Ground resonance and helicopter destruction

"When is a helicopter like a Patsy Cline song? When it falls to pieces." That's the darkly comedic subhed in a new Air & Space Magazine about ground resonance, a condition when a sitting helicopter's rotors become imbalanced while spinning. If the frequency of the now-vibrating rotor is close to the body of the chopper's normal vibration frequency, the oscillations increase. In seconds, the whole helicopter can just fall apart. (Ground resonance tore the helicopter above apart in just four seconds.) From Air & Space:

“I was standing right next to it,” says Frank Robinson, founder of the world’s leading helicopter company, describing a close call he had during a 1961 test of a gyroplane. “I had to grab hold of it and hang on and ride the damn thing down. You don’t want to be standing out there when it starts to jump around – it can jump on you. And there’s not a good way to get out of it. Just cut everything, hang on and hope..."

The destruction is wrought by the considerable energy stored in the rotor blades. The shaking rapidly grows in violence, exceeding the strength of the mast, transmission mounts, and landing gear. The cyclic control in the cockpit flails about so violently that the pilot cannot hold it, the rotor blades strike the tail boom or the cockpit, parts begin falling off, and moments later the helicopter may be a heap of scrap.

How Things Work: Ground Resonance

• [gollux]

  Good title, “Was a Robinson”

  Helicopters are merely an extremely tight flying formation of precision parts that’s attempting to rip itself apart at all times during operation. That’s why you want to see maintenance measured in cubic dollars if you are going to be flying one.

  One McGuyver has a helicopter landing that starts to shake on the helipad. The pilot immediately takes off while everyone ducks down the railing.

  • [Antinous]

    Am I the only person whose parents called it a Sikorsky?

• [GregLondon]

  Is ground resonance what does in Ospreys, or are those things uniquely blessed with a different problem?

  ground resonance means that your blades are no longer evenly spread out because one of them advanced at some point but then wont retreat. You can see in the chinook video just before it tears apart, that a couple of rotor blades have bunched together on one side. The whole thing is out of balance, and the frame can’t handle it.

  I think the osprey rotor is not an articulated rotor. I don’t think the individual blades can advance/retreat. Which means to get out of balance, something already broke.

  Even so, Ospreys have a lot stacked against them even when they work. They’re shaped in such a way that you’ve got the blades out at the tips left and right, but a whole lot of weight fore and aft.

  A single-masted helo, like a huey or apache or loach, has most of the mass directly under the center of the rotor. Imagine the rotor is a spinning plate you’ve balanced on a three foot long stick. Now, put the other end of the stick in the palm of your hand. You’re only input is to move the base of the stick left, right, fore, aft, to keep it balanced. It’s hard, but you can learn it and it’s like one of those magic acts you see on stage.

  I remember when I first started trainign on helos, I was a bit tense, so I’d grab the cyclic with an iron grip. Any time I’d try to hover, I’d overcorrect, we’d get pilot-induced-oscillations, and the instructor would take over and settle it out. I quickly got into the habit of holding the stick with a feather grip, and making constant micro-circular motions whenever I would hover. It was a way to remind myself to always keep moving the stick.

  So, once you get the hang of it, you’ve got all these little micro adjustments you’re doing. You start to drift right, nudge the cyclic left, but then bring it back to center.

  Then some corrections affect seemingly unrelated behaviour. If you’re hovering, and you start to sink, you add collective. When you add collective, you’re suddenly adding more torgue on the rotor to lift you up. When you add more torque, the fusalage wants to turn. So you need to nudge the pedal to stop the turn.

  When you nudge the pedal, you’re adding a sideways force at the tip of your tail. When you add a sideways force, you move sideways, so you need to nudge the cyclic the other way to stop the sideways drift.

  I look at the osprey, and I can’t even imagine what it would be like to try and balance the forces on something like that moment to moment.

