Mike Garton, 2733 NE 95th Ave., Ankeny IA 50021; E-mail: [email protected]
RADIO CONTROL SOARING
THIS MONTH I have a review of a new instructional moldmaking
DVD and a technical note submitted by Gregory Ciurpita
on the pitch stability of gliders at low speed.
Bill Haymaker made the new DVD, and it features Terry
Luckenbach’s method of making molded parts. In particular, Terry
shows how to make a Pretty Mantis glider fuselage. In the DVD,
Terry mentions that he has been making molded parts for
approximately 30 years; it shows. He goes into great detail about
the materials, methods, and the tricks he has developed.
I have often thought about making a molded fuselage for a
scratch-built glider, but the experienced fabricators have
consistently told me that it takes hundreds of hours to make a
single hardwood plug and an epoxy-fiberglass mold.
The unique thing about Terry’s method is that it takes just a
fraction of the normal time to create the plug and the mold. I
won’t give away his secrets, but the DVD convinced me to dive
into this technology for a current project. The information was
enabling.
My one complaint about the DVD is that Terry did not talk
about or demonstrate epoxy safety. The majority of modelers still
do not understand that all people are at risk for developing epoxy
allergies. Even if you have no other allergies and don’t react the
first 500 times you touch epoxy, eventually you will. It is
cumulative exposure that eventually triggers an allergy.
I can name three glider manufacturers who have had to stop
making epoxy parts within the last five years because they
developed epoxy allergies. It is essential to wear gloves when you
need to touch uncured epoxy. Disposable nitrile gloves cost
roughly $10 for 100 at any drug store.
In spite of my one gripe, the quality of the DVD is
extraordinarily high. It is 103 minutes long. I learned a great deal.
It would take an individual thousands of hours to develop a similar
process without the DVD. If you scratch-build, the information is
priceless. I put this DVD in my must-buy category.
Bill Haymaker also sells a great DVD on Phil Barnes’ methods
of making composite glider wings. You can reach Bill at
Bill Haymaker (East Berlin PA) has produced an excellent
instructional DVD about molding fiberglass parts.
Figure 1. Figure 2.
100 MODEL AVIATION
07sig4.QXD 4/23/04 12:37 pm Page 100
July 2004 101
Figure 3. Figure 4.
[email protected] or 107 Schofield Dr., East Berlin PA
17316.
What follows is technical information about glider stability by
Gregory Ciurpita. Greg wrote “Low-Speed Stability” for R/C
Soaring Digest (RCSD), which is the only American periodical
exclusively committed to RC Soaring. I obtained Greg’s and
RCSD’s permission to include it here.
I encourage glider fliers to subscribe to RCSD. It is inexpensive
and has regular contributions from Dave Register, Lee Murray,
Gordy Stahl, and Bill and Bunny Kuhlman, along with occasional
gems of information from Mark Drela. You can find out more
about RCSD at www.b2streamlines.com/RCSD.html or call (707)
578-7871.
“An aircraft can be stable at higher speeds but unstable at low
speeds. While a more rearward center of gravity (CG) may cause
instability, it not only reduces the lift force and induced drag
produced by the tail, but makes airspeed more sensitive to elevator
trim setting. Adding ballast may make an aircraft more unstable,
but a ballasted aircraft is normally flown faster. First a review of
an airfoil’s moment coefficient (Cm). Then, an analysis of an
Visit the MODEL AVIATION Digital Archives!
Featuring a searchable database of Model
Aviation issues and articles from 1975 to 2000.
This is by far one of the best
efforts AMA has made to
construct something that is for
every member.
—Marco Pinto
Peninsula Channel Commanders
San Francisco CA
“
” Find it at www.modelaircraft.org. On the main page, click
on the “Members Only” section, log in with your last name
and AMA number, then click on the “Visit the Digital
Archive” image.
07sig4.QXD 4/23/04 12:38 pm Page 101
aircraft’s pitching moments over a range of
CG positions.
“Airfoil Moment
“Figure 1 shows a typical airfoil
measurement from the UIUC [University
of Illinois at Urbana-Champaign] database.
It shows curves for the lift (Cl) and
moment (Cm) coefficients for various
angles of attack (AOA). The lift
coefficient (Cl) varies significantly,
steadily increasing until stall occurs. It is
common for an airfoil moment coefficient
(Cm) to be constant and negative.
“The actual lift (L) and moment (M) are
determined from the following well known
equations, where Q is the dynamic
pressure, rho (0.002378 slugs/ft3) is air
density, V is airspeed (ft/sec), S is the
wing area (ft2), and C the chord length (ft):
“Q = 0.5 rho V2
“L = Q * S * Cl
“M = Q * S * Cm * C
“The resultant of the lift force (L) is
generated through the aerodynamic center
(AC) of the airfoil, typically 25% of the
chord. The moment (M) is a rotational
force measured in foot-pounds or newtonmeters.
A wrench applies a moment on a
bolt; a motor generates a moment around a
shaft. A negative coefficient indicates a
nose-down direction, forcing the leading
edge of the wing down and the trailing
edge up.
“The relationship between pitching
moment and the total lift of an airfoil can
be confusing. As Figure 1 indicates, the Cl
and Cm coefficients are independent.
Consider a typical wing in a wind tunnel at
a constant airspeed. As the angle of attack
is increased, the lift will increase as
predicted by the equations. However, the
moment will remain constant, even when
the lift is zero. Also consider that the
moment does not change direction when
the lift coefficient becomes negative.
“Aircraft Moments
“Four forces affect the overall pitch of
the aircraft: the airfoil pitching moment
(Cm), the lift produced by the wing, the
lift force produced by the horizontal
stabilizer, and drag. Lift only affects pitch
when the CG is not located at the AC of
the wing. Likewise, drag produces a
moment when its center is either above or
below the CG. The moment produced by
drag will be ignored in this article.
