If you destroy the Ace planes (basically special enemies), you will unlock skins for your aircraft. It’s recommend that you play the game in Hard mode since some Aces won’t appear in other difficulty modes.
The video below will guide you how you can find them all. Also, if you unlock all of them, you will get the X-02S Strike Wyvern.
Gripen E Louveteau
Rafale M Chasseur
For more Ace Combat 7 guides, check out the following links: