88 MODEL AVIATION
THE RUDDER LINKAGE on a discus glider is one of the most
critical linkages in Radio Control Soaring. In this column I will
explain why the rudder system is so important on a discus-launch
glider (DLG) and review the pros and cons of several pushrod
systems.
When a discus glider does break, it will most likely happen on
launch. Often unnoticed minor damage from a rough landing will
cause a catastrophic failure on the next throw. Unlike other types of
gliders, a crash can be precipitated by almost any component of a
discus glider. The difference is that the DLG’s rudder system and
fuselage must work perfectly during launch or it will typically roll
inverted.
Imagine what happens when a rudder linkage breaks on launch;
you have high-speed flight a few feet off the ground, a pilot trying to
transition from spinning to standing, usually an up-elevator preset,
and an unexpected upside-down attitude. These factors add up to
make a high-energy crash likely. Launch velocity is often higher
than these same gliders can obtain in a vertical dive. You can guess
what happens when an 11-ounce, 11⁄2-meter glider structure hits the
ground at that speed.
When I refer to the rudder system I mean everything on the
airplane that must be in working order for the rudder to function
effectively. This includes the rudder, its hinges, the fin, the fintailboom
joint, the tailboom, the tailboom pod joint, the pod, the
control horn, the linkage connections, the pushrod, the servo gears,
and the servo mount.
If any one of these fails on launch, odds are that the discus glider
will go home in a trash bag. A simple linkage problem can cause a
high-speed crash, leaving the glider in a state not worth repairing.
I won’t go into the math, but it can be shown that the side force
on a typical DLG fin is 2-3 pounds. If there was doubt whether or
not a tail was strong enough for discus, a bench test would be wise.
Put the model together and have a friend hold it in a knife-edge
position with the left wingtip on the ground (for right-handed
throwers). Put a chair or something a couple
of inches under the tailboom for emergency
support. Try to stack bags of weight on the
side of the vertical fin and rudder. Tape may
be required to keep the weight from slipping
as the tailboom deflects. If the fin holds 21⁄2
pounds without making crunchy noises, it
will likely stand up to full-power discus
launching.
The bench test has a couple of benefits.
First, it is a known, repeatable load. You can
use it to make relative strength comparisons
between two designs. Second, if a
component does break, the chair catches the
weight, unloads the airplane, and no more
damage is done.
A part breaking during a real launch
usually leads to much more damage. If it
breaks during the static test, it was not strong
enough for competitive discus launching.
Finding this out in the static test will save
your wing, and possibly more, from breaking
on the first hard launch. Try this at your own
risk; a manufacturer is not at fault if you
break your new model.
My XP-3 deflected 4 inches at the tip of
Mike Garton, 2733 NE 95th Ave., Ankeny IA 50021; E-mail: [email protected]
RADIO CONTROL SOARING
Positive, slop-free rudder linkage is essential on discus glider.
XP-3 tail shows well-supported rudder pushrod.
Close-up of the balsa rudder support with fully deflected rudder. A piece of hook-up
wire insulation is glued on roughened wire to trap the pushrod.
90 MODEL AVIATION
the rudder with a 21⁄2-pound load. It was completely silent during the
test. I did not have any doubt about this stout airplane, but I was
curious how far it would deflect. Weaker airplanes will deflect
ridiculously or break with the same load. You would be surprised
how much the fuselage pods flex under the wing (and eventually
break) on non-carbon-reinforced pod designs.
The calculated 2- to 3-pound loads on the fin also explain why
most discus models use fiberglassed balsa surfaces. Non-discus
Hand-Launched Gliders (HLGs) can get by with just 1⁄8 contest balsa
surfaces glued to a boom. Discus airplanes need a decent-sized piece
of fiberglass to go over the fin-to-tailboom joint.
Last summer I forgot to reinstall a retainer on my XP-3’s rudder
pushrod. My preflight inspection should have caught it. Of course the
pushrod pulled loose on a hard launch. I let go of the up-elevator
preset as it rolled inverted and completed the 90 mph roll 4 feet off
the deck. The problem and the correction occurred in less than a
second. I wish I could say that I consciously thought that fast; it was
a combination of luck and reflexes that saved the airplane. I did count
my blessings and quit for the day.
