Chris Scott.
Take a look at the landing distance correction factors and the possible F/S combinations.
More is not always better.
Eg. E170 EASA Certification. Flap failed in position 1. The shortest landing distance correction factor comes with slats retracted. Factor is 1.46. If you choose to select slats (1,2,3) the correction factor increases to 2.0 as it does with slats (4,5,Full). In this case the Vref increment remains the same for all 3 options (VRef Full + 35) So same stall speed but higher pitch attitude resulting in longer landing distance. There are similar examples for all 4 versions both in EASA and FAA certifications.
In 2003 when we first started training the EJets on behalf of Embrear this was one of the first questions I brought to them. We asked if this might be a typo but the reply from their Perf Engineer was clear. Prolonged de-rotation due to High nose up at touchdown.
When discussing this with students I always draw a picture of the wing with both high lift devises in all positions and draw chord lines from Slat LE to Flap TE to show the different incidence on a fixed fuselage and then using Vref corrections discuss CL changes at different geometries.
This clearly shows those combinations where additional Slat does not increase Cl but simply changes the Incidence and thus the required attitude for a fixed Alpha.
Last edited by FE Hoppy; 20th Feb 2017 at 07:28.