Bob Kopski, 25 West End Dr., Lansdale PA 19446
RADIO CONTROL ELECTRICS
Plywood landing-gear mounting plate before cloth wrap. Long
battery floor of 1⁄16 cross-grain balsa sheet is also visible.
Shown is the revised, robust landing-gear installation. Some
stick-on weights are visible under the motor.
The attractive Goldberg Tiger ARF before its first flight. The
original landing gear proved to be very weak.
THIS COLUMN INCLUDES a review of ESC motor (voltage)
cutoff and some discussion of the Goldberg Tiger ARF.
Based on reader mail, it’s clear that there is some misunderstanding
about the motor (voltage)-cutoff function found in most contemporary
ESCs. Following is some background and present-day info on the
subject.
In the earliest days of electric power, little was thought of—and
much less done about—controlling motor power in flight. Roughly 30
years ago, many E-fliers just turned on a manual motor switch,
launched, and flew until the Ni-Cd pack ran dry.
This simplistic approach eventually gave way to the servo-driven
motor switch, where a standard servo was mechanically linked to an
ordinary toggle (or similar) switch. At last—the motor could be radio
controlled! An enhanced variation used a multiple switch configuration
to control a motor circuit resistor in one or more steps, and rudimentary
“speed control” was born.
Everyone agrees that early electric power was a comparatively
weak performer with short flights. Therefore, early focus was on
stretching flight time as much as possible. This natural desire for one
more go-round automatically led to well-drained packs at the end of
each flight. It was not always realized that deep discharge was actually
detrimental to Ni-Cd battery health.
The explanation for that is that when a battery of several cells is
drained all the way, some cells may deplete ahead of others, and those
are then subject to “reverse charge.” This means that the longer-lasting
cells of a pack continue to supply current to the motor through those
already dead ones, and the latter may thereby be driven “backward,” or
become “reverse charged.” This puts them at risk, and repeated uses
gradually cause cell damage.
Somewhere near the mid-1980s, MA presented an ESC
construction article by Joe Utasi. To the best of my knowledge, this
was a first. One could smoothly vary a motor’s speed in flight from full
off to full on. Joe later went into the business of manufacturing speed
controls under the brand name JOMAR.
These earliest controllers, by virtue of a quirk in the circuit
behavior, caused the motor to sputter and rapidly lose power as a pack
depleted to roughly 6.0 volts, as I recall. This encouraged pilots to
land, and this, in many cases, saved some packs from cell reversal—
especially the most popular battery configuration of that era: the sevencell
pack. However, larger-cell-count packs still had cells that were
subject to reverse-charge damage.
Since it usually required numerous deep-discharge flights before
degrading battery performance was noticed, the specific cause and
effect were not always obvious. It took years before this was widely
realized and accepted, and the delay resulted in many damaged Ni-Cd
packs in those earlier days. But seven- (or eight)-cell packs under
JOMAR control lasted longer—a clue for the future.
Fast-forward roughly a decade. Many more aeromodelers and
suppliers had emerged on the electric scene, and a new kind of ESC
appeared—ones with “BEC.” This Battery Eliminator Circuit, which
was a feature added to the basic ESC, permitted the radio stuff to be
powered from the motor battery. This was a big deal; the receiver
battery could be eliminated, and the weight savings were quite
welcome by the electric community.
But BEC quickly brought to the forefront another importance of not
running a motor pack all the way down; to do so also meant loss of
radio. “Rekitting” was rampant! Now there were two good reasons not
to drain a motor pack all the way!
Aeromodelers—historically an inventive lot—were quick to
address the latter problem, and BEC-equipped ESCs with “motor
cutoff” began to appear. These ESCs had circuitry that would
automatically shut down motor power as the pack depleted, to
conserve some power for radio operation and safe landing. Clearly, this
was a step in the right direction.
