Slingsby T-37 Skylark ~ 1/3-scale model by Peter Goldsmith (Build Thread by JimD)

[JimD]

Link to Peter Goldsmith Designs Web Page:

https://www.petergoldsmithdesigns.com/product-page/slingsby-skylark-1-type-37

This is the third giant scale sailplane designed by Peter Goldsmith that we have built. Many of the design elements were familiar to us having built the other two (40% Schweizer 1-26 and 1/3-Scale Slingsby Swallow), but the Skylark does have some unique features as well.

Span: 188" (4.7m)
Length: 85" (2.18m)
Weight: 12-13 lbs. (%.6kg)
Airfoil: Modified SD3021

Most of the structure was built using Deluxe Materials Aliphatic Resin. Wing, stab, and fin sheeting was applied using Deluxe Materials Speed Bond and a hot iron technique. Epoxy was used to laminate the spar joiners and center ribs. CA was used to tack several parts together to aid in assembly, and these joints were then reinforced using the Aliphatic Resin.

Our build started by separating all of the parts from their sheets, lightly sanding off the nubs, and then sorting them according to the structure they belong to.

Study the plan carefully so as to understand hos the parts fit together.

Fin/Rudder

The front of the fin has two pieces of balsa with predrilled holes for the hinge pins. One of these goes in front of the other. Use the rear piece (which has laser etched lines that mark the rib locations) and the rudder ribs R1-R8 to start construction. Laminate the plywood horn slot reinforcement piece to R9 making a left and right side. This assembly helps set the angle of the joint for R1.

Note that this picture does not have the front of the rudder aligned over the plan correctly! Note the vertical line on the plan that bisects the hinge pin pivots and then move aft. There is another solid vertical line, then a dotted vertical line, and then another solid vertical line. The vertical balsa "rudder spine" should be aligned over the plan between the first solid line (aft of the hinge pivot line) and the dotted vertical line.


Ribs R2-R8 attach to the vertical spine at a 90 degree angle (unlike R1 that angles upward). Note the "feet" on the ribs that enable this structure to be built on its side on a flat surface. Because of the direction of the grain, these feet can easily be broken off, so use a light touch during the assembly process. You may consider applying small pieces of masking tape over these feet to sturdy them during construction. Again, note that this picture also does not show the correct placement of the parts over the plan. The entire assembly should be moved forward so that the rear edge of the spine is aligned with the dotted vertical line. The rear most solid vertical line represents the edge of the 1/6" x 1/2" balsa cap strip applied over the spine.

1/16" X 1/2" balsa strips are added and then the balsa LE is laminated onto the front. Temporary dowel pins make handy alignment tools.

The Rudder LE piece gets sanded to a "V" to allow for the movement of the rudder.


Cut the leading edge spine and the spars for the fin. It is helpful to mark where the ribs will be positioned on these parts.


Fit the ribs into place and check alignment over the plan, CA glue can then be applied to tack all of these parts together.


This fin structure can then be place onto the Trailing Edge Fin Post using the laser etched reference lines. A small weight was used to hold the parts tight together while the glue cured.




Hinge Point "donuts" are added and sides of fin are sheeted with the pre-cut 1/16" balsa sheeting.




Front of rudder gets block sanded and Fin LE attached.

[JimD]

Stab/Elevator

Identify all the parts needed for the stab/elevator build. Note that the stab and elevators taper and build tabs (feet) are part of the parts design so structures can be built on a flat surface. These tabs are very delicate, so handle the parts gently. Small pieces of masking tape can be added to reinforce the tabs.




Start by laminating the center ribs together and gluing the "elevator servo plate" between ribs S3 and S4. Then add these to the Elevator TE.




Add the remaining ribs, spar, and leading edge. Note that the BOTTOM of the stab is facing upward here.




Carefully sand the leading and trailing edges to the contour of the ribs. You may want to put a balsa stick under the leading edge for support while you sand.



The stab gets a 1/16" balsa skin that is precut (but is slightly oversize). Locate the plywood servo cover door stop. This part will probably need to be trimmer a bit to fit between the spar and trailing edge. The key thing is for this part to fit tight against the S2 rib and for the three servo screw holes to be visible as you look down through this part into the servo mount. Note slight recess on the right side in the picture below. This allows access to the bottom servo mount screw (visible under this part in the picture. Hold the part against rib S2 and adjust the front and/or back sides to fit the part between the spar and TE and to center it over the servo mount tray below to allow access to all three servo mount screws.



