February 2011 29
This Valkyrie is mission status
ready for threechannel
electric fun
The XB-70 handles wel l—but
orientation can be difficult in a
banked turn when the thin wing is
edge on, so don’t fly too far out.
THE NORTH AMERICAN Aviation XB-
70 Valkyrie is the coolest-looking bomber
ever designed. In the mid-1970s,
Competition Models, a little-known
company in Long Beach, California, made a
kit of the aircraft; amazingly it was
manufactured for FF. The profile XB-70
was made entirely from sheet balsa and used
a .020-.049 glow engine.
Bob Linn designed the model, and Curt
Stevens drew the plans in January 1961. I
don’t know if Bob Linn is still with us, but I
admire him for being brave enough to make
the Valkyrie into an FF airplane. Although
it intrigued me, I didn’t see a way to convert
it to RC; the 1970s equipment was far too
large and heavy.
Fast-forward to 2007, when I received a
phone call from longtime flying buddy and
modeler extraordinaire, John Chapman. He
was on a business trip and, as usual, was
poking around in a hobby shop looking for
anything interesting.
He found a stash of old kits, and among
them was a Competition Models XB-70.
John likes the Valkyrie at least as much as I
do, so he bought it for less than $5.
While reviewing the plans, I realized that
devising an RC version of the aircraft would
me to hide the RC gear, battery, and ESC.
The finished XB-70 weighs 12.6 ounces
ready to fly, giving it a reasonable 8.1-
ounce-per-square-foot wing loading. Using
an AstroFlight brushless 010 motor and a 3S
700 mAh Li-Poly battery provides four to
five minutes of airtime with decent
performance.
Loops are often challenging for a deltawing
aircraft, because of the large increase
in drag at high angles of attack, but,
surprisingly, my Valkyrie will loop with a
bit of a dive. Perhaps the canard helps with
loops. Rolls, as expected, are blazing.
My design is unsuitable for novices, but
you should be able to fly it with no
problems if you can handle an aileron model
well. Little material is required to build this
aircraft and it’s easy to construct, so let’s get
started.
CONSTRUCTION
Study the plans to make sure you
understand how all the bits go together. All
of the balsa is medium density. All gluing is
done with thin CA unless otherwise stated.
Cut the fins from 1/8 C-grain sheet balsa.
Ensure that the grain is oriented vertically.
Round the LEs and TEs.
be possible with the modern electric power
systems and small equipment. I borrowed
the plans and ran to the copy shop.
I learned that the Competition Models
kit had been designed to duplicate an early
concept for the XB-70 rather than the one
that was built and flown. I also found that it
featured several departures from scale to
allow it to fly as an FF model, such as a
much larger canard with dihedral.
The wing was undercambered with
reflex and made from extremely thin 1/16
balsa. The large underbody of the full-scale
XB-70, which contained the jet engines,
was not reproduced.
I decided that I needed to start with a
clean-sheet design based on the
Competition Models iteration. My model is
roughly the same size, because that allowed
the fuselage to be built using 36-inch balsa
stock.
I reshaped and resized the fuselage,
wing, canard, and fins to match the scale
three-view, and I made the wing flat from
1/8 balsa. The canard (with no dihedral) is
1/16 plywood, because I figured that it would
take a beating on landing. I used 1/32
plywood fuselage doublers and added a
complete scale-sized underbody to allow
Park Flyer
XB-70 by Al Clark
02sig1.QXD_00MSTRPG.QXD 12/20/10 9:42 AM Page 29
30 MODEL AVIATION
Static photos by the author Flight photos by Mark McCutcheon
Study the plans to make sure that you understand how all the bits go together. All balsa is medium density. Gluing is done with thin CA
unless stated otherwise.
Glue the two rear underbody sides to the aft underbody
bottom, making sure that the pushrod exit slots are in the
correct locations.
Draw a centerline down the bottom of the wing. Carefully align
and glue the 1/4 sheet lower fuselage on the centerline, making
sure that the aft end is located as shown on the plans.
The wing is made from 3-inch-wide pieces of 1/8 C-grain balsa, with
the grain running spanwise. Ensure that the edges of all pieces are
straight before gluing them together.
Again, ensure that the edges of all
pieces are straight before adhering
them. Don’t forget the 1/8 square spruce
edges on the front and the 1/8 lightplywood
insert at the rear.
Cut the fuselage from a straight piece of 1/4 sheet balsa,
with the grain running lengthwise. Cut the slot for the
canard and check to make sure it fits.
