If it Flies-2011/08

82 MODEL AVIATION
CG and engine thrust
Dean Pappas | DeanF3AF2B@If It Flies ... pappasfamily.net
HI GANG. The last time we got together, we
had embarked on a discussion of five
commonly seen, and easily fixed,
maladjustments. Of course, it was necessary
to wax philosophical for a while about what a
well-tuned airplane can do for you.
As does a finely crafted sword (or any
well-built tool), it falls to hand readily and is
comfortable to use. It makes he who wields it
formidable in battle―or at least it helps him
or her to fly better.
The big five are:
1) Control surface hinge-line gaps
2) Heavy wingtip or lateral misbalance
3) Balance point, usually referred to as
CG
4) Engine thrust—both downthrust and
right thrust
5) Aileron
differential and
adverse yaw
We had discussed
the first two items on
the list the last time
we got together, so
let’s examine CG and
maybe a little bit
about downthrust.
Remember that
example of the
airplane that lands
nicely if the airspeed
is kept up a little bit,
but if slowed down
too much, the
elevator would lose
the control authority
needed to flare out
nicely? Last time we
sealed the hinge-line
gaps and made the
elevator work as it’s
supposed to, but that was only one element
that could have caused the problem. The
airplane also could have been extremely noseheavy.
The CG or balance point of an aircraft
affects every flight regime, as well as
operation on the ground. The effects of CG
position show up everywhere; this opens up
the possibility of checking several different
flight characteristics in order to determine the
optimal CG position.
There isn’t a single optimal CG position
for any particular design. It depends upon the
type of flying you are doing with the airplane,
what your piloting skills are, and to a certain
extent how your airplane is equipped. Some
designs have wide CG envelopes—that is the
range of useful CG positions—while others
will have a very narrow sweet spot. Within
that envelope, fine-tuning the CG’s position
may even be a matter of taste.
A wide sweet spot might actually be
necessary; take fuel consumption for example.
If the fuel tank is right behind the firewall, as
it should be for a non-pumped engine, then as
the fuel burns off, the CG will move aft.
If the tank is large (and we are often
tempted to install the biggest tank practical!),
then it will be necessary to compromise and
take off nose-heavy so that the airplane is not
dangerously tail-heavy when the tank is nearly
empty. Being tail-heavy can lead to a loss of
control and very short airframe life!
Let’s measure the CG. Before we go flight
testing, we should know where the CG is so
that we can make changes systematically.
Nothing rigorous, but we don’t want to lose
our place as we make changes.
If your airplane has a high or shouldermounted
wing, then you can hold it up using
the airplane and the tip; measure ¼ of the way
back from the LE, and you are done.
The MAC is actually the chord line where
half of the wing area is inboard and half is
outboard. That puts the MAC slightly inboard
of half way out.
CG positions near the front of the
envelope make it easier to fly smoothly; a CG
position near the back of the envelope gives
more maneuverability. What do you do if you
want to maneuver smoothly? Like so many
things in life―compromise is a necessity.
So where do we start? Make sure that
the balance point is where it says in the
plans or instructions for your airplane. If
the plans show a range of positions (as they
should), shoot for somewhere in the
forward half of that range.
A slightly noseheavy
CG is best for
test flying and for
familiarizing
yourself with an
airplane. Otherwise,
stick with the 1/4
MAC
recommendation.
Subsequent flight
testing will help you
determine whether to
move the CG aft and
how much.
CG Testing
Methods:
1. The climband-
glide test is
most useful with
trainers and aircraft
intended for a
loitering flight
regime. This test
gives us
information about engine downthrust, which
means we have to read the test correctly. A
better description will follow.
2. The inverted-flight test is useful for
airplanes intended for both sport Aerobatics
and Precision Aerobatics. Coupled with the
“chop-the-throttle” test for downthrust, this
gives a more complete picture of how CG and
pitch trim play together. A description of the
throttle chop will follow soon.
3. You should already be familiar with the
landing-flare/slow-flight check. If the elevator
power runs out before the airplane has slowed
enough to flare nicely, this indicates noseheaviness.
It is normal to lose some control
authority before stall, but as long as you have
enough to flare for a pretty landing, this
indicates that the CG is within the sweet spot.
Tail-heaviness makes the elevator plenty
powerful, even a bit touchy, until the last. An
airplane has to be quite tail-heavy before this
characteristic shows up.
one finger on each hand under the wing. If you
have a low-wing model, you may find it easier
to flip the airplane over and do this upside
down. Place both fingers the same distance
back on each wing panel, and move them back
and forth until the airplane hangs level.
A number of fancy balancing stands have
been designed and sold throughout the years
so that you won’t need to use your thumbs! A
typical safe starting point for almost any
airplane is when the CG is placed at 25% of
the Mean Aerodynamic Chord (MAC). The
CG on a typical trainer is normally between ¼
and 1/3 of the MAC. The balance point on 3-D
aerobatic airplanes often 40% of the MAC.
On a constant-chord wing, the 25% point is
exactly a quarter of the way back from the LE
to the TE. Most trainers are designed with
constant-chord wings.
For tapered and/or swept wings, try this
simple approximation. Find the chord on the
wing that is halfway between the centerline of
Like a good throwing knife, an airplane
should be properly balanced. The old
lift-by-the-thumbs method will always
work. Low-wingers are best balanced
upside down. Zachary Pappas photo.
08sig3.QXD_00MSTRPG.QXD 6/22/11 2:18 PM Page 82
84 MODEL AVIATION
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4. Jumpy pitch and yaw are results of tailheaviness.
This is not to be confused with
control sensitivity; airplanes with sensitive
controls still neutralize predictably, provided
the servos and linkages are tight and slop free.
