How to: Install Retractable Landing Gear - 2012/08

I suspect most modelers have their first experience
with retractable landing gear in some form of sport or
aerobatic model. In my case it was probably with one
of the 1970s vintage Pattern airplanes such as a Kaos or
Dirty Birdy.
The installation was straightforward: mount the main
gear retract unit perpendicular to the centerline of the
model and parallel to the lower surface of the wing. The
nose gear (and this is the last time I’ll mention that item)
was usually mounted directly to the firewall.
Figure 1 is a 3-D CAD drawing (top, front, and sideview)
of how the main gear mechanism, strut, and wheel
look in the extended and retracted positions for this type
of installation. The mechanism is elegant in its simplicity,
and everything is at right angles to everything else. In
keeping with the concept that right angles are good, the
strut swings through 90º. This works out nicely for sport
models.
Things become slightly more complex when we move
on to an accurate Scale model. World War II fi ghters are
popular builds; most are tail-draggers, and many have the
gear arranged so the struts rake forward relative to the
wing-chord line when extended, and rake aft from the
span line when retracted (Figure 2). Add the dihedral
angle seen in the front view and it becomes diffi cult to
visualize just how to fi t the retract mechanism in the
wing.
strut angles that I measured from my
collection of drawings. Don’t take
these angles as exact. The drawings
were small and I used a cheap plastic
protractor.
Let’s plan the retract installation
for a hypothetical model with the
strut raked forward 10º in the side
view, raked aft 20º in the plans
view, and perpendicular to a fl at
center section of the wing. These are
reasonable values considering the
data in Table 1, and will highlight all
the required adjustments.
With the power of the CAD
program, you can see how the gear
installation will look after making
any changes. All drawings will be of
the left wing.
It is clear that you will have to
tilt and rotate something to get the
strut where it belongs. What happens
if you tilt the entire mechanism
10º forward (clockwise) in the side
view and rotate it 20º backward
(clockwise) in the plans view?
That will create 20º of toe-in on
the extended wheel, but you can
fi x that by twisting the strut 20º
counterclockwise in the trunion.
That gives the result shown in Figure 3.
This may actually be acceptable,
depending on the size of the tire
and the thickness of the wing.
Looking at the strut in the front
view, you can see that there might
be circumstances where the wheel
would not completely retract into
Main landing gear strut rake angles
airplane side view plans view at strut from vertical
P-51 Mustang 11° 8 6° 0°
Spitfire 11° 22° 7° 5°*
Ki-61 Tony 10° 0° 5° 0°
Typhoon 16° 18° 0° 0°
Fw 190 10° 0° 5° -10°**
Me 109 8° 16° 5° 16°*
SBD 0° 0° 0° -6°***
for popular World War II airplanes
Table 1 shows some examples of
strut angles that I measured from my
collection of drawings. Don’t take
these angles as exact. The drawings
were small and I used a cheap plastic
protractor.
Let’s plan the retract installation
for a hypothetical model with the
strut raked forward 10º in the side
view, raked aft 20º in the plans
view, and perpendicular to a fl at
center section of the wing. These are
reasonable values considering the
data in Table 1, and will highlight all
the required adjustments.
With the power of the CAD
program, you can see how the gear
installation will look after making
any changes. All drawings will be of
the left wing.
It is clear that you will have to
tilt and rotate something to get the
strut where it belongs. What happens
if you tilt the entire mechanism
10º forward (clockwise) in the side
view and rotate it 20º backward
(clockwise) in the plans view?
That will create 20º of toe-in on
the extended wheel, but you can
fi x that by twisting the strut 20º
counterclockwise in the trunion.
That gives the result shown in Figure 3.
This may actually be acceptable,
depending on the size of the tire
and the thickness of the wing.
