TABLE OF CONTENTS Level flight performance may be described as steady state performance. It is a condition where all of the forces acting on the airplane are in balance or:
| (8.1) |  |
| (8.2) |  |
where
| L | = lift |
| W | = airplane weight |
| F | = thrust |
| ? F | = the thrust angle with the horizontal |
However, if we assume small angles we can say that thrust is equal to drag and lift is equal to weight.
In level flight performance we are primarily concerned with Eq. (8.2) or thrust F and drag D; however, we shall see that the lift L and weight W of Eq. (8.1) also play a part.
Level flight performance is essentially a determination of airplane drag as a function of velocity. Since the thrust must equal the drag it could also be called the thrust required for a given velocity.
The drag of an airplane is composed of two components. They are 1) parasite drag; and 2) induced drag.
The parasite drag is composed of all forms of drag other than the drag due to creating lift which is the induced drag.
Parasite drag can be defined by the equation:
| (8.3) |  |
where
| D P | = parasite drag |
| C DP | = parasite drag coefficient |
| S | = reference area (normally taken as the wing planform area) |
From this equation we can see that drag varies as a function of the true airspeed squared.
The induced drag is defined by the equation:
| (8.4) |  |
where
| D |