Bruce Davidson demonstrated an even better save at the 2002
Mid South contest. The rudder pull-pull
string on his polyhedral XP-3 broke on
launch. Using elevator control alone, Bruce
completed the four-minute max and landed
in bounds on the large sod farm. He said that
was the last time he ever used pull-pulls on a
discus glider.
Pull-pull strings are potentially the
lightest pushrod system. They have several
disadvantages. The lines need to be
periodically retensioned. There is a constant
compression force on the hinges. Even
exotic strings of Kevlar or Spectra cannot be
made as solid as a pushrod.
That can be demonstrated by trying to
XP-3 carbon stabilizer mount side view; SuperGee lineage shows.
Housing on this pushrod would ideally be slightly longer.
Notice the large amount of carbon in the XP-3’s fuselage pod. Flex in this area is
common on less expensive fiberglass discus-launch gliders.
move a servo (with power off) by moving
the control surface. If you don’t have to
move the surface very far before the servo
gears turn, it is a positive linkage. I have
never been successful at making pull-pulls
tight enough.
What pushrod systems work well for tail
surfaces on a discus airplane? That depends
on how meticulous you are at building and
how much you “baby” the model off the
field. Round .040-inch carbon in Teflon
spaghetti tubing housings are light. They
work great until you crack the carbon. Often
the damage is done in the back of a car,
carrying the airplane through a doorway, or
on a ground-loop landing. The crack will
break on the next full-power launch.
The carbon pushrods work fine as long as
you have the discipline to inspect them
carefully after transport and each rough
landing. Even with careful inspection there is
a risk that the damage will not be visible
until failure.
The SuperGee as designed and my
number-one XP-3 use .014-inch stainlesssteel
rods in Teflon tubing. The smalldiameter
stainless is more prone to
compression buckling than the .040 carbon.
My XP-3 pushrods are well supported and
still work great after almost a year of use.
The stainless does not hold a Z-bend
quite as well as music wire. Some pilots
have straightened out Z-bends on launch.
Wrapping the free end of the pushrod back
and twisting it around like a safety pin can
solve the problem. Besides being light, the
.014-inch stainless is also unobtrusive
running along the outside of a tailboom. The
.014-inch stainless works great if you
execute it perfectly.
Most builders and fliers find that .020
inch or larger in diameter music wire is the
most durable pushrod system for discus tails.
It is heavier than the other systems. Music
wire is not as prone to compression buckling
as the stainless. This makes it slightly less
sensitive to how it is installed. It also resists
transport and landing damage much better
than carbon. Current XP-3 kits ship with
these pushrods, and I have installed them on
my number-two model. (See pictures.)
I use Teflon spaghetti tubing for all three
of the preceding systems. It can be
purchased at Composite Structures
Technology. There are a couple of tricks to
using this tubing. First, it can be stretched to
three times its original length to reduce
weight and diameter. (This tip is from Mark
Drela.) Second, it must be supported along
its entire length because the housing is not
rigid enough to prevent pushrod buckling. If
the pushrods are run on the outside of the
boom (recommended), you can use Scotch
July 2003 91
July 2003 93
tape to secure the housings. Better yet is 1-
mil-thick bookbinding tape if you can find it.
A trick to making a straight pushrod
housing on the outside of a boom is to affix
one end and pull tension on the tubing. You
can wrap masking tape around the tubing at
places where you need to glue it. I use a
little bit of Goop glue to secure the Teflon
inside the pod.
Running the pushrods inside the boom
would be nice, but it is difficult to do
without excessive slop and/or weight. A
support every six or eight inches might have
been fine for a javelin-launch HLG, but it is
horribly inadequate for a discus airplane
with small-diameter pushrods.
Slop in a discus-glider rudder linkage can
cost 10s of feet on launch. A gyro working
through a sloppy rudder linkage is slow to
dampen yaw oscillations. Although it is
difficult to see, stopped video frames show
that most discus airplanes initially yaw 20°
on release. This creates a huge amount of
drag.
Ideally, a gyro working with a positive
linkage dampens the oscillation in
approximately one period. Even with a gyro,
slop and/or flex in a rudder linkage can
triple the time it takes to straighten out. MA
Sources:
XP3:
Pole Cat Aeroplane Works
www.polecataero.com/
Teflon tubing:
Composite Structures Technology
www.cstsales.com
SINCE 1936
Edition: Model Aviation - 2003/07
Page Numbers: 88,90,91,93
Edition: Model Aviation - 2003/07
Page Numbers: 88,90,91,93
88 MODEL AVIATION
THE RUDDER LINKAGE on a discus glider is one of the most
critical linkages in Radio Control Soaring. In this column I will
explain why the rudder system is so important on a discus-launch
glider (DLG) and review the pros and cons of several pushrod
systems.
