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?