Wow. Bad news. I crashed one of my RC aircraft in the mud/sand once and used a can of air to blow as much of the grit out as possible and then sprayed electric spray to further remove the grit. Oh. Do not rotate the motor until you flush the motor with the electric spray. Let the motor thoroughly dry before applying power. If there is any kind of grinding at all, repeat cleaning process. I would also order 4 new motors or make a decision where you can send for repair if cleaning does not work. Hopefully cleaning works. Test motors thoroughly before flying a mission if you elect to keep the original motors. If in doubt, replace all 4 motors. Good luck.