How To Do It The Tail of Two Airplanes - 2010/04

ALTHOUGH I OWN more than 60 flying
models, there are only a couple that I use
extensively during most of the flying season: the
numbers 1 and 2 RC Aerobatics (Pattern)
competition airplanes. This is mostly because I
need to practice a great deal since I don’t have the
world’s top Pattern pilots’ natural talents.
I also fly them a lot because nothing performs
as well as a modern trimmed and tuned Pattern
aircraft. However, Pattern models are difficult to
appreciate, because they are flown at high
altitudes. Purpose-built to perform gracefully, they
typically do that with exhausting repetition.
Larger sport-scale airplanes, especially
warbirds, present well and relate to the general
public. Many people can identify a P-51 Mustang
and have seen one fly. These aircraft are normally
bigger and impressive. People and pilots alike
enjoy watching them perform.
Power Upgrade Points to Consider
• When going to a larger-displacement engine, choose the next larger power plant in the same crankcase class. Do not
increase engine dimensions or weights. You could select an O.S. .90 two-stroke to replace the .60 or an Evolution .52
instead of the .46.
• Do not go overboard. Increased power means increased flight loads and stresses. Limit the power boost to roughly
20% as measured with the same propeller. A YS140 L turns the 15 x 8 9,800 rpm. That is only a growth of 12.6% rpm
compared with the 1.20, yet the performance increase is startling.
• Do not increase weight by more than 3%, for best results.
• A larger displacement means increased fuel consumption. However, manage the throttle correctly and the flight times
remain nearly the same if you effected only a 20% power increase. The Great Planes Curtiss P-6E Hawk’s flight times
did not change, because less “throttle” was needed in cruise.
• Keep propeller diameters in the same range (roughly 6% difference). Vertical side areas and landing gear are
designed for certain amounts of engine torque and propeller diameters. Too big of a diameter increase at higher rpm
could overpower the model’s ability to handle torque without the pilot’s always having to input extra rudder.
• Upgrading to electric power might seem easier, but it means employing a more robust ESC, extra cooling, and
possibly larger batteries. Approach this change carefully. Most e-power upgrades may also demand a stronger motor
mounting system and additional ground clearance.
spectators at the Top Gun Scale invitational
or a Warbirds Over “X” event.
My editorial and district associate vice
president duties often take me to fly-ins and
air shows. I travel often to visit and fly with
other clubs in my district.
Everyone I visit is kind enough to
politely watch my Pattern airplane do its
stuff—once. But more than once? It finally
occurred to me that flying Pattern
maneuvers with a larger sport-scale warbird
might make things less boring. But could it
be made to happen?
When built to sport specifications, these
models will fly but will not always perform.
To perform, sometimes they must be
modified, trimmed, and powered as Pattern
aircraft would be. “Patternized” airplanes
exhibit amazing flying abilities that will
surprise even the most experienced pilot.
Although this “tail” is about my two
models, all of the techniques I’ll write about
can be used to improve any sport aircraft’s
performance. This article contains some
“how-to”s , but its focus is on recognizing
your airplane’s performance or structural
deficiencies and using some of the remedies
to fix them. My test subjects are the two
models that I take with me from show to
show.
Warning: The following modifications
and trimming techniques are the ways I did
it. This does not mean that there are no other
ways to achieve these performance goals,
but I know that they work well.
Large warbirds can be crowd pleasers. No
matter where it is, Great Planes’ Curtiss P-
6E Hawk in its “Presidential Inauguration”
colors draws attention and questions from
spectators. Unfortunately, this model is no
longer available.
Likewise, the Hangar 9 P-47 Thunderbolt
150 looks so massive and big with bombs,
external fuel tank, retracts, and flaps that it
gets more than its share of attention. Both
models fly well as sport aircraft but have
severe limitations as true performers.
In sport trim, the Hawk is severely
underpowered, climbs slowly, can’t hold a
vertical, won’t roll worth a darn, drops the
right wing with heavy elevator input,
corkscrews in loops, has too much adverse
yaw, and won’t climb while inverted or hold
a knife edge. But it looks great flying by,
and the landings are slow-motion
spectaculars.
Let’s see what can be done.
To begin, I’ll solve the P-6E’s takeoff,
climb, and inverted-flight deficiencies.
The original engine was the reliable,
powerful O.S. Max 1.20 Surpass III. It’s a
great sport power plant, but it was not
designed to provide a 14-pound biplane with
Patternlike performance. Takeoff runs were
long and the initial climb rate was too low
for the more limited air show venues.
Verticals were impossible to hold.
The P-6E needed a more powerful
engine. But it had to be lightweight and fit in
the same space.
The O.S. 1.20 two-stroke would have
enough power but would not sound or look
right. The larger sport four-strokes were too
big and heavy to fit without cutting the heck
out of that beautiful cowl.
When I think of four-stroke engines
producing extra power in a small size while
having great reliability, YS comes to mind.
A YS140 L proved to be the perfect choice.
It fit in the same space and mounts as the
O.S. 1.20 did; only minor cowl cutting was
needed.
Central Hobbies sells an NMP sport
muffler system that fit perfectly while
exhausting under the cowling. I didn’t even
have to alter the throttle pushrod. The 1.20
and 1.40 weighed the same.
The net result was a power increase
(using Powermaster YS/Saito 20-20 fuel)
from 8,700 rpm on an APC 15 x 8 propeller
to 9,300 rpm on a 16 x 10 APC. That is a
huge increase at the top end. And it paid
huge dividends.
Now the Hawk will take off in less than
50 feet. The initial climb rate is more than
doubled. Small field size is no longer a
problem.
The model can hold a vertical up-line so
that tall stall turns are possible, as are Top
Hats with 1/4 or 1/2 rolls, Figure Ms, and
Humpty Bumps of all shapes. Level knifeedge
flight went from nearly impossible to
something that could be trimmed.
Best yet, the airplane will climb while
inverted and even perform Outside Loops
and Avalanches from the bottom. Airspeed
also increased, but the big biplane has so
much drag that it’s hardly noticeable.
And best of all, I no longer have to plan
“two moves ahead” to match available
energy to the planned maneuver schedule.
Excess power means a lot, but choose
carefully. Increasing power is a prerequisite
for the vertical up-lines needed, but it is only
the start.
Setting up the airframe is even more
critical. Although the Hawk could Snap Roll
like an Extra, it couldn’t outroll a trainer.
The solution was to not increase aileron
movement. Doing so, especially on a flatbottom
wing, also increases adverse yaw. In
the end, the roll slows while becoming ugly.
Sealing the aileron gaps was the answer.
Even if you can’t see through the aileron/
wing gap, air still passes through it. The
aileron and wings act as two separate
surfaces, reducing aileron effectiveness.
Use either clear or matching covering to
seal the underside of the gap. Do the same
for the elevators.
Sealing the gaps not only boosts control
effectiveness, but it also helps prevent flutter
and the annoying wing drop on sharp
pullouts. Do you ever wonder why your
model drops a wing on pullouts?
It’s because either an elevator or aileron
is “spilling more air” through its controlsurface
gap than the other side. Thus the
spilling wing, be it stabilizer or main
wing(s), has less lift on that side during the
pullout.
Sealing all the gaps on the Hawk
increased the roll rate without noticeably
increasing adverse yaw. It also stopped the
right wing drop on pullouts. Two problems
were solved.
I still had to fix the corkscrew loops and
the adverse yaw. If everything is built
straight, the most common cause of
corkscrew loops is poor lateral balance.
Every airplane must be balanced laterally for
good aerobatic performance.
Balancing laterally is the last step
before loading the car for the field, and
this is done indoors. Assemble the model,
remove the propeller (it is already
balanced, right?), and have a helper with you.
For sport aircraft, run thin nylon fishing
line through the rudder/fin gap under the top
hinge. And run fishing line under the
crankshaft. Then you and the helper lift the
model solely by the fishing line.
