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
4. GETTING BACK DOWN
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GETTING BACK DOWN Sooner or later we'll have to get the helicopter back onto the ground. A normal approach is nothing more than a gradual descent, gradually transitioning into a hover. First, however, we should get acquainted with something colorfully called the "dead man's curve." No, it's not that nasty switchback where someone went through the guardrail—it's more accurately called the "height-velocity diagram."
Here's a typical H-V envelope; it reads in skid height above ground from bottom to top, and airspeed from left to right. We'll get into energy management in more detail when we discuss autorotations further. For the moment, it's enough to know that as long as you operate the helicopter in the unshaded area of the chart, you should be able to make a safe autorotation (with average pilot technique) if the engine fails. If you're in the shaded area (for example, 100 feet off the ground at 20 knots), you can't make a safe autorotation no matter how good a pilot you might be. This isn't to say, of course, that helicopters are never operated in the "avoid" region; many types of operation (for example, takeoffs and landings from buildings or elevated platforms, or "external load" work like setting powerline pylons) require extended stays "on the wrong side of the curve." It's simply a matter of acceptable risk—as long as the engine is running, you're fine, so it's just a matter of your faith in Bell, Allison/Rolls Royce, and the deity of your choice! What it means for normal approaches, however, is that you should plan them to avoid the shaded area. Take a closer look. You can see that on a takeoff, you should stay below 25 feet until you reach a speed of 50 knots. Similarly, on an approach, it would be best not to decelerate below 50 knots until you were down to 25 feet skid height or lower. In the real world, that's probably pretty hard to manage unless your landing spot has very wide, clear approaches (like an airport). Still, you can try to minimize your exposure to the "avoid" area. 60 knots is a good speed with which to start a landing approach, since at that speed you're well outside the curve. Line up your landing spot and, holding 60 knots at first, adjust the collective to give you the desired rate of descent—the chosen spot should neither move up or down in the windshield. As you get closer, begin slowing by slight aft cyclic pressure. This will initially take a further collective reduction to avoid rising above your desired glide path. As you decelerate below about 30 knots, however, you'll feel the helicopter begin to settle— you're starting to lose the effects of translational lift. It'll also want to roll to the right, requiring a slight cyclic correction. Once you're below translational lift speeds, you'll have to start pulling collective back in. If you're approaching your landing spot too fast, you may have to apply even more aft cyclic pressure to reduce speed. Make sure you don't do this too close to the ground, as you risk striking the tail rotor if the helicopter is pitched up too high. Finally, as you settle toward your desired hover height, you'll need to add more collective (and left rudder) to establish a hover. Don't expect everything to come out right the first time; typically, a student will end up in a hover some distance away from the desired spot, then hover over to it.
Once over the desired spot, lower the collective to set down as we did before. STEEP DESCENTS Now, as we gain experience, we'll add another maneuver: a steep descent. This is what you'd use if you had to land in a confined area with tall obstacles. Lift off, climb out, and establish level flight at 500 feet above the ground and 60 knots. On our last approach, we made a slight collective reduction to start a very gentle descent. This time, make a considerably greater reduction (don't forget the pedals!) to descend more steeply, still maintaining 60 knots. When you reach 200 feet, smoothly pull the collective back up to maintain level flight at 60 knots, then add power and climb back up to 500 feet. Try this several times, making the descent steeper each time. SPLITTING THE NEEDLES There's no hard and fast dividing line between normal and steep descents, but as you get into the steeper ones, you may notice something interesting: the rotor and engine tach needles may "split" on the dual tachometer (this is less likely to happen with FADEC in automatic mode, you may want to try manual just for this demonstration). What's happened here is that during the descent, you demand so little power that the rotor system "decouples" from the engine, activating a clutchlike device installed between the engine and transmission. You can force this condition (starting at a safe altitude) by using the mouse to roll the throttle below the FLY detent to split the needles, simultaneously lowering the collective to maintain rotor RPM. Roll the throttle back up to FLY before you raise the collective to arrest the descent. Remember: in low-power (or no-power) situations, rotor speed is controlled by the collective. Raising the collective reduces rotor speed; lowering it increases rotor speed. QUICK STOPS Now we'll try another maneuver, one that most students find a lot of fun: the quick stop. Let's say we're zipping along a taxiway at low altitude and high speed when a jet pulls out in front of us. We want to slow down fast, but we don't want to climb. Establish flight at about 25 