FSF, March-April 2010 (29 page)

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Consider the rubber-band-powered plane.

For the past six months, we have been doing just that. In her current job as a writer and editor at Klutz, a publisher of children's books, Pat has been working with a team of model builders to develop a book about rubber-powered flying machines and a set of rubber-powered airplanes that will be included in the book. Paul has been working with the team and advising on physics along the way.

Some might dismiss a discussion of rubber-band-powered planes as childish, but we are confident that fantasy and science fiction readers are much smarter than that. After all, most of us have probably heard our favorite reading material dismissed as childish.

Of course, science fiction writers have examined the importance of toys. In “Mimsy Were the Borogoves,” by Lewis Padgett (a pseudonym for collaborations by Henry Kuttner and C. L. Moore), a box of toys from the future drastically change the thinking patterns of the children who find them, leading the kids to find a path to somewhere Euclidean geometry won't take you.

Though we can't claim that playing with rubber-powered planes has led us to find a path to another dimension, it has caused us to realize (yet again) that thinking about little things (like rubber-band-powered toys) leads to thinking about big ideas (which all science fiction readers like to do). In this column, we talk a bit about planes and a bit about the big ideas that have emerged from our exploration of toy planes.

In her misspent youth, Pat flew a few rubber-band-powered planes—the kind with a fuselage made of a balsa wood stick, a wing that slid through a slot through the middle of the stick, and a tail that fit into a slanting slot at the back of the stick. Put a propeller on the front and add a rubber band. Wind up the rubber band and the plane zipped away at top speed until it crashed into something and broke. Nu Craft Toys (the company that went on to become Guillow's) started selling this type of plane back in 1926. They still sell them today. Some are better than others, but most of them fly more like arrows than planes. Pat's planes were basically balsa projectiles that usually broke on their second or third flight.

Paul comes from a family that loves flying. His dad fell in love with flying in the 1920s and spent his life rebuilding aircraft, including the last remaining Sopwith Camel. Paul designed and built his own model planes. These planes seldom worked the first time, but after hundreds of test flights and modifications, they actually did fly.

For the Klutz book, Pat wanted something better than the balsa fliers of her youth. She'd heard of rubber-band-powered planes that stayed aloft for more than an hour. She knew a few fundamentals about flight—and she thought that a basic rubber-powered plane could be pretty straightforward: a rubber motor to store energy, a propeller to provide thrust, wings to supply lift, a tail for balance. So she located experts to advise her, bought a bunch of toy planes to consider, and got started. A simple project, right?

The early history of rubber-powered fliers is a lot weirder than you think. (When you get right down to it, we've found that most history is weirder than you think.)

It seems that rubber-powered model planes were incredibly important to the history of aviation. Back in 1871, a twenty-one-year-old Frenchman named Alphonse Pénaud built and successfully flew the Planophore—a bat-winged model with a propeller at the back that pushed the model through the air. At the Tuileries Gardens in Paris, in a demonstration for some members of the French Society of Aerial Navigation, the Planophore flew 131 feet—a remarkable achievement. This model wasn't just the first rubber powered airplane, it was also the first airplane capable of stable flight.

Over the next thirty years, lots of folks experimented with rubber-band-powered fliers including (you guessed it) Orville and Wilbur Wright. Before he came up with the Planophore, Pénaud had invented a rubber-powered helicopter that became a very popular toy. In 1878, Bishop Milton Wright of Cedar Rapids brought a rubber band-powered Pénaud-type helicopter for his sons: Orville and Wilbur.

The boys, ages seven and eleven, thought this helicopter was really cool. They flew it until it broke, then built another.

The rest, as they say, is history. Twenty-three years later, in 1902, Orville and Wilbur went on to build and fly the world's first successful airplane. Without that first rubber-powered helicopter toy, maybe they'd have just settled down to run a bicycle shop. They never would have created an invention that changed the world.

No, not the airplane. Just hold on—we'll get to what their invention was in a bit. First, let's talk about what's going on in that simple (yeah, right) rubber-powered model.

A rubber-powered model plane has all the essential bits of a real plane—and nothing but those essential bits. Like a full-size prop plane, a model in flight deals with five basic forces.

Number 1: Gravity pulls the plane toward the center of the earth (the direction most of us call
down
).

Number 2: The spinning propeller blows air backward, which creates the force known as thrust, which moves the plane forward. Newton's third law is: for every action there is an equal and opposite reaction. That's equal in size and opposite in direction. Air is pushed one way, the plane is pushed the other.

Number 3: The wings and tail create lift, the force that pushes the plane upward, as they move through the air.

Number 4: Drag, caused by air resistance, slows the plane's forward movement.

Number 5: Torque, a twisting force created by the unwinding rubber band, makes the propeller spin in one direction and pushes the plane to spin in the other. (Newton's third law at work again.) The plane's wings and tail extend out to the sides and resist that push, so that the propeller spins fast and the body of the plane doesn't spin much at all—but it does spin some.

That's the story in broad brush strokes. But the devil, as always, is in the details. You, being an astute reader, may have noticed that we were kind of vague about lift, the force that pushes a plane upward.

