The big moment arrives when you emerge from the shelter of your workshop
to put theory into practice. To get off to a good start, make it a
point to bring along the right equipment and a predetermined testing
procedure. For any model more complex than a hand-launched glider or
balsa ROG, the first necessity is a good flying box, containing a few
well-chosen tools and supplies, so you can make necessary adjustments
and minor field repairs without having to retire to the workbench, which
is sometimes miles from the flying field. A metal
fishing-tackle box can be easily adapted for the modeler's use, or you
can build a simple plywood box, like the one shown in Figure 33, with
room for everything needed.
Here are the items you shouldn't be without to be prepared for any kind of flying:
Fuel
(diesel or glow, to suit your engine), 1-1/2 volt battery (for glow
engines), Battery leads with alligator clips on both ends, Fuel pump, (a
hypodermic syringe with 1/16" O.D. needle ground square is good), Extra
fuel tubing, three sizes, Spare glow plug with washer, Glow plug
wrench, 6-inch adjustable crescent wrench, Long engine-mounting bolts
and nuts, Small wood screws (for emergency mounting of engine), Spare
engine (or rubber motor) to fit model.
Propellers and
reamer if needed to make it fit shaft, Fuel tank for testing or
emergency use, Razor blades and/or pocket knife, Slip-joint pliers,
Small screwdriver, Sheet lead or strip solder for weights, Fast-drying
fuelproof cement, Straight pins, Clear dope - 4 oz. bottle, Brush-flat
1/2", Thinner - 4 oz., Spare needle-valve assembly complete, Hacksaw
blade, Rubber bands in several sizes - plenty of them. A lump of
modeling clay (wrapped in waxed paper) for weight.
Sandpaper
in two grades, Thin aluminum sheet for trim tabs, 1/8-inch plywood for
emergency firewall repair, etc. Balsa, a few strips of each thickness,
Sheet balsa 3" x 12" in several thicknesses. One sheet of light-weight
tissue (in waxed paper envelope) for patching, Spare control lines,
Spare handle, Spare wheels to fit model, Spool of fine copper wire,
Small package of tissues or soft cloth, 1/16" and 1/8" drill bits -
use as augers to make emergency mounting holes.
Some
modelers carry more than this - but these items will cover most field
emergencies. Adapt the list to fit your situation; if you use only glow
engines, diesel fuel is unnecessary - except to lend to a pal in need.
The diesel flyer won't need batteries, and a rubber-power fan can get
along without fuel - but he needs a spare rubber motor. Get duplicate
tools and equipment for your flying box, rather than use your regular
workshop equipment, so you won't have the job of making up the kit each
time you go out to the flying field.
Fig.33
The
next consideration is a place to fly. This would be first, but for the
knowledge that a modeler armed with a ship that's ready to fly will
inevitably find a place to fly it. If you live in a small city or town,
or in a rural area, this may seem an unimportant consideration. The
big-city dweller may find it quite a problem - but it can be solved. If
you fly only C/L, you won't need a great deal of space, but it will be
necessary to have the permission of the owner of the site and an OK on
the noise from any near neighbors. To non-modellers, engines make quite a
racket. The use of small engines and mufflers helps on this point.
Your
C/L site should be at least half again as broad as the diameter of the
flight circle; i.e., 60 feet square for 20-foot lines, 210 feet square
for 70-foot lines. This will allow room for a few bystanders, and space
to maneuver if the lines go slack in a gust. Be sure to warn your
audience (you always have an audience) to stand well back, and keep the
model above head-level when in flight - lines have been known to break,
though rarely both at once.
Night flying - a C/L scale model.
For
free-flight you'll need a generous field, at least several acres in
extent, even for modest-sized rubber jobs. If your model stays aloft for
just one minute and a light breeze is blowing at ten miles per hour,
the ship will drift one-sixth of a mile, even though it flies in a
perfect circle. It's ground travel that counts, not air miles. Pick a
field without trees, fences, buildings, etc. and try to find one not
bordered by steep roofs, TV antennae, or tall trees. If your ship flits
past the cleared area, its going to be hard to find in that sort of
jungle.
