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Which is more common?


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#1 r22butters

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Posted 15 November 2017 - 13:18

Seems the first argument I hear when it comes to training in a Cabri (Schweizer in the old days) vs. a Robby, is the teetering rotor system. So which situation are students more likely to get into?




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#2 takefootoff

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Posted 15 November 2017 - 14:02

in my limited experience with the 300s, I only got into ground resonance like 4 times over the course of approx 700hrs. and it was always in one particular ship out of three available



#3 r22butters

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Posted 15 November 2017 - 14:26

in my limited experience with the 300s, I only got into ground resonance like 4 times over the course of approx 700hrs. and it was always in one particular ship out of three available


Around 700 hours between 35 different R22's,...zero low-G and/or mast bumping!

Looks like you got me beat! :D
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#4 DizzyD

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Posted 15 November 2017 - 14:28

Only one of these threats was deemed hazardous enough to require its own Special FAR.  Also, one of the local G2 operators has experienced no issues with ground resonance and I hear that having the skids connected to the fuselage by elastomeric mounts instead of being bolted directly on has reduced ground resonance occurrence. 

The 300 has some form of skid damping as well correct?


Edited by DizzyD, 15 November 2017 - 14:39.


#5 Eric Hunt

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Posted 15 November 2017 - 16:22

Ground resonance will only come if something is wrong with the aircraft - drag damper, oleo, tire pressure.

 

Zero G will only come if something is wrong with the pilot - didn't listen to thousands of people telling him DO NOT PUSH OVER!


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#6 Hand_Grenade_Pilot

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Posted 15 November 2017 - 20:42

Seems the first argument I hear when it comes to training in a Cabri (Schweizer in the old days) vs. a Robby, is the teetering rotor system. So which situation are students more likely to get into?


Its's pretty easy to get ground resonance in the S300. Experienced it dozens of times over the course of two years and about 1100 hours while instructing. Usually from a sloppy setdown on rough/uneven pavement and on a few occasions while doing run-on landings to a runway. Definetly a topic I stressed w/ my students (recognition and recovery). No expereince w/ the Cabri so can't comment on that.

Experienced it only once in an MD500. Never in the AS350 or 407, although it is possible.

Only encountered low-g roll once, while flying an R44 in Hawaii. A massive updraft caused the rotor to become unloaded resulting in a rapid, uncommanded right bank. Instinctively moved the cyclic back which immediately leveled the helicopter. Was a fascinating expereince to say the least; until that point I had thought it could only occur as a result of doing a hard pushover with the cyclic.

So I'd say ground resonance is a much more common experience. Also worth noting that any helicopter can experience a low-g induced roll (but only teetering rotor systems are subject to mast bumping).
Aviation is a cruel mistress. When she's not taking your money, she's coming up with creative ways to kill you.

#7 Wally

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Posted 16 November 2017 - 20:34

Ground resonance will only come if something is wrong with the aircraft - drag damper, oleo, tire pressure.
 
Zero G will only come if something is wrong with the pilot - didn't listen to thousands of people telling him DO NOT PUSH OVER!


I can't argue with your theory on ground resonance, except that it can be induced in an aicraft that has no apparent mechanical issue, and I think- in a sound aircraft if you work really hard at it. Seems to me that I encountered ground resonance on run-ups and then it doesn't just happen, it sort of indicates that something is wrong, rocking and rolling unusally before the full ground resonance thing starts happening.
Or it can happen as you land in an otherwise serviceable aircraft if you isolate part of the damping mechanism, the running landing with one skid down first and a jar to the system to upset the rotor. Soft/hard oleos on the 300 skids, hanging an Astar skid heel off the deck or the skid tub on a rock- and a jar to the airframe can set it off. Again, you just stop what you're doing and change things when you land again. None of this is uncommon in the real world, but it probably won't kill you. Aerospatiale says 'dynamic divergent excitation' (ground resonance) can happen in flight...

Teetering rotors and low G can be over much quicker- unloading the main rotor and not controlling the aircraft appropriately ended some Huey pilot's Vietnam tour early. Didn't this very problem underly the Robinson's SFAR?

Don't be stupid with either rotor and you'll be fine.

Just a pilot (retired, so I have a LOT of time)...


#8 RockinRob

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Posted 17 November 2017 - 10:55

I flew the 300 for about 100 hours of my initial training.

 

Experienced ground resonance once on a running landing, recovered to a hover and set back down.

 

Had one stick a valve in a hover and settle, ground contact lead to gr, recovered to a hover and set back down. (this one happened after a fuel stop on solo x/c)

 

I have 2k or so hours in 22s, 44s, and 66s at this point; including some more "aggressive" turning maneuvers in aerial application environments, and a fair amount of mountain flying.

 

Never experience low g in years in Robbies.

 

I think proper pilot education and training on avoidance, recognition, and recovery of these dangerous scenarios does wonders to aid in the overall safety and longevity of our rotorcraft brethren.






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