Autorotation
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Autorotation is the engineering term for the aerodynamics providing lift in a rotor-driven aircraft such as autogyro or helicopter.
Autorotations are used in helicopters to perform power off landings from altitude in the event of an engine failure. During an autorotation, the main rotor is not driven by a power plant, but by air flowing through the rotor disc bottom-up (imagine a windmill) while the aircraft is descending rapidly. About as much buoyancy is provided as a round parachute of the same diameter. The power required to keep the rotor spinning is obtained from the aircraft's potential and kinetic energy. An important contributing factor is the rotor's inertia. The descent is at a normal and familiar glide angle to a suitable landing spot assuming the flight plan stayed within gliding distance of a suitable landing spot.
Autorotation is also used in autogyro aircraft as the main means of achieving lift during normal operation.
[edit] Description
An autorotation is used when the engine fails in a helicopter, or when a tail rotor failure requires the pilot to shut down the engine. It is comparable to gliding in a fixed-wing aircraft without an operating power plant.
It is critical to immediately establish this upward airflow upon engine failure. Therefore, pilots diligently practice the series of maneuvers to fly a safe landing. This series of maneuvers is also called "an autorotation."
[edit] The entry
During entry into autorotation the objective is to establish the proper fuselage pitch attitude, forward airspeed and main rotor speed by manipulating the collective pitch control, cyclic control and pedals in a coordinated manner.
For the following discussion it is assumed that the helicopter is of the common main rotor and antitorque tail rotor configuration and the main rotor blades advance to the left, as is commonly found on American-built helicopters. Main rotors on most French-built and Russian-built helicopters advance to the right, thus causing opposite yaw torque forces.
To enter the autorotation, the pilot lowers the collective pitch lever all the way down, simultaneously adding right pedal (or left pedal for main rotor blades advancing to the right) and pulling the cyclic stick slightly aft.
Lowering the collective maintains RPM during the entry to autorotation, and keeps the rotor blade AOA (angle of attack) at a normal value during the glide. On some helicopters the collective is immediately raised a small amount to adjust rotor speed; this amount varies by design and is learned through experience, under the guidance of a qualified instructor.
Adding the right pedal is necessary because in autorotation no torque is being exerted on the fuselage. During power-on flight, the pilot uses left pedal force to counter the torque being produced by the engine. Once the helicopter is autorotating, the engine disengages and produces no torque.
While the collective is being lowered, the nose of the helicopter has a tendency to pitch down. The pilot will pull the cyclic stick aft slightly to counter this tendency and establish a stable attitude for descent. Allowing the nose to pitch down creates two problems: It reduces main rotor speed due to decreased airflow through the rotor disk, and it increases airspeed, usually far above the desired range for autorotation.
[edit] Establishing the glide
As the air starts flowing up through the rotor system, the RPM will start to increase, and depending on how the helicopter is rigged, the RPM may get too high. In this case, as RPM gets high the pilot can increase collective pitch to lower RPM.
The pilot should set up a normal autorotational attitude in order to get a normal airspeed. Although helicopters will autorotate at zero airspeed and even at negative airspeed, normally the pilot will want to hold between 60-70 knots of airspeed during the glide. The rate of descent will be between 800 and 1000 feet per minute. The glide ratio for minimum rate of descent will be about 3:1, considerably steeper than the 7:1 to 8:1 ratio for a fixed wing aircraft
[edit] Selecting a landing area
Within the first few seconds the pilot will establish autorotation and will have selected a landing area. The approach to the landing should almost always be into the wind, so the pilot needs to select a spot which will allow him to maneuver for an upwind approach.
The spot should normally be flat, firm, and fairly level.
One thing to quickly look for is poles which may have wires strung. The last thing the pilot needs on short final is trying to duck wires.
Once the pilot has selected a landing area, it is recommended he visualize a standard traffic pattern imposed on the landing area and aligned with the wind. The pilot should figure out which leg he is currently on, and then fly the pattern so that he arrives on final approach at an altitude and airspeed which will allow him to land in the selected area.
By flying a rectangular traffic pattern, the pilot can find himself on base leg, watching the angle to the landing area. When the angle is right, he simply turns final and will be very close to the desired spot. If the pilot starts to see the angle before he reaches the extended "centerline", he can simply turn final early. By cutting the corner he reduces the distance he has to fly, and makes it to the spot without ending up too low.
