Mast Bumping

[Iridus]

Why doesn't mast bumping always occur in semi rigid rotor systems? Specifically how can the fuselage change attitude without the stops on the blade hinges contacting the mast and physically moving the fuselage to match the attitude of the rotor disc?

[Radam]

I am thinking pendular action

[WolftalonID]

Thrust.

 

Those rotors are pulling that fuselage along with alot of thrust. It is following along not just gently hanging there.

 

Even with some rather sharp turns you never really move the tip path plane far enough to hit them in flight.

 

Now when you nose over hard or have a flight condition that causes the fuselage to push up into the rotors like low g, then its possible.

 

Also if your doing slips in an auto, your mast is much closer to the rotors than a standard straight in and care must be used to not over manipulate the cyclic.

[iChris]

Why doesn't mast bumping always occur in semi rigid rotor systems? Specifically how can the fuselage change attitude without the stops on the blade hinges contacting the mast and physically moving the fuselage to match the attitude of the rotor disc?

 

So, why don’t we hit the stops?

 

As in WolftalonID's post, it's about rotor thrust. Most flight maneuvers are conducted within the helicopters designed flight envelope; therefore, they are within the rotor systems maximum flapping limits. There is no contact made with the stops. The result, no fear of mast bumping during normal ≥1G flight. Inflight mast bumping (contact with the stops) is the result of excessive flapping, outside the design limits. It's normally caused by abrupt cyclic control inputs by the pilot during low G conditions. Aggressive slide slips and extreme CG position can also lead to problems.

 

The maximum flapping angle in most teetering rotor systems is about 12 ½°. Normal 1G level flight averages about 2° of flapping. All other normal maneuvering within the flight envelope don’t amount to more than about 5° flapping. As you can see it does not take much tilt of the thrust vector to create fuselage movement and avoid hitting the stops. In other words, it is not necessary to make contact with the stops in order to make attitude changes.

 

You asked specifically, how can the fuselage change attitude? Well, in order to make an attitude change, you must first create a moment about the helicopter’s CG. The moment being the product of a force working at a given distance from the CG. This moment causes the helicopter to change its attitude about its lateral, longitudinal, and vertical axis. These control moments are created mainly by the main and tail rotor; however, in the context of your question, we’ll concentrate on the main rotor.

 

To develop a control moment, the teetering rotor must first develop a force (trust). Therefore, if a controlled force or moment is needed to change the helicopter’s attitude, the rotor system must first develop thrust, accordingly the control moment becomes a byproduct of thrust.

 

You as the pilot control the thrust vectoring with the cyclic via the cyclical changes in rotor blade pitch that causes the blades to flap. The flapping action results in the tip-path plane tilting; therefore, the thrust vector which acts approximately perpendicular to the tip-path plane, will also tilt in the direction commanded by the pilot. As long as the flapping is within the design limit, there will be no mast bumping.

 

As an example, in hovering flight, the thrust vector passes through the CG of the helicopter and no moment is produced. When you enter forward flight by moving the cyclic forward, the trust vector no longer passes through the CG. A moment arm is created with respect to the CG, and thus, a moment is placed on the fuselage in a nose down direction as the helicopter starts to move forward. The control moment continues to tilt the fuselage until the thrust vector realigns itself as the helicopter reaches equilibrium in forward flight.

 

The articulated rotor system can develop control moments on the fuselage without developing trust. The teetering rotor is hinged at the center of the mast; the blades of an articulated rotor system are attached to the rotor hub at an offset distance from the mast. The offset hinging allows centrifugal force to produce a control moment. Centrifugal force is independent of trust; therefore, the articulated rotor system can produce a control moment even though it is not producing thrust. The development of thrust increases the magnitude of control moment that much more.

 

Inflight mast bumping is the result of excessive flapping. Normally caused by abrupt cyclic control inputs by the pilot during low G conditions.

 

The figure below shows that the moments about the aircraft’s CG, due to rotor flapping, are produced by the couple at the hub as a result of the rotor stiffness and the tilt of the thrust vector acting perpendicular to the tip-path plane. Note; This represents an articulated rotor, the couple at the hub would not be applicable to a teetering rotor.

Edited January 23, 2016 by iChris

[Eric Hunt]

As usual, Chris has hit the nail on the (rotor) head.

 

What hasn't been mentioned is that if the helo is on the ground, and the thrust is less than required for hover, it is possible to hit the stops if the cyclic is moved. This is because the thrust moment isn't big enough to make the fuselage follow, for example, if the lever is on the floor, and the blades can reach the stops quite easily.

 

You will feel a drumming through your seat and the stick, and your instructor will beat you about the ears and take control. You then realise that you had let the cyclic go while you were adjusting the volume of your iPod, or setting up your GoPro, or something trivial. KEEP THE DISK LEVEL WHEN ON THE GROUND, and don't let it go.

 

In a machine like the B206, there are Flap Restraints which prevent the teetering yoke from contacting the static stops when at rest or very low RRPM on startup or shutdown. These restraints swing out with increasing RRPM.

 

With an in-flight contact, the static stops are usually made of a softer metal than the mast, so they will deform first, giving the dopey pilot a few microseconds to fix the disk attitude before the mast itself starts to deform and break off.

[vortamock]

Lots of great information. Thanks guys!