Low-G

[r22butters]

I've heard a few people say that one of the reasons they prefer fully-articulated helicopters, is that they don't have to worry about Low-G Mast Bumping, like we in the semi-ridged world do.

 

Although I do understand that a fully-articulated rotor's blades are not underslug, and therefore will not bump the mast, can't it still get into Low-G?

 

If you abruptly push the nose forward in cruise, or during a climb, won't it still cause the rotor to become "unloaded", and the blades to swing down and cut the tail cone off, if you put in lateral cyclic to stop the accompanying roll?

 

Anyway, I don't have a lot of experience with fully-articulated rotors, so I just thought I'd ask.

[CharyouTree]

(UH-60) I can and do pull into a 30* climb at 120+ kts, (maintaining collective), and then at the top, (smoothly...if you can't do it smoothly, don't do it at all (thank you, Mr IP)) reduce collective and level the nose/dive. It's uhh...quite the low-g maneuver. Very fun for the kiddies getting their nickel ride, and not at all dangerous.

 

My experience with under-slung rotors is limited, but as I understood it (and not until after i was done in them) it's purely a mast bumping issue. An aircraft with an articulated rotor just can't "swing" the same way. (I can't think of how to describe the action, but I can see it in my head, sorry.)

 

Feel free to straighten me out on the issue.

[Trans Lift]

Of course it can get into Low G and what we can get in the "fully articulated world" is droop-stop pounding. Basically when the low g scenario is entered, instead of mast bumping, we get droop-stop pounding. This isn't a big deal in flight at normal operating RPM but when you are shutting down it will come into effect. If the droop stop is broken the blade will get lower in its plane of rotation as it is slowing down and could strike the tail.

[clay]

there is no such thing as mast bumpING... its mast bump...

[Goldy]

If you abruptly push the nose forward in cruise, or during a climb, won't it still cause the rotor to become "unloaded", and the blades to swing down and cut the tail cone off, if you put in lateral cyclic to stop the accompanying roll?

 

Think about how the blades are attached to the mast on an R22. Now look at say, an AStar. Where the R22 blades are essentially swinging from a teetering center point, the A Star blades are attached directly to a large fixed rotor head. So yes, you get Low G, you just dont have the same negative response in the rotor system. If the ship is weightless, but the blades are all attached to the fixed rotor head, so what?

 

It's the teetering arrangement that gets ya. I'm sure someone can post some great sketches that show the difference of the two.

[Goldy]

there is no such thing as mast bumpING... its mast bump...

 

Your right Clay, one bump is usually all you get. To think we used to practice that maneuver, just like you would auto's today. We had to induce it and correct it with a CFI before solo....what fun.

 

Goldy

[SBuzzkill]

You used to induce mast bump on purpose?

[Goldy]

You used to induce mast bump on purpose?

 

Yepp, we knew mast bumping was an issue. CFI's had to learn to demonstrate it to their students. As a student in the late 80's we got to experience it before solo. We did pushovers at around 40-45 knots to keep the low G under control. We knew if we tried it at 65 knots that right diving turn came pretty fast.

 

Anyway, thems were the days. Those of us still alive are a little older and a little wiser, but hey, I still fly an R22 now and again.

 

Goldy

[ascj]

Yepp, we knew mast bumping was an issue. CFI's had to learn to demonstrate it to their students. As a student in the late 80's we got to experience it before solo.

 

Goldy

 

Was it 'mast bumping' that used to be taught or low/neg G, which could result in mast bumping. I was taught they are two very different things that happen in this order 'Low G which if not dealt with correctly leads to Mast bumping which generally leads to you becoming a Lawn Dart..'

 

ASCJ

[MileHi480B]

I have talked to numerous CFIs and not one says they used to demonstrate "mast bumping". What they did demonstrate was a low-g maneuver and how to prevent the onset of a mast-bump.

 

Here's how it went (according to all of them who actually taught it).

 

Instructor induces a slight push over from the top of a climb.

 

As the ship goes "negative", it starts to turn right.

 

Instructor does not try to correct rolling tendency. Instead, he adds back pressure to "load up" the rotor. Then as the "Gs" normalize, he levels the ship and pulls it gently out of a dive.

 

It is not the negative g that kills you nor does it cause the mast to bump. It is the incorrect reaction to it.

 

BTW ... I am just spouting what I've leaned. I am a very new pilot who has had all of this taught and/or demonstrated. (Thanks Clay, Bob & Cayce)

Edited June 27, 2010 by MileHiR44

[Goldy]

I have talked to numerous CFIs and not one says they used to demonstrate "mast bumping". What they did demonstrate was a low-g maneuver and how to prevent the onset of a mast-bump.

 

Here's how it went (according to all of them who actually taught it).

 

Instructor induces a slight push over from the top of a climb.

 

As the ship goes "negative", it starts to turn right.

 

Instructor does not try to correct rolling tendency. Instead, he adds back pressure to "load up" the rotor. Then as the "Gs" normalize, he levels the ship and pulls it gently out of a dive.

 

It is not the negative g that kills you or bumps the mast. It is the improper reaction to it.

 

BTW ... I am just spouting what I've leaned. I am a very new pilot who has had all of this taught and demonstrated.

 

Yes, sorry I wasnt very clear. We would practice low G manuevers, which if not handled correctly, could lead to a mast bump. There are several incidents in the NTSB reports from the late 80's early 90's where this "practice" presumably resulted in an actual mast bump, resulting in the loss of lives and the ship. BTW Mile HI- you have it correct. Your butt can feel when the rotor is reloaded and you can level the ship.

[Spike]

Robinson factory used to demonstrate the low-G all the way through. That is, low-G to an unloaded system and subsequent immediate right roll. I can tell you, it was unnerving seeing the LB harbor up through the windscreen. Afterwards, they’d show you the correct way to do the maneuver which in my opinion was still a little extreme.

 

One of my common stage-check questions would be; with a semi-rigid system, what causes the mast to bump? The common incorrect answer would be; “low-G”. Wrong! As already stated; improper pilot reaction to low-G is what causes the mast to bump.