If the pilot finds himself slightly high on base, he can simply fly through the extended centerline, and turn a little late onto final. The extra distance uses up some extra altitude, and he still makes it to his spot.
A little overshoot is preferable to a little undershoot because it can be corrected easily still leaving sufficient energy. An undershoot normally requires going to best glide airspeed and dragging the rotor RPM down to the lowest allowable value. If the pilot is not careful, the result may be reaching the spot with low RPM. This is probably not a problem with a light inertia rotor system, but in a high inertia rotor system the RPM might not be recovered before touchdown.
[edit] The Flare
The pilot initiates the flare by using aft cyclic. No collective or pedal input is normally required. The height that the pilot should start to flare at depends on many factors, including the model of helicopter, the descent rate, the airspeed, the headwind component, and how rapidly the pilot is going to move the cyclic.
The purpose of the flare is twofold. First, it slows the descent rate of the helicopter, from 1,000 or 2,000 feet per minute to much less, so that a soft touchdown can be made. It also reduces the forward ground speed to just a few knots so that sliding on the landing gear is minimized.
The flare must be timed to not zero the descent rate, because the helicopter would be left hanging in the air bleeding RPM, but rather the flare should be timed to slow the descent rate so that the helicopter is approaching the ground at a manageable rate. The descent rate should be decreasing so that it either goes to zero just above the ground, or is low enough that a little collective pitch can bring it to zero.
[edit] The Landing
Touchdown is accomplished by (typically) putting the helicopter into a level attitude, and then using the collective to cushion the landing, just as in a hovering autorotation. The pedals are used to align the landing gear with the ground track.
[edit] Power Recovery
If the pilot is practicing an autorotation he may decide to recover to a hover, rather than touch down. The procedure is to start raising collective while still in the flare, just as flare effectiveness starts to go away, before any increase in sink rate is experienced. By starting the recovery early, the engine is not trying to play catch-up, and the recovery can be made with the RPM in the green range at all times.
[edit] Common Mistakes
[edit] Failure to Lower Collective all the way down
If the pilot forgets to lower collective and this is a real engine failure, it's a fatal mistake. Lowering collective is the most important part of doing an autorotation. If you remember to do that, you will probably walk away from the landing. Some pilots only put the collective pitch part of the way down. They get to "know" where it belongs. The only problem with this is that the position the collective needs to go to depends on many factors such as pitch link rigging, gross weight, and density altitude. These things can change from day to day. This method also delays recovery of rotor RPM, and there is no good reason to do that.
The best method is to lower collective all the way, and as RPM starts to build back up some collective should be raised to stop the RPM somewhere in the operating range.
[edit] Failure to trim with anti-torque pedals
For helicopters with a clockwise rotating main rotor, pilots may forget to push left pedal upon engine failure, push too much, or even push the right pedal. For helicopters with a counter-clockwise rotating main rotor, the reverse is true - right pedal is required on engine failure. In any case, the aircraft should be autorotated in trim.
[edit] Allowing the nose to drop
Do not let the nose drop during the entry. Whatever attitude the helicopter is in, enter the autorotation in that attitude, and then after the autorotation is established the pilot can make any attitude adjustments required for proper airspeed. Allowing the nose to pitch down delays the recovery of RPM (it's like an anti-flare) plus it is not uncommon for pilots to overspeed the rotor by waiting until the airspeed builds to 80 knots or more, and then suddenly trying to fix it by yanking back on cyclic. The result is an almost instantaneous rotor overspeed.
[edit] Failure to control Rotor RPM with collective
Most helicopters are rigged so that at normal weights the collective will have to be raised somewhat to keep rotor RPM in the normal operating area. Common mistakes are either to leave the collective full down so long that a rotor overspeed occurs, or to overcontrol the collective, moving it up and down during the entire glide. The proper way to manipulate collective is to lower it full down during the entry to autorotation. Then, as RPM starts to increase toward the normal operating area raise enough collective to stop the RPM from changing. Wait a few seconds until it stabilizes, and make one final adjustment to place the RPM exactly where it is desired. Normally no further manipulation of the collective will be required during the glide. One exception is that during turns, especially at high speed, some collective may be required to prevent the RPM from climbing too high. Rolling out of the turn, the pilot should put the collective back to where it was before the turn was entered. By performing turns at lower airspeeds, little or no collective will be required.
