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					               Sopwith Pup & Albatros
                   DIII "Foamies"

Ron Wanttaja
August 2011

• Two airplanes, nearly identical in construction
• Weight, with all electronics
    – 175 grams ready to fly with 500 ma battery – 6.2 oz
    – 145 grams (not including battery)
• Wing area: 1450 sq-cm (148 sq inches)
• Recommended motor/prop
    – Park 250 – 8 x 6 slow flyer prop
         • This is almost too much motor –produces ~230 oz of thrust with a fresh
           battery (greater than 1:1 thrust to weight ratio)
    – Can fly with small ~180-class motor…but the extra thrust is nice, when
      you need it.
• Construction material – Everything!
    –   Fuselage 6 mm Depron foam
    –   Wings 6 mm Depron foam (3 mm is OK)
    –   Fuselage stiffener/motor mount – ¼" x 5/8" bass or balsa stick
    –   Wing struts: 1/16" plywood
    –   Assembly: foam-safe CA, #4 nylon bolts/nuts
                               Design Philosophy

•   The design is a true "composite" using a variety of materials
     – Depron foam (3 mm & 6 mm) for large surfaces like wings and fuselages
     – Plywood for stiffening
     – Bass or Balsa "motor stick" to provide a basic structure
•   Biplane design gives good wing stiffness using even 3 mm Depron
     – Thin leading-edge reinforcement helps, too
•   Commercial kits/ready-to-fly designs put you at the mercy of the
    manufacturer for spare parts
     – These planes let you make all your parts
•   The airplanes are designed to be easily repaired
     – Wings are actually bolted to the fuselage using #4 sized nylon nuts/bolts
          • The ~5 nut/bolt sets used on the design weigh a total of only 2 grams
     – Break a fuselage? Glue it back together, or cut out a new one, bolt the motor
       stick and wings to it
     – Break a wing? Glue it back together, or unbolt the assembly, cut off the foam
       from the plywood struts, and glue on another wing
     – Break a motor stick? Glue it back together!
          • Never have broken any other wood part on a biplane!
•   Design supports experimentation, too
     – Can make other types of aircraft with just minor changes to wing or fuselage
                              The Real Machines

                                           • The Sopwith Pup was a predecessor to
                                             the famous Sopwith Camel
                                           • Its actual name was the Sopwith
                                             "Scout," but was called the Pup due to
                                             its similarity to larger Sopwith 2-seater
                                           • It entered combat in late 1916

• The Albatros DIII entered combat in
  early 1917
• It had a plywood semi-monocoque
  fuselage, similar to a modern aircraft
  only in wood
• Most of the WWI German pilots used
    • von Richthofen had more "kills" flying
      an Albatros than his Fokker Triplane
                  A Construction Note or Two

• The "glue" is "Foam Safe" Superglue (Cyanoacrylic, or CA)
   – Non-foam-safe kind can be used to bond the wood parts
   – Get the small spray bottles of "catalyst" or "Kicker" to get the CA to cure
• Cut the Depron foam with an Exacto knife
   – The 3mm foam can be easily cut with a good set of scissors
   – Buff the rough edges with ~80 grit sandpaper
   – Don't worry about cutting smooth curves…cut it out, then smooth it with
     the sandpaper
• When cutting foam, consider making spare parts
   – It's a lot easier when you're set up for it
   – The wings are quick and easy to cut out…make some spares
• These instructions assume using an interconnected rudder and
  aileron system
   – Can go to full four channel (throttle, aileron, elevator, rudder)
   – Can go rudder-only if desired, but need to build in a bit more dihedral
     angle (see notes on strut pages
   Front Half of Pup Fuselage Pattern

    Print out the two halves of the fuselage, tape together at the line

                               This notch sets the angle of
                              incidence for the wing system

Best Fuselage thickness seems to be 6 mm. 3 mm
          works, but is a bit more brittle
     Rear Half of Pup Fuselage Pattern

                                    Approximate Horizontal
                                        Stabilizer Slot

                                                        Rudder Line

Include the entire shape of the tail (rudder included) when
   cutting out the fuselage. Then cut off the end for the
  rudder as shown. If using 6 mm foam, use the cut-off
           rudder as a pattern for a 3 mm rudder.
Front Half of Albatros Fuselage

