There are two main things that make aircraft engineering difficult: the need to make every part as reliable as possible and the need to build everything as light as possible. They are to be combined (结合).The fact that an aeroplane is up in the air and can't stop, if anything goes wrong, makes it perhaps a matter of life or death that its performance is completely dependable.
Given a certain power of engine, and so a certain amount of oil, there is a practical limit to the total weight of aircraft that can be made to fly. Out of that weight as much as possible is wanted for fuel radio instruments, passenger seats, or freight(货物) room, and, of course, the passengers or freight themselves. So the aircraft has to be as small and light as safety will allow. The designer must work out the normal load that each part will bear. This specialist is called the "stress(压力) man." He takes notice of any unusual stress that may be put on the part as a means against errors in production, accidental damage, etc.
The stress man's decisions go to the designer of the part, and he must make it as strong as the stress man says is necessary. One or two small parts are always tested to prove that they are as strong as the designer wanted. Each separate part is tested, then a whole one-for example, a complete wing, and finally the whole aeroplane. When a new type of aeroplane is being made, normally only one of the first three made will be flown. Two will be destroyed on the ground in strict tests. The third one will be tested in the air.
When a plane has passed all the tests it can get a government permission of flight, without which it is illegal(非法的) to fly, except for test flying.
5. The main difficulty of aircraft design lies in________.
A. the combination of lightness and reliability
B. the speed and cheapness
C. reliability and passenger comfort
D. its engine power and oil burning
6. The stress man's job is to work out ________.
A. how safe the plane is
B. the load that each part bears
C. the possible errors made by designers
D. the amount of load the aircraft may carry
7. Which of the following statements is NOT true?
A. The total weight of an aircraft depends on the power of the engine.
B. An aircraft has to pass all the tests.
C. Permission of flight can be got from the government.
D. It is necessary for all the first planes to be tested in the air.