128 MODEL AVIATION
IN THE FEBRUARY column I requested
comments from aerodynamicists regarding
the apparent sudden tail-heaviness and control
difficulties that occurred when a model’s
covering released from the undercambered
portion of the wing. Thanks to Edmund (Ned)
Smith, Alan Brown, Basil Cooper, and Dick
Fischer for their responses.
The four were in basic agreement, and
their answers differed only in the amount of
detail. I had a rough idea of what had
happened, but I wanted to keep it simple and
accurate while sounding like an
aerodynamicist myself, by tossing in a proper
aerodynamic term or two. Following is a
condensed version of their composite answer.
A line drawn from the LE to TE of an
airfoil and centered between the top and
bottom surfaces is referred to as a “mean camber line.” An
undercambered airfoil is one with a high degree of camber, or
downward curvature in that line.
Most airfoils have a “pitching moment,” or tendency to pitch up or
down, pivoting at roughly the one-quarter chord point, or 25%, back
from the LE. Large or highly cambered airfoils (big downward
curvature) have a stronger tendency to pitch down. Symmetrical
airfoils (flat camber) have a zero-pitch tendency.
An aircraft nose’s down-pitching tendency is normally kept in
balance by downward pressure from the stabilizer/elevator assembly
being set at a slight negative incidence relative to the wing. In this case,
when the airplane’s undercamber covering came loose, the airfoil
became essentially a flat-bottomed wing, having less camber and
therefore less downward pitching tendency.
That allowed the nose to pitch up and stall. Stalls and their recovery
cause oscillation, resulting in control difficulty.
Loss of undercamber
Old-Timers Bob Angel | [email protected]
Also included in this column:
• The hot rods of SAM
• About balsa
• Triangular building aids
• Hannan’s Runway
• Coil saver test
A Hayseed on final approach over landmark Pichaco Peak at the annual
Southwest Regionals. Carl Hermes designed this model in 1940. Steve Roselle
took this photo at last year’s contest.
Pete Samuelsen’s Anderson Pylon
gets a strong start at the January
2010 Southwest Regionals near Eloy
AZ. Those ear protectors are highly
recommended. Doug Klassen photo.
Tom Empey of Eddyville OR preps his Comet Sailplane at an
event at the Schmidt Ranch in Northern CA. Mike Young assists.
Ned Nevels photo.
06sig4.QXD_00MSTRPG.QXD 4/22/10 11:08 AM Page 128
Also in that case, the pitch up during each
stall probably pressed the loose covering back
into at least a partial undercamber. That
resulted in an even more pronounced dropping
of the nose during stall recovery.
An FF model would continue to pitch up
and down all the way to the ground. An RC
airplane could fare either better or worse,
depending on pilot skill and ability to time
elevator movement with the pitching. By trying
to correct, the RC pilot could easily get out of
phase with the oscillations, making the pitching
more severe.
The hot rods of SAM (Society of Antique
Modelers) are the RC designs—and in
particular, those competing in the limitedengine-
run events. They use either the
strongest-running old spark-ignition engines or
strong-running modern glow (nitro) engines.
Those are basically racing engines, which are
usually run unmuffled and without RC
carburetors at top speed.
The high power-to-weight ratio results in
takeoff rolls that vary from roughly 1 inch to 3
feet, followed by a nearly vertical climb. The
models can climb vertically, but a climb angle
of slightly less than 90° allows the wing to do a
little work. Otherwise, the wing would be
trying to “lift” the airplane off of its vertical
course.
This type of performance was
uncharacteristic of the original designs, which
greatly disturbs some of the purists in the Old-
Timer (OT) movement. But there’s a positive
side. Many of the younger people (or young at
heart) who come into OT flying are at first
drawn in by the hot rod aspect of the
spectacular performance.
Once they have built and flown a few OT
models, they may branch out to add some of
the more “civilized” classes such as Texaco,
which emphasizes fuel economy. Those events
more closely re-create the original sights,
sounds, and gentle climbout of the majestic old
airplanes.