Looking at the strut in the front
view, you can see that there might
be circumstances where the wheel
would not completely retract into
Main landing gear strut rake angles
airplane side view plans view at strut from vertical
P-51 Mustang 11° 8 6° 0°
Spitfi re 11° 22° 7° 5°*
Ki-61 Tony 10° 0° 5° 0°
Typhoon 16° 18° 0° 0°
Fw 190 10° 0° 5° -10°**
Me 109 8° 16° 5° 16°*
SBD 0° 0° 0° -6°***
for popular World War II airplanes
Table 1
*The Spitfi3re and Me 109 have the axle approximately 102º from the strut
**The Fw 190’s axle is parallel to the ground; i.e. 110º from the strut
***The SBD’s axle is 84º from the strut
Note: The forward rake is expressed relative to the airfoil
chord line. The wing’s angle of incidence will make the strut
appear more raked when compared to the fuselage.
the wing. In the side view, the wheel is
at quite an angle compared to the lower
surface of the wing.
Now think about adding a cover
door to the strut. It ought to be roughly
parallel to the tire, but with this setup
it will either be way out of alignment
with the lower wing skin in the retracted
position or angled to the slipstream
when the gear is extended.
The full-scale aircraft manufacturers
made numbers like these work so
shouldn’t you be able to mimic that? Yes
you can, with the following three-step
procedure.
First, add the rake forward angle
(extended) to the rake aft angle
(retracted) and divide by two. In our
example, [(10º + 20º) ÷ 2 = 15º], the
mechanism—actually the pivot pin is
the key item here—will be mounted in
the wing rotated 15º clockwise in both
the side and plans views.
The second step is to rotate the strut
relative to the pivot pin. Subtract the
rake aft angle (retracted) from the rake
forward angle (extended) and divide by
two. In the example, it is (10º - 20º) ÷ 2
= -5º
The negative sign means the strut is
rotated 5º counterclockwise in the side
view. In other words, you need to put
a “kink” in the strut. This kink may be
the hardest part of the installation to
implement, and I’ll offer some ideas on
how to do it. Figure 4 is an exploded
view that shows two ways of making the
kink.
The third step of the procedure will be to calculate the required retraction
angle. In the example, you can see it
should be greater than 90º. Or the
opposite can happen. When I built my
Ki-61 Tony, the retraction angle needed
to be less than the 90º built into the
mechanism. I was unable to fudge it and
got downgraded at every meet I entered
because the strut was not at the proper
angle with respect to the lower surface
of the wing.
I had to splay the strut outward,
otherwise the retracted wheel would
have popped through the upper wing
skin!
You can fi nd the required retraction
angle using solid geometry, trigonometry,
and an $11 scientifi c calculator. First,
calculate the distance between the
lower end of the strut in the extended
and retracted positions. Then plug this
number into the Law of Cosines to
calculate the retraction angle.
This is only a “fi rst approximation,”
because it does not consider the kink
angle. In the example, the correct angle
is approximately half a degree larger.
This is insignifi cant given the tolerances
in the building and the manufacture of
the gear mechanism. See the sidebar for
these calculations.
Commercial retracts are available
with retraction angles varying in 5º
increments. The sidebar calculation
determined that roughly 94º of
retraction angle was needed, so let’s
install a 95º unit.
In the previous example it was
necessary to rotate the strut in the socket
of the trunion to avoid a huge amount of
toe-in on the wheel. That must be done
again, but you have two ways to do this.
You can either rotate the strut on the
kink, or rotate the kink in the trunion.
The better way is to fi x the strut to
the kink and rotate this assembly in the
trunion. This will keep the strut vertical
in the front view.
Figure 5 shows the installation if you
use a 5º kink and rotate the mechanism
15º clockwise in both the side and plans
views. This is essentially what you want
to achieve. The only deviation is that the
strut is raked slightly too far in the plans
view. I’m reasonably sure this is because
of the 95º retraction angle where the
method’s geometry is based on a 90º
angle.
The installation may still not be
exactly as the full-scale gear. To match
the full-scale geometry you would
have to have the point where the strut
intersects the pivot pin at the same
relative location in the wing as on the
full-scale aircraft. This may not be
possible given the relationship of
the strut socket, pivot pin, and
height of the model’s retract
mechanism.
This could also make the
gear door geometry even
more challenging. I can only
suggest some fi nessing and
fi nagling to fi ne-tune the
installation to meet your
standards.
