Question

In an analysis, the maximum endurance in cruise of a jet airplane is used to measure its performance. If just the parasite drag coefficient is reduced by 19 %, determine if this increases or decreases its performance, and if so determine by how much.

Answer 1

Given:

parasite Drag Coefficient,
`C _(D)` is reduced by 19% or 0.19 times

Solution:

Using Breguet's Endurance Eqn:

`E = t_(2) - t_(1) = - \int_{W_(1)}^{W_(2)}(dW)/(c_(t)D)`

or

`E = (1)/(c_(t))(C_(L))/(C_(D))\ln(W_(1))/(W_(2))` (1)

From eqn (1), other terms being constant, then

`E = K(1)/(C_(D))`

or

`E\propto(1)/(C_D)`

Therefore, the above relation shows that when drag coefficient
`C _(D)` decreases performance increases and vice-versa due to inverse relation between the.

So, 19% decrease will result in:

`E = (1)/(0.19)` = 5.26 times increase in performance