If It Flies ... - 2009/12

thank them. It’s nice to know that people
who already know this stuff read “IIF” too.
Now we get to do something practical. A
short while ago, I lost a crankshaft bearing in
the otherwise beautiful-running YS61 that has
been in the Carl Goldberg Tiger 2 for a few
years. That sort of thing makes a mess inside
an engine.
Maybe soon I will do an installment or
two about crankshaft
bearing
replacement; I have
the subject at hand.
Meanwhile, the O.S.
.91 Surpass II that is
slated to go into my
Nostalgia Patternlegal
Pulsar bipe
needed some breakin
time, and I
decided to make the
engine swap.
The problem is
that the four-stroke
and its muffler were
an ounce or two
Evidence of when the CG location is just right
Dean Pappas | DeanF3AF2B@If It Flies ... pappasfamily.net
Also included in this column:
• Swap out a two-stroke for a
four-stroke
• Knowing when your engine
thrust is correct
This test uncovers problems by removing engine thrust and, with it, the downthrust effect. Results are seen in a second as the
engine rpm drops. The model eventually settles into a normal glide angle.
Dave Brown Products’ T-Beam mount
dropped into the Tiger 60 with no fuss, and
the result is tidy. Soft mounts save on servo
wear and radiated airframe noise.
HI, GANG! All this theory of flight doesn’t
mean a thing unless it has meaningful
applications, and finally I get to some fun
stuff. This month I will write about flying
instead of writing about talking about
flying.
That’s the funny thing about writing a
column such as this; communication that
flows so naturally between flying buddies
hand-flying about how the last flight went
becomes a production when turned into the
written word. Such has been the case for the
last few months in “If It Flies” (“IIF”). But
I’d like to think that the process has taken
some of you to a place you might not have
visited otherwise.
A few dozen readers (that’s a bunch
compared to the usual) had written after the
first part of the discussion about how to find
the aft CG limit, and typically asked if
that’s all there was to it. They followed the
discussion, absorbed it, and it seemed so
simple.
For the most part, those fliers received
quick answers that it was not quite so
simple and that other important effects, such
as flying-surface aspect ratio and wing
downwash, had to be taken into account. I
asked them to wait for the October
discussion.
That’s the problem with tackling a
complicated problem in a step-by-step
fashion; if you tell people from the get-go
that it’s complicated and that it will require
them to put multiple installments together,
many will lose interest and the opportunity
is lost. That would be a shame.
As it turns out, a similar number of
helpful souls offered to educate me, a slowpaced
writer, and I take this opportunity to
80 MODEL AVIATION
The soft-mounting hardware is available with a pair of
aluminum T-beams or can be mounted to existing wooden
rails. The engine centerline winds up being 3/16 inch higher
than the mounting surface.
12sig3.QXD_00MSTRPG.QXD 10/23/09 10:08 AM Page 80
heavier than the YS and Hatori muffler they
were to replace. The Tiger flew sweetly and I
was happy with it. But I suspected that it was
the least bit nose-heavy, and this engine
change would make it more so. I’ll describe
(without the hand-flying) what made me think
that the CG was almost right.
The Tiger was balanced almost spot-on at
one-third of the wing chord. It flew nicely, the
elevator trim was centered, or at least looked
to be, and the airplane transitioned to a nice
glide when the engine was idled.
On top of that, straight and level inverted
flight required only a small amount of downelevator,
which tends to confirm that the CG is
not too far forward. The CG shift would be
small.
The problem was that while the Tiger
would spin and snap roll beautifully inverted,
or in the negative-G direction, both the spin
and snap were a bit “barrelly” in the positive-
G direction. While the inverted spin rotated
tightly and slowly with only elevator and
rudder application (indicating a deeply stalled
autorotation), the upright spin sometimes
unstalled and started to spiral out, unless
aileron control was held during the entire spin.
