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Construction Article Heading Blue Wrn Head

2m hand launch thermal glider

by John Lynch Blue Wren Pic

Introduction

Whilst not a serious competition type RC hand launch glider, the Blue Wren is a good all round fun type 2 metre glider which has been hand launched successfully.

Weighing only 17 oz ready to fly, and with a wing loading of a mere 3.75 oz per sq ft, it will stay aloft on the lightest lift, and the all-flying rudder gives excellent controllability at low speed, allowing it to be flown in very small diameter circles.

There is no greater thrill than thermalling away from a hand launch. As a matter of fact, the day after the initial test glides was one of those magic days with beautiful little fluffy cumulus clouds everywhere, and a 2 knot breeze. On my second hand launch I hooked into a little bubble of rising air and bingo, away it went, shrinking in silhouette, until it was just a speck in the sky. After 45 minutes and an aching neck, I found an area of sink and brought the Wren in for a landing just metres from where I was standing.

Construction

Blue Wren Pic

I find it best to cut out all the bits and pieces first; e.g. fuselage sides, bottom, top, all the formers, glue up F3 and F4 with dowels, wing ribs, dihedral braces, spars, trailing edges and so on. I sort of make a kit of it, then I can concentrate on the construction. I used a combination of glues: fast and slow cyano, Bondza and Loctite.

For the wing ribs, make a set of templates; i.e. two centre section ribs and one tip rib out of 1.5mm scrap sheet metal such as galvanised iron. Using the two centre section rib templates and the sandwich method, make sixteen 1.5mm ribs then three 3mm ribs. Then using one centre section template and the tip rib template, make seven 1.5mm ribs. Reverse the template positions and make another seven 1.5mm ribs of the opposite hand. Use the same templates and technique to make the riblets.

Having prepared all the parts, its time to start construction. Make sure that the building boards are straight and true with no twists or warps.

Tailplane and Rudder

Blue Wren Pic

Both are built over the plan in the usual way. Once the glue has dried, remove the parts from the building board, clean up the joints as required, then glue the opposite spar in place. After the glue has dried, sand to shape as shown on the plan.

To drill the holes for the 1.5mm i.d. aluminium tubes, place the elevator back over the plan and mark the centre lines of the tubes on both the outer and inner ribs, then carefully drill, starting with a 1 mm drill bit and working up to a 2 mm drill bit.

Cut the aluminium tubes to length, rough up the outside of the tube where it is to be glued, push the tubes into position and cyano in place. Crimp the inside ends as shown on the plan. The elevator is now ready for covering.

When the fuselage is built and ready for sanding, pin the rudder in its correct position, remembering the .8mm gap between the top of the fin and the rudder, then sand both the fuselage and the rudder for a perfect match.

Chamfer the leading edge of the lower section of the rudder at the hinge line at 45 degrees to the centre line, and fit the hinges.

Fuselage

Blue Wren Pic

Having already cut out all the parts and laminated formers F3 and F4, the job is easy.

Bend the 1.5 x 12.5mm ply doublers and the front of the fuselage sides to the required curve using a heat gun, then glue the doublers to the front of the inside of the fuselage sides.

Glue formers F1, F2, F3 and F4 to one fuselage side, making sure that they are set up .8mm from the bottom of the fuselage and that they are set at the correct angle. This is done by placing the fuselage side over the drawings.

Glue the ply doubler for the tow hook to the 0.8mm ply bottom. With the 0.8mm fuselage bottom aligned correctly to the fuselage side with the bulkheads attached, using fast cyano, glue the bottom of the fuselage to F3.

Slowly bring the fuselage side and bottom together, working towards the nose and gluing as you go. Make sure that the fuselage bottom is flush with the bottom of the fuselage side and square as you glue. Using the same technique, work from F3 to the tail, gluing as you go.

Glue in the fin post, top and leading edge, then the 5mm triangular longerons and the two rear 1.5mm laminated balsa formers.

Add the nyrods and elevator bellcranks, and run a thin wire through the rear section of the fuselage, exiting the fuselage just forward of the rudder hinge line through a small hole in the bottom of the fuselage just behind the tail skid. This is for pulling the antenna wire through later.

Glue the 5mm balsa wing saddle and the 5mm triangular section stringer in place to the top inside of the fuselage side making sure that the stringer is set down 0.8mm so that the ply top will fit flush. Glue the other side of the fuselage in place, making sure that the fuselage is straight and not twisted.

Glue the 0.8mm ply top in position, pulling the tops of the fuselage sides in to match the top at the base of the fin leading edge.

Glue the medium to hard balsa nose block in place. Hollow it out, as shown on the plan, in case nose weight is needed to achieve the correct balance.

Rough cut the balsa hatch block and tack glue it in position. Carve the hatch to shape, and sand the whole fuselage, including the hatch, to shape, as shown. Remove the hatch and hollow out the inside, leaving 8mm around the edges and 12.5mm at the front. Glue the ply hatch locater in place, and fit the hatch pin and brass tube to the front. Glue the retaining hook in place.

Blue Wren Pic

Glue the servo mounts in place. Add the false fin LE and the top of the dorsal fin.

Cover the fuselage and rudder separately, then fit the rudder and glue the hinges in place. I covered the whole airframe with Solarfilm because it's easy to use and relatively light.

For the front skid I used a length of rubber door seal, available from hardware shops. The profile is shown on the plan. It is hollow and has an adhesive backing. I cut the front to a point and glued it with cyano.

