An airplane inherently wants to fly, a helicopter inherently wants to thrash itself into a million burning pieces. Or a most airplane pilots say... 'a helicopter doesn't fly, it is so ugly the earth repulses it.
There was an episode of MacGyver where a helicopter lands, people get out, it enters ground resonance and the pilot lifts off immediately. Exactly what he was supposed to do.
I literally saw that video maybe 2 days before my first solo flight, and it saved me from a bad situation as I got into it for the first time while trying to pickup to a hover for the first time flying solo. You can checkout the video on my channel. I've experienced G/R dozens of times since, and now refer to the action of stopping it as, "The MacGyver Maneuver." LOL
Simply put, ground resonance is when one blade gets out of the plane of rotation and bites more air, causing the blade to move backward from it's normal position and now the main shaft is out of balance and it only gets stronger until it rips itself apart.
@Akhenaton nope these are fully articulating heads, the blades have lead lag, and flap, Ground resonance occurs when one of the blades lags further than the rest it bites more air throws the whole rotor out of balance. If the out of balance frequency matches the skid impact frequency then the whole vibration will continue to worsen. This is why ground resonance is far less of an issue with rigid rotor systems found on helicopters like the bo105
Actually ground residence is when an old Chevy passes by dropin that ghetto base and you turn to look and see that the trunk is shaking itself apart. This is also indicated by a rattling license plate.
Excellent review! My Commanding Officer with USMC HMM265 experienced this as a junior pilot - shaken but walked away from total destruction of CH46 ( Phrog ) His career survived since he was not POC and went on to be a very safety conscious career Heli pilot !
My 300 got into GR just after my examiner (for my CFII RH) got out with rotor turning saying “meet you inside”, leaving me to shut down. GR came on very quickly so I just grabbed collective and was able to move it off the crack in the asphalt that I believed caused it. Don’t land on cracks!
GREAT JOB! Your video was an amazingly easy way to understand the mechanics of ground resonance, making it easy to understand why mitigating actions (e.g. lifting off) and how mitigating devices (bushings, foam, etc) actually work. The video clips of helicopters being destroyed by GR were extremely helpful at showing how far ground resonance can take things.
In all my years of flying helicopters I never heard of a ground resonance accident that did not involve a 3 bladed rotor system. I have never heard of a 4 or more bladed helicopter involved in ground resonance. I personally never encountered it in the 800 or so hours I flew the Astar & Twinstar.
Just search: constructive wave interference. This principal can destroy all types of things! Useful applications exist as well. Love the frequency (no pun intended) of your uploads! Been watching for a long time. Cheers from Michigan
This may be why my RC Helicopter is a doing this when I spool up slowly. I was thinking it had to do with the onboard gyroscope acting wonky since the helicopter was still in contact with the ground and not responding to it's attitude changes the way it expected. I just got in the habit of spooling up and lifting off the ground a foot and that solved the problem. I was unfamiliar with ground resonance. I also tend to keep the screws that retain the main blades to the rotor a bit on the loose side, think I will snug them up a bit as well. Thanks for the video!
This is great knowledge I've gained from you, I had no idea this resonance thing existed, I've only heard of it once a long time ago when I was chatting with a friend who is a navy helicopter pilot; he only mentioned "vibration frequency after landings" and I want able to fully appreciate what he was talking about until I watched this video. Thanks so much. I wanna see more unusual stuff about helicopters from you (i.e. unique engine start input sequences/flows by pilots, etc.). Cheers 👍🏾
Good explanation of a phenomenon which is hard to understand from just watching videos of destruction. The Mc Gyver episode was a great one of quick correction saving the day and being included in the show ( already mentioned below)
Good video. The only thing I would add would be an “emergency shutdown” for a low RPM ground resonance situation. IE: Chopper start up and the blade is un-phased, or the uneven terrain, causes resonance. Increasing the RPM to break ground contact will exasperate the situation and destroy the aircraft.
Disclaimer: I'm not a pilot of full scale helicopters, but in the past I have had this issue at low RPM on my large size scale helis. Even at low RPM just adding pitch increased the drag on the rotor blades enough to slow them down without vibrating my heli to death
Not being a heli pilot, I had no idea of what "ground resonance" is. However, by means of your short, but very succinct video, I now both know and understand what it is. Moreover I also learned exactly how to alleviate this phenomenon. If only you could put together a full series of such training videos, I feel I would be a competent candidate for rotary wing flight training school. Thank you sir, for a most excellent training video.
Ground resonance can start without any landing at all, often when starting - before liftoff or hover - from a resonance being set up while running, sitting on the pad.
Thanks for this one. You cleared up some misconceptions I had on the subject. I've seen the videos you showed. There's another one I saw of a Schweizer 300 that the mechanic was running up after a maintenance cycle, I think. He had no intention to actually lift off but he had the rotor system at or near flight rpms and held it there for a couple of minutes with no apparent problems until something happened and all hell broke loose. In seconds, the aircraft was destroyed. Very scary.