The perfect F/F flying site is an abandoned or
little-used airport. Some fields with only one or two scheduled flights
per day will allow modelers to use the runways between flights. Once you
have gotten permission to use a field, don't abuse the privilege;
observe whatever rules or limitations the owner imposes, and see to it
that other modelers do the same. A good flying site is too valuable to
lose by carelessness.
Once on the field (pick a calm day
for testing) with your flying box, run through a preflight check. If the
model is a free-flight, rubber or engine powered, look at the flying
surfaces (wing, tail assembly) to make sure you have no unintentional
warps. If you discover a warp, you'll have to correct it before you can
fly successfully.' For an all-balsa surface, simple twisting may do the
trick. Bend the warped member strongly against the direction of warp,
and hold it for thirty seconds. If this doesn't work, wet it with water
or clear dope and prop it in position until dry.
For a
doped tissue-covered wing, brush on thinner and hold a reverse twist
until dry. Recheck these surfaces after a flight or two, to be sure the
warp hasn't crept back in.
With all surfaces flat, look the
model over for alignment; the wing and elevator should be parallel,
both from top and front views, with the rudder at right angles and on
center. Be sure the engine has the proper thrust angle. You can check
side-thrust by setting the propeller horizontally and measuring back
from each tip to the rear of the fuselage. Most free-flight ships
require down-thrust and offset to the right. For pylon models the latter
rule is reversed, because of the prop wash against the pylon, which
tends to turn the model to the right.
If no balance point
was shown on your plans, assume one at the one-third point of the wing.
If the model doesn't assume a level position when supported at this
point (or near it), you'd better add some weight to the proper end to
start with. The first test glide will show whether it's needed. If you
have removable wings and tail, be sure they're adequately held down.
It's better to use short rubber bands stretched well out, than longer
ones with less tension.
For test glides, remove the
propeller and substitute an equal weight; props tend to break easily on
nose-overs, dives, etc., which are to be expected at first. Gliding over
tall grass is a good way to minimize this kind of damage. Cover all
engine openings with masking tape to keep dirt out. Aim the model into
the wind, slightly down to avoid starting off in a stall position, and
toss the model lightly forward toward a spot on the ground about twenty
feet away. The idea is to give the ship flying speed and the proper
flight angle; it may help you to trot into the wind with the model until
you feel it beginning to lift, then let it slide forward. Throw it as
you would a light spear - not a baseball.
If the model is
properly trimmed fore and aft, it will continue at the angle at which
you started it off, flattening out as it nears touch-down (Fig. 34). If
the model bores steadily downward and smacks the ground head first, it's
nose-heavy - or else you threw it too weakly to give it flying speed.
You can overdo it, too. By hurling a nose-heavy model at high speed you
can make it whistle along in a flat glide, until drag slows it down. But
we're assuming you've done your homework and are gliding the model
properly. Put an incidence block under the leading edge of the wing, or
the trailing edge of the elevator to correct nose heaviness (Fig. 35).
This is where the balsa strips in your flying box came in handy. Start
off with a thin strip and increase as needed.
Fig.34
The
point at which many beginners become confused is with a stall in the
test glide, because, although a stall results from tail heaviness, the
model will still hit nose first. But the condition is easy to
distinguish from nose heaviness. If the model dips in its glide,
starting out flat or even climbing a bit - then dropping its nose and
descending suddenly - that's a stall. The heavy tail causes the nose to
move up, putting the model in a mushy attitude, with wings and elevator
at an angle to the direction of flight. This extra drag slows the ship
below flying speed. The flying surfaces become no better than dead
weights, the weight of the nose asserts itself, and down toward the
earth the plane goes - bang!
A stall is cured by placing an
incidence block under the trailing edge of the wing or the leading edge
of the elevator. Be very careful not to set up a negative angle between
the wing and elevator, with the leading edge of the wing relatively
lower than that of the elevator (Fig. 35); under power this will cause a
spectacular power dive.
If you need more correction of
trim than you can get with a reasonable amount of added incidence, start
adding weight to the appropriate end of the model. Clay packs easily
into any space available, and bits of lead can be embedded in it as
needed. Once the balance is established, cement some balsa over the
ballast to hold it in position.