When a discus glider does break, it will most likely happen on
launch. Often unnoticed minor damage from a rough landing will
cause a catastrophic failure on the next throw. Unlike other types of
gliders, a crash can be precipitated by almost any component of a
discus glider. The difference is that the DLG’s rudder system and
fuselage must work perfectly during launch or it will typically roll
inverted.
Imagine what happens when a rudder linkage breaks on launch;
you have high-speed flight a few feet off the ground, a pilot trying to
transition from spinning to standing, usually an up-elevator preset,
and an unexpected upside-down attitude. These factors add up to
make a high-energy crash likely. Launch velocity is often higher
than these same gliders can obtain in a vertical dive. You can guess
what happens when an 11-ounce, 11⁄2-meter glider structure hits the
ground at that speed.
When I refer to the rudder system I mean everything on the
airplane that must be in working order for the rudder to function
effectively. This includes the rudder, its hinges, the fin, the fintailboom
joint, the tailboom, the tailboom pod joint, the pod, the
control horn, the linkage connections, the pushrod, the servo gears,
and the servo mount.
If any one of these fails on launch, odds are that the discus glider
will go home in a trash bag. A simple linkage problem can cause a
high-speed crash, leaving the glider in a state not worth repairing.
I won’t go into the math, but it can be shown that the side force
on a typical DLG fin is 2-3 pounds. If there was doubt whether or
not a tail was strong enough for discus, a bench test would be wise.
Put the model together and have a friend hold it in a knife-edge
position with the left wingtip on the ground (for right-handed
throwers). Put a chair or something a couple
of inches under the tailboom for emergency
support. Try to stack bags of weight on the
side of the vertical fin and rudder. Tape may
be required to keep the weight from slipping
as the tailboom deflects. If the fin holds 21⁄2
pounds without making crunchy noises, it
will likely stand up to full-power discus
launching.
The bench test has a couple of benefits.
First, it is a known, repeatable load. You can
use it to make relative strength comparisons
between two designs. Second, if a
component does break, the chair catches the
weight, unloads the airplane, and no more
damage is done.
A part breaking during a real launch
usually leads to much more damage. If it
breaks during the static test, it was not strong
enough for competitive discus launching.
Finding this out in the static test will save
your wing, and possibly more, from breaking
on the first hard launch. Try this at your own
risk; a manufacturer is not at fault if you
break your new model.
My XP-3 deflected 4 inches at the tip of
Mike Garton, 2733 NE 95th Ave., Ankeny IA 50021; E-mail: [email protected]
RADIO CONTROL SOARING
Positive, slop-free rudder linkage is essential on discus glider.
XP-3 tail shows well-supported rudder pushrod.
Close-up of the balsa rudder support with fully deflected rudder. A piece of hook-up
wire insulation is glued on roughened wire to trap the pushrod.
90 MODEL AVIATION
the rudder with a 21⁄2-pound load. It was completely silent during the
test. I did not have any doubt about this stout airplane, but I was
curious how far it would deflect. Weaker airplanes will deflect
ridiculously or break with the same load. You would be surprised
how much the fuselage pods flex under the wing (and eventually
break) on non-carbon-reinforced pod designs.
The calculated 2- to 3-pound loads on the fin also explain why
most discus models use fiberglassed balsa surfaces. Non-discus
Hand-Launched Gliders (HLGs) can get by with just 1⁄8 contest balsa
surfaces glued to a boom. Discus airplanes need a decent-sized piece
of fiberglass to go over the fin-to-tailboom joint.
Last summer I forgot to reinstall a retainer on my XP-3’s rudder
pushrod. My preflight inspection should have caught it. Of course the
pushrod pulled loose on a hard launch. I let go of the up-elevator
preset as it rolled inverted and completed the 90 mph roll 4 feet off
the deck. The problem and the correction occurred in less than a
second. I wish I could say that I consciously thought that fast; it was
a combination of luck and reflexes that saved the airplane. I did count
my blessings and quit for the day.