Same applies to RC model helicopters. With rigid heads and stiff blades i just give alot of negative pitch so the helis is pressed to the ground - that trick only works with very stiff blades otherwise you easily get a boom-strike.
Good presentation. I would suggest including the fact that ground resonance which is often encountered during landing, it can also occur during takeoff. I if you want to save the day,and just like landing you’ve gotta either get it in the air or slam it down and hope it stays together. Your quite correct identifying shock struts on the under carriage and lead lag dampers on the rotor head as possible causes, and usually are the primary cause, however don’t forget to check the rest of the rotor head and undercarriage carefully. The early Sikorskys, eg S 55 and S58 S would go into ground resonance due to differential tire pressure..
What you failed to mention is why it only happens with 3 blade or more (fully articulated rotors) and not 2 blade is because the 2 blade doesn’t lead and lag (semi rigid system) If the 3 blade gets out of phase (120 degrees apart) it causes the vibration in the rotor head
Suppose you detect ground resonance beginning, and you get off the ground, then is there something you should do differently when you attempt to land again? If you were on hard concrete, should you next try landing on grass? And vice versa?
Yes. An awesome saying my first CFI offered me was, "Treat every landing like a mini run-on landing." and I still pass that on today. I've always hovered right behind my intended spot to set down prior to doing so, and for good reasons. First, I can clearly see my spot as I'm setting on it (given). Also the forward motion helps for a smoother set down and allows you to set the front of the skids first (depending on the aircraft). This really helped in the S300s for preventing G/R.
Magnífica explicación, volé Hughes 500 C. Ahora lo entiendo mejor. A ver q día habla sobre la "Disimetria sustentacional. " Lo felicito, d gran ayuda. Saludos desde Nicaragua.
Happens on RC model helicopters too. My old nitro 60 class models would do this if kept on the ground too long at spool up. This was with two blades and a flybar.
In spansh we call it Gomas de impacto. long time ago I watched a TV program when a person came up with a plastic or rueber that was capable to absorb all the energy in a hit. He placed is hand on a table and then put the rubber or damper over his hand and then he took a hammer and he hit his hand and, but that thing was capable to absorb all the energy of the hit. The purpose of this ruber or damper was to put in inside of the toe in the tennis shoes to absorb the impact in excercices or jumps to avoid knee injuries I did not know this characteristic of helicopters vibration in the skids. Awesome!
Correct me if I'm wrong ---- as I usually am --- but it sounds to me like constructive interference between the rotor head and the ground. As the vibration/pressure waves from the rotor head, to the ground via the skids/wheels of the helicopter rebounds off of the ground, those propagating waves collide the ones still being generated. Some of those many be canceled out, due to being the equal, but opposite frequency, and waveform (deconstructive interference). However, some waves are of similar, if not exactly the same frequency, wavelength, etc, and what you get is bigger waves. Thus, in a cycle of positive feedback, the rapid increase in resonance overcomes the natural frequency of the materials used, thus causing catastrophic metal fatigue, as, as soon as the first micro-craking begins, those crakes rapidly propagates, tearing right through the material in fractions of a second ... it's a similar effect of how a glass can shatter by sound waves alone, as high speed footage shows the glass literally flexing like jelly, until the stress harmonics cause material fatigue ... then boom, glass everywhere ... At least that is what I think is happening. Although I'm a (very ...) amateur astronomer, at best, my day job was in veterinary laboratory science, where the only physics I encountered was in autoclaves, and ultra-centrifuges, especially a continuous flow centrifuge, which I didn't understand how one worked, until I got to use one myself. Very interesting bit of kit, that had to be in a kept in, more or less, strengthened fume cabinate, with a high strength, polycarbonate vertical sliding door, that had to be closed, as much as possible, allowing for the in- and out- feed tubing, just incase one day it decided to fly apart at, iirc, ≈20K RPM. But watching it spin up, was fun, although a bit scary as the meter just kept climbing ... interestingly, a by product of it spinning so fast is the pitch climbed so high, that, once at full speed, it went near silent, which was damn eerie the first time ...
Hey guy, cool topic. One note: Dampers damp motion and provide damping to oscillatory systems. Liquids dampen surfaces and are the source of dampening i. Just something to remember. Otherwise, nice video!
I love this channel... I help applicant prepare for the SIFT test. This is the exam the U.S. Army gives you before you go to helicopter flight training. I hold a CFII MEI, ATP fixed wing ...These guys are awesome instructors. KEEP UP THE GOOD WORK!!!!!!
I don't have a lot of time in fully-articulated machines, but while flying them I actually feared this more than Low-G mast-bumping in the Robby. Mostly because it just seems so much easier to get into (like I have no control over when and if its going to suddenly get me) while mast-bumping seems easier to avoid (because I'm only going to get it if I do what I'm not supposed to do).