Somewhere along the way, this step led to a larger leap. Soon an
even better motor-cutoff configuration began to appear: an adjustablevoltage
cutoff. I do not recall the “who” and the “when,” but this new
feature permitted users to choose where during motor-battery rundown
the actual motor cutoff would occur.
A guideline began to win favor. It was to set the cutoff to occur at
the Ni-Cd pack equivalent voltage of 0.9 volt per cell. So if you had a
Greatly improved steering is possible with simple rudder and
tail-wheel modifications. Note the added hinge.
Simple plastic parts permit rapid canopy access. Cowl covering
is removed for finger access to battery/connectors.
10-cell pack, the ESC might be adjusted for motor shutdown to occur
at a pack voltage of 9.0.
The nominal 0.9-volt-per-cell value would preclude cell reversal
and early battery failure for the typical motor pack. This would be the
case for any pack in good condition; i.e., where all cells were
reasonably matched in the first place. Furthermore, this guideline
number would typically assure continuing radio operation for safe
landing where BEC was used. It was a twofer!
Although much of the preceding evolution happened during the
dominant reign of Ni-Cd power in E-aeromodeling, the same
guidelines were extended to NiMH as that battery technology emerged.
Its emergence took place during the past decade or so.
This brings me to now, and enter the latest and greatest battery for
electric power: Li-Poly. Just a few years past introduction and rapidly
growing in popularity, Li-Poly has mandated an even newer voltagecutoff
standard.
Mandated? Yes! Historic Ni-Cd and NiMH batteries could
eventually be damaged by repeated deep discharge, but Li-Poly
batteries may be destroyed by just one incident.
The magic number is 2.5 volts per cell for Li-Poly, although a more
conservative 3.0 volts per cell is gaining popularity. So rundown must
cease at at least 7.5 volts for a three-cell Li-Poly pack, but I use the 9.0-
volt number.
Remember that with just one overdischarge, the Li-Poly is dead!
So, as in the preceding, voltage cutoff of motor operation is mandated!
This brings me to the latest ESCs: those that automatically set their
cutoff values by first sensing how many cells are connected.
This feature is primarily aimed at the Li-Poly-powered systems to
help assure a safe operating environment for these packs. However, it
also sets up a generally suitable cutoff value for Ni-Cd and NiMH
systems. Hence the user need not manually set (program) a proper
voltage-cutoff value, and this mostly eliminates the chance of operator
error in this process.
To summarize, it is advisable to have ESCs with a suitable voltage
(motor)-cutoff function for two reasons. The first is to prevent
overdischarging the motor battery. This is imperative with Li-Polypowered
systems and extremely beneficial with Ni-Cd and NiMH.
For BEC installations, the second reason is to assure adequate
remaining power for continuing radio operation and safe landing after
the motor is shut down. For most commonly used cell counts, the first
reason generally assures the second one. Motor control has certainly
come a long way in 35 years!
A local E-aeromodeler asked for some assistance with his new
Goldberg Tiger. It is an attractive Speed 400-class ARF, and he wanted
a checkout and a test flight before he took the controls. Several things
came out of this experience that are worth passing on.
The Tiger is a low-wing design, which, for the most part, dictates a
rise-off-ground (ROG) takeoff. It is challenging to hold a transmitter in
one hand and safely hand launch a low-winger with the other! The
ROG requirement dictates other needs, such as a suitable surface,
adequate power, and reasonable ground handling and control.
As with all new airplanes, it’s prudent to check out balance first.
This Tiger was tail-heavy, and a reasonable solution was to move the
seven-cell 500 AR pack far forward. This necessitated modifying the
inner fuselage structure to include a battery floor extending through the
LE former—which had to be cut out to some extent. This modification
would prove to be valuable later.
Accessing the replaceable pack was made easier by changing the
screwed-on canopy to a rapid-access type. The front edge of the
canopy was “toed in” to an added plastic holder piece in front and held
in place with two foreshortened nylon landing-gear straps in the rear.
These could be swung out of the way to slip the canopy off and access
the pack.
