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HomeAircraftHelicopterHelicopter Main Discussion › Why we dont tie down our helicopters
10-14-2005 05:20 AM  13 years agoPost 41
dkshema

rrMaster

Cedar Rapids, IA

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I have pictures of the testing of one of the apache prototypes, totally tied down, if its ok for them then its ok for us
The Apache would be a rigid rotor system, no lead/lag hinge, no ground resonance.

Our helis have that nastly single bolt attachment, lead-lag hinge results, ground resonance a real possibility.
The wobble that model helis exhibit is an "out of balance" condition. Nothing more.
Go read about ground resonance. It is the result of that out of balance condition. And it will destroy your heli, left unchecked.
It can not occur in even numbering blade heads.
Baloney. Go read about ground resonance.

The statement that "two-bladed rotor systems can't exhibit ground resonance" is often quoted from various texts on the web. The one that is most commonly used seems to have been introduced in one place, and then propagated all over the web. The wording never changes, no matter whose site you read it from. It appears as follows:
Note that two bladed helicopters are not susceptible to ground resonance because they do not require lag-lead hinges. Two-bladed rotors remain balanced through all flight conditions.
What people who quote this passage when stating that our models CAN'T exhibit ground resonance conveniently ignore is the caveat that I highlighted in bold type above.

"...do not require" does not mean "do not have".

Since our rotor heads have the single bolt blade attachment, and that attachment point is NOT rigid, the blades have a lead-lag hinge, and our blades DO lead and lag. Since our blades can, and do, lead/lag, the rotor head IS susceptible to the imbalance (or "bunching" of blades, as the commonly quoted text refers to) the rotor head does NOT remain balanced through all flight conditions (that "wobble" we all see as we spool up, and down).

One heli that exhibited ground resonance very nicely is the original Kyosho Concept 30 DX. I owned one, and flew from an asphalt pad. I also used silicone skid stops (those little rubber things on the skids). As the heli would spool up and begin to get light on the skids, there was a definite band of RPM where the entire heli would start gyrating exactly as that Chinook does in the video. Lower the RPM, the gyration stops, lift off, the gyration immediately stopped. If I did nothing and left the heli in this condition, it was obvious that the heli would thrash itself to pieces.

And yes, the Chinook in the pics did enter ground resonance because it was tied down. And this is precisely the reason you don't tie your heli down to run it up and tweak things.

Dave

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10-14-2005 05:36 AM  13 years agoPost 42
dkshema

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Cedar Rapids, IA

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its just that they are a mass off centre causing a wobble, not a resonance.
It is the off-balance condition that starts the resonance phenomenon. If the skids of the heli are restrained, whether tied down physically, or simply due to the friction between the skids and the ground, once you get the imbalance condition, the only other thing you need to do is to hit the proper RPM where the off-balance condition coupled with the stiffness of the skids/strut/frame system and the restraint allow the entire system to enter into resonance. If you maintain that RPM (and it is a band of RPM, not a specific RPM) while maintaining the restraints, the physics of the situation will tear your heli apart.

The physics do not change simply because our helis are models, as someone suggested earlier. Our helis are subject to the same aerodynamic forces and conditions as their full-scale counterparts. Phsyics is physics, an exact science, not subject to the whims of the size or scale of the object in question.


Maybe as an experiment, the next time your heli begins that nasty wobble as you spool up, maintain the RPM at that speed. Don't throttle up or down, don't break contact with the ground. See what happens.

Dave

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10-14-2005 05:59 AM  13 years agoPost 43
shuttlepilot

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Mullins, South Carolina

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dkshema
I totally agree!!! That's what I was trying to get across, but don't have the patience to get into all of the explanation behind it....you read my mind.

Gas is Great
Camper Fuel is Better!!
QWW Helis

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10-14-2005 06:06 AM  13 years agoPost 44
Mark C

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Houston, TX - USA

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I'm sorry but my one-track mind kicks in when I watch that video. It really needs music from a porn video soundtrack. It just starts movin' and shakin' and movin' and shakin' more and more and more until it makes one final thrust then smoke comes out of it and then the rotors just go limp and fall backwards.

I must really need psychiatric help.

Mark C.

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10-14-2005 06:08 AM  13 years agoPost 45
dkshema

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Cedar Rapids, IA

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Maybe, but there is no pizza delivery guy in it.

