Saturday, December 1, 2012
Leading Edge: Ground Effect Revisited
There will always be aviation incidents or accidents due to circumstances beyond the control of the pilot. Unforeseen component failure has terminated many a flight prematurely. But some of the hardest accident reports for me to read are the ones that involve true pilot error—the type of error that can come from years of incident-free flying bringing insidious complacency or oversight.
Student pilots go to great lengths to learn lessons involving physics and aerodynamics for their checkride, and then something happens. They pass the ride, and some of them never revisit those lessons again.
To a great extent, staying airborne comes down to this: power required versus power available. It is a concept that should not be overlooked in recurrent training. Unfortunately too often, not being mindful of this ends up as a causal factor in many helicopter accidents, when the former exceeds the latter.
To a similar extent, the environment that hovering helicopters operate in comes down to this: out of ground effect (OGE), versus in ground effect (IGE). “Ground effect” is an aerodynamic condition that reduces the power requirements of the main rotor when it is operating in close proximity to the ground. It is a physics lesson in action as it can readily, and sometimes acutely demonstrate the power required versus power available concept.
The benefits of ground effect begin at the surface, are greatest when the helicopter is closest to it, and are completely gone by about 1.5 rotor diameters above it. Its strength and height is variable, depending on atmospheric conditions, surface conditions, disk loading, and rotor-to-ground distance. Smooth flat surfaces provide the most benefit, while tall grass or bodies of water rob you of it.
For any helicopter to hover it must produce enough vertical lift to equal its weight no matter what the condition. How hard the engine must work to turn the rotor and create that lift force is measured as power required.
Consider this: most of the power required to hover comes from overcoming induced drag (drag due to lift). At subsonic speeds, air molecules behave nicely with one another, telegraphing their intent to their surrounding peers. As a result, when the rotor approaches to within 1.5 rotor disks of the surface, the air hitting the ground has an effect on the air above it by slowing the induced flow. Slowing the induced flow lessens the rearward tilt of the resulting lift vector, which lessens induced drag. This means that for a given condition, less power is required to produce the same amount of lift IGE, as would be required OGE. Realize that this does NOT mean there is more power available for the pilot to use!
This may seem obvious to most, but I wonder how apparent it was to the pilots that have successfully picked up into a hover on a pier or elevated helipad, nearly at or slightly over gross weight, only to find out that the very same helicopter required more power to hover, beyond what was available, once they moved off the platform?! Sadly, some are no longer here to explain their thoughts.
It can take over 10 percent more power to hover OGE than IGE. As an exercise, on a day with little or no wind, you can make what flight testers call a “hover ladder” and see this effect for yourself (please realize that you will be in the shaded area of height-velocity diagram). Get the helicopter steadied after just breaking ground and note the power setting. Now increase power by a couple percent. The helicopter will begin climbing but will stop at a slightly higher altitude. Note this increased power setting and altitude. Continue adding power incrementally, wait for the helicopter to come to a stop, and each time note the increase in power required versus the increase in altitude. At some point, the power required to hold a new altitude will become a constant. Any additional power will cause the helicopter to continue upward at constant rate. The altitude at which you first reached this constant power setting marks the transition from IGE to OGE.
It is entirely possible to obtain a combination of hot, high, or heavy conditions where you have IGE power available, but your OGE capability is nonexistent. In that case, it’s not hard to imagine the rapid evolution of events. A “routine” flight with many successful takeoffs in the past might preempt a weight and balance check, or reference to any hover ceiling chart. But on the day the scales literally tip in the wrong direction, a power requirement that critically exceeds the power available will ultimately be answered with a drooping rotor, loss of lift, and possible catastrophe. To every extent possible, remember to revisit the basics and make it your business to know the limits of your aircraft. Everyone is depending on it.