[edit] Failure to maneuver to the point of intended landing
Many pilots get quite proficient at autorotating to the runway at their home airport, but have more trouble when trying to make a specific landing area in the off-airport environment. It is best to set up a (tight) traffic pattern to the landing area, just as is done at an airport. The pilot should figure out the wind, and therefore where "final" will be. Then the pilot should figure out where he currently is with respect to the traffic pattern (is he already on downwind, base, or final?). Once he knows what leg he is on, he can manipulate the length of the remaining legs to arrive on final at the proper altitude. A very short final is suggested. The longer final is, the bigger the chance is of over or undershooting, with no easy way to correct once the under or overshoot is recognized. Instead, fly a very tight base and time your turn onto short final to give you the desired distance to the touchdown spot. If you are a little low, turn final slightly early. If you are a little high, delay the turn to final, overshoot the centerline somewhat, and use up the additional altitude on base. For gross errors, S-turns or zero (or negative) airspeed may be required. One final - never do a 360 degree turn. You lose track of your approach angle for too long. Instead, if you have massive amounts of altitude to lose, perform a figure-8 pattern on final. This way the spot is always visible, and you can turn back onto final when the angle begins to look right.
[edit] Flaring at the wrong altitude
Each helicopter has a range of altitudes it needs to be flared at. The altitude will change from flight to flight based on gross weight, density altitude, wind, and airspeed. Generally, aircraft with higher disk loadings require a higher flare. If the pilot flares too high, the helicopter will stop its descent too high above the ground to make a safe landing. If the pilot flares too low, he will be forced to level the helicopter (get rid of the flare) too early (to avoid hitting the tail on the ground). The result will be a high rate of descent (which he can probably fix by raising collective) and high forward ground speed (which he can't fix, so he'll slide hundreds of feet).
Assuming a perfect flare cannot be made, which way the pilot should err depends on the surface being landed on. If the surface is firm and level, some slide probably won't hurt, and it would be best to be a little bit low to give a soft touchdown, followed by a little slide. If the surface does not appear to allow a slide (swamp or such which will cause the skids to dig in) the flare should probably be a little high to insure removal of all forward speed. The touchdown may be a little harder, but, by being more vertical, the chance of rolling over is reduced. One caveat is that human beings do not take vertical accelerations well, so to avoid back injuries, the flare should not be too high.
[edit] Flaring too aggressively or not aggressively enough
The speed with which the nose of the aircraft needs to be pitched up is related to gross weight, density altitude, wind, and airspeed. Generally if gross weight is high, a more aggressive flare will be required. If density altitude is high, a more aggressive flare is required. If wind is high, a less aggressive flare is required. And if airspeed is high, a less aggressive flare is required. Pilots can adjust for minor airspeed deviations by flaring at different altitudes, or with different amounts of aggressiveness. For instance, if the airspeed is 10 knots below optimal, a more aggressive flare will help to make up for this. Of course there are limits to the amount of correction that is possible.
[edit] Failure to level the aircraft
Some aircraft land in a slightly tail low attitude, but with many others it is critical to have the landing gear level before touchdown. Failure to do so can result in tail boom strikes and porpoising (where you hit on the heels, and then roll up onto the toes and flip over forward).
[edit] Failure to maintain heading during the slide
There are a couple reasons that heading might not be maintained during any ground slide. One is just that the pilot fails to manipulate the pedals correctly, the other is that if rotor RPM gets too low the tail rotor may lose effectiveness. Failure to maintain heading can cause a skid gear to catch and roll the aircraft over on its side. Most aircraft can perform fairly high speed slides if the skids are pointed in the direction the aircraft is moving.
[edit] Using aft cyclic to minimize sliding distance
Moving the cyclic aft during ground slide is an improper technique to minimize sliding distance. During in-flight conditions, a helicopter is suspended from the main rotor by the rotor mast and is therefore free to move underneath the spinning rotors as the disc tilts from pilot control inputs. During conditions when the helicopter is in contact with the ground (such as during a ground slide following a full-down autorotation), the helicopter is not free to move underneath the main rotor disc. Therefore, the angle between the tailcone and main rotor disc created with aft cyclic input is much smaller when the helicopter is grounded than it is during flight, creating a very real possibility for a tailboom strike. The proper technique to minimize sliding distance is a smooth lowering of collective while maintaining neutral cyclic.