I've had a quite expensive scale model ah-1w cobra destroy itself due to ground resonance in the past. I quickly learned the foam trick on the skid mounts, and fly from grass where possible. It seems the softer grass is much more forgiving than pavement.
You put a bit of foam on skids? Might try it on mine. Got new one and did slow spool up on kitchen table and spooling down it started a chicken dance. Read it was to do with blade tightness so mostly gone now but hopefully will be much better on grass.
@@mrbushpilot I would suggest to always fly from grass when possible. Make sure your blades are balanced both statically and dynamically, and check that they track properly. I was able to create a spacer between the frame of the heli and the skids out of foam from some old blade holders (the ones that keep the blades folded above the tail boom). It is stiff, but provides a buffer between the frame and skids. Edit: I actually run my blades tighter on my Cobra than I have on any of my other helis, and I was nervous about that at first. But it seemed to help mine. Your results may vary, so idk how helpful that may be.
1. If you're successful in recovering from a GR event by taking off, how confident can you be that it won't merely repeat the GR when you touch down a second time? 2. What effect does the surface (concrete, asphalt, grass, dirt, etc.) have on GR? I suppose it would depend on the nature of the resonant frequency itself. So, if I get GR on a concrete slab, should I try to set it down on another nearby surface? This is great stuff!! Thanks.
It depends on what caused it. Normally it won't repeat unless something has broken on the ship which induced the first event. (broken tooth on drivetrain gear, or something weird like that) Think of ground resonance as being like striking a bell. Don't strike the bell and it won't ring (resonance). The energy which keeps the resonance going or building comes from the rotating rotor head. If that energy source is removed (rotor is stopped), the resonance immediately dies out. Also, increasing the damping will kill it immediately. This is what lifting off does - it breaks the coupling to the ground and drastically increases damping.
Once you are back in the air... ya gotta come back down. and do it all over again. You can't gently touch down, so the only way out, is to get the machine within an inch or two of the ground, and gently, but assertively .. dump it onto the ground. Happened to me in an AS350. I've heard that a two or three inch off the ground hover chop will have the same effect... never had the opportunity to try that. That might work better if you only have grass or gravel to put down on..? Maintenance guy thought I was nuts... but he did find a weak damper. He had never heard of that process before.
Hi .. I love helicopters .. but far far from being able to learn how to fly it. Anyway, that was a fine n clear information on ground resonance. My wee query is, would ground resonance happen to landing on softer surface like grass field etc.. tQ from Malaysia. take care n stay safe.
Would be very interesting to listen about aircraft/helicopter insurances in general. What can you insure, what is mandatory, when your wrecked aircraft can be reimbursed etc etc.
I would add that the cause is that the lead/lag of the blades get out of sync such that the center of rotation of the blades is no longer over the shaft. It’s offset
Ok, well explained what happened at ground resonance. Is there any trick to avoid this happens while landing at all? Touch ground with just one side of skid? Or with Endings (Skid) touch the ground first? Or any other hint?
Interesting video. I have to say though, it's damping not dampening. You want to damp the vibration not make it slightly wet. It strange that you would get that wrong considering you named the dampers correctly.
@@prehensileman7254 you are incorrect. Ground resonance is not alleviated by the tires and shocks. An example would be if a helo is tied down and the attachments are above the shocks this can cause ground resonance.
You see in the example videos that external events trigger the resonance (plane rolling by or other helis warm up on equal distance parking positions). So, don't park helis near such triggers and not on hard ground, better on meadow and not on a eaqual distance row. This does not look so proper but is the better choice.
Question: If instead of a skid, the helicopter equiped with wheel and suspension like the mi-8/mi-17. Will that prevent ground resonant from happening? To me it looks like the suspension and the wheel will absorb the resonant
I have seen so many helicopter videos of these choppers tearing themselves apart. Robinsons don’t do that. They have the mast bumping risk... but interesting to see their pros and cons.
So could we consider the case: carrying a load underneath through hard cables have the same effect if we fixed the copter to ground? .. i mean ground resonance.
I wonder if it would help to add some additional material to the landing pad itself to potentially absorb any resonance before it can begin. Probably not.
I'd be curious to know if a thick rubber mat bolted down to landing pad would eliminate the chances of ground resonance to begin to start with. To me is sounds like GR begins when the skids make contact with a hard solid surface. I'm interested in this as my favorite helicopter to ride in is the 4-blade Bell 407GX Jet Ranger. The last one I rode in, the pilot landed on a small slightly elevated wooden platform cushioned by rubber and I noticed nothing unusual. Just had to watch your step from the platform to the tarmac. He was an exceptionally smooth pilot to begin with which really helps.
Thank you Mischa, an excellent tutorial. Yes I should have been more specific with regards to ground resonance affecting all multi bladed rotor system helicopters and not just specific to the Cabri.