During first flight attempts, it quickly became clear that the singlestrut
landing gear could stand some improvement. The landing-gear
wire in the Tiger was extremely soft and much too easy to bend. In
addition, landing-gear attachment to the fuselage quickly deteriorated,
and the entire landing gear became “wiggly.” Photos show how the
landing gear was revised.
Somewhat longer (for better propeller clearance) double struts
using K&S 0.078-inch-diameter wire were formed and securely
attached to a revised fuselage bottom. A 1⁄16 plywood bottom plate was
glued to the fuselage side edges with gusseting added at the internal
interfaces of these surfaces. A cloth wrap of iron-on patch material
(from a sewing store) was added to this area. The result was a suitably
robust landing-gear installation.
The tail skid was replaced with a steerable tail wheel. Some balsa
was added to build up the bottom of the rudder, and an additional
hinge was installed. A newly formed tail-wheel strut of 0.032-inchdiameter
wire was then attached to the rudder with 2-56 hardware.
Several flights were made with the seven-cell 500 AR pack, and it
became clear that the Tiger was marginally powered. Flight was
feeble.
One option was to use a three-cell 1320 Li-Poly pack and
appropriate propeller. Here’s where that battery floor really paid off
because it was even harder to achieve balance with this lighter pack.
Fortunately, the extended floor allowed this pack to be moved forward,
and balance was achieved by adding some stick-on nose weight.
This revised power system made a huge difference in flyability; the
Tiger was much more spirited. But there was one side effect: a
scorching-hot motor upon landing. It’s clear that the power needed to
make the Tiger really perform was too much for the economy geared
motor. The next step is a different motor! Stay tuned.
Thus ends one more column—as my favorite time of year is
approaching; the end of winter is in sight! Come ooooon, springtime!
Please enclose an SASE with any correspondence for which you’d
like a reply. Everyone so doing does get one. Meanwhile, happy Elandings
everyone! MA
Edition: Model Aviation - 2005/03
Page Numbers: 89,90
Edition: Model Aviation - 2005/03
Page Numbers: 89,90
Bob Kopski, 25 West End Dr., Lansdale PA 19446
RADIO CONTROL ELECTRICS
Plywood landing-gear mounting plate before cloth wrap. Long
battery floor of 1⁄16 cross-grain balsa sheet is also visible.
Shown is the revised, robust landing-gear installation. Some
stick-on weights are visible under the motor.
The attractive Goldberg Tiger ARF before its first flight. The
original landing gear proved to be very weak.
THIS COLUMN INCLUDES a review of ESC motor (voltage)
cutoff and some discussion of the Goldberg Tiger ARF.
Based on reader mail, it’s clear that there is some misunderstanding
about the motor (voltage)-cutoff function found in most contemporary
ESCs. Following is some background and present-day info on the
subject.
In the earliest days of electric power, little was thought of—and
much less done about—controlling motor power in flight. Roughly 30
years ago, many E-fliers just turned on a manual motor switch,
launched, and flew until the Ni-Cd pack ran dry.
This simplistic approach eventually gave way to the servo-driven
motor switch, where a standard servo was mechanically linked to an
ordinary toggle (or similar) switch. At last—the motor could be radio
controlled! An enhanced variation used a multiple switch configuration
to control a motor circuit resistor in one or more steps, and rudimentary
“speed control” was born.
Everyone agrees that early electric power was a comparatively
weak performer with short flights. Therefore, early focus was on
stretching flight time as much as possible. This natural desire for one
more go-round automatically led to well-drained packs at the end of
each flight. It was not always realized that deep discharge was actually
detrimental to Ni-Cd battery health.
The explanation for that is that when a battery of several cells is
drained all the way, some cells may deplete ahead of others, and those
are then subject to “reverse charge.” This means that the longer-lasting
cells of a pack continue to supply current to the motor through those
already dead ones, and the latter may thereby be driven “backward,” or
become “reverse charged.” This puts them at risk, and repeated uses
gradually cause cell damage.
