Helicopter Autorotations

Photos by Jon Davison.

As a future helicopter pilot, you will become very familiar with the maneuver known as an autorotation. Every student pilot practices autorotations over and over and over again, as well they should: being able to successfully autorotate a helicopter to a suitable landing spot after an engine failure can mean the difference between walking away and eating hospital food.

In an autorotation (or auto), the helicopter transfers kinetic energy from altitude and airspeed into the spinning main rotor. The energy stored here is crucial because you will call upon it at the end of the auto as you flare the aircraft to cushion your landing.

If you’ve ever watched a pinwheel spinning in the breeze, you can grasp how a helicopter’s rotor blades can autorotate without engine power.  There are complex aerodynamic forces at work during an autorotation including the rotation of and lift generation by a rotorcraft‘s primary rotor.  Truthfully, getting the hang of doing a good auto is not that hard.  What’s hard is doing an auto to an exact spot.

Student pilots in the Robinson R-22 are taught to lower the collective pitch as soon as they detect a problem with the engine and/or transmission.  The problem can manifest itself as a loud bang, as a reduction of power and/or as drooping rotor RPM and is accompanied by a yawing of the aircraft as torque from the engine is reduced or eliminated.  Lowering the collective will reduce the pitch of the rotor blades, allowing wind from below to now drive and maintain rotor RPM as the helicopter descends.

When you begin to practice helicopter autorotations, your instructor will generally take you away from the airport to practice entries and glides from 3 or 4,000 feet above ground.  This will give you time to learn the balance of controls and how to maintain an airspeed and a rotor RPM setting with opportunity to gain an understanding of the basics.

After a day or two of practicing autos from altitude, you are introduced to the airport environment, combining the art of autorotative descent with the flare and recovery.  For this, your instructor will have you point the aircraft into the wind, lined up with the runway.  Your entry altitude will be somewhere between 500 and 700 feet AGL, your airspeed about 65 to 70 knots.  You will then smoothly lower the collective pitch and, simultaneously, add right pedal to maintain trim and pull back just a bit on the cyclic control to maintain level attitude.  The rotor and engine RPM needles will usually split at this point, but if they don’t, reduce the throttle slightly.

Once your descent has been established, you’ll want to slowly reduce your airspeed to 60 knots and then keep it there for the remainder of your glide.  For the most part, changes to the collective and cyclic pitch are necessary during the maneuver to manage the energy of the rotor and the airspeed of the aircraft.  Small, gentle movements are sufficient.  When you over control, you can create a whipsaw effect that can cause you to “chase the needle,” which makes your job more difficult.

Assuming your attitude, trim, rotor RPM and airspeed are acceptable; you’ll want to begin your flare at about 40 feet AGL.  You do this with aft cyclic, the amount you use depending on wind conditions and your aircraft gross weight.  In a practice auto, you will then commence your power recovery at maybe 10 feet above the ground.  You do this initiating with an increase in throttle to above 80% engine RPM then allowing the governor to return the RPM to 104%.   Level the aircraft with forward cyclic, increase the collective and maintain your heading with left pedal as the power returns.

At no time should the rotor RPM descend below the top of the green arc; this can be prevented by a sure recovery of engine RPM when the rotor reaches its peak.  Example: rotor 108%, engine 104%.  The engine then, through the sprague clutch, drives the rotor and restores powered flight when the rotor RPM drops to 104%.

You always want to land into the prevailing wind, preferably not more than 30 degrees off the nose, to reduce your speed over the ground, aid in the flare and to still have effective translational lift to further reduce the power required to transition back to a hover.

Engine failures rarely occur when you are flying directly into the wind 700 feet above the ground with an airspeed of 70 knots and a nice, long runway below.  The trick, then, is to practice autos to the point that you are able to abbreviate or lengthen your glide, able to turn the aircraft into the prevailing wind, and still hit your spot.  And that, dear student, is why we practice these things so frequently.

Engine failures in modern aircraft are almost unheard of.  But they do happen, so it behooves you to be prepared so that if the unthinkable happens, you can achieve a controlled landing in a safe spot.

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