02sig1.QXD_00MSTRPG.QXD 12/20/10 9:46 AM Page 30
February 2011 31
Left: Mark the positions of the servos,
ESC, battery, and receiver. Apply a very
thin layer of 5-minute epoxy to the balsa
at each position to make the servo tape
and Velcro squares stick better.
Left: Cut the four underbody side pieces from 1/8 balsa, with the
grain running lengthwise. Glue a 1/8 square spruce piece to the
front edge of each forward underbody side. Bevel the spruce
piece at a 45° angle toward the inside of the underbody.
Below: Try for a total gap of 1/64 inch on the hatch. When
satisfied with the alignment, glue the assembly to the lower
fuselage and wing bottom.
Install the 1/8 x 1/4 balsa stiffeners and the
1/32 plywood tabs to the inside of the
hatch, taking care to leave sufficient
room for the thickness of the underbody
sides. Magnets secure the hatch.
This is also a good time to make the two
1/8 triangular-balsa reinforcing pieces. This
is easy to do by sanding the edge (along the
grain) of a sheet of 1/8 balsa to a 45° bevel,
and then cutting the beveled edge from the
sheet.
Cut the canard from 1/16 plywood with
the grain running spanwise. Round the LE
and TE. Do not substitute balsa for the
canard. The extra strength is needed for
damage resistance, and the extra weight
helps balance the model.
The wing is made from 3-inch-wide
pieces of 1/8 C-grain balsa, with the grain
running spanwise. Make sure that the edges
of each piece are straight before you glue
them together. I used CA, but you can also
use Titebond if you don’t mind the drying
time and want to make the sanding slightly
easier.
Add the 1/8 square spruce LE. It prevents
dings from forming during landings.
Round the LEs and TEs, and cut the
elevons free. Bevel the front edge of each
elevon per the plans. After covering, the
elevons will be hinged with covering
material.
Drill a hole in each elevon for the Du-
Bro Micro Control Horn. Make sure to
locate the hole per the plans.
Cut the fuselage from a straight piece of
1/4 sheet balsa, with the grain running
lengthwise. Make a slot for the canard and
check to ensure that it fits.
Fashion two doublers from 1/32 plywood,
making them slightly oversize all around.
Make the canard slots in the doublers,
Check the wing alignment on the
fuselage, making sure that the
fuselage is perpendicular to the wing
top surface. When satisfied, glue the
wing and underbody assembly to the
fuselage.
02sig1.QXD_00MSTRPG.QXD 12/20/10 9:48 AM Page 31
making sure that the
location matches the slot on
the fuselage.
Adhere the doublers to the
fuselage using thick CA.
When it has cured, sand the
edges of the doublers flush
with the fuselage.
Cut the 1/4 sheet balsa for
the lower fuselage, with the
grain running lengthwise.
Use thick CA to glue two
pieces of 1/8 square spruce,
side by side, to the front
edge.
Bevel the spruce pieces as
shown on the plans top view.
Punch the 3/8-inch-diameter
hole for the battery lead, as
the plans show.
Cut the four underbody side pieces from
1/8 balsa, with the grain running lengthwise.
Use thick CA to adhere a 1/8 square spruce
piece to the front edge of each forward
underbody side.
Bevel that square spruce section at a 45°
angle toward the inside of the underbody.
Refer to the plans top view to see the
correct angle.
Slice the pushrod exit slot into each of
the rear underbody sides, locating them
exactly as shown on the plans. These slots
should be slightly wider than 1/32 inch to
provide good pushrod support.
The bottom of the underbody is built up
in the same manner as the wing, using 3-
inch-wide pieces of 1/8 C-grain balsa with
the grain running spanwise. Make sure that
the edges of each piece are straight before
gluing them together. Don’t forget the 1/8
square spruce edges on the front and the 1/8
light-plywood insert at the rear.
After trimming and sanding the bottom
to the outline shown on the plans, cut the
hatch portion loose. You now have three
32 MODEL AVIATION
The author recommends launching the model using a
small bungee. It is difficult to launch by hand, because the
elevons lose their effectiveness at slow speeds and high
angle of attack. With a bit of practice, the Valkyrie will set
down softly in a nose-high attitude at a low forward speed.
Right: The author used UltraCote on his prototype and
recommends it because it has no tendency to wrinkle over time
on sheeted surfaces, as other covering brands might.
Below: The direction of motor rotation should be clockwise,
viewed from the rear of the XB-70. Install the AstroFlight 010
using a bit of Loctite on the motor screws.
pieces that make the bottom of the
underbody.
Draw a centerline down the bottom of
the wing. Carefully align and glue the 1/4
sheet lower fuselage on the centerline,
ensuring that the aft end is located as the
plans show. Glue the forward piece of the
underbody bottom to the 1/4 sheet lower
fuselage, positioned even with the front.