We each enjoy reliving the past on our own
terms. I’ve heard many older FF fliers describe,
in nostalgic terms, an aircraft “that climbed
straight up.” Today that’s a repeatable
performance. And some of us can imagine
what heroes we might have been in the 1930s
with such a flight.
Fast or slow, I enjoy both types of flying.
But the best part comes after engine shutdown
when you seek out and quietly “dance with the
thermals.”
An accompanying photo is of Pete
Samuelsen’s Anderson Pylon design, which
fits into the hot rod category. It’s a class C
model powered by a Nelson .60 engine burning
60% nitro. It can usually perform nine-minute
max flights with an engine run limited to just
18 seconds.
Balsa wood is still the material of choice for
those of us who build our models. But we’re in
competition with heavy industry for the best
and lightest grades.
During World War II, large life rafts were
made from balsa. Today, oil tankers contain
balsa as insulation between double-walled
hulls. And huge wind-turbine blades are now
using balsa in their cores. Modelers might have
been the dominant users at one time, but not
today.
As a result, much of our sheet balsa is no
longer one piece; it is made from smoothly
joined smaller sections. Commercial suppliers
do an excellent job of connecting the pieces,
and you have to look closely to see the joints.
However, many 3-foot by 36-inch sheets
seem to have at least one joint, and it runs
parallel to the grain. This is easier to notice
when you try to cut across that grain with a
hobby knife. The glue leaves a hard spot in the
grain.
And when cutting diagonally to the grain,
the slight shift in grain direction can throw
your knife off course. So it pays to develop an
“eye” for those joints and plan your cuts
accordingly.
Bob Holman’s Plans has recently released a
simple, inexpensive little aid for those who
build with balsa. Bob is well known for
supplying scale and other plans to the
modeling community. Perhaps less known is
the fact that he and others in his family are
active OT model builders and fliers.
Bob laser-cuts precision 30°-60°-90°
plywood building triangles. They have several
uses, but the most obvious is for aligning
fuselage sides. These tools can be stood
vertically alongside a fuselage during
construction, to encourage making square
assemblies.
An interlocking crosspiece at the base
allows a triangle to either stand alone or be
pinned to a building board or held in place with
magnets. Bob sells these 51/2-inch-long
triangles in a packet of 10 for $5, plus $2
shipping.
In the December 2009 column I mentioned
the Frank Zaic Year Books as a great source of
OT information. Later I received a nice note
from Bill Hannan, who wrote the discontinued
Model Builder magazine’s “Hannan’s Hangar”
column for almost 25 years.
Bill and Joan Hannan run a small business
called “Hannan’s Runway,” which sells an
assortment of books aimed primarily at those
who still construct their own models. The
Hannans were friends with the Zaics and still
keep in touch with Frank’s wife, Carmen.
Hannan’s Runway stocks all of the Zaic
books (roughly 10) that are still in print. The
company also sells several other authors’
collections that cover a variety of specialized
subjects about building models. This includes
10 books that Bill has written throughout the
years, and they are modestly priced at
approximately $12 each.
Postage is reasonable, starting at $3.75 for
any size order. That’s media rate, if you’re not
in a hurry. If you’re a builder, you might find a
visit to the Hannans’ Web site interesting. It
includes a photo gallery of quirky little models
that Bill has enjoyed.
In the April column I mentioned Larry
Davidson’s new transistorized ignition circuit,
which he designated “SSIGNCO.”
When an engine stops with the ignition
points closed, a coil can be burned out or
batteries damaged if the current isn’t shut off
within a reasonable time. Larry’s unit
automatically cuts off current after two seconds
of inactivity with the points closed. It
reactivates immediately when the points are
reopened.
The engine-run timer in most FF models
shuts off the circuit, so burnout isn’t a problem
unless the timer fails. But for Texaco designs,
both FF and RC, the power plant normally stops
on its own after exhausting the fuel supply.
