A question for those who fly with fully articulated rotors
 

In a semi rigid rotor system we know that low to negative g's will unload the rotor and the tail rotor thrust being above the center of gravity will cause a rapid roll to the right (with counter clockwise main rotor rotation.) Improper application of left cyclic without first reloading the rotor disk with aft cyclic can cause mast bumping and other bad things. I know mast bumping is not an issue with a fully articulated rotor system, but if the disk is unloaded will the helicopter still roll due to tail rotor thrust? If not, does any one know why?

Regards,
Phil

Good question! Wish I knew the answer - over to you Nick!

If a rotor disc is unloaded in the sense of providing no lift or horizontal stability (due to negative g or otherwise), then I'd've thought the effect on the ship from the tail rotor would be no different irrespective of the rotor system?

IMHO helicopter still rolls in low-G, no matter what rotor system. This is because the TR still provides thrust, but the unloaded MR doesn't provide any thrust to counteract.

Since fully articulated rotor systems don't teeter, all blades would be able to independently flap up or down within their limits in a low-G condition. As I understand it, the limits of certain semi-rigid systems are the droop stops hitting the mast (= mast bumping) or the blades hitting the tail boom or airframe.

Remember, a teetering system depends on positive-G for control power. An articulated system, with its flapping hinge-offset does not. Articulated systems retain their control power even at low G-loading.

When you fly a bit slower with a semi-rigid (teethering system) you won't experience a roll-to the right, maybe very slightly. With a fully articulated rotorhead the roll is almost none anyway (it can be to the left obviously with a clockwise turning main rotor) The fact that the rotorhead is rigid to the mast and the rotordisc has a certain mass there is basically no roll. There is a yaw however.

The design of the rotorhead will not affect the thrust of the TR and therefore the answer is yes it will still roll. The difference is that with a teetering head in low G you can't do anything to oppose it, which is why it is so dangerous - with an articulated head you retain control power and therefore can oppose the roll with cyclic.

With a fully articulated rotor, the offset of the flapping hinge exerts a mast moment. When the blades are rotating, they will always try to keep the mast perpendicular with the disk. The greater the offset, the greater the mast moment. This gives you positive control of the fuselage attitude as long as the blades are spinning.

To demonstrate this look at this video. Griffin Helicopters | Video Player

Even without forward speed the Hughes is almost able to roll over. This is a good demonstration of mast moment gone bad:eek:

Re the video:

Where's the low/negative G?

This appears to be a loss of control issue, mechanical or otherwise.

I seem to remember this video, was some time back in late 90's seem to think either in maxico or Hawai and it was some linkage that detached from the cyclic, the driver was only rescued by the heroic efforts of one man who held the Heli up so others could drag the injured pilot out!

Vfrpilotpb
PeterR-B:eek:

First time I displaced the cyclic while at flat pitch 100% on the ground in an articulated rotor system proved it to me. You can damn near roll the helicopter over without any collective lifting force from the rotor system. Try that in a teeter system and it'll just tilt the disc. Made it easy for my stupid brain to understand! Articulated rotors don't care about the g loading, they always provide a roll/pitch moment with the cyclic.

The video link I posted was not to show neg-g, but rather to show the differences in the rotorsystem. It shows how the disk exerts a mast moment..... A two bladed ship would not do this. On a two bladed ship the fuselage basically hangs like a pendulum. (which is why it can make contact when unloaded)

Sorry if there was confusion...

I hadn't thought about the moment being applied to the mast through the flapping hinges - I didn't realize that the fully articulated system is always acting on the mast though it makes sense to me now that Induced Drag mentions it. I had asked an Apache pilot who has quite a bit of NOE flying in his background and he said he had never noticed a rolling tendency. I image the rate of roll would be damped somewhat by the mass of the helicopter, and the Apache is not a particularly light bird. Perhaps that combined with the lateral controllability even at negative g makes it a non issue.

Thank you all for the information!

In the fully articulated system we can still get droop stop pounding, which will break the droop stops with that left cyclic correction before reloading the disc. This won't be really noticed until you're shutting down, slow blades, low drooping blades, loss of a tail boom, etc!!!:}