After three or four glides
and some juggling of incidence and weight, you will have arrived at a
satisfactory initial trim. If any marked turning tendency has shown up,
correct it by offsetting the rudder tab slightly away from the direction
of turn. If this isn't enough, set the wing at a slight angle, as
viewed from above, with the tip toward which the model has been turning
slightly forward (Fig. 36). An alternate method is to block up one side
of the elevator; this will cause the model to turn toward the high tip
(see the Adjustment Check List at the end of the chapter).
Now
comes the moment you've been waiting for - the first powered flight.
Whether your ship is powered by rubber or an engine, all your power
testing and preparation should have been done at home before coming out
to fly. You'll have plenty to occupy you with adjusting the ship without
also having to tinker with a cranky engine or make up a rubber motor.
For your first tests with a rubber job, a hundred or so turns without
stretching out the motor will do. If you use a glow engine, run it as
rich (needle valve open) as possible with steady running. A diesel can
be throttled back by slacking off the compression. Position the prop on
the shaft so that it will rest horizontally as it begins to come up
against compression. Put a little fuel in the tank, start up and launch
the model just as for a test glide. This low-power flight will give you a
much better opportunity to observe your model's flight habits. Here is
where a turn is likely to show up. If it is not too pronounced, leave
the rudder setting as is until you see how it looks under full power.
Correct any power stalling or diving by adjusting the angle of thrust of
the engine. In most cases, the trouble if any will be stalling, and a
washer under the proper engine mounting bolts (or for rubber, a wedge
under the thrust button) will correct the situation.
Fig.35
With
only a few turns, a rubber model will probably land with the prop still
turning, so you won't be able to study the glide until you try more
power. An engine-powered job will stay in the air as long as the motor
turns, and give you a chance to watch a longer glide down. Make any
further glide-trim adjustments now to achieve a smooth flat glide with a
gentle turn. Be careful about adding left turn at this point, since
with engine torque it may add up to a spiral dive under high power. A
right turn is preferable, canceling out torque effects. Some turn is not
only inevitable, it is highly desirable to keep the model in the
vicinity of the modeler. Lost models and long chases are not generally
the result of a straight fly-away, but of wind, which causes the
circular flight path to move away in toto. That's why you shouldn't try
to fly in a wind of more than about 10 mph. The wind also tends to
buffet the model about and makes for hard landings. To combat wind
drift, fly from the upwind end of the field, and launch the model
crosswind, slightly into the wind; so that its natural turn will bring
it into the wind. Of course, it will continue around and make a dash
downwind before coming around into the wind again, but it will have
gained its altitude and used up some motor run bucking the breeze, and
won't be so likely to be a quarter of a mile downwind when power cuts.
Now
you can hazard a flight under full power. Use only enough fuel for a
five-to-ten-second motor run, and be certain there is no unexpended fuel
from a previous motor run in the tank. An engine running a little off
proper setting may quit early several flights in a row, then perversely
burn it all on the next flight and end up in the next county. Even a
low-powered sport model gets upstairs fast under full power, and it's
amazing how quickly your model can dwindle away to a mere speck in the
sky. Any time your model seems to be headed away from home don't
hesitate to take off in pursuit in a hurry. Keep an eye on the model,
and if it disappears from view behind an obstruction, stop and study the
situation; fix in mind the position and direction of the ship as last
seen. Then head for the spot where you calculate it should be, and start
hunting. Most contest-type models use a de-thermalizer to bring the
ship down abruptly but safely after a specified time. Commonest is
the pop-up elevator, released by the burning of a timer fuse.
Fig.36
The
chief risk on the first full-power flight is a spiral dive, or loops
ending in a nose-up power cut-off and a subsequent failure to pull out.
The spiral dive is sometimes the result of excessive torque (assuming
your glide is correct), and you can remedy this by using a lower-pitch
prop and by offsetting the engine farther. More down-thrust will cure
the loops. If the loops are straight-away, a little turn will convert a
loop into a nice climbing spiral.
Use the tissue or soft
rag in your flying box to wipe the model carefully after a motor run.
