Autorotation Explained

A primer for non-pilots.

One of my pet peeves is finding inaccurate information in works of fiction (or non-fiction, for that matter). You can argue all day long that fiction is fiction and the writer can write whatever he wants. After all, fiction, by definition, is a made up story. That gives the author license to make things up as he goes along.

I agree that it’s fine to make up the story, but unless it’s a work of science fiction or fantasy (where it might be acceptable to change the laws of physics), it’s not okay to make up the details of how existing things work. I explored this theme in my post “Facts in Fiction,” and picked apart the work of a bestselling author in “Dan Brown Doesn’t Know Much about Helicopters.” Both posts were triggered, in part, by basic errors about how helicopters work that appeared in works of fiction.

The Question

“Facts in Fiction” was also triggered by an email message I received from a writer looking for facts about how helicopters fly. Oddly, I just received another one of those messages not long ago:

I’ve recently been writing a novel in which I have to describe the sound a helicopter makes, how they fly and things along these lines.

But there is a section of my book where a helicopter runs out of fuel and begins to drop. However, below them is a forest and they crash into the canopy. But in order to minimize damage the pilot uses autorotation to make the helicopter somewhat stable. I don’t want to be an ignorant writer that makes stuff up at the expense of fact. I’ve looked up autorotation but it’s still not clear to me- would you be able to help me out with how a pilot would initiate autorotation (in simple terms!)

Again, I applaud this writer’s desire to get it right. The aviation community certainly doesn’t need yet another work of fiction that misrepresents basic aerodynamic facts.

Unfortunately, it’s pretty clear that this writer does not understand how helicopters fly. This is common among non-pilots. Some folks think that the rotor disc — when the blades are spinning — works like a giant fan that keeps the helicopter in the air. Other folks — well, I don’t know what they think. But very few seem to realize that like airplanes, helicopters have wings.

Yes, wings. What do you think the rotor blades are?

Helicopters are rotary wing aircraft. This means that they have wings that rotate.

The Real Question

Although this writer seems to want an explanation of “how a pilot would initiate autorotation,” he has a bigger misunderstanding to clear up first. It all stems around these two phrases:

…a helicopter runs out of fuel and begins to drop.

and

…in order to minimize damage the pilot uses autorotation to make the helicopter somewhat stable.

The problem is that if a helicopter ran out of fuel and the engine quit (assumed), the pilot has only about 2 seconds to enter an autorotation to prevent a catastrophic crash. You don’t enter an autorotation to “make the helicopter somewhat stable.” You enter an autorotation to maintain a controlled glide to the ground that, hopefully, concludes with a landing everyone can walk away from.

Or, put it another way, in the event of an engine failure, the pilot must perform an autorotation if he wants to survive.

So in order to answer the question this writer asked, I need to first address his misunderstanding of how helicopters fly and what autorotation does.

How Helicopters Fly

Let’s start with something most people do understand — at least partially: how an airplane flies.

An airplane has at least one pair of wings that are fixed to the sides of the fuselage. The wings have a specific shape called an airfoil that makes lift possible.

When the pilot wants to take off, he rolls down the runway, gathering speed. This causes wind to flow over and under the airfoil. After reaching a certain predetermined minimum speed, the pilot pulls back on the yoke or stick which lifts the airplane’s nose. This also changes the angle of attack of the relative wind on the wings. That change produces lift and the plane takes off.

Obviously, this is an extremely simplified explanation of how airfoils, relative wind, and angle of attack produce lift. But it’s really all you need to know (unless you’re a pilot).

A helicopter’s wings — remember, they’re rotary wings — work much the same way. But instead of moving the entire aircraft to increase the relative wind over the airfoil, the wings rotate faster and faster until they get to 100% (or thereabouts; long story) RPM. Then, when the pilot wants to take off, he pulls up on a control called the collective which increases the pitch or angle of attack of all the rotor blades. That change produces lift and the helicopter takes off.

It’s important to note here that when you increase angle of attack, you also increase drag. Whether you’re in an airplane or in a helicopter, you’ll need to increase the throttle or power setting to overcome the increased drag without decreasing forward speed (airplane) or rotor RPM (helicopter).

Rotorcraft Flying HandbookIf you’re interested in learning more about lift and how helicopters fly, I highly recommend a free FAA publication called Rotorcraft Flying Handbook. This is a great guide for anyone interested in learning more about flying helicopters. You don’t need to be an aeronautical engineer to understand it, either. If the text isn’t enough to explain something, the accompanying diagrams should clear up any confusion. I cannot recommend this book highly enough.

