Editor: With this article we launch a series on helicopter flight from the perspective of a real chopper pilot. Here is Part I

Do helicopters really FLY or do they beat the air into submission?

In this edition of "Pulling pitch with Blades", I am going to cover a few basics of how helicopters work (very briefly). After that, I will present a feature article on "The art of the autorotation". Future articles will go into more detail on the individual components and aerodynamics of helicopter flight.

How does a helicopter work? How does this "hog" get in the air? You would be amazed to find out what is actually going on in this mechanical marvel.

As you have probably seen on most helicopters, there is a main and tail rotor (the exceptions being the Kaman, the CH-46/47 tandem type, and the MD/Boeing NOTAR). On most American helicopters, the main rotor rotates counter-clockwise (if you are looking at it from above). So if the main rotor rotates counter-clockwise (by power supplied by the engine in normal flight), the airframe is going to want to spin the opposite direction (Newton’s law of physics which states for every action, there is an equal and opposite reaction). That’s where the tail-rotor comes into play. The tail rotor serves several functions. First, it counteracts the torque of the engine onto the airframe. Second, it allows the pilot to control the heading of the aircraft during slow flight and a hover. Third, it allows the helicopter to fly in trim while in high-speed flight.

Why is all this important? The main and tail rotors are the main control surfaces you have at your disposal to tell the helicopter just what to do! By skillfully maneuvering these adjuncts, you can pretty much have the helicopter do what your heart desires.

So, what do the controls do? The stick between your legs is called the "cyclic". The lever you pull to your left is called the "collective". The throttle may either be located as a twist-grip control at the end of the collective or as a lever on another part of the helicopter. The pedals are called...well, "pedals".

The cyclic is used to change the pitch in specific portions of the main rotor (also referred to as the "rotor disk"). In further editions, we'll get into the specifics of how it works, but for now, realize that the cyclic tilts the rotor disk in the direction the pilots wants. So, you can equate the cyclic in the helicopter to the yoke in an airplane.

The collective, the lever in your left hand, collectively changes the pitch of the blades. When I am on the ground, I start pulling the collective up, which in turn starts producing positive pitch about the blades. The more I pull, the greater the pitch, and the greater the engine output. That means when I want to go quicker and maintain the same altitude, I pull a little collective and push the cyclic forward a little.

The pedals change the pitch of the tail rotor blades. More on this at another juncture. In further updates, we will go into a lot more detail about helicopter aerodynamics, advanced maneuvers, and other interesting topics.

The Art of the Autorotation

Fundamentals:

During powered flight, the rotor drag is overcome with engine power. When the engine fails, or is deliberately disengaged from the rotor system, some other force must be used to sustain rotor RPM so controlled flight can be continued to the ground. This force is generated by adjusting the collective pitch to allow a controlled descent. Airflow during helicopter descent provides the energy to overcome blade drag and turn the rotor. When the helicopter is descending in this manner, it is said to be in a state of autorotation. In effect the pilot gives up altitude at a controlled rate in return for energy to turn the rotor at an RPM which provides aircraft control.

Stated another way, the helicopter has potential energy by virtue of its altitude. As altitude decreases, potential energy is converted to kinetic energy and stored in the turning rotor. The pilot uses this kinetic energy to cushion the touchdown when near the ground. Most autorotations are performed with forward airspeed. (From Army Training Manual)

What sets one helicopter pilot apart from another? The autorotation! That’s not the only thing, but it does tell a lot about how a pilot flies. The autorotation, while simple in theory, takes a lot of coordination and judgement along with a gentle but definitive feel of the ship. There are four basic parts to an autorotation:

• 1. The entry
• 2. The descent
• 3. The flare
• 4. The recovery or landing

Let's assume this is not an actual engine failure and we have time to "plan" this simulated engine out. First, pick out a spot you believe you can make (that means reach). Keep that spot in mind throughout your descent. The entry is just like it sounds. The pilot transitions from powered flight to non-powered flight. Most pilots do this purposely; some pilots are forced into this (either by their instructor or a critical component failure). In the entry, three things happen simultaneously:

1. Down Collective
2. Aft cyclic (Aft goes towards your body)
3. Right Pedal (in helicopters where the main rotor spins counterclockwise)

After you do these, you tweak off some throttle so the engine is no longer powering the rotor. You don’t want to roll too much throttle off because it takes longer for the engine to wind back up. . . (especially in a turbine) and there’s a higher likelihood of an engine stall or engine failure at lower RPM’s. Helicopter pilots call this "separation of needles". Once we have verified that there is a separation of the needles (The Rotor RPM and Engine RPM are not at the same percent), then we know the two are disconnected and we are truly autorotating. Oh, this all happens in a matter of a second or two…so think quick but smart!

