Posted on 02/28/2023 8:42:22 AM PST by Paal Gulli
First reported here (twice), complete with video, when it happened last December at NAS JRB Fort Worth. The mishap was remarkable because the pilot ejected while the a/c was on the ground.
The news reports that followed all spoke to the engine as the cause -- which would seem to indicate that neither the pilot nor other aspects of the aircraft were under suspicion -- and DoD ordered engine manufacturer Pratt & Whitney to stop delivering new engines for the F-35B until the problem was sorted.
Last Friday, a news source that may not be quoted here for copyright reasons reported that P&W had identified the cause of the mishap as "a rare system phenomenon involving harmonic resonance" (in the engine). Not stated directly but P&W must have told DoD the problem is fixed because they have approved resuming engine deliveries. However, for reasons the story does not address (possibly waiting on a retrofit of "the fix" to all F-35Bs already in the field), the a/c are still grounded, which means the Marines won't be getting any new F-35Bs for the timebeing.
Galloping Gertie was a special case of induced resonance. (I am an EE not an ME.) The light poles on the roadway caused turbulence in the airflow around them, aeoli. When the roadbed tipped they shed the aeoli, which induced an aerodynamic impulse exactly in phase with the resonant frequency of the bridge. The tipping of the roadbed caused the impulse, which reinforced the tipping of roadbed, like the escapement on a pendulum in a clock, only a clock pendulum stabilizes when the energy dissipated on each cycle equals the energy supplied by the escapement. For poor Gertie, mechanical failure preceded stabilization.
https://www.youtube.com/watch?v=j-zczJXSxnw
That's what I thought. He was staying with it till it got back up on it's wheels. Couldn't throttle down so he thought it was going to keep going thru that fence.
Would the clip-on or the stick-on work better at 20,000 RPM?
John McGriftCain
Picked up an Engineering class a few years ago where the instructor showed a clip of a CH-46 (chained down) and it just beat itself to pieces during ground runs.
From the link…
Initial assessments in the F-35B mishap investigation found a high-pressure fuel tube had failed.
They’re blaming the fuel tube failure on the vibrations, looks like.
IIRC, harmonic resonance was tearing the wings off of early Lockheed Electras.
Looks to me like as soon as that cockpit was a bit more upright the pilot got the hell out of there intentionally.
If that’s the case its lucky there wasn’t a fire
You can hear the engine going at high thrust when he punched out.
Brain lock.
Depends on whether on the compressor or turbine blade. I would go with the stick on in the lower temperature and clip on for the higher temperature blades.
Lead substitute for environmental considerations.
FTA: ... while the a/c was on the ground
They cannot even spell aircraft.
The writer is texting too much.
“Remarkable” because he ejected while on the deck? Not exactly the first time that’s happened.
The plane was not under his control anymore, and was an instant from flipping. No good reason to burn to death at that point. Good decision...
God only knows what demons are lurking in the 35B lift jet, driveshaft and main engine. It’s pretty intact so I’m guessing they have some good data inbound.
A balance issue cropping up with the engine or the fan itself would be almost impossible, unless one or the other threw a blade.
I’m thinking some sort of acoustic resonance. There’s a lot of very high energy airflows going on in and around around that fan. And there’s could be interesting stuff with that high-bypass engine. An acoustic resonance could drive mechanical vibration in turn.
Ever hear of a phenomenon in helicopters called "ground resonance"?
https://en.wikipedia.org/wiki/Ground_resonance
It only happens to 'copters with articulated rotors and some sort of impact damping system on the landing gear (pneumatic wheels, oleo struts, or even thick rubber bushings on the saddle tubes of skids). The a/c enters a positive feedback loop and when it's pronounced enough to be noticeable, it's probably irreversible and the aircraft is going to shake itself to death.
https://www.youtube.com/results?search_query=ground+resonance
Frightening to watch, especially if you've ever been inside on of them thangs.
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