Dave

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10-14-2005 06:42 AM  13 years agoPost 46
3dwanabe

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Huntersville, NC

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Even if this "ground resonance" condition did not exist in our model helicopters, I would never put my body in such close proximity to a spooled up set of blades to set a needle valve, or any other reason.

Roy

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10-14-2005 10:09 AM  13 years agoPost 47
zoom boy

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N.E. Lincolnshire UK

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The Apache would be a rigid rotor system, no lead/lag hinge, no ground resonance.

Our helis have that nastly single bolt attachment, lead-lag hinge results, ground resonance a real possibility.
Guess you missed the point there, the apaches rotorhead is more similar to our model rotor heads (having rigidity in it, and it was ok with it) and that doesnt have ground resonance, and neither do model heli rotorheads (the most common ones)
The wobble that model helis exhibit is an "out of balance" condition. Nothing more.

Go read about ground resonance. It is the result of that out of balance condition. And it will destroy your heli, left unchecked.
Thanks, have gone and read about it, learn what the term resonance means and understand that "ground resonance" is a specific term, not a general catch all.

Ground resonance: resonant condition in the undercarrage struts, cause by the lead and lag difference in the blades of a FULLY articulated rotorhead.

Vibration doesnt have to mean its resonating at all.
Baloney. Go read about ground resonance.

The statement that "two-bladed rotor systems can't exhibit ground resonance" is often quoted from various texts on the web. The one that is most commonly used seems to have been introduced in one place, and then propagated all over the web. The wording never changes, no matter whose site you read it from. It appears as follows:
Please try to post sperate replys when resonding to what different people have said.
Since our rotor heads have the single bolt blade attachment, and that attachment point is NOT rigid, the blades have a lead-lag hinge, and our blades DO lead and lag. Since our blades can, and do, lead/lag, the rotor head IS susceptible to the imbalance (or "bunching" of blades, as the commonly quoted text refers to) the rotor head does NOT remain balanced through all flight conditions (that "wobble" we all see as we spool up, and down).
Sorry but your missing something there, in our helicopters, the blades cant do that independant of each other, if one blade starts to lag, that puts load on the spindle, then that causes the other to lag, now the concepts rotorhead is different, in that case, one can lead and lag and because its a two piece spindle one is independant of the other.
One heli that exhibited ground resonance very nicely is the original Kyosho Concept 30 DX. I owned one, and flew from an asphalt pad. I also used silicone skid stops (those little rubber things on the skids). As the heli would spool up and begin to get light on the skids, there was a definite band of RPM where the entire heli would start gyrating exactly as that Chinook does in the video. Lower the RPM, the gyration stops, lift off, the gyration immediately stopped. If I did nothing and left the heli in this condition, it was obvious that the heli would thrash itself to pieces.
See above, it allowed full lead/lag, rise and fall.
And yes, the Chinook in the pics did enter ground resonance because it was tied down. And this is precisely the reason you don't tie your heli down to run it up and tweak things.

Dave
BTW, maybe the reason that it says the same thing everywhere about ground resonance is because that is what is known as a defining description of its properties.

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10-14-2005 10:17 AM  13 years agoPost 48
zoom boy

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N.E. Lincolnshire UK

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The physics do not change simply because our helis are models, as someone suggested earlier. Our helis are subject to the same aerodynamic forces and conditions as their full-scale counterparts.
Wrong, they are not subject to the same aerodynamic forces as full scale, I know your an avionics tech, but, are you an aerodynamicist?

As you reduce the scale of an aircraft, relative air viscosity is increased, fortunatly we can compensate for that, and the reynolds number formula still works, so wind tunnel models are still useful, we just scale them up in calculations.
Phsyics is physics, an exact science, not subject to the whims of the size or scale of the object in question.
Quantum scale

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10-14-2005 02:02 PM  13 years agoPost 49
Max_Power

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Savage,Minnesota

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Well, I think other than "resonance" or "ground resonance we pretty much know that we have a out of balance in the rotor due to lead/lag......interesting thread...My first rc heli was a old nexus 30.....I test han it in restrained and it DEFINITELY had issues...Twice during my first flight with it at about a 10' hover it developed the shakes in flight!!! looked like it was made of jello!! Amazingly it survived....I attribute it to very low RRPM due to rich mixture and loose rotor bolts...I think in moderate wind advancing/retreating blade issues can be present as well. I do know that you I can almost always get my rappy to resonate at 1500rps or so on the asphalt strip I fly at...The only way to calm it is to hit hold and spool down with pitch, or get into hover....

