NEGATIVE G AND MAST BUMPING
This article was written by Becker Helicopters Pty Ltd and reprinted with their permission - March 3/08

All objects are subject to the gravitational pull of the earth relative to their mass. This includes you and me and it includes a helicopter.

As we are standing on the surface of the earth we will feel the weight of our body, which we can count as 1 G force or 1 positive G force. If we were able to conduct a manoeuvre that causes our apparent weight to increase we would increase the G force felt, in effect increasing our weight at that point in time. If we double our weight we double the G force or experience 2 Gs and so on.

If we were able to conduct a manoeuvre that reduced our apparent weight we would decrease the G force felt, in effect decreasing our weight at this time. Any reduction in the standard 1 G force we usually experience is known as a low G or a negative G situation. Reduce our weight by 1 we have experienced 1 negative G, reduced our weight by 2 we have experienced 2 negative G and so on.

In flight we are able to produce both a negative or low G (weightless feeling) and a positive of high G (increasing weight feeling) condition. Because G force deals with the laws of attraction then any manoeuvre that tries to pull us away from the earth (the body with the greatest mass we are closest to therefore the body with the greatest attraction) then we will have a temporary increase in the G force as we experience this acceleration away from the surface. Any manoeuvre that tries to accelerate our movement towards the centre of the earth (in other words assist the attraction) will lead to a temporary reduction in the pull of the earth and we will feel slightly weightless. The faster and harsher these manoeuvres the greater the change in the G force.

For example

You are travelling along in your new Mad Max Charger doing 300kph when you come to a small hump in the road. As you hit the hump you are forced away from the earth so you experience an increase in the G force and you temporarily feel heavy. Very quickly though the earth pulls the Charger back towards it and you start to fall towards the road again and now feel a reduction in the G force and you feel lighter.

Another example of G force is a typical giant drop rollercoaster. You experience a positive G force as you are propelled away from the surface, and you experience a negative G force as you fall back towards the earth.

Playing around with G force is great as a teenager and great in an aerobatic aircraft but in a helicopter with a two bladed teetering type rotor head it can have serious consequences.

Mast bumping

Mast bumping is generally a result of pilot induced over controlling of the cyclic leading to a negative G situation, however the negative G can also be caused by other factors without input by the pilot such as severe turbulence or a rapid lowering of the collective. It is a condition applicable to two bladed helicopters with a teetering rotor head such as the R22/R44, Bell47, Bell 206 Bell205/212 series and other similar designs. Mast bumping is a result of the helicopters main rotor hub (head) making contact with the main rotor mast. The head literally 'bumps' the mast and can damage or snap it off. For this to happen excessive flapping of the disc must occur and this is impossible if the helicopter is flown within its designed tolerances.

Excessive flapping may be the result of

• Flight resulting in a negative or low G situation, this may be due to manoeuvring, turbulence or similar

• Sudden, abrupt and large changes made to the cyclic, especially in the fore/aft direction

• Sudden and unanticipated lowering (or dropping) of the collective

• Strong gusty winds (especially updraughts associated with hovering or landing on a cliff edge)

• Excessive sideways flight beyond the maximum allowable limits

• Landings on an excessive slope beyond the design limits of the helicopter

In normal flight when the cyclic is moved it then tilts the rotor disc, which in turn produces a horizontal component of rotor thrust (HRT). This produces a moment between the Horizontal component of rotor thrust and the centre of gravity of the fuselage, which acts as a lever to roll the fuselage, and the helicopter as a unit (rotors and fuselage) will roll in the desired direction.

If the helicopter for some reason is put into a negative G situation, then several things happen.

The angle of attack on the rotor blades changes and the amount of total rotor thrust at that moment reduces The apparent weight of the helicopter reduces therefore the moment required between the horizontal component of rotor thrust and the centre of gravity of the helicopter is temporarily no longer there. At this stage the pilot still has control of the rotor disc by use of the cyclic but he no longer has control of the fuselage because there is no moment and therefore no force to make it follow the disc. At this point the fuselage will be influenced by other forces.

In normal powered flight the rotor disc in a conventional helicopter will always be displaced slightly to the left to counter the effects of tail rotor drift.