The airplane will probably drop a wing
toward the muffler side. Use a variety of
finishing nails taped to one wingtip until the
aircraft balances and remains level. Remove
the tape and insert the finishing nails into the
wingtip, leaving 1/4 inch of them exposed.
Now go fly!
After all else is trimmed using the
transmitter trim adjusters, fly loops toward
you on a calm day. Go upright and inverted,
keeping the wings level.
Once you are convinced that the loops
remain on line, you can fully insert the nails
and secure them with a drop of CA. Conceal
the area with a patch of matching covering.
If you really want your model to
perform, insert the line under the bottom
rudder hinge—not the top one. This test is
more sensitive and achieves an even better
balance. However, the more exact balance
does not seem to make a difference on sport
airplanes, especially those with generous
dihedral.
I’m now down to fixing the Hawk’s
adverse yaw. Its ailerons are only on the top
wing, so although distracting, the adverse
yaw is not that bad.
Still, I had to repair it or slow-flight
maneuvers and vertical rolls would wiggle
too much. Because the “down” aileron has
more drag, causing the nose to first drift in a
direction opposite the intended roll, adverse
yaw is usually trimmed out by making the
“up” aileron travel more than the “down”
aileron.
If you have a computer transmitter with a
differential function, try it. But that is not
always the ideal solution.
Unless you have an expensive unit such
as the JR 12X, JR 8103, Futaba 14MZ or
12Z, or others, it does not have separate
differential. Once dialed in, the
differential is applied equally in both
directions (right and left).
Most airplanes, the P-6E included, need
more differential in one direction than in the
other. Left rolls with the Hawk wiggled like
in an old Elvis movie, while right rolls
needed only the artist’s lightest touch.
Adding enough equal differential killed the
right rolls.
Spotting this particular demon is easy.
Each pilot has his or her own way, but mine
is to fly a wings-level vertical up-line,
stabilize it, and then apply full aileron in a
given direction.
Repeat, rolling in the other direction;
watch the tail. If it wiggles, you have the
demon. The answer is to use your
transmitter’s travel function. Start by
identifying which rolling direction needs the
help most.
Use the equal differential function to dial
out adverse roll effects in that direction.
Measure each aileron’s travel in the problem
direction, up and downward. Eliminate the
differential you dialed in. Adjust each
aileron’s travel, only in the problem
direction, to your measurements.
If right roll was the problem, match the
right-wing aileron so it travels the same
amount upward as it did when differential
was used. Do the same for the downward,
left aileron.
Go flying again (tough assignment,
huh?), and do the same for the other, less
troublesome direction. While you are
adjusting the differential function, the
previous adverse roll direction you had
already eliminated is going to return with a
vengeance. It will go away once the
differential function is removed during the
final step.
Even upgraded with the YS, the Hawk
could hold only a 250- to 300-foot vertical
up-line. It had to be dived to excess airspeed
to hold the verticals needed for adverse yaw
trimming.
You might need to do the same for your
airplane. That’s fine, but remember to enter
every vertical with the wings level.
Great Planes’ P-6E Hawk is extremely
robust and built to handle flight stresses that
far exceed those encountered when sportflying.
However, two reinforcements were
made to handle excess stress.
First, the directions are to assemble the
two elevator pushrods with two wheel
collars before going to the single elevator
servo. Aerobatic routines, especially with
the larger engine, caused me some concern.
There was more than enough room to
install a second elevator servo. This separate
unit added control authority while also
providing extra trimming capabilities.
The second problem area was the cabane
attachment. Great Planes provided wood
screws into hardwood. That is good enough
for sport-flying, but for maneuver schedules
with many Snap Rolls and outside
Avalanches? I was not sure, even though the
wood screws held well during the airplane’s
sport days.
Instead of wood screws I installed 4-40
blind nuts and bolts. Then I bonded them
firmly in place using thread-locking
compound.
Examine your model, looking for weak
spots such as those I’ve mentioned. You
may choose to install extra firewall gussets,
to hold that larger engine (or motor) in
place. Or you might have to upgrade older
servo mounts.
If your airplane has a single aileron, go
to the newer dual-servo system. This is
shown in Part 22 of MA’s “From the Ground
Up” series. The Web address at the end of
this article, in the “Sources” list, will take
you to that feature.
It is a good idea to use more powerful,
nonsport, digital servos on all control
surfaces, especially if you stepped up the
power. I later upgraded all of the P-6E’s
servos to digital, at roughly 85 ounce-inch,
while the digital rudder servo produced 155
ounce-inch of output. While you’re
upgrading, ensure that the flight battery is up
to the task.
I’ll do the final trimming on both models
at once.
Now my task is to get the P-47D
Thunderbolt ready. It already had a more
powerful engine; the Saito 2.20 cu. in. was
originally installed in place of the 1.50-1.80
power plant that was specified. Flying 300-
to 400-foot up-lines was routine.
The smallish ailerons caused slow roll
rates, while their far-outboard positioning on
the 81-inch-span wing made adverse yaw
obvious in both directions. The complex
rudder control system limited rudder
movement to only 1 inch. Holding level
knife-edge flight was impossible.
Unlike the Hawk, the P-47D’s higher
engine torque and larger propeller diameter
caused the up-line to bend left during fullpower
verticals. The fuselage servo mount
was weak.
The first fix had to be the rudder. A
movement of only 1 inch was unacceptable.
The rudder linkage passed through the tail
wheel and then on to the rudder, limiting
movement.
I cut the control rod just beyond the tail
wheel connection. Then I installed a pullpull
system directly to the rudder. This
required some internal fuselage work.
The Thunderbolt’s internal fuselage is
constructed from lightened plywood
formers, with many crossbraces. All of the
braces crossed the fuselage center exactly
where the cables had to pass. I removed the
stock braces and installed twin substitutes
just above, below, or aside the original brace
positions.
Since exact rudder centering is
extraordinarily important, there is a
geometry that must be observed in pull-pull
systems. The total width of the rudder
attachment points must match the servo
arm’s length.
The cables must exit the fuselage at the
point where its width is the same distance.
This allows the cables to be straight from
servo to rudder horn. Any kinks will prevent
48 MODEL AVIATION
the rudder from centering perfectly, as it
must for best performance.
With this change, the Thunderbolt
now climbs in knife-edge. However, this
heavy airplane puts a strain on the
fuselage servo mounts during extreme
performances; a few did come loose.
I reinforced these mounts, as shown.
Remember to examine your airplane for
such weak points, as I have mentioned.
I braced the fuselage servo mounts. A
cap on one side prevents the mounting
rails from lifting away from the side
fuselage braces. Triangle stock did the
same on the other, less critical, side.
The rails flexed in the middle, so I
fitted a hardwood brace that tied the two
rails together, adding strength, and then
glued them to the former just forward of
the rails. This eliminated the servo
flexing, which causes pitch hunting.
My big four-stroke engine, mounted
upside-down, had issues. Raw fuel pooled
into the large head area, extinguishing the
glow plug at low rpm. The YS 2.20 ran
well and idled fine for a day of sportflying.
But air show dead-sticks are only
exciting once, and I am already excited
enough for three pilots at the usual
performance.
To extinguish this type of exhilaration,
I installed a Maxx Products International
Super Glow MX9900 onboard glow-plug
driver. It can be set to light the glow plug
at any throttle setting.
The MX9900 uses a single-cell, 1300
mAh Ni-Cd battery for power and works
directly from the receiver’s throttle port.
The throttle servo plugs into the Super
Glow. Since I installed the unit in 2008,
there has been no engine failure at idle.
I increased roll rate by sealing the
aileron gaps. That worked even better on
the P-47 than it did on the Hawk.
My experience has been that closing
gaps increases roll rates more on
symmetrical wings than on flat-bottom
airfoils. The P-47 has a semisymmetrical
wing.
