Energy. “It cannot be created or destroyed, only changed from one form to another.” You should remember your high school physics teacher making a statement like this. You should also remember your primary flight instructor making a statement like, “Rotor speed is life.” The relevance of these two statements could not be more important, especially when the engine in your whirlybird stops.
Simply defined, energy is the capacity to do work. To make helicopters fly, the engine converts potential energy stored in fuel into rotational kinetic energy that powers the rotor system. The rotor then imparts its energy to the air by accelerating it through the disk, in order to create the thrust that provides airspeed and altitude. The rate at which this energy conversion must be done is measured as power required.
When the engine suddenly stops providing power, the rotor begins to feed on its own rotational energy, and RPM begins to drop. The decrease in thrust starts the helicopter descending. Now it is gravity and the act of falling that dictates the transfer of energy. Even in autorotation, the power required to turn the rotor and keep it “in the green” remains about the same as it was in level flight. However, instead of the rotor doing work on the air, it is the air that must do work on the rotor. Therefore, the power required can only be achieved by attaining a certain rate of descent. When the potential energy of altitude and kinetic energy of airspeed is used up, it is the kinetic energy of rotor speed that we will once again convert back to thrust, in the final moments before the touchdown that, without an operating engine, will happen.
With training and physics on our side, the odds are good that our touchdown will be successful; but sometimes fate and physics work against us. This is why engineers and test pilots have brought us the Height Velocity (H-V) diagram, or “Dead Man’s Curve.” While every pilot should know they don’t want to be caught in it when the engine goes silent, also knowing the basis for it can be helpful.
The H-V diagram starts off as a mathematical estimate by designers, who compute the inertial properties of the rotor as well as its torque requirements at certain speeds, weights and lift coefficients. This gives an idea of the time it would take for the rotor to decay to an unrecoverable RPM under certain conditions. Armed with this, they first calculate, then very cautiously confirm key points on the curve with an incremental buildup of flight testing.
The “knee” of the curve is the first of three points, and is of particular importance when building the avoid area. It is defined by a “critical” speed (Vcr), above which an autorotative landing can be made at any height. Surprisingly, for most helicopters the height at which this point occurs (hcr) remains approximately the same, between 80-100 feet. The second key point is the high hover point. This is the point at which one can expect to achieve steady autorotation and adequately flare to a safe landing from a zero airspeed condition. The third point is the low hover point. This point is dictated by the energy absorbing qualities of the landing gear and rotor inertia, since RPM will be decaying all the way to the ground. As speed increases from this point along the curve to Vcr, any flare will be ineffective and only add little, if any, energy to the rotor, bumping the height up to hcr.
It’s helpful to know the conditions dictated during H-V testing, as they are not always listed on the chart. For FAA certification purposes, the curve is usually developed in little, if any wind, at max gross weight and high density altitude conditions. There is a 1-second delay by the pilot before applying corrective action when demonstrating points along the curve above the knee, and no delay when demonstrating points below it. (Some military specs use 2 seconds all around!). All points are demonstrated from level flight, and therefore may be conservative in a descent, but overly hopeful if in a climb, when the failure occurs. In fact, variables such as pilot reaction time, actual aircraft weight, CG, technique, etc., will cause the H-V diagram to grow or shrink depending on the conditions you are at on that fateful day when things get quiet, so “your mileage may vary.”
One thing that cannot be over-stressed is that no matter how good the pilot, the deeper you are into the “avoid” area at the time of failure, the larger the mess will be when you reach the ground. The laws of physics cannot be outwitted, so respect and manage them the best you can.