Exercise 26 - Vortex Ring

The aerodynamic stresses to which an airframe and rotor system is exposed during Vortex Ring State are virtually unknown. Exercise 26 which deals with this condition of flight has been retained, but the emphasis should be placed on the early recognition and avoidance rather than practising a fully developed Vortex Ring State. Controlling the rate of descent should be stressed in situations where Vortex Ring is likely to develop.

BECAUSE OF THE UNKNOWN STRESSES PUT ON THE HELICOPTER MANY SCHOOLS DO NOT FLY THIS AS A FULL EXERCISE, USING CLASSROOM DISCUSSION AND SCENARIOS TO COVER THE SUBJECT. RECOVERY CAN STILL BE PRACTISED IN THE AIR BY SIMULATING VORTEX RING. THIS CAN BE ACHIEVED IF IN TEACHING POINTS EX. 4C THE RATE OF DESCENT IS REDUCED TO 300-400 FEET PER MINUTE.

GROUND SCHOOL POINTS

Theory of Flight - Vortex Ring

PREPARATORY INSTRUCTION

Aim

For the student to learn the practical avoidance of, and recovery from, vortex ring state.

Motivation

It is preferable that the pilot altogether avoids the flight conditions that result in vortex ring. It is nonetheless desirable that he recognizes the symptoms of the incipient stage and can prevent it from developing into the full state, which in most operational situations, will result in an accident.

Airmanship

    1. Aircraft limitations
    2. Lookout

Teaching Points

    1. Review the causes, conditions, and symptoms of vortex ring (see Tips for Instructors, 3 and 4).
    2. Describe typical operational situations where the condition could be encountered, such as a steep approach at high all-up weight and density altitude conditions, with nil wind or a tail wind.
    3. Review recovery action discussing the advantages and disadvantages of:
      1. increasing the airspeed; or
      2. entering autorotation.
    4. Describe the techniques for initiating and recovering from vortex ring safely under training conditions, as follows:
      1. in nil or light wind conditions, climb to an altitude sufficient for safe recovery;
      2. carry out a good lookout, particularly below;
      3. initiate a rate of descent in the order of 500-1,500 feet per minute with an airspeed of 0-20 MPH;
      4. apply power;
      5. observe symptoms and rate of descent as appropriate to type; and
      6. recover by transition to forward flight, or entering autorotation.
    5. Stress that avoidance is preferable and that by being constantly aware of W/V and rates of descent, it should be possible to prevent it from occurring.

Confirmation

PRE-FLIGHT BRIEFING

AIR LESSON
    1. Demonstrate incipient (and developed if desired {NOT RECOMMENDED}) vortex ring together with safe recovery procedures.
    2. Student practice.

POST-FLIGHT DEBRIEFING

TIPS FOR INSTRUCTORS
    1. This exercise need not necessarily involve a separate air lesson but can be combined with another dual flight.
    2. The airflow conditions which give rise to the formation of vortex ring will only occur if the following are present:
      1. the helicopter has induced flow passing down through the rotor system (IN POWERED FLIGHT);
      2. there is an external flow directly opposing the induced flow (HIGH RATE OF DESCENT); and
      3. the indicated airspeed is low (DOWNWIND OR CALM WIND CONDITIONS).
    3. Brief the student on the following symptoms of vortex ring:
      1. judder and stick shake;
      2. random yawing off heading;
      3. rapid increase in rate of descent;
      4. cyclic stick less effective; and
      5. random rolling and pitching.
    4. Students should avoid situations likely to cause vortex ring state, or if this is impossible, restrict the rate of descent to a low figure (200 FPM) when the airspeed is low (between 0-15 kts). This is because the most likely flight conditions occur when within 500 feet of the ground, when recovery techniques are unlikely to be successful.
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