A Fly in the Ointment
I was flying last month to attend a graduation and it occurred to me that I was, in fact, flying in a large metal tub with metal flappy bits bolted on. How do airplanes fly? Well, they have wings. Duh! But why do wings fly? What makes a wing a wing? Can any flat sheet of metal be used? Why do planes only fly when moving? This is quite an interesting question indeed. The answer is quite subtle – subtler than you may think. It is in fact confusing enough that a lot of high school text books and a good chunk of introductory university physics textbooks explain it incorrectly. You may have been taught the answer in high school and you have just accepted it without thinking too hard about it. Read on if you wish to check if you were indeed taught correctly.
Aircraft and Treadmills
In this post, I want to address an interesting thought experiment that I “found on the internet”™. The problem is usually phrased in a few different ways but here is the version I have chosen to attack.
It was a calm, sunny day at Horsehead Airport. An aircraft, ready to fly to a foreign country lines up on the airport runway, carrying out final preparations for takeoff. On this calmest of days, the evil Dr. Horrible has decided to discombobulate the pilot of our aircraft by swapping out the runway for a long conveyor belt. This belt is powered by a very powerful motor that can drive the conveyor belt and anything on it at enormous speeds. As the pilot throttles up for takeoff, he powers up his conveyor belt to move backwards. His aim is to move the conveyor belt backwards just fast enough to cancel the aircraft’s forward motion, hence preventing the aircraft from achieving takeoff velocity. Does he succeed in doing so? Can the pilot still takeoff his aircraft or is he at the mercy of the evil Dr. Horrible?
What do you think would happen? Take an intuitive guess. Part of being a good scientist is getting good at making defensible guesses. Also part of being a good scientist is realizing when your common sense and gut feeling are going to fail and training to guess more systematically. When I first ran into this problem many years ago, my gut feeling was “Hmm.. if the conveyor belt moves as fast as the aircraft but in the other direction, then the two would cancel each other out and the aircraft would be at rest, unable to take off!”. My second thought was “Wait a minute.. maybe the engines can produce enough airflow over the wings that the plane can lift off even if stationary!”. The Mythbusters investigated a similar question a while back (here is a sneak peek).