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HomeAircraftHelicopterHelicopter Main Discussion › Why we dont tie down our helicopters
10-13-2005 08:47 PM  12 years agoPost 21
vaportrail OLD

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Anchorage, Alaska

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Well if it isn't 'ground resonance' it is some other type of resonance and since it only occurs on the ground that makes it 'ground resonance.' Just because it may have a different exact cause (which I don't totally believe) it still is 'ground resonance' for the purpose of this discussion. Call it 'harmonic vibration' if you like but that is not specific enough.


The real point of terrorism is not the act itself, but our reaction to the act.

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10-13-2005 08:52 PM  12 years agoPost 22
zoom boy

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N.E. Lincolnshire UK

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Ground resonance is when the heli is bouncing off the ground, where the vibration has made the gear start vibrating at its natural frequency.

Where it "ground resonance" comes from is one place, the fact is that if it comes from the rotorhead then its called ground resonance, when that vib starts the gear vibrating.

If it comes from somewhere else and appears like ground resonance but the gear isnt at its natural frequency then its not "ground resonance"

The key is the word resonance, but it comes from one place, and just does NOT happen with a helicopter with the wrong type of head.

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10-13-2005 09:01 PM  12 years agoPost 23
helichulo

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Queens, New York

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AMA March 1978 shows how "the master" Dieter Schluter holds the Heli-Baby with one hand above the head and gives it full throttle/collective as he is preparing for his first flight. Is this something simular to tying down the heli? Also, in that article Mr. Walt Schoonard (X-Cell founder) states that Mr. Schluter uses a mixture of alcohol, castor oil and "unleaded gasoline" for his helicopter.

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10-13-2005 09:04 PM  12 years agoPost 24
zoom boy

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N.E. Lincolnshire UK

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Doing that, I think I know where the alcohol was

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10-13-2005 09:13 PM  12 years agoPost 25
zoom boy

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N.E. Lincolnshire UK

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Yes, but he did mention alcohol, and that makes dumb things seem smart remember

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10-13-2005 11:03 PM  12 years agoPost 26
helichulo

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Queens, New York

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I don't know about the alcohol but I'm thinking about adding a few cc's of unleaded gas to my fuel to see what it does but I don't know when that's going to happen. It's been raining non-stop in NY for quite a while.

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10-13-2005 11:32 PM  12 years agoPost 27
shuttlepilot

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Mullins, South Carolina

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I always thought of it as a harmonic vibration. I think anything that has parts that move in some sort of circular motion there is a point where things get kind of "unbalanced". You notice this with engines, cars rolling down the road, and so on..... Even after I have spooled up my heli and the blades had straightened out, if I slow it down and then spool back up there is still a spot that things will vibrate. When you leave it at that point, things get more stressed and will eventually fail if left at that speed. I don't know if it's resonance or not, but I do know it can tear up your heli, just like overcontrolling, the shakes just get worse and worse until it contacts the ground. Just my 2c.

Gas is Great
Camper Fuel is Better!!
QWW Helis

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10-13-2005 11:52 PM  12 years agoPost 28
helidad2

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potosi mo. usa

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i have flown off concrete for years and i have never had a problem with this

even a blind hog finds an acorn now and then

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10-14-2005 12:17 AM  12 years agoPost 29
zoom boy

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N.E. Lincolnshire UK

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Shuttlepilot, that is resonance, but thats not the same thing as what "ground resonance" means unless its coming from the rotorsystem with a fully articulated system, and its also not "ground resonance" unless the resonance vibration is happening in the undercarrage, if its not those things, it may be resonance, but its not the same thing as what is meant when the term "ground resonance" is used.

Basically ground resonance is a specific type of resonance, by a specific cause, anything else is just resonance or un-tuned vibrations.

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10-14-2005 01:05 AM  12 years agoPost 30
shuttlepilot

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Mullins, South Carolina

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Zoom Boy, I don't doubt you a bit on the explanation, and would not dispute your argument. I just know what I see and experience.....results can be pretty similiar no matter what the cause. Ground resonance or not, things still can deterioate pretty quickly.

