# How does deploying flaps change the gradient and rate of descent?

My understanding is:

Using the gradient formula "(T-D)/W x 100", we can see that with the deployment of flaps, the gradient would reduce (due to the drag added). And since Rate of Descent (RoD) = gradient x Ground Speed (GS), the RoD will naturally decrease.

However, when doing questions, it is apparent that the gradient increases rather than the reduction I expected.

Can anyone provide some explanation on it? Am I using the wrong formula or viewing it in the wrong way?

• More drag makes the gradient steeper, so it increases, no? And then obviously so does rate of descent, at least until the pilot reduces speed. Apr 5, 2021 at 21:42
• >More drag makes the gradient steeper. This is intuitive, however where I'm getting hung up on is the fact that the formula doesn't seem to work in this senario. Once again it could be me misunderstanding the formula, but it'd but nice to know where I'm going wrong.
– De_
Apr 6, 2021 at 11:00
• Are you simply missing the sign? In descent, thrust is less than drag, so $T-D$ is negative. Say $T=20$ (units don't matter) and $D=30$, so $T-D = -10$. Now you increase drag to $D=40$, so $T-D$ becomes $-20$. It is lower number, but has larger absolute value, and means steeper descent. $0$ means level flight, the larger the difference, the further from level the flight is, either way. Apr 7, 2021 at 18:51