Even fuelproof finishes can soften up some if fuel is allowed to remain
on them indefinitely. By taking a moment to clean the ship, you'll save a
lot of time later in scraping off sticky paint and embedded dirt.
Recheck all settings and incidence blocks after each flight, and shake
the tank dry, just in case. Be careful about preflighting the ship, and
you'll save most of your crack-ups and flyaways.
Control-line
test procedures are simpler, since you rely much less heavily on
built-in flight patterns. Start by laying out and checking your lines.
Use lines that are heavy enough, but don't load the ship down with
oversized ones.
For small C/L jobs No. 30 linen thread is
heavy enough. Single-strand steel wire, available at hobby shops, should
be used for the bigger models. Stranded steel cable is needed for the
biggest monsters, and for high-powered speed ships. Don't let steel
lines get kinked, and try to keep spectators from walking on your lines;
something they seem to love to do, claiming they can't see them when
they're laid out in the grass. The audience also needs to be discouraged
from stepping on models, kicking over fuel cans, and testing fabric
("Ooh, I didn't know it was only paper!"). Next, check the balance of
the model. Most C/L jobs balance at about the leading edge of the wing.
This nose-heavy condition gives the pilot better control; you don't want
your tethered job to start floating like a F/F while the lines hang
slack.
Usually both an offset rudder and a weighted outer
wing tip are used to keep the model out on the lines. Engine offset
helps, too. It is customary to fly counterclockwise, but there is no
reason not to install your controls so as to fly clockwise if you prefer
- and you'll have torque helping to hold the model out. After a few
flights, when you have the feel of the model, you may want to reduce
your rudder and engine offset to increase efficiency.
If
you're a novice at flying on lines, try to get an old hand to test-hop
the ship for you. When he's satisfied all is well, you try it.
If
possible, a C/L ship should be allowed to take off under its own power
so as to enable the flyer to get the feel of the controls and lift the
ship at will. For the initial flight, it's wise to set the control
linkage for minimum control surface movement - particularly down - since
the chief danger is overcontrol with an unfamiliar ship. After a few
practice flights, set them as sensitive as you want them, but don't
forget you've done so.
Make a last-minute, but very
important, check of your controls to be sure up is up, have your crew
chief start the engine (with a modest fuel supply - you may get tired),
and let her go. Hold the controls neutral until the ship is skimming
along ready to rise. If it's sluggish about getting up on the main gear,
try just a little down elevator to get the tail up - then neutral again
as it picks up speed. When you're ready, and not before, give a touch
of up, and back to neutral. (Be prepared for the model to take off
before you give it any control, by the way; it may not wait for that
touch of up.)
Once in the air and flying straight and
level, cautiously try a delicate maneuver or two. Of course, if your
ship is a bit tail heavy, or overly responsive, you may already have
your hands full trying to keep it flying level; in this case, omit the
maneuvers.
C/L models can't fly or be blown away, but a
high wind can cause the ship to come in, on the upwind leg of the
circle, so be prepared for this when you try flying in breezy weather.
If you back-pedal in a hurry, you can keep the lines taut. This is where
that extra room counts.
When the engine begins to change
tone preliminary to stopping, bring the model down about waist high, and
hold it in level flight. Let it glide in naturally, with a little up
only if obviously needed. A big stall at this point can wash out the
model. Give the ship full up as soon as it touches the ground firmly, to
avoid a nose-over. As you become more practiced, you'll find you can
whip the model around after the engine cuts and set it down at any point
you wish.
The average sport model is capable of doing a
wide loop; trainers usually are not, unless a bigger power-plant is
installed. A true stunt model can do consecutive tight loops, and almost
anything else you can think of. Work your way into these stunts
carefully, and don't try maneuvers that are beyond the model on the
lines. It's a lot more fun to watch your ship perform properly, doing
what it was designed to do, than to see it laboring to do the
impossible. Sport models don't climb like contest jobs; C/L trainers
aren't noted for stunting. The real thrill of flying your ship is in
obtaining correct, controlled flight - when you make that assembly of
inanimate parts behave almost as though it had life and intelligence of
its own.