Print out the two halves of the fuselage, tape together at the line
Approximate Horizontal
    Stabilizer Slot
                                           Pup Wings
•     Basic wing shape is a rectangle – 4.25" high, 26 inches wide
•     Use pattern to shape the wingtip
•     Need two wings!
•     6 mm depron, can go 3 mm for
      less weight but more fragile
       –   Surprisingly little weight advantage

                                                                     Leading Edge

     Wing Construction – Make Two
    Cut Rectangle 26" long and 4.25" Wide                     Wingtip Shape
          from 3mm or 6 mm foam                                      Wings are 26" span

    Lay template at each end and cut wingtip
                              Albatros Wings
• Basically identical to the Pup, just
  with a different wingtip shape and
  the short length of the wing
• I actually made the first Albatros'
  wings from a set of spare Pup
  wings                                        Leading Edge
    – Just re-shaped the tip

  Wing Construction – Make Two
 Cut Rectangle 26" long and 4.25" Wide
       from 3mm or 6 mm foam

 Lay template at each end and cut wingtip       Albatros Wingtip
                                                     Wings are 26" span
                       Pup Horizontal Stabilizer/Elevator
   • 3 mm depron
   • Notch at leading edge is the width of the fuselage foam



Cut out from 3mm   Cut 1" wide strip at   Join with hinge material    Bond in wire "Bridge"           Cut out elevator
 foam (scissors      end (elevator)        (Packing tape or 1/8"                                       "notch" to let
                                                                     using foam-safe CA on
   works fine)                             strips of hinge fabric)                                     rudder swing
                                                                          sides ONLY
                                                                         No Glue Here!

                                                                      ~.032" piano wire       Glue here
                      Albatros Tail
• Elevator shown in actually a bit too big!
   – A bit sensitive in flight
   – Cut a bit off the trailing edge of the elevator if your controller
     won’t reduce travel
                           More on Hinges
                                             Strips of ½A control
      Strips of clear plastic               hinge fabric - Internal
          packing tape                            • Cut to 1/8" wide
                                                  • Use exacto to cut slot
                                                  • CA in place

Strips of ½A control hinge              Hybrid
                                            • Glue hinge material to control
     fabric - External                        surface only
                                            • Use packing tape to attach surfaces
            • Cut to 1/8" wide              • Easily removed/repaired
            • Lay on surface
            • CA in place
Wing System
              Center Struts and Wing Struts
             are the same height and slant –
               easy to keep straight during

     Leading Edge/Landing Gear
       Brace installs in slot in
            Center Strut
                                                                                                               Glue to plywood with rubber cement, or
                                                                                                               print on label stock

                                         Pup Wing Struts
                                                               Odd bumps
                                                              and bulges at
                                                               the ends of
                                                                                                              Holes for Gear Bolts
                                                              the struts are
          basically identical, so the

                                                                 just for
          struts can go either way

          Top and bottoms are


                                                                                                                    Center Strut

                                                                                                                                                        Leading Edge & Landing Gear
                                        Formerly had a triangular
All are one-piece                       brace in the middle of the             Cut slot to the thickness of
                                        lower wing - Deleted                   the wood used
1/16" plywood
            Alternate Pup Wing Struts                 Glue to plywood with rubber cement, or
            (Easier to cut out)                       print on label stock

                                               Deeper brace for more dihedral
                    Structurally, these are       (rudder-only systems)
                    identical, but they are
                    different appearance if

                                              Leading Edge & Landing Gear


                                                        Same 1/16" plywood, cut
                                                         to match motor mount.
                                                          Can be trimmed after

All are one-piece
1/16" plywood
Albatros Wing Struts
                                                       Leading Edge & Landing Gear

                                               Albatros has less wing spacing than
                  Formerly had a triangular       Pup…these struts are shorter
                  brace in the middle of the
                  lower wing - Deleted
Simple Albatros Wing Struts

                                        Deeper brace for more dihedral
                                      (rudder-only systems-either plane)

                                      Leading Edge & Landing Gear

                                                       Strut Assembly                                                Center Strut
                                                                                                                 Off-Center, since it bolts to
                                                                                                                 the SIDE of the fuselage!

