As I've recently started time building in DA20, I've noticed the AFM suggests flaps at T/O for a glide. Being very intrigued by this I run across this post. Although I appreciate Peter's reply that does throw some light I'm not quite clear with it and I'd like to elaborate on it further. As the topic is closed, and the forum advises against asking for more clarification, I open a new topic here.

Diamond DA20-A1 FM says:


  • Wing Flaps: T/O or UP
  • Airspeed: 65 kts

further elaborating with a table:

  • altitude: 0-4000 ft, flaps T/O: 65 kts, flaps UP: 69 kts
  • altitude: 4000-7000 ft, flaps T/O: 63 kts, flaps UP: 65 kts
  • altitude: 7000-10000 ft, flaps T/O: 62 kts, flaps UP: -- kts
  • altitude: above 10000 ft, flaps T/O: 59 kts, flaps UP: -- kts


  • Throttle: as required
  • Prop. Speed Lever: 1900 - 2400 RPM
  • Wing Flaps: UP


  • Wing Flaps: T/O
  • Airspeed at 1609 lbs (730 kg) (vIAS): 72 kts

I deduce there is a speed below which flaps at T/O create more lift than drag and it is therefore beneficial to have them down, however in the whole ATPL syllabus I've never run across such a thing - a specific speed at which lift/drag ratio for flaps down changes.

I can understand it's either up or down for the best L/D ratio but I don't get why does it change at a specific speed? Unless the reason is the limitation by Vfe (81 kts) when pilot eventually must retract the flaps even at the cost of lower L/D ratio. If that is the reason, then I don't understand why would someone design a wing that's less efficient to fly in Cruise then in Climb?

So, my final question: does the flap settings for the best lift/drag ratio change with speed, or the wing is not flying at its best efficiency in cruise?

Besides, if we compare climb with gliding speed, we can see that in a climb flaps are to be retracted at 69kts (and even lover as altitude increases), while in glide they are to be at T/O position at 72kts. Where does this difference come from?

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    $\begingroup$ Ok, whole two hours, no answers and there's a question regarding the whereabouts of ppl. It's weekend dude, I hope folks are outside having fun 😃 But seriously, a good question (the one about flaps that is), I was stupid enough not to wonder this myself some years ago. I do not have a definitive answer, but my intuition is that it has to do with the DA's being very slippery planes. They just love to fly, where some other planes I'm familiar with tend towards the gnd when not powered... I thinks @Addullah might be onto smthng in his answer. $\endgroup$
    – Jpe61
    May 24, 2020 at 17:44
  • $\begingroup$ The linked question definitely isn't closed, and asking Peter to elaborate further in a comment on his answer is probably most likely to get you the clarification you want. $\endgroup$
    – Jan Hudec
    May 24, 2020 at 19:09
  • $\begingroup$ @JanHudec You're right, the post is still open. Initially I tried to comment on Peter's answer however I was unable to as I need at least 50 reputation. Then I tried to give an answer however the pop up message advised against answers which actually ask for clarification, instead of giving an answer. $\endgroup$
    – Flion
    May 24, 2020 at 20:04

1 Answer 1


You did the right thing when asking a new question. Comments are poorly suited for the explanation of non-trivial issues.

Now follow several links to older answers. They should help to explain a couple of things, such as:

does the flap settings for the best lift/drag ratio change with speed?

Below you see the polar plot from the first linked answer. It shows the airfoil drag in 2D flow. Clearly, the best L/D is achieved with the +10° flap setting, but at this setting the airfoil becomes rather poor below a lift coefficient of 0.3. In order to reduce drag, the flap should be raised when flying fast enough so that the lift coefficient drops below that value.

flap polar

So the answer is yes, the optimum flap setting does change with speed. Only the flap setting for the overall best L/D will be found at the same flap setting, but then this best L/D can only be reached at one specific lift coefficient (which is equivalent with one specific speed if mass does not change).

I can understand it's either up or down for the best L/D ratio but I don't get why does it change at a specific speed?

It does not change at a specific speed. It changes continuously with lift coefficient (and, implicitly, speed). If you compare specific flap angles, you will find crossover points when the L/D of one flap setting drops below that of another flap setting as the lift coefficient changes. Normally, airplanes only allow a few specified flap angles, so it might seem that L/D changes abruptly when changing flap angles. However, flaps could also move continuously, adapting the wing to its optimal setting for each operating point.

why would someone design a wing that's less efficient to fly in Cruise then in Climb?

Cost. GA aircraft don't have the complex flap systems of airliners and, consequently, have a much larger wing than what is required for cruise. Wing area is sized for the mandated minimum airspeed of 61 kts or less and installed power means that the airplane at low level cruises at a speed where friction drag dominates and induced drag is almost negligible. While technically possible, the complex high-lift systems of airliner wings would make any GA airplane a lot more expensive, a death trap for untrained pilots and a maintenance nightmare.

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    $\begingroup$ @Flion The syllabus is right: The best L/D is indeed achieved with the clean configuration. Vary speed and you will vary L/D, and now there might be some flap setting where L/D at that speed is better with flaps than in the clean configuration. This is possible at very low speed when the clean configuration almost stalls. $\endgroup$ May 25, 2020 at 22:16
  • $\begingroup$ Thanks Peter. This was very informative and quite strange that in the whole ATPL syllabus there's not even a slight hint that the best L/D flap setting changes with speed. It always teaches that the best LD ratio for the aircraft is always in clean configuration. Interesting to see the polar diagram with a double change in CL (0.4 to 0.9 for flaps +10) for no change Cd. I've always observed it as a curved line with interdependence of coefficients. I wonder now how the polar diagrams for DA-20 and C-172 compare, as the latter glides with flaps up. Any source for these? $\endgroup$
    – Flion
    May 25, 2020 at 22:27
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    $\begingroup$ @Flion: The polar here is for the airfoil only. A full airplane polar is dominated by induced drag which has that curved line as drag increases with the square of lift. This induced drag is missing here to make more obvious what flap settings do. $\endgroup$ May 26, 2020 at 4:21
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    $\begingroup$ @Flion: The difference between C-172 and DA-20 is how zero flaps are defined. In the C-172, zero is when the flap is in line with the rest of the wing. In the DA-20 zero is actually a negative setting and the equivalent to the C-172's zero is a positive deflection. The original DA-20 airfoil was designed for high lift and is unsuitable for fast flight. HOAC added a 20% flap to widen its useable lift coefficient range. It needs that negative deflection to improve drag in cruise and in order to not confuse pilots, the zero point was shifted to that position. $\endgroup$ May 26, 2020 at 4:38
  • $\begingroup$ Thanks a million Peter. I'm curios, why would a full airplane polar add more induced drag? Shouldn't a fuselage add more parasitic drag? I think of induced drag as something that comes as an adverse effect of lift, which is what wings create (although a fuselage can create some lift as well). $\endgroup$
    – Flion
    May 27, 2020 at 9:41

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