I'm not a helicopter pilot but I think it's something to do with 2 bladed rotor systems being rigid/semi rigid in design and 3 or more bladed systems tend to be articulated (independent of each other) in design causing them to become out of synchronisation with one another hence causing the resonance. I may be talking complete nonsense but I remember reading this somewhere.
My dad’s 300 (I still call them Hughes because I’m old and that’s what they were when I learned to fly) got into GR on startup and destroyed itself around him. He was chasing the mixture around the cockpit. Turned out to be bad maintenance... I can’t remember exactly what, but I seem to remember it was actually tail rotor related.
I know nothing about helicopters so my question is why are there no replaceable rubber bumpers built onto the base of the skids - they seem to be bare metal which surely reduces any cushioning ability and would accentuate the ground resonance ?
The skids need to be able to ... skid. In an emergency autorotation landing you could have quite a bit of forward momentum. If the skids were rubber they would suddenly grip and flip the helicopter.
Brantley helicopters effectively resolved this resonance by using a double articulating rotar blade design on both their models. The inner short shaft design wasn’t taken up be other designers; the outer blade sections Cary the flight loads.
Mellow Yellow have seen this happen but did not know why. Thank you for a detailed explanation. So now a question, the small landing pad by your video prop....what if it is made of say 2-3 inches of rubber (like a kids school jungle Jim area) would that help in the shock transmission to the rotor head. It would be like the foam dampener od
Is this phenomenon only when there are skids that contact the ground? For instance can a copter with wheels such as Marine One experience ground resonance, or do the wheels act like the foam dampers you showed us?
If you didn't have enough power to lift would rapid small difference in movement of the collective or the cyclic knock off the resonance? Just thinking the resonance or rebound maybe a blade working against the skids/ground and that it can't overcome the resistance.
like you and your Capri 3 isn't it, and enjoy your video's. wish you said how you should stop this on the ground, maybe the fuel off collective down and the rotor brake friction applied ? thankyou interesting subject.
Everything I learn about helicopters makes me appreciate airplanes more.
Especially true if you know helicopter mechanics
An airplane inherently wants to fly, a helicopter inherently wants to thrash itself into a million burning pieces.
Or a most airplane pilots say... 'a helicopter doesn't fly, it is so ugly the earth repulses it.
🤣😂😃😄😅
@@zacharyrec1872 some Pilots say the helicopter flies by beating the air into submission
@@Dra741
Also how Chuck Norris flies.
=)
YT put this video in my feed and I found your explanation of something I never knew existed very clear and informative.
Spot on🚀
Great video, very clearly explained.
RIP the Chinook that was given a Viking funeral.
Made me cry a bit.
But, as long as it was about to be retired, good video evidence.
🤣🤣🤣
A true berserker.!!!!
There was an episode of MacGyver where a helicopter lands, people get out, it enters ground resonance and the pilot lifts off immediately. Exactly what he was supposed to do.
Yes have seen that Video.!
I literally saw that video maybe 2 days before my first solo flight, and it saved me from a bad situation as I got into it for the first time while trying to pickup to a hover for the first time flying solo. You can checkout the video on my channel. I've experienced G/R dozens of times since, and now refer to the action of stopping it as, "The MacGyver Maneuver." LOL
ruclips.net/video/6vICf8l-KV0/видео.html
@@Stellicopter I watched your video earlier this week, then got recommended this one! Scary first solo flight, glad you sorted it!!
that inly happens in the movies. Real life human error is the danger. People are not all there.
Simply put, ground resonance is when one blade gets out of the plane of rotation and bites more air, causing the blade to move backward from it's normal position and now the main shaft is out of balance and it only gets stronger until it rips itself apart.
@Akhenaton nope these are fully articulating heads, the blades have lead lag, and flap,
Ground resonance occurs when one of the blades lags further than the rest it bites more air throws the whole rotor out of balance.
If the out of balance frequency matches the skid impact frequency then the whole vibration will continue to worsen.
This is why ground resonance is far less of an issue with rigid rotor systems found on helicopters like the bo105
Actually ground residence is when an old Chevy passes by dropin that ghetto base and you turn to look and see that the trunk is shaking itself apart. This is also indicated by a rattling license plate.
@Akhenaton that would it’s street name. 😂
At least I know what "ground resonance" is now.
I never heard of it before.
I never heard of it either, and I thought I knew something
I just have to say this is the BEST helicopter channel on the entire internet.
Great explanation and very clear speaking, makes it much easier for non-english speaking people to understand. Very nice!
WOW! I knew they could tear themselves apart but I’ve never actually seen it or known why. This was very informative!!!
They give a washing machine competition
Excellent review! My Commanding Officer with USMC HMM265 experienced this as a junior pilot - shaken but walked away from total destruction of CH46 ( Phrog ) His career survived since he was not POC and went on to be a very safety conscious career Heli pilot !