“The horizontal stabilizer and wing lift
forces produce moments determined by
multiplying each with their respective
moment arms. Their moment arms are the
distance between the aircraft CG and AC
of the tail and wing respectively. The sum
of all three moments must balance (equal
zero) for the aircraft to maintain its pitch.
Otherwise, it will constantly rotate upward
or downward.
“Balanced pitch does not mean that the
pitch angle will remain unchanged. For
stability, there must be some mechanism to
maintain the pitch orientation of the
aircraft. This orientation may be affected
by turbulence or a change in airspeed. A
conventional approach to maintain stability
is to have the horizontal stabilizer generate
negative lift (a downward force). As the
airspeed increases, the tail lift increases
pushing the tail down and slowing the
aircraft.
“Figure 2 plots the three moment forces
vs. airspeed for a CG located at 30% of the
chord length (10”). The lift and its moment
are constant since it must balance the
weight (38 ounces) of the aircraft. The lift
moment produces a nose up force, and is
therefore positive.
“The airfoil moment (M) is dependent
on airspeed, wing area (900 sq. in.) and
chord length. Cm is negative as well as its
moment (M). It produces a nose down
force. As the airspeed increases it produces
a greater negative moment.
“The tail moment must balance (equal
but opposite) the sum of the lift and airfoil
moments. In this example, the horizontal
stabilizer always produces a nose up force.
Even though the horizontal stabilizer
produces a negative (downward) lift force,
the moment is positive (nose up).
“The aircraft becomes unstable in pitch
if the tail is required to produce positive
(upward) lift in order to balance the sum of
the airfoil and lift moments. This would be
indicated by a negative (nose down) tail
moment, and is most likely to occur at low
airspeeds.
“Low-Speed Instability
“Figure 3 plots the sum of the airfoil
and lift moments versus airspeed, for CG
ranging from 25 to 33% of the chord. For
all cases, the airfoil moment contribution
is the same as in Figure 2. As the CG is
moved more rearward the lift moment arm
and its moment increase.
“The lowermost curve is for the case
where the CG is at the AC of the wing and
the lift moment arm is zero. Therefore, the
lift moment is zero, and this curve is
purely the airfoil moment. The uppermost
curve is for the case where the CG is at its
most rearward position. At each CG
position, the lift moment is constant with
airspeed, and simply shifts the airfoil
moment curve upward the same amount at
all airspeeds.
102 MODEL AVIATION
07sig4.QXD 4/23/04 12:38 pm Page 102
“As the CG is moved rearward, it is
more likely to cause an unstable situation
at low speeds. As the 33% case shows, a
negative tail moment is required below an
airspeed of 20 feet per sec. A negative tail
moment requires a positive (upward) tail
lift, which no longer provides a pitch
correcting mechanism.
“Airspeed Sensitivity
“Figure 4 shows the corresponding tail
lift coefficients for the cases shown in
Figure 3. The lift coefficient is calculated
from the required tail moment. The plot
for the 33% case clearly shows that the lift
coefficient becomes positive at low
airspeeds, producing an unstable, noncorrecting
situation.
“When the CG is at the AC and the lift
moment is zero, the horizontal stabilizer
and wing pitching moment must balance
one another (the lowermost curve). Both
are equally affected by airspeed. There is
only one trim setting where they both
balance one another, and will do so at all
airspeeds.
“For all other curves in Figure 4, when
the lift moment is not zero, the aircraft
will be balanced at only one airspeed
depending on the tail lift coefficient. The
lift coefficient depends on the elevator
trim setting.
“The figure also shows that as the CG
is moved rearward there is a greater
change in lift coefficient per a change in
airspeed. This means that for a more
rearward CG, there will be a greater
corrective force for a smaller change in
airspeed. In other words, stable airspeed
will be more sensitive to elevator trim
setting.
“Summary
“1. The pitching moments of an aircraft
can be balanced with a horizontal stabilizer
that provides positive lift, but negative
(downward) lift, producing a positive
moment, is required to provide pitch
stability.
“2. Because the lift moment is constant
but the airfoil and horizontal stabilizer
moments are airspeed dependent, a negative
tail moment may be required at low
airspeeds. This is more likely to happen as
the CG is moved more rearward. The
aircraft can therefore be unstable at low
airspeeds, but stable at higher speeds.
“3. A more rearward CG results in a
corrective force more airspeed dependent,
making the stable airspeed more sensitive
to elevator trim setting.
“Adding ballast is more likely to cause
low-speed instability, but a ballasted
aircraft is flown faster. In flight camber
adjustments will also affect balance and
stability.
“This article came about after studying
tail lift forces and CG position. The
analysis helps me understand why a
seemingly stable aircraft can behave poorly
at low airspeeds. Unlike other technical
articles dealing with aircraft design, CG
position is something every pilot must
consider. I hope others will contribute
articles on this and other technical subjects.
My sincere thanks to Dave Register for his
review and discussion of this article.” MA
104 MODEL AVIATION
OVER 100 LITHIUM-ION
BATTERY PACKS TO
CHOOSE FROM
MULTI-VOLTAGE MODULES
POWER REGULATORS
CHARGERS
CLIP-ON-COCKPIT
TELEMETRY SYSTEM
R/C Power Solutions
“...Great Products...
Great Customer Service...”
2002 TOC CHAMPION
CHIP HYDE
FROM MICRO-FLYERS...
...TO GIANT SCALE...
...LAND, SEA AND AIR...
WE HAVE A POWER SYSTEM
THAT’S “LITE” FOR YOU!
