The “Tales from Nick’s FARRP” series are a fictionalized version of real events and are dedicated to the memory of friends and classmates from the Class of 1969.
“SO, ROD, WHAT MAKES A HELICOPTER FLY, ANYWAY?”
I was talking with some regulars at the bar I had recently inherited from my late Uncle Nick. It’s known locally as “the FAARP,” some sort of Army aviation term. Just outside Fort Bragg, the FARRP is a hang-out for off-duty paratroopers and aviators, all Vietnam combat veterans who tell some crazy stories. Since I know nothing about the Army, these guys must get tired of my questions. Chief Warrant Officer Rod Jordan is an old-timer, a flight school classmate and for years a super buddy of my Uncle Nick, who has taken me under his wing since I arrived here in Fayetteville, NC.
“Well, New Guy, it’s really pretty simple,” Chief Rod replied to my question. “You surely know that a helicopter has an engine, right?”
“Of course,” I said. “I may not know much about the Army, but I’m not a total idiot.”
“Well,” he went on. “Maybe you didn’t know that the engine has a very important purpose. Its job is to make lots and lots of noise and vibration. Because the earth doesn’t like noise or vibration at all. So when the helicopter engine makes enough noise and vibration, the earth can’t stand it anymore, and it tells the helicopter to get away. So the helicopter comes up and starts to fly. And it keeps on flying as long as the engine makes enough noise and vibration. But if the engine ever stops, the noise and vibration stop too. So, the earth says, ‘OK, you can come back now.’ And down the helicopter comes.”
“Dang, Rod!” injected Major Tony Williams. “Gil was asking a serious question, and you treat him like a child!”
“All right, hot dog. You are such a smart ass, being a recent graduate of flight school and all. You tell the lad what you know about it.”
“OK, I will. Gil, the big rotor on top blows air down. That’s called rotor wash – the wind you feel when you are near a helicopter that is hovering. The helicopter has a tail rotor, which pushes the tail sideways to keep the aircraft from spinning. When a helicopter is hovering, it is really balanced on its rotor wash, which is extremely hard work for the aircraft.
“It actually takes more power for a helicopter to hover than it does to fly. So, the normal way to take off is to lift up to a hover, which ensures you have enough power. Then very gently you use the control stick to barely tilt the rotor disk forward, which makes the aircraft begin to drift forward, faster and faster. As the bird gets ten or fifteen knots airspeed, the rotor disk begins to overrun its turbulent downwash and bite into undisturbed air. When this happens, the effectiveness of the blades increases on the forward part of the rotor disk, so the disk tries to pitch up in front, tipping the bird back into its own turbulence.
“So as soon as you begin to feel the nose pitch up, you tip the rotor forward more, so the whole rotor disk slides completely into undisturbed air. This is called translational lift. At this point the helicopter begins to accelerate and climb out. Usually the power setting for a hover is enough to enable you to climb at cruise speed. Once you get to your intended altitude you can reduce power and maintain level cruising airspeed.”
“Wow,” I exclaimed. “I always thought helicopters took off straight up.”
“Well, they can,” replied Chief Rod, “but it’s pretty dangerous. It requires pulling in a lot more power, which increases the chances that your engine will quit. And if that happens, having altitude but no airspeed means most likely you will break the bird when you crash. And aviators always hate it when that happens.”
“Did you know that it’s possible for a helicopter to take off when there’s not enough power even to pick it up off the ground?” interrupted Major Tony.
“How could that even be possible?” I asked, very puzzled.
“In Vietnam, sometimes the Huey gunships were loaded so heavy with rockets, ammunition and fuel that they could not lift the skids off the ground. So the technique known as a ‘Charlie-model takeoff’ was developed. The crew chief and the door gunner would stand beside the loaded aircraft, plugged in to the intercom with long lines. The pilot would pull in max power until the rotor RPM just began to bleed back, making the aircraft light on its skids. Then he would ease the rotor disk forward, making the bird slide on its skids. With the crew chief and door gunner running beside the helicopter, the aircraft would gradually accelerate into the wind. When it reached translational lift, the helicopter would begin to pick itself up, but the pilot had to hold it just off the ground, staying in ground effect to gain enough airspeed to finally begin a slow climb out. The crew chief and door gunner running beside the bird had to watch the toes of the skids carefully, because as soon as the skids tipped up they had to jump into the bird, or get left behind.
“If that happened, their intercom cords would unplug. So, they had to call to the pilot immediately that they were on board, or else he had to abort the takeoff. Kept those guys in pretty good shape. It wasn’t a problem landing after the mission, because the birds were always lighter in fuel and ammo.”
“Well,” I concluded, “that makes sense, if you think about it, I guess.”