Still, aileron movement had to be set
near 1 inch and then adjusted for adverse
yaw. On these aircraft, the best roll rate
for aerobatic performance has proven to
be the old standard of three rolls in five
seconds.
Sealing control-surface gaps had an
unexpected—but welcome—effect on the
Thunderbolt. It eliminated the airplane’s
left wing drop, and overall lift seems to
have increased. The airplane was a
“floater,” but now it repels the ground,
especially in ground effect.
To regain precision landing spots, I
had to increase flap deployment by 5°.
Without flaps, landing approaches that
start in New Jersey might end in
Pennsylvania.
I trimmed out the P-47D’s
considerable adverse yaw the way I did
the Hawk’s. It just took probably 15
flights longer.
After all that work, the Thunderbolt
still flies like a baby carriage. But now it
will slow roll as if Col. Bob Johnson were
at the controls. Loops track well, as do
Vertical Figure Eights. Inverted flight
requires only a touch of down-elevator
but tracks as if upright.
Because the P-47D flies so well and is
as honest as they come, I moved the CG
1/8 inch aft of the rearmost setting. That
improved Snap Rolls and Spins but kept
the airplane fully controllable. I don’t
recommend this practice until you have at
least a few hundred flights on the model
and know it well.
“A few hundred flights?” you might ask.
“You’re kidding, right?”
No. Trimming the airplane will require
roughly 40 flights. Practice time will
easily use the remaining airtime before
you know it.
If you follow the National Society of
Radio Controlled Aerobatics (NSRCA)
trim chart, you will need approximately
100 flights and adjustable wing/stabilizer
incidences that the P-6E and the P-47D
are missing.
Although the NSRCA guide is the
best, it may be overkill for those models. I
have found a few trim adjustments to be
the most important for a sport-type
aircraft’s optimum aerobatic performance.
Most important is knife-edge flight trim.
Sport airplanes are going to “walk” in
this flight zone. When rudder is applied in
Knife-Edge or Slow Rolls, the aircraft will
pull toward the belly or the canopy (usually
the belly). Moving the CG, usually rearward,
or adjusting wing incidence (awkward on
these models) will usually help trim out this
condition.
Try mixing the rudder to elevator. Use a
direct mix—no curves. The goal is straight
flight in knife-edge.
If less than 20 elevator points are needed,
okay. More than 20 points can mess up some
maneuvers, so slightly adjust the CG or wing
incidence (using shims) until the elevator mix
is less than 20 points.
Second in importance is eliminating roll
coupling. This occurs when rudder input also
rolls the wings. Point Rolls and Stall Turns
require that there be little or no coupling.
Mix opposite ailerons to rudder if the
coupling is proverse (in the rudder’s direction)
or vice versa for adverse, opposite-direction,
coupling. The ideal trim condition results in a
slightly descending flat turn on rudder input
alone.
The third important adjustment is downline
trim. Take the airplane high, go to idle,
and push the nose down to 90°. Watch the
track. Most sport models will begin to pull out
as airspeed increases.
Eliminate this by mixing down-elevator
with low throttle only. If your transmitter does
not have a curve mix that allows mixing only
at idle, skip this step unless the pullout is
extremely noticeable (roughly 10°). If so, you
might have to reduce the wing incidence,
assuming that your aircraft’s stabilizer is
glued in place.
Be careful here; a little goes a long way.
Start with a 1/8° change.
Most sport airplanes will not pull to the
canopy in vertical up-lines. Pattern models
need to trim this out, but a sport aircraft
won’t. However, I’ll bet you will need to trim
in right rudder on the up-lines.
Adjusting engine thrust to compensate for
this is a hassle, and it was impossible on the
Hawk because of the tiny crankshaft hole in
the cowling. Instead, try curve mixing (also
called “step”)—one to two points of right
rudder at half throttle, up to four to five points
at full power.
Although this condition is most apparent
in the vertical up-line, it also exists in level
flight. The leftward-nose-pointing tendency
can make it difficult to hold straight lines from
one maneuver to the next. This increases the
pilot’s workload.
The tendency we want to trim out occurs
when the model is moving near high speed
while in the up-line. All airplanes will go
“nose left” under full power once airspeed
drops. This is not a trim problem, but rather a
pilot who might need more rudder practice.
I have my air show aircraft to prove that any
sport model (except a basic trainer) can be
improved and trimmed to provide air showlike
performance with little work. If a giant
biplane with a flat-bottomed airfoil can be
improved to near-Extra 300 performance, so
can your sport airplane.
I’d bet that your model will be easier to
prepare for stunning airborne performance
than my Thunderbolt was. It might even fly
better. But that P-47D is amazing, so no bets
on that score.
Try it, just once, and you will never
want to fly a stock, out-of-trim sport
airplane again! MA
Frank Granelli
[email protected]
Sources:
Great Planes
(217) 398-3630
www.greatplanes.com
Hangar 9
(800) 338-4639
www.hangar-9.com
Central Hobbies
(406) 259-9004
www.centralhobbies.com
“From the Ground Up” Index
www.modelaircraft.org/mag/FTGU/titlespag
eftgu.htm
Maxx Products International
(800) 416-6299
www.maxxprod.com
National Society of Radio Controlled
Aerobatics
www.nsrca.us

ALTHOUGH I OWN more than 60 flying
models, there are only a couple that I use
extensively during most of the flying season: the
numbers 1 and 2 RC Aerobatics (Pattern)
competition airplanes. This is mostly because I
need to practice a great deal since I don’t have the
world’s top Pattern pilots’ natural talents.
I also fly them a lot because nothing performs
as well as a modern trimmed and tuned Pattern
aircraft. However, Pattern models are difficult to
appreciate, because they are flown at high
altitudes. Purpose-built to perform gracefully, they
typically do that with exhausting repetition.
Larger sport-scale airplanes, especially
warbirds, present well and relate to the general
public. Many people can identify a P-51 Mustang
and have seen one fly. These aircraft are normally
bigger and impressive. People and pilots alike
enjoy watching them perform.
Power Upgrade Points to Consider
• When going to a larger-displacement engine, choose the next larger power plant in the same crankcase class. Do not
increase engine dimensions or weights. You could select an O.S. .90 two-stroke to replace the .60 or an Evolution .52
instead of the .46.
• Do not go overboard. Increased power means increased flight loads and stresses. Limit the power boost to roughly
20% as measured with the same propeller. A YS140 L turns the 15 x 8 9,800 rpm. That is only a growth of 12.6% rpm
compared with the 1.20, yet the performance increase is startling.
• Do not increase weight by more than 3%, for best results.
• A larger displacement means increased fuel consumption. However, manage the throttle correctly and the flight times
remain nearly the same if you effected only a 20% power increase. The Great Planes Curtiss P-6E Hawk’s flight times
did not change, because less “throttle” was needed in cruise.
• Keep propeller diameters in the same range (roughly 6% difference). Vertical side areas and landing gear are
designed for certain amounts of engine torque and propeller diameters. Too big of a diameter increase at higher rpm
could overpower the model’s ability to handle torque without the pilot’s always having to input extra rudder.
• Upgrading to electric power might seem easier, but it means employing a more robust ESC, extra cooling, and
possibly larger batteries. Approach this change carefully. Most e-power upgrades may also demand a stronger motor
mounting system and additional ground clearance.
spectators at the Top Gun Scale invitational
or a Warbirds Over “X” event.
My editorial and district associate vice
president duties often take me to fly-ins and
air shows. I travel often to visit and fly with
other clubs in my district.
Everyone I visit is kind enough to
politely watch my Pattern airplane do its
stuff—once. But more than once? It finally
occurred to me that flying Pattern
maneuvers with a larger sport-scale warbird
might make things less boring. But could it
be made to happen?
When built to sport specifications, these
models will fly but will not always perform.
