Helicopter shakes itself to pieces
Discussion
We used to have to guard against this in the Wessex, it would do it at approx 150rpm, a few ended up on their sides/upside down in service. It was called Ground resonance and would start woth the 'padding' that you see this helicopter doing.
To counter it we would start the engines with the rotor brake on and then up the torque and release the rotor brake. The Captain would count the blades go past the window and if the 4th wasn't a blur the brake went back on pronto! This was the only part of the flight where the crewman had to be actually strapped into a seat, we just used to be loose on a Monkey Harness the rest of the time.
On shut down I would stand out the front and hand signal the stops going in as the power is reduced (centrifugally operated locks to stop the flapping hinges from drooping the blades too much) as soon as they were in then the brake would be applied.
if a stop didn't click back in then the head was spun up again so that they were all out and then the process tried again...and again...
To counter it we would start the engines with the rotor brake on and then up the torque and release the rotor brake. The Captain would count the blades go past the window and if the 4th wasn't a blur the brake went back on pronto! This was the only part of the flight where the crewman had to be actually strapped into a seat, we just used to be loose on a Monkey Harness the rest of the time.
On shut down I would stand out the front and hand signal the stops going in as the power is reduced (centrifugally operated locks to stop the flapping hinges from drooping the blades too much) as soon as they were in then the brake would be applied.
if a stop didn't click back in then the head was spun up again so that they were all out and then the process tried again...and again...
Fat Albert said:
We used to have to guard against this in the Wessex, it would do it at approx 150rpm, a few ended up on their sides/upside down in service. It was called Ground resonance and would start woth the 'padding' that you see this helicopter doing.
To counter it we would start the engines with the rotor brake on and then up the torque and release the rotor brake. The Captain would count the blades go past the window and if the 4th wasn't a blur the brake went back on pronto! This was the only part of the flight where the crewman had to be actually strapped into a seat, we just used to be loose on a Monkey Harness the rest of the time.
On shut down I would stand out the front and hand signal the stops going in as the power is reduced (centrifugally operated locks to stop the flapping hinges from drooping the blades too much) as soon as they were in then the brake would be applied.
if a stop didn't click back in then the head was spun up again so that they were all out and then the process tried again...and again...
Would taking off again not have helped?To counter it we would start the engines with the rotor brake on and then up the torque and release the rotor brake. The Captain would count the blades go past the window and if the 4th wasn't a blur the brake went back on pronto! This was the only part of the flight where the crewman had to be actually strapped into a seat, we just used to be loose on a Monkey Harness the rest of the time.
On shut down I would stand out the front and hand signal the stops going in as the power is reduced (centrifugally operated locks to stop the flapping hinges from drooping the blades too much) as soon as they were in then the brake would be applied.
if a stop didn't click back in then the head was spun up again so that they were all out and then the process tried again...and again...
Typical on helicopters with Oleos (slight suspension on the skids), and those with fixed rotor heads with more than three blades.
One solution is to take off again as soon as its noticable that its happening, and the other if you can't get it back in the air is to just lose all pitch from the blades.
The chinook video is nice, but this vid is an impressive rescue of Ground Resonance effect on top of a building. Very switched on pilot, could have been a disaster!
http://www.youtube.com/watch?v=6vICf8l-KV0
ETA: if you can't get it back in the air due to lack of rotor speed, just to answer the question above.
One solution is to take off again as soon as its noticable that its happening, and the other if you can't get it back in the air is to just lose all pitch from the blades.
The chinook video is nice, but this vid is an impressive rescue of Ground Resonance effect on top of a building. Very switched on pilot, could have been a disaster!
http://www.youtube.com/watch?v=6vICf8l-KV0
ETA: if you can't get it back in the air due to lack of rotor speed, just to answer the question above.
Edited by anonymous-user on Friday 24th February 15:03
bulldong said:
Typical on helicopters with Oleos (slight suspension on the skids), and those with fixed rotor heads with more than three blades.
GR is very unusual, and would be most likely following a 'shock' landing on a hard surface, with 'out of spec' oleos (dampers).Recovery simply requires a quick lift to the hover, in most cases. But avoidance is the best policy. On some helis, in ground idle, shut down and rotor brake suffice.
Contrary to the above, it is the fully articulated rotor systems that can be prone. Rigid or semi-rigid rotor systems can't get GR. One condition that Robinson drivers can be smug about
Eric Mc said:
I was going to say I assume it's knackered having watched half of the video and seeing a bit of shaking expecting that to be it..........It was once the tail fell off
Having flown model heli's for 25 years I can confirm this can also (to some degree, however not normally shaking itself back to kit!) happen to a smaller heli. However body wobble is normally reduced by opening the throttle as instant power and rotor speed reduces it to normal vibration levels.......full size clearly take a bit more than that!