Bruce Davidson demonstrated an even better save at the 2002
Mid South contest. The rudder pull-pull
string on his polyhedral XP-3 broke on
launch. Using elevator control alone, Bruce
completed the four-minute max and landed
in bounds on the large sod farm. He said that
was the last time he ever used pull-pulls on a
discus glider.
Pull-pull strings are potentially the
lightest pushrod system. They have several
disadvantages. The lines need to be
periodically retensioned. There is a constant
compression force on the hinges. Even
exotic strings of Kevlar or Spectra cannot be
made as solid as a pushrod.
That can be demonstrated by trying to
XP-3 carbon stabilizer mount side view; SuperGee lineage shows.
Housing on this pushrod would ideally be slightly longer.
Notice the large amount of carbon in the XP-3’s fuselage pod. Flex in this area is
common on less expensive fiberglass discus-launch gliders.
move a servo (with power off) by moving
the control surface. If you don’t have to
move the surface very far before the servo
gears turn, it is a positive linkage. I have
never been successful at making pull-pulls
tight enough.
What pushrod systems work well for tail
surfaces on a discus airplane? That depends
on how meticulous you are at building and
how much you “baby” the model off the
field. Round .040-inch carbon in Teflon
spaghetti tubing housings are light. They
work great until you crack the carbon. Often
the damage is done in the back of a car,
carrying the airplane through a doorway, or
on a ground-loop landing. The crack will
break on the next full-power launch.
The carbon pushrods work fine as long as
you have the discipline to inspect them
carefully after transport and each rough
landing. Even with careful inspection there is
a risk that the damage will not be visible
until failure.
The SuperGee as designed and my
number-one XP-3 use .014-inch stainlesssteel
rods in Teflon tubing. The smalldiameter
stainless is more prone to
compression buckling than the .040 carbon.
My XP-3 pushrods are well supported and
still work great after almost a year of use.
The stainless does not hold a Z-bend
quite as well as music wire. Some pilots
have straightened out Z-bends on launch.
Wrapping the free end of the pushrod back
and twisting it around like a safety pin can
solve the problem. Besides being light, the
.014-inch stainless is also unobtrusive
running along the outside of a tailboom. The
.014-inch stainless works great if you
execute it perfectly.
Most builders and fliers find that .020
inch or larger in diameter music wire is the
most durable pushrod system for discus tails.
It is heavier than the other systems. Music
wire is not as prone to compression buckling
as the stainless. This makes it slightly less
sensitive to how it is installed. It also resists
transport and landing damage much better
than carbon. Current XP-3 kits ship with
these pushrods, and I have installed them on
my number-two model. (See pictures.)
I use Teflon spaghetti tubing for all three
of the preceding systems. It can be
purchased at Composite Structures
Technology. There are a couple of tricks to
using this tubing. First, it can be stretched to
three times its original length to reduce
weight and diameter. (This tip is from Mark
Drela.) Second, it must be supported along
its entire length because the housing is not
rigid enough to prevent pushrod buckling. If
the pushrods are run on the outside of the
boom (recommended), you can use Scotch
July 2003 91
July 2003 93
tape to secure the housings. Better yet is 1-
mil-thick bookbinding tape if you can find it.
A trick to making a straight pushrod
housing on the outside of a boom is to affix
one end and pull tension on the tubing. You
can wrap masking tape around the tubing at
places where you need to glue it. I use a
little bit of Goop glue to secure the Teflon
inside the pod.
Running the pushrods inside the boom
would be nice, but it is difficult to do
without excessive slop and/or weight. A
support every six or eight inches might have
been fine for a javelin-launch HLG, but it is
horribly inadequate for a discus airplane
with small-diameter pushrods.
Slop in a discus-glider rudder linkage can
cost 10s of feet on launch. A gyro working
through a sloppy rudder linkage is slow to
dampen yaw oscillations. Although it is
difficult to see, stopped video frames show
that most discus airplanes initially yaw 20°
on release. This creates a huge amount of
drag.
Ideally, a gyro working with a positive
linkage dampens the oscillation in
approximately one period. Even with a gyro,
slop and/or flex in a rudder linkage can
triple the time it takes to straighten out. MA
Sources:
XP3:
Pole Cat Aeroplane Works
www.polecataero.com/
Teflon tubing:
Composite Structures Technology
www.cstsales.com
SINCE 1936
Edition: Model Aviation - 2003/07
Page Numbers: 88,90,91,93
88 MODEL AVIATION
THE RUDDER LINKAGE on a discus glider is one of the most
critical linkages in Radio Control Soaring. In this column I will
explain why the rudder system is so important on a discus-launch
glider (DLG) and review the pros and cons of several pushrod
systems.
