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ARTICLES
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Get started building with these basic tools
How to choose a set of plans for your first building project
An easy, great looking color layout for maximum
visibility
Build swept back wings to avoid tail heaviness
Install your wings with bolts when the plans show rubber bands
How to flight trim a model airplane
How to build a straight fuselage
without
using a jig
How to build a simple cowl out of balsa blocks
Fixing construction mistakes
Set your engine thrust angle using higher math
How to iron Monokote into tight spaces
How to get longer run time from a Cox engine by
using a remote tank
Compatibility of various paints and finishing materials
Propeller selection guide for 2 stroke and 4 stroke glow engines
robbie@balsaworkbench.com
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A plane that's correctly
trimmed is a pleasure to fly, but if you don't know how to do it the
process can be a bit of a mystery because there are several factors
that influence the flight trim of a model plane. These factors
also influence each other, so it's important to work them out in the
proper order.
Your first trimming effort should be aimed at choosing the right
propeller. Fit the propeller you think you want to use, but
remember that this may change. If your plane is slow and draggy,
no amount of propeller is going to make it go fast, so don't try.
A slow, draggy plane such as a biplane needs a long, low pitch
propeller, so it will at least have good acceleration. A racing
plane will need a short, high pitch propeller to get its speed, which
will come at the expense of acceleration. But this won't be a
great loss because the sleekness of the plane will allow it to take off
better than a draggy one anyway.
Most planes you fly will be somewhere between these extremes, so be
willing to try different propellers to get the plane to perform in a
way that makes you happy. If your choice is way off, switching to
a more appropriate propeller will make a huge difference. Also,
choose an efficient propeller. I like APC because of the
noticeable performance advantage that they have over the more old fashioned brands.
For your first flight, set the controls and thrust angle as suggested
on your plans, or if you're the one making up the numbers take your
best guess. Take off, climb to a comfortable altitude, set the
engine at medium speed, and trim the controls for reasonably level
flight. Now check speed and acceleration. If you like to
zoom around fast, that's OK. It's your plane. If you don't
care about top speed but you like zippy performance and vertical
maneuvers, that's your choice. If you don't like the way the
plane performs, try a different propeller. Choose a size that
allows the engine to run fast enough to develop full power, and choose
your pitch-to-diameter ratio to allow your preferred balance of speed
vs. acceleration. The reason why you choose the propeller first
is because your propeller choice will influence the thrust angle
setting.
Once you have your favorite propeller picked out, move on to the engine
thrust angle. Set the throttle at full speed and trim the
controls for level flight. Then cut the engine to idle. If
the plane turns left when you cut the power, you have too much right
thrust. If it turns right when you cut the power, you need more
right thrust. Shim the engine accordingly, retest in the air,
adjust shims again, test again, etc, until you are satisfied with the
thrust angle.
Your next step is to determine the correct trim of the rudder and
ailerons. Let's say the plane was steering to the left so you
want to trim to the right. Should you trim the rudder, or should you trim the
ailerons? If you trim the wrong one you'll notice unusual
performance whenever you deviate from straight and level flight.
Get your plane at a comfortable altitude at medium high speed, then
pull the elevator to make the plane go straight up, while
simultaneously reducing the throttle to idle. If your plane goes
up and yaws left, you need to trim the rudder to the right. If it
yaws to the right, you'll need to trim the rudder to the left. You know the rudder setting is correct when the plane stalls straight forward when you cut power and pull the nose up.
When you get the rudder straightened out, you'll need to return to straight, level flight and retrim the ailerons if
necessary.
The reason you cut the engine for this maneuver is to remove the engine
torque, P-factor, and spiral slipstream from the equation as much as
possible. After you complete the trimming sequence, go back and perform
the right thrust test again and see if you still think that you got it
right. If not, go through the sequence again until your plane will
fly straight at any throttle setting from idle to maximum, and in any attitude.
Now the main thing
that's left is the elevator and throttle trim. I always like to
trim the throttle so it idles dependably with the stick all the
way down and the trim tab up. When I move the trim tab down I want the engine to
die. That's simple enough. The elevator is trimmed
according to your flying style. You'll probably want to trim to maintain
altitude at whatever speed you fly the most. If you like to make your little pilot
throw up by constantly doing loops, rolls, spins, and stall turns,
maybe you want to set the elevator for a very slight nose-up attitude,
so you will automatically gain altitude when you come out of a
maneuver. If you like to fly inverted a lot, you'll probably want
to set your center of gravity and elevator trim for level flight
whether it's upright or inverted.
I always try to emulate the planes I see at small airports. I
like to set my trim tab so I can adjust it a few clicks for cruising,
then simply reduce throttle for a smooth descent with minimal elevator
adjustment, then use the stick for the flare. A couple of clicks
of up trim will ready the plane for the next takeoff and cruise.
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