Contact:
Skyborn Electronics
3405 Express Dr.
Garland, TX 75041
972-267-5099 Fax: 972-271-3529
www.rcpowerflite.com
[email protected]
J
Li
For m
Jac
Rad
The
07sig4.QXD 4/23/04 12:38 pm Page 104
Edition: Model Aviation - 2004/07
Page Numbers: 100,101,102,104
Edition: Model Aviation - 2004/07
Page Numbers: 100,101,102,104
Mike Garton, 2733 NE 95th Ave., Ankeny IA 50021; E-mail: [email protected]
RADIO CONTROL SOARING
THIS MONTH I have a review of a new instructional moldmaking
DVD and a technical note submitted by Gregory Ciurpita
on the pitch stability of gliders at low speed.
Bill Haymaker made the new DVD, and it features Terry
Luckenbach’s method of making molded parts. In particular, Terry
shows how to make a Pretty Mantis glider fuselage. In the DVD,
Terry mentions that he has been making molded parts for
approximately 30 years; it shows. He goes into great detail about
the materials, methods, and the tricks he has developed.
I have often thought about making a molded fuselage for a
scratch-built glider, but the experienced fabricators have
consistently told me that it takes hundreds of hours to make a
single hardwood plug and an epoxy-fiberglass mold.
The unique thing about Terry’s method is that it takes just a
fraction of the normal time to create the plug and the mold. I
won’t give away his secrets, but the DVD convinced me to dive
into this technology for a current project. The information was
enabling.
My one complaint about the DVD is that Terry did not talk
about or demonstrate epoxy safety. The majority of modelers still
do not understand that all people are at risk for developing epoxy
allergies. Even if you have no other allergies and don’t react the
first 500 times you touch epoxy, eventually you will. It is
cumulative exposure that eventually triggers an allergy.
I can name three glider manufacturers who have had to stop
making epoxy parts within the last five years because they
developed epoxy allergies. It is essential to wear gloves when you
need to touch uncured epoxy. Disposable nitrile gloves cost
roughly $10 for 100 at any drug store.
In spite of my one gripe, the quality of the DVD is
extraordinarily high. It is 103 minutes long. I learned a great deal.
It would take an individual thousands of hours to develop a similar
process without the DVD. If you scratch-build, the information is
priceless. I put this DVD in my must-buy category.
Bill Haymaker also sells a great DVD on Phil Barnes’ methods
of making composite glider wings. You can reach Bill at
Bill Haymaker (East Berlin PA) has produced an excellent
instructional DVD about molding fiberglass parts.
Figure 1. Figure 2.
100 MODEL AVIATION
07sig4.QXD 4/23/04 12:37 pm Page 100
July 2004 101
Figure 3. Figure 4.
[email protected] or 107 Schofield Dr., East Berlin PA
17316.
What follows is technical information about glider stability by
Gregory Ciurpita. Greg wrote “Low-Speed Stability” for R/C
Soaring Digest (RCSD), which is the only American periodical
exclusively committed to RC Soaring. I obtained Greg’s and
RCSD’s permission to include it here.
I encourage glider fliers to subscribe to RCSD. It is inexpensive
and has regular contributions from Dave Register, Lee Murray,
Gordy Stahl, and Bill and Bunny Kuhlman, along with occasional
gems of information from Mark Drela. You can find out more
about RCSD at www.b2streamlines.com/RCSD.html or call (707)
578-7871.
“An aircraft can be stable at higher speeds but unstable at low
speeds. While a more rearward center of gravity (CG) may cause
instability, it not only reduces the lift force and induced drag
produced by the tail, but makes airspeed more sensitive to elevator
trim setting. Adding ballast may make an aircraft more unstable,
but a ballasted aircraft is normally flown faster. First a review of
an airfoil’s moment coefficient (Cm). Then, an analysis of an
Visit the MODEL AVIATION Digital Archives!
Featuring a searchable database of Model
Aviation issues and articles from 1975 to 2000.
This is by far one of the best
efforts AMA has made to
construct something that is for
every member.
—Marco Pinto
Peninsula Channel Commanders
San Francisco CA
“
” Find it at www.modelaircraft.org. On the main page, click
on the “Members Only” section, log in with your last name
and AMA number, then click on the “Visit the Digital
Archive” image.
07sig4.QXD 4/23/04 12:38 pm Page 101
aircraft’s pitching moments over a range of
CG positions.
“Airfoil Moment
“Figure 1 shows a typical airfoil
measurement from the UIUC [University
of Illinois at Urbana-Champaign] database.
It shows curves for the lift (Cl) and
moment (Cm) coefficients for various
angles of attack (AOA). The lift
coefficient (Cl) varies significantly,
steadily increasing until stall occurs. It is
common for an airfoil moment coefficient
(Cm) to be constant and negative.
“The actual lift (L) and moment (M) are
determined from the following well known
equations, where Q is the dynamic
pressure, rho (0.002378 slugs/ft3) is air
density, V is airspeed (ft/sec), S is the
wing area (ft2), and C the chord length (ft):
“Q = 0.5 rho V2
“L = Q * S * Cl
“M = Q * S * Cm * C
“The resultant of the lift force (L) is
generated through the aerodynamic center
(AC) of the airfoil, typically 25% of the
chord. The moment (M) is a rotational
force measured in foot-pounds or newtonmeters.
A wrench applies a moment on a
bolt; a motor generates a moment around a
shaft. A negative coefficient indicates a
nose-down direction, forcing the leading
edge of the wing down and the trailing
edge up.
“The relationship between pitching
moment and the total lift of an airfoil can
be confusing. As Figure 1 indicates, the Cl
and Cm coefficients are independent.
Consider a typical wing in a wind tunnel at
a constant airspeed. As the angle of attack
is increased, the lift will increase as
predicted by the equations. However, the
moment will remain constant, even when
the lift is zero. Also consider that the
moment does not change direction when
the lift coefficient becomes negative.
“Aircraft Moments
“Four forces affect the overall pitch of
the aircraft: the airfoil pitching moment
(Cm), the lift produced by the wing, the
lift force produced by the horizontal
stabilizer, and drag. Lift only affects pitch
when the CG is not located at the AC of
the wing. Likewise, drag produces a
moment when its center is either above or
below the CG. The moment produced by
drag will be ignored in this article.
