Tuesday, August 18, 2009

How does an airplane fly? Think about it - By Judge Jim Gray

As my wife and I in a few days are going to get onto an airplane and take off for a vacation, I again started wondering how an airplane actually flies.

To me it seems like an unnatural thing for a long and heavy canister with wings to be able to get off the ground and fly through the air. So I did a little research on the subject.

The three major factors of airplane flight are the engines that produce thrust, the wings that produce lift, and the flaps, elevators, and rudders that are used to control the direction in which the aircraft travels. All of those are probably self-evident, except for the concept of lift.

Many flat surfaces like a piece of plywood, a Frisbee, or even a flat stone, will glide for short distances on a fluid like air or water, and will even rise or dive, depending upon the wind and their angle of attack. But they will not produce lift. For that to occur, the objects must have a different configuration.

The next time you are around an airplane, or helicopter either for that matter, look at how the wings and rotors are formed. Their bottoms are basically flat, and go straight to the back, but their top surfaces are curved. This design forces the air on the top of the wing or rotor to travel farther than the air on the bottom during the same amount of time to reach the same spot at the back.

This is important because when the speed of a fluid increases, the pressure exerted by that fluid on the surface of a nearby object actually decreases. In our case, that means that there is more pressure pushing up from the bottom of the wing or rotor than there is pressure on the top pushing down. Therefore, assuming all other factors are equal, when pushed through the air the wings and rotors have a tendency to rise, or to produce lift.

This is Bernoulli’s Principle. Daniel Bernoulli was a Dutch/Swiss mathematician who lived from 1700 to 1782, and he discovered that as the speed of a moving fluid increases, the pressure exerted by that fluid decreases. And this principle holds true as long as the fluids flow smoothly and consistently, and are of the same density.

Furthermore, the more dense the air is with an airplane, the more lift will be generated. That means that the wings will generate more lift in cold weather than warm, because cold air is denser, and the wings will also generate more lift at sea level than at higher altitudes for the same reason.

As I understand it, this can be analogized to a situation in which people would throw large numbers of golf balls in many different directions inside a small container. The balls would spend a lot of energy bouncing into each other and onto the walls of the container, and that would generate higher amounts of pressure on those inner walls. But if the balls were all thrown in the same direction, the inner pressure would be reduced because the balls would not be hitting the walls or each other, and the faster they go, the less they would be inclined to hit anything.

So what about stunt pilots that fly their planes upside down? Why don’t the wings then “lift” the airplane toward the ground? The answer is that the pilots counteract the downward thrust from the wings by over-adjusting the elevators on the back wings of the plane, which changes the angle of attack by keeping the front of the plane higher than the back.

Another application of Bernoulli’s Principle explains why windows in houses tend to explode outward instead of implode inward during a hurricane. The reason is that since the winds are going so fast outside the house, there is less pressure outside the house than inside. That is why people who are knowledgeable about surviving hurricanes suggest that you open all of the windows in your house when one occurs.

I don’t know about you, but I think it is fun to understand the various phenomena that explain the everyday things that are all around us (For example, how does a flush toilet work?). Think about it; it’s interesting.

I also found it interesting to learn that eastbound aircraft are required to fly at an odd number of thousands of feet, and westbound aircraft at an even number. So, for example, eastbound aircraft will fly at 31,000 feet, and westbound will fly at 30,000. Obviously that regulation is meant to decrease the chances of head-on collisions.

Accordingly, as my wife and I get onto our airplane, we can be comfortable not only that Mr. Bernoulli’s Principle will still be fully engaged, but so will many other features that will increase our chances of a safe flight. In fact, studies show that airplane travel is about 20 times safer than automobile travel, as computed in deaths per passenger mile. (You probably know that the even-numbered interstate highways in our country generally go east and west, and the odd numbered go north and south. But that is just for convenience, and regretfully doesn’t have any effect on head-on collisions.)

So the odds are good that we will return safely from our travels, and you can look forward to many more installments of this column in the coming months. Happy summer travels to us all!

JAMES P. GRAY is a retired judge of the Orange County Superior Court, the author of “Wearing the Robe – the Art and Responsibilities of Judging in Today’s Courts.” He can be contacted at jimpgray@sbcglobal.net or via his website at www.judgejimgray.com .

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