Possible Clues:

• Propellerless planes
• Rapid streams of liquid or gas
• Boeing 747s, e.g.
• What's up with the travellers
• They're grounded when they're misbehaving
• Sooty and Sweep's original careers?
• Fast airplanes
• Early '60s Polo Grounds team, nowadays
• Movie foes of the Sharks
• They leave trails