“The horizontal stabilizer and wing lift
forces produce moments determined by
multiplying each with their respective
moment arms. Their moment arms are the
distance between the aircraft CG and AC
of the tail and wing respectively. The sum
of all three moments must balance (equal
zero) for the aircraft to maintain its pitch.
Otherwise, it will constantly rotate upward
or downward.
“Balanced pitch does not mean that the
pitch angle will remain unchanged. For
stability, there must be some mechanism to
maintain the pitch orientation of the
aircraft. This orientation may be affected
by turbulence or a change in airspeed. A
conventional approach to maintain stability
is to have the horizontal stabilizer generate
negative lift (a downward force). As the
airspeed increases, the tail lift increases
pushing the tail down and slowing the
aircraft.
“Figure 2 plots the three moment forces
vs. airspeed for a CG located at 30% of the
chord length (10”). The lift and its moment
are constant since it must balance the
weight (38 ounces) of the aircraft. The lift
moment produces a nose up force, and is
therefore positive.
“The airfoil moment (M) is dependent
on airspeed, wing area (900 sq. in.) and
chord length. Cm is negative as well as its
moment (M). It produces a nose down
force. As the airspeed increases it produces
a greater negative moment.
“The tail moment must balance (equal
but opposite) the sum of the lift and airfoil
moments. In this example, the horizontal
stabilizer always produces a nose up force.
Even though the horizontal stabilizer
produces a negative (downward) lift force,
the moment is positive (nose up).
“The aircraft becomes unstable in pitch
if the tail is required to produce positive
(upward) lift in order to balance the sum of
the airfoil and lift moments. This would be
indicated by a negative (nose down) tail
moment, and is most likely to occur at low
airspeeds.
“Low-Speed Instability
“Figure 3 plots the sum of the airfoil
and lift moments versus airspeed, for CG
ranging from 25 to 33% of the chord. For
all cases, the airfoil moment contribution
is the same as in Figure 2. As the CG is
moved more rearward the lift moment arm
and its moment increase.
“The lowermost curve is for the case
where the CG is at the AC of the wing and
the lift moment arm is zero. Therefore, the
lift moment is zero, and this curve is
purely the airfoil moment. The uppermost
curve is for the case where the CG is at its
most rearward position. At each CG
position, the lift moment is constant with
airspeed, and simply shifts the airfoil
moment curve upward the same amount at
all airspeeds.
102 MODEL AVIATION
07sig4.QXD 4/23/04 12:38 pm Page 102
“As the CG is moved rearward, it is
more likely to cause an unstable situation
at low speeds. As the 33% case shows, a
negative tail moment is required below an
airspeed of 20 feet per sec. A negative tail
moment requires a positive (upward) tail
lift, which no longer provides a pitch
correcting mechanism.
“Airspeed Sensitivity
“Figure 4 shows the corresponding tail
lift coefficients for the cases shown in
Figure 3. The lift coefficient is calculated
from the required tail moment. The plot
for the 33% case clearly shows that the lift
coefficient becomes positive at low
airspeeds, producing an unstable, noncorrecting
situation.
“When the CG is at the AC and the lift
moment is zero, the horizontal stabilizer
and wing pitching moment must balance
one another (the lowermost curve). Both
are equally affected by airspeed. There is
only one trim setting where they both
balance one another, and will do so at all
airspeeds.
“For all other curves in Figure 4, when
the lift moment is not zero, the aircraft
will be balanced at only one airspeed
depending on the tail lift coefficient. The
lift coefficient depends on the elevator
trim setting.
“The figure also shows that as the CG
is moved rearward there is a greater
change in lift coefficient per a change in
airspeed. This means that for a more
rearward CG, there will be a greater
corrective force for a smaller change in
airspeed. In other words, stable airspeed
will be more sensitive to elevator trim
setting.
“Summary
“1. The pitching moments of an aircraft
can be balanced with a horizontal stabilizer
that provides positive lift, but negative
(downward) lift, producing a positive
moment, is required to provide pitch
stability.
“2. Because the lift moment is constant
but the airfoil and horizontal stabilizer
moments are airspeed dependent, a negative
tail moment may be required at low
airspeeds. This is more likely to happen as
the CG is moved more rearward. The
aircraft can therefore be unstable at low
airspeeds, but stable at higher speeds.
“3. A more rearward CG results in a
corrective force more airspeed dependent,
making the stable airspeed more sensitive
to elevator trim setting.
“Adding ballast is more likely to cause
low-speed instability, but a ballasted
aircraft is flown faster. In flight camber
adjustments will also affect balance and
stability.
“This article came about after studying
tail lift forces and CG position. The
analysis helps me understand why a
seemingly stable aircraft can behave poorly
at low airspeeds. Unlike other technical
articles dealing with aircraft design, CG
position is something every pilot must
consider. I hope others will contribute
articles on this and other technical subjects.
My sincere thanks to Dave Register for his
review and discussion of this article.” MA
104 MODEL AVIATION
OVER 100 LITHIUM-ION
BATTERY PACKS TO
CHOOSE FROM
MULTI-VOLTAGE MODULES
POWER REGULATORS
CHARGERS
CLIP-ON-COCKPIT
TELEMETRY SYSTEM
R/C Power Solutions
“...Great Products...
Great Customer Service...”
2002 TOC CHAMPION
CHIP HYDE
FROM MICRO-FLYERS...
...TO GIANT SCALE...
...LAND, SEA AND AIR...
WE HAVE A POWER SYSTEM
THAT’S “LITE” FOR YOU!
