New movieclip code

1

Hi i am trying to create a new movieclip instance of Letter1 onto the stage,
at the moment if i put

mc = new Letter1();

it does this, but i want to increase the number so i created a number var called letterDrop, as you can see below i tried to create a new mc using this but it doesnt seem to work, when i trace it i get Letter1 but when i try create it i get the error Letter1
TypeError: Error #1007: Instantiation attempted on a non-constructor.
at flashgame_fla::MainTimeline/gameEngine()

any help appreciated

   if (randomNum==1) { //creates a new movieclip on the right column
            trace("Letter"+letterDrop);  // returns Letter1
            mc = new ("Letter"+(letterDrop))();
            mc.name = "letter"+clip;
            this.addChild(mc);
            mc.x=728;
            mc.y=50;