As mentioned, making
the kink in the strut may be
the biggest challenge in this project. On
a small model with 5/32- or 3/16-inch wire
gear, it is only necessary to bend the wire
that inserts into the trunion.
I did this on a Hurricane, but
unfortunately, the wire would bend
on rough landings and I would have to
remove it and adjust the bend angle. I suspect the wire lost its
temper when I soldered washers on the kink to set the length
of insertion into the trunion and strut.
The retract mechanism in my 86-inch span Ki-61 was set up
for a 1/2-inch diameter strut. The strut itself was hollow tubing.
I used some 1/2-inch aluminum bar
stock to make a fi tting such as the
one shown in Figure 4. First, I made a
fi xture (see Figure 6) from a piece of
1-inch hex stock.
I drilled a 1/2-inch hole in the face
of a short length of stock, but at a
5º angle. (The kink for Tony’s Ki-61
gear also worked out to be 5º, but 5º
forward.) A piece of the aluminum bar stock roughly 1 inch
long was inserted to half its length in the fi xture and held with
a set screw.
The fi xture was mounted in the three-jaw chuck of my
mini lathe and the exposed portion of the aluminum turned
down until it would just fi t inside the strut. The axis of this
necked-down portion is rotated 5º from the axis of the 1/2-inch
diameter section. When the parts were aligned as well as I
could get them, I drilled the strut, kink, and trunion for bolts
to hold them together.
I hope this technique will save you some frustration and
yield a more accurate model on your next scale build remove it and adjust the bend angle. I suspect the wire lost its
temper when I soldered washers on the kink to set the length
of insertion into the trunion and strut.
The retract mechanism in my 86-inch span Ki-61 was set up
for a 1/2-inch diameter strut. The strut itself was hollow tubing.
I used some 1/2-inch aluminum bar
stock to make a fi tting such as the
one shown in Figure 4. First, I made a
fi xture (see Figure 6) from a piece of
1-inch hex stock.
I drilled a 1/2-inch hole in the face
of a short length of stock, but at a
5º angle. (The kink for Tony’s Ki-61
gear also worked out to be 5º, but 5º
forward.) A piece of the aluminum bar stock roughly 1 inch
long was inserted to half its length in the fi xture and held with
a set screw.
The fi xture was mounted in the three-jaw chuck of my
mini lathe and the exposed portion of the aluminum turned
down until it would just fi t inside the strut. The axis of this
necked-down portion is rotated 5º from the axis of the 1/2-inch
diameter section. When the parts were aligned as well as I
could get them, I drilled the strut, kink, and trunion for bolts
to hold them together.
I hope this technique will save you some frustration and
yield a more accurate model on your next scale build. Set up a coordinate system as shown in the graphic on the
left, with the intersection of the strut and pivot pin at the
origin. Assume that the strut is 10 inches long. Call the rake
when forward extended angle “A.” The rake backward when
retracted will be angle “B.”
The (x,y,z) coordinates of the end of the extended strut are
(-10sinA, 0, -10cosA) and coordinates of the retracted strut are
(10sinB, 10cosB, 0.) Note: the example assumes no dihedral
in the wing center section. Make sure to include the effect of
your model’s dihedral angle when calculating the location of
the end of the strut.
The extended position is (-1.74, 0, -9.85) and retracted is
(3.42, 9.40, 0). Find the difference between the retracted and
extended x values: (3.42 - (-1.74)) = 5.16. Likewise for y values:
(9.40 - 0) = 9.40, and for z value: (0 - (-9.85)) = 9.85.
Square each of the differences:
5.162 = 26.63
9.402 = 88.36
9.852 = 97.02
Sum the squares:
26.63 + 88.36 + 97.02 = 212.01
Take the square root of this number:
Square root of 212.01 = 14.56
This is the distance the end of the strut moves when the gear is
retracted. And it is also “c” in the Law of Cosines: c2 = a2 + b2
-2ab(Cos C) where “c” is the desired retraction angle and “a”
and “b” are the length of the strut:
(14.56)2 = 100 + 100 -200(Cos C)
(212 - 200) ÷-200 = Cos C
-0.06 = Cos C
C = 93.4º

I suspect most modelers have their first experience
with retractable landing gear in some form of sport or
aerobatic model. In my case it was probably with one
of the 1970s vintage Pattern airplanes such as a Kaos or
Dirty Birdy.