Contact:
Skyborn Electronics
3405 Express Dr.
Garland, TX 75041
972-267-5099 Fax: 972-271-3529
www.rcpowerflite.com
[email protected]
J
Li
For m
Jac
Rad
The
07sig4.QXD 4/23/04 12:38 pm Page 104
Edition: Model Aviation - 2004/07
Page Numbers: 100,101,102,104
Mike Garton, 2733 NE 95th Ave., Ankeny IA 50021; E-mail: [email protected]
RADIO CONTROL SOARING
THIS MONTH I have a review of a new instructional moldmaking
DVD and a technical note submitted by Gregory Ciurpita
on the pitch stability of gliders at low speed.
Bill Haymaker made the new DVD, and it features Terry
Luckenbach’s method of making molded parts. In particular, Terry
shows how to make a Pretty Mantis glider fuselage. In the DVD,
Terry mentions that he has been making molded parts for
approximately 30 years; it shows. He goes into great detail about
the materials, methods, and the tricks he has developed.
I have often thought about making a molded fuselage for a
scratch-built glider, but the experienced fabricators have
consistently told me that it takes hundreds of hours to make a
single hardwood plug and an epoxy-fiberglass mold.
The unique thing about Terry’s method is that it takes just a
fraction of the normal time to create the plug and the mold. I
won’t give away his secrets, but the DVD convinced me to dive
into this technology for a current project. The information was
enabling.
My one complaint about the DVD is that Terry did not talk
about or demonstrate epoxy safety. The majority of modelers still
do not understand that all people are at risk for developing epoxy
allergies. Even if you have no other allergies and don’t react the
first 500 times you touch epoxy, eventually you will. It is
cumulative exposure that eventually triggers an allergy.
I can name three glider manufacturers who have had to stop
making epoxy parts within the last five years because they
developed epoxy allergies. It is essential to wear gloves when you
need to touch uncured epoxy. Disposable nitrile gloves cost
roughly $10 for 100 at any drug store.
In spite of my one gripe, the quality of the DVD is
extraordinarily high. It is 103 minutes long. I learned a great deal.
It would take an individual thousands of hours to develop a similar
process without the DVD. If you scratch-build, the information is
priceless. I put this DVD in my must-buy category.
Bill Haymaker also sells a great DVD on Phil Barnes’ methods
of making composite glider wings. You can reach Bill at
Bill Haymaker (East Berlin PA) has produced an excellent
instructional DVD about molding fiberglass parts.
Figure 1. Figure 2.
100 MODEL AVIATION
07sig4.QXD 4/23/04 12:37 pm Page 100
July 2004 101
Figure 3. Figure 4.
[email protected] or 107 Schofield Dr., East Berlin PA
17316.
What follows is technical information about glider stability by
Gregory Ciurpita. Greg wrote “Low-Speed Stability” for R/C
Soaring Digest (RCSD), which is the only American periodical
exclusively committed to RC Soaring. I obtained Greg’s and
RCSD’s permission to include it here.
I encourage glider fliers to subscribe to RCSD. It is inexpensive
and has regular contributions from Dave Register, Lee Murray,
Gordy Stahl, and Bill and Bunny Kuhlman, along with occasional
gems of information from Mark Drela. You can find out more
about RCSD at www.b2streamlines.com/RCSD.html or call (707)
578-7871.
“An aircraft can be stable at higher speeds but unstable at low
speeds. While a more rearward center of gravity (CG) may cause
instability, it not only reduces the lift force and induced drag
produced by the tail, but makes airspeed more sensitive to elevator
trim setting. Adding ballast may make an aircraft more unstable,
but a ballasted aircraft is normally flown faster. First a review of
an airfoil’s moment coefficient (Cm). Then, an analysis of an
Visit the MODEL AVIATION Digital Archives!
Featuring a searchable database of Model
Aviation issues and articles from 1975 to 2000.
This is by far one of the best
efforts AMA has made to
construct something that is for
every member.
—Marco Pinto
Peninsula Channel Commanders
San Francisco CA
“
” Find it at www.modelaircraft.org. On the main page, click
on the “Members Only” section, log in with your last name
and AMA number, then click on the “Visit the Digital
Archive” image.
07sig4.QXD 4/23/04 12:38 pm Page 101
aircraft’s pitching moments over a range of
CG positions.
“Airfoil Moment
“Figure 1 shows a typical airfoil
measurement from the UIUC [University
of Illinois at Urbana-Champaign] database.
It shows curves for the lift (Cl) and
moment (Cm) coefficients for various
angles of attack (AOA). The lift
coefficient (Cl) varies significantly,
steadily increasing until stall occurs. It is
common for an airfoil moment coefficient
(Cm) to be constant and negative.
“The actual lift (L) and moment (M) are
determined from the following well known
equations, where Q is the dynamic
pressure, rho (0.002378 slugs/ft3) is air
density, V is airspeed (ft/sec), S is the
wing area (ft2), and C the chord length (ft):
“Q = 0.5 rho V2
“L = Q * S * Cl
“M = Q * S * Cm * C
“The resultant of the lift force (L) is
generated through the aerodynamic center
(AC) of the airfoil, typically 25% of the
chord. The moment (M) is a rotational
force measured in foot-pounds or newtonmeters.
A wrench applies a moment on a
bolt; a motor generates a moment around a
shaft. A negative coefficient indicates a
nose-down direction, forcing the leading
edge of the wing down and the trailing
edge up.
“The relationship between pitching
moment and the total lift of an airfoil can
be confusing. As Figure 1 indicates, the Cl
and Cm coefficients are independent.
Consider a typical wing in a wind tunnel at
a constant airspeed. As the angle of attack
is increased, the lift will increase as
predicted by the equations. However, the
moment will remain constant, even when
the lift is zero. Also consider that the
moment does not change direction when
the lift coefficient becomes negative.
“Aircraft Moments
“Four forces affect the overall pitch of
the aircraft: the airfoil pitching moment
(Cm), the lift produced by the wing, the
lift force produced by the horizontal
stabilizer, and drag. Lift only affects pitch
when the CG is not located at the AC of
the wing. Likewise, drag produces a
moment when its center is either above or
below the CG. The moment produced by
drag will be ignored in this article.