To perform, sometimes they must be
modified, trimmed, and powered as Pattern
aircraft would be. “Patternized” airplanes
exhibit amazing flying abilities that will
surprise even the most experienced pilot.
Although this “tail” is about my two
models, all of the techniques I’ll write about
can be used to improve any sport aircraft’s
performance. This article contains some
“how-to”s , but its focus is on recognizing
your airplane’s performance or structural
deficiencies and using some of the remedies
to fix them. My test subjects are the two
models that I take with me from show to
show.
Warning: The following modifications
and trimming techniques are the ways I did
it. This does not mean that there are no other
ways to achieve these performance goals,
but I know that they work well.
Large warbirds can be crowd pleasers. No
matter where it is, Great Planes’ Curtiss P-
6E Hawk in its “Presidential Inauguration”
colors draws attention and questions from
spectators. Unfortunately, this model is no
longer available.
Likewise, the Hangar 9 P-47 Thunderbolt
150 looks so massive and big with bombs,
external fuel tank, retracts, and flaps that it
gets more than its share of attention. Both
models fly well as sport aircraft but have
severe limitations as true performers.
In sport trim, the Hawk is severely
underpowered, climbs slowly, can’t hold a
vertical, won’t roll worth a darn, drops the
right wing with heavy elevator input,
corkscrews in loops, has too much adverse
yaw, and won’t climb while inverted or hold
a knife edge. But it looks great flying by,
and the landings are slow-motion
spectaculars.
Let’s see what can be done.
To begin, I’ll solve the P-6E’s takeoff,
climb, and inverted-flight deficiencies.
The original engine was the reliable,
powerful O.S. Max 1.20 Surpass III. It’s a
great sport power plant, but it was not
designed to provide a 14-pound biplane with
Patternlike performance. Takeoff runs were
long and the initial climb rate was too low
for the more limited air show venues.
Verticals were impossible to hold.
The P-6E needed a more powerful
engine. But it had to be lightweight and fit in
the same space.
The O.S. 1.20 two-stroke would have
enough power but would not sound or look
right. The larger sport four-strokes were too
big and heavy to fit without cutting the heck
out of that beautiful cowl.
When I think of four-stroke engines
producing extra power in a small size while
having great reliability, YS comes to mind.
A YS140 L proved to be the perfect choice.
It fit in the same space and mounts as the
O.S. 1.20 did; only minor cowl cutting was
needed.
Central Hobbies sells an NMP sport
muffler system that fit perfectly while
exhausting under the cowling. I didn’t even
have to alter the throttle pushrod. The 1.20
and 1.40 weighed the same.
The net result was a power increase
(using Powermaster YS/Saito 20-20 fuel)
from 8,700 rpm on an APC 15 x 8 propeller
to 9,300 rpm on a 16 x 10 APC. That is a
huge increase at the top end. And it paid
huge dividends.
Now the Hawk will take off in less than
50 feet. The initial climb rate is more than
doubled. Small field size is no longer a
problem.
The model can hold a vertical up-line so
that tall stall turns are possible, as are Top
Hats with 1/4 or 1/2 rolls, Figure Ms, and
Humpty Bumps of all shapes. Level knifeedge
flight went from nearly impossible to
something that could be trimmed.
Best yet, the airplane will climb while
inverted and even perform Outside Loops
and Avalanches from the bottom. Airspeed
also increased, but the big biplane has so
much drag that it’s hardly noticeable.
And best of all, I no longer have to plan
“two moves ahead” to match available
energy to the planned maneuver schedule.
Excess power means a lot, but choose
carefully. Increasing power is a prerequisite
for the vertical up-lines needed, but it is only
the start.
Setting up the airframe is even more
critical. Although the Hawk could Snap Roll
like an Extra, it couldn’t outroll a trainer.
The solution was to not increase aileron
movement. Doing so, especially on a flatbottom
wing, also increases adverse yaw. In
the end, the roll slows while becoming ugly.
Sealing the aileron gaps was the answer.
Even if you can’t see through the aileron/
wing gap, air still passes through it. The
aileron and wings act as two separate
surfaces, reducing aileron effectiveness.
Use either clear or matching covering to
seal the underside of the gap. Do the same
for the elevators.
Sealing the gaps not only boosts control
effectiveness, but it also helps prevent flutter
and the annoying wing drop on sharp
pullouts. Do you ever wonder why your
model drops a wing on pullouts?
It’s because either an elevator or aileron
is “spilling more air” through its controlsurface
gap than the other side. Thus the
spilling wing, be it stabilizer or main
wing(s), has less lift on that side during the
pullout.
Sealing all the gaps on the Hawk
increased the roll rate without noticeably
increasing adverse yaw. It also stopped the
right wing drop on pullouts. Two problems
were solved.
I still had to fix the corkscrew loops and
the adverse yaw. If everything is built
straight, the most common cause of
corkscrew loops is poor lateral balance.
Every airplane must be balanced laterally for
good aerobatic performance.
Balancing laterally is the last step
before loading the car for the field, and
this is done indoors. Assemble the model,
remove the propeller (it is already
balanced, right?), and have a helper with you.
For sport aircraft, run thin nylon fishing
line through the rudder/fin gap under the top
hinge. And run fishing line under the
crankshaft. Then you and the helper lift the
model solely by the fishing line.
The airplane will probably drop a wing
toward the muffler side. Use a variety of
finishing nails taped to one wingtip until the
aircraft balances and remains level. Remove
the tape and insert the finishing nails into the
wingtip, leaving 1/4 inch of them exposed.
Now go fly!
After all else is trimmed using the
transmitter trim adjusters, fly loops toward
you on a calm day. Go upright and inverted,
keeping the wings level.
Once you are convinced that the loops
remain on line, you can fully insert the nails
and secure them with a drop of CA. Conceal
the area with a patch of matching covering.
If you really want your model to
perform, insert the line under the bottom
rudder hinge—not the top one. This test is
more sensitive and achieves an even better
balance. However, the more exact balance
does not seem to make a difference on sport
airplanes, especially those with generous
dihedral.
I’m now down to fixing the Hawk’s
adverse yaw. Its ailerons are only on the top
wing, so although distracting, the adverse
yaw is not that bad.
Still, I had to repair it or slow-flight
maneuvers and vertical rolls would wiggle
too much. Because the “down” aileron has
more drag, causing the nose to first drift in a
direction opposite the intended roll, adverse
yaw is usually trimmed out by making the
“up” aileron travel more than the “down”
aileron.
If you have a computer transmitter with a
differential function, try it. But that is not
always the ideal solution.
Unless you have an expensive unit such
as the JR 12X, JR 8103, Futaba 14MZ or
12Z, or others, it does not have separate
differential. Once dialed in, the
differential is applied equally in both
directions (right and left).
Most airplanes, the P-6E included, need
more differential in one direction than in the
other. Left rolls with the Hawk wiggled like
in an old Elvis movie, while right rolls
needed only the artist’s lightest touch.
Adding enough equal differential killed the
right rolls.
Spotting this particular demon is easy.
Each pilot has his or her own way, but mine
is to fly a wings-level vertical up-line,
stabilize it, and then apply full aileron in a
given direction.
Repeat, rolling in the other direction;
watch the tail. If it wiggles, you have the
demon. The answer is to use your
transmitter’s travel function. Start by
identifying which rolling direction needs the
help most.
Use the equal differential function to dial
out adverse roll effects in that direction.
Measure each aileron’s travel in the problem
direction, up and downward. Eliminate the
differential you dialed in. Adjust each
aileron’s travel, only in the problem
direction, to your measurements.
If right roll was the problem, match the
right-wing aileron so it travels the same
amount upward as it did when differential
was used. Do the same for the downward,
left aileron.