What Happens when the Engine Quits

Things get a bit more interesting when an aircraft’s engine quits.

On an airplane, the engine is used for propulsion. If the engine stops running, there’s nothing pushing the airplane forward to maintain that relative wind. Because it’s the forward speed that keeps an airplane flying, its vital to maintain airspeed above what’s called stall speed — the speed at which the wings can no longer produce lift. To maintain airspeed, the pilot pushes the airplane’s nose forward and begins a descent, thus trading altitude for airspeed. The plane glides to the ground. With luck, there’s something near the ground resembling a runway and the airplane can land safely.

On a helicopter, the engine is used to turn the rotor blades. If the engine stops running, there’s nothing driving the blades. Because it’s the spinning of the rotor blades or rotor RPM that keeps a helicopter flying, its vital to keep the rotor RPM above stall speed. The pilot pushes the collective all the way down, thus reducing drag on the rotor blades — this is how he enters autorotation. (The helicopter’s freewheeling unit has already disengaged the engine from the drive system, so the blades can rotate on their own.) The reduction of the angle of attack of the blades starts a descent, trading altitude for airspeed and rotor RPM. The helicopter glides to the ground. With luck, there’s a clearing or parking lot and the helicopter can land safely.

It’s extremely important to note that as long as the pilot maintains sufficient rotor RPM, he has full control of the helicopter all the way down to the ground. He can steer in any direction, circle an appropriate landing zone, and even fly sideways or backwards if necessary (and he has the skill and nerve!) to make the landing spot. So to say “the pilot uses autorotation to make the helicopter somewhat stable” shows complete ignorance about how autorotation works.

About 30 feet above the ground, the pilot pulls back on the cyclic to slow his forward airspeed. The resulting flare trades airspeed for rotor RPM, thus giving the main rotor blades extra speed. That comes in handy when he levels the helicopter and pulls the collective full up — thus bleeding off RPM, which he won’t need on the ground — to cushion the landing before touching the ground.

The point that needs to be made here is that helicopter engine failures and autorotations don’t always end in a crash. In fact, with a skilled pilot and a suitable landing zone, there’s no reason why it should end in a crash. So in the example presented by this writer, the helicopter doesn’t have to crash at all. It could have an engine failure and safely land in a clearing.

And here’s another newsflash: every helicopter pilot not only knows how to perform an autorotation, but he’s tested on it before he can get his pilot certificate. He’s also required to prove he can do one every two years during a biennial flight review. And if he’s like me, he’s tested annually by an FAA inspector for a Part 135 check ride.

Writers: Do Your Homework!

It’s good to see this writer trying to get the information he needs. But in my opinion, he went about it all the wrong way.

It’s been over a month since I got his emailed request for information. I never replied by email; this is my reply. Has he written his passage without the answers to his question? I have no idea. He never followed up.

But wouldn’t it have been smarter to simply talk face-to-face with a helicopter pilot? Any helicopter pilot could answer these questions and set him straight. Helicopter pilots aren’t so hard to find. Flight schools, tour operators, medevac bases, police helicopter bases, etc. Not only could the writer get his questions answered by someone who knows the answers from experience, but he could gather a wealth of information about helicopters, including their sound, why they don’t usually take off straight up, and other operation aspects. And if he visited a flight school or tour operator and had some extra money to spend, he could even go on a flight to see what it’s like from the inside of the aircraft.

Emailing a blogger who happens to write a lot about helicopters and complain when novelists get it wrong [hand raised] is downright lazy.

And despite what you might think, writing is not a job for lazy people.

9 thoughts on “Autorotation Explained

  1. Very cool, thanks for that. I’m too interested in fixed-wing flight to have ever spent much time learning about autorotation, so this is a great primer. I had to chuckle at how you keep referring to the pilot as “he” :-)
    So, since you brought it up… why *don’t* helicopters usually take off straight up?

    • I use “he” because, as a writer, I think it sounds stupid to keep saying “he or she” or “he/she” and sexist to say “she.” Let’s face it: most pilots are men (or at least male). So “he” seems to make sense.

      As for why we don’t usually take off straight up, I allude to one reason for that in an older post, The Deadman’s Curve. I should probably revisit the topic to more directly answer your question. That’s something for another post.