When a discus glider does break, it will most likely happen on
launch. Often unnoticed minor damage from a rough landing will
cause a catastrophic failure on the next throw. Unlike other types of
gliders, a crash can be precipitated by almost any component of a
discus glider. The difference is that the DLG’s rudder system and
fuselage must work perfectly during launch or it will typically roll
inverted.
Imagine what happens when a rudder linkage breaks on launch;
you have high-speed flight a few feet off the ground, a pilot trying to
transition from spinning to standing, usually an up-elevator preset,
and an unexpected upside-down attitude. These factors add up to
make a high-energy crash likely. Launch velocity is often higher
than these same gliders can obtain in a vertical dive. You can guess
what happens when an 11-ounce, 11⁄2-meter glider structure hits the
ground at that speed.
When I refer to the rudder system I mean everything on the
airplane that must be in working order for the rudder to function
effectively. This includes the rudder, its hinges, the fin, the fintailboom
joint, the tailboom, the tailboom pod joint, the pod, the
control horn, the linkage connections, the pushrod, the servo gears,
and the servo mount.
If any one of these fails on launch, odds are that the discus glider
will go home in a trash bag. A simple linkage problem can cause a
high-speed crash, leaving the glider in a state not worth repairing.
I won’t go into the math, but it can be shown that the side force
on a typical DLG fin is 2-3 pounds. If there was doubt whether or
not a tail was strong enough for discus, a bench test would be wise.
Put the model together and have a friend hold it in a knife-edge
position with the left wingtip on the ground (for right-handed
throwers). Put a chair or something a couple
of inches under the tailboom for emergency
support. Try to stack bags of weight on the
side of the vertical fin and rudder. Tape may
be required to keep the weight from slipping
as the tailboom deflects. If the fin holds 21⁄2
pounds without making crunchy noises, it
will likely stand up to full-power discus
launching.
The bench test has a couple of benefits.
First, it is a known, repeatable load. You can
use it to make relative strength comparisons
between two designs. Second, if a
component does break, the chair catches the
weight, unloads the airplane, and no more
damage is done.
A part breaking during a real launch
usually leads to much more damage. If it
breaks during the static test, it was not strong
enough for competitive discus launching.
Finding this out in the static test will save
your wing, and possibly more, from breaking
on the first hard launch. Try this at your own
risk; a manufacturer is not at fault if you
break your new model.
My XP-3 deflected 4 inches at the tip of
Mike Garton, 2733 NE 95th Ave., Ankeny IA 50021; E-mail: [email protected]
RADIO CONTROL SOARING
Positive, slop-free rudder linkage is essential on discus glider.
XP-3 tail shows well-supported rudder pushrod.
Close-up of the balsa rudder support with fully deflected rudder. A piece of hook-up
wire insulation is glued on roughened wire to trap the pushrod.
90 MODEL AVIATION
the rudder with a 21⁄2-pound load. It was completely silent during the
test. I did not have any doubt about this stout airplane, but I was
curious how far it would deflect. Weaker airplanes will deflect
ridiculously or break with the same load. You would be surprised
how much the fuselage pods flex under the wing (and eventually
break) on non-carbon-reinforced pod designs.
The calculated 2- to 3-pound loads on the fin also explain why
most discus models use fiberglassed balsa surfaces. Non-discus
Hand-Launched Gliders (HLGs) can get by with just 1⁄8 contest balsa
surfaces glued to a boom. Discus airplanes need a decent-sized piece
of fiberglass to go over the fin-to-tailboom joint.
Last summer I forgot to reinstall a retainer on my XP-3’s rudder
pushrod. My preflight inspection should have caught it. Of course the
pushrod pulled loose on a hard launch. I let go of the up-elevator
preset as it rolled inverted and completed the 90 mph roll 4 feet off
the deck. The problem and the correction occurred in less than a
second. I wish I could say that I consciously thought that fast; it was
a combination of luck and reflexes that saved the airplane. I did count
my blessings and quit for the day.