                                                                    Hold gear in place with
                                                                      #4 nylon bolt with
                                                                       plywood washer                        Leading Edge & Landing
                                                                                                                   Gear Brace

                                                Do not glue Mid Brace until
                                                  final assembly of wing

        Leading Edge & Landing
              Gear Brace                                                               Landing Gear (0.049"
                                                                                           piano wire)
                                                                                               Why, yes, it IS

                                                                                    #4 nylon bolts attach center strut
                                                                                    and motor stick to opposite sides
                                                                                            of foam fuselage

 Leading Edge of upper wing                                                              Asymmetrical nature
is even with the upper part of                                                     accommodates for the offset of the
       the center strut                                                               foam fuselage from the strut

                                                                    Bottom Posts go through
               Leading Edge of lower wing butts up
                                                                    slits made in Wing. Trim
               against (and is glued to) the forward
                                                                        off excess with side
              Wing Assembly                                                         Position Center Strut assembly just to the right of the
                                                                                  centerline, with the landing gear brace flush against the
                     Score shallow cut in top of lower                            leading edge of the wing. Press to leave dents from the
1                    wing, down the centerline. This is
                       to let the wing bend in Step 4
                                                                                   bottom posts. Leave off the mid brace from the center
                                                                                               strut until the slits are finished


        Albatros is identical, except the wing is the other
                                                                                                                                     Slots MUST be parallel to
                           way around
                                                                                                                                           the centerline!

                                   Shove Center Strut through slits (it'll pop
                               through). Forward edge of wing should be flush
                 4                   against back of landing gear brace.
                                                                                             3   Remove center strut, use exacto knife to cut slits
                                 Add the Mid Brace to the center strut, press                        where the bottom posts made dents.
                                against the wing to mark its location, then cut

                                                                                                                                 6 mm wing shown. For
                                                                                                     4                          3mm wing, the bottom of
                                                                                                                                  the wing doesn’t go to
                                                                                                                                 the bottom of the brace

                                                                                  Bend up each side of wing, use CA to bond it parallel to the bottom of the
   Apply foam-safe CA to the center strut ONLY (not the                           Landing Gear Brace. This sets the dihedral. Bond the top and bottom of
landing gear or mid braces). Keep strut vertical to wing until                                the wing leading edge to the back of the brace
                         CA sets
                                                                                    Bond the mid brace also to the wing, trim off excess with side cutters
                       More on Attaching the Center Strut
• Wing alignment isn’t hyper-critical, but attaching the struts to the
  wings needs a bit of care. The Center Strut is the key component

                                                         The slots for center strut MUST be parallel to the centerline!
                                                         Draw a guideline perpendicular to the wing, press the forward tab
                                                         into the wing, then carefully set the tab location for the rear tab at
                                                         the same offset to the guide line

If the strut is at an angle, the overall wing alignment to the fuselage will be at an angle, too. In flight, the
airplane will tend to roll away from the wing that’s “forward.” If necessary, add a shim to where the wing
strut bolts to the fuselage to take out the angle

                                                                Center Strut installed at an
  Shim added between
  strut and fuselage to set
  wing at right angle to

                                                       Wing-attach bolts (nylon)
                                      Upper Wing System
                                                        1    Lightly score top of upper wing
                                                                to allow flex for dihedral

                                           Set the wing lightly atop the strut in the
                                            right position (leading edge of wing
                                             even with top part of center strut)

                                          Remember to offset wing slightly to left
                                                to match lower wing

                                          Apply pressure to lightly dent the bottom
                                           of the wing to show where the tabs go

                                       3 Use Exacto to cut out tab slots at the dents

                                              Check fit (no glue), alter slots as necessary

                                          4    Glue wing struts to lower wing and repeat

                                              Don't glue the upper wing in place until the
Pup and Albatros built the same way
                                               entire airplane is done (easier access)
               Lower Wing Leading Edge Reinforcement

  •   The "Box" of the biplane wing structure is pretty strong, but there still can be some
      flex (especially with 3 mm wings)
  •   Bonding reinforcement to lower-wing leading edges adds more stiffness
  •   Use 1/16 x 1/8" spruce strip (available at hobby stores) or thin carbon-fiber strip
  •   Should go at least 3/4ths of the wingspan
  •   IMPORTANT: Need "bridge strip" across front of main strip and landing gear brace to
      carry load

                                  Without bridge strip, loads are
                                  concentrated here

                             Bridge Strip is bonded to both the center
                             section and the wing reinforcement strip
Pup and Albatros built the same way
 Landing Gear Attachment – Leading Edge
                             Graphite Strip Leading Edge

