From Jason Wentworth: The Thermal Scout

Here are plans (see the attached PDF file) for a high-performance yet simple-to-build Hand-Launched Glider (HLG) called the "Thermal Scout," which would make a great laser-cut glider kit for beginners.  Its wings have simple dihedral rather then polyhedral, which makes them easier to build.  Also, it has short moment arms, which give it bouncy and forgiving flight characteristics.

 

The "Thermal Scout" has never been kitted, having been designed by a British Boy Scout group leader (hence its name) for his scout group to build.  Here is a link to the online illustrated construction article (see: www.f4bscale.worldonline.co.uk/scouts.htm ).

 

I have also reproduced the text of the building and flying instructions below, in order to provide fractional inch conversions for the metric balsa wood thicknesses that are given in the article.  Two other points: [1] The instructions call for using cyanoacrylate glue and epoxy glue at various steps, but aliphatic resin glue (yellow carpenter's wood glue) will also work fine for all of the glue joints;  [2] The instructions call for sanding the flat, rectangular cross-section fuselage to an oval cross-section, but this would weaken the fuselage--it's better to leave it "as is" or to just slightly round off the fuselage's square corners (as viewed from the front or rear).  Here are the instructions:

 

THERMAL SCOUT

 

 

Fuselage: 3/16" (5 mm) thick hard balsa (C-grain balsa, also called quarter-grain balsa)

 

Fuselage stiffener strip: 3/32" X 3/16" OR 1/16" X 3/16" (2 mm X 5 mm OR 1.5 mm X 5 mm) basswood or spruce strip

 

Wings: 3/16" (5 mm) thick medium/soft balsa (B-grain balsa)

 

Rudder: 1/16" (1.5 mm) thick medium/soft balsa (B-grain balsa)

 

Stab: 1/16" (1.5 mm) thick medium/soft balsa (B-grain balsa)

 

Grip: 3/16" (5 mm) thick medium/soft balsa (B-grain balsa)

 

BUILDING

Start by cutting out a ‘kit’ of parts.

Cut out the wing in one piece - plane and sand it to shape before cutting at the centre and then rejoining with Epoxy (50 mm dihedral each wing tip).

Cut out the tailplane and fin, sand their edges to about 0.5 mm thickness, leave the centre 4 mm of the tailplane untouched (flat) as it will be glued to the underside of the fuselage.

Cut the fuselage from 5 mm balsa sheet (hard C-grain balsa), cut it accurately as there is incidence built into the wing seat.  If you measure the wing seat position (on the fuselage) you will find that the rear of the wing is 1 mm lower than the front.

[More experienced builders will perhaps like to build up the fuselage as follows: - From the bottom up - 1.5 mm X 5 mm basswood or spruce strip, 5 mm X 5 mm hard balsa strip, the upper part of the fuselage can be Medium/Soft grade balsa.  This method will result in a strong/stiff/ light fuselage.]

Before sanding the rear of the fuselage to an oval section, glue ('Cyano’) the basswood or spruce strip to the bottom of the fuselage.  It is shown on the plan as 2 mm X 5 mm but 1.5 mm X 5 mm will suffice.  Round all the edges of the fuselage.

Join the wing to the fuselage with Epoxy glue.

Before fitting the tailplane sand the bottom of the fuselage rear end to accommodate the tail tilt (3 mm).  Use 'Cyano’ glue to attach the fin and tailplane.  Ensure that the fin is vertical.

Cut the finger grip from 5 mm balsa - bevel and taper to fit the underside of the right hand wing - ensure a good fit between the wing and fuselage as this will impart added strength to the wing / fuselage joint.

Finish the model with 2 to 3 coats of non-shrinking dope, sand with fine ‘wet and dry’ paper between coats.

FLYING

Build your model exactly to the plan, the scales should show an all-up weight of around 16 grams.  A visit to the local field (with long grass) will be required for testing, calm evenings are the best times for first flights.

Using Plasticine clay stuck to the nose, an initial balance point at around 50% of the wing chord should be established.  Check also that neither wing is heavier than the other - add a little plasticine clay tip weight to balance if necessary.  Test glide using shallow gentle hand launches, add or remove Plasticine clay until a flat shallow glide is achieved.

Once a flat glide is established - ensure that the model is also turning to the left.  If it is not then gently bend the rear of the fin to encourage a left turn.  When all is correct Thermal Scout should be launched up at an angle of about 40 degrees and to the right of the wind (with the wind in your face).  Launch by trotting forward a few steps and use a whipping arm action to launch.