(Multi Engine Rubber Power... pg.2)
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CONSTRUCTION |
BOB:
Building a twin means having to build
3 fuselage type structures (the fuselage plus 2
nacelles). No way around it, so if you hate building
fuselages I suggest that you build them first to
add incentive to complete your project! Another
consideration is the method that you choose for
winding the motors. Unless you are equipped with
3 hands, you should provide a means to “peg”
one of the motors after it is wound so you can wind
the second motor. A short piece of aluminum tubing
in the front of the nacelles will simplify this
requirement. If you look carefully at the picture
below, you will see a piece of 1/8” tubing
mounted in the front noseblock at about 7 o’clock
position (just left of the rubber hanging from the
nose.)
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Prop-stop tube in noseblock |
I
attach two pieces of heavy monofilament (think
weed-eater cord) to a 1/2" dia. dowel,
and space them apart to line up with the tubes
in the noseblocks. The monofilament pieces
insert into the tubes to stop the props. I
can wind and peg both motors with this system,
and a firm pull forward releases both motors
simultaneously. |
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consider the method you will use to hold the airplane
for winding... remember that your wife, girlfriend,
or flying buddy may not be available when you are
ready to wind! |
Alignment is even more critical on a twin
than it is on a single engine bird. Remember
that you have two engines to deal with so
you want them to provide equal thrust in
the same direction!
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Bob's A-20 Havoc |
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The full
size concept of pointing each engine outboard to
counteract drag created by a dead engine will not
work on a model. If you lose an engine on a rubber
powered twin, get ready for some serious repair
work!
Give some thought to how you'll launch your model.
It is very easy to grip a twin too hard and crush
some fuselage structure. Reinforce the gripping
area during construction in order to save some repair
work later.
Don't forget to build in “washout”
in your wings (T.E. higher than L.E. at the wing
tips) Make sure that you have the SAME amount
in both tips - 3/16” on a 36” span
airplane is about right.
Last but not least, build in lots of downthrust
in your noseblocks. Start with 6 degrees and anticipate
the need for more later.
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ANDY:
Of course, it is generally advisable to build rubber-powered
models light, but on multiengine models, light weight
is especially important. In many cases, you can
save weight by building the nacelles from wood one
size smaller than the wood in the fuselage.
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Andy's Fairchild Metro |
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of it this way: a peanut scale twin would
have nacelles the size of a pistachio-scale
fuselage each. On my Fairchild Metro
peanut, everything is 1/20”, but I could
have gone to 1/32” on the nacelles. |
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When you’re trying to get a long run from
a short motor, you will have to use the prop to
slow the rubber down – wide blade, high pitch.
As the props are smaller and off-center, torque
is not such a big deal. If the nacelles or motorsticks
are less than ½ length of the fuselage, you
mave have to use a small rubber size to get a sufficient
number of turns (recall that I prefer 1000 turns
in each motor).
If you’re building a multi with motorsticks,
heed the rules for saving weight on those. A good
motorstick is built from an unusually stiff and
light piece of wood – better larger and lighter
than smaller, because larger pieces tend to be stiffer.
Don’t forget to make the sticksmore tall than
wide – I used 1/4” x 1/8” on my
Tremendous Four to resist the tension
of the motor. Finally, make sure the motor sticks
join to the wings very securely. Some of mine break
off every time I fly – not good.
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FLIGHT TRIMMING |
BOB:
I do not find that the trimming process is any more
difficult on a twin than it is on a single engine
bird; in fact it is somewhat easier due to the fact
that I build counter-rotating props, and their torque
reactions cancel out. When viewed from the front
of the airplane, the engine on the left (starboard
wing) rotates counterclockwise. The engine on the
right (port wing) rotates clockwise. Think “outboard
over the top” and you will have it correct.
If you use “Z” hooks to prevent the
motor from climbing off the prop shaft then you
must have an “S” hook on the opposite
motor.
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Start glide testing over long grass. I test
glide with motors and props installed, Bill
Henn, Midkiff and others test glide with props
off; pick one and go with it! I try to get
a straight ahead, no stall or dive glide.
If the C/G is okay, adjust the stab to get
the desired glide profile. The first powered
flight should consist of several hundred hand
winds in each engine..Look for the airplane
to fly straight ahead with NO turn evident.
If the bird wants to turn, find out why. A
warp? One wing heavier? Crooked rudder? Misaligned
nacelle? FIX IT NOW!! |
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Fokker 27
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Continue to add turns and closely observe the flight
profile. If it is climbing out straight ahead, all
is well. You can add a turn by tweaking the rudder,
but only after the bird is climbing out with no
tendency to fall off on one wing. Most birds will
have a natural tendency to turn, as the torque of
the motor winds down. My suggestion is to fly the
bird in that direction.
Experiment with more or less cross section on your
motors as well as increasing or decreasing motor
length.
Remember: when in doubt, power up!
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ANDY:
Unlike Bob, I prefer to deal with the torque and
let both props spin the same way. It makes winding
easier but it requires that you fly unstable airplanes
to the right, against the torque. Use right rudder,
and put just a few more turns into the right motor.
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Andy's Dash 8 |
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Both the Dash 8 and the Tremendous
Four fly to the right only, and are
reasonably stable that way. I did cause the
Dash 8 to spiral in a couple
of times when I tried to fly it to the left
under equal turns in each motor. |
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As Bob said, downthrust is very important to multis.
Because the props are blowing are right over the
wing, the multiengine airplanes produce lift just
sitting there with the props spinning. As a result,
when they fly, they can really produce an immense
amount of lift and the usual amounts of downthrust
just won’t be enough.
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I wind and launch my multis with a human helper
and a styrofoam cradle. A two-part interlocking
cradle could be made in order to wind without
the help of another person - with a bottom
piece that attaches to your stooge, and a
top piece to firmly hold the airplane down
in place.
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styrofoam cradle |
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The cradles shown here sit between each wing and
prop. Pieces of hard 1/8” square balsa extend
from the cradle into the prop arcs to stop the props
from rotating. The wound props will usually keep
the cradle in place.
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styrofoam cradle |
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I carry the airplane out to my favorite spot
on the field, pull the cradle away, and release
the airplane. This works well for me. |
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Bonus Feature:
Tremendous
Four video!
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