My 300 got into GR just after my examiner (for my CFII RH) got out with rotor turning saying “meet you inside”, leaving me to shut down. GR came on very quickly so I just grabbed collective and was able to move it off the crack in the asphalt that I believed caused it. Don’t land on cracks!
GREAT JOB! Your video was an amazingly easy way to understand the mechanics of ground resonance, making it easy to understand why mitigating actions (e.g. lifting off) and how mitigating devices (bushings, foam, etc) actually work. The video clips of helicopters being destroyed by GR were extremely helpful at showing how far ground resonance can take things.
nice and concise explanation, no stupid music. Thanks.
In all my years of flying helicopters I never heard of a ground resonance accident that did not involve a 3 bladed rotor system. I have never heard of a 4 or more bladed helicopter involved in ground resonance. I personally never encountered it in the 800 or so hours I flew the Astar & Twinstar.
I don’t have a pilot’s license, will never have a pilot’s license but enjoy flying ( as a passenger )and watching this channel. Excellent job.
I hope you are hitting the flight sims then.
Got into it in an AW139 once. Very briefly before I quickly yanked it back into the air, but damn it’s violent.
What an ignoble ending to that Chinook. One for the team, I guess.😢
No better ending really. Sacrifice itself for knowledge, so that this may never happen with lives on the line.
Much like a crash test car, or hitting a retired ship with a missile to see how survivable it is for crew
That's anew word, what exactly does it mean (the "ig" part)
@@kerbalairforce8802 still makes me sad, so much history and information only for it to get obliterated
@@midgetman4206 Sure would be nice if there was a place you could look up words and their meanings...
Just search: constructive wave interference. This principal can destroy all types of things! Useful applications exist as well. Love the frequency (no pun intended) of your uploads! Been watching for a long time. Cheers from Michigan
This may be why my RC Helicopter is a doing this when I spool up slowly. I was thinking it had to do with the onboard gyroscope acting wonky since the helicopter was still in contact with the ground and not responding to it's attitude changes the way it expected. I just got in the habit of spooling up and lifting off the ground a foot and that solved the problem. I was unfamiliar with ground resonance. I also tend to keep the screws that retain the main blades to the rotor a bit on the loose side, think I will snug them up a bit as well. Thanks for the video!
Very important information ..Resonance is a tragic moment for helicopter . To get it off from the ground is the best solution ... Thanks .👍👍
clear and good demo to understand the ground resonance. Thanks for sharing
This is great knowledge I've gained from you, I had no idea this resonance thing existed, I've only heard of it once a long time ago when I was chatting with a friend who is a navy helicopter pilot; he only mentioned "vibration frequency after landings" and I want able to fully appreciate what he was talking about until I watched this video. Thanks so much. I wanna see more unusual stuff about helicopters from you (i.e. unique engine start input sequences/flows by pilots, etc.). Cheers 👍🏾
Good explanation of a phenomenon which is hard to understand from just watching videos of destruction. The Mc Gyver episode was a great one of quick correction saving the day and being included in the show ( already mentioned below)
That was the best and most simple explanation of ground resonance.
Good video. The only thing I would add would be an “emergency shutdown” for a low RPM ground resonance situation. IE: Chopper start up and the blade is un-phased, or the uneven terrain, causes resonance. Increasing the RPM to break ground contact will exasperate the situation and destroy the aircraft.
Disclaimer: I'm not a pilot of full scale helicopters, but in the past I have had this issue at low RPM on my large size scale helis. Even at low RPM just adding pitch increased the drag on the rotor blades enough to slow them down without vibrating my heli to death
Not being a heli pilot, I had no idea of what "ground resonance" is. However, by means of your short, but very succinct video, I now both know and understand what it is. Moreover I also learned exactly how to alleviate this phenomenon.
If only you could put together a full series of such training videos, I feel I would be a competent candidate for rotary wing flight training school.
Thank you sir, for a most excellent training video.
Ground resonance can start without any landing at all, often when starting - before liftoff or hover - from a resonance being set up while running, sitting on the pad.
he said that
Thanks for this one. You cleared up some misconceptions I had on the subject. I've seen the videos you showed. There's another one I saw of a Schweizer 300 that the mechanic was running up after a maintenance cycle, I think. He had no intention to actually lift off but he had the rotor system at or near flight rpms and held it there for a couple of minutes with no apparent problems until something happened and all hell broke loose. In seconds, the aircraft was destroyed. Very scary.
Same applies to RC model helicopters. With rigid heads and stiff blades i just give alot of negative pitch so the helis is pressed to the ground - that trick only works with very stiff blades otherwise you easily get a boom-strike.