If you feel that the airplane doesn’t reliably
settle down after you have returned to neutral
elevator, then it’s time to look at both the
control setup and at tail-heaviness as potential
problems.
Let’s discuss those first two flight tests in
more detail. In the climb-and-glide test, start
with trimming the airplane for level, cruisespeed
flight. Once cruising, smoothly advance
the throttle to full. Without making any
elevator corrections, but still keeping the
wings level, watch the resulting climb.
Is the climb too steep? Watch to see if the
climb is so steep that the airspeed decays and
control authority suffers. This indicates that
the airspeed is too low, because of an overly
steep climb. In this case, we will do one of
two things: either we will make the airplane
less speed sensitive, by moving the CG aft and
adding some down-elevator trim, or we will
add more downthrust.
Is the climb too shallow and fast? If this is
the case, we would do the opposite. How do
we decide whether to change the engine
downthrust or the balance of aerodynamic
trim and balance point? We need more
information, and to get it, we will use the lowthrottle
glide test.
For the low-throttle test, establish cruise
flight again at roughly 100 feet up. With the
airplane trimmed for cruise-power level flight
and your hand off the elevator stick, smoothly
reduce the power to slightly above idle. This
is the throttle setting that most of us use
during final approach, at least right up until
the threshold of the runway, after which the
engine is slowed to its lowest idle.
Watch the glide slope after the airplane
slows. Keep the wings level, but don’t make
any elevator corrections. Does the airplane
settle into a nice glide, or does it assume the
glide path of Thor’s Hammer? (Okay, it’s not
exactly a sword, but we’re in the
neighborhood!) Maybe the glide slope is very
shallow.
Now that we’ve done both tests, it’s time
to assemble what we have observed, and
make changes. CG and elevator trim both
have a strong effect during the glide. We set
the elevator trim for cruise flight, so let’s
assume that glide behavior is strongly
influenced by CG. This next bit might sound
backward on first reading, but stick with me.
If the airplane climbs too steeply under
full power and glides a little steeply, then the
airplane may be nose-heavy. Nose-heaviness
makes the airplane overly stable in pitch,
which means that it responds to more airspeed
by trying to climb too much at full power.
If the airplane has insufficient downthrust,
the same steep climb will result, but the glide
should not be too steep.
If, at full throttle, the airplane’s climb is
slightly shallow and then it glides nicely or a
bit shallow, then what you want is a model
that changes pitch trim with airspeed more so
than it already does. This is accomplished by
pushing the CG forward and then re-trimming
with a small amount of elevator for level
flight.
In either case, each time you make an
adjustment, repeat the entire test: trim for
cruise-power level flight, and perform the
full-throttle and low-throttle tests.
If the problem is downthrust rather than
CG, then the climb will be more affected and
the glides less so. For example: if the airplane
has insufficient downthrust, then the elevator
will have to be trimmed slightly down for
level flight, as compared to where it would be
with the correct downthrust.
At full power, an airplane that needs more
downthrust will climb too steeply when
engine thrust is great. If it has too much
downthrust, then the climb will be shallow.
This will force the pilot to add some up
elevator trim for cruise so the glide may be a
bit shallow.
It can take a few adjustments to tease these
two effects apart, but it’s fun to experiment.
Chop-the-Throttle Test: This test is most
useful for high-performance sport, scale, and
some aerobatic airplanes. This applies to most
designs with semisymmetrical and
symmetrical airfoils.
Assuming that you generally fly at full
throttle, trim the elevator for a hands-off level
pass on the far edge of the runway at
approximately 50 feet up. As you fly, nice and
straight with the airplane trimmed out,
suddenly pull the power to idle.
For only a second or two the airplane will
still be zipping along at cruise speed.
Downthrust is taken out of the picture for
those seconds; the aerodynamic trim at full
speed predominates, and the difference (if
any) is the effect of the downthrust.
Ideally, the airplane will fly straight and
level, without a twitch for a second or two,
and then slow and settle into a glide.
If, after you pull the throttle back, the nose
twitches up and then the airplane slows down
into a glide, you have too much downthrust.
The downthrust has forced you to add up trim
to counteract it, and once the thrust goes, the
trim makes the nose rise. Of course, the
airplane decelerates quickly and there is only a
flicker of a climb.
On the other hand, if the nose abruptly
drops—even just a tiny bit—and the airplane
instantly assumes a fast, nose-down glide, then
you need more downthrust. That’s because
your elevator trim has been fighting down
against the engine-induced climb the entire
flight. If the downthrust is correct, the airplane
continues straight for a mere second or two,
and gradually fades into the glide angle.
You will also see the effect of wrong
downthrust when the power is applied rather
than pulled back. If the downthrust is
insufficient, then the airplane will go abruptly
nose-up when you pour the coals to it for a
missed approach. This can be bad news with a
heavy scale model.
Inverted helps you see things right side up!
For high-performance airplanes with
semisymmetrical or symmetrical airfoils, the
CG is the dominant factor determining how
much elevator input is needed to sustain
inverted flight.
If it takes too much down elevator to fly
inverted, then the airplane is likely noseheavy.
If it takes no down elevator or even
climbs sometimes, then it is definitely tailheavy!
It is not normal to fly inverted without
needing any down elevator input, even for the
Precision Aerobatics or Pattern fliers, many of
whom trim as tail-heavy as they dare.
The next time we get together we will discuss
adverse yaw and aileron differential. These are
key to good directional control, especially at
low speeds.
I’m out of room, for now, but the next time
we get together, we will spend some more
time sharpening our flying weapons.
See you next time. Until then, have fun,
and do take care of yourselves. MA
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