Bruce Davidson demonstrated an even better save at the 2002
Mid South contest. The rudder pull-pull
string on his polyhedral XP-3 broke on
launch. Using elevator control alone, Bruce
completed the four-minute max and landed
in bounds on the large sod farm. He said that
was the last time he ever used pull-pulls on a
discus glider.
Pull-pull strings are potentially the
lightest pushrod system. They have several
disadvantages. The lines need to be
periodically retensioned. There is a constant
compression force on the hinges. Even
exotic strings of Kevlar or Spectra cannot be
made as solid as a pushrod.
That can be demonstrated by trying to
XP-3 carbon stabilizer mount side view; SuperGee lineage shows.
Housing on this pushrod would ideally be slightly longer.
Notice the large amount of carbon in the XP-3’s fuselage pod. Flex in this area is
common on less expensive fiberglass discus-launch gliders.
move a servo (with power off) by moving
the control surface. If you don’t have to
move the surface very far before the servo
gears turn, it is a positive linkage. I have
never been successful at making pull-pulls
tight enough.
What pushrod systems work well for tail
surfaces on a discus airplane? That depends
on how meticulous you are at building and
how much you “baby” the model off the
field. Round .040-inch carbon in Teflon
spaghetti tubing housings are light. They
work great until you crack the carbon. Often
the damage is done in the back of a car,
carrying the airplane through a doorway, or
on a ground-loop landing. The crack will
break on the next full-power launch.
The carbon pushrods work fine as long as
you have the discipline to inspect them
carefully after transport and each rough
landing. Even with careful inspection there is
a risk that the damage will not be visible
until failure.
The SuperGee as designed and my
number-one XP-3 use .014-inch stainlesssteel
rods in Teflon tubing. The smalldiameter
stainless is more prone to
compression buckling than the .040 carbon.
My XP-3 pushrods are well supported and
still work great after almost a year of use.
The stainless does not hold a Z-bend
quite as well as music wire. Some pilots
have straightened out Z-bends on launch.
Wrapping the free end of the pushrod back
and twisting it around like a safety pin can
solve the problem. Besides being light, the
.014-inch stainless is also unobtrusive
running along the outside of a tailboom. The
.014-inch stainless works great if you
execute it perfectly.
Most builders and fliers find that .020
inch or larger in diameter music wire is the
most durable pushrod system for discus tails.
It is heavier than the other systems. Music
wire is not as prone to compression buckling
as the stainless. This makes it slightly less
sensitive to how it is installed. It also resists
transport and landing damage much better
than carbon. Current XP-3 kits ship with
these pushrods, and I have installed them on
my number-two model. (See pictures.)
I use Teflon spaghetti tubing for all three
of the preceding systems. It can be
purchased at Composite Structures
Technology. There are a couple of tricks to
using this tubing. First, it can be stretched to
three times its original length to reduce
weight and diameter. (This tip is from Mark
Drela.) Second, it must be supported along
its entire length because the housing is not
rigid enough to prevent pushrod buckling. If
the pushrods are run on the outside of the
boom (recommended), you can use Scotch
July 2003 91
July 2003 93
tape to secure the housings. Better yet is 1-
mil-thick bookbinding tape if you can find it.
A trick to making a straight pushrod
housing on the outside of a boom is to affix
one end and pull tension on the tubing. You
can wrap masking tape around the tubing at
places where you need to glue it. I use a
little bit of Goop glue to secure the Teflon
inside the pod.
Running the pushrods inside the boom
would be nice, but it is difficult to do
without excessive slop and/or weight. A
support every six or eight inches might have
been fine for a javelin-launch HLG, but it is
horribly inadequate for a discus airplane
with small-diameter pushrods.
Slop in a discus-glider rudder linkage can
cost 10s of feet on launch. A gyro working
through a sloppy rudder linkage is slow to
dampen yaw oscillations. Although it is
difficult to see, stopped video frames show
that most discus airplanes initially yaw 20°
on release. This creates a huge amount of
drag.
Ideally, a gyro working with a positive
linkage dampens the oscillation in
approximately one period. Even with a gyro,
slop and/or flex in a rudder linkage can
triple the time it takes to straighten out. MA
Sources:
XP3:
Pole Cat Aeroplane Works
www.polecataero.com/
Teflon tubing:
Composite Structures Technology
www.cstsales.com
SINCE 1936
Edition: Model Aviation - 2003/07
Page Numbers: 88,90,91,93
88 MODEL AVIATION
THE RUDDER LINKAGE on a discus glider is one of the most
critical linkages in Radio Control Soaring. In this column I will
explain why the rudder system is so important on a discus-launch
glider (DLG) and review the pros and cons of several pushrod
systems.
