Ground Resonance
Thread Starter
Joined: Feb 2004
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From: Either side of the Tasman
Ground Resonance
To all out there, I've heard sometime down the line that there is a video of a helicopter shaking itself to pieces due to resonance.It would be appreciated if anyone does have a link to the video or videos.........for the purpose of showing students the effects of what WILL happen if they dont pick the machine off the ground.
Any help would be appreciated.
Any help would be appreciated.
Iconoclast
Joined: Sep 2000
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From: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
To: whopwhop
Here is an example of ground resonance after every thing stops moving.
http://home.earthlink.net/~pandata/wolters.html
Here is an example of ground resonance after every thing stops moving.
http://home.earthlink.net/~pandata/wolters.html
Joined: Jan 2003
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From: Australia
It's no suprise to see that the machine was a 269 or similar.
I have suffered the onset of ground resonance in the 269 on several occasions. Most of the time for no reason. i.e overcontroling etc.
Happens very quick, and if you don't get it in the air..................
Luckily I was taught very early that the machine very capable of this occurance!
I have suffered the onset of ground resonance in the 269 on several occasions. Most of the time for no reason. i.e overcontroling etc.
Happens very quick, and if you don't get it in the air..................
Luckily I was taught very early that the machine very capable of this occurance!
Joined: Dec 2002
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From: World Wide
Mmmmm... a very long time ago ...when I first learnt to fly helicopters Hu269 As, Bs and a TH55 that somehow got away from its original keeper, my then instructor (a very experienced ex US Army IP) showed me that even after "phasing" the blades manually prior to start up it was often necessary to lift off, move the a/c over to the grass and doing a hovering auto to rephase the rotor dynamically to get the little sucker to "smooth out" for continued flight .... that was of course back in the days of the hydraulic/friction dampers which I believe have now been superceded by the elastomeric jobs???
Arrrr... the joys of technological advancement!
Arrrr... the joys of technological advancement!
Iconoclast
Joined: Sep 2000
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From: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Yeah but what about.....?
To: spinningwings
There is a problem with elastomeric dampers as well. The elastomeric elements must be within 5% of each other relative to their compressibility and tensile capabilities. If they are not you can get into a condition called air resonance, which is as bad as ground resonance. This same condition can manifest itself on rotorheads with hydraulic dampers.
Arrrr... the joys of technological advancement!
Joined: Jul 2002
Posts: 230
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From: Murica.
corrective action....
The proper corrective action in case of ground resonance in a 269 series helicopter is not to simply "get it in the air"...
Granted, that is one of them...you get it in the air if the RPM is in the green operating range....2500-2700 if I remember correctly (300CB/CBi numbers)
If the RPM is not in the green you immediately roll the throttle off, to cut the power, that will also stop the resonance.
But be quick
Timts
Granted, that is one of them...you get it in the air if the RPM is in the green operating range....2500-2700 if I remember correctly (300CB/CBi numbers)
If the RPM is not in the green you immediately roll the throttle off, to cut the power, that will also stop the resonance.
But be quick
Timts
Iconoclast
Joined: Sep 2000
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From: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Another way to enter ground resonance with every thing working normal.
We were operating off the back end of an icebreaker with a Sikorsky HO3S on tricycle gear and a Bell 47 on floats.
The Sikorsky had three blades and with the cold you could not be sure that the blades were in phase during start-up. Because the Sikorsky only had parking brakes (which were not very effective) we had to keep the chocks in place and several cable tie-downs attached. Once the clutch was engaged the pilot would bring on the throttle and when he hit 100% NR we would release the tie-down and the pilot would take off. Of course this would only be when we were in rough seas or breaking ice as the ship would really roll. This rolling moment would perturb the rotor system and the helicopter would start working the shock struts.
Although the Bell would not normally go into ground resonance the rolling of the ship would perturb the spinning rotor and the helicopter would start working on the floats. To minimize this from happening we would hold the blades in the neutral flap position to facilitate the run-up of the blades.
One word of caution when the blade is snapped out of your fingers get your hand out of the way FAST, as the next blade will hit you. That was over fifty years ago and my hand still hurts.
The Sikorsky had three blades and with the cold you could not be sure that the blades were in phase during start-up. Because the Sikorsky only had parking brakes (which were not very effective) we had to keep the chocks in place and several cable tie-downs attached. Once the clutch was engaged the pilot would bring on the throttle and when he hit 100% NR we would release the tie-down and the pilot would take off. Of course this would only be when we were in rough seas or breaking ice as the ship would really roll. This rolling moment would perturb the rotor system and the helicopter would start working the shock struts.
Although the Bell would not normally go into ground resonance the rolling of the ship would perturb the spinning rotor and the helicopter would start working on the floats. To minimize this from happening we would hold the blades in the neutral flap position to facilitate the run-up of the blades.
One word of caution when the blade is snapped out of your fingers get your hand out of the way FAST, as the next blade will hit you. That was over fifty years ago and my hand still hurts.
Joined: Nov 2001
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From: San Jose, CA
Lu mentioned that elastomeric dampers must be fairly "matched". Fairly shortly after I got my private helicopter rating in 2001, I was spooling up the Schweizer 300C that I was flying at the time. At some point during rotor spool-up the ship started shaking something fierce, and to be honest I wasn't quite sure what to do. I believe I pulled the mixture control to shut the engine down, disengaged the rotor, waited for everything to stop, and then went through the startup again and experienced no problems.
Shortly thereafter the owner of the ship elected to replace the main rotor dampers (perhaps some other renters had similar problems? I never heard why he decided on this action), which was roughly $12,000. That was my first lesson in unscheduled helicopter maintenance costs 8^) . I have to say that it made me a little less inclined to buy a 300 model as a personal ship, but I still love to fly 'em.
Dave Blevins
Shortly thereafter the owner of the ship elected to replace the main rotor dampers (perhaps some other renters had similar problems? I never heard why he decided on this action), which was roughly $12,000. That was my first lesson in unscheduled helicopter maintenance costs 8^) . I have to say that it made me a little less inclined to buy a 300 model as a personal ship, but I still love to fly 'em.
Dave Blevins
Joined: Sep 2001
Posts: 301
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From: Australia.
TIMTS
I agree that if you don't have enough RRPM to get off the ground, cut the throttle.
I didn't think cutting the throttle will stop ground resonance, more a case of hang on until everything stops and/or disintegrates.
I agree that if you don't have enough RRPM to get off the ground, cut the throttle.
I didn't think cutting the throttle will stop ground resonance, more a case of hang on until everything stops and/or disintegrates.
Joined: Feb 2004
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From: Great Southern Land
http://www.google.com.au/search?q=%2...-8&hl=en&meta=
... if you were looking for lots of interesting links for more info on the topic.
Cheers,
SLC.
... if you were looking for lots of interesting links for more info on the topic.
Cheers,
SLC.
Joined: Dec 2003
Posts: 168
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From: At home
On the TH55 we were just taught to poke the cyclic forward onto the droop stops if there was any hint of G/R. Made a kinda grinding noise but any incipient resonance stopped. After the flight we would have to get the wrenchies to check the droop stop nut torque. Seemed it didn't like the procedure and the nut would loosen off. Better than a debris circle though.
