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Subject: R/C Flying: FAQ Part 2 of 2/rec.models.rc
Summary: A Beginner's Guide to Radio Controlled Flying
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Date: 7 Dec 92 15:00:13 GMT
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Archive-name: RC-flying-FAQ/part2
Last-modified: May 13 1992

============================== Part 2 ========================================

              ::::::  -- Powered (gas)  -- ::::::

Even though "wet" power is called "gas", it's not the same as car
gasoline. Model fuel is usually a mixture of a lubricant (synthetic or
castor oil), methanol and nitromethane. The power plants are usually
called engines, as opposed to electrics, which use motors (see below).
Engines are available in 2-stroke (louder, cheaper, and more powerful
for the same displacement) and 4-stroke (a more scale sound, less
vibration, but more expensive). Engine displacements are usually
measured in cu. in. the US (A 60 engine = 10cc [actually 0.61 cu. in.]).

Compared to beginner's gliders, powered trainers are more difficult to
master.  This means that everything about instructors and equipment
checks goes DOUBLE for powered planes. There are many, many ways a
beginner can make mistakes and destroy a model that he/she has spent
alot of time and money on. With the typical powered trainer, going it
alone is foolhardy and will likely end with a destroyed model and a
very disappointed modeler.

If you can't find an expert that is willing to teach you, it is best to
start with a 2-3 channel model with a long wingspan and alot of
dihedral. The ideal thing to start with here would be a 2 channel
glider. If you must start with a powered plane, a Sig Kadet is one of
the more docile trainers.

If you have an instructor, but have not flown R/C before, you can start
with something a bit more advanced. The Great Planes PT-20/40/60 series
are good.  You can build these with ailerons, but due to their large
dihedral, they can also be flown without ailerons. It won't hurt to
have them built-in. Even though they will not be very effective, they
will get you used to using them. Other recommended planes are the
Midwest Aerostar and the Goldberg Eagle.

If you have an instructor, and have flown R/C gliders, you might want
to start with something still more advanced, say a Great Planes Trainer
20/40/60 or the like. These have a fully symmetrical airfoil and less
dihedral. They are capable of more in the way of aerobatics, but are
trickier to fly due to higher speed and less stability.


             ::::::  -- Electric Flight  -- ::::::

>I didn't know that you could put an electric motor and batteries
>in an airplane.  Isn't that kind of heavy?

Modern NiCd batteries are pretty amazing.  You can charge them in 15
minutes, take power out of them at up to 50 amps or so, and do it all day.
That capability is what makes electric flight possible.  Electric power can
be used for any kind of flying---gliders, aerobatics, even racing.  It's an
excellent choice for sport flying.

>What are the advantages and disadvantages of electric flight
>compared to wet power?

Electric power systems are heavier for a given power output.  This means
that planes must be built lighter, which may be more challenging
(especially for the beginner).  That's really the only significant
disadvantage.  The big advantages are that electrics are quiet and clean.

To me, the biggest advantage of all is that electric flight is unusual and
interesting.

>What is the best way to get started in electric flight?

That depends on what you want to do and where you're starting from.

If you've never flown RC before, and you want to start with an electric
plane rather than a pure glider, I recommend an electric glider like the
Airtronics Eclipse.  This will give you the best chance to stay ahead of
the plane.  In the sport/trainer category, I hear a lot of good things
about the Leisure Amptique.

If you know how to fly RC, you have a lot of choices.  The simplest and
most available electric power systems use six or seven cells.  These are
called "05" systems, and are very similar to the power system of an RC car.
You can find all sorts of planes in kit or plan form which will work well
with these systems.  Outstanding examples are the aforementioned Eclipse
and Amptique, old timers such as the Leisure Playboy and Astro Viking, a
variety of semi-scale kits from Davey Systems, all sorts of gliders, and
the aerobatic ElectroStreak from Great Planes.  Any two-meter glider kit
can be easily adapted to six or seven cell electric by a moderately
competent builder.  Just stick a motor in the nose, battery under the wing,
and go.

If you want more performance, good ground handling, or just like larger
planes, there are larger power systems available, all the way up to systems
which will handle a 60-sized power plane.  The cost and complexity, of
course, go up with size.  Any reasonably well-designed power plane kit or
plan can be adapted to an appropriately chosen electric power system.  The
first step is to leave out half the wood---all power planes are grossly
overdesigned.  Electric motors generate very little vibration, which helps
you get away with lighter structures.

>What are the elements of an electric power system?

The power system includes a battery, a motor, a control, and wiring.  The
battery is almost always made up of Sanyo NiCd cells in the appropriate
number.  Motors vary from the simple, cheap "can" type (otherwise known as
"540" or "550" style), through more sophisticated styles adapted from RC
car motors, up to the cobalt powerhouses.