Go flying again (tough assignment,
huh?), and do the same for the other, less
troublesome direction. While you are
adjusting the differential function, the
previous adverse roll direction you had
already eliminated is going to return with a
vengeance. It will go away once the
differential function is removed during the
final step.
Even upgraded with the YS, the Hawk
could hold only a 250- to 300-foot vertical
up-line. It had to be dived to excess airspeed
to hold the verticals needed for adverse yaw
trimming.
You might need to do the same for your
airplane. That’s fine, but remember to enter
every vertical with the wings level.
Great Planes’ P-6E Hawk is extremely
robust and built to handle flight stresses that
far exceed those encountered when sportflying.
However, two reinforcements were
made to handle excess stress.
First, the directions are to assemble the
two elevator pushrods with two wheel
collars before going to the single elevator
servo. Aerobatic routines, especially with
the larger engine, caused me some concern.
There was more than enough room to
install a second elevator servo. This separate
unit added control authority while also
providing extra trimming capabilities.
The second problem area was the cabane
attachment. Great Planes provided wood
screws into hardwood. That is good enough
for sport-flying, but for maneuver schedules
with many Snap Rolls and outside
Avalanches? I was not sure, even though the
wood screws held well during the airplane’s
sport days.
Instead of wood screws I installed 4-40
blind nuts and bolts. Then I bonded them
firmly in place using thread-locking
compound.
Examine your model, looking for weak
spots such as those I’ve mentioned. You
may choose to install extra firewall gussets,
to hold that larger engine (or motor) in
place. Or you might have to upgrade older
servo mounts.
If your airplane has a single aileron, go
to the newer dual-servo system. This is
shown in Part 22 of MA’s “From the Ground
Up” series. The Web address at the end of
this article, in the “Sources” list, will take
you to that feature.
It is a good idea to use more powerful,
nonsport, digital servos on all control
surfaces, especially if you stepped up the
power. I later upgraded all of the P-6E’s
servos to digital, at roughly 85 ounce-inch,
while the digital rudder servo produced 155
ounce-inch of output. While you’re
upgrading, ensure that the flight battery is up
to the task.
I’ll do the final trimming on both models
at once.
Now my task is to get the P-47D
Thunderbolt ready. It already had a more
powerful engine; the Saito 2.20 cu. in. was
originally installed in place of the 1.50-1.80
power plant that was specified. Flying 300-
to 400-foot up-lines was routine.
The smallish ailerons caused slow roll
rates, while their far-outboard positioning on
the 81-inch-span wing made adverse yaw
obvious in both directions. The complex
rudder control system limited rudder
movement to only 1 inch. Holding level
knife-edge flight was impossible.
Unlike the Hawk, the P-47D’s higher
engine torque and larger propeller diameter
caused the up-line to bend left during fullpower
verticals. The fuselage servo mount
was weak.
The first fix had to be the rudder. A
movement of only 1 inch was unacceptable.
The rudder linkage passed through the tail
wheel and then on to the rudder, limiting
movement.
I cut the control rod just beyond the tail
wheel connection. Then I installed a pullpull
system directly to the rudder. This
required some internal fuselage work.
The Thunderbolt’s internal fuselage is
constructed from lightened plywood
formers, with many crossbraces. All of the
braces crossed the fuselage center exactly
where the cables had to pass. I removed the
stock braces and installed twin substitutes
just above, below, or aside the original brace
positions.
Since exact rudder centering is
extraordinarily important, there is a
geometry that must be observed in pull-pull
systems. The total width of the rudder
attachment points must match the servo
arm’s length.
The cables must exit the fuselage at the
point where its width is the same distance.
This allows the cables to be straight from
servo to rudder horn. Any kinks will prevent
48 MODEL AVIATION
the rudder from centering perfectly, as it
must for best performance.
With this change, the Thunderbolt
now climbs in knife-edge. However, this
heavy airplane puts a strain on the
fuselage servo mounts during extreme
performances; a few did come loose.
I reinforced these mounts, as shown.
Remember to examine your airplane for
such weak points, as I have mentioned.
I braced the fuselage servo mounts. A
cap on one side prevents the mounting
rails from lifting away from the side
fuselage braces. Triangle stock did the
same on the other, less critical, side.
The rails flexed in the middle, so I
fitted a hardwood brace that tied the two
rails together, adding strength, and then
glued them to the former just forward of
the rails. This eliminated the servo
flexing, which causes pitch hunting.
My big four-stroke engine, mounted
upside-down, had issues. Raw fuel pooled
into the large head area, extinguishing the
glow plug at low rpm. The YS 2.20 ran
well and idled fine for a day of sportflying.
But air show dead-sticks are only
exciting once, and I am already excited
enough for three pilots at the usual
performance.
To extinguish this type of exhilaration,
I installed a Maxx Products International
Super Glow MX9900 onboard glow-plug
driver. It can be set to light the glow plug
at any throttle setting.
The MX9900 uses a single-cell, 1300
mAh Ni-Cd battery for power and works
directly from the receiver’s throttle port.
The throttle servo plugs into the Super
Glow. Since I installed the unit in 2008,
there has been no engine failure at idle.
I increased roll rate by sealing the
aileron gaps. That worked even better on
the P-47 than it did on the Hawk.
My experience has been that closing
gaps increases roll rates more on
symmetrical wings than on flat-bottom
airfoils. The P-47 has a semisymmetrical
wing.
Still, aileron movement had to be set
near 1 inch and then adjusted for adverse
yaw. On these aircraft, the best roll rate
for aerobatic performance has proven to
be the old standard of three rolls in five
seconds.
Sealing control-surface gaps had an
unexpected—but welcome—effect on the
Thunderbolt. It eliminated the airplane’s
left wing drop, and overall lift seems to
have increased. The airplane was a
“floater,” but now it repels the ground,
especially in ground effect.
To regain precision landing spots, I
had to increase flap deployment by 5°.
Without flaps, landing approaches that
start in New Jersey might end in
Pennsylvania.
I trimmed out the P-47D’s
considerable adverse yaw the way I did
the Hawk’s. It just took probably 15
flights longer.
After all that work, the Thunderbolt
still flies like a baby carriage. But now it
will slow roll as if Col. Bob Johnson were
at the controls. Loops track well, as do
Vertical Figure Eights. Inverted flight
requires only a touch of down-elevator
but tracks as if upright.
Because the P-47D flies so well and is
as honest as they come, I moved the CG
1/8 inch aft of the rearmost setting. That
improved Snap Rolls and Spins but kept
the airplane fully controllable. I don’t
recommend this practice until you have at
least a few hundred flights on the model
and know it well.
“A few hundred flights?” you might ask.
“You’re kidding, right?”
No. Trimming the airplane will require
roughly 40 flights. Practice time will
easily use the remaining airtime before
you know it.
If you follow the National Society of
Radio Controlled Aerobatics (NSRCA)
trim chart, you will need approximately
100 flights and adjustable wing/stabilizer
incidences that the P-6E and the P-47D
are missing.
Although the NSRCA guide is the
best, it may be overkill for those models. I
have found a few trim adjustments to be
the most important for a sport-type
aircraft’s optimum aerobatic performance.
Most important is knife-edge flight trim.
Sport airplanes are going to “walk” in
this flight zone. When rudder is applied in
Knife-Edge or Slow Rolls, the aircraft will
pull toward the belly or the canopy (usually
the belly). Moving the CG, usually rearward,
or adjusting wing incidence (awkward on
these models) will usually help trim out this
condition.
Try mixing the rudder to elevator. Use a
direct mix—no curves. The goal is straight
flight in knife-edge.
If less than 20 elevator points are needed,
okay. More than 20 points can mess up some
maneuvers, so slightly adjust the CG or wing
incidence (using shims) until the elevator mix
is less than 20 points.
Second in importance is eliminating roll
coupling. This occurs when rudder input also
rolls the wings. Point Rolls and Stall Turns
require that there be little or no coupling.
