Helicopter Safety - Dangerous Flying Manoeuvres
Settling with power
Settling with power (or vortex ring state) describes the situation in which a helicopter is descending rapidly without much lateral (e.g. forward)
speed, but with substantial engine power. Adding power will make things worse and won't stop the descent. This settling with power condition
occurs when the helicopter is descending in the vortex that is created by its own rotorblades. In normal circumstances, these vortices are of no
influence as the helicopter leaves them behind (normal flight). However, when the helicopter is hovering and descending, it will stay in its own
vortices, and the addition of power will stall the blades, making the speed of descent even worse. The settling with power condition occurs as a
result of two phenomena; firstly there is the vortex itself, which leads to a large amount of induced flow. And secondly, the angle of attack is
such that blade stall is reached even with modest collective settings. The blade stall is concentrated towards the centre of the rotor disc. The
angle of attack at the blade tips is very low in these circumstances, and so no thrust is generated there either. During settling with power
conditions, large and sudden variations in the angle of attack take place, and the helicopter will pitch and roll.
- Try to fly out of the vortex by adding forward cyclic
- Do not increase power or collective, as this will encourage blade stall.
- Never make a rapid descent without forward speed. Do not start to make a purely
vertical descent.
Helicopters cannot perform such a manoeuvre.
Ground resonance
A situation exists in which a helicopter can suffer from very severe resonances. The severity of these is such that the helicopter can be destroyed.
The resonance is powered by the rotor system, and only develops to complete amplitude when the helicopter makes ground contact. Hence, the name
ground resonance. The rotor system generates the resonance when the centre of gravity of the rotor system is not aligned with the centre of rotation.
As this can happen more easily with fully articulated rotor systems, ground resonance mainly occurs with these systems. When detecting ground
resonance, you can basically do two things.
- Leave the ground immediately, since the resonance will disappear in the absence of ground contact.
- Going through the landing procedure as quickly as possible while removing power should be your main concern.
Dynamic rollover
When a helicopter comes into contact with the ground on one side of the machine (e.g. the left or right skid), there is a possibility that it
will roll over onto that side. Dynamic rollover happens when the rotor disc is applying a sideways force (thrust vector) while the helicopter is on
the ground on one side. The only movement the machine can make in this situation is to roll over. Dynamic rollover can easily happen when a skid
is accidently stuck or attached to the ground. When dynamic rollover starts it will be over in a few seconds, and so you must either react very
quickly, or prevent it from happening.
- Be aware of lateral (sideways) movements when still on the ground. A dynamic rollover
is easily started when the banking angle becomes too great.
- Apply corrective cyclic.
- Lower the collective; it will remove the thrust vector, thus removing the rolling
momentum. This is more effective than applying opposite cyclic.
- Inspect the terrain the helicopter is resting on. Look for obstructions,
wet ground,
and things you could get stuck in.
Wire stikes
It is difficult to imagine helicopters striking a wire when you have never flown one. Wires are easily seen from the ground when set against
a clear blue sky. However, when in the air, power lines have the earth’s surface as their background, which makes them much more difficult to
detect. Moreover, wires are thin and therefore not easily noticeable. As helicopters regularly fly into wires, a strategy against doing so is
needed. It is based on a simple idea. Avoid them.
- Do not trust that you know where the wires are. Ultimately you will forget about them, or otherwise
miss them (but your helicopter will not).
- Realise that you cannot see the wires properly. Imagine that between every two obstructions,
there is a wire (potentially).
- Avoid flying lower than the highest man-made obstruction or mast.
- If you do not fly below 500 feet you will not encounter wires between man-made constructions
(if this is possible for your flying mission).
Mast bumping
When a teetering rotorhead is 'flapping' beyond its mechanical limits, it will bump against the mechanical stops on the rotorshaft. This
mast bumping will, inevitably, lead to disaster (broken mast, separation of rotor blades). This condition can occur when the rotor disc
is no longer loaded. An unloaded rotor disc cannot produce any thrust vector, so it is no longer under control. In an attempt to regain
control, the necessary extreme control input will also lead to an extreme tip path, causing mast bumping. Mast bumping can also occur on
fully articulated rotor heads, although, it is then called 'droop stop pounding' and does not generally result in a disastrous ending.
- Avoid manoeuvres (with a two bladed rotorhead) with low disc loading, such as negative
G.
- Tie-down the rotorblades of a landed two bladed helicopter, as mast bumping can
be caused by wind (and not only strong winds!)
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Don't Get Caught in a Fatal Trap!
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