Landing gear mounting bolt
   with plywood washer

                                  "Bridge" to center section

                                              Receiver velco'd to
                                               bottom of wing

   •     Cut out 1" x 3" notch in trailing edge
   •     Make 3mm aileron to fit notch (leave room to move)
   •     Install with hinges
   •     Bend root end of aileron torque wire as shown (~0.038" piano wire)
   •     Cut plywood strip~ 1/8-1/4" wide, 1.25" long, with small holes top and bottom 1" apart ("Horn")
   •     Slide over wire
   •     Slide two bearings over torque wire
           –    Anything that goes loosely around tube
   •     Bend the outer wire to fit under aileron
   •     Cut thin slot near root of wing for the horn, and insert under wing
   •     Position the wire so that it runs from the root to the front part of the notch
   •     Tape the bearings to the bottom of the wing so the wire can still turn                    plywood
   •     Tape the aileron to the bend end of the wire                                              aileron horn

                  4"                                                         CA wire to
                                                                             aileron horn

                                                                                     Plywood strip with two
                                                                                     small holes 1" apart
                                         Bearings (section of thin tubing, like
                                         wire insulation or nylon tubing)
Pup and Albatros built the same way
                                                                     Also can use commercial aileron system
                              Aileron Close-up

                                                         Fabric Hinge Material

                                 Bent end of torque wire, taped to aileron

"Bearing" (piece of plastic
tube) wire, taped to lower
     surface of wing
                                      Wing Struts Attachment

                                                                  About 3½" inboard of wingtip
                                                                         – not critical!

 Both wing struts parallel with
         Center Strut

                                            ½" behind the leading edge of
                                          wingtip – not critical, but both sides
                                                  should be the same
Pup and Albatros built the same way
                                      Upper Wing Attachment
                                             Place top wing on Center Strut,
                                                      hold in place

Re-assemble wing,
 repeat on other
    wing strut
                                                      Press top tabs of one wing strut
                                                       into bottom of upper wing to
                                                        mark position, then slit with

                                                  • This method works very well at making a box wing
                                                    structure with minimum warp
                                                  • Key factor: top wing should be resting LOOSELY
                                                    on top of wing-strut tabs prior to marking
                                                     • Flick the upper wing up slightly before marking
                                                        to be sure it isn't caught on a sharp point of the
Pup and Albatros built the same way                     strut
          Assembly Sequence
Glue Center Strut First
            Top wing is just resting
           atop tabs on outer struts

                   Lift one wingtip gently so that the strut tabs no longer touch, then
                    set it gently just atop the strut tabs. Then press lightly atop the
                     wing, and cut out slots for the tabs based on the impressions

                    The basic goal is to find where the struts want to attach without
                     friction causing them to touch in a different spot. The foam is
                            stiff, both wings will want to be at the same angle.

                   Glue the wing to the outer strut.

                    Repeat the lifting/gentle touch on the
                      other wingtip, and cut the slots

                  Do NOT glue the second outer strut yet
                             Bonding the final strut
• Attachment point for second outer strut determines if there’s any
  twist in the wing


        Looking at Wing assembly from directly in Front


                                                                                                    slots for strut
Twist means you see less of
the underside of the wing on
         one side                           If the wing is twisted, lengthen the attachment
                                            slots for the unglued outer wing strut and slide
                                                  the attachment to minimize the twist
           The airplane is tolerant of a surprising amount of twisting, as long as you can adjust the
                                        ailerons or rudder to compensate
                                      Motor Stick

   • Main structural member in fuselage is the "Motor stick"
           – Supports the motor, and provides the solid mounting for the
   • Made of ~1/4" x 5/8" basswood or balsa running length
     of fuselage
           – Hobby stores sell stock pieces of these dimensions… don't have
             to cut them out
           – Exact dimensions don't really matter
           – Basswood is more durable…have broken many balsa sticks, but
             few bass ones

Pup and Albatros built the same way
                                      Making the Motor Stick

   • Cut stick length equal to nose to tail plus a bit extra
   • Cut two short pieces ~ 1" long
   • Use CA to glue them both to ONE SIDE of motor stick
                                           Top View

     • Use Razor saw & miter box, band saw, or very good
       eyeballs to cut stacked end straight
                                           Top View

