R-22 accident in NC?

[Timebuilder]

(bump for my mast bumping explanation request)

 

[EagleRJ]

FracCapt already explained it.

Mast bumping is still very much a problem - but the training these days adressing it is much better. When an abrupt maneuver is made that places low G loads on the rotor, the aircraft is no long pivoting underneath the rotor because it's not supporting the weight. Abrupt or excessive control inputs cause the rotor to move very quickly, and it can go beyond its design limits and part of the rotor head will actually make contact with the rotor mast. If this happens too much or too hard, the mast cracks....and when the mast finally fails, bye bye rotor. This is not just a problem with the Robinson line, but with all helicopters with a semi-rigid rotor system - including all of the Bell 200 series, the Huey, and several other types. Mast bumping was actually discovered on the Hueys back in the Vietnam time frame...and pilots were trained to avoid situations that cause it whenever possible.

Mast bumping occurs more on the Robinsons than other types because the most inexperienced pilots are flying the Robinsons. By the time somebody is experienced enough to be flying most of the other helicopter suseptible to this problem(which are mostly turbine), they know what to avoid, and how to correct it if you get into a low G situation.

 

[Timebuilder]

When an abrupt maneuver is made that places low G loads on the rotor, the aircraft is no long pivoting underneath the rotor because it's not supporting the weight. Abrupt or excessive control inputs cause the rotor to move very quickly, and it can go beyond its design limits and part of the rotor head will actually make contact with the rotor mast. If this happens too much or too hard, the mast cracks....and when the mast finally fails, bye bye rotor.

OK. I was hoping for a clarification geared towrd someone who might not be familiar with the concepts, such as myself.

What kind of "abrupt maneuver" would place "low G loads" on the rotor? Is the pivoting mentioned refering to the use of the antitorque pedals, or their incorrect or ineffective use?

Are "abrupt of excessive control inputs" a referece to "yanking" on the cyclic or collective?

Where does the rotor head make contact with the rotor mast? Are they not already spinning together? Is there a diagram that shows where the "bumping" occurs?

Is this something that happens when we would move the collective to one extreme or the other in pitch, or when the cyclic positions the swash plate so that an extreme of pitch is made in each rotation or passage of the rotor past that point on the swash plate?

Yes FracCapt explained it, but I need some additional help in understanding it.

 

[Birdstrike]

Mast Bumping

One typical maneuver that can cause mast bumping in semi-rigid rotor systems like the Robby or Huey would be an abrupt forward or aft cyclic application, say leveling after a climb or over-compensating when caught in moderate turbulance. At the top of a climb (commanded or not), if the cyclic is "dumped" (pushed forward too abruptly and too far) the rotor system "unloads" and goes from pulling positive G's to a negative G situation.

You can do the same thing in an airplane by abruptly nosing it over at the top of a climb (go from positive to negative Gs). Everything in a semi-rigid helo has to be done smoothly. You put in a cyclic input and there is a brief pause before the desired movement of the aircraft takes affect. This is due to gyroscopic precession but we won't go there. Suffice to say that when an overly abrupt input is made, a student may not recognize it instantly because of the delayed application of the input before it is relayed down to the pivoting or pendulating (better word) fuselage. Then the input it kicks in and it is too late; the sides of the rotor head that are designed to spin without contacting the rotor mast now do make "bumping" contact with it because of the execessive rotor plane angle demanded by the overly abrupt and excessive cyclic input. A bump equals a fracture equals mast failure.

Cobra and even Huey pilots in Vietnam knew how to control this just within limits to avoid mast bumping (most of the time) and still make the evasive maneuvers they needed to survive. Guys in my reserve unit years later could still demonstrate steep-angle dives followed by dumping the collective and pulling up on the cyclic for a max angle climb with no power trading airspeed for altitude. At the top of the arc they would (carefully) dump the cyclic over, load up the rotor with collective and go in for another run. It's a timing maneuver and totally illegal in the Army today, but it saved lives in wartime.

 

[FracCapt]

What kind of "abrupt maneuver" would place "low G loads" on the rotor? Is the pivoting mentioned refering to the use of the antitorque pedals, or their incorrect or ineffective use?

Are "abrupt of excessive control inputs" a referece to "yanking" on the cyclic or collective?