Wing

Blue Wren Pic

As the wing is one piece, I find it easier to make up a simple wing jig from pieces of 5 mm thick lamiboard (masonite) 40 cm wide and cut to length to suit the dihedral and polyhedral joints. Off-cuts of this material can be picked up for nothing from most cabinet makers.

Set these up with the correct dihedral and polyhedral (dimensions as shown on the plan), cover with gladwrap and you are ready to start building. For securing the spars, TE and so on, I use a small drill in my Dremel, which is slightly smaller than the pins being used so that I get a nice tight fit using a small hammer to tap the pins in.

Pin the trailing edge in position, and then the lower 3 x 6.5mm main spar, which has to be packed up by a strip of 1.5 x 6.5mm balsa.

As the centre panels of the wing are constant chord, the 1.5mm vertical grain balsa shear web is a constant height, so glue up the shear web separately, cut it to the correct size, then glue it to the bottom spar after the 2.5mm ply dihedral brace has been glued in place. Make sure that it is not crooked and that it lines up with the centre of the top spar.

Cut a nice tight slot for the 1.5mm ribs using two fin-tooth hacksaw blades clamped or taped together. To do this, obtain a hardwood cube with sides of approximately 50mm and line this up against the spar at a rib location. Using the hacksaw blades, gently cut a slot in the shear web at the rib location, using the block of wood as a guide. Repeat this at each rib position, then slide the ribs down into the slots and glue them in position. This is a very fast, easy and accurate technique. Don't forget to slide the rear spar into the ribs as you go.

Use the same technique to do the outer panels, cutting the shear web with the correct mount of taper.

After all the ribs, spars, leading and trailing edges have been glued up, added the 1.5mm triangular gussets, and the riblets (top surface only). On either side of the 3mm ribs at the polyhedral join, glue 3 x 3mm balsa ribs (made using metal templates). This is done to increase the surface area for attaching the covering material.

Make a sanding straight edge by gluing a strip of medium grade sandpaper to a nice straight piece of hardwood approximately 300 x 40 x 20mm. Gently work the sanding straight edge backwards and forwards in a chordwise direction to remove any high spots. It doesn't take much effort, and you end up with a nice straight wing.

Gently remove the wing from the building jig, and clean it up, removing any excess glue. Using your straight edge sanding tool, lightly sand the lower surface of the wing.

Remove the 1.5 x 6.5mm packing strip from under the main spar and replace it with a 0.8 x 6.5mm strip.

Add the under surface of the leading edge of the wing sheeting first, using 0.8mm sheet balsa. Before you sheet the upper surface, make sure that neither the jig nor the wing is twisted, because once the upper sheeting has been glued in place it is not possible to correct any unwanted twists. You can build a 6.5mm washout into the outer panels of the wing if you so desire.

After the wing has been completely skinned, remove it from the jig, glue on the 12.5mm balsa tip blocks and shape as shown. Give the whole wing a gentle sanding as required.

Feather the 0.8mm sheet leading edge skins to the leading edge. The 0.8mm sheeting will sit proud on the main spar so cut strips 0.8 x 1.5 x 75mm and, using cyano, glue these strips onto every rib, top and bottom, butting the strips up against the 0.8mm leading edge sheeting. Using the straight edge sanding block, feather these strips so that there is a nice flowing contour from the leading edge skin to the trailing edge of the wing.

I used Solarfilm for the covering, because it is reasonably light and durable. Covering material is important, because it is easy to add a lot of weight if you use a heavier one. For example, clear Solarfilm is approximately half the weight of coloured Solarfilm, and Litespan is half that of clear Solarfilm. I guess it's a compromise between appearance, durability and weight. The choice is yours, but the lighter the model the better it will soar.

When the model has been completely covered, and the radio installed (a standard receiver fits fine), check that the balance point is in the correct position and add weight if required. Use the length of wire that was left in the fuselage to pull the radio antenna through. It exits at the base of the rudder leading edge, as shown on the plan.

Flying

Blue Wren Pic

Pick a reasonably calm day for the initial flights. Gently launch the model straight ahead and check the control responses. Once satisfied, keep increasing the intensity of your hand launches and you will be able to climb more steeply. As the glider approaches the top of the launch, get ready to feed in a little bit of down elevator, otherwise it will run out of airspeed and the glider will stall.

I made a simple, lightweight bungee using 30m of 5mm commercially available bungee cord and 100m of 25 lb fishing line. It works really well with the Blue Wren, and it's no problem to get 300 foot launches. I've also been experimenting with a device similar to the woomera that the aborigines use to launch their spears. It's just a short length of broom handle with a ring on one end and a small U-shaped bracket lined with felt in which the glider fuselage sits. The ring hooks onto the glider's tow hook, and "whoosh", away she goes. I've had some good launches using this device.

The Blue Wren is a very stable, predictable glider that is a pleasure to fly. It will stay airborne on a very small bubble of rising air, will turn effortlessly on a wing tip, and will manoeuvre like a bird.

Have a go; put one together. It won't cost much to build, and you will have a lot of fun flying it.



SPECIFICATIONS

Blue Wren


Designed by John Lynch
Type: 2 Channel Lightweight Hand Launched Glider
Wingspan: 2 metre
Length: 1.1 metre
Weight: 500g
Radio: 2 Channel Radio Control with micro servos


Blue Wren plans are available from:
Airborne Plans Service

AU$22.00 plus AU$2.00P&H1
Plan No. 632

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This page was last modified on the 19-May-02