feet along an easily-followed reference—a road, taxiway, or runway. Now, to stop in as short a distance as possible, smoothly and simultaneously lower the collective to the bottom and apply just enough back pressure on the cyclic to maintain your altitude, neither climbing nor sinking. You may notice the dual tach needles splitting during this maneuver; Np will remain at 100%, but Nr may climb briefly toward 105% or so. As the helicopter declerates and starts to settle, use forward cyclic pressure to bring it back to a level attitude, and collective as necessary to hold altitude. Fun, isn't it? Practice it until you're proficient at stopping the helicopter in minimum distance without losing altitude. PUTTING IT ALL TOGETHER There's a reason we've practiced these particular maneuvers in this particular order: you've been gaining the skills you need to perform a safe autorotation! We'll start by doing a few practice autorotations to what's called a "power recovery"—i.e., rather than landing, we'll terminate the maneuver in a hover. Begin at 500 feet, flying at 100 knots. Now we're going to simulate an engine failure by smoothly but rapidly lowering the collective all the way to the bottom (or reducing throttle to minimum on your joystick). In addition to various horns and warning lights, which are the least of our concerns for the moment, you'll notice a few things: the helicopter will yaw to the left (as in a hovering autorotation), it'll tend to drop its nose, and it'll start a pretty rapid descent. We're already at the maximum authorized speed for autorotation, and we want to get slower—the speed for maximum glide distance over the ground is 80 KIAS, and that speed for minimum descent rate is 55 KIAS. We'll use 80, so apply aft cyclic pressure to decelerate to that speed. The helicopter will just about hold altitude while you're decelerating (which won't take long), then resume it's descent. Take a look at the dual tach; Nr will probably be higher than 100%. As long as it's below 107%, it's fine for the moment. Now hit [P] to pause the simulator while we discuss autorotations. AUTOROTATIONS AND BANK ACCOUNTS I'm indebted to the remarkable Frank Robinson, of Robinson Helicopters, for this method of explanation. In addition to being a brilliant designer and a very savvy businessman, Frank is probably the best helicopter instructor I've ever encountered. To keep a helicopter's rotor turning, and to keep it supporting the helicopter, requires energy. In normal flight, that energy comes from the powerplant. Where does it come from during an autorotation? Actually, it can come from three different sources, and you can compare them to bank accounts, with the energy taking the place of money. Think of the rotor's energy needs as a mortgage—you gotta pay it, and keep paying it, or you're in trouble. To draw the analogy even farther, you can transfer "funds" between these "accounts" without any additional charges—although if you make the wrong choice at low altitude, at the end of an autorotation, "there may be a substantial penalty for early withdrawal." Let's label the accounts "altitude," "forward speed," and "rotor energy." In steady forward flight, with the engine running, the balance in all three accounts remains the same, with constant deposits from the engine matching the constant withdrawals through the rotor. Now the engine quits—no more deposits. As in real life, however, the skinflints at the Rotor Mortgage Company insist that their payments continue (and we sure don't want to be foreclosed at this point!), so we'll need to transfer funds from other accounts. We have a few bucks saved up in our rotor energy account, but in a low-inertia ship like the 407, it's not much—just enough to keep us afloat while we figure out which funds to transfer. Since we're at altitude, the altitude account is flush...so let's start "transferring funds" to the rotor by descending. This is what's happening when we bottom the collective—we're transferring energy from the altitude account into the rotor energy account to balance the constant drain from the rotor. If we keep the rotor speed up, we're keeping its energy account up, as well as replacing any losses we may have incurred right after the engine failure. Obviously, we can't keep this up forever—sooner or later, we'll reach the ground. At this point, our altitude account is pretty well tapped out...but we still haven't touched our reserve in the forward speed account. Oh, well...there went the kids' trip to an Ivy League college...By reducing our forward speed, we can start transferring energy from that account into the voracious mortgage...er, rotor. In fact, with a low-inertia rotor system like the 407s, this speed-reducing maneuver, called a "flare," will actually increase rotor speed, so we're building up a little cushion in the rotor energy account. By the time the flare is complete, we're within a few feet of the ground (which means the altitude account is tapped out), and we've either slowed to minimal forward speed, or stopped altogether (which means the forward speed account is broke, too). But the repo man isn't here yet—we still have a decent balance in our rotor energy account. Now is when we can spend it by raising the collective to cushion the touchdown. With no deposits, that account will dwindle the instant we start to raise the collective (particularly since we're now adding rotor pitch, an expensive luxury)...but before we're completely bankrupt, we'll have the helicopter on the ground. SAVE AND SPEND WISELY We can actually