Inside and outside snap rolls told a similar
story. Outsides started and stopped crisply,
while inside (or up-elevator) snaps barrelrolled
for maybe one-quarter of a turn before
the Tiger would “wind in” and rotate quickly.
The exits were still nice and clean.
Many models spin and snap roll better in
the negative-G direction. It’s a consequence of
almost every design feature that gives an
airplane stability, such as dihedral.
As part of the .91 four-stroke installation, I
moved the CG aft 1/4 inch for starters. That
works out to a lousy 2% of the mean
aerodynamic chord. However, if you are
moving the CG aft in nonrisky, incremental
steps, this is a good step size. Fliers who finetune
subtle flying characteristics often take
steps half this big. As it turned out, I won’t
bother moving it any more.
The original 60-size engine installation in
the Tiger used a Dave Brown Products
VibraDamp Sport Beam, but that would not
do for the four-stroke. Those engines often
require some sort of nose-ring restraint, and I
did not want to add that structure to the front
of the model. Someday after the O.S. migrates
to its intended home, I will probably replace
the rebuilt YS in its old home.
I removed the YS and the mounting
hardware, and I replaced it with a VibraDamp
T-Beam mount. That mount was originally
intended for 1.20 four-strokes, but the rails
and predrilled holes fit the .91 Surpass
perfectly.
The only change I had to make was to cut
the upper arms of the aluminum Ts shorter
and drill a new mounting hole that lined up
with the predrilled holes in the firewall. The
changeover was painless, and the Surpass
looks good in its temporary home.
I accomplished the 1/4-inch rearward CG
shift by removing the 4-ounce brass weight I
had mounted in the front of the tank
compartment. I left the other 3-ouncer in there.
Yes, with the YS and lightweight muffler I
used slightly less than a half pound of nose
weight to balance the Tiger originally.
It was time to go flying and evaluate the
change, but first I put a couple tanks of fuel
through the Surpass at a dead-rich needle
setting on the ground. Since it’s a ringed
engine, as are almost all four-strokes, it never
hurts to do this.
From the beginning, it was apparent that
the small CG shift aft was the right thing to
do. The elevator trim didn’t appear to change,
but I had messed around with my radio’s trimposition
memory during the control checks, so
all I can say is that the elevator still lined up
nearly perfectly with the stabilizer tips.
The spins and snap rolls were crisp in all
directions, with no tendency to over-rotate. I
won, but a few adjustments were in order.
To begin with, I added a bit of downthrust,
and after a fair bit of checking and crosschecking,
I left the right thrust alone. The
right-thrust angle built into the Tiger 60
ARF’s firewall was just right.
I’ll bet that the downthrust was correct too,
but when I tightened the screws holding the
shortened upper mounting arms of the
aluminum T-beams, I crushed the plywood
firewall a bit. That probably added a bit less
than 1° of upthrust.
Now for the in-flight tests I did to check the
thrust angles. This is where we get to exercise
our Walter Mitty urges and become a test
pilot. Okay, maybe it isn’t the full Walter
Mitty-esque fantasy of a test pilot taking up a
radical new prototype for the first time, but it’s
the staple of real-life test pilots: checking the
airplane in a methodical manner, to evaluate
some step-by-step changes that have been
made to it.
Let’s look at the right-thrust tests. Yes,
“tests,”—plural. There are several ways to
check right thrust, depending on the aircraft’s
type and purpose.
In general, once aileron and rudder trim
are set correctly for straight and level flight,
the right-thrust adjustment is made to account
for most models’ tendency to wander to the
left at full throttle in a climb.
Have you heard the term cross-trimmed?
It’s commonly used when an airplane is
trimmed for straight flight, but the aileron and
the rudder trims are fighting each other.
Therefore, although the aircraft isn’t turning,
it isn’t going straight like an arrow. The other
term I’ve heard is dog-tracking, even though
this is properly called a sideslip or forward
slip.
The problem is that even tiny slip angles
make a mess of the model’s flying
characteristics. And because we are standing
on the ground looking up at our airplanes, it is
nearly impossible to detect those tiny angles.