In a negative G situation the total rotor thrust is reduced however the torque produced by the engine and the amount of tail rotor thrust being produced is unaffected, therefore the fuselage while still moving forward will also want to drift to the right and yaw to the left. This right drift and left yaw will cause an airflow to act against the fuselage, which will then cause it to roll to the right without any input from the pilot.

The pilot will instinctively use left cyclic to counter the unanticipated right roll. With the small amount of rotor thrust still being produced by the disc the rotors will flap to follow the cyclic pitch change and the rotor disc will start to tilt left but the fuselage, not being influenced by the moment between the horizontal component of rotor thrust and the centre of gravity (because there is no apparent weight) will not follow and continue to roll right. At this point it is still recoverable. All the pilot needs to do is take away the negative G and replace it with a positive G and the moment will be restored.

Lets assume though that this does not happen and the pilot continues to use left cyclic to counter the continuing right roll of the fuselage. There will now come a point where the rotor head strikes the mast. The severity of the striking will determine whether the mast separates from the helicopter or is just damaged.

Below is the end result of a Robinson R22 that suffered mast bumping several years ago. The picture is compliments of Greg Whyte who wrote the book "Fatal Traps" for helicopter pilots and is a definite must read. http://www.fataltraps.com

Recovery action

Prevention is the best rule. Never get into a negative G situation in a two bladed helicopter system by intentionally using abrupt control inputs. If you do experience a negative G for any reason then the obvious solution is to reload the rotor disc so that you are experiencing positive G. This can be done by

• Using aft cyclic to increase the G force and use right cyclic to follow the roll

• Raise collective to increase the total rotor thrust and help increase the G force

• Once the G force has been restored then you can recover to straight and level flight.

Common scenario

One of the most common scenarios causing mast bumping, is that of a middle aged person who has some fixed wing experience converting over to a helicopter. Fixed wing being much more stable allows the pilot to look longer and more often at maps, and generally allows the fixed wing to fly itself. Also most emergencies in a fixed wing required that the pilot push the control column forward to maintain airspeed.

In a helicopter if the pilots attention is not on flying it is very common for the helicopter to be slowly pitching up, when the pilot does eventually look out side and sees a nose high attitude the natural response is to push the cyclic forward. But as we know this can cause a negative G situation and lead to catastrophe.

This is more common in a Bell47 with no hydraulic collective but can be possible in any helicopter if the collective is not rigged correctly although the dropping collective only leads to a mast bump in two bladed helicopters.

If the pilot releases his hold on the collective and does not secure it (by putting on some friction or placing his leg against it) then the collective has the potential to move. The movement may be caused by aerodynamic forces on the rotor blades overcoming the collective setting by the pilot and this can happen suddenly and without warning. If the collective drops rapidly to the floor there will be an instant reduction in the flapback of the main rotor disc. Because the pilot has forward cyclic to counter this flapback the helicopter will instantly dive towards the ground and cause a negative G situation. This can have two catastrophic results. First of all mast bumping as described above and secondly the pilots natural response will be to use a large aft cyclic input to correct the dive, at this point the tail is going up, the disc starts to come back and the potential is there for the disc to strike the tail boom.

Although negative G can be caused by turbulence and in extreme cases lead to some form of mast bumping, the negative G usually experienced is not catastrophic. This is because the turbulence is fast acting. Often a negative G is very quickly followed by a positive G therefore there are no 'prolonged' effects.

If it is that turbulent you probably shouldn't be flying anyway!! If you are flying in turbulent conditions and experience a negative G then hold the controls steady and wait for the turbulence to subside. It is also best in these situations to slow down (reduce IAS) so that you not hit the turbulence so fast and also bring the RPM down to the mid green to help reduce RPM overspeeds.

Negative G is commonly caused by pilots inducing a pull up followed by a harsh push over (harsh use of the cyclic) resulting in a negative G situation and the fuselage rolling to the right with no input form the pilot. This roll causes an immediate response by the pilot who uses opposite cyclic (left) this in turn leads to the rotor head striking the mast and may cause it to separate.

This article was written by Becker Helicopters Pty Ltd and reprinted with their permission - March 3/08
Graphics were done by a pilot we trained Capt. Sam Jenkins
Web Site: www.beckerhelicopters.com