Helicopter Flight Training Manual (TP 9982)
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EXERCISE 26 - VORTEX RING
If the helicopter pilot chooses a flight path, airspeed and a rate of descent that coincides with the aircraft’s downwash, the helicopter could enter a condition known as the Vortex Ring state. The stall condition formed by the rate of descent flow in opposition to the induced flow, combines with the tip vortices present in all regimes of flight to produce a turbulent rotational flow on the blades and an unsteady spanwise shifting of that flow. This condition induces a very rapid rate of descent, vibrations, excessive flapping and a reduction in cyclic authority that could result in an accident. Obviously, this condition is to be avoided and the helicopter pilot should be able to recognize the incipient stage and be able to affect a safe recovery.
Your instructor will review the causes, conditions and symptoms of vortex ring. During a steep approach, at a high gross weight, high-density altitude and in a downwind or light wind condition; the helicopter may enter its own downwash and the development of vortex ring state. This situation would certainly contribute to the onset of vortex ring, but not necessarily cause it. The phenomenon is most likely to occur when all the conditions listed below are present:
- in powered flight;
- high rate of descent, in excess of 500 feet per minute; and
- low airspeed, less than 20 MPH indicated.
Almost every transition from forward flight to a hover utilizes a powered approach, a rate of descent and a reduced airspeed. To prevent the occurrence of vortex ring, control your rate of descent less than 300 feet per minute.
Recovery Techniques. There are two methods of effective recovery from the vortex ring state. Both change the airflow conditions causing it and both involve a loss of height:
- Dive out. Normally this technique will result in less altitude loss than with the autorotational recovery. The pilot should apply forward cyclic while reducing the collective; the vortices will leave the disc as the airspeed increases and the helicopter will move forward of its downwash. Normal flight may then be resumed.
- Enter autorotation. By this method, the airflow through the rotor changes from the disturbed flow of the vortex ring to the upward autorotational flow. Once autorotational descent has commenced then the pilot may ease the cyclic forward to gain airspeed while power is increased and normal flight resumed.
You should note that an increase in collective alone may not result in a recovery and indeed may only serve to increase the rate of descent. This increase in blade pitch will cause the vortices to intensify in strength and will result in a more rapid descent.
There are some uninformed pilots who use “settling with power” to describe vortex ring, in fact some publications use the terms interchangeably. Confusion results when symptoms are related that do not describe true vortex ring but rather describe “settling with insufficient power”. This may occur when a pilot attempts to arrest a rapid, low power descent only to find that he has insufficient power available to bring the helicopter to either a hover or a no-hover landing without exceeding the engine limits. However, this is not a vortex ring situation.
Another situation, ‘over-pitching’ is often misinterpreted as vortex ring. This is where the pilot rapidly increases collective considerably and the engine cannot produce enough power to overcome the large, swift increase in drag on the rotor system. The result is that the rotor system quickly slows down and loses efficiency causing the helicopter instantly to sink. Again, this is not vortex ring.
The most common situations, where you would be most likely to encounter vortex ring, are usually when you misjudge the wind with a heavy load on a hot day. Downwind approaches to a confined area, or a mountain pad, are two good examples. Always control your rate of descent carefully on these occasions, and make sure an escape route is available. Your instructor will discuss the symptoms and recovery techniques more fully. Demonstration of this exercise is not generally performed, as the stresses on the airframe and rotor system are unknown.
PREVENTION IS BETTER THAN CURE!!!
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