So let's talk about lift—or at least let's talk about people talking about lift. (Pat recommends that anyone who is scheduled to take a flight anytime soon skip ahead to the next section. The discussion of lift that follows is likely to make you consider taking the train.)

In the ranks of physics teachers, there are two opposing camps, each of which claims to have the one true explanation for the lift of a wing. Paul, with Solomon-like wisdom, says that both explanations are approximations of the truth. (And that, after all, is what science is about: approximations to the truth.) With un-Solomon-like glee, Paul also explains how to piss both sides off.

First, the explanations.

One explanation attributes lift to the Bernoulli effect, a difference in pressure created by air flow. The opposing group focuses on Newton's third law, noting that the wing throws air down and that a reaction force pushes the wing upward. Each group claims to have the one true view. Paul believes both groups espouse ideas that are completely true and at the same time completely useless.

Paul explains the disagreement as a difference in viewpoint. To clarify, he uses another situation in which there are different ways of understanding and calculating what's going on.

Imagine dropping a ball to the ground from a height of one meter. Suppose you want to know how fast the ball is moving when it hits the ground.

You could use Newton's law to find the acceleration of the ball due to the force of gravity. Then you could calculate the speed from the acceleration. Or you could use conservation of energy to find the speed of the ball, setting the gravitational potential energy at one meter equal to the kinetic energy at the ground.

Whichever method you use, the velocity you calculate will be the same—as long as you do the math correctly. Both approaches are 100 percent correct and physicists agree that either one completely explains the speed of the ball.

In the case of lift, the Bernoulli camp is using arguments of conservation of energy, while the Newton camp is using Newton's laws. Basically, it's a problem of point of view. Both approaches produce an explanation of lift if applied correctly. But neither camp can actually calculate the lift of a wing given the shape of the wing, its angle of attack, and the airspeed. And that's a problem: Without the test of experimental predictions, you can't do real science.

What do aerodynamicists say? After all, you figure they'd know the real answer.

Aerodynamicists avoid the whole mess by using yet another approximation to the true equations. They use the circulation of air around the wing to calculate lift. They start by mapping the wing into a cylinder, which reminds Paul of the apocryphal physicist trying to understand a cow who began by saying: Assume a spherical cow. Except in this case it actually works! From the air flow around the cylinder, aerodynamicists can calculate the pressure on the surface of the wing and then calculate the lift.

Here's the part that Pat thinks is really scary for anyone who was under the impression that we actually know why planes fly. Aerodynamicists use an approximation because the real calculation would involve solving the Navier-Stokes equations, which have never been solved. The Navier-Stokes equations mathematically describe the motion of fluids. They are used for a lot of things, including modeling the weather, the water flow in a pipe, and the air flow around a wing. To encourage people to work on the Navier-Stokes equations, the Clay Mathematics Institute has offered a million dollar prize for a solution to the equations—or proof that they can't be solved. But no one has claimed the prize.

Now take a deep breath and put that controversy about lift out of your head. After all, you don't necessarily need to know how something works to make use of it. Other than that pesky business about lift, the basics of flight seem pretty straightforward. The propeller pulls the plane through the air, the wings and tail (somehow) generate lift, and the lift sends the plane into the wild blue yonder.

It sounds so simple—like you can slap any propeller on the front of a fuselage, put on some wings, maybe add a tail for decoration, power up and off you go.

Pat says, just try that and see what happens. As Paul likes to say (with a smile...always with a smile), it's complicated. But it's also where things start to get interesting.

At the beginning of the rubber-powered plane project, Pat, optimistic soul that she is, bought a bunch of rubber-band-powered planes—toys, model planes from the hobby shop, and an assortment of fliers available online. Most of them didn't fly as well as she wanted. The balsa models (similar to the ones she remembered) flew like arrows. The toys flew kind of like bricks (not so well).

So Pat started swapping things around to try to improve performance, and all heck broke loose. Just about any modification to the planes seemed to make a lousy aircraft even worse. That's when Pat gathered a team of experts.

One of the great things about being a writer is you can pretty much call anyone and ask a bunch of questions. If you're writing a book, most people will do their best to answer. And if you find an expert who is passionate about sharing information on a particular subject, stand back.

Pat found Lou Young and Gary Hinze of the Oakland Cloud Dusters, a model airplane club that's been around since 1937, and Michael Norcia, a high school student who has been building model planes since he was seven. All three were delighted to build and fly planes and talk for hours about propellers, torque, rubber, wings, lift, and everything else aeronautical.

They explained, through experiment and discussion, that every doggone part of a plane—from the rubber motor to the tail—had to work with every other doggone part of the plane. Change any part and it'll be out of whack with all the other parts. Consider each part of a basic rubber-band-powered plane individually, and it's easy to understand what each one does. It's when you look at them together that the true complexity comes to light.

That brings us back to the Wright Brothers and their true accomplishment. Their flight at Kitty Hawk wasn't just about getting up in the air. It was about controlling the flying machine once they were up. Getting off the ground was the (comparatively) easy part; stability and control were (and still are) the tricky bits. The Wright brothers were the first to build an aircraft that could be controlled while in the air.

Why is control so tricky? Because there are so many ways to go wrong.

On the ground, you have your choice of going forward, going left, or going right. Life is so easy on a two-dimensional surface.