I have been designing a hydrofoil lately and have tried to optimize it for the lift over drag ratio. Up to this point, I have only been designing the main wing. The requirements for the main wing is for it to have a wingspan of 1m and an area of 0.1m^2. I have also designed it to have an elliptical lift distribution to minimize induced drag. The root cord is 12.7cm and I am designing it to foil at 5m/s.
For the airfoil initially, I was using the airfoil with the highest L/D at my Reynolds number (which goes from 280 000 to 650 000, since the chord of the wing changes along the wingspan). The airfoil that had the best L/D ratio was the FX76MP120 for Reynolds numbers around 500 000. Therefore I designed the wing with the FX76MP120 airfoil.
However, when I simulate this wing in XFLR5 I get a L/D ratio of 26 which is quite a low value.
Then I simulated the exact same wing with the NACA4412 airfoil (which has a lower L/D ratio than the FX76MP120) and this wing surprisingly had a L/D ratio of 31. I have added screenshots of the results of the XFLR5.
This result confused me since I expected the wing with the FX76MP120 airfoil to have a better L/D ratio. I am not sure how to select the right airfoil for my wing now.
For these tests, i used the XFLR5 software. Below are some screenshots of the tests with the two different airfoils.