Mechanics of flight

We do not pretend to have covered all the principles of mechanics, nor even to have explained fully those that have been covered. All we have done has been to select some aspects of the subject which seem to form the chief stumbling blocks in the understanding of how an aeroplane flies; we have attempted to remove them as stumbling blocks, and perhaps even so to arrange them that, instead, they become stepping stones to the remainder of the subject. In the next chapter we will turn to our real subject – the Mechanics of Flight.

Before continuing, try to answer some of the questions below, and the numerical questions in Appendix 3.

Can you answer these?

These questions are tests not so much of mechanical knowledge as of mechan­ical sense. Try to puzzle them out. Some of them are easy, some difficult; the answers are given in Appendix 5.

1 A lift is descending, and is stopping at the ground floor. In what direction is the acceleration?

2 What is the difference between –

(a) Pressure and Force?

(b) Moment and Momentum?

(c) Energy and Work?

3 Why does it require less force to pull a body up an inclined plane than lift it vertically? Is the same work done in each case?

4 Distinguish between the mass and weight of a body.

5 If the drag of an aeroplane is equal to the thrust of the propeller in straight and level flight, what makes the aeroplane go forward?

6 Is the thrust greater than the drag during take-off?

7 Can the centre of gravity of a body be outside the body itself?

8 Is an aeroplane in a state of equilibrium during –

(a) A steady climb?

(b) Take-off?

9 Are the following the same, or less, or more, on the surface of the moon as on the surface of the earth –

(a) The weight of a given body as measured on a spring balance?

(b) The apparent weight of a given body as measured on a weigh-bridge (using standard set of weights)?

(c) The time of fall of a body from 100 m?

(d) The time of swing of the same pendulum?

(e) The thrust given by a rocket?

10 In a tug-o’-war does the winning team exert more force on the rope than the losing team?

11 Are the following in equilibrium –

(a) A book resting on a table?

(b) A train ascending an incline at a steady speed?

12 A flag is flying from a vertical flag pole mounted on the top of a large balloon. If the balloon is flying in a strong but steady east wind, in what direction will the flag point?

For solutions see Appendix 5.

For numerical examples on mechanics see Appendix 3.