Action League Now! S01 E05 No Fly Zone

A mysterious liquid is squirted onto Thundergirl by The Mayor in a sabotage attempt, resulting in her losing the ability to fly. Once the Action League find out that he has two children kidnapped in a birdhouse, only the power of flight can save them, so the League gets to work—using balloons and a slingshot, among other devices, to try to get Thundergirl airborne.