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?
Simple.
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.
Technique
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