Mix opposite ailerons to rudder if the
coupling is proverse (in the rudder’s direction)
or vice versa for adverse, opposite-direction,
coupling. The ideal trim condition results in a
slightly descending flat turn on rudder input
alone.
The third important adjustment is downline
trim. Take the airplane high, go to idle,
and push the nose down to 90°. Watch the
track. Most sport models will begin to pull out
as airspeed increases.
Eliminate this by mixing down-elevator
with low throttle only. If your transmitter does
not have a curve mix that allows mixing only
at idle, skip this step unless the pullout is
extremely noticeable (roughly 10°). If so, you
might have to reduce the wing incidence,
assuming that your aircraft’s stabilizer is
glued in place.
Be careful here; a little goes a long way.
Start with a 1/8° change.
Most sport airplanes will not pull to the
canopy in vertical up-lines. Pattern models
need to trim this out, but a sport aircraft
won’t. However, I’ll bet you will need to trim
in right rudder on the up-lines.
Adjusting engine thrust to compensate for
this is a hassle, and it was impossible on the
Hawk because of the tiny crankshaft hole in
the cowling. Instead, try curve mixing (also
called “step”)—one to two points of right
rudder at half throttle, up to four to five points
at full power.
Although this condition is most apparent
in the vertical up-line, it also exists in level
flight. The leftward-nose-pointing tendency
can make it difficult to hold straight lines from
one maneuver to the next. This increases the
pilot’s workload.
The tendency we want to trim out occurs
when the model is moving near high speed
while in the up-line. All airplanes will go
“nose left” under full power once airspeed
drops. This is not a trim problem, but rather a
pilot who might need more rudder practice.
I have my air show aircraft to prove that any
sport model (except a basic trainer) can be
improved and trimmed to provide air showlike
performance with little work. If a giant
biplane with a flat-bottomed airfoil can be
improved to near-Extra 300 performance, so
can your sport airplane.
I’d bet that your model will be easier to
prepare for stunning airborne performance
than my Thunderbolt was. It might even fly
better. But that P-47D is amazing, so no bets
on that score.
Try it, just once, and you will never
want to fly a stock, out-of-trim sport
airplane again! MA
Frank Granelli
[email protected]
Sources:
Great Planes
(217) 398-3630
www.greatplanes.com
Hangar 9
(800) 338-4639
www.hangar-9.com
Central Hobbies
(406) 259-9004
www.centralhobbies.com
“From the Ground Up” Index
www.modelaircraft.org/mag/FTGU/titlespag
eftgu.htm
Maxx Products International
(800) 416-6299
www.maxxprod.com
National Society of Radio Controlled
Aerobatics
www.nsrca.us

ALTHOUGH I OWN more than 60 flying
models, there are only a couple that I use
extensively during most of the flying season: the
numbers 1 and 2 RC Aerobatics (Pattern)
competition airplanes. This is mostly because I
need to practice a great deal since I don’t have the
world’s top Pattern pilots’ natural talents.
I also fly them a lot because nothing performs
as well as a modern trimmed and tuned Pattern
aircraft. However, Pattern models are difficult to
appreciate, because they are flown at high
altitudes. Purpose-built to perform gracefully, they
typically do that with exhausting repetition.
Larger sport-scale airplanes, especially
warbirds, present well and relate to the general
public. Many people can identify a P-51 Mustang
and have seen one fly. These aircraft are normally
bigger and impressive. People and pilots alike
enjoy watching them perform.
Power Upgrade Points to Consider
• When going to a larger-displacement engine, choose the next larger power plant in the same crankcase class. Do not
increase engine dimensions or weights. You could select an O.S. .90 two-stroke to replace the .60 or an Evolution .52
instead of the .46.
• Do not go overboard. Increased power means increased flight loads and stresses. Limit the power boost to roughly
20% as measured with the same propeller. A YS140 L turns the 15 x 8 9,800 rpm. That is only a growth of 12.6% rpm
compared with the 1.20, yet the performance increase is startling.
• Do not increase weight by more than 3%, for best results.
• A larger displacement means increased fuel consumption. However, manage the throttle correctly and the flight times
remain nearly the same if you effected only a 20% power increase. The Great Planes Curtiss P-6E Hawk’s flight times
did not change, because less “throttle” was needed in cruise.
• Keep propeller diameters in the same range (roughly 6% difference). Vertical side areas and landing gear are
designed for certain amounts of engine torque and propeller diameters. Too big of a diameter increase at higher rpm
could overpower the model’s ability to handle torque without the pilot’s always having to input extra rudder.
• Upgrading to electric power might seem easier, but it means employing a more robust ESC, extra cooling, and
possibly larger batteries. Approach this change carefully. Most e-power upgrades may also demand a stronger motor
mounting system and additional ground clearance.
spectators at the Top Gun Scale invitational
or a Warbirds Over “X” event.
My editorial and district associate vice
president duties often take me to fly-ins and
air shows. I travel often to visit and fly with
other clubs in my district.
Everyone I visit is kind enough to
politely watch my Pattern airplane do its
stuff—once. But more than once? It finally
occurred to me that flying Pattern
maneuvers with a larger sport-scale warbird
might make things less boring. But could it
be made to happen?
When built to sport specifications, these
models will fly but will not always perform.
To perform, sometimes they must be
modified, trimmed, and powered as Pattern
aircraft would be. “Patternized” airplanes
exhibit amazing flying abilities that will
surprise even the most experienced pilot.
Although this “tail” is about my two
models, all of the techniques I’ll write about
can be used to improve any sport aircraft’s
performance. This article contains some
“how-to”s , but its focus is on recognizing
your airplane’s performance or structural
deficiencies and using some of the remedies
to fix them. My test subjects are the two
models that I take with me from show to
show.
Warning: The following modifications
and trimming techniques are the ways I did
it. This does not mean that there are no other
ways to achieve these performance goals,
but I know that they work well.
Large warbirds can be crowd pleasers. No
matter where it is, Great Planes’ Curtiss P-
6E Hawk in its “Presidential Inauguration”
colors draws attention and questions from
spectators. Unfortunately, this model is no
longer available.
Likewise, the Hangar 9 P-47 Thunderbolt
150 looks so massive and big with bombs,
external fuel tank, retracts, and flaps that it
gets more than its share of attention. Both
models fly well as sport aircraft but have
severe limitations as true performers.
In sport trim, the Hawk is severely
underpowered, climbs slowly, can’t hold a
vertical, won’t roll worth a darn, drops the
right wing with heavy elevator input,
corkscrews in loops, has too much adverse
yaw, and won’t climb while inverted or hold
a knife edge. But it looks great flying by,
and the landings are slow-motion
spectaculars.
Let’s see what can be done.
To begin, I’ll solve the P-6E’s takeoff,
climb, and inverted-flight deficiencies.
The original engine was the reliable,
powerful O.S. Max 1.20 Surpass III. It’s a
great sport power plant, but it was not
designed to provide a 14-pound biplane with
Patternlike performance. Takeoff runs were
long and the initial climb rate was too low
for the more limited air show venues.
Verticals were impossible to hold.
The P-6E needed a more powerful
engine. But it had to be lightweight and fit in
the same space.
The O.S. 1.20 two-stroke would have
enough power but would not sound or look
right. The larger sport four-strokes were too
big and heavy to fit without cutting the heck
out of that beautiful cowl.
When I think of four-stroke engines
producing extra power in a small size while
having great reliability, YS comes to mind.
A YS140 L proved to be the perfect choice.
It fit in the same space and mounts as the
O.S. 1.20 did; only minor cowl cutting was
needed.
Central Hobbies sells an NMP sport
muffler system that fit perfectly while
exhausting under the cowling. I didn’t even
have to alter the throttle pushrod. The 1.20
and 1.40 weighed the same.