Somewhere near the mid-1980s, MA presented an ESC
construction article by Joe Utasi. To the best of my knowledge, this
was a first. One could smoothly vary a motor’s speed in flight from full
off to full on. Joe later went into the business of manufacturing speed
controls under the brand name JOMAR.
These earliest controllers, by virtue of a quirk in the circuit
behavior, caused the motor to sputter and rapidly lose power as a pack
depleted to roughly 6.0 volts, as I recall. This encouraged pilots to
land, and this, in many cases, saved some packs from cell reversal—
especially the most popular battery configuration of that era: the sevencell
pack. However, larger-cell-count packs still had cells that were
subject to reverse-charge damage.
Since it usually required numerous deep-discharge flights before
degrading battery performance was noticed, the specific cause and
effect were not always obvious. It took years before this was widely
realized and accepted, and the delay resulted in many damaged Ni-Cd
packs in those earlier days. But seven- (or eight)-cell packs under
JOMAR control lasted longer—a clue for the future.
Fast-forward roughly a decade. Many more aeromodelers and
suppliers had emerged on the electric scene, and a new kind of ESC
appeared—ones with “BEC.” This Battery Eliminator Circuit, which
was a feature added to the basic ESC, permitted the radio stuff to be
powered from the motor battery. This was a big deal; the receiver
battery could be eliminated, and the weight savings were quite
welcome by the electric community.
But BEC quickly brought to the forefront another importance of not
running a motor pack all the way down; to do so also meant loss of
radio. “Rekitting” was rampant! Now there were two good reasons not
to drain a motor pack all the way!
Aeromodelers—historically an inventive lot—were quick to
address the latter problem, and BEC-equipped ESCs with “motor
cutoff” began to appear. These ESCs had circuitry that would
automatically shut down motor power as the pack depleted, to
conserve some power for radio operation and safe landing. Clearly, this
was a step in the right direction.
Somewhere along the way, this step led to a larger leap. Soon an
even better motor-cutoff configuration began to appear: an adjustablevoltage
cutoff. I do not recall the “who” and the “when,” but this new
feature permitted users to choose where during motor-battery rundown
the actual motor cutoff would occur.
A guideline began to win favor. It was to set the cutoff to occur at
the Ni-Cd pack equivalent voltage of 0.9 volt per cell. So if you had a
Greatly improved steering is possible with simple rudder and
tail-wheel modifications. Note the added hinge.
Simple plastic parts permit rapid canopy access. Cowl covering
is removed for finger access to battery/connectors.
10-cell pack, the ESC might be adjusted for motor shutdown to occur
at a pack voltage of 9.0.
The nominal 0.9-volt-per-cell value would preclude cell reversal
and early battery failure for the typical motor pack. This would be the
case for any pack in good condition; i.e., where all cells were
reasonably matched in the first place. Furthermore, this guideline
number would typically assure continuing radio operation for safe
landing where BEC was used. It was a twofer!
Although much of the preceding evolution happened during the
dominant reign of Ni-Cd power in E-aeromodeling, the same
guidelines were extended to NiMH as that battery technology emerged.
Its emergence took place during the past decade or so.
This brings me to now, and enter the latest and greatest battery for
electric power: Li-Poly. Just a few years past introduction and rapidly
growing in popularity, Li-Poly has mandated an even newer voltagecutoff
standard.
Mandated? Yes! Historic Ni-Cd and NiMH batteries could
eventually be damaged by repeated deep discharge, but Li-Poly
batteries may be destroyed by just one incident.
The magic number is 2.5 volts per cell for Li-Poly, although a more
conservative 3.0 volts per cell is gaining popularity. So rundown must
cease at at least 7.5 volts for a three-cell Li-Poly pack, but I use the 9.0-
volt number.
Remember that with just one overdischarge, the Li-Poly is dead!
So, as in the preceding, voltage cutoff of motor operation is mandated!
