The last five in this series have all gone Under, including Army's 17-14 win last year. It's no surprise considering these are both triple option teams, play at a slow pace and this is a heated rivalry. 
Not many teams run an option. But Army and Air Force each do so its defenses are familiar with that kind of unique attack. 
Army is averaging just 19.3 points in its last three games. 
Note the teams will be playing on a grass field, which is a plus for the Under.