The installation was straightforward: mount the main
gear retract unit perpendicular to the centerline of the
model and parallel to the lower surface of the wing. The
nose gear (and this is the last time I’ll mention that item)
was usually mounted directly to the firewall.
Figure 1 is a 3-D CAD drawing (top, front, and sideview)
of how the main gear mechanism, strut, and wheel
look in the extended and retracted positions for this type
of installation. The mechanism is elegant in its simplicity,
and everything is at right angles to everything else. In
keeping with the concept that right angles are good, the
strut swings through 90º. This works out nicely for sport
models.
Things become slightly more complex when we move
on to an accurate Scale model. World War II fi ghters are
popular builds; most are tail-draggers, and many have the
gear arranged so the struts rake forward relative to the
wing-chord line when extended, and rake aft from the
span line when retracted (Figure 2). Add the dihedral
angle seen in the front view and it becomes diffi cult to
visualize just how to fi t the retract mechanism in the
wing.
strut angles that I measured from my
collection of drawings. Don’t take
these angles as exact. The drawings
were small and I used a cheap plastic
protractor.
Let’s plan the retract installation
for a hypothetical model with the
strut raked forward 10º in the side
view, raked aft 20º in the plans
view, and perpendicular to a fl at
center section of the wing. These are
reasonable values considering the
data in Table 1, and will highlight all
the required adjustments.
With the power of the CAD
program, you can see how the gear
installation will look after making
any changes. All drawings will be of
the left wing.
It is clear that you will have to
tilt and rotate something to get the
strut where it belongs. What happens
if you tilt the entire mechanism
10º forward (clockwise) in the side
view and rotate it 20º backward
(clockwise) in the plans view?
That will create 20º of toe-in on
the extended wheel, but you can
fi x that by twisting the strut 20º
counterclockwise in the trunion.
That gives the result shown in Figure 3.
This may actually be acceptable,
depending on the size of the tire
and the thickness of the wing.
Looking at the strut in the front
view, you can see that there might
be circumstances where the wheel
would not completely retract into
Main landing gear strut rake angles
airplane side view plans view at strut from vertical
P-51 Mustang 11° 8 6° 0°
Spitfire 11° 22° 7° 5°*
Ki-61 Tony 10° 0° 5° 0°
Typhoon 16° 18° 0° 0°
Fw 190 10° 0° 5° -10°**
Me 109 8° 16° 5° 16°*
SBD 0° 0° 0° -6°***
for popular World War II airplanes
Table 1 shows some examples of
strut angles that I measured from my
collection of drawings. Don’t take
these angles as exact. The drawings
were small and I used a cheap plastic
protractor.
Let’s plan the retract installation
for a hypothetical model with the
strut raked forward 10º in the side
view, raked aft 20º in the plans
view, and perpendicular to a fl at
center section of the wing. These are
reasonable values considering the
data in Table 1, and will highlight all
the required adjustments.
With the power of the CAD
program, you can see how the gear
installation will look after making
any changes. All drawings will be of
the left wing.
It is clear that you will have to
tilt and rotate something to get the
strut where it belongs. What happens
if you tilt the entire mechanism
10º forward (clockwise) in the side
view and rotate it 20º backward
(clockwise) in the plans view?
That will create 20º of toe-in on
the extended wheel, but you can
fi x that by twisting the strut 20º
counterclockwise in the trunion.
That gives the result shown in Figure 3.
This may actually be acceptable,
depending on the size of the tire
and the thickness of the wing.
