The ground lesson is partly about energy management in the event of an engine failure and partly about not getting the helicopter into unusual attitudes.
As with airplanes, much of the key to safety in a helicopter is energy management. In an airplane you have potential energy (altitude) and kinetic energy (forward speed) that can be traded off against each other to bring the airplane down gently in the event of an engine failure or ordinary landing. The helicopter has three kinds of energy: potential (altitude), kinetic (forward speed), and angular momentum (blade speed).
In an airplane you can make decisions about trading forms of energy very late in the day. For example, if you pull the stick all the way back at 6000' above the ground you will gradually slow down and eventually stall and perhaps enter a spin. With many airplanes you could spin nearly all the way to the ground before applying forward stick and opposite rudder to get back to a normal flight condition. All without an engine.
In a helicopter, by contrast, if the blades spin down more than 10-15% from their normal velocity, there is no way to convert potential or kinetic energy into spinning such that the helicopter will start to fly again. If you can't restart your engine, therefore, your helicopter can very quickly become a rock.
In a turbine-powered helicopter like a Bell 206 JetRangers the blades are heavy and the blades won't slow down for several seconds after an engine failure. In the flyweight Robinson, however, after an engine failure you have no more than 1.2 seconds to take exactly the right actions or the helicopter cannot be recovered.
What if you do take all the right actions? Suppose that you're up at 4000' and the engine quits. You lower the collective pitch (lever on your left) immediately to flatten the blades and allow them to be driven by the wind through which the helicopter is now falling at 2000 feet-per-minute. You adjust the cyclic (stick in front of you) for about 65 knots of forward speed. You aim for a landing zone. The good news is that you don't need a very large one but the bad news is that the glide ratio is between 2:1 and 4:1 instead of an airplane's 10:1 and therefore you don't have as large an area from which to choose. As you get within about 50' from the ground you pull back the cyclic to flare the helicopter and shed most of the forward speed. Just as in an airplane this flare also arrests most of the vertical speed. At the second to last moment you stop flaring and return the helicopter to being parallel to the ground. Ideally at this point you are hovering 5' or so above a soccer field and the blades are still spinning. Finally you raise the collective as the helicopter falls, using the stored energy in the blades against the force of gravity. You land gently on the skids. (In practice the cyclic flare is more important than the "hovering autorotation" at the end; a lot of people walk away from helicopter engine failures if they get the cyclic flare right but can't manage to pull the collective smoothly at the last moment.)
This all sounds good until you look at the "deadman's curve". The marketing literature for helicopters says "if the engine fails, you can autorotate down to a smooth landing." The owner's manual, however, contains a little chart of flight conditions from which it is impossible to landing without at least bending the helicopter. Unfortunately these conditions are the very ones in which nearly all helicopters seem to operate. If you're above 500', for example, you're pretty safe. But TV station helicopters are often lower than that when filming. Flying along at 65 knots at any altitude is safe, but if the camera needs the pilot to hover the helicopter slows to a crawl and may get under the "height-velocity diagram", as the FAA calls the deadman's curve.
No comments:
Post a Comment