"Hoot" noise?

We've had a few F-16s in town for the past week or two, and I've occasionally heard a short "hoot" for "fweeep!" noise coming from them if I happen to be outside while they're taxiing or starting up. What is that noise? I'm not sure what block they are (and I can't get close enough to read the tail markings), but I think they have Pratt engines.

Thanks!

You are probably talking about the CENC - Engine Nozzle Control. This unit is operated by air pressure to actuate the exhaust nozzles; the air pressure creates a distinct whistle noise.

It's an F100-PW-229 thing, in particular, that you are probably hearing. Not nearly as pronounced on -220's, and I believe GE is hydraulic or fuel. You can also hear it quite well from -229's on final approach.

GE uses fuel pressure - if you are hearing "fweeeep" then thats Pratt powered baby

GE uses fuel pressure - if you are hearing "fweeeep" then thats Pratt powered baby

Actually the GE's use engine oil to provide the pressure to the augmentor actuators. The device that provides this is the Engine Driven Hydrualic Pump.

In the Pratt and Whitney's -220's more then the -229's the noise you here is 7th or 13th stage bleed air being pushed thru the CENC to actuate the Primary and Secondary Aug Actuators and the noise is exiting from the CENC air exhaust tube.

All versions of the F100 make the noise.

The Convergent Exhaust Nozzle Control (CENC) uses 13th stage bleed air from the engine's N2 compressor to drive its dual-helical-gear air-motor, providing rotational power to the Primary Nozzle Actuator. The Primary Nozzle actuator then shares this rotating power through to the other 4 Secondary nozzle actuators (so 5 total). These actuators are driven and interconnected by 'flex-shafts' (like over sized speedometer cables) to turn the rotational energy into linear energy (the actuators are like screw-jacks) to push the nozzle synchronizing ring forward/aft inside the nozzle support assembly. The sync-ring then pushes/pulls on the long links, that turn the convergent lever arms, which push/pull the short links to expand/contract the convergent segments that are hinged inside the nozzle assembly.... and so on and so forth.

So yes, the Pratts all use high-pressure air from the compressor to drive an air-motor driven system to position the nozzle. This varies the exhaust nozzle exit - AKA 'Aj' or Area, jet.

Keep 'em flyin'
TEG

[quote="That_Engine_Guy"]All versions of the F100 make the noise.

TEG is spot on as always! This is a different design than the GE engines. The F100 family of engines use the convergent / divergent balanced beam exhaust nozzle system.
Essentially as the engine receives requests for changes, it adjusts the exit area of the nozzle according to its conditions.
When on approach, as the pilot makes subtle changes to the engine power lever while lining up with the runway, the engine is respondiing by optimizing his request. (not only adjusts Engine Pressure Ratio but also takes into consideration fan stall margin) This results in small movements in the exhaust nozzle which is driven by this very high speed air motor resulting in that voop or hoot sound. It only takes 500 revolutions of the final drive from fully open (in Max Augmentor) to fully closed and that air motor covers that ground in a hurry. The proximity to the ground is the reason you hear it.
While mechanical in nature it is a double redundant drive system so that any one cable failing in the system will not stop the nozzle from properly positioning itself.