Even if you're a solitary type, you'll find
there's more pleasure in flying with other modelers. They're a congenial
bunch for the best of reasons; a genuine common interest in the sport.
It's always interesting to see what the other guy has built, to see
other models and engines in action, to learn new angles and tricks to
help your own flying. Then, too, it's nice to have a little help with a
cranky engine, or getting a C/L job aloft, or chasing a runaway. Don't
be stingy with your own time and help, either. You can even carry extras
in your flying box, including some items you don't use yourself, just
in case somebody needs them. He'll pay you back when you're about to be
grounded for want of a mounting bolt or a spare prop some day. Like
flyers in the early days of aviation, modelers stick together.
If
there's a club in your vicinity, join it. Clubs frequently have good
flying sites, quantity purchase arrangements, and other advantages to
offer, in addition to the company of other modelers. If the city fathers
(or cranky citizens) object to flying in the park or on a vacant lot, a
club can present a better case for a change of heart than an
individual. Often the local hobby shop or department is the sponsor of a
club. If yours isn't, suggest it. The more the merrier!
Businesses
with large parking lots, like supermarkets, can often be persuaded of
the public relations value of letting the club use the paved area for
C/L flying on Sundays. Don't be bashful; scout around and get things
lined up for more flying fun.
The Society of Model
Aeronautical Engineers is the national organisation for aeromodelling in
Britain. It has an average annual membership of 4,000 active
aero-modellers and conducts a very active contest programme which runs
from March to October each year. About 150 clubs are affiliated to the
SMAE and this collective representation is responsible for the facility
of using Royal Air Force airfields. For details of membership, contact
the Secretary (a voluntary post, there is no permanent paid official) of
the SMAE, c/o Royal Aero Club, 116 Pall Mall, London, SW1Y 5EB. A
feature of SMAE activity is the regular National Championships held over
each Spring Bank Holiday (Whitsun).
While some model
builders think in terms of free-flight endurance competition as the only
real measure of a model, the great majority of modelers fly for the
pleasure of making something with their hands, tinkering with engines,
watching their jobs buzz around, and associating with other modelers.
While staying up ten minutes on a fifteen-second motor run is an
interesting trick, it depends completely on the presence of thermals;
the best model will glide down in a minute or so without a rising air
current. There are other feats a model can perform that are at least as
difficult - realistic take-offs, flying a predetermined pattern, spot
landings, free-flight speed - the possibilities are endless. Try out all
the ideas you can think of, and if bad luck strikes and you clobber in -
it'll happen now and then - head back to the old drawing board
philosophically; that's part of the challenge and the sport of modeling.
I
am going to be looking at the most common aircraft configuration, i.e.,
a 'tractor' type with the tailplane to the rear of the wing, as seen in
the plan view picture, left. Starting with the wing, there is a
simple ratio that you'll need to know. The ratio of the wingspan to the
wing chord is known as the 'aspect ratio'. This is a very important ratio,
since the greater the aspect ratio the more efficient the wing will be
(hence gliders have long skinny wings). This is down to tip vortices,
basically the greater the aspect ratio the smaller the percentage of wing
that is affected by the tip vortices, thus more lift is produced. In general
you may find 4:1 to 8:1 maybe typical for a sports model and perhaps 8:1
to 15:1 for a model glider. The drawing shown has an aspect ratio of 4:1,
i.e. the span is 4 times that of its chord. The tailplane area is calculated
by a simple function of the wing. Tailplane area should be around 15-25%
of the wing area depending upon the moment arm length. The longer the
moment arm the smaller the tailplane can be. A good starting point for
the moment arm would be 1.5 times the wing chord combined with a tailplane
at 20% of the wing area.The fin size is again calculated via a percentage
of the wing. As a rule it should be about 10-15% of the wing area, but
again it does depend on the moment arm length. If in doubt it is always
best to start with a fin that is too big and possibly trim it down after
if it is oversized. Once again apply the rule of thumb, if it looks
right ...! Moving on; the length of the nose is not too important
but is usually at least one wing chord long. Generally the nose is made
long enough to get the model to balance correctly but remember that too
long a nose only adds weight and is more vulnerable to damage when landing.