Adhere the two rear underbody sides to
the aft underbody bottom, making sure that
the pushrod exit slots are located correctly.
Turn this assembly over and carefully
center it on the 1/4 sheet lower fuselage,
using the hatch as a guide for fore and aft
position.
Try for a total gap of 1/64 inch on the
hatch. When you are satisfied with the
alignment, glue the assembly to the lower
fuselage and the wing bottom.
Now is a good time to mark the
positions of the servos, ESC, battery, and
receiver; apply a very thin layer of 5-
minute epoxy to the balsa at each position.
That will make the servo tape and Velcro
squares stick much better when you install
them later. Position the two forward
underbody sides and glue them in place.
Install the 1/8 x 1/4 balsa stiffeners and
the 1/32 plywood tabs to the inside of the
hatch, taking care to leave sufficient room
for the thickness of the underbody sides.
Drill a 1/16-inch-diameter hole at the rear of
the hatch, slightly off center, as shown on
the plans. This will be used later, to remove
the hatch with a paper clip.
Cut two 3/32 plywood hatch magnet
holders, and drill a 3/16-inch-diameter hole
into each for the RadioShack rare earth
magnets. Glue one magnet into each hole,
flush with the surface.
Adhere the two magnet holders to the
underbody bottom, locating them per the
plans. Mark a center point on the inside of
the hatch at each magnet location.
Use a sharpened 3/16-inch-diameter brass
tube to drill a hole partway into the hatch at
each location. You need only make the
hatch holes deep enough to mount the
magnets flush. If you end up drilling all the
way through, it’s no problem; you can plug
the holes after you install the magnets.
02sig1.QXD_00MSTRPG.QXD 12/20/10 9:50 AM Page 32
and sand the entire XB-70 with 220-grit
paper, followed by 400 grit.
Vacuum off the dust, and you are ready for
covering.
Covering and Decals: I used Hangar 9
UltraCote on my prototype. I recommend it
because it does not tend to wrinkle in time on
sheeted surfaces, as do some other covering
brands. Make sure that you completely iron
the covering onto all of the wood; doing so
will give additional
strength to the airframe,
especially the wing.
The black areas on the
underbody and forward fuselage
are standard UltraCote that I
masked and sprayed with Testors
ModelMaster Dullcote flat paint. I simulated
the windows using Sky Blue UltraCote.
The correct window outlines are shown on
the plans. You can make a copy of the window
outlines, glue them to UltraCote with rubber
cement, and cut out the pieces. Then remove
the paper pattern and apply the windows. I
also cut the red stripes between the fins from
UltraCote, but striping tape would work.
After everything is covered, hinge the
elevons using 1/2-inch-wide strips of covering
material on the top of the hinge line and one
1/4 x 1/2 square strip of covering on the bottom
at each end of both elevons.
Before gluing in the magnets, check
their orientation to ensure that they will be
attracted to the other magnets rather than
repelled. When you confirm this, glue one
magnet into each hatch hole, making sure
that they are flush.
Put the hatch in place and round the
outer edges of the underbody, referring to
cross-section view A-A on the plans.
Assembly: Trial-fit the canard into the
fuselage, adjusting the slot if necessary to
get it to fit. Align the canard and apply
glue.
Check the wing alignment on the
fuselage, making sure that the fuselage is
perpendicular to the wing top surface.
When you’re satisfied, glue the wing and
underbody assembly to the fuselage.
Drill the firewall holes per the plans
and check the fit of the motor. (I used an
AstroFlight 010.) Check the fit of the
firewall to the wing, lower fuselage, and
underbody bottom, ensuring that it contacts
all three surfaces. You don’t want down,
up, left, or right thrust built in. Use thick
CA to adhere the firewall in place.
Mark the positions of the two fins on
top of the wing and glue them in place,
making sure that they are perpendicular to
the wing top surface and have no left or
right offset. Add the two 1/8 triangularbalsa
reinforcing pieces.
This completes assembly. Fill any dings
with spackle and sand smooth. Slightly
round the edges of the fuselage, as shown
on cross-section view A-A on the plans,
The author (shown) encourages builders to try some
aerobatics with the XB-70. Rolls are easy with full
aileron, and add a touch of down-elevator as it rolls
through inverted.
Once you have flown this aircraft for a
while and feel comfortable, it will fly
around fine at reduced throttle. But pay
attention not to get too slow and behind
the power curve in the turns.
