They should all have flaps almost touching the ground. Even in dry conditions it keeps the chain cleaner. Most of the dirt on the drivetrain comes from the bottom of the front wheel. A flap almost reaching the ground needs to be flexible to go over things. I use wide MTB tubes folded over to glue to itself making the flap two layers of tube. One layer will just return to the original tube shape. My long flaps will go up over snow. And up or down stairs, rolling over something like a small log, etc.
When going fast the flaps move back a little and they catch a little less air. Very little.