I want to simulate the Cessna citation M2 in XFLR as a student homework assignment. This aircraft has a NASA high-speed 0213 airfoil (which is not symmetric) and a winglet, which can be seen in the picture below. Is the airfoil of the winglet the same as the main wing or a symmetric one is used?
*If you do not know for the Cessna M2 specifically, what is the common practice?
There is no reason to assume that winglets have symmetrical airfoils -- in this case or in general.
Furthermore, the wing is likely twisted, and the winglet likely also has twist that manifests as toe-in or toe-out.
Generally speaking, very few aircraft have sufficient information published in order to perform a high quality aerodynamic analysis.
At the same time, it is great to learn about how sensitive the wing's performance is to these different parameters. You might try starting with symmetrical winglet airfoils and zero toe-in/out. Look at the lift distribution and compare to an elliptical distribution.
Then, add some toe-in/out and see what changes.
You should be able to achieve the same effects using either twist or camber of the winglet. So for simplicity, I would use a symmetrical foil and conduct an early study just changing twist.
It is not unreasonable to continue into the winglet with the wing tip airfoil. In order to work best, the circulation (local lift coefficient times local chord) at the root of the winglet should continue with the same value as on the wingtip. A steep gradient in the circulation distribution would increase induced drag.
Since a change in the angle of attack will mainly affect the wing circulation, the winglet incidence and airfoil must be matched to the desired working point of the wing. This can both be done by cambering the winglet airfoil and by selecting the proper winglet incidence.
Note that a high lift coeffcient at the wingtip needs to be bought with a steeper pressure gradient in the pressure recovery zone. If you now add the same gradient at the winglet root, flow separation in the corner between wingtip and winglet will be hard to avoid. This will show up as a drag increase and is normally counted as interference drag. Make sure that the pressure gradients at the operating point are still shallow enough to avoid separation.
