R/C Flying: rec.models.rc FAQ
(From Greg Johnson)
Getting the hang of flying an R/C heli is a fairly challenging undertaking.
It's like riding a bike: when you first start trying it seems impossible,
but with enough practice it starts to seem easy, like second nature. It
may take 5 or 10 sessions to get to the point of being able to hover with
some consistency. Helicopters provide a long sequence of challenges, and
the corresponding satisfactions of mastering them. After hovering, there
is forward flight, nose-in hovering and flight, auto-rotation, aerobatics,
inverted flight, etc.
There are several good helicopters on the market. It's a bit like Ford
people versus Chevy people: different people develop preferences for
different helis. Good ones to learn on include the Hirobo Shuttle, Kyosho
Concept .30, and Kalt Enforcer. An excellent although somewhat more
advanced heli is the X-Cell .40. Also, Shluter makes first-rate R/C helis.
Check out the local hobby shops to see what the well-supported helis are in
your area, and if possible find where the locals fly. Hang out at the
flying field for an afternoon or two, and see what the locals are flying.
The helicopter itself will cost from $250 to $400 for a good starter heli.
A radio will cost $200 to $450 or so. Gyro is about $70. Engine is about
$130. Starter box, starter battery, etc. will probably be at least another
There are two excellent books. Paul Tradelius's book (available through
Model Airplane News) is particularly good for beginners. He presents the
material in an order and a depth that is well suited to getting started. A
more encyclopedic book is the one by Ray Hostetler. This book goes into
great detail on all topics, and is a book to grow into. Ray's book mixes
beginner info and info necessary only for advanced pilots, and consequently
can be a bit overwhelming at first. There's a lot of stuff in there that
you won't need to delve into for quite a while. I would recommend getting
both of these books.
There are a million accessories that you can buy. There are a relative few
that are indispensable, or almost so. I'd put the following items on the
short list: a prop balancer, a pitch gauge, a pair of ball link pliers, and
a receiver battery tester. You will need a standard assortment of tools
such as needle nose pliers, screw drivers, hex wrenches, etc. You'll also
need a starter and starter battery.
There are a couple of pretty good electric helis on the market. One is
made by Kyosho (the Concept EP), and one is made by Kalt (the Kalt
Whisper). These machines are small, light, delicate, and squirrely. Not
the thing to try to learn on. They are more novelty items for experienced
R/C heli pilots.
On most R/C helis (and full-scale helis for that matter), the main blades
can change their (so-called) pitch angle. What this means is that if you
sit the heli on a table and look at the tip of one of the main blades, the
chordline of the blade can be tilted through a range of angles by the
servos. In this sense, the rotor disk of a heli is a bit like a
variable-pitch prop on an airplane. If the heli is hovering and you wish
to make it climb straight up, you increase the pitch of the main blades,
and increase the throttle so that the engine can overcome the increased
drag and keep the blades turning at the same speed. The increased blade
pitch results in more lift, and so the heli climbs. (With R/C helis,
unlike R/C airplanes, engine RPM's are supposed to stay the same over (most
of) the throttle range. At high throttle the engine puts out more power,
but there is a corresponding increase in the load on the engine due to
increased main rotor blade pitch, and so the engine stays at the same
RPM's.) This overall increase in pitch that makes the heli climb is called
To get the heli to pitch forward or back, and to roll left and right, there
are controls that are analogous to airplane elevators and ailerons. These
controls are referred to as cyclic controls. The idea is to set up
asymmetric lift on the rotor disk. (This is similar to what ailerons do to
an airplane-one wing can be made to generate more lift than the other, and
so the airplane rolls.) If there's asymmetric lift on the rotor disk, the
plane of rotation of the rotor disk is going to change. For instance, the
rotor disk (and the heli that is attached to it) might go a bit nose-down.
In that case, the heli will transition out of a hover and start flying
forward. Similarly, the heli can be made to lean back (nose-high), left,
right, or any combination of these. The way this asymmetric lift is set up
is to vary the pitch of each blade as it goes around. For instance, say
you push forward on the cyclic control stick (the right one on your
transmitter, which does the same thing as an aileron/elevator control stick
on an airplane radio). This will make the blade pitch down as it travels
through the forward-moving part of the rotor disk (usually the left side of
the rotor disk), and it will make the blade pitch up as it travels through
the backward-moving part of the rotor disk (usually the right side of the
This is a counter-intuitive aspect of helicopters, that even many advanced
pilots don't clearly understand. In order to get the helicopter's rotor
disk to tilt (for example) downward at the front, you increase the lift on
the right side of the rotor disk and decrease the lift on the left side of
the rotor disk. (This is assuming the standard clockwise main rotor
rotation.) To see why this is so, consider the following example. If the
heli is in a nose-down attitude, the forward moving blade travels downhill,
and the aft-moving blade travels uphill. The blades travel level at the
front and back. To get a hovering heli to go into a nose-down attitude,
you need to encourage the forward-moving blade to start going downhill and
the aft-moving blade to start going uphill. Hence, pushing the cyclic
stick forward causes lift to be killed on the forward-moving (left) part of
the rotor disk and increased on the aft-moving (right) part of the rotor
There are usually five servos on an R/C heli. One controls throttle, one
controls collective, one controls fore-aft cyclic (analogous to elevator),
one controls left-right cyclic (analogous to aileron), and one controls
tail rotor pitch (analogous to rudder).