Now glue the door stop to the inside of the precut bottom 1/16" balsa skin positioning it so the servo door will be supported by it.

[JimD]

Installing the Spektrum A7020 elevator servos.

The long double arm (included with the servo) was used and one side was cut off. Servo was plugged into the receiver and centered so arm could be correctly positioned.

Servos were screwed to the mounting plate using the screws that came with the servos. (You may want to add a dot of Zap Goo to the bottom of the servo mounting screws.)

Next, route the servo leads out through the openings before adding the lower skins.

[JimD]

Use a scrap piece of 1/16" balsa to make a spacer between E2 ribs.

[JimD]

Locate the stab spar joiner (there may be two of these in the kit but only one is needed). Trim it to fit the taper of the stab and cut the openings in the S1 and S2 ribs to make a slot for the joiner to fit into between the top and bottom spars.



Add top spar and sand leading and trailing edges to the contour of the ribs.



Add the hinge point reinforcement donuts.



Add the top skins to finish.

[JimD]

Wings

There are two ways to approach building the wings.

1) Build over the plan and use the plan to set rib spacing.
2) Build over the bottom sheeting and use the shear webs to set rib spacing. (This is the method that Peter prefers)

This sequence shows using the plan. Note that it will be necessary to sand some of the shear webs slightly in order to get the rib spacing to match the plan.

First, find all of the wing parts, remove them from the carrier, sand off the nubs, and sort them into right and left wing parts.








Epoxy was used to laminate the center rib and the wing joiners. Note that there are pre-drilled alignment holes. Stick a large T-pin through these holes to maintain alignment while gluing.




Locate bottom spar and pace it over the plan.


Layout the ribs.


...and the shear webs.


Glue the spoiler servo plate between ribs, place this assembly over the plan, then work both directions from there adding shear webs and then the next rib.

[JimD]

Once all of the ribs and shear webs are glued in place, cut out the middle of the root and tip ribs to make the slot for the wing joiners.






We used the joiners to sent the spacing of the shear webs. The center joiner gets glued in while the outboard joiners are to be removable.




These TE parts can be installed in one of two ways. Building over the plan, they were used to maintain rib spacing and top rear sheeting was glued over them. If you are building over the bottom sheeting, they can be glued in standing vertically like mini shear webs at the front edge of the rear sheeting.




Wing bolt plates get laminated together and a taper sanded so they fit in place and follow the rib contour.

[JimD]

Rear sheeting, top spar, spoiler bay stringers, and leading edge are added. Then leading edge gets sanded to the contour of the ribs.






Cut a piece of 1/16" x 6" sheeting to size, fitting it around the spoiler bay.


Trim the pre-cut center sheeting to fit between the front and rear sheeting. Adding tabs helps reinforce the butt joint between these pieces of sheeting.


Deluxe Materials Speed Bond was used to apply the top sheeting. Apply glue to both sides of the joint, allow it to dry (turns clear), then use a hot iron to bind sheeting to structure.


Add cap strips.


Install Spektrum A7020 spoiler servo. Plug the servo into the receiver and check centering before adding the servo arm. The long double arm that comes with the servo is used with one side removed. Mounting screws that come with the servo are used to screw the servo to the mount plate.


A dab of Zap Goo was applied to the bottom of the mounting screws. Next, route the extension cable for the aileron servo through the inboard wing panel rib bays.

[JimD]

Once the wiring is completed, the bottom sheeting can be added.














Wing center panels are then permanently joined together. Center was allowed to rest on the workbench surface while both wing panels were elevated outboard to accommodate the dihedral. Note reference line on the workbench surface that helped to maintain the straight line of the trailing edge. Plywood end cap ribs were added. Center joint is reinforced with a wrap of fiberglass tape (0.75oz is fine, about 3" wide at front tapering to about 2" at the rear). Outer wing panel construction will continue after the fuselage section.


Ailerons are built over the pre-cut sheeting. Parts placement is laser etched onto the lower sheeting. Glue the ribs, LE and horn reinforcement plate in place and then sand the rib contour onto the LE.


