Sunday, May 1, 2016
Vortex Ring State: That Sinking Feeling
I guess the difference between flying and crashing is all relative. By relative, I am referring to relative motion. Of the airflow, that is.
In its normal propeller working state, the helicopter rotor produces thrust by accelerating a mass of air downward, dutifully confirming Isaac Newton’s teachings. This downwash will have a velocity that depends on the aircraft’s gross weight, the rotor disk’s diameter and the air’s density (i.e. disk loading).
Problems occur when the downward speed of the helicopter matches the rotor’s downwash velocity (like driving at highway speeds next to another car doing the same thing; you appear not to be moving). This is the case with the air mass that is trying to accelerate through the disk away from the surrounding air when a helicopter descends at a similar rate to its own downwash.
With power applied, a low or zero airspeed, and a descent rate between 50% and 150% of the particular helicopter’s downwash speed, the air sees little relative downward motion. It is simply pushed outward, wraps around the rotor tips and gets re-ingested, creating a donut-shaped ring of air. Lift is lost quickly, and the bottom falls out. Congratulations, you have entered VRS.
This state only exists when all three of the above conditions are met. Low descent rates keep rotors in propeller working state and out of VRS. If descent rates go exceedingly higher, then the freestream air upflow through the rotor simply puts it into windmill brake state, or autorotation — again, where VRS cannot occur. For example, a Robinson R22’s descent rates lower than 700 fpm and higher than 2,100 fpm, and a Sikorsky Aircraft H-60 Black Hawk’s descent rates lower than 1,200 fpm and higher than 4,000 fpm would keep you out of VRS.
Pilots are generally taught to initiate recovery by reducing collective and lowering the nose 20 to 30 deg, allowing the helicopter to fly straight out of the condition. Given enough time, space and altitude, this recovery will work.
Consider the regimes of flight in which VRS is often encountered: downwind approaches to landing, excessively “hot” approaches with an aggressive flare and “pitch pull” at the bottom and long-line or vertical reference work with a lot of fore-aft, left-right translating movements. All can cause an unhealthy vertical descent rate and ingestion of downwash. In these, recovery with the traditional method may not be possible in time.
This brings us to the “Vuichard Recovery,” named for Claude Vuichard, an inspector for the Federal Office of Civil Aviation in Switzerland. Vuichard developed this technique after years of performing long-line work in the Alps. As an added surprise during Tucker’s presentation, Vuichard himself was there to further explain his findings.
For counterclockwise-rotating rotors, a Vuichard recovery is initiated with 10 to 20 deg of right cyclic bank and a simultaneous increase of collective to climb power while applying left pedal to maintain heading. This will allow you to sidestep out of VRS into clean air. (For opposite turning rotors, such as that of the Eurocopter AS350, the technique is done with application of left cyclic with right pedal.) When the rotor reaches the vortex’s upwind, recovery is completed. With practice, the technique is said to produce an altitude loss of only 20 to 50 ft.
You can see why this is a viable alternative. It combines the translating tendency of tail-rotor thrust with a sideward-angled lift vector to maximize sideforce and push the helicopter out of VRS.
Since there is no real “front” to the rotor, recovery in any direction is possible. However, the traditional, straight-ahead recovery can be a disadvantage when entering VRS due to a tailwind condition, as downwash will move in the same direction of recovery and delay it. True, lowering the collective will increase descent rate and decrease vortex generation, both of which can move you out of VRS. But this also moves you closer to the ground.
The science behind a topic such as VRS cannot be done justice in such a short column. The virtues of Vuichard’s alternate recovery give pilots another tool in their toolbox and may even leave many of us wondering, “Why didn’t I think of that?”