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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:

Climb

  • 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

Cruise

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

Gliding

  • 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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  • $\begingroup$ where is everyone $\endgroup$ – Abdullah May 24 at 17:27
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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 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 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 at 20:04
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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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  • $\begingroup$ So, the actual optimum cruise/minimum drag speed for GA is lower than the flown cruise speed. Interesting. $\endgroup$ – Abdullah May 25 at 8:35
  • $\begingroup$ why are my comments disappearing $\endgroup$ – Abdullah May 25 at 8:39
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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. $\endgroup$ – Peter Kämpf May 25 at 22:16
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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. $\endgroup$ – Peter Kämpf May 26 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. $\endgroup$ – Peter Kämpf May 26 at 4:38
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This is more a suggestion than an answer, as I am not an aviator. There are those better qualified to give a definitive answer, such as Peter.

why does it change at a specific speed?

As you probably know, there are different forms of drag. Pressure, friction, vortex drag...... At higher speeds, pressure drag dominates more, at lower speed, friction and vortex drag more. So you can understand that L/D can change with speed. And in general, that change is for the worse.

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

At higher speeds, you have to spend less time exerting lift force, so increasing speed significantly for a little lower L/D will give you better gallon/mile, actually increasing efficiency. Besides, a small gallon/mile penalty may be accepted for a large drop in flight time (I don't know).

And again, as Peter said in the answer to the previous post, the DA20 has fixed gear, bringing the best performance down to lower speeds.

Indeed, wings are not the only thing that may not be at best efficiency at cruise

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

It looks like a certain yes. In your plane, as I understand, there are only 3 flap settings. But if we do not let ourselves to be limited just by these settings, then we can see that in order to fly at different speeds at the same altitude, we have 2 options: change angle of attack, or change flaps. If having the plane fly at a higher AoA creates more drag than having it fly with flaps down, then one must lower flaps and maintain a reduced AoA for best L/D.

And this is certainly the case here. Why? If I understand Peter's post correctly, when the flaps are at 0, they would actually have to be negative to reduce high high-speed drag of the high-lift airfoil's downturned trailing edge. Thus, the T/O setting is needed to restore the airfoil to it's "default" shape. Looking at the DA20's airfoil, this does not seem strange to me.

.....ition at 72kts. Where does this difference come from?

Flaps limit climb rate. That may be a reason.

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  • $\begingroup$ I'm not sure I understood your points quite. The way I understood Peter is that there are some aerofoils who are optimized for high lift in clean configuration (flaps up) while in cruise the flaps would go negative in order to get the best lift/drag ratio. If I understand correctly, such airplane would use flaps negative in a glide, in order to get the longest possible glide. $\endgroup$ – Flion May 24 at 20:23
  • $\begingroup$ That is what I said. However, remember, due to high drag of things like landing gear, the optimum L/D was at a lower speed for this plane, and at that lower speed, flaps negative presumably don't produce enough lift......... $\endgroup$ – Abdullah May 25 at 8:20
  • $\begingroup$ Anyway, listen to Peter and the manual $\endgroup$ – Abdullah May 25 at 8:21

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