Actually, if you fly airplanes at the student pilot level or better, you already know what's making it fly because you already know how airfoils work. That helicopter flies for the same reasons that your airplane does - it just applies those principles differently.
For one thing, that set of blades whirling around on top is not a fan meant to keep the pilot cool, although you can bet that he or she will start sweating if those blades stop turning. No, those blades are a set of wings, and they work in the same way that airplane wings do: They create lift by means of relative wind and angle of attack.
The wings of a helicopter come in two basic styles - symmetrical and asymmetrical. Symmetrical blades are the more common type because their center of pressure moves less when the pitch angle changes. If I just lost you there, don't worry. We don't need to get that far into it. Just remember that symmetrical blades produce less lift and therefore don't have to be as strong as asymmetrical blades. Think of symmetrical blades as having a cross-section resembling an elongated teardrop - the same on both sides.
The RotorWay Exec 162F in which I train pilots uses the other kind of blade - asymmetrical. These are shaped more like the wings of an airplane and they do the same things - use relative wind and angle of attack to create lift. Asymmetrical blades produce more lift and have to be tougher. Blades like this have cut down a sapling four inches in diameter and then flown home.
So let's talk about how we get helicopters with either type of rotor blades off the ground. In an airplane, when you want to get some relative wind moving across the wing, you need to get the entire airplane moving. That may be the biggest difference between fixed-wing and rotor-wing (please don't call them fling-wing) aircraft. In the airplane, you advance the throttle and start moving down the runway. The wings get more air flowing across them, and the Bernoulli effect - decreasing air pressure above the wing as airflow over the wing increases - starts to become evident.
The helicopter needs to achieve that same relative wind to get it into the air. Since the helicopter's wings are moving in a circle above the aircraft, the pilot just needs to make the wings - and not the whole aircraft - go faster. The helicopter pilot adds throttle, the rotor (wing) speeds up, and the result is decreasing pressure on the upper surface of the wings, just as in an airplane.
Of course, helicopter and fixed-wing pilots alike would have to get their wings going mighty fast if we were using nothing but Bernoulli effect to get us into the air. So now we're going to change the angle of attack.
In an airplane, you do this by allowing the nose to rise as you gain speed, thereby changing the angle of attack of the wings and of the entire aircraft along with them. Helicopters handle this a little differently. They use a flight control called a collective to change the pitch angle of the blades relative to the plane of rotation. The collective is the control in the pilot's left hand, and it moves either up or down. It works by changing the pitch of each of the blades by the same amount at the same time, or collectively. The fact that there's relative wind moving across the blades also causes a change in the angle of attack, and the result is more lift.
So there we are - you in your airplane and me in my helicopter - both heading away from the ground, using different methods but the same principles to fly. Are you with me so far? Good, because it's going to get just a little more complicated.
Helicopters wouldn't be of much use if they could only go straight up. In hovering flight, the lifting force is moving in exactly the opposite direction of gravity. We're going to have to tilt that lift in some direction in order to go somewhere. When we do, we're going to sacrifice some of that lift in order to get some traveling to happen. In other words, we give up some of our upward motion in exchange for some forward motion. Here's how.
We're going to tilt the entire plane of rotation of the rotor relative to the rest of the helicopter by use of a control called the cyclic. This is the control in the pilot's right hand, and it looks like a stick from an old-fashioned airplane.
An interesting thing about helicopters is that the collective and the cyclic both do the same thing - they change the pitch on the main rotor blades. The difference between them is how they do it. While the collective changes the pitch of all of the blades at the same time and by the same amount (collectively), the cyclic changes the pitch of the blades as they move along their rotation track (cyclically).
When we want the disk described by the rotor blades - let's call it a rotor disk - to tilt forward, we move the cyclic forward. As the blades travel about their cycle, in one place a little pitch is subtracted, causing that blade to produce less lift and fly down. At exactly the same time and on the other side of the disk, the pitch is being adjusted to give the blade more lift so that it flies upward.
The places where the pitch changes are probably not where you'd expect them to be - along the centerline of the helicopter. They would be except for a little thing called gyroscopic precession.
Remember that a quick pitch change in your airplane causes a yaw movement? When a lateral force is applied to a spinning body like your propeller or my rotor disk, the resulting movement actually occurs 90 degrees later in the plane of rotation. So when we want the rotor disk to tilt forward, the pitch is added 90 degrees before the blade passes over the nose.
Now, back to the heart of the matter. When we want to go somewhere, we tilt the rotor disk in the direction that we want to travel, and some of our lift is spent in that direction. This produces some effects that you don't have to deal with in an airplane.
In straight-and-level flight in an airplane, when your airspeed indicator shows 80 knots, you know that your wings are getting about 80 kt of wind across them, and you can be pretty sure that they're both producing about the same amount of lift.
When my helicopter's airspeed indicator shows 80 kt, the fuselage is getting 80 kt of wind up the nose, but the blades are not. The blade that is headed into the wind is getting that 80 kt plus the speed at which it is rotating. At the same time, the opposite blade has that 80 kt subtracted from its rotation speed. You can see that we're getting into a problem here, right?
The blade that's headed into the wind is going to produce a whole lot more lift than the other one. To keep the helicopter right side up, we're going to have do something about it. What we're going to do is allow the blades to flap. Flapping means that the blade that is headed into the wind doesn't just go forward, it also goes up - just as your airplane would if it encountered an 80-kt gust. More relative wind equals more lift, right? When the blade flaps up, the upward movement is factored into the forward movement (relative to the wind) and the resulting forward-and-upward movement causes a decrease in the angle of attack, reducing the excessive lift.
On the other side of the disk, the blade that just experienced an 80-kt decrease in relative wind doesn't have as much lift as it did a moment ago. And like everything else, including your airplane, when it loses lift, it comes down. As the blade flaps down, the downward movement is factored into the blade's relative movement into the wind, and the result is an increase in angle of attack.
So you can see that what the advancing blade gains in the amount of relative wind, it loses in decreased angle of attack. The lift that the retreating blade loses because of decreased relative wind is made up by increased angle of attack. Lift is maintained equally on both sides of the rotor disk, which is why helicopters don't roll over when they go forward.
And that's more or less how helicopters fly - the same way that airplanes do. Of course, I've left a few things out. For example, the scenario that I described applies to two-blade rotors. And we haven't even begun to talk about autorotation, translational lift, transverse flow effect, and the rest. But that's OK. It's just like learning to fly - one thing at a time.
Darren Raleigh is chief pilot at RotorWay International.
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