Birdstrike addressed it very well. I should have stated Low G maneuvers rather than low G loads. Just like an airplane...in forward flight, push the cyclic forward quickly, and you create low or negative G's. The lower the G's, the less effective the rotor disc is(which controls all movement except for yaw). Everything I'm referencing here is strictly cyclic inputs - not collective or anti-torque pedal. When the disc is lightly loaded(therefore ineffective), the limits of the teetering hinge can be exceeded, which allows the lower part of the rotor head(a few inches below where the rotor blades attach to the hub) to contact the mast.

If you're not familiar with the teetering hinge, it is what allows the blades to adjust to create symmetrical lift between the two sides. Lets say, for simplicity, the main rotor turns at 200mph. When in a hover, the rotor blades are each traveling 200mph throughout the entire plane of rotation. When you're moving forward at 100mph, the advancing blade is going 300mph, and the retreating blade is going 100mph(airspeed). With the same pitch on the blades, obviously the one going 300mph is going to produce much more lift than the one going 100mph. The teetering hinge allows the blades to move up and down to reduce the AOA on the advancing and increase the AOA on the retreating blade to make the lift symmetrical. On 2 blade systems, when one blade moves up, the other moves down.

Check out http://www.helicoptersonly.com/sayagain_MastBumping.html .

Also, http://www.helicfi.com/sfar.htm gives some good info.

 

[Timebuilder]

Thanks for the expnded explanation; I really appreciate it.

Helos have always fascinated me. When I was a kid in the 50's, a show called Whirlybirds that featured a Korean veteran and his younger colleague flew about in a "MASH" type bubble front piston helo (a 47?) having weekly adventures. I knew the word "autorotation" before I was in third grade. I toyed with the idea of Army helo school in 1970, but so many returning vets advised me that we were not being permitted to win the war because of political considerations, and that I should pass on participating.

Tapes of the show are being sold at the bottom of the list on this site:

http://www.swapsale.com/50's_action_adventure.htm

and this is a site about the show:

http://cellmath.med.utoronto.ca/B47/history/wbGuide/wbLaunchIt.html


One question: why is the Vietnam maneuver you described "illegal" today?

Okay, two questions.

Do modern helicopters used by the military have an improved rotor system that avoids the mast bumping problem?

Thanks.

 

[Timebuilder]

I intend to.

A few national spots for GM or several dozen horse farms sold might put me within reach of it. I want to get a 172 first, and either build or buy the helo, which would be ideal for taking clients for a view of their new "Southfork Ranch."

 

[FracCapt]

One question: why is the Vietnam maneuver you described "illegal" today?

Because the maneuver he described - dumping the cyclic and loading the disc by pulling more collective - is very much a perfect timing maneuver. Screw it up, and you end up with mast bumping...possibly more severe than if you had just dumped the cyclic and left the collective in position. It's best to just avoid the potential situation. Of course, when you're in combat - you do what it takes to survive, and if that means doing something you were taught never to do because it "could cause a problem"....so be it.

Do modern helicopters used by the military have an improved rotor system that avoids the mast bumping problem?

Only helicopters with two rotor blades are subject to mast bumping. Anything with more than two blades on the rotor system is either a rigid or articulated system, and therefore not subject to mast bumping. They each have their own issues, though. Not all two blade rotor systems are semi-rigid - some are rigid systems(which are not subject to mast bumping). I don't know of any helicopters in the military that are semi rigid that haven't been operated in the military for many, many years. Some of these military guys might know of some...but I know they still operate OH58's, AH1's, and UH1's.

 

[Birdstrike]

I remember Whirlybirds also. Was on about the same time as Ron Reagan hosting Death Valley Days and some show about a guy screaming through the everglades in an airboat...Lincoln Vale was his name I think.

FracCapt answered the mail. "Illegal" means that the Army doesn't teach the maneuver and it used to, and may still be, prohibited in writing -- was known as a "return to target" maneuver, I believe. The last major fatality that I recall involving this maneuver was at Fort Hood in 1987 - 11 deaths in a UH-1 belonging to a USAR unit in OK. The PIC was apparently demonstrating the maneuver on a hot day with a full load. He ran out of altitude at the bottom and tried to recover with power but it was like pounding a nail with a hammer at that rate of descent and that was it.

I'm surprised that there isn't a UH-1 on display at the Smithsonian. It's been flying since 1954 and is still in service today. The design was ahead of its time and belongs right up there with the DC-3 as an a American classic as well as an icon.

 

[sky37d]

Don't gyro copters have the same problem with unloading the disc, which causes it to stop turning, or something like that.