Good presentation. I would suggest including the fact that ground resonance which is often encountered during landing, it can also occur during takeoff. I if you want to save the day,and just like landing you’ve gotta either get it in the air or slam it down and hope it stays together. Your quite correct identifying shock struts on the under carriage and lead lag dampers on the rotor head as possible causes, and usually are the primary cause, however don’t forget to check the rest of the rotor head and undercarriage carefully. The early Sikorskys, eg S 55 and S58 S would go into ground resonance due to differential tire pressure..
Excellent video on a widely confusing subject. I learned a bunch.
Excellent explanation of what it is and how to mitigate.
What you failed to mention is why it only happens with 3 blade or more (fully articulated rotors) and not 2 blade is because the 2 blade doesn’t lead and lag (semi rigid system)
If the 3 blade gets out of phase (120 degrees apart) it causes the vibration in the rotor head
Suppose you detect ground resonance beginning, and you get off the ground, then is there something you should do differently when you attempt to land again?
If you were on hard concrete, should you next try landing on grass? And vice versa?
Yes. An awesome saying my first CFI offered me was, "Treat every landing like a mini run-on landing." and I still pass that on today. I've always hovered right behind my intended spot to set down prior to doing so, and for good reasons. First, I can clearly see my spot as I'm setting on it (given). Also the forward motion helps for a smoother set down and allows you to set the front of the skids first (depending on the aircraft). This really helped in the S300s for preventing G/R.
This actually helped me more than you realise. Currently in the beginning of doing my license
VERY well done!
Clear, concise, well edited...
Magnífica explicación, volé Hughes 500 C. Ahora lo entiendo mejor. A ver q día habla sobre la "Disimetria sustentacional. " Lo felicito, d gran ayuda. Saludos desde Nicaragua.
Happens on RC model helicopters too. My old nitro 60 class models would do this if kept on the ground too long at spool up. This was with two blades and a flybar.
In spansh we call it Gomas de impacto. long time ago I watched a TV program when a person came up with a plastic or rueber that was capable to absorb all the energy in a hit.
He placed is hand on a table and then put the rubber or damper over his hand and then he took a hammer and he hit his hand and, but that thing was capable to absorb
all the energy of the hit. The purpose of this ruber or damper was to put in inside of the toe in the tennis shoes to absorb the impact in excercices or jumps to avoid knee injuries
I did not know this characteristic of helicopters vibration in the skids.
Awesome!
That is an incredibly aesthetic helicopter
I love it
Correct me if I'm wrong ---- as I usually am --- but it sounds to me like constructive interference between the rotor head and the ground.
As the vibration/pressure waves from the rotor head, to the ground via the skids/wheels of the helicopter rebounds off of the ground, those propagating waves collide the ones still being generated. Some of those many be canceled out, due to being the equal, but opposite frequency, and waveform (deconstructive interference).
However, some waves are of similar, if not exactly the same frequency, wavelength, etc, and what you get is bigger waves. Thus, in a cycle of positive feedback, the rapid increase in resonance overcomes the natural frequency of the materials used, thus causing catastrophic metal fatigue, as, as soon as the first micro-craking begins, those crakes rapidly propagates, tearing right through the material in fractions of a second ... it's a similar effect of how a glass can shatter by sound waves alone, as high speed footage shows the glass literally flexing like jelly, until the stress harmonics cause material fatigue ... then boom, glass everywhere ...
At least that is what I think is happening. Although I'm a (very ...) amateur astronomer, at best, my day job was in veterinary laboratory science, where the only physics I encountered was in autoclaves, and ultra-centrifuges, especially a continuous flow centrifuge, which I didn't understand how one worked, until I got to use one myself. Very interesting bit of kit, that had to be in a kept in, more or less, strengthened fume cabinate, with a high strength, polycarbonate vertical sliding door, that had to be closed, as much as possible, allowing for the in- and out- feed tubing, just incase one day it decided to fly apart at, iirc, ≈20K RPM. But watching it spin up, was fun, although a bit scary as the meter just kept climbing ... interestingly, a by product of it spinning so fast is the pitch climbed so high, that, once at full speed, it went near silent, which was damn eerie the first time ...
Thank you for share your knowledge!! You are an amazing pilot and person!
Hey guy, cool topic. One note: Dampers damp motion and provide damping to oscillatory systems. Liquids dampen surfaces and are the source of dampening i. Just something to remember. Otherwise, nice video!
Only ever experienced it in the H269/S300 Most multi blade machines need a positive landing and lift. Don’t stay light on the skids too long.
Very well described and understanding this could save someones life plus not destroying an helicopter.
I know nothing about flying period......but found this a fascinating watch....thanks!
I read that unequal tyre pressures on an aircraft fitted with wheels can cause it too. Great videos!
I love this channel... I help applicant prepare for the SIFT test. This is the exam the U.S. Army gives you before you go to helicopter flight training. I hold a CFII MEI, ATP fixed wing ...These guys are awesome instructors. KEEP UP THE GOOD WORK!!!!!!