This brings me to the latest ESCs: those that automatically set their
cutoff values by first sensing how many cells are connected.
This feature is primarily aimed at the Li-Poly-powered systems to
help assure a safe operating environment for these packs. However, it
also sets up a generally suitable cutoff value for Ni-Cd and NiMH
systems. Hence the user need not manually set (program) a proper
voltage-cutoff value, and this mostly eliminates the chance of operator
error in this process.
To summarize, it is advisable to have ESCs with a suitable voltage
(motor)-cutoff function for two reasons. The first is to prevent
overdischarging the motor battery. This is imperative with Li-Polypowered
systems and extremely beneficial with Ni-Cd and NiMH.
For BEC installations, the second reason is to assure adequate
remaining power for continuing radio operation and safe landing after
the motor is shut down. For most commonly used cell counts, the first
reason generally assures the second one. Motor control has certainly
come a long way in 35 years!
A local E-aeromodeler asked for some assistance with his new
Goldberg Tiger. It is an attractive Speed 400-class ARF, and he wanted
a checkout and a test flight before he took the controls. Several things
came out of this experience that are worth passing on.
The Tiger is a low-wing design, which, for the most part, dictates a
rise-off-ground (ROG) takeoff. It is challenging to hold a transmitter in
one hand and safely hand launch a low-winger with the other! The
ROG requirement dictates other needs, such as a suitable surface,
adequate power, and reasonable ground handling and control.
As with all new airplanes, it’s prudent to check out balance first.
This Tiger was tail-heavy, and a reasonable solution was to move the
seven-cell 500 AR pack far forward. This necessitated modifying the
inner fuselage structure to include a battery floor extending through the
LE former—which had to be cut out to some extent. This modification
would prove to be valuable later.
Accessing the replaceable pack was made easier by changing the
screwed-on canopy to a rapid-access type. The front edge of the
canopy was “toed in” to an added plastic holder piece in front and held
in place with two foreshortened nylon landing-gear straps in the rear.
These could be swung out of the way to slip the canopy off and access
the pack.
During first flight attempts, it quickly became clear that the singlestrut
landing gear could stand some improvement. The landing-gear
wire in the Tiger was extremely soft and much too easy to bend. In
addition, landing-gear attachment to the fuselage quickly deteriorated,
and the entire landing gear became “wiggly.” Photos show how the
landing gear was revised.
Somewhat longer (for better propeller clearance) double struts
using K&S 0.078-inch-diameter wire were formed and securely
attached to a revised fuselage bottom. A 1⁄16 plywood bottom plate was
glued to the fuselage side edges with gusseting added at the internal
interfaces of these surfaces. A cloth wrap of iron-on patch material
(from a sewing store) was added to this area. The result was a suitably
robust landing-gear installation.
The tail skid was replaced with a steerable tail wheel. Some balsa
was added to build up the bottom of the rudder, and an additional
hinge was installed. A newly formed tail-wheel strut of 0.032-inchdiameter
wire was then attached to the rudder with 2-56 hardware.
Several flights were made with the seven-cell 500 AR pack, and it
became clear that the Tiger was marginally powered. Flight was
feeble.
One option was to use a three-cell 1320 Li-Poly pack and
appropriate propeller. Here’s where that battery floor really paid off
because it was even harder to achieve balance with this lighter pack.
Fortunately, the extended floor allowed this pack to be moved forward,
and balance was achieved by adding some stick-on nose weight.
This revised power system made a huge difference in flyability; the
Tiger was much more spirited. But there was one side effect: a
scorching-hot motor upon landing. It’s clear that the power needed to
make the Tiger really perform was too much for the economy geared
motor. The next step is a different motor! Stay tuned.
Thus ends one more column—as my favorite time of year is
approaching; the end of winter is in sight! Come ooooon, springtime!
Please enclose an SASE with any correspondence for which you’d
like a reply. Everyone so doing does get one. Meanwhile, happy Elandings
everyone! MA