The simplest way I know to detect them is
to compare left and right turns. The test is to
fly horizontal figure 8s at your normal flying
speed, without changing altitude, and
carefully using the same bank angle for the
left and right turns. I feel the need to digress.
One evening I was on the phone with my
flying buddy, Bryan, in Louisiana, who is a
well-recognized authority on trimming
aircraft for RC Aerobatics. Maybe I can get
him to write a piece about that for MA.
As we spoke, he noted that one of the
problems in helping others learn to optimally
trim their models is that in-flight trim checks
require a fair amount of piloting skill. If a
pilot fails to fly with his or her aircraft’s
wings level, the test that follows could be
invalid.
It’s like a musician practicing scales, I
opined. The moral of this story is that there is
no substitute for the basics. End of digression.
So you are making figure 8s, trying to hit
the same 45° bank in both directions. If the
rudder trim is incorrect, the airplane will tend
to roll out of the turn in one direction and tend
to tuck into the turn in the other direction.
Does your model do this? Adjust the
rudder until both direction turns behave the
same, and call that rudder-trim position
centered. From now on, try to keep the rudder
trim centered.
Now do the climb test. Climb the aircraft
as steeply as it will tolerate at full throttle, and
look to see if it yaws to the side as it slows to
the sustained climb speed. Make no rudder or
aileron corrections in the climb.
The Tiger climbed steadily at a nearly 45°
angle with the engine set rich for break-in.
But if you have power enough for protracted
vertical climbs, go for it.
If the airplane wanders left after it starts
slowing, add more right thrust. Now go all the
way back to the figure 8 test, because even
more than they are daring, test pilots are
methodical. By iterating these tests and
adjustments, you will optimize both
adjustments together.
The downthrust test is simpler to perform
and more fun to argue about with your flying
buddies who are helping/watching/heckling/
criticizing. It’s a sort of exercise in testing
what isn’t there.
Start by carefully trimming the elevator for
your normal cruise-power level flight. If you
intend to fly this model often at full power,
use full power; if you cruise around more
slowly, consistently use that throttle setting.
Once that is done, make a nice, long, level
pass over the runway so that the aircraft’s
behavior is easier to see. Get those hecklers to
watch too.
Just as the airplane passes in front of you,
with your hand off of the elevator stick,
quickly chop the throttle to idle and watch
what happens in the first second as the engine
rpm spools down. After maybe two seconds,
the model will settle into a fast glide, but that
is not what you are looking for; you are
looking for what it does as the engine rpm
decays.
If the airplane noses up abruptly, even just
a few degrees, there is too much downthrust.
If the eventual glide is too shallow and slower
than will provide good controllability, there is
too much downthrust.
If the nose abruptly drops a few degrees
before the model settles into the glide, you
probably need more downthrust. The same is
true if the glide is overly steep and fast.
In thinking this through, in the ideal case,
the engine thrust is not fighting the elevator
trim in cruise-power flight. The moment the
engine power is reduced, the aircraft’s pitch
trim does not change until the speed decays
and the airplane gently drops off into a glide.
If you tested at less than full power, you
are not finished. Double-check by going to
full power and waiting to see how the
eventual hands-off climb angle looks. If it’s
too steep and the aircraft handles mushily, you
still need to add downthrust.
The full-blown test for models with tons of
horsepower involves executing a hands-off
vertical climb after it is trimmed for hands-off
level flight. That falls under the heading of
aerobatics trimming and is best left for
another day.
I’m not sure what I will write about in the
next column. If that stainless-steel crankshaft
bearing arrives in time, it might be twostroke-
engine rebuilding. Until then, have fun
and do take care of yourself. MA
Sources:
Carl Goldberg Products
(800) 637-7660
www.carlgoldbergproducts.com
O.S. Engines
(800) 637-7660
www.osengines.com
Dave Brown Products
(513) 738-1576
www.dbproducts.com