“The horizontal stabilizer and wing lift
forces produce moments determined by
multiplying each with their respective
moment arms. Their moment arms are the
distance between the aircraft CG and AC
of the tail and wing respectively. The sum
of all three moments must balance (equal
zero) for the aircraft to maintain its pitch.
Otherwise, it will constantly rotate upward
or downward.
“Balanced pitch does not mean that the
pitch angle will remain unchanged. For
stability, there must be some mechanism to
maintain the pitch orientation of the
aircraft. This orientation may be affected
by turbulence or a change in airspeed. A
conventional approach to maintain stability
is to have the horizontal stabilizer generate
negative lift (a downward force). As the
airspeed increases, the tail lift increases
pushing the tail down and slowing the
aircraft.
“Figure 2 plots the three moment forces
vs. airspeed for a CG located at 30% of the
chord length (10”). The lift and its moment
are constant since it must balance the
weight (38 ounces) of the aircraft. The lift
moment produces a nose up force, and is
therefore positive.
“The airfoil moment (M) is dependent
on airspeed, wing area (900 sq. in.) and
chord length. Cm is negative as well as its
moment (M). It produces a nose down
force. As the airspeed increases it produces
a greater negative moment.
“The tail moment must balance (equal
but opposite) the sum of the lift and airfoil
moments. In this example, the horizontal
stabilizer always produces a nose up force.
Even though the horizontal stabilizer
produces a negative (downward) lift force,
the moment is positive (nose up).
“The aircraft becomes unstable in pitch
if the tail is required to produce positive
(upward) lift in order to balance the sum of
the airfoil and lift moments. This would be
indicated by a negative (nose down) tail
moment, and is most likely to occur at low
airspeeds.
“Low-Speed Instability
“Figure 3 plots the sum of the airfoil
and lift moments versus airspeed, for CG
ranging from 25 to 33% of the chord. For
all cases, the airfoil moment contribution
is the same as in Figure 2. As the CG is
moved more rearward the lift moment arm
and its moment increase.
“The lowermost curve is for the case
where the CG is at the AC of the wing and
the lift moment arm is zero. Therefore, the
lift moment is zero, and this curve is
purely the airfoil moment. The uppermost
curve is for the case where the CG is at its
most rearward position. At each CG
position, the lift moment is constant with
airspeed, and simply shifts the airfoil
moment curve upward the same amount at
all airspeeds.
102 MODEL AVIATION
07sig4.QXD 4/23/04 12:38 pm Page 102
“As the CG is moved rearward, it is
more likely to cause an unstable situation
at low speeds. As the 33% case shows, a
negative tail moment is required below an
airspeed of 20 feet per sec. A negative tail
moment requires a positive (upward) tail
lift, which no longer provides a pitch
correcting mechanism.
“Airspeed Sensitivity
“Figure 4 shows the corresponding tail
lift coefficients for the cases shown in
Figure 3. The lift coefficient is calculated
from the required tail moment. The plot
for the 33% case clearly shows that the lift
coefficient becomes positive at low
airspeeds, producing an unstable, noncorrecting
situation.
“When the CG is at the AC and the lift
moment is zero, the horizontal stabilizer
and wing pitching moment must balance
one another (the lowermost curve). Both
are equally affected by airspeed. There is
only one trim setting where they both
balance one another, and will do so at all
airspeeds.
“For all other curves in Figure 4, when
the lift moment is not zero, the aircraft
will be balanced at only one airspeed
depending on the tail lift coefficient. The
lift coefficient depends on the elevator
trim setting.
“The figure also shows that as the CG
is moved rearward there is a greater
change in lift coefficient per a change in
airspeed. This means that for a more
rearward CG, there will be a greater
corrective force for a smaller change in
airspeed. In other words, stable airspeed
will be more sensitive to elevator trim
setting.
“Summary
“1. The pitching moments of an aircraft
can be balanced with a horizontal stabilizer
that provides positive lift, but negative
(downward) lift, producing a positive
moment, is required to provide pitch
stability.
“2. Because the lift moment is constant
but the airfoil and horizontal stabilizer
moments are airspeed dependent, a negative
tail moment may be required at low
airspeeds. This is more likely to happen as
the CG is moved more rearward. The
aircraft can therefore be unstable at low
airspeeds, but stable at higher speeds.
“3. A more rearward CG results in a
corrective force more airspeed dependent,
making the stable airspeed more sensitive
to elevator trim setting.
“Adding ballast is more likely to cause
low-speed instability, but a ballasted
aircraft is flown faster. In flight camber
adjustments will also affect balance and
stability.
“This article came about after studying
tail lift forces and CG position. The
analysis helps me understand why a
seemingly stable aircraft can behave poorly
at low airspeeds. Unlike other technical
articles dealing with aircraft design, CG
position is something every pilot must
consider. I hope others will contribute
articles on this and other technical subjects.
My sincere thanks to Dave Register for his
review and discussion of this article.” MA
104 MODEL AVIATION
OVER 100 LITHIUM-ION
BATTERY PACKS TO
CHOOSE FROM
MULTI-VOLTAGE MODULES
POWER REGULATORS
CHARGERS
CLIP-ON-COCKPIT
TELEMETRY SYSTEM
R/C Power Solutions
“...Great Products...
Great Customer Service...”
2002 TOC CHAMPION
CHIP HYDE
FROM MICRO-FLYERS...
...TO GIANT SCALE...
...LAND, SEA AND AIR...
WE HAVE A POWER SYSTEM
THAT’S “LITE” FOR YOU!
