Honestly, I never gave the whirly-gigs that much thought, back before my son opted to fly them.
Now I find that they’re on my mind pretty much all the time.
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Don’t worry – the H-60 series is not overly prone to ground resonance. An impressive demonstration of the forces involved can be seen in this CH-47 test.
That Chinook vid is why we guarded against BM’s putting high point tie downs on turning helos. 22 years, 4000 hours, 1700 shipboard landings, 12 rescues. It sure was fun when it wasn’t scaring the pants off of you. Now my oldest son is doing it. I guess we are not too smart.
You had to post that, didn’t you? Makes me cry every time I see it.
From what I understand, HFS, that was the best thing to do with that particular airframe. “…uncommanded roll to inverted…” indeed. They should have named it Christine.
I know. But still…it’s painful to watch. Poor thing just eats itself.
Think I’d rather do a cruise of night traps than have to worry about that phenomenon. @Pogue thanks for the CH-47 vid. What, pray tell, brings this on?
The rotor becomes unbalanced – instead of having 3 blades 120 degrees apart one will lead or lag excessively causing the CG of the rotor disk to no longer be at the rotor shaft. It is usually caused by bumping a skid or wheel on touchdown, knocking a blade out of phase. Improperly serviced struts or rotor dampers can also contribute. The cure is to pick back up into a hover and let the blades settle down if you have sufficient rotor rpm, or to apply rotor brake if available if you don’t have sufficient rpm to fly. Seems to happen to three bladed systems more often than rotors with four or more blades.
Would a two blade system be immune?
No. None of them are. If the blades are poorly tracked, or if tracking us upset, you don’t even need to be tied down. Just like the one in the clip.
As Dan mentions below two bladed systems are generally (always?) semi-rigid where they flap as a unit – one goes up the other must go down. They are unable to lead/lag like an articulated system so ground resonance can’t happen with them. More than you ever wanted to know about rotors is here.
Not always. As I recall, the Bell Jet Ranger had flapping hinges as did the UH-1.
It’s both on the Robinsons – the hub itself can tilt from side to side, and each blade is attached to the hub with a big bolt that lets the blade itself flap upwards with respect to the hub. The root of each blade has a little “tusk” that won’t let the blade droop when it’s sitting on the ground.
But the blades are always 180deg opposed, so you don’t get ground resonance (there is a brief sympathetic “wiggle” when the rotor rpm matches the natural freq of the skids on spoolup/spooldown).
Gonna send that to my old Captain, he was a LOH crewman
Loved OH-6′s really hated CH-47′s saw one fall apart in mid air in Vietnam..
Just doesn’t seem natural.
Flight is a crime against nature.
It sure is fun, though, ain’t it? Probably less dangerous than sodomy, what with all the STDs these days.
JTG; you’re still OK with me, despite what the others say ;
It can happen to any fully articulated rotor system. Pogue is right however about 3 blade rotors. It’s seen more often in any rotor with odd numbers of blades. It will also happen to untracked, or poorly tracked, rotors.
Lex, have you sent this onto your son so he knows just how poorly he’s chosen?
Why? He didn’t choose Brazilian helos.
Saw a few similar vids when I was at Robinson. The R22/R44 two-blade teetering rotor has its own well-documented sensitivities, but ground resonance isn’t one of them. Because the blades are fixed (only allowed to flap vertically) the center-of-gravity of the rotor system can’t become offset from the center-of-rotation, as it can with a 3-rotor system with lead-lag.
Ground resonance, settling-with-power/ring-vortex, they’re high-strung critters….
Glue.
Next time, need to use more glue.
It might be more effective if they were introduced to some of R. G. LeTourneau’s technology.
Have just got back from a ride out and it suddenly dawned on me; – that helo must’ve had a pre 1980 Harley engine.
An airplane is a structure; an helicopter is a machine, with all kinds of little and big moving parts interacting with each other all the time, and just waiting to break. This is why helicopter pilots tend to be nervous, and strange. ( I direct y’all’s attention to Our Bill.)
We really do need to perfect that sideways ejection seat.
Or as I heard it… “A thousand and one parts all flying in close formation”
I resemble that remark! We like our checklists. And hate almost imperceptible oscillations of any sort going through the seat of your pants, no matter how tiny, that make you nervous. When you really think about how a helo flies, it really is trying to wring itself apart. Without the damping system, that’s exactly what she’ll do.
These AS350 Ecureuils were decent, reliable helos. One of them actually landed on Mount Everest a few years back.
“Maintenance control….we’re gonna need some troubleshooter out to spot 5. Looks like we’ll need a tron and perhaps a few framers.”
And a butt-load of duct tape.
At least he’s not squirrel suit base jumping,
My father worked for Sikorsky and they had a saying there, “A helicopter is a machine that spends its life beating itself to death.”
For another example, check this vid which I encountered while looking for just what ground resonance is:
http://www.youtube.com/watch?v=6vICf8l-KV0
You can skip to about the 35 second mark. It’s the intro to an episode of MacGyver in which a helo lands on top of a building to drop passengers. After the passengers disembark the vibration starts and the pilot quite properly takes off even though the passenger door is still open. The director must have said “cut and print” so it stayed in the episode.
George V.
What nobody has posted the old canard yet.
An airplane soars in Harmony with the air, a helicopter on the other hand beats the air into submission.
Ok, it looks like that helicopter lost the fight.
Precession… always amazing to watch just how powerful a force it is.
You could just feel the pilot thinking “There’s no place within 100m safer than where I am just this second… and that sucks, cuz it ain’t very safe here.”