E 　　A wind tunnel¹ (风洞) is used for teasing² planes or plane models. In a wind tunnel air is blown over a plane or a model placed in a test section(区).Wind tunnel test sections are different in size depending on airflow speed requirements³. Some low-speed tunnels⁴ have test sections large enough for a complete small plane. In the very high speed tunnels used for space testing, however, the model may be as small as a pencil. 　　There are two ways of feeding air to the test section. In the constant⁵ flow tunnel, airflow 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 reuse⁶. The other way involves (卷入) storing high-pressure air in a box, sending it out by a controlling device(装置) to pass through the tunnel and test section into another box. This way is usually used for a very short period of high-speed airflow. 　　Wind tunnels are usually used to test planes before they are flown. Problems in a plane can be found when it meets the high-speed airflow in the tunnel. For example, if the wings are built so that they would weaken(become weak) at high speeds, this fact can be discovered in the tunnel, saving⁷ many lives and millions of dollars. 　　In a wind tunnel, scientists also find out how different planes will act different speeds. They find out such things as how the air dives by the wing surfaces, how slowly a plane can fly before it starts to drop, and how fast it can climb.