[deelowe]

The same seal that contacts the rotor likely seals the face as well. That's how wankels work.

[mannykannot]

The 'seals' section in this article suggests that wankels need separate apex and side seals:

http://what-when-how.com/automobile/wankel-engines-automobil...

See also Pengaru's comment about problems with side seals, specifically:

https://news.ycombinator.com/item?id=25451923