Multiplex 21 4091, Building Instructions

9

Now let’s get going …
Completing the wing panels:
1.

 Glue the wingtips 

15

 to the wing panels and sand smooth.

Fig. 01

2.

 Fit the capstrip 

39

 in the position shown. Place the wing strut support

53

 on the wing sheeting in the stated position and cut round the support

using a balsa knife. Lever out the loose piece of sheeting using a
chisel.

Fig. 02

3.

 Fix the strut support to the wing using the screws 

58

; you will need

to drill 1.5 mm Ø pilot-holes beforehand. Remove the support again
before continuing work on the wings. It should not be installed
permanently until the model has been covered.

Fig. 03

4.

 We strongly recommend the use of our ready-made servo mounts

for Profi servos, # 8 5080. Place the servo mount centrally over the
sheeted rib bay and cut carefully all round it. Trim the opening neatly
and glue the mount in place from the underside. Reinforce the joint
by gluing the servo plates 

33

 to the underside of the mount, running

from the right rib to the left rib (please note: two parts 

33

 are required

for each servo).

Fig. 04

5.

 The inboard and outboard wing panels 

4 + 6 / 5 + 7

 can now be

joined: cut a 3 x 37 mm slot adjacent to the spar in the facing ribs of
the inboard and outboard panels. Trim the pairs of wing panels to fit
together accurately, joining them with the dihedral braces 

42

. When

you are satisfied, glue the panels together using epoxy or white glue.
Use paper masking tape to hold the parts together while the glue
hardens.

Figs. 05 / 06

6. Trimming the ailerons and landing flaps to fit

You will find factory-cut hinge slots in the ailerons 

8

.

Fig. 07

Fit the hinges 

73

 in the control surfaces ”dry” (no glue) and temporarily

attach them to the wings.
Glue the reinforcements 

38

 in place. Temporarily screw the offset

hinges

75

 for the landing flaps 

9

 (3 each side) to the wings and flaps,

and check that they deflect correctly. Carry out any adjustments
required. Don’t forget to deploy the servo leads in the wings before
applying the covering film.

Figs. 08 / 09

7. The wing / fuselage transition

The root facing ribs 

34

 should be copied from the wing root ribs 

34

:

position the root ribs, draw round the outline, cut out the facing ribs
and trim them to fit accurately. Glue the locating pegs 

78

 and the

retaining hooks 

79

 in the root ribs. The locating pegs should project

by about 30 mm. The free length of the retaining hooks should be
about 40 mm, measured to the curved section.
Don’t glue the facing ribs to the fuselage until you have checked the
longitudinal dihedral (see section (8)).

8. Preparing the tail panels

Sand the fin 

10

, rudder 

12

, tailplane 

13

 and elevators 

14

 smooth

overall, and round off the tips. Assemble the parts ”dry” using the
hinges

73

. Trial-fit both tail panels in the slots in the fuselage.

This is your best chance to check the longitudinal dihedral (wing / tail
incidence), as you can still make any minor corrections required without
major effort: lay a straight strip of wood (a length of steel strip is even
better) on the fuselage side, resting flat on the tailplane root. Mark the
line of the straight edge on the fuselage below the wing and measure
from this line up to the wing root locating peg holes: the front hole
should be 9 mm higher than the rear one, although a tolerance of 1
mm is acceptable. If the deviation is greater than this, adjust the
position of the root facing ribs 

34

 before gluing them to the fuselage.

Figs. 10 / 11

9. Top fuselage hatch

The hatch over the wing is assembled from parts 

30, 31, 32

 and 

35

.

Note that the cover plate 

32

 should be trimmed and glued in place so

that the large hatch 

31

 fits exactly in the free space in the bay. Glue

the reinforcing ribs 

30

 to the hatch 

31

, glue the cabin latch spacer 

35

in place, and install the cabin latch 

69.

Figs. 12 / 12.1

10. Fitting out the fuselage

Trim the servo plate 

36

 to fit in the fuselage. Cut the apertures for the

servos, then glue the servo plate support strips 

40

 to the plate. Posi-

tion this assembly in the fuselage and glue it in place. The elevators
can be actuated by a single servo or two servos. If you are using two
servos install two separate control ”snakes” 

76

. If only one servo is to

be used, join the two snake inner rods before they reach the servo,
and connect them using a single clevis 

67.

Fig. 13

11. Trimming the windscreen to fit

Carefully cut the windscreen 

17

 to the marked outline, trimming the

moulding in small increments.
Note: the windscreen must fit between the fuselage and the facing
ribs at the top. When the model is finally assembled, the windscreen
is fixed in place using the screws 

71.

Fig. 14

Cut slots below the elevators through which the snakes 

76

 will pass.

The slots can be adjusted to the correct angle using a 3 mm Ø needle
file.

12. Installing the servos, connecting the linkages

Glue the snake outers 

76

 in the fuselage slots using cyano. Install the

snake supports 

19 + 20

 in the fuselage; 5-minute epoxy is best for

this.
Fix a horn 

65

 to each control surface, taking care to align each one

accurately with the associated pushrod.

Fig. 15

13. Aero-tow coupling

Attach the bracket 

81

 to the former 

(3)

 using the screws 

71

, with the

angled end of the bracket facing the tail. Drill a 2 mm Ø hole through
the top hole in the bracket and into the fuselage top decking. Uns-
crew the bracket again. Open up the hole in the top decking to 3 mm
Ø. Glue the brass tube 

80

 in the hole, flush with the top of the fuselage.

Apply glue to the rear face of the former 

(3)

 at the same time to

reinforce the joints.
Form a Z-bend in one end of the steel pushrod 

82

, and connect the

pre-formed end to the servo output arm. Slip the steel rod into the
brass tube 

80

 from the underside. Install the servo with the output

shaft facing the tail, using the mounting rails 

41

. When the model is

finally assembled, the pushrod can be cut to the correct length and
the retaining screws secured.

Figs. 16 / 16.1

14. Installing the main undercarriage

The main undercarriage unit 

16

 is supplied in the kit as a ready-made

item. Locate the plywood-reinforced area on the underside of the
fuselage and place the undercarriage unit on it, with the rear screw
hole position forward of the fuselage former 

(2)

. Set the undercarriage

unit at 90° to the fuselage centreline and mark the position of the two
front screw holes. Drill 6.5 mm Ø holes at the marked points. Fit the
captive nuts 

72

 in the holes from the inside of the fuselage and tighten

the undercarriage screws 

61

 to pull the nuts into the plywood. Secure

the nuts with glue, then remove the undercarriage again.

15. Preparing the tailwheel unit

Drill a 4 mm Ø hole in the centre of the radius of each of the tailwheel
unit mounting lugs. Sand the inside of the two side panels 

60

 and

clamp the tailwheel leg 

60

 and the tube 

60

 in the correct position

between the outside parts. Glue the tailwheel unit components together
using UHU plus, and hold the assembly together with clothes pegs
while the glue is hardening. Take particular care to avoid glue getting
between the wheel leg and the tube. Leave the adhesive to set hard
(overnight), then trim the mounting lugs to fit the shape of the fuselage
and fix the assembly to the fuselage using the retaining screws 

58

. If

your flying strip is rough, we recommend fitting hardwood dowels
through the fuselage to accept the retaining screws. When the
tailwheel unit is finally installed, apply glue to the screws to prevent
them shaking loose.

Fig. 17