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. Keeping your speed up till the Flare!

One of the most important things about speed is how it affects the flare at the bottom of the autorotation. If I have 70 knots airspeed [in most helicopters, I will really be able to reduce my rate of descent when I start flaring. Remember that airspeed is a source of energy. When we flare at the bottom of an autorotation, we are converting that airspeed to lift and rotor RPM.

Most autorotations in flight ideally take place at 60-90 knots; of course there may be helicopters that fall above or below that range. So, on the way down during the autorotation, we are keeping our airspeed consistent (for argument sake, 65 knots) with the cyclic and our RPM in the green with the collective. We continue a scan of the cockpit instruments and our intended landing spot. Depending on the wind and judgment, we might over shoot or fall short of the spot we picked out. If I see my airspeed start drooping down, I push the cyclic forward a little and ease down on the collective some. If I see the rpm start getting low, I lower the collective a little.

During the descent, the airflow keeps the main rotor spinning (the tail rotor spins because the main and tail rotors are connected through the same transmission). We are keeping our pedals almost all the way to the right since there is no torque on the ship from the engine (remember, the engine is temporarily disconnected). We still have ample ability to control our tail since the tail rotor is still spinning along with the main rotor.

The Flare

In an actual autorotation, start a gentle flare when you get close to the ground. The height at which you execute the flare depends greatly on the type of helicopter, the airspeed, and other important factors. Some helicopters require that you start the flare at over 100 feet, while others recommend you start at 50 feet or below. For argument sake, we’ll say this particular flare starts at 50 feet.

We start gently pulling aft cyclic so as to slow the helicopter down, reduce the rate of descent, and increase the rotor RPM. We don’t want to pull back too much on the cyclic [which would cause the helicopter to balloon, but on the other hand, we want to pull enough so we greatly reduce our vertical descent speed. But remember what we said about pulling aft cyclic? We are going to have an RPM increase! That means we probably need to pull up on the collective a little to keep the RPM from getting too high. When you feel the helicopter is about to drop from under you (in other words, you are going to start descending), you push the cyclic forward to level the ship and apply full collective so as to cushion your landing.

I want to mention one more thing about timing. When you pull aft cyclic, you want to initiate the pull so that when you slow down to 10 or 15 knots, you are pretty close to the ground. If you flare too high, you will have a pretty long drop when you level the ship out. If you flare too low, you run this risk of either having an awesome auto where the tail boom is 1 foot from the ground, or striking the tail boom on the ground (usually a pretty expensive repair). When you pull the collective, pulling it too early can make the ship rise, hence bleeding off your RPM and control of the aircraft. In some helicopters, depending on atmospheric conditions, the type or rotor system, and the weight of the ship, you may not have to pull the collective all the way up right away.

Some more interesting variables to the "art of the autorotation":

· Depending upon which helicopter you are in, the inertia of the rotor system may either be high, low, or somewhere in between. As an example, a high inertia rotor system (such as a Bell JetRanger 206B3) simply means the blades and connecting components weigh a lot; the rotor RPM is going to be harder to change. The high inertia rotor systems are more forgiving at the end of an autorotation and will provide lift for a longer period of time than a low inertia system. Other helicopters that have a lower inertia rotor system (such as the R22) require that the pilot really time the pull of the collective at the end of the autorotation perfectly.

· You always want to do an autorotation into the wind (if possible) – Never attempt them with the wind to your tail

· Try not to "chase the RPM" with the collective. You want to be smooth with the controls.

· Make small changes when at all possible. Large and abrupt changes are not desirable.

· The RPM will increase when you make a left or right turn in an autorotation…and what does that mean? Add a little collective as you enter the turn. The opposite holds true when you get out of a turn; more on this at a later date.

· Try to land on a smooth surface and keep your skids inline with the direction of travel. If you don’t keep your skids straight, you may rollover when you hit the ground. Depending on the conditions, you may opt to do a running landing at the end of an auto as opposed to coming to a complete stop. In helicopters, airspeed is a good thing. If possible, have at least 15 knots of airspeed when landing at the end of the auto. . .I’ll discuss why in a later issue.

· Timing of the collective pull is crucial! If you pull too early, you are going to balloon up then hit hard. If you pull to late, you are going to hit hard as well.

Buckets - What do they have to do with helicopters?

Finally, I would like to bring up the famous "bucket theory" of flight. During an autorotation, you are managing your energy. Your energy stores are in three buckets:

1. The Rotor RPM
2. Altitude
3. Airspeed

So, we call all these buckets our "potential energy". Potential meaning it is a source of energy we can convert to kinetic energy or "energy of motion". So, at the top of an autorotation, we have energy in our altitude, rotor RPM, and airspeed. We bleed off the altitude in the beginning (the altitude is converted to forward airspeed and rotor RPM). At the bottom, we have no altitude left (or very little), so we next go to our airspeed bucket and bleed it off with the gentle flare. Now, we have very little to no airspeed and altitude and about 10-20 feet between us and the ground (at least you hope you do). We have one bucket left…the rotor RPM. If we screw this one up, guess what, we are bucketless! Make it count and apply collective just before impact.