, and the force of the collisions represents the pressure between those atoms. In the first case, when the speed of the group as a whole was zero, the jostling (or pressure) was high.

In the second case, when the speed of the group as a whole was large, the jostling (or pressure) was low.”

Get it?  So when the air is moving quickly it doesn’t exert as much pressure on the plane as the slower air below.  Confused?  I thought the kids might be too so you’ll have to trust me on that one.  The real takeaway is that the air pressure from below the wings is holding the plane up.

Here’s what I tell the kids-

The wing of the airplane is designed to make the air travel farther and therefore faster over the top of the wing, making just a little bit of pressure pushing the plane down and a whole lot of pressure pushing the wings up.  That’s why there is a big, rounded bump on the top of the wing.  Bernoulli’s principle tells us that the air from below the plane’s wings is pushing the plane up. 

When people say “lift off” or “blast off” they are actually saying that it’s time to thrust the plane so that it gets lift.  Lift is a word that means that high air pressure required to overcome gravity.

We’re going to first practice lift off and think about how much pressure we need. I take a paper towel and put it on my arm.  I blow on the top and it flutters around.   Then I blow on it softly from underneath my arm.  The “wings” go out straight (versus folding over my arms naturally).  Oooh.  Then I tell the kids that I’m going to apply a lot of pressure from below and….lift off.  I then blow a paper towel to each kid and they try to make enough pressure to get liftoff by blowing straight at the paper and blowing up from underneath.

Then we spend some time trying to blow our planes at each other just for fun.

In looking up fun ways to talk to kids about Bernoulli’s principle – which did not yield a good experiment for toddlers, so I made up the paper towel thing – I found a fun fact that I thought the kids would love.  A racecar’s back actually has the same design as an airplane wing only upside down.  That way when the racecar starts going really fast it’s also being pushed downwards so that it can drive along and make sharp turns without flying up in the air.  I explain my cool car fact to the kids excitedly, pointing to a large Lightening McQueen toy that we have.  Briana tells me I am wrong. “McQueen sticks his tongue out and flies.  Yeah.  In the beginning part of the movie.  He does fly Mommy so you’re wrong about that part.”  I fight the urge to contest this.  Technically Lightening jumped to achieve liftoff which would constitute using an additional force to overcome the grounding effects of the rear design, but I think this will distract and maybe confuse everyone so I just say ok and move on.

I put Lightening away and make a couple of paper airplanes with the kids while we talk about folding the wings just right so that the air can push them upwards.  I tell the kids that in addition to the lift, and airplane needs an engine to get thrust.  Thrust is technically the forward force required to move an aircraft through the air, overcoming drag (and providing sufficient speed for a wing to develop enough lift to fly).  I simplify for the kids and say that thrust pushes the plane up in the air.  When birds fly, their muscles thrust them.  When airplanes fly their engines thrust them.  When paper airplanes fly we thrust them.  So we did.

Bree likes the big wind-up when she thrusts her plane.  She also requires that we beautify the plane, so we do that too.

While we color and sticker I tell the kids that Wilbur and Orville Wright made the first airplane in Dayton, OH where Daddy’s office is.  I make a mental note that we need to get to the airplane museum one of these days.

When I told the kids that we were finished talking about airplanes for today they insisted we do something else.  I gave them the choice between making gliders out of shopping bags or making a giant airplane out of the big box we had waiting to go to the trash.  They picked the giant plane so we did that.

Later that afternoon Abby rolls a crayon down the wing of the plane a couple of times in a row and marvels that "it keeps going down!". She was young when we last did gravity so I explain that everything falls down to the ground because of gravity.  Abby asks if I can make it go up. So I say I can use force to make it go up and I demonstrate and Abby replies "That's thrust! Like an airplane to make it go”.  My little engineer-to-be. :D

<3 Pedigreed Housewife