Flying Bell 407
From Fly! II simulator's documentation by Terminal Reality Inc.
It might be interesting to read how to fly a helicopter. Just ignore all those "Press Ctrl-PgDn" etc.
1. Rotary-Wing Aerodynamics | 2. Cockpit Tour | 3. Let's Fly ! | 4. Getting Back Down
1. Rotary-Wing Aerodynamics
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INTRODUCTION TO ROTARY-WING AERODYNAMICS
You're about to embark on an entirely new area of flight: the helicopter. Although flying a helicopter may not be inherently more difficult than flying a fixed-wing airplane (after all, look at all the high-school grads who learned to fly them very well during the Vietnam era), it can be very different. In fact, it can be so different that, given the choice, many helicopter instructors (including me) find it easier to teach someone to fly a helicopter from scratch than to cross-train a fixed-wing airplane pilot. That being said, however, there are quite a few parallels—particularly if you look at the helicopter in the most literal interpretation of its generic name: a rotary-wing aircraft. Like any other heavier-than-air machine, a helicopter flies because of its wings. Taken individually, a rotor blade is nothing more than a skinny wing flying at a very high airspeed; and, taken individually, it obeys exactly the same aerodynamic rules and principles as any other wing. Like any other wing, it can glide without power if necessary; like any other wing, it can be stalled. It's when we examine this wing in the context of a helicopter, on which it's spinning around, that things become a bit more complex; and they become more complex yet when we start looking at the helicopter as a whole, and the way its various dynamic components interact both with the atmosphere and with each other. A LITTLE HISTORY The dream of a heavier-than-air machine capable of hovering motionless in the air has been with us at least since the days of Leonardo da Vinci (if not, indeed, since the first time some caveman watched a hummingbird). With the appearance of the gasoline engine in the late 19th century, a power source was finally available that was both light and strong enough to allow for some serious experimentation. By the time of the First World War, a few experimental test rigs (with rigid rotorlike horizontal propellers) had actually managed to heave themselves a couple of feet off the ground for a few moments at a time. Some could even hover, after a fashion—but, invariably, as soon as they attempted to move horizontally at any kind of speed, they became uncontrollable. It wasn't until after World War I that a Spaniard, Juan de la Cierva, realized that the blades of any sort of lifting rotor or propeller had to be able to deal with very widely varying airspeeds as the whole assemblage moved through the air. His innovation of a hinge, allowing individual blades to flap up and down while rotating, allowed the construction of aircraft called "autogyros," which flew with a combination of unpowered rotors and airplane propellers. While capable of flying quite slowly, they still could not hover, or take off or land vertically. The step to actual helicopters came with the realization that it would be necessary to control each blade individually. Both the US and Germany had workable helicopters by World War II, although their use in combat was negligible, and development of helicopters has continued unchecked since then. A SPLIT PERSONALITY In a sense, a helicopter behaves like two entirely different aircraft, depending on whether it's hovering or flying at appreciable speed (which is called, in helicopterese, in translation.) When it's hovering or flying very slowly, you could equate its rotor system to a group of very small airplanes, flying in very tight circles. Once it gets some speed, however, it's more like an airplane suspended beneath a single circular wing (although one with some odd characteristics). Dealing with the change between these modes is one of the challenges of helicopter design and piloting. Let's look at the hovering situation first. GOING NOWHERE The helicopter's rotor system is, indeed, similar to a very large propeller, and it runs at a constant speed. Like a constant-speed propeller on an airplane, the amount of thrust it delivers to the surrounding air is controlled by varying the pitch of its blades, all at the same time. This is called collective pitch control, and it's controlled by a cockpit lever called, appropriately enough, the collective. It's mounted along the left side of each pilot seat, somewhat like a handbrake lever; pulling it upward increases blade pitch.
Of course, if this rotor were just spinning in a rigid horizontal plane atop the aircraft, and doing nothing more than supporting its weight, the helicopter would be completely at the mercy of any movement of the air around it—it would drift about uncontrollably, like a balloon (as, in fact, you'll probably find it doing on your first attempts at hovering)! What's necessary is some way to tilt the rotor disc slightly, so that a small component of its lift can be directed in any desired direction, whether to keep the helicopter in one place (hovering), or to accelerate it in a desired direction (forward, backward, or sideward flight).
This is achieved by changing the pitch of each rotor blade, one at a time, as it sweeps through the applicable part of the circle. (For example, to fly forward, we'd want to tilt the rotor disc in that direction, so we'd want to decrease the pitch of each blade as it sweeps through the forward part of the circle, and increase it as it sweeps through the rear part.) Since this pitch change occurs once per circle, or cycle, for each blade, it's called cyclic pitch control, and the cockpit control that affects it is called the cyclic (pronounced either "sigh-click" or "sick-lick"—take your choice!) and mounted vertically in front of the pilot. (Actually, since the whole rotor system acts like a big gyroscope, the desired pitch change is applied 90 degrees before the point at which it finally occurs.) The way this is done is by means of a device called a swashplate. We don't need to go into too much detail here; suffice it to say that it's a device on the rotor mast to which the pitch control links of each individual blade are attached. Moving the cyclic in the cockpit tilts the swashplate in the required direction, changing the pitch of each blade in turn, while moving the collective moves the entire swashplate up and down, changing the pitch of all the blades at once. Do you start to understand why a rotor hub looks so mechanically complex? LET'S (NOT) TWIST AGAIN... Of course, keeping the rotor turning means that the powerplant is putting out a lot of work—and since the helicopter isn't attached to anything, Newton's Third Law tells us that the engine is working just as hard to turn the rest of the helicopter the other way! Why doesn't the whole thing just spin round and round? (Early ones did, until they figured this part out!) Because of the efforts of the antitorque rotor, located at the tail of the helicopter and often called the "tail rotor." This closely resembles an airplane propeller, and works exactly the same way, producing thrust to offset the torque that's constantly trying to spin the helicopter around. Like the main rotor, it runs at constant speed (in fact, it's geared to it mechanically), so thrust is controlled by varying its pitch. The cockpit control for this is the pair of antitorque pedals.