The gyro is positioned so that it senses yaw. It then feeds small inputs
to the tail rotor servo to counter the yaw that it detects. This keeps the
helicopter from yawing to the left and right when you don't want it to.
Left-right movement of the left stick also supplies input to the tail rotor
servo; so you and the gyro are both giving control inputs to the tail. A
gyro is a MUST. It's probably not an exaggeration to say that gyro-based
stabilization of the tail rotor made R/C heli flying feasible. It is
possible to fly an R/C heli without a gyro, and it's also possible to
juggle seven balls. It's just darn hard! Furthermore, it's definitely not
something you want to try tackling when you're just getting started.
Without a gyro, the heli can begin to whip around wildly as soon as the
skids leave the ground. The heli will do a 180-degree turn and you're
looking at an angry helicopter coming right at you before you know what
happened. Definitely not something for a beginner to tackle.
Helicopters with collective are now inexpensive and reliable. Every
reasonable modern heli, from beginner-trainers up to FAI world-beaters, has
collective. In a fixed-pitch heli, lift is controlled by varying engine
RPM, just as in an airplane. This is an outdated technology, and you will
outgrow such a heli very soon. Virtually no aerobatics, no auto-rotation
(if the engine quits at altitude, the heli becomes a brick), not as much
You need five channels to control a heli. You need one each to control
pitch, roll, and yaw. You need one to control throttle, and you need one
to control collective. You might think that one servo could control both
throttle and collective, since they are related. There are several reasons
this wouldn't work, however. The main rotor disk of a heli is huge and
generates a correspondingly huge amount of drag compared to an airplane
prop. (If you think of the heli rotor disk as a big propeller, its
actually pretty amazing that a tiny little .32 engine can turn it at all.
There's about a 10:1 gear down from the engine to the main rotor, which
makes it possible for the engine to turn the main rotor.) So, you have to
have fairly fine control over the relationship between the collective pitch
(and corresponding drag) and the throttle setting. If you get it wrong,
the engine bogs badly or races wildly. Also, auto-rotation is an important
maneuver, and this entails control of collective pitch while the throttle
is set to idle. Finally, for inverted flight you want to have full
throttle both at maximum up collective and maximum down collective.
- Pitch curves and throttle curves
- You can adjust the amount of servo travel
at 0% stick, 25%, 50%, 75%, and 100%, both for throttle servo and
collective servo. This feature is a must.
- Throttle hold
- Flip this switch to practice auto-rotation; the throttle is
reduced to idle. All the other controls still work normally.
- Idle up
- This is an alternate mode, usually used for aerobatics. You can
set throttle and pitch curves, mixes, etc., and change over to the
different setup at altitude or whenever.
- Programmable mixing
- This neat feature lets you establish a relationship
between channels. One channel is designated as the input or master
channel. As the master channel varies, it causes small changes to the
output channel. This is an advanced feature.
- Revolution mixing
- This feature causes increases in tail rotor as throttle
and pitch increase. This is useful to compensate for the increased torque
the engine produces. I feel that this is a somewhat over-rated feature,
and that it only really comes into its own when you're doing aerobatics.
Even then, a programmable mix may be better.
- Electronic trim adjustment
- Similar to and augments mechanical trim
- End point travel adjustment
- Sets where servos go at max stick displacement
- Can be used to make cyclic less sensitive in midrange.
It is possible to control a helicopter with a 4-channel airplane radio.
You can master hovering and move into elementary forward flight this way.
For anything beyond that, you will need a helicopter radio. If you do try
to use a 4-channel airplane radio, build a Y-connector, and control two
separate servos (collective and throttle) off the throttle channel. Then
adjust control arms to get a form of mechanical throttle and pitch curve
adjustment. It's not too hard to set a heli up so that it will hover
tolerably well at mid-stick this way, and you can contrive to increase lift
above mid-stick and lose lift below mid-stick.
If a heli's engine quits in flight or you simulate this by going to
throttle hold mode, it is still possible to glide the helicopter down
safely. As the helicopter descends, the wind flows up through the rotor
disk from below. At a low or negative collective pitch setting, the wind
flowing up through the rotor disk keeps the blades spinning. Heli blades
usually have lead weights epoxied into the tips, so as the blades spin they
build up a fair amount of rotational inertia. When you are near the ground
and ready to land, you add in collective to increase lift, and the inertia
maintains head speed sufficient to execute a controlled landing. In
theory. ;-) Auto-rotative glides and landings are beautiful to watch. A
helicopter can sustain as much as a 4:1 glide ratio in auto-rotation.