Contact:
Skyborn Electronics
3405 Express Dr.
Garland, TX 75041
972-267-5099 Fax: 972-271-3529
www.rcpowerflite.com
[email protected]
J
Li
For m
Jac
Rad
The
07sig4.QXD 4/23/04 12:38 pm Page 104
Edition: Model Aviation - 2004/07
Page Numbers: 100,101,102,104
Mike Garton, 2733 NE 95th Ave., Ankeny IA 50021; E-mail: [email protected]
RADIO CONTROL SOARING
THIS MONTH I have a review of a new instructional moldmaking
DVD and a technical note submitted by Gregory Ciurpita
on the pitch stability of gliders at low speed.
Bill Haymaker made the new DVD, and it features Terry
Luckenbach’s method of making molded parts. In particular, Terry
shows how to make a Pretty Mantis glider fuselage. In the DVD,
Terry mentions that he has been making molded parts for
approximately 30 years; it shows. He goes into great detail about
the materials, methods, and the tricks he has developed.
I have often thought about making a molded fuselage for a
scratch-built glider, but the experienced fabricators have
consistently told me that it takes hundreds of hours to make a
single hardwood plug and an epoxy-fiberglass mold.
The unique thing about Terry’s method is that it takes just a
fraction of the normal time to create the plug and the mold. I
won’t give away his secrets, but the DVD convinced me to dive
into this technology for a current project. The information was
enabling.
My one complaint about the DVD is that Terry did not talk
about or demonstrate epoxy safety. The majority of modelers still
do not understand that all people are at risk for developing epoxy
allergies. Even if you have no other allergies and don’t react the
first 500 times you touch epoxy, eventually you will. It is
cumulative exposure that eventually triggers an allergy.
I can name three glider manufacturers who have had to stop
making epoxy parts within the last five years because they
developed epoxy allergies. It is essential to wear gloves when you
need to touch uncured epoxy. Disposable nitrile gloves cost
roughly $10 for 100 at any drug store.
In spite of my one gripe, the quality of the DVD is
extraordinarily high. It is 103 minutes long. I learned a great deal.
It would take an individual thousands of hours to develop a similar
process without the DVD. If you scratch-build, the information is
priceless. I put this DVD in my must-buy category.
Bill Haymaker also sells a great DVD on Phil Barnes’ methods
of making composite glider wings. You can reach Bill at
Bill Haymaker (East Berlin PA) has produced an excellent
instructional DVD about molding fiberglass parts.
Figure 1. Figure 2.
100 MODEL AVIATION
07sig4.QXD 4/23/04 12:37 pm Page 100
July 2004 101
Figure 3. Figure 4.
[email protected] or 107 Schofield Dr., East Berlin PA
17316.
What follows is technical information about glider stability by
Gregory Ciurpita. Greg wrote “Low-Speed Stability” for R/C
Soaring Digest (RCSD), which is the only American periodical
exclusively committed to RC Soaring. I obtained Greg’s and
RCSD’s permission to include it here.
I encourage glider fliers to subscribe to RCSD. It is inexpensive
and has regular contributions from Dave Register, Lee Murray,
Gordy Stahl, and Bill and Bunny Kuhlman, along with occasional
gems of information from Mark Drela. You can find out more
about RCSD at www.b2streamlines.com/RCSD.html or call (707)
578-7871.
“An aircraft can be stable at higher speeds but unstable at low
speeds. While a more rearward center of gravity (CG) may cause
instability, it not only reduces the lift force and induced drag
produced by the tail, but makes airspeed more sensitive to elevator
trim setting. Adding ballast may make an aircraft more unstable,
but a ballasted aircraft is normally flown faster. First a review of
an airfoil’s moment coefficient (Cm). Then, an analysis of an
Visit the MODEL AVIATION Digital Archives!
Featuring a searchable database of Model
Aviation issues and articles from 1975 to 2000.
This is by far one of the best
efforts AMA has made to
construct something that is for
every member.
—Marco Pinto
Peninsula Channel Commanders
San Francisco CA
“
” Find it at www.modelaircraft.org. On the main page, click
on the “Members Only” section, log in with your last name
and AMA number, then click on the “Visit the Digital
Archive” image.
07sig4.QXD 4/23/04 12:38 pm Page 101
aircraft’s pitching moments over a range of
CG positions.
“Airfoil Moment
“Figure 1 shows a typical airfoil
measurement from the UIUC [University
of Illinois at Urbana-Champaign] database.
It shows curves for the lift (Cl) and
moment (Cm) coefficients for various
angles of attack (AOA). The lift
coefficient (Cl) varies significantly,
steadily increasing until stall occurs. It is
common for an airfoil moment coefficient
(Cm) to be constant and negative.
“The actual lift (L) and moment (M) are
determined from the following well known
equations, where Q is the dynamic
pressure, rho (0.002378 slugs/ft3) is air
density, V is airspeed (ft/sec), S is the
wing area (ft2), and C the chord length (ft):
“Q = 0.5 rho V2
“L = Q * S * Cl
“M = Q * S * Cm * C
“The resultant of the lift force (L) is
generated through the aerodynamic center
(AC) of the airfoil, typically 25% of the
chord. The moment (M) is a rotational
force measured in foot-pounds or newtonmeters.
A wrench applies a moment on a
bolt; a motor generates a moment around a
shaft. A negative coefficient indicates a
nose-down direction, forcing the leading
edge of the wing down and the trailing
edge up.
“The relationship between pitching
moment and the total lift of an airfoil can
be confusing. As Figure 1 indicates, the Cl
and Cm coefficients are independent.
Consider a typical wing in a wind tunnel at
a constant airspeed. As the angle of attack
is increased, the lift will increase as
predicted by the equations. However, the
moment will remain constant, even when
the lift is zero. Also consider that the
moment does not change direction when
the lift coefficient becomes negative.
“Aircraft Moments
“Four forces affect the overall pitch of
the aircraft: the airfoil pitching moment
(Cm), the lift produced by the wing, the
lift force produced by the horizontal
stabilizer, and drag. Lift only affects pitch
when the CG is not located at the AC of
the wing. Likewise, drag produces a
moment when its center is either above or
below the CG. The moment produced by
drag will be ignored in this article.