This can be a problem for the FF airplane,
since it is seldom recovered quickly after the
engine quits. But it can also be a problem for
an RC aircraft if the pilot forgets to switch off
ignition after the Texaco run, or in case of an
early flameout during a timed-engine-run
event. (It happens!)
I wrote the report on Larry’s unit from his
press release, and unfortunately I made an
untrue assumption. So an apology and a
correction are in order.
I mentioned that the unit would replace a
servo and microswitch, which it does not. Later
I received a test unit and found that an
auxiliary arming or on/off switch of some kind
is needed for either FF or RC. A radio-operated
switch would be unnecessary for the FF flier.
The unit I received for testing did
everything as specified. It provided a hot,
continuous spark; it shut down after close to
two seconds of point closure; and it fired back
up immediately upon reactivating point
movement.
The on/off switch for RC use requires
either a servo/microswitch combination or an
electronic (E) switch of some type.
Since E switches have sometimes caused
RF (radio frequency) interference when used
with spark ignition, I tested for that possibility
using an old 72 MHz AM radio. I wired one of
Marvin Stern’s ES-1 switching units between
the receiver’s throttle port and Larry’s unit.
The spark was steady, and the unit switched
off and on as it should have with point position.
Two servos also plugged into the receiver
didn’t twitch, despite the batch of loose hookup
wires.
One should always do an engine-running
range check before flying, to make sure there
isn’t some gremlin in a particular aircraft
installation. MA
Sources:
Bob Holman Plans
(909) 885-3959
www.bhplans.com
Hannan’s Runway
(530) 873-6421
www.hrunway.com
Larry Davidson
(540) 721-4563
www.modelflight.com/larrydavidson.html
Marvin Stern
(732) 928-0884
[email protected]
SAM
www.antiquemodeler.org
Edition: Model Aviation - 2010/06
Page Numbers: 128,129
Edition: Model Aviation - 2010/06
Page Numbers: 128,129
128 MODEL AVIATION
IN THE FEBRUARY column I requested
comments from aerodynamicists regarding
the apparent sudden tail-heaviness and control
difficulties that occurred when a model’s
covering released from the undercambered
portion of the wing. Thanks to Edmund (Ned)
Smith, Alan Brown, Basil Cooper, and Dick
Fischer for their responses.
The four were in basic agreement, and
their answers differed only in the amount of
detail. I had a rough idea of what had
happened, but I wanted to keep it simple and
accurate while sounding like an
aerodynamicist myself, by tossing in a proper
aerodynamic term or two. Following is a
condensed version of their composite answer.
A line drawn from the LE to TE of an
airfoil and centered between the top and
bottom surfaces is referred to as a “mean camber line.” An
undercambered airfoil is one with a high degree of camber, or
downward curvature in that line.
Most airfoils have a “pitching moment,” or tendency to pitch up or
down, pivoting at roughly the one-quarter chord point, or 25%, back
from the LE. Large or highly cambered airfoils (big downward
curvature) have a stronger tendency to pitch down. Symmetrical
airfoils (flat camber) have a zero-pitch tendency.
An aircraft nose’s down-pitching tendency is normally kept in
balance by downward pressure from the stabilizer/elevator assembly
being set at a slight negative incidence relative to the wing. In this case,
when the airplane’s undercamber covering came loose, the airfoil
became essentially a flat-bottomed wing, having less camber and
therefore less downward pitching tendency.
That allowed the nose to pitch up and stall. Stalls and their recovery
cause oscillation, resulting in control difficulty.
Loss of undercamber
Old-Timers Bob Angel | [email protected]
Also included in this column:
• The hot rods of SAM
• About balsa
• Triangular building aids
• Hannan’s Runway
• Coil saver test
A Hayseed on final approach over landmark Pichaco Peak at the annual
Southwest Regionals. Carl Hermes designed this model in 1940. Steve Roselle
took this photo at last year’s contest.
