| Vortex - Meatloaf |
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It's Just Rock ‘n' Roll By Jannie Matthysen "I want you, I need you, but there aint no way I'm ever gonna love you..." Meatloaf. Why are we singing rock if we need to be talking helicopters? Meatloaf is definitely oblivious to the fact that his song called "Two out of three aint bad" kept this pilot from hitting the rocks more than once... let's find out why. The Americans call it "settling with power" while the term "vortex ring state" sends shivers down the spine of many helicopter pilots outside the USA. The universally accepted definition of vortex, as we commonly refer to it, is the rapid loss of height associated with random pitching, yawing, and rolling. It sounds simple enough, but after many years of training pilots, conducting flight tests, and generally just asking silly questions, I've realised that this is one of the most misunderstood phenomena in helicopter aerodynamics. Before we plummet into a vortex ring state, we need to get the helicopter in a condition of flight where three factors affect the airflow over the main rotors. These are: 1. Power "on" 2. Low airspeed 3. High rate of descend. So far so good. The combination of these factors cause random yawing, pitching, and rolling accompanied by an increase in the rate of descent. Any attempt to reduce the rate of descend by raising the collective (increasing power and main rotor pitch angle), only exaggerates the rolling, yawing, pitching and plummeting from the sky! Does this sound like rock ‘n' roll or what? Pitching, rolling, yawing, and falling are the last things you need when trying to land a helicopter. Clever aerodynamicists, rocket scientists, and other ‘cists will get technical and tell you that the rotor system is affected in two regions during vortex, namely at the root, and at the tip. The angle of attack on any helicopter blade is larger at the root than at the tip. The value of induced flow is quickly matched by the velocity of the airflow from below. This causes the root area to stall. At the same time, vortices around the tips increase in intensity, causing the angle of attack to reduce dramatically. The end result is a near-complete loss of lift at the tip. This leaves only a very small section of the rotor blade to produce the required lift and keep the plummeting craft from a nasty impact. Let's keep it simple. A helicopter in flight creates a downwash of air as the rotors turn, and with a positive rotor blade pitch angle displaces a column of air downwards. This constant downward movement of air sucks more air into the rotors from above and thereby forms a giant vortice. This downward movement of air is called induced flow. All is good until this induced column of air collides head-on with an opposing column from below caused by a high rate of descent. However, these columns collide only when the airspeed is low enough for the helicopter not to fly away from its own downwash. This is about as much as the average helicopter jock can tell you about vortex, but the subtle details make all the difference. As Mr. ‘Loaf said: two out of three aint bad". We need all three factors to create vortex! The power "on" part creates the downward velocity or induced flow. The high rate of descend causes the opposing rush of air, and the low airspeed ensures that all this havoc occurs around the rotor system. A silly question: why don't we get vortex during autorotation? We have a low forward speed and high rate of descent? The absence of the power "on" part says that we won't. In a state of autorotation, there is no induced flow because the rotors are driven solely by the airflow from below caused by the high rate of descend. No vortex. Let's look at the practical implications. A classic scenario where vortex has threatened many of us is during a badly planned and executed approach to landing. I start the approach a little too high and too fast. No problem, I just lower the collective a bit more. Increased rate of descend. It's not looking any better yet; maybe I should flare a bit. Reduced airspeed. Now it looks like I'll make it. I bet this steep, fast approach looks great to those mortals on the ground watching this master at work. Whoa!! The ground is rushing towards me a bit faster than I expected! Pull collective... and lots of it! Induced flow. What's that vibration? Why aren't we slowing down? What the... Three out of three IS bad! Now that we are experts in creating vortex, how do we get out of it? Simple. We only need to take away any one of the three factors that caused it in the first place. The easiest way is to apply a small amount of forward cyclic in order to increase airspeed. Forward cyclic movement reduces the flare, which is in itself not a bad thing. In addition to cyclic input, most textbooks tell us to also lower the collective. In my opinion; easier said than done. It goes against our basic will to lower the collective close to the ground while rushing towards the earth in an excessive vibratory descent. By lowering the collective, we are also increasing the rate of descend - the last thing we need at the time. Apart from the factors already discussed, several other nasties plot in order to increase our chances of flying into vortex. High density altitudes (high temperature, humidity, and altitude) cause thin air and, obviously, less efficient aerodynamic performance from the rotor system. High gross weight puts the helicopter near the limits of its performance envelope. What are our allies in this battle against vortex? Knowledge! Know that an approach directly into wind effectively increases your airspeed and delays the onset of vortex. Know that slow, smooth control inputs cause a gradual aerodynamic change where it is easy to anticipate and correct for any signs of vortex. Know your helicopter. All helicopters communicate a very distinctive vibration when the airflow over the main rotor is disrupted. Get familiar with how the type you fly communicates with you. And most importantly: know your rock ‘n' roll, ‘cause "TWO OUT OF THREE AINT BAD." |