Speed Bond was applied and top sheeting and it was applied using the hot iron technique.

[MarkY]

Jim, just a question, I have never used the Deluxe Materials Aliphatic resin is it any better than any other aliphatic glue? Thanks for putting this thread together, it's a lot of work, it's appreciated. Mark

[JimD]

Thanks guys!

Mark, the Deluxe Materials Aliphatic Resin is in a class by its self...no comparison to the other aliphatics. All I can say is, get a bottle and try it. You will be a fanboy after the first use!

[MarkY]

Will do. Mark

[JimD]

Fuselage

First step is to make mirror image fuselage sides. Since these sides are straight along nearly the entire length of the bottom, They can be built next to each other along a straight reference line on your workbench.

Each side has a top and bottom 1/4" x 1/4" stringer that runs its length. Front half of stringer is balsawood and the rear half is basswood. Make a scarf joint to join these two parts together. Lay these on the plan and mark the location of the side panels, formers, and cross members.

T-pins and bricks make good holding devices.





Copy marks to the opposite side.



Support the stringers with 1/16" shims between the front and rear 1/16" plywood fuselage sides. Align the stringers on the laser etched marks on the plywood side panels and add weights to keep joints tight and side panels flat on the workbench.





F1, F2, F3, F4, and F5 formers are used to set spacing of top and bottom stringers. They are NOT glued in place at this point.



Thin CA was used to tack the stringers along this curved section.



1/16" x 1/2" balsa cap strips are added under the top and bottom stringer between the front and back 1/16" plywood fuselage sides.



The vertical 1/4" x 1/4" balsa truss parts were measured over the plan and pre-cut in matching sets for the left and right sides.

[JimD]

Joining the fuselage sides can be done over the plan starting at the rear and working forward.

[JimD]

While the fuselage is flat on the workbench, the rear top formers and spine are added and the stab mounting plate is added. Then fin is added and finally the stab is drilled and bolted to the stab mounting plate using a blind nut.

[JimD]

Having completed everything that can be done with the fuselage flat on the bench, it is time to move on to the installation of the front formers. F1 is taped in place and F4 is installed in a friction fit, neither is glued at this point.




A fuselage stand was made using landscape bricks and scrap wood. Formers F2, F3, F5, and F6 are firction fit into place and masking tape used to hold fuselage sides against the formers. Again, bricks are used as fixing devices.



Before any of the front formers are glued in place, the fuselage is aligned over a straight reference line drawn down the center of our workbench, blocked in place with weights, the fin is checked for plumb, and the fuselage checked for level.



Looking down over F1, note that it is angled to the left. This is because the right fuselage side stringers are a bit less flexible than the left side. If you note this on your build, use a razor saw to make partial cuts in the stringers on the side that bends less.



After making a series of cuts, F1 is now perpendicular to the centerline. After checking again to make sure that all of the formers were aligned with the fuselage inside laser etched lines, thin CA was used to tack the formers in place. A bead of Aliphatic Resin was then applied to all sides to make good strong joints.



The battery box parts were added between F2 and F3 and the battery was checked for proper fit. This box needs to be slightly oversize to account for any "puffing" of the battery over time.



Spektrum A6320 tow release servo was mounted in place.



Spektrum A5060 rudder servo was mounted in place (note that there is a plywood doubler that is added to the bottom of the rudder mount).



Tow release supports were added between F1 and F2 and included wire loop glued in place.

[JimD]

F11 and F12b parts are glued onto the F10 wing mounting plate.





F15 is added to the underside of F10. This is support for the area where the blind nuts for the wing bolts will be added.



Top stringers between F1, F2, and F3 are added and sanded flat along the top, then the pre-cut top 1/32" plywood sheeting is glued on.





F13 is added to the front of F4.





Pre-cut 1/32" plywood cockpit sheeting is added.





F4B doubler is added to the front of F4 (note that the wing center rib can be used to align these parts).

[JimD]

Use a sanding bar to sand a flat on the fuselage between the top and side sheeting, then add the 1/32" precut plywood parts.







A word to the wise here...don't add the sheeting to the other side until the wing attachment bolt holes are drilled and the blind nuts installed. Also, it is helpful to install the receiver, ESC, and flight battery while there is easy access through the one side.

F16 is added to the bottom rear of the fuselage to form the tail skid mount.