Originally Posted by Phoq Af
... instead of 80kt, Flaps 10, id go 87+flaps 0 to clear obstacles... as I only see 1 and only Vx under DH certif/AFM/etc manuals. Am I wrong?
Moreover, 80kt is Vyse @ 12500lb, so the correct reply should be Vyse for Flaps 10 (ie 80-4/1000lb lighter than MTOW) and best RoC (SE+Flaps10 config). Computed accordingly. Wrong again?
It depends on what you are trying to accomplish, and what the circumstances are.
The aircraft achieves best rate of climb when in the flaps 10 configuration at 80 KIAS. It doesn't matter how many engines you have - 0, 1, 2, or 3. The aircraft also achieves best angle of climb (when in the flaps 0 configuration) at 87 KIAS.
If you wanted to fly up the side of the Matterhorn, and you were already established in level flight a few miles away from the base of the mountain, then for sure flaps 0 and 87 KIAS would give you the best angle of climb. That is the configuration I would choose for that task, and that configuration corresponds to the Vx published in the AFM.
On the other hand, if I had a short runway with a tall tree at the end, that means I am going to be configured with flaps 10 the moment I become airborne, because flaps 10 is the only approved takeoff configuration for the DHC-6 Series 300 or 400 landplane. Personally, I would leave the flaps at 10 and target 80 KIAS (best rate of climb for that flap configuration) in order to get over the tree. I would not retract the flaps and target 87 KIAS (best angle of climb), simply because the performance loss during flap retraction would greatly outweigh any performance gain arising from climbing in the flaps 10 / 87 KIAS configuration.
Michael