“The horizontal stabilizer and wing lift
forces produce moments determined by
multiplying each with their respective
moment arms. Their moment arms are the
distance between the aircraft CG and AC
of the tail and wing respectively. The sum
of all three moments must balance (equal
zero) for the aircraft to maintain its pitch.
Otherwise, it will constantly rotate upward
or downward.
“Balanced pitch does not mean that the
pitch angle will remain unchanged. For
stability, there must be some mechanism to
maintain the pitch orientation of the
aircraft. This orientation may be affected
by turbulence or a change in airspeed. A
conventional approach to maintain stability
is to have the horizontal stabilizer generate
negative lift (a downward force). As the
airspeed increases, the tail lift increases
pushing the tail down and slowing the
aircraft.
“Figure 2 plots the three moment forces
vs. airspeed for a CG located at 30% of the
chord length (10”). The lift and its moment
are constant since it must balance the
weight (38 ounces) of the aircraft. The lift
moment produces a nose up force, and is
therefore positive.
“The airfoil moment (M) is dependent
on airspeed, wing area (900 sq. in.) and
chord length. Cm is negative as well as its
moment (M). It produces a nose down
force. As the airspeed increases it produces
a greater negative moment.
“The tail moment must balance (equal
but opposite) the sum of the lift and airfoil
moments. In this example, the horizontal
stabilizer always produces a nose up force.
Even though the horizontal stabilizer
produces a negative (downward) lift force,
the moment is positive (nose up).
“The aircraft becomes unstable in pitch
if the tail is required to produce positive
(upward) lift in order to balance the sum of
the airfoil and lift moments. This would be
indicated by a negative (nose down) tail
moment, and is most likely to occur at low
airspeeds.
“Low-Speed Instability
“Figure 3 plots the sum of the airfoil
and lift moments versus airspeed, for CG
ranging from 25 to 33% of the chord. For
all cases, the airfoil moment contribution
is the same as in Figure 2. As the CG is
moved more rearward the lift moment arm
and its moment increase.
“The lowermost curve is for the case
where the CG is at the AC of the wing and
the lift moment arm is zero. Therefore, the
lift moment is zero, and this curve is
purely the airfoil moment. The uppermost
curve is for the case where the CG is at its
most rearward position. At each CG
position, the lift moment is constant with
airspeed, and simply shifts the airfoil
moment curve upward the same amount at
all airspeeds.
102 MODEL AVIATION
07sig4.QXD 4/23/04 12:38 pm Page 102
“As the CG is moved rearward, it is
more likely to cause an unstable situation
at low speeds. As the 33% case shows, a
negative tail moment is required below an
airspeed of 20 feet per sec. A negative tail
moment requires a positive (upward) tail
lift, which no longer provides a pitch
correcting mechanism.
“Airspeed Sensitivity
“Figure 4 shows the corresponding tail
lift coefficients for the cases shown in
Figure 3. The lift coefficient is calculated
from the required tail moment. The plot
for the 33% case clearly shows that the lift
coefficient becomes positive at low
airspeeds, producing an unstable, noncorrecting
situation.
“When the CG is at the AC and the lift
moment is zero, the horizontal stabilizer
and wing pitching moment must balance
one another (the lowermost curve). Both
are equally affected by airspeed. There is
only one trim setting where they both
balance one another, and will do so at all
airspeeds.
“For all other curves in Figure 4, when
the lift moment is not zero, the aircraft
will be balanced at only one airspeed
depending on the tail lift coefficient. The
lift coefficient depends on the elevator
trim setting.
“The figure also shows that as the CG
is moved rearward there is a greater
change in lift coefficient per a change in
airspeed. This means that for a more
rearward CG, there will be a greater
corrective force for a smaller change in
airspeed. In other words, stable airspeed
will be more sensitive to elevator trim
setting.
“Summary
“1. The pitching moments of an aircraft
can be balanced with a horizontal stabilizer
that provides positive lift, but negative
(downward) lift, producing a positive
moment, is required to provide pitch
stability.
“2. Because the lift moment is constant
but the airfoil and horizontal stabilizer
moments are airspeed dependent, a negative
tail moment may be required at low
airspeeds. This is more likely to happen as
the CG is moved more rearward. The
aircraft can therefore be unstable at low
airspeeds, but stable at higher speeds.
“3. A more rearward CG results in a
corrective force more airspeed dependent,
making the stable airspeed more sensitive
to elevator trim setting.
“Adding ballast is more likely to cause
low-speed instability, but a ballasted
aircraft is flown faster. In flight camber
adjustments will also affect balance and
stability.
“This article came about after studying
tail lift forces and CG position. The
analysis helps me understand why a
seemingly stable aircraft can behave poorly
at low airspeeds. Unlike other technical
articles dealing with aircraft design, CG
position is something every pilot must
consider. I hope others will contribute
articles on this and other technical subjects.
My sincere thanks to Dave Register for his
review and discussion of this article.” MA
104 MODEL AVIATION
OVER 100 LITHIUM-ION
BATTERY PACKS TO
CHOOSE FROM
MULTI-VOLTAGE MODULES
POWER REGULATORS
CHARGERS
CLIP-ON-COCKPIT
TELEMETRY SYSTEM
R/C Power Solutions
“...Great Products...
Great Customer Service...”
2002 TOC CHAMPION
CHIP HYDE
FROM MICRO-FLYERS...
...TO GIANT SCALE...
...LAND, SEA AND AIR...
WE HAVE A POWER SYSTEM
THAT’S “LITE” FOR YOU!
Contact:
Skyborn Electronics
3405 Express Dr.
Garland, TX 75041
972-267-5099 Fax: 972-271-3529
www.rcpowerflite.com
[email protected]
J
Li
For m
Jac
Rad
The
07sig4.QXD 4/23/04 12:38 pm Page 104