  Two big prop/rotors behind me to my left and right. I’m sitting in a cockpit way out in front of them, and I’ve got a fuselage and tail that’s hanging way back behind them. The rotors act like massive gyroscopes when you try to turn them, or when you try to pitch or roll. Then there’s that whole weirdness that must go on when it transitions from helo to airplane.

• [Takuan]

  ouch! that was a test pilot in that one.

• [Oren Beck]

  The quiet miracle is how rare fatalities from resonance are. Resonance in structural complex systems is often damnably variable from mathematical models. That being due to trivial differences in Static Vs Dynamic balances get multiplied in non-linear fashions. Making something that “models” as a worst-case failure safe design into grim death.

  The underlying reason can be found in how the math model’s information diverges from real factors. In a finished product the differences begin stacking up quickly. One builder of kit copters found that fatal. http://en.wikipedia.org/wiki/Revolution_Helicopter_Corporation_Inc

  As for the Robinson bird? The production R22 has an elegant design for compensation. It’s a weighted “bean bag” on the stick assembly. And it just plain works.

• [gollux]

  Oops, three blade, definitely not a Robinson.

  N3194K is listed as an Aerospatiale Alouette out of Cedar City, Utah.

  http://en.wikipedia.org/wiki/A%C3%A9rospatiale_Alouette_III

  And here’s the Department of the Interior report on the incident.

  http://amd.nbc.gov/safety/mishaps/06/121405.htm

• [eander315]

  This phenomenon can be observed in remote control helis as well. During spinup, the blades sometimes begin to shake at a specific RPM. If that RPM is gently increased, the blades will sometimes begin to violently shake, and can even bend the drive shaft, possibly resulting in a tail boom strike, which in turn results in a large replacement parts bill.

• [Antinous]

  How is there not a video? If anything ever cried out for a video, this is it.

• [Takuan]

  you mean a trim weight on the control feeds back to the main rotor? That’s all?

• [dbarak]

  As some in the Navy (and elsewhere?) say, a helicopter is just 10,000 parts flying in close formation.

  But they’re still fun.

• [Takuan]

  wish we had big L-5 colonies with pedal powered copters…

• [frankieboy]

  I had a friend who flew helicopters to ocean oil rigs, and remember him saying the number of hours of maintenance for each hour of flight was crazy, like 15 to 1 or something. He claimed things didn’t often go wrong, but that when they did, “they go horribly wrong”.

• [avraamov]

  Antonius:

  allow me.

  http://www.youtube.com/watch?v=-LFLV47VAbI&feature=related

  • [Antinous]

    avraamov,

    Even better that it looks like a giant, freaked-out goldfish.

• [Anonymous]

  wow … it’s a good thing *I’m* not a helicopter! O_O

• [dbarak]

  @ #10 posted by avraamov

  That might be the most expensive “America’s Funniest Video” I’ve seen…

• [Anonymous]

  That helicopter broke up faster than the wind could blow it. Therefore: free lunch! Q.E.D.

• [Takuan]

  heh! Got any of the Osprey?

• [avraamov]

  i think the DWFTTW team could learn from this.

• [Alpinwolf]

  Oh, snap!! =P

  Sorry, had to after that Chinook vid.

  OK, bye.

• [Oren Beck]

  RE: Trim weight on the control. Consider it more of a damping weight. Which in one mode of interaction shifts the resonance. Putting it out of a destructive range.

• [Takuan]

  same feeling as dumping your bike in a speed wobble.

• [Scuba SM]

  Helos are weird….

  For instance, the gearbox on the AH-64 Apache reportedly can function for a half-hour without lubricant. It was designed that way so if the lubricant lines were hit, or the housing was cracked by enemy fire, the pilot could get the aircraft out of harm’s way, or back home, before ditching it. Personally, I wouldn’t like to test it.

  I new a guy that spent a lot of time around Hueys in Vietnam. He said that whenever they got a new guy on the line, they’d send him with a bag of “Jesus Bolts” to have them blessed by the Chaplain. They called them Jesus bolts because they were what held the rotor hub assembly to the shaft, and if they let go, “you were going straight to Jesus.”

