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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?

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  • $\begingroup$ 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. $\endgroup$
    – Jan Hudec
    Apr 5, 2021 at 21:42
  • $\begingroup$ >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. $\endgroup$
    – De_
    Apr 6, 2021 at 11:00
  • $\begingroup$ 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. $\endgroup$
    – Jan Hudec
    Apr 7, 2021 at 18:51

1 Answer 1

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Yes you’re viewing it the wrong way. Deployment of flaps changes all sorts of flight parameters, not just one. The wing area may increase, the camber and angle of incidence of the wing increase and so the engine thrust line is effectively tilted downwards. The aspect ratio changes, as does the relative angle of the horizontal stabiliser. The net result is that the flight efficiency is reduced (i.e. the amount of energy required to maintain height increases) but the stall speed is reduced. There’s a lot happening and you have to take it all into account. Typically on approach you’ll want a low airspeed and relatively high rate of descent but without compromising control authority, so the flap layout is designed to provide this.

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