Gas is Great
Camper Fuel is Better!!
QWW Helis

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10-14-2005 01:18 AM  12 years agoPost 31
Max_Power

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Savage,Minnesota

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seems like some folks are confusing articulation terms. Our helis definitely lead and lag,(blade bolts) they can feather(change pitch)...The one trait they don't have compare to the Ch-47 is the ability to "flap", or aise and lower without affecting the other blade. In RW heli training you are taught that only fully articulated heads suffer from ground resonance, and that semi-rigids and rigids do not, although I have seen this on my own RC helis and would be hard to convince this isnt GR. My .02 cents is that is has something to do with the scale of the forces of RC compared to the real ones...I also find the explanation that what we are feeling is only that "the blades havent spread out yet...Isn't that in itself exactly the definition of ground resonance? when the blades are both in one side of the disk creating a imbalance and because the heli can't dampen this energy on hard ground it finds a resonant frequency and worsens because of the imbalances multiplying force when it comes"in phase"?
??but who knows....

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

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10-14-2005 01:58 AM  12 years agoPost 32
zoom boy

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N.E. Lincolnshire UK

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The difference there is that once up to speed (so that they have spread out) this shouldnt happen, if you leave it at idle and the blades are slowly rotating, then you will get the blades throwing the balance off, its just that they are a mass off centre causing a wobble, not a resonance.

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10-14-2005 02:11 AM  12 years agoPost 33
Leif

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USA

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The one trait they don't have compare to the Ch-47 is the ability to "flap", or raise and lower without affecting the other blade.
That's funny... My Concept VR has a full flapping head. Independent feathering shafts for each rotor that can flap up and down.

I understand that some people like to nitpick terminology. The physics are the same, the effect is the same and the results are the same.

I call it "Ground Resonance", even if it's a model.

Leif

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10-14-2005 02:22 AM  12 years agoPost 34
zoom boy

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N.E. Lincolnshire UK

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Thats true, the concept did have that, I guess with that model ground resonance is possible.

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10-14-2005 02:32 AM  12 years agoPost 35
Hawk4flyer

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Deland,Florida

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The wobble that model helis exhibit is an "out of balance" condition. Nothing more.

Ground resonance can only occur in rotor head of odd numbering blades. (i.e. 3, 5, 7, 9) It can occur in a flapping fully articulated head, semi-rigid, and rigid of odd numbering blades.

It can not occur in even numbering blade heads.

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10-14-2005 02:59 AM  12 years agoPost 36
Castlebravo

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Hillsboro,Oregon

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It (ground resonance) can happen to any helicopter that is securely tied down. I had the brilliant idea to tie my Gas Alpha to a workmate bench with clamps to set the high end needle valve. I got the idea from Rays Helicopter Manual when I saw the GMP Cobra secured with paint cans.

I ran it up and almost immediately it started rocking and shaking violently! It was really scary because I was directly below it (I know but I was new and dumb)! I was able to reduce throttle and stop the oscillation before it broke the skids. When all was said and done, I had a bent main shaft but no other damage. I still shudder when I think what coud have happened.

So dont be fooled.....it can happen with any helicopter!

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10-14-2005 03:12 AM  12 years agoPost 37
minuteman42

rrNovice

Stockton, CA USofA

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helicopter tie down
Castlebravo;
Did you ever consider the fact that your main shaft was bent before you ran it up"

tandem rotor nut

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10-14-2005 03:35 AM  12 years agoPost 38
Castlebravo

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Hillsboro,Oregon

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I know it was not! It was brand new and It was built, balanced and flown by an experienced person (way back when). However,if you dont believe me.....then by all means, go ahead and strap your helicopter to a workbench and see what happens. I'll be standing waaaaay over there behind the brick wall! Call me when you find all the pieces.

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10-14-2005 04:34 AM  12 years agoPost 39
tadawson

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Lewisville, TX

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Nuts! DP'd!

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10-14-2005 04:34 AM  12 years agoPost 40
tadawson

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Lewisville, TX

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OK, I guess I will be the rebel here. Once in a while when testing, I do have a set of hold downs that I will use for runups. Unlike what others have said, I allow the skids to come up about one inch before they hit the retainers, and as such, are not "hard" clamped, but still will not allow the bird to tip over or blade strike. I have used this on all new models when setting up baseline engine tuning (it is high enough off the ground, I can get a reasonable approximation of hover behaviour) and since it is not hard, have never had a bit of uncontrolled resonance, and no vibration issues other than what would be seen in normal flight. So, with care, I feel that it can be done, but once the bird is flyable, I do all my testing in the air. For tracking, you basically let it get "light" and then check. Since it is basically in a controlled hover, no problems . . .

- Tim

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HomeAircraftHelicopterHelicopter Main Discussion › Why we dont tie down our helicopters
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