• [AirPillo]

  There are a couple well-circulated videos of ground resonance testing being done on a US military Chinook helicopter, if anyone has a penchant for watching aircraft disembowel themselves.

  side view http://www.youtube.com/watch?v=RihcJR0zvfM

  closer rear view http://video.google.com/videoplay?docid=-6055303336171660489&ei=Uik3Sa-zJIb-qAP6t8zFCA&q=ground+resonance&hl=en

  The scary thing, of course, is that by the time a pilot notices this there’s a chance that it’s too late to avoid it, and both of the two ways to get out of the situation have the potential to only make the problem worse.

• [GregLondon]

  My understanding is this:

  The blades in an articulated rotor will “flap” because they sometimes produce more lift on one side than the other. They also advance/retreat from each other because they flap.

  You know when an ice skater starts spinning and then pulls their arms/legs in tight and they spin faster? that’s part of what’s happening.

  On one side of the rotor, the blade is up high, deflected a lot because it’s creating a lot of lift, on the other side, not so much.

  Imagine a skater that puts their arm straight out when their arm is pointing west and then puts their arm down along their side when their arm is pointing east.

  Their arm will want to speed up and slow down as they turn.

  so the individual blades advance/retreat from each other. If you have two rotor blades, I believe it works out that they balance out and stay in sync with each other, speeding up and slowign down at the same time. I think if you have an odd number of blades, or maybe anythign more than two (I forget), then you can have individual blades advancing and retreating, and doing all sorts of intersting things.

  As far as I know, the only way you get destructive resonance is if you’re on the ground or if you touched the ground hard and then try to hover. It can knock the blades out of whack, and they sometimes can’t correct themselves and instead get worse. I think the basic solution is to land and reduce power so you’re not hitting the resonant frequency, or (maybe, again, cant remember) to pull up, get away from the ground and increase your rpm/power/pitch and maybe it sorts out.

  I don’t recall the exact training anymore. I do remember helos shaking a lot when they’re on the ground and then running a lot smoother when you got off the ground.

  I believe there was an old “Hiller” brand helicopter that had the nickname “hiller killer” because it had a tendancy to flap a rotor right into the tailboom.

• [GregLondon]

  hm, actually, I think that’s a Hiller in the picture. I believe they had a three-bladed rotor like the one in the pic.

• [avraamov]

  #16 posted by Antinous

  good lord, you’re right. new tactics. paralyze the enemy with zoomorphic physical comedy. genius.

  that vid is now ruined/enhanced for me b.t.w.

• [Daemon]

  Resonance is fun. At least one bridge was destroyed by the resonance caused by soldiers walking accross it in lockstep. (The Angers Bridge in 1850)

• [Guysmiley]

  That’s why on helicopter rotor RPM gauges there is a “caution” zone where you should not operate for sustained periods.

  Example: http://www.cmaviation.com/maylan/shop/liit/r2244_er_rpm_2normal%5b1%5d.jpg notice the yellow area between 60% and 70% for engine and rotor RPM.

• [V]

  After all, helicopters don’t so much fly as beat the air into submission….

• [Adam Stanhope]

  There are a pantload of videos on YouTube that show it happening:

  http://www.youtube.com/results?search_query=ground+resonance+helicopter

• [Guysmiley]

  #28: Nope, that crash was due to a cross-wired flight control system. As in commanding right roll resulted in the a/c rolling left.

• [soupisgoodfood]

  @18, GregLondon

  2-blade helis often don’t have lead-lag hinges to begin with. I’m not sure if 2-blade rotors with lead-lag are naturally balanced, my first thought is that it doesn’t make sense to me, but I’ve read enough discussions about the complexities of rotor dynamics to know that things are often not what one may expect.

• [Teresa Nielsen Hayden / Moderator]

  The Vario footage is fun. Takes a while, but when it goes, it rips itself apart.

  Is ground resonance what does in Ospreys, or are those things uniquely blessed with a different problem?