@Deputy-Nick first of all read topic that @Reacon linked you before you continue reading...

now to your problem, I had it many times so should know what is problem even without description. Write to the search name of that helicopter and add ' .ytd' . Now check in which all folders is this file located and delete it from any of your addon folders where it is but you didn't add it there. Should be any pack as for example WOV.

Reason is that this dlc folder is wrote in 'dlclist' later after the folder that you was replacing the helicopter and game is loading it one by one and in case the file is in more folders, it use last one. So it use textures from different file that you replaced.

Don't forgot read that topic before creating next topic when you will have any other problem in future please!