Looking at the strut in the front
view, you can see that there might
be circumstances where the wheel
would not completely retract into
Main landing gear strut rake angles
airplane side view plans view at strut from vertical
P-51 Mustang 11° 8 6° 0°
Spitfi re 11° 22° 7° 5°*
Ki-61 Tony 10° 0° 5° 0°
Typhoon 16° 18° 0° 0°
Fw 190 10° 0° 5° -10°**
Me 109 8° 16° 5° 16°*
SBD 0° 0° 0° -6°***
for popular World War II airplanes
Table 1
*The Spitfi3re and Me 109 have the axle approximately 102º from the strut
**The Fw 190’s axle is parallel to the ground; i.e. 110º from the strut
***The SBD’s axle is 84º from the strut
Note: The forward rake is expressed relative to the airfoil
chord line. The wing’s angle of incidence will make the strut
appear more raked when compared to the fuselage.
the wing. In the side view, the wheel is
at quite an angle compared to the lower
surface of the wing.
Now think about adding a cover
door to the strut. It ought to be roughly
parallel to the tire, but with this setup
it will either be way out of alignment
with the lower wing skin in the retracted
position or angled to the slipstream
when the gear is extended.
The full-scale aircraft manufacturers
made numbers like these work so
shouldn’t you be able to mimic that? Yes
you can, with the following three-step
procedure.
First, add the rake forward angle
(extended) to the rake aft angle
(retracted) and divide by two. In our
example, [(10º + 20º) ÷ 2 = 15º], the
mechanism—actually the pivot pin is
the key item here—will be mounted in
the wing rotated 15º clockwise in both
the side and plans views.
The second step is to rotate the strut
relative to the pivot pin. Subtract the
rake aft angle (retracted) from the rake
forward angle (extended) and divide by
two. In the example, it is (10º - 20º) ÷ 2
= -5º
The negative sign means the strut is
rotated 5º counterclockwise in the side
view. In other words, you need to put
a “kink” in the strut. This kink may be
the hardest part of the installation to
implement, and I’ll offer some ideas on
how to do it. Figure 4 is an exploded
view that shows two ways of making the
kink.
The third step of the procedure will be to calculate the required retraction
angle. In the example, you can see it
should be greater than 90º. Or the
opposite can happen. When I built my
Ki-61 Tony, the retraction angle needed
to be less than the 90º built into the
mechanism. I was unable to fudge it and
got downgraded at every meet I entered
because the strut was not at the proper
angle with respect to the lower surface
of the wing.
I had to splay the strut outward,
otherwise the retracted wheel would
have popped through the upper wing
skin!
You can fi nd the required retraction
angle using solid geometry, trigonometry,
and an $11 scientifi c calculator. First,
calculate the distance between the
lower end of the strut in the extended
and retracted positions. Then plug this
number into the Law of Cosines to
calculate the retraction angle.
This is only a “fi rst approximation,”
because it does not consider the kink
angle. In the example, the correct angle
is approximately half a degree larger.
This is insignifi cant given the tolerances
in the building and the manufacture of
the gear mechanism. See the sidebar for
these calculations.
Commercial retracts are available
with retraction angles varying in 5º
increments. The sidebar calculation
determined that roughly 94º of
retraction angle was needed, so let’s
install a 95º unit.
In the previous example it was
necessary to rotate the strut in the socket
of the trunion to avoid a huge amount of
toe-in on the wheel. That must be done
again, but you have two ways to do this.
You can either rotate the strut on the
kink, or rotate the kink in the trunion.
The better way is to fi x the strut to
the kink and rotate this assembly in the
trunion. This will keep the strut vertical
in the front view.
Figure 5 shows the installation if you
use a 5º kink and rotate the mechanism
15º clockwise in both the side and plans
views. This is essentially what you want
to achieve. The only deviation is that the
strut is raked slightly too far in the plans
view. I’m reasonably sure this is because
of the 95º retraction angle where the
method’s geometry is based on a 90º
angle.
The installation may still not be
exactly as the full-scale gear. To match
the full-scale geometry you would
have to have the point where the strut
intersects the pivot pin at the same
relative location in the wing as on the
full-scale aircraft. This may not be
possible given the relationship of
the strut socket, pivot pin, and
height of the model’s retract
mechanism.
This could also make the
gear door geometry even
more challenging. I can only
suggest some fi nessing and
fi nagling to fi ne-tune the
installation to meet your
standards.
