阅读理解 A wind tunnel is a device used for testing airplanes or plane models． In a wind tunnel air is blown over an airplane or a model placed in a test section．Wind tunnel test sections are different in size depending on airflow speed requirements．Some low-speed tunnels have test sections large enough for a complete small airplane．In the very high-speed tunnels used for space testing, however, the model may be as small as a pencil． There are two methods of feeding air to the test section．In the continuous flow tunnel air-flow is produced by electric fans．It is forced through the tunnel to the test section, then through another tunnel section back to the starting point for refuse． The other method involves storing high-pressure air in a box, sending it out by a control device to pass through the tunnel and test section into another box．This type of system is generally used for a very short period of high-speed airflow． Wind tunnels are usually used to test airplanes before they are flown．Problems in an airplane can be found when it meets the high-speed airflow in the tunnels．For instance, if the wings are built so that they would weaken at high speeds, this fact can be discovered in the tunnel, saving many lives and millions of dollars． In a wild tunnel scientists also find out how different planes will act at different speeds．They find out such things as how smoothly an airplane is flown, how slowly a plane can fly before it starts to drop, and how fast it can climb． | 