Take heed in thy rotor speed, Lest the Earth reach up and smacketh thee!!

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10-17-2005 05:57 AM  13 years agoPost 50
joelduval

rrApprentice

Puyallup, Wa (south hill)

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The hook in this video is doing what I call a sevear case of the chinook shimmy.

2 rotors are better than one

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10-17-2005 01:02 PM  13 years agoPost 51
dkshema

rrMaster

Cedar Rapids, IA

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So what is it about our helicopters that they don't follow the laws of physics as we know them, or that they don't react to aerodynamic forces the same as their large scale counterparts?

Coefficients may differ, but when do you switch to a different set of unrelated equations based on different physics to describe what makes them fly the way they do?

And it is still ground resonance we are seeing.

Dave

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10-17-2005 02:36 PM  13 years agoPost 52
zoom boy

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N.E. Lincolnshire UK

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They obey the same laws, but you have to compensate for the scale factor when making calculations.

If you re-read my post, I didnt say they are subject to different laws, just different forces.

For example, if you have a scaled down model of a full scale rotorblade, if you put that in a wind tunnel obviously the forces like lift and drag are going to be smaller

But the thing thats really important is the reynolds number, which tells you if the flow is streamlined or turbulent, this is dependant on the length along the surface, and which percentage (0%, 25%, 50% etc) you are looking at from the leading edge.

The reason why the reynolds number is important is that if you can find a model that in the wind tunnel has a similar (99% the same) reynolds number when scaled up to the real one, then that model can be used to make realistic predictions of the full scales performance.

However, having a similar reynolds number doesnt mean they are a similar shape.

This is related to the viscosity of air, and obviously as the scale of the aerofil goes down the relative scale of the air molecules goes up, so if you have a model thats scaled down to 10%, then at the same time the air molecules are scaled up to 1000%, which affects the viscosity, which is a factor in calculating the reynolds number.

So, bascially if you scaled up a model helicopter it wouldnt fly the same because its reynolds number will have changed due to this effect.

Also they still dont have fully articulated rotorheads

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10-17-2005 03:22 PM  13 years agoPost 53
xrbeast

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Texas, Irving

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Dynamic rollover .. A term used for this incident.
It will happen to any helicopter...if a resonance were left unattended .. amplitude of resonance increases with the momentum of the blades.

I hate to see the bird break into pieces ... becos i fly one too...

flown a CH47SD .. super dees ( class cockpit )
Now a Ch47D ... (analogue , boring )

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10-18-2005 04:00 AM  13 years agoPost 54
helihobbit

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Dublin, Arizona

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· ground resonance of a kind can and will occur in a model under certain conditions
· whether or not it is the identical phenomenon is of no practical concern
· viscosity effects probably have nothing whatsoever to do with the phenomenon
· a very big consideration is the relative rigidity of model blades compared to the fullsize
· which is why you will never see a fullsize do anything like 3D flying
· you can tie a model down safely to a workbench (15 years experience using restraining holes in timber, giving loose contact with the skids), although this cannot be a complete explanation, given that sometimes models on a hard surface can resonate
· you can also tie it down unsafely, according to the experience of some people here

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10-18-2005 04:27 AM  13 years agoPost 55
ShellDude

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East Coventry, PA

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So I've got this lazy susan and a couple bricks....


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10-18-2005 05:01 AM  13 years agoPost 56
Stet

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Key Largo FL

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To all of you who think it is ok to tie down your heli then run it at speed and/or load:

Go right ahead, you know better. Your stuff is balanced. Your stuff does not flex and does not act as a spring-mass-damper system.

Even though you have been warned repeatedly by people like me who also thought we were too smart and totally balanced and did not heed the warning. Until of course we saw our heli voilently rip itself off it's skids in about half of a second. Suddenly and for no reason. Arrogant engineers like me who worked on systems like the Apache, MD80, F22 etc. So smart, but so stupid as not to listen to those more experienced who warned us in the same tone I am warning you.

Sooner or later you will join the chorus of those who warn in no uncertain terms that tying down your heli is unacceptable.

In the mean time, please have your video camera running while you run up your 90 machine to speed. And make sure those tie-downs are nice and tight. Maybe you can post the video and the warning will finally be universally believed.

keepin' it real

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HomeAircraftHelicopterHelicopter Main Discussion › Why we dont tie down our helicopters
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