Thinking Quick!

Now, you have full down collective, right pedal, and aft cyclic and have entered into an autorotation. Depending on the helicopter you are flying and the conditions, you may or may not have to pull in a little bit of collective to keep the rotor from overspeeding. In every helicopter I have ever flown, I have had to pull anywhere from ½ inch to 2 inches to keep the RPM "In the green". The green means the safe operating region for the rotor and engine RPM.

At this point, we are not worried about the engine RPM since we already tweaked it down a little. On the other hand, we are worried about keeping the Rotor RPM "in the green". Some helicopters allow the engine-off RPM to exceed the maximum engine-on RPM. For purposes of this article, we will assume that the RPM shall be kept at or around 100%. Remember, at this point, the sprague clutch has totally disconnected the engine from the rotor.

That is to say, you are controlling your rotor RPM by simply increasing or decreasing the overall pitch in the blades through the use of the collective; the engine plays no part in this at all. Normally, when the engine is on and you apply up-collective, the engine delivers more power because you are passing more fuel to the injector(s) or carburetors. If you run out of fuel while in flight, the engine will quit and you'll get a quick test regarding your autorotation abilities. The next time you use the engine will probably be when the mechanic along with the FAA say, "Hmm I wonder why this thing quit". I'll bet you they'll check your gas tank first!

Why is RPM so important? The RPM of the rotor system is like a bucket of energy. You have to make sure you don’t have too much or too little. Either one can be a deadly recipe!

All About RPM

How do we control the RPM? The collective and cyclic help us control the airspeed, vertical rate of descent, and Rotor RPM. Remember when I said we usually have to pull a little collective a few seconds after the entry? That’s so we don’t overspeed the main rotor. By pulling the collective up a little, we are creating pitch in the main rotor blades (uniformly about the whole rotor system), thus creating more drag, hence slowing the rotor system down.

Now, here’s the tricky part! When you push the Cyclic (the stick between your legs) forward, you will increase you’re airspeed BUT you’re RPM will start decreasing. This phenomena happens because the greater the angle you tilt the rotor system forward, the more of an angle the rotor system makes with the path of flight, hence the volume of air per second going through the rotor disk is decreased; the opposite is true for pulling aft cyclic (pulling the stick towards your body).

So, if you are going too slow in an autorotation, you are going to push the cyclic forward (which speeds the helicopter up and lowers the RPM a little) and lower the collective a little (to decrease the pitch of the main rotor blades) to increase the RPM a little. We generally try to coordinate these events so as to minimize the change of RPM.

Why is speed important in an autorotation? As helicopter pilots, we generally like to descend in an autorotation at slow vertical yet brisk horizontal speed. Generally, a helicopter has a slowest rate of descent airspeed and best glide distance airspeed. That means that the particular helicopter might only descend at 900 feet per minute when autorotating at 65 knots but might fall at 1700 feet per minute when travelling at 35 knots. At the other extreme, say closer to 110 knots, you might be descending at 1700+ feet per minute (these rates of descent are just examples and not actual figures from any particular helicopter) . . .so there is a "sweet spot" speed for autorotation.

Uh oh! The Engine is Too Quiet!

If I lose my engine over a large plot of empty land, I am probably not going to be as worried about extending my glide path. If I was over a forest and lose my engine ½ mile from an opening or clearing, I’m going to want to descend at the airspeed that will give me the furthest gliding distance. There are other ways to extend glide distance as well!

Helicopters also have a sweet spot in the RPM band for best glide distance. Some helicopters, like the Robinson R22 glide the furthest with the RPM at 90%. What does that mean though? You are playing with a low inertia rotor system that can change its RPM rather quickly. If it is a real emergency and you want to make the distance, you might have no choice but to put the RPM down there. . .but if you make it a habit of practicing autorotations at 90%, you have about 10% of RPM left (assuming you are not at high density altitude) before you run the risk of entering catastrophic rotor stall.

Another important thing to do is look to your left and right (assuming you are doing your autorotation into the wind). If you find a more desirable place to your left or right, there is nothing that says you can't do your autorotation with a crosswind. It's advisable NOT to do your autorotation with the wind to your tail, but if you have no choice, you do what's necessary.

Finally, there are some tricks you can practice to nail your landing spot. If you lose your engine, have some altitude, and just passed over a good spot to land, you can do some spirals and turns to "make it" to your spot.

Go to Part II

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Last Updated March 16th, 1998