I don't have a lot of time in fully-articulated machines, but while flying them I actually feared this more than Low-G mast-bumping in the Robby. Mostly because it just seems so much easier to get into (like I have no control over when and if its going to suddenly get me) while mast-bumping seems easier to avoid (because I'm only going to get it if I do what I'm not supposed to do).
A very good explaination . Thanks so much.
Choppers have regular maintenance intervals, like any aircraft, so aren’t these dampeners included in that inspection?
I've had a quite expensive scale model ah-1w cobra destroy itself due to ground resonance in the past. I quickly learned the foam trick on the skid mounts, and fly from grass where possible. It seems the softer grass is much more forgiving than pavement.
You put a bit of foam on skids? Might try it on mine. Got new one and did slow spool up on kitchen table and spooling down it started a chicken dance. Read it was to do with blade tightness so mostly gone now but hopefully will be much better on grass.
@@mrbushpilot I would suggest to always fly from grass when possible. Make sure your blades are balanced both statically and dynamically, and check that they track properly. I was able to create a spacer between the frame of the heli and the skids out of foam from some old blade holders (the ones that keep the blades folded above the tail boom). It is stiff, but provides a buffer between the frame and skids.
Edit: I actually run my blades tighter on my Cobra than I have on any of my other helis, and I was nervous about that at first. But it seemed to help mine. Your results may vary, so idk how helpful that may be.
Great explanation, thank you. Never heard of this before.👍🇨🇦
1. If you're successful in recovering from a GR event by taking off, how confident can you be that it won't merely repeat the GR when you touch down a second time?
2. What effect does the surface (concrete, asphalt, grass, dirt, etc.) have on GR? I suppose it would depend on the nature of the resonant frequency itself. So, if I get GR on a concrete slab, should I try to set it down on another nearby surface?
This is great stuff!! Thanks.
It depends on what caused it. Normally it won't repeat unless something has broken on the ship which induced the first event. (broken tooth on drivetrain gear, or something weird like that)
Think of ground resonance as being like striking a bell. Don't strike the bell and it won't ring (resonance). The energy which keeps the resonance going or building comes from the rotating rotor head. If that energy source is removed (rotor is stopped), the resonance immediately dies out. Also, increasing the damping will kill it immediately. This is what lifting off does - it breaks the coupling to the ground and drastically increases damping.
Wow.... something new every day... thanks
We were taught to avoid letting the helicopter (TH-55) get light on the skids. "pull pitch" to hover, or keep the collective down.
Once you are back in the air... ya gotta come back down. and do it all over again. You can't gently touch down, so the
only way out, is to get the machine within an inch or two of the ground, and gently, but assertively .. dump it onto the ground.
Happened to me in an AS350. I've heard that a two or three inch off the ground hover chop will have the same effect... never had the opportunity to try that. That might work better if you only have grass or gravel to put down on..? Maintenance guy thought I was nuts... but he did find a weak damper. He had never heard of that process before.
Hi .. I love helicopters .. but far far from being able to learn how to fly it. Anyway, that was a fine n clear information on ground resonance. My wee query is, would ground resonance happen to landing on softer surface like grass field etc.. tQ
from Malaysia.
take care n stay safe.
Thanks.. great description of ground resonance.
Would be very interesting to listen about aircraft/helicopter insurances in general. What can you insure, what is mandatory, when your wrecked aircraft can be reimbursed etc etc.
Yes
Yes
I would add that the cause is that the lead/lag of the blades get out of sync such that the center of rotation of the blades is no longer over the shaft. It’s offset
Ok, well explained what happened at ground resonance. Is there any trick to avoid this happens while landing at all? Touch ground with just one side of skid? Or with Endings (Skid) touch the ground first? Or any other hint?
Interesting video, well structured and really good videography. What camera are you using for this footage?
We used a Panasonic GH5
Interesting video. I have to say though, it's damping not dampening. You want to damp the vibration not make it slightly wet. It strange that you would get that wrong considering you named the dampers correctly.
Nice presentation as always. Enjoy your positive attitude!
does helicopter that have retractable landing gear ( i.e one with shock absorbers ) have this problem as well ?
No, the tires and shocks prevent that
Yes they do.
@@prehensileman7254 you are incorrect. Ground resonance is not alleviated by the tires and shocks. An example would be if a helo is tied down and the attachments are above the shocks this can cause ground resonance.
@@skipstreet yes shocks help with ground resonance. That’s why newer AS350’s and all H125’s have absorbers on the front attachments for the skids.
@@randallranchnorth8840 did you read my response? You are correct if the helo is tied down below the shock so it ls stroke is not impeded.
Would helicopters with wheels be less susceptible to ground resonance? How often does it happen in real life?
You see in the example videos that external events trigger the resonance (plane rolling by or other helis warm up on equal distance parking positions). So, don't park helis near such triggers and not on hard ground, better on meadow and not on a eaqual distance row. This does not look so proper but is the better choice.