Helis can do awesome aerobatics: loops, rolls, pirouettes, you name it. My
personal favorite is inverted flight. If looks 'way cool to see a
helicopter hovering inverted right above the grass. I've seen guys do
aerobatic routines flying the whole thing BACKWARD. With a helicopter you
have unbelievable versatility.
Helicopters can go so high they are out of sight. Being able see the thing
in order to control it is the only limit on how high they can fly. R/C
helis can go 60-80 MPH or more.
(From Shamim Mohamed)
The aircraft can rotate around three axes: the fore-and-aft axis (or the
ROLL axis); the span-wise (nose-up/nose-down) axis or the PITCH axis;
and the nose-left/nose-right, or YAW axis.
The cross-section of the wing has a shape called an "airfoil". It has the
property that when it meets the air (usually at some small angle, called
the "angle of attack") it generates an upward force (lift) for a small
backward force (drag). The amount of lift (and drag) depends on the
airspeed and a value called the "lift coefficient" (and a few other
things like surface area and density of the air). If the plane is in
unaccelerated flight, the upward force (approximately equal to the lift)
is equal in magnitude to the weight of the plane, which is a constant. It
thus follows that the total lift generated by the wing is always constant
(at least in unaccelerated flight). [One example of accelerated flight is
turning - see below]
The above mentioned "coefficient of lift" (abbreviated Cl) depends on the
angle of attack. Usually, as the A-of-A is increased, Cl increases; to
keep the lift force constant, speed can decrease. So to fly fast, we
decrease Cl (and A-of-A); to slow down, increase Cl (and A-of-A). Since
the wings are fixed, we alter the A-of-A by pitching the entire plane up
or down. This is done with the elevator. The elevator is thus the speed
To turn a body moving in a straight line, a sideways force must be
applied to it. For a plane, the best method for generating a force is to
use the wings. To get them to act sideways, we roll the plane: now part
of the lift is acting sideways and voila! a turn. To roll the plane, we
use the ailerons (the movable surfaces at the wingtips). Also, notice
that now since part of the lift is acting sideways, the lift force in the
upward direction is reduced; but the upward component of the lift needs
to be equal to the weight of the plane i.e. we need a little more lift
from the wings, which we can do by increasing Cl - i.e. by pulling a bit
of up-elevator. That's why to turn in a plane you push the stick sideways
in the direction of the turn and then pull back a bit to keep the nose
What happens if you try to turn with the rudder alone? The application of
the rudder will cause the aircraft to yaw, and it will continue to travel
in the same straight line (more or less), skidding. (Think of a car on a
perfectly slippery road - if you try to turn just by turning the wheel,
you'll skid but won't turn). So we need a roll to turn.
But most of the trainers we see don't have any ailerons! How do they
turn? They use a configuration of the wings called "dihedral" (or, for most
Flat Dihedral Polyhedral
-------O-------- ~~~----___O___----~~~ ~~~~~~~----O---~~~~~~
^ ^ ^ ^ ^
0 angle between small angle between small angle between 2 wing
2 wing panels 2 wing panels panels and also small angle
within each panel (Gentle Lady)
0 angle between 2 wing panels
and small angle within each
panel (Olympic 650)
When we apply rudder (say left rudder) to a plane with dihedral, what
happens? The plane yaws; the right half of the wing then sees a greater
angle of attack than than the left half:
/ / / / / / <--- airflow direction
left wing right wing
(You can try this out if you don't believe it: take a piece of paper and
fold it slightly, like dihedral; then look at it end on, but slightly
off-center, i.e. from the point of view of the approaching airflow. You
will see that you can see more of the underside of one half than you can
of the other.) And what does an increased angle of attack do? It
increases the Cl and the lift generated by that half! So we now have the
right wing generating more lift and the left less; the result is a roll
to the left. With polyhedral we get the same effect, only to a larger
If you try to fly slower and slower by pulling back on the stick (i.e.
applying up-elevator) you will reach a point where the plane "falls out
of the sky" or the stall. What happens is that an airfoil will only
"work" up to a certain angle of attack. When that angle is exceeded, the
airflow above the airfoil breaks up and the result is an increase in drag
and a drastic decrease in lift, so that the wings can no longer support
the plane. The only remedy is to reduce the A-of-A i.e. to push the nose
down. This may be a little difficult to do when you see your plane
falling---the natural tendency is to pull back on the stick, to "hold the
A development of the stall is the spin. Volumes can be written about it,
and have been; go to the library and check any book on introductory
If you want to know more about Aerodynamics as it applies to Model
Aircraft (the small Reynolds' number regime, as it is sometimes called)
check "Model Aircraft Aerodynamics" by Martin Simons [Argus Books,
ISBN 0 85242 915 0].