When a discus glider does break, it will most likely happen on
launch. Often unnoticed minor damage from a rough landing will
cause a catastrophic failure on the next throw. Unlike other types of
gliders, a crash can be precipitated by almost any component of a
discus glider. The difference is that the DLG’s rudder system and
fuselage must work perfectly during launch or it will typically roll
inverted.
Imagine what happens when a rudder linkage breaks on launch;
you have high-speed flight a few feet off the ground, a pilot trying to
transition from spinning to standing, usually an up-elevator preset,
and an unexpected upside-down attitude. These factors add up to
make a high-energy crash likely. Launch velocity is often higher
than these same gliders can obtain in a vertical dive. You can guess
what happens when an 11-ounce, 11⁄2-meter glider structure hits the
ground at that speed.
When I refer to the rudder system I mean everything on the
airplane that must be in working order for the rudder to function
effectively. This includes the rudder, its hinges, the fin, the fintailboom
joint, the tailboom, the tailboom pod joint, the pod, the
control horn, the linkage connections, the pushrod, the servo gears,
and the servo mount.
If any one of these fails on launch, odds are that the discus glider
will go home in a trash bag. A simple linkage problem can cause a
high-speed crash, leaving the glider in a state not worth repairing.
I won’t go into the math, but it can be shown that the side force
on a typical DLG fin is 2-3 pounds. If there was doubt whether or
not a tail was strong enough for discus, a bench test would be wise.
Put the model together and have a friend hold it in a knife-edge
position with the left wingtip on the ground (for right-handed
throwers). Put a chair or something a couple
of inches under the tailboom for emergency
support. Try to stack bags of weight on the
side of the vertical fin and rudder. Tape may
be required to keep the weight from slipping
as the tailboom deflects. If the fin holds 21⁄2
pounds without making crunchy noises, it
will likely stand up to full-power discus
launching.
The bench test has a couple of benefits.
First, it is a known, repeatable load. You can
use it to make relative strength comparisons
between two designs. Second, if a
component does break, the chair catches the
weight, unloads the airplane, and no more
damage is done.
A part breaking during a real launch
usually leads to much more damage. If it
breaks during the static test, it was not strong
enough for competitive discus launching.
Finding this out in the static test will save
your wing, and possibly more, from breaking
on the first hard launch. Try this at your own
risk; a manufacturer is not at fault if you
break your new model.
My XP-3 deflected 4 inches at the tip of
Mike Garton, 2733 NE 95th Ave., Ankeny IA 50021; E-mail: [email protected]
RADIO CONTROL SOARING
Positive, slop-free rudder linkage is essential on discus glider.
XP-3 tail shows well-supported rudder pushrod.
Close-up of the balsa rudder support with fully deflected rudder. A piece of hook-up
wire insulation is glued on roughened wire to trap the pushrod.
90 MODEL AVIATION
the rudder with a 21⁄2-pound load. It was completely silent during the
test. I did not have any doubt about this stout airplane, but I was
curious how far it would deflect. Weaker airplanes will deflect
ridiculously or break with the same load. You would be surprised
how much the fuselage pods flex under the wing (and eventually
break) on non-carbon-reinforced pod designs.
The calculated 2- to 3-pound loads on the fin also explain why
most discus models use fiberglassed balsa surfaces. Non-discus
Hand-Launched Gliders (HLGs) can get by with just 1⁄8 contest balsa
surfaces glued to a boom. Discus airplanes need a decent-sized piece
of fiberglass to go over the fin-to-tailboom joint.
Last summer I forgot to reinstall a retainer on my XP-3’s rudder
pushrod. My preflight inspection should have caught it. Of course the
pushrod pulled loose on a hard launch. I let go of the up-elevator
preset as it rolled inverted and completed the 90 mph roll 4 feet off
the deck. The problem and the correction occurred in less than a
second. I wish I could say that I consciously thought that fast; it was
a combination of luck and reflexes that saved the airplane. I did count
my blessings and quit for the day.