Great video. Couldn't understand your term "lead-leg" but then figured out your Canadian dialect was saying "lead-lag." Hahh!
Question: If instead of a skid, the helicopter equiped with wheel and suspension like the mi-8/mi-17. Will that prevent ground resonant from happening? To me it looks like the suspension and the wheel will absorb the resonant
I have seen so many helicopter videos of these choppers tearing themselves apart. Robinsons don’t do that. They have the mast bumping risk... but interesting to see their pros and cons.
Good explanation.
So could we consider the case: carrying a load underneath through hard cables have the same effect if we fixed the copter to ground? .. i mean ground resonance.
Fantastic explanation. (S.E.L. and Sailplane here).
I wonder if it would help to add some additional material to the landing pad itself to potentially absorb any resonance before it can begin. Probably not.
I'd be curious to know if a thick rubber mat bolted down to landing pad would eliminate the chances of ground resonance to begin to start with. To me is sounds like GR begins when the skids make contact with a hard solid surface. I'm interested in this as my favorite helicopter to ride in is the 4-blade Bell 407GX Jet Ranger. The last one I rode in, the pilot landed on a small slightly elevated wooden platform cushioned by rubber and I noticed nothing unusual. Just had to watch your step from the platform to the tarmac. He was an exceptionally smooth pilot to begin with which really helps.
Thank you for an excellent educational video that was actually understandable. You are a great teacher.
Thank you Mischa, an excellent tutorial. Yes I should have been more specific with regards to ground resonance affecting all multi bladed rotor system helicopters and not just specific to the Cabri.
What I don‘t understand? Why is this not happening with 2-bladed Rotors?
I'm not a helicopter pilot but I think it's something to do with 2 bladed rotor systems being rigid/semi rigid in design and 3 or more bladed systems tend to be articulated (independent of each other) in design causing them to become out of synchronisation with one another hence causing the resonance. I may be talking complete nonsense but I remember reading this somewhere.
Is there a reason why multi blade helicopter can suffer from ground resenance but not the duel blade helicopter?
Is this equally likely when landing on a softer surface, ie...sand or grass, as when landing on hard surfaces like concrete pads?
More likely on a hard surface but still possible on grass.
My dad’s 300 (I still call them Hughes because I’m old and that’s what they were when I learned to fly) got into GR on startup and destroyed itself around him. He was chasing the mixture around the cockpit. Turned out to be bad maintenance... I can’t remember exactly what, but I seem to remember it was actually tail rotor related.
Whoa, your dad is 300 and still flies helis?
@@Obelisk57 sadly my Dad died last year. 300 as in Hughes 300 helicopter.
Fixed wing aircraft are an elegant solution to flight. Helicopters are taking physics and smashing it in the face.
Love your videos! Thank you for all the information!
I know nothing about helicopters so my question is why are there no replaceable rubber bumpers built onto the base of the skids - they seem to be bare metal which surely reduces any cushioning ability and would accentuate the ground resonance ?
The skids need to be able to ... skid. In an emergency autorotation landing you could have quite a bit of forward momentum. If the skids were rubber they would suddenly grip and flip the helicopter.
Thanks for sharing your knowledge.
Thanks for the explanation.
Very interesting and (IMO) well explained.
Thank you.
Brantley helicopters effectively resolved this resonance by using a double articulating rotar blade design on both their models. The inner short shaft design wasn’t taken up be other designers; the outer blade sections Cary the flight loads.
If a heli was on water, with one under inflated pontoon, could it enter ground resonance?
How is ground resonance depending on collective pitch ? Is there any with zero pitch ?
This really resonated with me - I got good vibrations from the whole video.
⭐⭐⭐⭐⭐ dad comment for the gold medal
Beach Boys
Mellow Yellow have seen this happen but did not know why. Thank you for a detailed explanation. So now a question, the small landing pad by your video prop....what if it is made of say 2-3 inches of rubber (like a kids school jungle Jim area) would that help in the shock transmission to the rotor head. It would be like the foam dampener od
If the engine is already shut down, can increasing collective pitch slow the rotor rapidly?
Is this phenomenon only when there are skids that contact the ground?
For instance can a copter with wheels such as Marine One experience ground resonance, or do the wheels act like the foam dampers you showed us?
Nothing like a semi truck bellow, bud.
Great video, thanks.
If you didn't have enough power to lift would rapid small difference in movement of the collective or the cyclic knock off the resonance? Just thinking the resonance or rebound maybe a blade working against the skids/ground and that it can't overcome the resistance.
Sir what are the possible causes of ground resonance and how to correct it
like you and your Capri 3 isn't it, and enjoy your video's.
wish you said how you should stop this on the ground,
maybe the fuel off collective down and the rotor brake friction applied ?
thankyou interesting subject.
Excellent explanation!
Wow I didn’t know that!