There are 17 words that start with 'flap'.
| flap | flaps | flappy | 
| flapped | flapper | flaperon | 
| flapjack | flapless | flappers | 
| flappier | flapping | flaperons | 
| flapjacks | flappable | flappiest | 
| flapdoodle | flapdoodles | |
Note: These 17 words that start with 'flap' are found from the USA & Canada version of the tournament Scrabble word list.