Lesson 10: Autorotation

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Lesson 10: Autorotation


Autos 101

As most of the club members know, I just started doing successful autos, so I may not be an expert on the finer points, but what I lack in experience, I hope I can make up for in freshness of the experience.

Although I wasn't sure I was ready to begin autos, I felt I had to learn them as all of my recent crashes have been engine out affairs (Except one while learning autos, but don't be scared).

I crashed my Xcell .60 that way, and I felt that if I had just a little practice, I may have been able to save it.

If you can save one bird from a crash due to a flame out, it's worth your time to learn, plus they are a lot of fun and not as hard as they seem, just a little scary.


Pre- Auto Skills

To begin, you must be absolutely comfortable in hovering from front and side.

Nose in would be helpful, but I'm not nose-in proficient, so it isn't a necessity. You must be very comfortable in forward flight from all angles. Although my nose-in hovering is shaky, my nose in forward flight is fine.

Then you should start practicing "airplane" style approaches. When I first started in forward flight, when I wanted to stop, I would come a dead stop at about 30 to 40 feet high, then hover down. This is safe, but useless for autos and makes you look like a wimp. Practice coming in with an angle of decent like a plane, flaring a few feet off the ground, then stopping in a hover at 3 or 4 feet.

Don't worry if you can't do this right off, try it at 20 feet first, then work your way down. It's scary at first being close to the ground and still moving that fast, especially with your tail boom low in the flare. Start high and work your way down.

This is actually the exact same skills you'll be using in an auto!


Auto Theory

Most of you have collective pitch birds.

You cannot auto a fixed pitch bird like a LMH-100.

To give some basic physics:
In an auto, you are converting the energy of your altitude (Remember potential energy=mass x gravity x height) into kinetic energy, stored in the form of rotating rotor blades (I don't remember the exact formula for kinetic energy of a rotating body, but it's roughly r.p.m.^2 x mass x radius) Since RPM is a squared, it is where the real energy comes from.

If you double the RPM, you get 4 times the energy. If you double either the mass or length, you only get double the energy. This will be needed later.

Okay, you need RPM. How is this done?

When the rotor blades are under power, and providing lift, they have a positive angle of attack. They are blowing wind down.

However, with no power, the RPM decays to zero due to drag and the fact that, as the helicopter falls, the moving air now acts in the opposite direction of their rotation due to the positive angle of attack! So, you shift them to a negative angle of attack and the wind blowing up through them spins them faster!

You need to be able to shift the angle of the blades, which is why a fixed pitch machine will not auto.

Now you are going down with the blades at negative pitch because your didn't panic and you lowered your collective stick. The helicopter keeps falling like a rock right? Wrong.

The rotating blades act as a brake! They are soaking up kinetic energy, thus they are slowing you down. Aerodynamic theory of lift from negative pitch is beyond me, but it works. Important to note here is that the more negative pitch you have, the faster the helicopter falls, but the more RPM is built up.

More RPM=more potential energy.

So now the ground is approaching rapidly. You have built up a lot of energy in your blades, which have peaked at a certain RPM depending on the negative pitch.

Well before you land, remember you are also moving forward at this point. You must flare to kill of any forward speed, or as much as possible. You pull back on the cyclic.

You can actually gain altitude at this point if you are not careful. Here again you convert the kinetic energy of forward flight into blade RPM. Not nearly as much as the descent, though, just a bit.

Now you have a lot of energy stored in your blades, but you are still sinking. You start adding pitch to cushion your fall (you may have already added some in the flare), and if it all works out right, your rate of decent will be zero at touchdown, and if your set-up allows it, you may be able to hover for a few seconds before gently touching down.


Mechanical Set-Up

Most important: make sure you engine is running right.

Flameouts are not what you want here. Your model needs to have a nice, fat 4-cycle idle, well below the point where the centrifugal clutch engages. It should drop into that idle quickly and not buzz for too long, and then it should then transition instantly when you give it gas.