Geneve said:
bulldong said:
Typical on helicopters with Oleos (slight suspension on the skids), and those with fixed rotor heads with more than three blades.
GR is very unusual, and would be most likely following a 'shock' landing on a hard surface, with 'out of spec' oleos (dampers).Recovery simply requires a quick lift to the hover, in most cases. But avoidance is the best policy. On some helis, in ground idle, shut down and rotor brake suffice.
Contrary to the above, it is the fully articulated rotor systems that can be prone. Rigid or semi-rigid rotor systems can't get GR. One condition that Robinson drivers can be smug about
Max_Torque said:
Can some one clever please explain the dynamics behind GR ?? I'd guess at some gyroscopic effect resulting in excessive blade trail unbalancing the disc? But i'd be just guessing ;-)
The blades are hinged so that they can pivot - the centrifugal force usually stretches them out and both blades are perfectly in balance.On a hard/dodgy landing the knock to the helicopter gets transmitted to the boss - the shaft - in the middle of the blades. Imagine looking down, a blade to the east and west, and imagine giving the shaft a knock to the north. Now the blade system is out of balance, and if causes the helicopter to wobble.
At a certain frequency this wobble causes further knocks to the shaft which keeps it out of balance, or even makes it worse. If this carries on for long enough the out of balance rotor destroys the craft. The safe method for avoiding is to leave the ground ASAP so no more knocking occurs and the rotor will then self balance again.
bulldong said:
I think you mean "in addition to" as opposed to contrary... Are there any helicopters with more than three blades without a fixed rotor head?
Most of them. There are very few truly rigid rotor systems available. The Bolkows almost are, but the blades are free to feather while being rigid in the dragging and flapping planes. Most multi blade heads are fully articulated using bearings or elastomerics to allow movement.The Hughes 269/369 variants, Enstroms, most, perhaps all Sikorskys,and the Puma/ Super Puma to name but a few all have articulated systems
Edited by Makila2A on Saturday 25th February 02:10
So when on the ground, the weight of the airframe on the LG/skids results in the natural frequency of the airframe being within the same frequency as a single (or harmonic) revolution of the disc. So the mast gets a knocks sideways, the rotor moves, unbalances the disc, which shunts the mast back the other way, that force gets transmitted to the ground, bounces back and pushes the mast back the otherway again, and when all those events occur at a critical frequency, it all goes a bit pearshaped ?
When the airframe is flying and suspended from the mast, the natural frequency of the airframe is higher than the natural frequency of the disc so this phenomina is self damping?
Clever stuff, never knew about that effect!
When the airframe is flying and suspended from the mast, the natural frequency of the airframe is higher than the natural frequency of the disc so this phenomina is self damping?
Clever stuff, never knew about that effect!
Edited by anonymous-user on Saturday 25th February 14:14
Max_Torque said:
When the airframe is flying and suspended from the mask, the natural frequency of the airframe is higher than the natural frequency of the disc so this phenomena is self damping?
When in the air there is nothing to keep knocking the disk out of centre!Check this out:
http://www.youtube.com/watch?v=6vICf8l-KV0
And that means getting into the air ASAP!!, smart pilot there saved a big accident.
dazco said:
Fat Albert said:
We used to have to guard against this in the Wessex, it would do it at approx 150rpm, a few ended up on their sides/upside down in service. It was called Ground resonance and would start woth the 'padding' that you see this helicopter doing.
To counter it we would start the engines with the rotor brake on and then up the torque and release the rotor brake. The Captain would count the blades go past the window and if the 4th wasn't a blur the brake went back on pronto! This was the only part of the flight where the crewman had to be actually strapped into a seat, we just used to be loose on a Monkey Harness the rest of the time.
On shut down I would stand out the front and hand signal the stops going in as the power is reduced (centrifugally operated locks to stop the flapping hinges from drooping the blades too much) as soon as they were in then the brake would be applied.
if a stop didn't click back in then the head was spun up again so that they were all out and then the process tried again...and again...
Would taking off again not have helped?To counter it we would start the engines with the rotor brake on and then up the torque and release the rotor brake. The Captain would count the blades go past the window and if the 4th wasn't a blur the brake went back on pronto! This was the only part of the flight where the crewman had to be actually strapped into a seat, we just used to be loose on a Monkey Harness the rest of the time.
On shut down I would stand out the front and hand signal the stops going in as the power is reduced (centrifugally operated locks to stop the flapping hinges from drooping the blades too much) as soon as they were in then the brake would be applied.
if a stop didn't click back in then the head was spun up again so that they were all out and then the process tried again...and again...
Here's an incident report on one in 1990, I flew this one 9 times in '88 and '89!
http://www.ukserials.com/pdflosses/maas_19901207_x...
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