Btw, are you a fan of Toa Plan? I definitely need some experts to add more information to their games.
What I read was Tiger Trilogy was made up of the said games (
Tiger Heli,
Sky Shark,
Twin Cobra).
In this liner note, Hisayoshi Ogura called those three as "Tiger Series", and M.Y & T.U (I'm sure they are abbreviation of Masahiro Yuge and Tatsuya Uemura, respectively) even confessed they wondered why Sky Shark was seen as part of "Three Tiger Brothers", even though the protagonist wasn't a helicopter, etc..
Then again, I won't argue, since I'd like to decide the product relationship based on how our users like. Also, there is some arbitrariness with which we can treat products, I think (for instance, I lumped
Xenoblade as other Xeno- games, although the only shared elements were the prefix and some developer names). We can make two more franchises ("Twin Cobra", "Shark", or such) and if Twin Hawk is also known as an alternative constituent game of the Tiger trilogy, we can just drop Sky Shark from there, and then replace it by Twin Hawk.