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I know that deploying flaps will increase both drag and lift, thus increasing L/D ratio. But how exactly does that correlate with the increase/decrease of the rate-of-descent?

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    $\begingroup$ Normally, deploying flaps will reduce L/D ratio due to more drag than lift contributions. Nothing’s as good for normal flying as a well-designed clean wing, aerodynamically... $\endgroup$ – Cpt Reynolds Aug 15 '18 at 18:44
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According to the FAA, lowering or extending flaps allows one to increase drag without an increase in speed. This increases descent rate for a given speed.

One must be careful to consider what factors are held constant to evaluate the effect. For example, one can lower flaps during level flight. This will result in a lower pitch attitude--since they produce more lift, less angle of attack is required. If power/thrust is not increased, speed will decrease due to increased drag.

However, an aircraft flying at a constant speed will descend faster (higher rate of descent) if the flaps are extended compared to when they are not.

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    $\begingroup$ Your last statement is not correct. If you maintain your speed and lower the flaps, you will gain Lift and therefore gain Altitude. $\endgroup$ – Hubschr Aug 15 '18 at 13:41
  • $\begingroup$ @Hubschr, that last statement is absolutely correct. If you maintain speed and lower the flaps, the aircraft will accelerate up very briefly until the angle of attack reduces back to the previous lift. That makes the plane climb, but climbing requires power, so it'll promptly start losing speed, and with that the lift again, which will in turn make it accelerate down, until it regains the speed. Eventually, it will stabilize at higher rate of descent than without flaps—assuming you don't add power. $\endgroup$ – Jan Hudec Aug 16 '18 at 5:55
  • $\begingroup$ "If you maintain speed [...]" is not "[...] so it'll promptly start losing speed [...]" $\endgroup$ – Hubschr Aug 16 '18 at 8:35
  • $\begingroup$ @Hubschr, remember, it is a dynamic process. You initially do something (well, it's too fast for the speed to change yet anyway), but then the system forces something else anyway. $\endgroup$ – Jan Hudec Aug 16 '18 at 19:05
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Flaps reduce L/D. The lift increase is not as much as the drag increase, and ratio of lift over drag goes down. So if L/D flaps up is 12 and I am descending in a glide at X knots with a vertical speed of Y (Y being 1/12th of my forward speed), and I deploy flaps and L/D is now 8, and I'm still at X knots, vertical speed will be higher, with the glide angle now much steeper at 8:1 vs 12:1 (vertical speed will have increased from 1/12th to 1/8th of my forward speed). I can reduce the descent rate, but for that I have to slow the airplane down but the new vertical descent rate rate will still be 1/8th of my reduced forward speed, which will generally still be higher than it was flaps up.

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  • $\begingroup$ This statement is not generally true. Lower flap settings, such as those used for takeoff, increase lift substantially at lower speeds but not drag. Higher flaps settings usually create much more drag. $\endgroup$ – Mark Jones Jr. Aug 17 '18 at 1:27
  • $\begingroup$ If that was true you would leave takeoff flap on all through the climb, since if, say, 10 degrees extension improves L/D, then I'll have a better climb rate with them out. Or if my engine quits and I'm gliding for distance, I can increase my gliding distance by extending takeoff flap. Not aware of many airplanes where that works. The L-13 Blanik sailplane has flaps that only go out a few degrees and are used for thermaling and landing because they lower the sink rate slightly and allow a bit slower speed for turning/landing. But if u are running between thermals at best L/D, they come in. $\endgroup$ – John K Aug 17 '18 at 1:54
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If your hold your Speed and Angle of Attack constant and deploy your flaps, you will gain Altitude. That means, that you can reduce your Speed OR your Angle of attack (or both), while maintaining your altitude constant.

So depending on what situation, Flaps does not have to increase/decrease your rate-of-descent. It is more an interaction between Flaps, angle of Attack and Speed.

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  • $\begingroup$ The question kinda implies that the thing you don't touch is the power levers. $\endgroup$ – Jan Hudec Aug 16 '18 at 5:49
  • $\begingroup$ The questions says nothing about the power levelers... $\endgroup$ – Hubschr Aug 16 '18 at 8:37
  • $\begingroup$ I didn't say it says it, I said it implies it. Because it does not make any sense otherwise. $\endgroup$ – Jan Hudec Aug 16 '18 at 14:26

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