1/3-Scale Cessna L-19 "Bird Dog" Kit by Peter Goldsmith Designs

[JimD]

***Note that the lower rear section of 1/32" plywood wing skin extends beyond the trailing edge of the wing.***

• This extension acts as a gap seal for the aileron (see the plan view).
• There is extra length to the lower 1/32" wing skin so this extension can be customized to your preference.
• It is helpful to trim the lower wing skin before adding all of the other structures to it.
• After building the first outer panel, we took measurements and trimmed the second panel accordingly.

Here are our measurements and how we elected to do this step:

Plan shows 9/32" extension at the inboard end and 5/32" at the outboard end.



We measured and marked this on our completed wing panel. You can see the amount of excess at the inboard and outboard ends.





Knowing the amount of excess, we measured and marked the second wing panel lower skin at the inboard and outboard ends. This excess was then removed using a utility knife to cut the plywood skin.






There are three laser etched lines on the bottom wing skin trailing edge. The inner most is where a line drawn down the inside slope of the trailing edge would contact the skin (see picture below).



The middle line is the important one. The rear edge of the 3/8" triangle is aligned with this line (see plan view at top of this post).



The rear line is the approximate cut line for the excess 1/32" lower wing skin. Our measurements were close to this line but not exactly on it as you can see in my pictures above.

[JimD]

Moving on, tape the front and rear 1/32" plywood bottom wing skins together and align them with the inboard panel.

We have a straight line drawn on our workbench surface. The edge of the spar line of the inboard panel was aligned on this reference line. The spar line of the outboard wing skins was likewise aligned on this reference line.

Note the three T-pins along the spar...these basswood spars will likely have a slight bow. The three pins act to hold this spar in a straight line. Set these in place and test fit the spar before gluing.

[JimD]

The 1/4"x1/2" basswood spar and the plywood Aileron Hatch Support are glued in place on the bottom skin as shown by the laser etched lines. Bricks were used to hold them securely in place while the glue dries.





Locate the ribs and shear webs. Add the aileron servo doubler to rib wR11.





Note that the shear webs taper from root to tip and there is a square corner (that goes on the bottom) while the top corner is not square.





Starting with wR9, sequentially add a rib and then shear web, next rib and then shear web, etc. on to rib wR17. Use the laser cut LE and TE parts to set fore and aft spacing between ribs. Use clamps and weights as needed to keep all joints tight together and ribs tight down against the wing skin.

[JimD]

Insert the 3/8" balsa triangle under the TE.



A small filler piece will need to be made from scrap balsa to fill in at the inboard end of the Tip TE where it connects to rib wR8.



Glue on the LE laser cut balsa piece and clamp it to the ribs while glue sets.



Cut out the slot between wR7 and wR8 and epoxy in place the joiner shear web Sh10.









Add the top basswood spars using epoxy for the root wing tube area and the Sh10 joiner area.

[JimD]

Top of trailing edge needs to be cut down to match the contour of the ribs. Use a straight edge to mark a line.







The balsa hinge pin "donuts" need to be cut or sanded to match the angle of the trailing edge. We typically use a 3/16" dowel as a locator pin when gluing these on.





Note the balsa "filler" piece at the inboard end of the trailing edge. Use a scrap piece of balsa to make this. Sand the top of the trailing edge and donuts level with the top of the ribs.



Install servos and extension wire. We cut the servo lead down to 5" so there would not be excess wire coiled up under the access door.

[JimD]

Wings are now ready for the top 1/32" plywood skins to be added.

[JimD]

Flaps

Flaps initially can be started flat of the workbench, but will need to be elevated when the ribs with the fowler hinges are added because the hinge extends below the bottom skin.

Locate the bottom skins and add the 1/8"x1/4" basswood. Use one of the ribs to help set the correct locatio

Laminate ribs Fl1 to each side of the G10 flap control horn rib Fl2.

Add this assembly plus the other Fl1 ribs. We used two 1/2"x1" hard balsa sticks to elevate the flap assembly and then added the Fl3 ribs.







Top 1/8"x1/4" basswood spar is added. You may need to slightly sand the notches so the upper spar sits flush with the top of the ribs.



Add the balsa leading edge, make sure you have a nice straight and flat joint here.





Lightly sand the top of the ribs and the top spar line and then carve the top of the leading edge to match the contour of the ribs.







Flap is now ready to add the top sheeting.