If it stumbles, you are asking for a kiss from terra-firma. If your engine doesn't transition flawlessly, adjust your idle mixture, glow plug, fuel, pipe length, etc. until it does.

Set your throttle hold point so that it drops the engine to this real nice idle point.

Second, if your clutch drags at a low idle, fix it now, then make sure the one-way auto clutch between your main gear and your drive shafts doesn't drag. This eats up energy. You guys with driven tails during autos, make sure you system is silky smooth.

Your tail rotor eats enough energy without wasting it.

Blades: Remember how I said rotor RPM was the biggest factor?

Well it is, but rotor weight is important too. I am doing autos on a Shuttle with 98 gram NHP symetrical blades. (Some said it couldn't be done! Nyah Nyah!) With covering, they weigh out to about 103 grams.

I also HAD a nice pair of Hirobo glass over wood symmetrical blades that were around 120 grams that auto'ed a little better, but they list for about $130. I also HAD a pair of Xcell blades on my Xcell .60 that weighed around 235 grams.

I don't know how they auto'ed, but they exploded great when they hit the ground!

My point is that heavy blades auto better, as they store more energy, but don't get hung up on it. Also, remember that the Shuttle Z and ZX head do not have thrust bearings in the blade holders, so I wouldn't go much above 100 grams for those blades.

The ZXX heads do have thrust bearings and I've had no problem with the heavier blades. Most Xcell blades are just under 200 grams, (I think), but the 235 grams blades didn't cause any problems. I do not know about Concept .30 blades or stock (non-symmetrical) Shuttle blades, but I will let you know.

Your blades should also be rather tight in the grips. The accepted rule is that, while gripping the tip of the blade, and the tip of the fly bar, you should be able to bend the fly bar at least an inch toward the blade without it slipping in its grip for a .30 machine and 2 inches for a .60.

Don't worry about too tight. The centripetal force on the blades WILL straighten them out, but you may get some shake as the blades initially spool up to speed.

The reason for this is to prevent boom strikes. I learned the hard way that, as you add pitch at the very end of an auto, blade drag can be greater than centripetal force, pulling the blades back if the holder is not tight enough.

This pulling back, combined with the fact that the blades are at about 10 degrees of pitch causes them to be angled right down at the boom. I found this out when I was already safe on the ground after a good auto, I continued pullin

g pitch and the whole thing exploded as the blade hit the boom.

Pitch curves: This is where I may get the most flack, but I know what worked for me.

I set up my throttle Hold curve for autos to be almost exactly like my Normal curve. My low pitch is -3 degrees, mid-stick at 4 degrees, and high stick at about 10 degrees (Normal high stick is 8 or 9 degrees, but all the rest are the same).

This is for a Shuttle, and the Xcell guys may disagree, but this definitely works. Why this way? Okay, -3 degrees gives a reasonable rate of descent in Shuttle, not too fast, but still plenty of rotor speed. Remember that you have to stop the falling machine at the bottom, and you have to have time to react.

I have not yet experimented with more negative pitch, but I know this set up works. Some guys swear by -5 or -6 degrees, but my feeling is that you'll have enough to do without having it going so fast that you don't have time to do it.

Why 4 degrees at center stick instead of 0 degrees? Practice purposes.

First, when just learning autos, you do power recovery autos, which will be explained, and they are done in the Normal flight mode. Second, I also practice "low" autos a few feet off the ground, and when I flip the switch to Hold, if the curves are the same, there is little jump.

Top end pitch in Normal mode is, of course, set by engine strength and the RPM you are happy with.

Why did I go for 10 degrees in Throttle Hold?


I only have about 19 degrees total pitch range in my Shuttle, and if I want to allow -9 degrees for inverted flight (someday) the most I could go at the top was +10 degrees. This seems to be plenty. With minor modifications, I could get more range, but for now, this seems like enough.