      • Use CA to glue "firewall" to front of stick
         • Note the center of the firewall is even with the
           CENTER short stick
                                          Top View

Pup and Albatros built the same way
                                        Motor Stick

   •     Press Motor stick lightly in place, make impression with "side sticks" in

       • Cut out foam to clear Motor Stick "side sticks"

       • Drill hole through motor stick near tail, bolt to
         fuselage using #2 or #4 nylon bolt and nut

Pup and Albatros built the same way

•   Slip fuselage over wing, with center strut on the OPPOSITE side of the wing
    from the Motor Stick

    Cut slot in fuselage to clear
    center support

•   Drill hole through Motor Stick, through Fuselage, and into wing strut on
    opposite side – install #4 Nylon bolt/nut

                                                                           Cut slot for
                                                                           parallel to
                                                                           motor stick
                                                          Other Details
                             Two-sided arm used for rudder/aileron servo. One side has two
                             pushrods, one to aileron, one to rudder

                                                                         Cut out Fuselage foam
                                                                         and mount rudder/aileron
                                                                         servo on right side of
                                                                         motor stick with foam
                                                                         double-faced tape

                           Battery                                                                                  Stub of toothpick for tail skid,
                                                                                                                    glued into a hole drilled into
                                                                                                                    motor stick
                                                                                         Mount elevator servo on left
                                                         Aileron Pushrod (both           side of motor stick with foam
Long strip of Velcro for
                                                         sides)                          double-faced tape
battery to allow CG

                                        Bend landing gear legs
                                        slightly forward
                 Aileron Pushrod

Commercial aileron hardware
shown…this piece is normally
   the thin plywood strip
                     Other Side of Servo

Inboard easy-link is connected
          to rudder

     Outside (both sides of servo
           arm) to Aileron
                            Other Details/2

                                                                        Velcro for Speed

                           Rudder Pushrod (one side)

                          Aileron Pushrod (both

                                                       Radio Receiver

•   Radio receiver installed under bottom wing, near center. Cut holes in lower
    wing as necessary for running servo wires
                                     Other Details/3

•   Glue a thin strip of wood (1/16 ply, leading edge strip, etc. on trailing edge of lower
    wing, bridging the junction between left and right wings
     –   Due to the scoring to allow the wings to bend to the dihedral angle, this is a stress point…the
         wood keeps the wing from splitting
•   Pop holes in the upper wing near the center to allow a long screwdriver to reach the
    quick-adjusts on the aileron/rudder servo arm

                                                                     Thin Strip of wood (like leading
                                                                      edge) bonded to junction at
                                                                               trailing edge
                             Sopwith Markings

•   Print this page on a color printer
•   Cut out roundels, glue to surface using thin line of foam-safe
    CA just inside the outer edge edge
     – By putting the bit roundels ONLY on the top wing, you can tell
       from a distance if the plane is turning towards you or away from
•   Color on the rudder is blue-white-red
     – Make rudder from white foam, color with magic markers
                        Albatros Markings
• If installing on Gray foam, cut out iron crosses right at the square to
  give white background

• CG should be roughly midway between forward edges of
  the two wings
   – Built as shown, CG should be about right with the battery near
     the motor
• Should easily take off at 3/4th power with Park 250 motor
   – Drag of wheels can cause slight nose-over tendency for
     takeoff…hold a little back pressure
   – Full power with a Park 250 gives very short takeoff roll
• If turning tendency, leave ailerons set neutral and adjust
  out with Rudder

• Can fix most foam damage with heavy clear packing
  tape and/or CA
• Most common damage is the wingtips breaking off
  outboard of the aileron
   – Airplane will still fly with one wingtip gone!
   – However, easy fix with tape or superglue
• Most common problem is an aft CG
   – Version V4 moves the wing attachment ~1/2” further aft to help
• Wing and tail shapes are just cosmetic…can reproduce
  other aircraft very easily (or do your own design)
• Big motors can sometimes be more trouble than they’re
   – More torque tendency, harder to compensate for
• Wing attach system works for monoplanes as well
   – Leave off struts that go above fuselage
   – Works a bit better with 6 mm foam for the wings
       • If use 3 mm foam, put reinforcement strips on trailing edge as well
         as leading edge
   – Easier to build, harder to fly (higher wing loading)

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