As mentioned, making
the kink in the strut may be
the biggest challenge in this project. On
a small model with 5/32- or 3/16-inch wire
gear, it is only necessary to bend the wire
that inserts into the trunion.
I did this on a Hurricane, but
unfortunately, the wire would bend
on rough landings and I would have to
remove it and adjust the bend angle. I suspect the wire lost its
temper when I soldered washers on the kink to set the length
of insertion into the trunion and strut.
The retract mechanism in my 86-inch span Ki-61 was set up
for a 1/2-inch diameter strut. The strut itself was hollow tubing.
I used some 1/2-inch aluminum bar
stock to make a fi tting such as the
one shown in Figure 4. First, I made a
fi xture (see Figure 6) from a piece of
1-inch hex stock.
I drilled a 1/2-inch hole in the face
of a short length of stock, but at a
5º angle. (The kink for Tony’s Ki-61
gear also worked out to be 5º, but 5º
forward.) A piece of the aluminum bar stock roughly 1 inch
long was inserted to half its length in the fi xture and held with
a set screw.
The fi xture was mounted in the three-jaw chuck of my
mini lathe and the exposed portion of the aluminum turned
down until it would just fi t inside the strut. The axis of this
necked-down portion is rotated 5º from the axis of the 1/2-inch
diameter section. When the parts were aligned as well as I
could get them, I drilled the strut, kink, and trunion for bolts
to hold them together.
I hope this technique will save you some frustration and
yield a more accurate model on your next scale build remove it and adjust the bend angle. I suspect the wire lost its
temper when I soldered washers on the kink to set the length
of insertion into the trunion and strut.
The retract mechanism in my 86-inch span Ki-61 was set up
for a 1/2-inch diameter strut. The strut itself was hollow tubing.
I used some 1/2-inch aluminum bar
stock to make a fi tting such as the
one shown in Figure 4. First, I made a
fi xture (see Figure 6) from a piece of
1-inch hex stock.
I drilled a 1/2-inch hole in the face
of a short length of stock, but at a
5º angle. (The kink for Tony’s Ki-61
gear also worked out to be 5º, but 5º
forward.) A piece of the aluminum bar stock roughly 1 inch
long was inserted to half its length in the fi xture and held with
a set screw.
The fi xture was mounted in the three-jaw chuck of my
mini lathe and the exposed portion of the aluminum turned
down until it would just fi t inside the strut. The axis of this
necked-down portion is rotated 5º from the axis of the 1/2-inch
diameter section. When the parts were aligned as well as I
could get them, I drilled the strut, kink, and trunion for bolts
to hold them together.
I hope this technique will save you some frustration and
yield a more accurate model on your next scale build. Set up a coordinate system as shown in the graphic on the
left, with the intersection of the strut and pivot pin at the
origin. Assume that the strut is 10 inches long. Call the rake
when forward extended angle “A.” The rake backward when
retracted will be angle “B.”
The (x,y,z) coordinates of the end of the extended strut are
(-10sinA, 0, -10cosA) and coordinates of the retracted strut are
(10sinB, 10cosB, 0.) Note: the example assumes no dihedral
in the wing center section. Make sure to include the effect of
your model’s dihedral angle when calculating the location of
the end of the strut.
The extended position is (-1.74, 0, -9.85) and retracted is
(3.42, 9.40, 0). Find the difference between the retracted and
extended x values: (3.42 - (-1.74)) = 5.16. Likewise for y values:
(9.40 - 0) = 9.40, and for z value: (0 - (-9.85)) = 9.85.
Square each of the differences:
5.162 = 26.63
9.402 = 88.36
9.852 = 97.02
Sum the squares:
26.63 + 88.36 + 97.02 = 212.01
Take the square root of this number:
Square root of 212.01 = 14.56
This is the distance the end of the strut moves when the gear is
retracted. And it is also “c” in the Law of Cosines: c2 = a2 + b2
-2ab(Cos C) where “c” is the desired retraction angle and “a”
and “b” are the length of the strut:
(14.56)2 = 100 + 100 -200(Cos C)
(212 - 200) ÷-200 = Cos C
-0.06 = Cos C
C = 93.4º