First, practice on a day with a 10 to 15 mph breeze. This will help keep the tail straight and the nose pointed into the wind (remember, most of you do not have tail control in an auto, so the bird will always weathervane into the wind).

Second : Don't touch the Hold switch yet.

Instead, use your throttle trim switch! I normally fly with my trim switch almost all the way up, set to where the clutch does not dis-engage in a descent at low stick, but keeps the rotor speed constant.

With the bird sitting on the ground, move the trim switch to the point where the motor is happily idling, as discussed earlier, but the clutch is not engaging. (This is also where your throttle Hold should also be set.) If you give it a little up stick, the motor will rev and it will act as it normally does, but at full low stick, the rotor will spin free.

Now, remember how I told you to practice your "airplane" style approaches?

Start doing this. Your approach should be fairly steep 45 degrees or more, beginning at as high an altitude as you feel comfortable with and still see the machine, in fast forward flight. Drop the collective all the way down, and keep the cyclic forward.

Here's what will happen: The machine will drop suddenly at first, say 10 or 15 feet, but as the rotor picks up speed (Which you will hear, as the engine is idling), the descent will stabilize and slow down and it will act just like it did before under power when you were doing airplane approaches.

Remember to keep the nose down, and it will fly just like it always has! You will lose tail control, but the wind will keep it straight.

Now, as you get down to 75 feet or so, do a flare. Pull back on the cyclic. The bird will come to a stop, all under no power! Rather than describe this flare, just try them. You'll quickly get a feeling for how they should be. Now, add collective before the bird starts to fall again and go around for another try!

The tail may twitch some as power is brought back in, so be forewarned. This is called a power recovery.

The advantages to this method of practice are as follows:

First, you can't "forget" where the hold switch is at a critical time.

When you get low, you add stick just like you normally would and the machine acts normally, no surprises. Yet, you get the feel for a bird in an auto descent, and the flare. Once you have this mastered, you are two-thirds there! Practice this over and over, leaving the hold switch alone, until you can flare and end up just a few feet above the ground.

It may seem scary, but start high and work down! Practice, practice, practice. This should be automatic. As a matter of fact, I always do a few power recoveries before I try any auto's, just to warm up, so to speak.

The other thing I practice are what I call "low" autos. From a 2 or 3 foot hover, hit the hold switch.

If you do not have the rudder offset set properly for your hold mode, be prepared to feed in rudder.

Add pitch and hold the bird off the ground as long as possible. I always practice this at the end of fuel tank, to burn out the fuel. It's a safe way to run the tank dry.

This is also why my Hold pitch curve and my Normal pitch curves are set the same, the machine doesn't jump at hover when I hit the Hold switch! You may note that a two foot hover is the last third of the auto, the first two thirds you have been practicing as power recoveries above.

Okay, you are comfortable with power recoveries, now learn to work the Hold switch.

Set up just like you have been for the power recoveries, go to low stick, then hit the hold.

Note I said hit the hold AFTER going to low stick?

This conserves rotor RPM, Thy Staff of Life. Descend for a short while until you are stabilized, i.e. no more RPM, then turn Hold off, go around and do it again. Practice this over and over, until the Hold switch is completely automatic.

When it is, start coming in lower and lower and flaring too, just like you had been doing without the Hold switch, with just one more step. Start high, work your way down.

Soon, when nobody is out on the field, or the other guys have been egging you on, and you are feeling totally confident with power recoveries and low autos, you will go to low stick, hit the hold, descend, flare, and then add pitch and bring the machine totally to the ground, blades still turning!

It will actually be rather anti-climatic after all the practicing, but it will feel good!

If anybody would like to discuss autos, or wants whatever help I can offer, let me know, I'm more than happy to help. I also suggest you talk to some of the club gurus and read Ray Hosteler's book. He has a great section on auto's.

You can borrow my copy if you leave me your transmitter as a security deposit! ; )

The following article was contributed by Kenneth Rudd