How to Avoid a Vortex Ring State, Part 2

Trim Strings provide information of the VRS envelope

The trim string hangs straight down during hover and low speed flight due to the rotorwash’s prevailing downward motion. This provides vital information to a pilot of the proximity to the VRS envelope. Photo Credit: Vuichard Recovery Aviation Safety Foundation

This is part of a three-part article series. The first part discusses trim strings.

When learning about Vertical Ring State (VRS), another significant revelation is the sudden low-g flight condition (0.2-0.8 g) at the initial entry into VRS. Under the mildest conditions, a pilot will feel a lightness in the seat. This is likely the downward acceleration that a pilot encounters when practicing VRS recoveries under the carefully controlled conditions during training.  

In contrast, the real world often doesn’t mimic the carefully controlled conditions of the training environment. An inadvertent entry into VRS often results in an abrupt downward acceleration. I had not been trained in the Vuichard Recovery so I sought out a flight school that said it was qualified to teach this technique. The flight instructor in a Robinson R22 stated, “Let me show you a REAL vortex ring state.” Suddenly it felt as if we were in an elevator whose suspension cables had been cut. The abrupt downward acceleration was unlike any sensation I had ever experienced in an aircraft before. None of the ground training devices used for spatial disorientation training such as a Barony Chair simulate this abrupt downward acceleration. 

One positive aspect of this horribly botched “training maneuver” was that it helped me realize the significant human factors with VRS. In a terrestrial environment under 1g, the human body is well adapted to maintain the perception of the correct orientation provided by the body’s visual, vestibular and proprioceptive systems. Conversely, during flight, the sensory systems are poorly suited to these abnormal accelerations and can easily trick the body’s balance and motor mechanisms. This abrupt downward acceleration opens up a Pandora’s Box of visual, vestibular and proprioceptive illusions that can hinder a pilot’s ability to make a timely recovery.   

Other symptoms of a VRS are the lack of response when increasing power by pulling the collective, and the helicopter exhibits random uncontrolled pitch, roll and yaw oscillations. As soon as the helicopter transition from a hover into a descent, the blades will hit the vortices of the preceding blades, which causes vibration that can be felt and heard in the helicopter. The cyclic will shake and has less control authority.   

A fully developed VRS is signified by a reduction in the vibration level of the main rotor when the induced flow circulates in a closed circuit and the rotor blades encounter less turbulent air. This will also be accompanied by a low frequency of random buffeting.

Descent Rate

The combination of airspeed and descent rates that form the VRS envelope are specific to each helicopter make/model. In general, helicopters with higher disk loading will produce higher induced velocities. Therefore, the envelope of airspeed versus rate of descent (ROD) will generally be larger, and secondly, the descent rate envelope for VRS will begin at a faster rate. 

The video compares and contrasts the VRS entry parameters for three categories of disk loading.  The R-22, which has a disk load of approximately 4 psf (pounds per square ft.), is in the low disk loading category. The H125, with a disk loading between 4-6 psf, falls within the medium category, and the AW139 fits the high disk load category with 6-10 psf. 

For example, at sea level at the maximum takeoff weight, the range of descent rates for entry into the VRS envelope in the R-22 are 500-2,000 fpm. The H125’s range is 800-3,000 fpm. The heavily disk loaded AW139’s range is 1,200-4,000 fpm. 

A vital revelation is the immense descent rate that occurs when a helicopter enters into a full VRS. At sea level and the maximum takeoff weight, an R22’s descent rate in a fully enveloped VRS would be 3,600 fpm. This equates with losing six building floors in a second.  

By the way, the R22 flight instructor’s “recovery” wasn’t effective until we were less than 300 ft. above the ground. Ironically I had asked this flight school to teach me the Vuichard Recovery because I had been trained long ago in the conventional recovery technique. That experience certainly calls into question how a flight school can deem itself “specially qualified” to teach this recovery technique.  That demonstration placed us in a dangerous situation and resulted in negative training. 

This is yet another point that illustrates the safety and efficiency afforded by proper training in a flight training device whose software models the aerodynamics of the specific helicopter model. It allows students to safely practice these reflexes in repetitions to formulate the correct and nearly instantaneous control inputs in the incipient stage. The simulator can fly the helicopter into a VRS condition automatically to enable the pilot to practice recovering from VRS in a more efficient way.  

The heavier disk loading in an H125 results in a final descent rate of 4,900 fpm, which is the equivalent of losing eight floors per second. This equates to 53 mph. An AW139 with an even heavier disk loading would plummet out of the sky at 6,600 fpm, which is the equivalent of losing 11 floors per second or a speed of 74 mph. In basic terms, impacting the ground at these speeds is not likely survivable.

The descent rates worsen with altitude.  At a density altitude of 13,000 feet, the H125 plummets at a speed of 67 mph (equivalent of 10 floors in a second).  The AW139 would plummet at 94 mph (14 floors in a second). 

The emphasis in this section of the video is absolutely clear. Due to the tremendous sink rates in the VRS, it is essential to reflexively apply a recovery technique because every second lost results in an enormous loss of altitude. When practicing, recovery should be initiated at the first sign (lightness in the seat) in the incipient stage of the vortex ring.