Controls can be a simple on-off switch controlled by a servo, a directly
controlled on-off switch, or a proportional electronic control.

If you are going to fly a glider or old-timer type plane with less than a
500-watt motor, think seriously about getting battery packs made of Sanyo
900 SCR cells.  They are significantly lighter than the more usual 1200 mAH
(sub-C) cells and give excellent performance.

>  What do the various letters used to refer to NiCd cells mean?

A:  SC is the basic cell.  SC cells will take fast charging and have
reasonably low internal resistance.  SCR cells have lower internal
resistance and a somewhat flatter discharge curve, that is, they put
out nearly the same voltage from beginning to end of the discharge.
SCRs are best for high current drain applications.  SCE cells have
somewhat more capacity for the same physical size, but also have
higher internal resistance.  They are best for low current drain
use (less than about 10 Amps.)  The higher capacity of SCE cells
will not be realized at high current drains, and they will heat up
more than SCR cells.


Many kits nowadays come with a power system.  In most cases, these systems
are adequate for the application.  It won't hurt to try what's there to
start with, you can always experiment later.  If the kit you choose doesn't
come with a motor, of course, you'll have to choose one.  If you are a
beginner, go with the recommendations of the kit manufacturer.  If you are
an experienced RCer, you probably don't need my help.

For a six- or seven-cell glider or old-timer with a cheap motor, an on-off
switch is sufficient control.  For anything else, you will have much
greater enjoyment with a proportional throttle.  Get a high-rate control,
they are much more efficient at part throttle.  There are several good
brands out there, but I like Jomar for good controls at good prices.

>What support equipment do I need?

You need a charger of some sort.  If you are using six or seven cells, any
RC car charger will do the job.  You don't need peak detecting or any of
that fancy stuff to start with.  For larger packs, there are good
high-voltage chargers around.  Check out Astro Flight and TRC, among
others. Remember, the biggest enemy of NiCads is heat, so try and keep those
batteries cool when charging. Expect to pay about $40.

>How are motor sizes specified?

Motors are traditionally specified by a system which attempts to equate
them to wet engines.  There are significant problems with this, but they
probably aren't of concern to beginners.  An "05" motor takes a six or
seven cell battery and puts out 75 to 120 watts, and so on up to a "60"
which takes 28 cells and puts out 1200 watts.  Incidentally, there are
about 750 watts in a horsepower.

The actual power output for a given voltage (number of cells) depends on
the load.  Unlike wet engines, electric motors put out more power with more
load.  If you don't like the performance you get from your plane, you can
try a bigger propellor---up to a point.  More power, of course, means less
run time.

In the ideal world, motors would be specified by the total power they are
capable of supporting and by the number of cells (or voltage) with which
that power is produced.

>What's a cobalt motor and why would I want one?

Rare-earth magnets, of which the most common type is samarium cobalt, are
stronger for a given weight and volume than ferrite magnets.  Perhaps an
even more important reason for getting a cobalt motor is that they also
have better brushes, bigger shafts, better bearings, are built more
carefully, and so on.  For the serious electric flier, they are worth the
extra expense.

>Where can I get this stuff?

Electric equipment is somewhat specialized, and most hobby shops aren't yet
sufficiently enlightened to carry very much.  You can use RC car equipment
for a lot of things (after all, they developed this stuff in the first
place) and your local hobby shop will have lots of that.  If you want to
get more sophisticated, get the catalogues from Hobby Lobby and Hobby Horn
(both have ads in all the usual magazines.)  Both catalogs contain a lot of
detailed information that I can't fit in here.  Hobby Horn has good prices
on mainstream stuff.  Hobby Lobby sells the lines of several European
manufacturers, and tends to have higher prices for fancier (or at least
more unusual) stuff.  I haven't dealt with CS Flight Systems on the East
Coast, but I read good things about them.


           ::::::  --  Some Aerodynamics  --  ::::::

The aircraft can rotate around three axes: the fore-and-aft axis (or the
_roll_ axis); the spanwise (nose-up/nose-down) axis or the _pitch_ axis;
and the nose-left/nose-right, or _yaw_ axis.

Speed:

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
control.

Turning:

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
level.

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
gliders, _polyhedral_).

     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)
                                OR
                         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
extent.

The Stall:

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
plane up."

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
aerodynamics.

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.

--
--
  Shamim Mohamed / {uunet,noao,cmcl2..}!arizona!shamim / [e-mail]
  "Take this cross and garlic; here's a Mezuzah if he's Jewish; a page of the
    Koran if he's a Muslim; and if he's a Zen Buddhist, you're on your own."
   Member of the League for Programming Freedom - write to [e-mail]