Bruce Davidson demonstrated an even better save at the 2002
Mid South contest. The rudder pull-pull
string on his polyhedral XP-3 broke on
launch. Using elevator control alone, Bruce
completed the four-minute max and landed
in bounds on the large sod farm. He said that
was the last time he ever used pull-pulls on a
discus glider.
Pull-pull strings are potentially the
lightest pushrod system. They have several
disadvantages. The lines need to be
periodically retensioned. There is a constant
compression force on the hinges. Even
exotic strings of Kevlar or Spectra cannot be
made as solid as a pushrod.
That can be demonstrated by trying to
XP-3 carbon stabilizer mount side view; SuperGee lineage shows.
Housing on this pushrod would ideally be slightly longer.
Notice the large amount of carbon in the XP-3’s fuselage pod. Flex in this area is
common on less expensive fiberglass discus-launch gliders.
move a servo (with power off) by moving
the control surface. If you don’t have to
move the surface very far before the servo
gears turn, it is a positive linkage. I have
never been successful at making pull-pulls
tight enough.
What pushrod systems work well for tail
surfaces on a discus airplane? That depends
on how meticulous you are at building and
how much you “baby” the model off the
field. Round .040-inch carbon in Teflon
spaghetti tubing housings are light. They
work great until you crack the carbon. Often
the damage is done in the back of a car,
carrying the airplane through a doorway, or
on a ground-loop landing. The crack will
break on the next full-power launch.
The carbon pushrods work fine as long as
you have the discipline to inspect them
carefully after transport and each rough
landing. Even with careful inspection there is
a risk that the damage will not be visible
until failure.
The SuperGee as designed and my
number-one XP-3 use .014-inch stainlesssteel
rods in Teflon tubing. The smalldiameter
stainless is more prone to
compression buckling than the .040 carbon.
My XP-3 pushrods are well supported and
still work great after almost a year of use.
The stainless does not hold a Z-bend
quite as well as music wire. Some pilots
have straightened out Z-bends on launch.
Wrapping the free end of the pushrod back
and twisting it around like a safety pin can
solve the problem. Besides being light, the
.014-inch stainless is also unobtrusive
running along the outside of a tailboom. The
.014-inch stainless works great if you
execute it perfectly.
Most builders and fliers find that .020
inch or larger in diameter music wire is the
most durable pushrod system for discus tails.
It is heavier than the other systems. Music
wire is not as prone to compression buckling
as the stainless. This makes it slightly less
sensitive to how it is installed. It also resists
transport and landing damage much better
than carbon. Current XP-3 kits ship with
these pushrods, and I have installed them on
my number-two model. (See pictures.)
I use Teflon spaghetti tubing for all three
of the preceding systems. It can be
purchased at Composite Structures
Technology. There are a couple of tricks to
using this tubing. First, it can be stretched to
three times its original length to reduce
weight and diameter. (This tip is from Mark
Drela.) Second, it must be supported along
its entire length because the housing is not
rigid enough to prevent pushrod buckling. If
the pushrods are run on the outside of the
boom (recommended), you can use Scotch
July 2003 91
July 2003 93
tape to secure the housings. Better yet is 1-
mil-thick bookbinding tape if you can find it.
A trick to making a straight pushrod
housing on the outside of a boom is to affix
one end and pull tension on the tubing. You
can wrap masking tape around the tubing at
places where you need to glue it. I use a
little bit of Goop glue to secure the Teflon
inside the pod.
Running the pushrods inside the boom
would be nice, but it is difficult to do
without excessive slop and/or weight. A
support every six or eight inches might have
been fine for a javelin-launch HLG, but it is
horribly inadequate for a discus airplane
with small-diameter pushrods.
Slop in a discus-glider rudder linkage can
cost 10s of feet on launch. A gyro working
through a sloppy rudder linkage is slow to
dampen yaw oscillations. Although it is
difficult to see, stopped video frames show
that most discus airplanes initially yaw 20°
on release. This creates a huge amount of
drag.
Ideally, a gyro working with a positive
linkage dampens the oscillation in
approximately one period. Even with a gyro,
slop and/or flex in a rudder linkage can
triple the time it takes to straighten out. MA
Sources:
XP3:
Pole Cat Aeroplane Works
www.polecataero.com/
Teflon tubing:
Composite Structures Technology
www.cstsales.com
SINCE 1936
