flying dog Posted February 24, 2012 Posted February 24, 2012 Brazil. 4 people on a helicopter. Sorry can't give more details, as it was only a 20 second video. But the helpicopter shook itself appart after/upon landing.
Tex Posted February 24, 2012 Posted February 24, 2012 Ground Resonance - only remedy is to lift off...
Doug Evans Posted February 24, 2012 Posted February 24, 2012 What ground Resonance could u enlighten me on what it is and why it dose that too choppers ?
Tex Posted February 24, 2012 Posted February 24, 2012 Hi Doug, Basically the helicopter, often caused by blades out of position, gets out of balance while in contact with the ground and develops an increasing 'resonance' which exacerbates and multiplies on itself. Essentially an ever increasing vibration, but through the WHOLE helicopter, driven by an engine no less, but it needs something solid to 'bounce off' to cause the increase in frequency and amplitude... the ground! Hence the need to get airborne and let gravity and physics balance things out again. All helicopters are prone to it but some more susceptible than others because of they way they are constructed. 3 bladed helicopters are more likely to experience it than two..... hmmm....actually two bladed helis may be close to immune to it, not sure on that... and those with particular types of damping systems on the skids, which can both assist and contribute to the problem. I understand Squirrels have had some issues with it... Therefore always touch down as gently as possible and be very controlled with collective, even once both skids are on the ground (all helicopters land one skid first) be prepared to get it airborne again...
Tex Posted February 24, 2012 Posted February 24, 2012 BAH! wiki gives a far better explanation http://en.wikipedia.org/wiki/Ground_resonance
eightyknots Posted February 24, 2012 Posted February 24, 2012 And a good example of saving it! Thanks for finding and posting this video, Tex: very instructive. I think it's worthwhile to 'paste' the first part of the Wikipedia article into this thread because it explains it well: Ground resonance is a hazardous condition that can occur any time the rotor of a helicopter or gyroplane is turning while the aircraft is on the ground. Similar in concept to the behavior of a washing machine when the clothes are concentrated at one point during the spin cycle, ground resonance can occur with a rotorcraft when the spacing between blades becomes irregular or the damping system, including drag hinge dampers, landing gear oleo struts, or wheel tire pressure, is operating out of limits. Ground resonance occurs at three rotor rpm bands, the first at about one-third of normal rotor speed, the second at a range including normal rotor speed, and the last in a range above normal rotor speed. Ground resonance is precipitated by a shock to the aircraft arising from excessive motion of a rotor blade in its plane of rotation thereby moving the rotor center of gravity from the axis of rotation. Inadequate damping allows the rotor center of gravity to spiral away from the rotor axis of rotation, causing the rotor to generate unbalanced rotating moments beyond the compensating ability of the damping system. In addition to damping system malfunctions mentioned above, such blade movement can occur as a result of taxiing over rough ground with the rotor turning at a speed within a susceptible range, or takeoff/landing in which a shock occurs to one side of the landing gear. Under extreme conditions, the initial shock can cause violent oscillations that quickly build and result in catastrophic damage of the entire airframe. In some cases, complete destruction occurs, e.g. body panels, fuel tanks, and engines are torn away, even at normal rotor speed. Recovery is possible in some cases. If sufficient rotor RPM exists, immediate takeoff can restore rotor balance. If rotor RPM is low, complete shutdown may be sufficient.
Doug Evans Posted February 24, 2012 Posted February 24, 2012 Thank fellows I have never heard or seen anything like that before ,
eightyknots Posted February 25, 2012 Posted February 25, 2012 This ground resonance is pretty dramatic; it shakes the entire helicopter to pieces. Here is some amateur footage of the event: https://www.youtube.com/watch?v=FeIbf3xRF_Y
Robert Posted February 25, 2012 Posted February 25, 2012 Ground Resonance - only remedy is to lift off... Look like it happened that quick he didnt have time
Ultralights Posted February 25, 2012 Posted February 25, 2012 No, the AS350 or better know as "the Squirrel" is french made. also, the entire front canopy is one piece, made from 1 piece polycarbonate, just like a remote control car body, and its bonded onto the rear half, also, one piece of polycarbonate. hence under such forces it separated at the join. there is really not much to the structure at all, as you can see here.. this part, bolts onto this part.. and you have a complete helicopter..... and also, explains why they come apart so easily in such resonance situations..
Guest Maj Millard Posted February 25, 2012 Posted February 25, 2012 Mmmmmmmm.....and I once had the Squirrel on my 'ok to fly in list'....ah well back to the good old Iroquos I suppose !................................Maj...
facthunter Posted February 25, 2012 Posted February 25, 2012 I don't have any helicopter in my "Ok to fly in" list. Nev 1
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