ONE GOOD TURN DESERVES ANOTHER... Just to keep things interesting, the amount of power (and hence engine torque) required by the main rotor system varies constantly. If you want to climb, for instance, you'll pull in more collective. This applies more power to the rotor system and, in turn, increases the torque that's trying to turn the helicopter the other way. Thus, in an American helicopter, an increase in power (collective setting) also requires an increase in left pedal deflection, and vice versa. (Many European and Russian helicopters turn their main rotor the other way, so they'd require right pedal with a power increase. There's no particular advantage to one way or the other—it's just one of those weird things, like driving on the right in England.) MOVING OUT Things become even more complex when we start to fly the helicopter in any particular direction at a speed faster than, say, a brisk jog. Let's say we're flying forward (the most common direction, after all) at 100 knots. Now, even in a hover, the helicopter's little "wings" are clipping along at quite an airspeed. In the Bell 407, for example, at normal RPM the airspeed at the tips of the blades is almost 450 knots! When we're moving forward, though, the forward-going blades on the right side of the helicopter "see" a higher airspeed (550 knots at the tip), while the aft-going blades on the left side "see" a correspondingly lower airspeed (350 knots at the tip).
Obviously, the side with the forward-going blades is going to produce a lot more lift than the one with aft-going blades—and this is why the first attempts at helicopters would invariably fall over (to the aft-going side) as soon as they moved off from a hover. It wasn't until Cierva's flapping hinge that rotor systems could accommodate this asymmetric lift. But wait...there's more... Once a helicopter is moving at decent speed, its motion through the air produces additional lift over most of the rotor disc as a whole. This is called translational lift, and it's why it takes much less power to support a helicopter in forward flight than in a hover. Where you'll feel this is on both takeoffs and landings. As you lift into a hover, then gradually begin to gain speed, you'll feel the helicopter suddenly "come to life" and gain performance as it moves into translational lift. Similarly, as you begin slowing up toward a hover during a landing approach, you'll find yourself needing to add quite a bit of power as the helicopter begins to settle out beneath you. (If this proves to be more power than you have available at some particular combination of altitude and temperature, you're in trouble.) Don't forget, too, that every power change requires its corresponding change on the antitorque pedals. DON'T FLY TOO FAST The faster you fly forward, the greater this dissymmetry of lift becomes, and the more cyclic pressure you'll need (to the right on American helicopters) to counteract it. Before you'd run out of control, however, something even more significant starts to occur at excessive speed. Remember, our forward speed is added to the rotor speed on the forward-going side, subtracted from it on the aft-going side. At some speed, we'll reach the point at which so much speed is being subtracted from the aft-going blades that they begin to stall. This is called retreating blade stall, and it's what sets the absolute upper speed limit for just about any helicopter (other factors may set lower limits). If you ever are so foolhardy as to seek it out, the first signs will be very heavy vibration, followed by a tendency to roll toward the stalling side (left in American helicopters). Since the air is thinner at high altitudes, a higher blade angle of attack (more collective) is required to produce the same amount of rotor thrust. This reduces stall margins, which is why most helicopters (including the 407) have restricted high speed ranges depending on altitude. WHAT IF IT QUITS? This is the question that's almost invariably asked by the uninitiated—and their expectation is that if the engine fails, the helicopter will drop like a stone. Luckily for us, it won't. Remember, the total aerodynamic force produced by any wing, whether fixed or rotary, is composed of both a lift and a drag component—and remember that if the relative wind is coming from below the chord line, the lift component is directed at least slightly forward. That's how a fixed-wing airplane glides—and if you think of rotor blades as little airplanes flying in a tight circle, there's no reason they can't glide in a circle, either!
What's critical, in an airplane as well as in a helicopter, is to reduce the angle of attack to prevent the wing (or blade) from stalling. In an airplane, that would mean lowering the nose; in a helicopter, it means to immediately reduce the collective all the way to the bottom of its range. Sure, the helicopter will descend—but it won't "fall out of the sky," but descend relatively gently. During this descent, the air flowing upward through the center part of the rotor, where blade speeds are relatively low, drives the blades and keeps them turning (while still contributing its share of lift). Out near the tips, where blade speeds are higher, even more lift is produced, although the blades themselves are also producing drag. This situation is called autorotation, partly because the blades are "turning themselves" and partly because, as old instructors like to say, they "ought'a rotate if you do everything right." The inner part of the blade is now considered the driving portion, while the outer part is considered the driven portion. We'll cover autorotations in much more detail once we've gained a little experience with the helicopter. For the moment, let's get into the cockpit and start looking around. |
1. Rotary-Wing Aerodynamics | 2. Cockpit Tour | 3. Let's Fly ! | 4. Getting Back Down