  2. Hi, thanks for a great explanation. I’m not a pilot, but extremely interesting in flying and aviation for years. Still autorotation isn’t completely clear to me. On a basic level I understand it, but something is missing – maybe you could help clarify even more:
    I understand the need to lower the AOA, by reducing collective to minimum in order to reduce drag, and keep as much RPM as possible – it means, if I’m correct, that it’s only possible to conserve as much RPM for as long as possible, but not regain more RPM somehow? Many sources claim that the air coming from under the disk rotates the blades – but if that were through according to my logic, the blades would either have to start rotating the other side (nonsense), or the AOA would have to be completely reversed – but I don’t think that happens, plus then the lift would be created to the wrong side!!
    This brings me to the tougher question – lift is created downwards in normally operating (powered) helicopter blades. Once autorotating, if AOA is reduced to minimum, what lift is created now?? and if air from underneath turns the blades, how would lift then be created, since lift comes with pushing air downwards and not upwards??
    What am I missing?? am I completely off??
    From my research about the topic what I found was that maybe the answer lies in different ranges of the disk, something along the lines of: center of disk is stalled anyways, middle created the lift (pushes air downward) and outer part (fastest speed of airfoil movement) is the one creating the RPM (so pushing air upwards?)

    As you can see I’m pretty confused, would really appreciate if you could help me out here! thanks!!! :)

  3. Hi Maria. First of all, I’ll have you know I’m a big fan. My daughter is also interested mechanical engineering and she sees you as one of her inspirations. Anyway, on the topic of autorotation, I am also at a bit of a loss one 1 small aspect, and that is how the airflow actually spins the blade. Like one of the commenters ahead of me, my question would be: can the autorotative rotor rpm be increased by the airflow (by adjusting the collective pitch, of course)? Since I now know that there is no such thing as ‘negative pitch’ on a real helicopter, how is the rpm increased using the air flow? At first, I was of the impression that autorotation was done by using negative pitch, thereby causing the upward airflow to force the rotors to spin. Then, upon realizing there isn’t a ‘negative pitch’, I thought the idea now was to ‘conserve’ as much rotational energy by decreasing the drag on the rotors as the helicopter descended to the ground. In your article however, you mention how the cyclic is pulled back to flare the helicopter, causing airflow to increase the rotor rpm. Bottomline is my confusion comes from the fact that since there is no ‘negative pitch’ setting on the blades, how is the upward rushing air causing the blades to spin faster?

    • First of all, thanks for taking the time to write — and for saying such nice things. Comments like yours really make my day.

      As for your question, I’m not a CFI, so I’m not sure that I can explain it properly. Hopefully, someone will correct me if I screw up. That said, you’re right: there is no negative pitch in a helicopter. By reducing the collective to full-down, we are reducing the amount of drag on the blades. The blades spin because they are weighted and because there’s energy stored in them. There’s also energy stored in the altitude, airspeed, and weight of the aircraft. At least this is how it was explained to me. When you descend in autorotation you exchange the energy of your altitude and your airspeed for the energy needed to turn the blades. Pulling the cyclic back to flare the helicopter reduces the forward airspeed and move some of the energy from the airspeed up into the rotor disc. So the wind is not actually turning the blades.

      I’m sure I screwed up this explanation pretty badly. There’s an excellent resource on the FAA website that explains it better and provides a wealth of information about how helicopters fly, the Helicopter Flying Handbook: http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/

      I highly recommend it.

  4. Thanks for sharing your knowledge in the interests of accuracy. As a writer, I try to check all details carefully but the actual experiences of some things are too expensive to attain, like the luxury of a helicopter flight to determine the sound it makes while in the air. It’s great that you are willing to communicate with those seeking accurate and detailed information. :)

    • I think my bigger point is that writers need to get off their butts and talk to the people who can provide the information they need. Too many writers work in their own little world, typing away at a word processor and using Google to get all their answers. Too often, they rely on cliches to come up with the descriptive phrases they need for something they simply have no first-hand knowledge of. A helicopter flight might not be as much of a “luxury” as you think — only 10 days ago, I gave about 100 people helicopter rides for only $35/person. That’s less than the cost of a good dinner out. A visit to a local flight school to watch the helicopters come and go can also provide a glimpse of reality — for free! A chat with the people who fly them can also provide free and accurate information.

What do you think?