Pete Samuelsen’s Anderson Pylon
gets a strong start at the January
2010 Southwest Regionals near Eloy
AZ. Those ear protectors are highly
recommended. Doug Klassen photo.
Tom Empey of Eddyville OR preps his Comet Sailplane at an
event at the Schmidt Ranch in Northern CA. Mike Young assists.
Ned Nevels photo.
06sig4.QXD_00MSTRPG.QXD 4/22/10 11:08 AM Page 128
Also in that case, the pitch up during each
stall probably pressed the loose covering back
into at least a partial undercamber. That
resulted in an even more pronounced dropping
of the nose during stall recovery.
An FF model would continue to pitch up
and down all the way to the ground. An RC
airplane could fare either better or worse,
depending on pilot skill and ability to time
elevator movement with the pitching. By trying
to correct, the RC pilot could easily get out of
phase with the oscillations, making the pitching
more severe.
The hot rods of SAM (Society of Antique
Modelers) are the RC designs—and in
particular, those competing in the limitedengine-
run events. They use either the
strongest-running old spark-ignition engines or
strong-running modern glow (nitro) engines.
Those are basically racing engines, which are
usually run unmuffled and without RC
carburetors at top speed.
The high power-to-weight ratio results in
takeoff rolls that vary from roughly 1 inch to 3
feet, followed by a nearly vertical climb. The
models can climb vertically, but a climb angle
of slightly less than 90° allows the wing to do a
little work. Otherwise, the wing would be
trying to “lift” the airplane off of its vertical
course.
This type of performance was
uncharacteristic of the original designs, which
greatly disturbs some of the purists in the Old-
Timer (OT) movement. But there’s a positive
side. Many of the younger people (or young at
heart) who come into OT flying are at first
drawn in by the hot rod aspect of the
spectacular performance.
Once they have built and flown a few OT
models, they may branch out to add some of
the more “civilized” classes such as Texaco,
which emphasizes fuel economy. Those events
more closely re-create the original sights,
sounds, and gentle climbout of the majestic old
airplanes.
We each enjoy reliving the past on our own
terms. I’ve heard many older FF fliers describe,
in nostalgic terms, an aircraft “that climbed
straight up.” Today that’s a repeatable
performance. And some of us can imagine
what heroes we might have been in the 1930s
with such a flight.
Fast or slow, I enjoy both types of flying.
But the best part comes after engine shutdown
when you seek out and quietly “dance with the
thermals.”
An accompanying photo is of Pete
Samuelsen’s Anderson Pylon design, which
fits into the hot rod category. It’s a class C
model powered by a Nelson .60 engine burning
60% nitro. It can usually perform nine-minute
max flights with an engine run limited to just
18 seconds.
Balsa wood is still the material of choice for
those of us who build our models. But we’re in
competition with heavy industry for the best
and lightest grades.
During World War II, large life rafts were
made from balsa. Today, oil tankers contain
balsa as insulation between double-walled
hulls. And huge wind-turbine blades are now
using balsa in their cores. Modelers might have
been the dominant users at one time, but not
today.
As a result, much of our sheet balsa is no
longer one piece; it is made from smoothly
joined smaller sections. Commercial suppliers
do an excellent job of connecting the pieces,
and you have to look closely to see the joints.
However, many 3-foot by 36-inch sheets
seem to have at least one joint, and it runs
parallel to the grain. This is easier to notice
when you try to cut across that grain with a
hobby knife. The glue leaves a hard spot in the
grain.
And when cutting diagonally to the grain,
the slight shift in grain direction can throw
your knife off course. So it pays to develop an
“eye” for those joints and plan your cuts
accordingly.
Bob Holman’s Plans has recently released a
simple, inexpensive little aid for those who
build with balsa. Bob is well known for
supplying scale and other plans to the
modeling community. Perhaps less known is
the fact that he and others in his family are
active OT model builders and fliers.