The net result was a power increase
(using Powermaster YS/Saito 20-20 fuel)
from 8,700 rpm on an APC 15 x 8 propeller
to 9,300 rpm on a 16 x 10 APC. That is a
huge increase at the top end. And it paid
huge dividends.
Now the Hawk will take off in less than
50 feet. The initial climb rate is more than
doubled. Small field size is no longer a
problem.
The model can hold a vertical up-line so
that tall stall turns are possible, as are Top
Hats with 1/4 or 1/2 rolls, Figure Ms, and
Humpty Bumps of all shapes. Level knifeedge
flight went from nearly impossible to
something that could be trimmed.
Best yet, the airplane will climb while
inverted and even perform Outside Loops
and Avalanches from the bottom. Airspeed
also increased, but the big biplane has so
much drag that it’s hardly noticeable.
And best of all, I no longer have to plan
“two moves ahead” to match available
energy to the planned maneuver schedule.
Excess power means a lot, but choose
carefully. Increasing power is a prerequisite
for the vertical up-lines needed, but it is only
the start.
Setting up the airframe is even more
critical. Although the Hawk could Snap Roll
like an Extra, it couldn’t outroll a trainer.
The solution was to not increase aileron
movement. Doing so, especially on a flatbottom
wing, also increases adverse yaw. In
the end, the roll slows while becoming ugly.
Sealing the aileron gaps was the answer.
Even if you can’t see through the aileron/
wing gap, air still passes through it. The
aileron and wings act as two separate
surfaces, reducing aileron effectiveness.
Use either clear or matching covering to
seal the underside of the gap. Do the same
for the elevators.
Sealing the gaps not only boosts control
effectiveness, but it also helps prevent flutter
and the annoying wing drop on sharp
pullouts. Do you ever wonder why your
model drops a wing on pullouts?
It’s because either an elevator or aileron
is “spilling more air” through its controlsurface
gap than the other side. Thus the
spilling wing, be it stabilizer or main
wing(s), has less lift on that side during the
pullout.
Sealing all the gaps on the Hawk
increased the roll rate without noticeably
increasing adverse yaw. It also stopped the
right wing drop on pullouts. Two problems
were solved.
I still had to fix the corkscrew loops and
the adverse yaw. If everything is built
straight, the most common cause of
corkscrew loops is poor lateral balance.
Every airplane must be balanced laterally for
good aerobatic performance.
Balancing laterally is the last step
before loading the car for the field, and
this is done indoors. Assemble the model,
remove the propeller (it is already
balanced, right?), and have a helper with you.
For sport aircraft, run thin nylon fishing
line through the rudder/fin gap under the top
hinge. And run fishing line under the
crankshaft. Then you and the helper lift the
model solely by the fishing line.
The airplane will probably drop a wing
toward the muffler side. Use a variety of
finishing nails taped to one wingtip until the
aircraft balances and remains level. Remove
the tape and insert the finishing nails into the
wingtip, leaving 1/4 inch of them exposed.
Now go fly!
After all else is trimmed using the
transmitter trim adjusters, fly loops toward
you on a calm day. Go upright and inverted,
keeping the wings level.
Once you are convinced that the loops
remain on line, you can fully insert the nails
and secure them with a drop of CA. Conceal
the area with a patch of matching covering.
If you really want your model to
perform, insert the line under the bottom
rudder hinge—not the top one. This test is
more sensitive and achieves an even better
balance. However, the more exact balance
does not seem to make a difference on sport
airplanes, especially those with generous
dihedral.
I’m now down to fixing the Hawk’s
adverse yaw. Its ailerons are only on the top
wing, so although distracting, the adverse
yaw is not that bad.
Still, I had to repair it or slow-flight
maneuvers and vertical rolls would wiggle
too much. Because the “down” aileron has
more drag, causing the nose to first drift in a
direction opposite the intended roll, adverse
yaw is usually trimmed out by making the
“up” aileron travel more than the “down”
aileron.
If you have a computer transmitter with a
differential function, try it. But that is not
always the ideal solution.
Unless you have an expensive unit such
as the JR 12X, JR 8103, Futaba 14MZ or
12Z, or others, it does not have separate
differential. Once dialed in, the
differential is applied equally in both
directions (right and left).
Most airplanes, the P-6E included, need
more differential in one direction than in the
other. Left rolls with the Hawk wiggled like
in an old Elvis movie, while right rolls
needed only the artist’s lightest touch.
Adding enough equal differential killed the
right rolls.
Spotting this particular demon is easy.
Each pilot has his or her own way, but mine
is to fly a wings-level vertical up-line,
stabilize it, and then apply full aileron in a
given direction.
Repeat, rolling in the other direction;
watch the tail. If it wiggles, you have the
demon. The answer is to use your
transmitter’s travel function. Start by
identifying which rolling direction needs the
help most.
Use the equal differential function to dial
out adverse roll effects in that direction.
Measure each aileron’s travel in the problem
direction, up and downward. Eliminate the
differential you dialed in. Adjust each
aileron’s travel, only in the problem
direction, to your measurements.
If right roll was the problem, match the
right-wing aileron so it travels the same
amount upward as it did when differential
was used. Do the same for the downward,
left aileron.
Go flying again (tough assignment,
huh?), and do the same for the other, less
troublesome direction. While you are
adjusting the differential function, the
previous adverse roll direction you had
already eliminated is going to return with a
vengeance. It will go away once the
differential function is removed during the
final step.
Even upgraded with the YS, the Hawk
could hold only a 250- to 300-foot vertical
up-line. It had to be dived to excess airspeed
to hold the verticals needed for adverse yaw
trimming.
You might need to do the same for your
airplane. That’s fine, but remember to enter
every vertical with the wings level.
Great Planes’ P-6E Hawk is extremely
robust and built to handle flight stresses that
far exceed those encountered when sportflying.
However, two reinforcements were
made to handle excess stress.
First, the directions are to assemble the
two elevator pushrods with two wheel
collars before going to the single elevator
servo. Aerobatic routines, especially with
the larger engine, caused me some concern.
There was more than enough room to
install a second elevator servo. This separate
unit added control authority while also
providing extra trimming capabilities.
The second problem area was the cabane
attachment. Great Planes provided wood
screws into hardwood. That is good enough
for sport-flying, but for maneuver schedules
with many Snap Rolls and outside
Avalanches? I was not sure, even though the
wood screws held well during the airplane’s
sport days.
Instead of wood screws I installed 4-40
blind nuts and bolts. Then I bonded them
firmly in place using thread-locking
compound.
Examine your model, looking for weak
spots such as those I’ve mentioned. You
may choose to install extra firewall gussets,
to hold that larger engine (or motor) in
place. Or you might have to upgrade older
servo mounts.
If your airplane has a single aileron, go
to the newer dual-servo system. This is
shown in Part 22 of MA’s “From the Ground
Up” series. The Web address at the end of
this article, in the “Sources” list, will take
you to that feature.
It is a good idea to use more powerful,
nonsport, digital servos on all control
surfaces, especially if you stepped up the
power. I later upgraded all of the P-6E’s
servos to digital, at roughly 85 ounce-inch,
while the digital rudder servo produced 155
ounce-inch of output. While you’re
upgrading, ensure that the flight battery is up
to the task.
I’ll do the final trimming on both models
at once.
Now my task is to get the P-47D
Thunderbolt ready. It already had a more
powerful engine; the Saito 2.20 cu. in. was
originally installed in place of the 1.50-1.80
power plant that was specified. Flying 300-
to 400-foot up-lines was routine.
The smallish ailerons caused slow roll
rates, while their far-outboard positioning on
the 81-inch-span wing made adverse yaw
obvious in both directions. The complex
rudder control system limited rudder
movement to only 1 inch. Holding level
knife-edge flight was impossible.
Unlike the Hawk, the P-47D’s higher
engine torque and larger propeller diameter
caused the up-line to bend left during fullpower
verticals. The fuselage servo mount
was weak.