Type: RC electric profile scale
Skill level: Beginner builder; intermediate pilot
Wingspan: 20.5 inches
Wing area: 224 square inches
Wing loading: 8.1 ounces/square foot
Length: 56.5 inches
Weight: 12.6 ounces
Power: AstroFlight 010 brushless motor; 9-
amp ESC; 11.1-volt, 700 mAh Li-Poly battery
Construction: Balsa sheet, bass, plywood
Finish: UltraCote
Radio: Two Hitec HS-55 servos, microreceiver
February 2011 33
XB-70 Park Flyer
Specifications
02sig2_00MSTRPG.QXD 12/20/10 10:04 AM Page 33
34 MODEL AVIATION
I made all of the markings using Testors
ink-jet decal paper and my ink-jet printer. I
drew the decals using a combination of
AutoCAD and PowerPoint.
After printing the images onto the decal
paper, I sprayed them with three coats of
Krylon Crystal Clear. Then I cut them out and
applied them the way I would normal waterslide
decals.
I have duplicated all of the markings on
the plans, so you should be able to scan them
and make your own decals.
Equipment Installation: Trim the pegs on
the Du-Bro Micro Control Horns so that they
protrude slightly from the tops of the elevons,
and glue them to the elevons. Make sure that
they are angled to align with the pushrods.
Fabricate the two pushrods from 1/32-inchdiameter
music wire. Make a Z-bend at one
end and an L-bend at the other.
Try to make the length between the bends
as close as possible to what the plans show. If
you don’t like using pliers to fashion your Zbends,
Radical RC sells a nifty set of Z-bend
pliers for 1/32-inch-diameter music wire.
Plug the servos into the receiver and center
the arms. Make sure that the arms are oriented
on the correct side of each servo.
Clean the servo cases with alcohol and
apply 1/16-inch-thick servo mounting tape
to each. Attach a pushrod to a servo and
control horn, position the servo so that the
elevon is neutral, and stick the servo to its
mounting area.
Repeat this for the other pushrod and
servo. Each pushrod is secured to the
control horns using a Du-Bro Micro EZ
Link.
Check the length of the ESC wires (I
used a Castle Creations Thunderbird-9) to
see if they will reach the motor when
Full-Size Plans Available—see page 171
02sig2_00MSTRPG.QXD 12/20/10 10:15 AM Page 34
installed. You will likely have to add length
to the ESC wires.
Solder the wires to the motor, plug the
ESC into the receiver, and check the direction
of rotation of the motor. It should turn
clockwise when viewed from the rear of the
XB-70.
Install the motor using a bit of Loctite on
the motor screws. Install the ESC using
Velcro self-adhesive squares. Plug the servos
into the receiver and mount the receiver using
Velcro self-adhesive squares.
Trim an APC 6 x 3 propeller to 5.5 inches
in diameter, balance it, and install it on the
motor. Make sure that the front of the blades
are facing forward—not aft!
Use thick CA to adhere two pieces of
Velcro (part hook and part loop) to the bottom
of the wing where the battery is positioned.
These Velcro pieces will hold the battery in
place.
Glue a piece of 1/4 square balsa at the front
and rear of the battery pack, to keep it from
shifting fore and aft. Run the battery lead
through the hole in the lower fuselage so it
can be connected to the ESC.
Drill a small hole in the bottom of the
forward fuselage, and screw the “L” hook
(from a home-improvement store) in the
position shown on the plans. Remove the “L”
hook, apply 5-minute epoxy, and reinstall.
Install the hatch cover.
Check the balance point. My prototype
balanced spot on, but you might need to add a
small amount of weight to the nose or the tail.
If nose weight is required, use a Dremel
tool or drill to make a small pocket in the
forward fuselage (from the bottom) and glue
the weight into the pocket. Then cover with a
small piece of covering material. If tail weight
is required, glue it to the inside of the
underbody at the rear.
Set the elevon throws per plans note #6.
The elevons have no reflex at neutral; they are
set to 0°. Set the exponential for elevator and
aileron near 20%.
These are good starting points; you can
adjust the settings later to suit your taste.
Make sure that the elevons move in the proper
directions for both elevator and aileron
commands.
Flying: I wish I could tell you that my
Valkyrie flew directly off of the board, but it
didn’t; initial flights were thrilling. When I
commanded a turn, the model initially rolled
in the opposite direction and then came back
for an erratic turn. At high angles of attack it
went out of control in yaw and could not be
recovered.
All of this ended in a crash into a pond
near the edge of the park at which I fly. After
waiting a half-hour for the XB-70 to drift to
shore, I took it back to the shop and dried it
out. The ESC was shot, but the rest of the gear
survived. Amazingly, it dried out and looked
as good as new.
