Why Does The F-15 Make THIS Strange Noise Before Takeoff?

Hear that strange sound from this F-15? That weird vwoop-thweep-whistle noise you hear while the jet is taxiing or preparing for takeoff? It's one of the most distinctive sounds the F-15 makes. But it's not actually coming from the engine the way most people think. So what are you really hearing? The answer is hidden underneath the aircraft. In the F-15's exhaust nozzle control system. The F-15's Pratt & Whitney F100 engines don't use fixed exhaust nozzles. Instead, the nozzle pedals constantly open and close to optimise engine performance, airflow and thrust. To control that movement, the aircraft uses something called the Convergent Exhaust Nozzle Control System, or CENC. This is where the famous sound comes from. The system uses highly pressurised bleed air to rapidly adjust the engine nozzle position as the pilot changes throttle settings. During taxi, line-up, overhead brakes and pre-takeoff power changes, those nozzles are constantly moving. When they rapidly expand or contract, pressurised air vents from ports on the underside of the aircraft. And that escaping air creates the sound you're hearing. That strange wheep, whoop, almost servo-like whistle. Think of it like blowing across the top of a Coke bottle. Except instead of your breath, you're hearing high-pressure air venting from a fighter jet's nozzle control system. If you look underneath the aircraft, these exhaust ports sit near the engine nozzle. And in cold or humid conditions, you can sometimes even see the venting air. So next time you hear an F-15 making that strange whistle on the ground, listen carefully. You're not just hearing a jet engine spool up. You're hearing the F-15's CENC system, working to control the variable exhaust nozzles on the F-100 engines. And once you know what causes it, you'll probably never unhear it.