The first fix had to be the rudder. A
movement of only 1 inch was unacceptable.
The rudder linkage passed through the tail
wheel and then on to the rudder, limiting
movement.
I cut the control rod just beyond the tail
wheel connection. Then I installed a pullpull
system directly to the rudder. This
required some internal fuselage work.
The Thunderbolt’s internal fuselage is
constructed from lightened plywood
formers, with many crossbraces. All of the
braces crossed the fuselage center exactly
where the cables had to pass. I removed the
stock braces and installed twin substitutes
just above, below, or aside the original brace
positions.
Since exact rudder centering is
extraordinarily important, there is a
geometry that must be observed in pull-pull
systems. The total width of the rudder
attachment points must match the servo
arm’s length.
The cables must exit the fuselage at the
point where its width is the same distance.
This allows the cables to be straight from
servo to rudder horn. Any kinks will prevent
48 MODEL AVIATION
the rudder from centering perfectly, as it
must for best performance.
With this change, the Thunderbolt
now climbs in knife-edge. However, this
heavy airplane puts a strain on the
fuselage servo mounts during extreme
performances; a few did come loose.
I reinforced these mounts, as shown.
Remember to examine your airplane for
such weak points, as I have mentioned.
I braced the fuselage servo mounts. A
cap on one side prevents the mounting
rails from lifting away from the side
fuselage braces. Triangle stock did the
same on the other, less critical, side.
The rails flexed in the middle, so I
fitted a hardwood brace that tied the two
rails together, adding strength, and then
glued them to the former just forward of
the rails. This eliminated the servo
flexing, which causes pitch hunting.
My big four-stroke engine, mounted
upside-down, had issues. Raw fuel pooled
into the large head area, extinguishing the
glow plug at low rpm. The YS 2.20 ran
well and idled fine for a day of sportflying.
But air show dead-sticks are only
exciting once, and I am already excited
enough for three pilots at the usual
performance.
To extinguish this type of exhilaration,
I installed a Maxx Products International
Super Glow MX9900 onboard glow-plug
driver. It can be set to light the glow plug
at any throttle setting.
The MX9900 uses a single-cell, 1300
mAh Ni-Cd battery for power and works
directly from the receiver’s throttle port.
The throttle servo plugs into the Super
Glow. Since I installed the unit in 2008,
there has been no engine failure at idle.
I increased roll rate by sealing the
aileron gaps. That worked even better on
the P-47 than it did on the Hawk.
My experience has been that closing
gaps increases roll rates more on
symmetrical wings than on flat-bottom
airfoils. The P-47 has a semisymmetrical
wing.
Still, aileron movement had to be set
near 1 inch and then adjusted for adverse
yaw. On these aircraft, the best roll rate
for aerobatic performance has proven to
be the old standard of three rolls in five
seconds.
Sealing control-surface gaps had an
unexpected—but welcome—effect on the
Thunderbolt. It eliminated the airplane’s
left wing drop, and overall lift seems to
have increased. The airplane was a
“floater,” but now it repels the ground,
especially in ground effect.
To regain precision landing spots, I
had to increase flap deployment by 5°.
Without flaps, landing approaches that
start in New Jersey might end in
Pennsylvania.
I trimmed out the P-47D’s
considerable adverse yaw the way I did
the Hawk’s. It just took probably 15
flights longer.
After all that work, the Thunderbolt
still flies like a baby carriage. But now it
will slow roll as if Col. Bob Johnson were
at the controls. Loops track well, as do
Vertical Figure Eights. Inverted flight
requires only a touch of down-elevator
but tracks as if upright.
Because the P-47D flies so well and is
as honest as they come, I moved the CG
1/8 inch aft of the rearmost setting. That
improved Snap Rolls and Spins but kept
the airplane fully controllable. I don’t
recommend this practice until you have at
least a few hundred flights on the model
and know it well.
“A few hundred flights?” you might ask.
“You’re kidding, right?”
No. Trimming the airplane will require
roughly 40 flights. Practice time will
easily use the remaining airtime before
you know it.
If you follow the National Society of
Radio Controlled Aerobatics (NSRCA)
trim chart, you will need approximately
100 flights and adjustable wing/stabilizer
incidences that the P-6E and the P-47D
are missing.
Although the NSRCA guide is the
best, it may be overkill for those models. I
have found a few trim adjustments to be
the most important for a sport-type
aircraft’s optimum aerobatic performance.
Most important is knife-edge flight trim.
Sport airplanes are going to “walk” in
this flight zone. When rudder is applied in
Knife-Edge or Slow Rolls, the aircraft will
pull toward the belly or the canopy (usually
the belly). Moving the CG, usually rearward,
or adjusting wing incidence (awkward on
these models) will usually help trim out this
condition.
Try mixing the rudder to elevator. Use a
direct mix—no curves. The goal is straight
flight in knife-edge.
If less than 20 elevator points are needed,
okay. More than 20 points can mess up some
maneuvers, so slightly adjust the CG or wing
incidence (using shims) until the elevator mix
is less than 20 points.
Second in importance is eliminating roll
coupling. This occurs when rudder input also
rolls the wings. Point Rolls and Stall Turns
require that there be little or no coupling.
Mix opposite ailerons to rudder if the
coupling is proverse (in the rudder’s direction)
or vice versa for adverse, opposite-direction,
coupling. The ideal trim condition results in a
slightly descending flat turn on rudder input
alone.
The third important adjustment is downline
trim. Take the airplane high, go to idle,
and push the nose down to 90°. Watch the
track. Most sport models will begin to pull out
as airspeed increases.
Eliminate this by mixing down-elevator
with low throttle only. If your transmitter does
not have a curve mix that allows mixing only
at idle, skip this step unless the pullout is
extremely noticeable (roughly 10°). If so, you
might have to reduce the wing incidence,
assuming that your aircraft’s stabilizer is
glued in place.
Be careful here; a little goes a long way.
Start with a 1/8° change.
Most sport airplanes will not pull to the
canopy in vertical up-lines. Pattern models
need to trim this out, but a sport aircraft
won’t. However, I’ll bet you will need to trim
in right rudder on the up-lines.
Adjusting engine thrust to compensate for
this is a hassle, and it was impossible on the
Hawk because of the tiny crankshaft hole in
the cowling. Instead, try curve mixing (also
called “step”)—one to two points of right
rudder at half throttle, up to four to five points
at full power.
Although this condition is most apparent
in the vertical up-line, it also exists in level
flight. The leftward-nose-pointing tendency
can make it difficult to hold straight lines from
one maneuver to the next. This increases the
pilot’s workload.
The tendency we want to trim out occurs
when the model is moving near high speed
while in the up-line. All airplanes will go
“nose left” under full power once airspeed
drops. This is not a trim problem, but rather a
pilot who might need more rudder practice.
I have my air show aircraft to prove that any
sport model (except a basic trainer) can be
improved and trimmed to provide air showlike
performance with little work. If a giant
biplane with a flat-bottomed airfoil can be
improved to near-Extra 300 performance, so
can your sport airplane.
I’d bet that your model will be easier to
prepare for stunning airborne performance
than my Thunderbolt was. It might even fly
better. But that P-47D is amazing, so no bets
on that score.
Try it, just once, and you will never
want to fly a stock, out-of-trim sport
airplane again! MA
Frank Granelli
[email protected]
Sources:
Great Planes
(217) 398-3630
www.greatplanes.com
Hangar 9
(800) 338-4639
www.hangar-9.com
Central Hobbies
(406) 259-9004
www.centralhobbies.com
“From the Ground Up” Index
www.modelaircraft.org/mag/FTGU/titlespag
eftgu.htm
Maxx Products International
(800) 416-6299
www.maxxprod.com
National Society of Radio Controlled
Aerobatics
www.nsrca.us