“But it’s even more complicated than that,” continued Tony. “When you are flying forward, the rotor blade advancing on the right side of the aircraft has more relative airspeed than the retreating blade on the left. The faster the helicopter flies, the greater the difference between the airspeed of the blade on the right and the left. The amount of lift a wing has is related to its airspeed, so the faster you fly the stronger the lift on the right side gets and the weaker the lift on the left side.”
Captain Kenny Wayne broke in. “Hey, maybe that explains why it’s so dangerous to parachute from the left door of a helicopter. I’ve made a bunch of chopper blasts, but always from the right door. Early on I watched a jumper ahead of me going out the left door of a Huey get caught in the turbulence, so his parachute static line got snarled in the skids of the helicopter. He was hanging helpless, face down under the bird.
“Since it was impossible for anyone to climb outside and try to free him, they had to fly the helicopter back for landing at a high hover, while people on the ground got him untied. If he had come loose from the skids while the descending helicopter was below 300 feet, there wouldn’t have been enough altitude for his parachute to open. In that case, he would have splatted into a giant pizza on the drop zone. So my personal rule is: Never jump the left door.”
“Thank you very much for that extraordinarily superb insight, Kenny,” Tony resumed explaining. “As I was saying, the helicopter would flip over if it didn’t reduce the right-side lift and increase the left-side lift. The aircraft does that by flattening the pitch angle on the rotor blade advancing on the right side and increasing the pitch on the left side to keep the lift balanced.
“Just one problem though. The blade on the left side already has less relative airspeed, and when you increase the pitch angle as the airspeed decreases, eventually you reach the point where the airflow across the top of the retreating blade breaks turbulent and you lose lift completely. That’s called a stall.
“The faster an airplane flies, the better the lift from its wing. But in a helicopter the faster you fly, the closer you get to ‘retreating blade stall,’ which means the rotor disk violently flips the bird left and back. That indicates the moment before the simultaneous finale of the flight, the aircraft and all those misfortunate enough to be on board. Bad news,” Tony concluded.
“Wow!” I exclaimed. “Sounds like there are a lot of ways to die in a helicopter.”
“That’s not the end of it, young lad,” continued Chief Rod. “When a helicopter flies slow enough, there is another hazard. You can lose translational lift, which means the rotor, instead of finding undisturbed air for lift, can wind up in its own turbulence. That’s called ‘settling with power.’ The sudden loss of lift is indicated by an instantaneous plunge. If you have enough altitude you can get out of settling with power by nosing the bird forward to get the rotor back into undisturbed air. But if it happens at low altitude, you will probably break the bird, and likely some of the people on board. We always hate it when that happens.”
Tony interrupted Rod. “In flight school they taught us a lot of simple rules which it really pays to remember. For one, crashes normally only happen at or near the ground. For another, it is always a good idea, before you take off, to have a plan for where you intend to land.”
“Gee,” I said. “No wonder people are scared of flying in helicopters. Too many ways to die.”
“Yeah,” Rod replied, “you could say that about a lot of things. But helicopters are way safer than fixed wing planes, as far as I am concerned. I’m a rated instructor in all of them, but airplanes scare the hell out of me. Not enough moving parts for my taste. Tony, what’s the very first rule they taught you in flight school?”
“That’s easy,” Tony replied. “The engine is mandatory for takeoff, optional for landing.”
“What does that mean?” I asked, astonished.
“It means, my fine young friend,” replied Chief Rod, “that your engine can quit on you any time it feels like, and probably will chose to do so at the very worst moment possible.”
“Well, doesn’t that mean your helicopter will crash when it does?” I asked.
“Quite the contrary, lad,” replied Rod. “The beauty of the helicopter is that, as long as the rotor is turning and you still have controls over it, you should be able to bring the bird down and walk away from it. We call it autorotation. You may be coming down pretty steeply, compared to the glide in an airplane, but you can still fly the bird. If you have already selected a fairly clear and level spot to land in, there is enough energy in the momentum of the spinning rotor to cushion your touchdown to the point that you should be able to walk away.”
“But how can you find a clear and level spot after the engine quits? What if there isn’t any close by?” I asked.
“Remember I told you the engine is guaranteed to quit on you? You should never be surprised when it does, because you are always picking out safe landing areas all the time you are flying. Tony, what always happens every time you fly with someone who is instructor-rated?”
“They’re going to chop the throttle on you, sending you into autorotation. At least once, every flight, guaranteed. After a while you learn to fly expecting the engine to quit at any time, so it’s never a surprise. You get to where you can do an autorotation in your sleep.”
“Now, New Guy,” Chief Rod concluded. “How did I originally answer your question about how a helicopter flies?”
“The engine makes noise and vibration that the earth hates. So, when the engine quits, that’s what you meant. Down it comes. Actually, I guess, that seems like a pretty good explanation after all.”
IN MEMORY OF BILL AND TERRY AND EDDIE AND JERRY