manage these accounts during an autorotation, and adjust our spending plan to best deal with the situation. For example, if we have a nearby landing spot made (which means you can just about see it between your feet through the chin bubble), we may want to come down as slowly as possible. This would dictate a forward airspeed of only 55 knots, so we might not have as much energy in our "speed" account to transfer to our "rotor" account during the flare. On the other hand, we can keep the rotor account as high as possible by leaving the collective all the way down, and maintaining 107% rotor RPM. On the other hand, if the only available landing spot is farther away, we may want to fly at "best glide" speed of 80 knots. Our sink rate will be significantly higher—but we'll be coming into the "flare" transaction with a full "speed" account. This means we'll have plenty of energy to transfer into the rotor account at that time, so we might consider reducing our rate of descent a bit by pulling on just a bit of collective to bring rotor speed back to 100%—possibly even a bit lower. You'll notice that the green arc on the dual tach goes all the way down to 85%...but that doesn't leave you much for those "unexpected last-minute expenses!" Bell recommends no less than 90% for training autorotations (and remember that the low RPM warning light and horn come on at 95%). During the flare, we may actually want to lower the collective again to build rotor speed as high as possible. TOO MUCH OF A GOOD THING What we want to avoid is over-flaring. Not only does this risk leaving us up higher than we want to be with rapidly dwindling rotor energy reserves, but if we're low enough it puts the tail rotor awfully close to the ground. As long as the surface is reasonably smooth, you'll get a perfectly good autorotative landing by running the skids on at 10 to 15 knots, and you'll have much more time to "play" the final collective pull and touchdown. AUTOROTATION AND POWER RECOVERY Now do you see why we've introduced maneuvers in the particular order we chose? Seen alone, an autorotation looks difficult and scary...but if you think of it as nothing more than "a steep descent followed by a quick stop followed by a hovering autorotation," you can see that it's just a series of maneuvers in which you're already proficient. We'll do a few for practice first, leaving off the touchdown at the end. Ready? Tap [P] to resume the simulation—we're in a steep unpowered descent, with the engine running at 100% RPM but the collective bottomed all the way. Hold 80 knots. The helicopter will certainly seem to be swooping down at an impressive rate, but it's equally certain that it's not "falling out of the sky." At about 50 feet, apply smooth, but fairly decisive, back pressure on the cyclic to flare. The nose will come way up, the rate of descent will slow., .and you'll see and hear the rotor speed up to near its 107% redline (the needles will split for a moment). As the descent stops, apply forward pressure to level the fuselage, and as the helicopter settles, smoothly raise the collective again to transition to a hover. Of course, all these large collective movements will require corresponding pedal inputs. Don't worry about getting the helicopter to stop dead—anything below translational lift speed is OK. Do enough of these until you're comfortable. Then do a couple of hovering autorotations, just as a refresher. IT'S THE REAL THING Now you're ready for this series of lessons' "graduation exercise:" an actual autorotation to touchdown. Let's start at 800 feet and 100 knots, just to give you a little more room. Ready? Take a deep breath and hit [Ctrl+pg dn] to shut down the engine. The needles will split right away, and you'll get the "engine out" light. Simultaneously apply back cyclic pressure to start the speed toward 80 knots, and smoothly lower the collective. If your reactions are reasonably fast, you probably won't get the low RPM light or horn. Once the descent has stabilized, you can experiment with rotor speed control. A slight pull on the collective will bring Nr back toward 100%. Ease the collective back down to build rotor speed back up; you'll notice that the sink rate increases as you do so. At about 50 feet, smoothly and firmly apply back pressure to the cyclic to arrest the descent and reduce your forward speed. Nr should be right at its maximum. As the helicopter levels out, you should be down to around 15 to 20 feet. Make a definite forward cyclic correction to level the skids ("rocking the ship forward" is a good image). As it begins to settle, smoothly pull in more and more collective to cushion the touchdown. It's important to keep the helicopter coming down (although not too fast) all the way through the final settle-and-touchdown phase. Pull too much collective too early, and you'll run out of blade energy and fall the last few feet—that's the "early withdrawal penalty" I mentioned earlier. In a really hard touchdown, it's not unusual for the main rotor blades to flex down far enough to chop off the tailboom. It's much better to settle on gently, even if you're still moving forward at 10 or 15 knots. Congratulations! You've mastered the most critical maneuver in helicopter landings. It's often said that "A good landing is one you can walk away from; a great one is one after which the aircraft is still flyable." There's no reason you shouldn't make great landings in the 407, power on or power off, from here on. |
1. Rotary-Wing Aerodynamics | 2. Cockpit Tour | 3. Let's Fly ! | 4. Getting Back Down