Bob laser-cuts precision 30°-60°-90°
plywood building triangles. They have several
uses, but the most obvious is for aligning
fuselage sides. These tools can be stood
vertically alongside a fuselage during
construction, to encourage making square
assemblies.
An interlocking crosspiece at the base
allows a triangle to either stand alone or be
pinned to a building board or held in place with
magnets. Bob sells these 51/2-inch-long
triangles in a packet of 10 for $5, plus $2
shipping.
In the December 2009 column I mentioned
the Frank Zaic Year Books as a great source of
OT information. Later I received a nice note
from Bill Hannan, who wrote the discontinued
Model Builder magazine’s “Hannan’s Hangar”
column for almost 25 years.
Bill and Joan Hannan run a small business
called “Hannan’s Runway,” which sells an
assortment of books aimed primarily at those
who still construct their own models. The
Hannans were friends with the Zaics and still
keep in touch with Frank’s wife, Carmen.
Hannan’s Runway stocks all of the Zaic
books (roughly 10) that are still in print. The
company also sells several other authors’
collections that cover a variety of specialized
subjects about building models. This includes
10 books that Bill has written throughout the
years, and they are modestly priced at
approximately $12 each.
Postage is reasonable, starting at $3.75 for
any size order. That’s media rate, if you’re not
in a hurry. If you’re a builder, you might find a
visit to the Hannans’ Web site interesting. It
includes a photo gallery of quirky little models
that Bill has enjoyed.
In the April column I mentioned Larry
Davidson’s new transistorized ignition circuit,
which he designated “SSIGNCO.”
When an engine stops with the ignition
points closed, a coil can be burned out or
batteries damaged if the current isn’t shut off
within a reasonable time. Larry’s unit
automatically cuts off current after two seconds
of inactivity with the points closed. It
reactivates immediately when the points are
reopened.
The engine-run timer in most FF models
shuts off the circuit, so burnout isn’t a problem
unless the timer fails. But for Texaco designs,
both FF and RC, the power plant normally stops
on its own after exhausting the fuel supply.
This can be a problem for the FF airplane,
since it is seldom recovered quickly after the
engine quits. But it can also be a problem for
an RC aircraft if the pilot forgets to switch off
ignition after the Texaco run, or in case of an
early flameout during a timed-engine-run
event. (It happens!)
I wrote the report on Larry’s unit from his
press release, and unfortunately I made an
untrue assumption. So an apology and a
correction are in order.
I mentioned that the unit would replace a
servo and microswitch, which it does not. Later
I received a test unit and found that an
auxiliary arming or on/off switch of some kind
is needed for either FF or RC. A radio-operated
switch would be unnecessary for the FF flier.
The unit I received for testing did
everything as specified. It provided a hot,
continuous spark; it shut down after close to
two seconds of point closure; and it fired back
up immediately upon reactivating point
movement.
The on/off switch for RC use requires
either a servo/microswitch combination or an
electronic (E) switch of some type.
Since E switches have sometimes caused
RF (radio frequency) interference when used
with spark ignition, I tested for that possibility
using an old 72 MHz AM radio. I wired one of
Marvin Stern’s ES-1 switching units between
the receiver’s throttle port and Larry’s unit.
The spark was steady, and the unit switched
off and on as it should have with point position.
Two servos also plugged into the receiver
didn’t twitch, despite the batch of loose hookup
wires.
One should always do an engine-running
range check before flying, to make sure there
isn’t some gremlin in a particular aircraft
installation. MA
Sources:
Bob Holman Plans
(909) 885-3959
www.bhplans.com
Hannan’s Runway
(530) 873-6421
www.hrunway.com
Larry Davidson
(540) 721-4563
www.modelflight.com/larrydavidson.html
Marvin Stern
(732) 928-0884
[email protected]
SAM
www.antiquemodeler.org