After pondering the ill handling for
quite awhile, I surmised that the profile
fuselage was the culprit. The thin forward
fuselage was acting as fin area ahead of the
balance point. This was too much for the
scale-sized fins to handle. The model was
essentially underfinned, causing the erratic
behavior during the first test flights.
I scaled up the fin areas by 50%, keeping
the same proportions. It is almost
unnoticeable unless you compare with the
factory three-view. Larger fins cured the
problem, and the aircraft now handles
normally. Although the construction photos
show smaller fins, the flight photos and plans
depict the bigger ones.
I recommend launching the XB-70 using a
small bungee. It is difficult to hand launch,
because the elevons lose their effectiveness at
slow speeds and high angles of attack, causing
loss of control. This airplane makes an
excellent lawn dart with its pointy nose, and I
know from experience.
I made my bungee from part of an old
mini hi-start that I had, but anything that will
give 5-6 pounds of pull will work. I used
yellow surgical tubing that measures 7/32 inch
OD, 1/8 inch ID, and 25 feet long. Attached to
that is 25 feet of braided nylon line, 30- or 40-
pound test, with a small ring on the end.
Stretch the bungee to get 5-6 pounds of pull.
Launching the model from some type of
small, flat surface works well; use a card
table, cardboard box, or similar. Make sure
that the launching surface is angled up
approximately 20° or the Valkyrie won’t clear
the grass before gaining flying speed. I use a
24-inch-high cardboard box with a smaller
box stuffed under the front edge to get the 20°
angle.
Launch the model with power off. As soon
February 2011 35
02sig2_00MSTRPG.QXD 12/20/10 10:30 AM Page 35
36 MODEL AVIATION
as you release it, go to full throttle and
hold close to one-third up-elevator. Be
ready for a quick pitch up when the speed
increases and the elevons become
effective, after which you can go to neutral
elevator.
Don’t try to make the aircraft climb too
steeply; a shallow climb works best. The
little AstroFlight 010 has good power, but
this is no 3-D model. If you allow the nose
to get too high, the drag will increase
significantly, causing the XB-70 to lose
speed and “get behind the power curve.”
When that happens, the elevons become
ineffective and the model will roll to the
right, causing it to fall off into a spiral.
You can recover from that by letting the
airplane dive to gain speed, but it does
require some altitude so you don’t want to
let that happen right after launch.
Attain a comfortable altitude following
launch, and set the trims for level flight.
The XB-70 handles well, but orientation
can be difficult in a banked turn when the
thin wing is edge on, so don’t fly too far
out on the first flights. Have someone let
you know when 4 minutes has passed, so
you can set up the landing approach while
there is still battery power.
The aircraft will fly dead-stick, but it
comes down fast and doesn’t have much
flare out without power, so it is better to
avoid running out of power. Fly a normal
landing pattern and throttle back some once
you are on the final approach leg. Adjust
power to establish a smooth descent.
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When the XB-70 is close to the ground,
throttle back and add up-elevator for a
nose-high flare. With a bit of practice it
will set down softly in a nose-high attitude
with a low forward speed.
Remember to cut power immediately
before touchdown so you don’t break the
propeller. I have made many flights so far
and am still on the original propeller.
Once you have flown the model for a
while and feel comfortable with it, you
will be able to fly it around at reduced
throttle. But pay attention in the turns; do
not let the airplane get too slow and get
behind the power curve. Try some
aerobatics too.
Rolls are easy. Merely apply full
aileron and add a touch of down-elevator
as the aircraft rotates through the inverted
part of the roll.
A bit of dive is required for a loop.
Achieve some altitude and establish level
flight at full throttle; set up a shallow dive
and pull full up-elevator to get a loop.
Thanks to Mark McCutcheon, a longtime
flying buddy and an extraordinary
modeler, for taking the flight photos. He
has that magic touch and gets the
necessary angles and lighting for the
perfect shot.
The XB-70 is a nice change of pace
from the usual park flyer. It looks great in
the air and is a good conversation piece at
the flying field. Read up on your Valkyrie
history; you will get many questions when
people see you flying yours. Enjoy! MA
Al Clark
[email protected]
Sources:
AstroFlight
(949) 855-9903
www.astroflight.com
Du-Bro
(800) 848-9411
www.dubro.com
RadioShack
(800) 843-7422
www.radioshack.com
UltraCote
(800) 338-4639
www.hangar-9.com
Radical RC
(937) 256-7727
www.radicalrc.com
Castle Creations
(913) 390-6939
www.castlecreations.com
APC
(530) 661-0399
www.apcprop.com
Testors
(800) 837-8677
www.testors.com
02sig2_00MSTRPG.QXD 12/20/10 10:32 AM Page 36