[JimD]

Test fit the upper flap skin (in a few of the first kits, the slot for the flap control horn may be offset slightly. If needed, widen this slot so that skin fully covers the flap from end to end.



We used Deluxe Materials Speed Bond to apply the top skin.



Start by applying to both a bead of glue to all of the frame area where the skin will make contact.



Center the skin over the framework and lightly press it down over the entire surface so some of glue transfers to the skin.



Spread the glue out so there is a good coat on both sides to be joined and allow the glue to dry until it turns clear.



Carefully align the skin over the framework, making sure the horn is aligned with the slot in the skin. We used binder clips to hold the skin in place once the alignment was verified. Use a covering iron set to the highest (hottest) setting. gently push the skin down at the front (but not too far) and run the hot iron along the top spar line. Once a good bond is established, run the iron down each rib and finally across the trailing edge. We added binder clips to hold the trailing edge joins tight together (but this is probably not really necessary).

Use a sponge of wet paper towel to wet the forward half of the skin.



Bend the skin down over the front of the ribs until it touched the top of the leading edge. Use the iron to seal along the top of the leading edge holding each area until the heat cools and the bond is good. Take your time here (doesn't really take long, but no need to rush along). If there is any area that does not stay tight together, you can reapply the hot iron and reseal the joint.


Sand the front edge to a rounded shape per the cross section shown on the plans and sand the ends so wing skin is flush with the end ribs.

[JimD]

Ailerons

Locate the bottom aileron skins, laser cut leading edge, 1/4" balsa triangle, ribs, aileron horn mount plate and all of the ribs.

Align the slot in the aileron horn mount plate with the slot in the lower skin. Glue the mount plate to the skin.

Glue the 1/4" balsa triangle along the laser etched line on the skin.


Glue the leading edge to the skin and back side of the triangle. Use ribs A11 and A12 to center the leading edge with the aileron horn mount plate.





Add remaining ribs.


Add hinge pin donuts.

[stevekremer]

Very nice Jim. Looks great!

[JimD]

Continuing on with the ailerons, the LE gets sanded down to the contour of the ribs and then top sheeting is applied using Speed Bond and hot iron technique.

[JimD]

With the flaps and ailerons mostly completed, we switch back to the wings.There are a couple of details to tend to and then we can go back and sheet the top of the wings. The steel anti-rotation pins need to be epoxied into the wing root. Check that they are square with the root rib both vertically and horizontally.





Peter designed the pushrod that operates the flaps to have a steel clevis on each end. This allows the pushrod to exit the back of the wing through a narrow slot. After flying the prototype, Peter commented that it would be nice to have more flap deflection. We elected to change the pushrod ends to ball links in order to facilitate increased travel for greater flap deflection. This requires a wider exit hole to accommodate the width of the ball link assembly.

We fabricated parts using scrap materials to support the sides of the sider opening.

[JimD]

The leading and trailing edges of the upper wing surfaces need to be sanded to the contour of the ribs and then the upper wings skins can be added. Just as with the lower wing skins, the 1/32" plywood skins are in four pieces. We started with the inner rear section, then added the inner front section, then outer rear, and finally the outer front section.

[JimD]

The wing skins are slightly oversize, so there will be about a 1/8" overhang along the leading edge. The easiest way we have found to trim this is with a construction grade utility knife (like those used to cut drywall). Sand the LE so the edges of the skins are flush. Then the balsa LE cap can be glued on.



The easiest way to trim this balsa cap to the contour of the wing is with a small razor plane.


Next, sand out the balsa LE cap to a flat that follows the contour of the airfoil top and bottom.



Final step rounds the remaining edges to make the finished LE shape.

[JimD]

The flap hinge assemblies are fabricated from G10 and plywood parts. Note the small "step" on the bottom edge of the G10 parts (edge with tab that inserts into the bottom of the wing). In a few early kits, the plywood HG Support parts do not have this step so you might need to add it.









To assemble the flap hinges, we pinned one side of the G10 parts to the workbench and (using the pins for alignment) glued on the plywood HG Support pieces.





Then the other side of the G10 was added.



The tabs insert into the slots in the bottom wing sheeting. Note how the bottom edges of the flap and wing (along the hinge line) are sanded slightly round.



Here you can see why the "step" is needed in the bottom edge of the flap hinge assembly. It allows the flap to achieve the full up position without stalling the servo.


In this view, you can see the flap pushrod and the ball link attachment to the srevo arm.