Helicopter mast bumping in low g condition

Question

The helicopter shown in the concrete pouring video seems to taking off from a hill and descending down in a way which can be classified as low G with low rotor loading. Although, the helicopter is three-bladed, mast bumping should be less of an issue, but from my understanding still worth considering.

Could a maneuver like this be executed in a similar way with a two-bladed helicopter such as a Bell 206 or Robinson. Furthermore, what is the reason for mast bumping being less of an issue for helicopters with more than two blades?

The famous video of the US Army seems to be describing exactly the condition as shown in the first video as potentially dangerous.

Answer 1

The answer is yes it could be used in that flight profile as the pilot of that AS300 was never in a low G condition with the rotor unloaded during those maneuvers. Once he dropped the concrete, though he’s descending, he’s also accelerating at a high power setting, which is keeping the rotor loaded. However, zero G mast bumping is still a risk that must be considered and thoroughly mitigated for during preflight for all helos with a semi rigid rotor system.

Mass bumping only occurs on helicopters with a semi rigid or teetering rotor system. It has to do with how a semi rigid rotor is supported by the mast in an under slung condition, where excess flapping can result in mast bumping. It will generally occur in conditions like zero-G pushovers at low power settings or when attempting a steep slope landing. Mast bumping is not possible on rigid, fully articulated, or elastomeric rotor systems where the rotor mounts cannot strike the mast, even with excess flapping.

Here is a pretty good recap on the subject of mast bumping.

Even helos with two bladed semi-rigid rotor systems can be flown through some pretty extreme maneuvers, provided the pilot flies within the design limits and does not unload the rotor. I remember West Coast JetCopters, who provided the Bell 222 for the television series Airwolf, did some pretty extreme flying on those choppers and a 222 does use a teetering rotor system. They just had really, really good pilots at the controls. See 0:57 in the video below where the stunt pilots put a 222 through the helo equivalent of a hammerhead stall.

Answer 2

Clearly you are speaking of what happens after the helicopter dumps the concrete. A transition to a dive, even a rather steep dive, need not involve a reduction in G-load too much below 1 G as long as it is accomplished smoothly.

Answer 3

Mast bumping is not an issue on articulated rotors (3 blades and up) because the system is not completely dependent on pendulum effect to keep the fuselage/rotor disc relationship within limits like with teetering systems.

And even with the disc completely unloaded, the articulated rotor still has some (although much degraded) roll authority due to the blade root offset from the center of the rigidly attached rotor hub, so that a pilot input to correct a roll induced by the tail rotor when zero G will limit the amount of roll.

Even if the rotor disc's displacement of the vertical axis did become excessive, the result is just the blade roots running into the flapping limits in their root attachments. While it puts a lot of bending stress on the mast as the rolling moment bending is transferred through the mast and transmission mountings to the body, the system is designed to handle that and, it won't cause the mast to snap off like on the teetering rotor, where there is a concentrated bending load applied directly to the mast tube by the teeter stops.

So the AS-300 in the video, while it didn't go zero G in that departure, having a 3 blade elastomeric articulated rotor meant it wasn't in any risk of that kind of disaster even if it did go zero G. Problem is, while you get some peace of mind when it comes to 0 G flight in your 3 blade Squirrel, now you have to worry about something the teetering pilot doesn't, ground resonance.