A 1/4-Scale Model of the 16 metre Club Class Sailplane
designed by CLIFF CHARLESWORTH
originally published in the RCM&E magazine August 1979
I was on a gliding holiday with my son when we I first came across the ASK-18 in the hangar. It was the first that the Club had purchased andnaturally it was a valuable addition to theClub's fleet. It was, to me, like a overgrown model glider and just asked to be reduced in size. It is a simple pleasing design and so practical in these days of modern high-speed glass fibre sailplanes. It's a nice change to see a manufacturer come out with something quite basic like the K-18.
The model was based upon K-18 No. 174 based at Dunstable and I am indebted to London Sailplanes Ltd.. and the London Gliding Club for their assistance.
The model is not for the faint hearted, but a good average builder should not have any problems. If you like big sailpianes without breaking the bank, then have a go at this one and have a ball. Already I am building 174's twin sister. No. 294 - see you on the slope.
Commence by cutting all ply formers, keel and wheel box parts to shape as indicated on the plan. Ensure that all the locking slots are cut in the ply parts and carry out a dummy run on the centre assembly. Ensure all stringers are made from good and straight grained spruce and the 1/4 x 1/2in. spars are medium hard balsa.
Lay the keel pieces on the plan and glue together. Scarf joint the 3/8 x 1/4 spars and taper them towards the nose as shown Mark all the former positions onto the keel, then position the keel vertical with support front and rear to prevent any droop. Add the forrners beginning with the centre sub-assembly. Main longeron slots should be parallel with the worktop. Add the longerons and stringers making sure that no bowing or drooping is evident. I found it best to steam bend all stringers and main longerons. Add the wheelbox inner parts and 1/4in. sheet filler pieces between the nose formers. Drill the axle holes in the wheel box and insert a 31/2in. airwheel and axle. Fix a length of 8swg tube at the rear forthe fin fixing, but file an opening in the tube to line up with a self tapping retainer screw mounted in small hardwood block. Cut and shape a hardwood tailskid mounting block and locally build up the keel in thickness to carry this, then glue the block in position. Sprung tailwhecl assembly is optional on the full size glider so you can take your choice with either a hardwood stump or a sprung tailwheel; the one on the prototype was made from an old clockwork motor spring and works well. Add a filler piece at the top of the fuselage to form a mounting for the tailplane. Add a 1 mm ply piece at the bottom to take the heads of the two bolts, then add rudder and elevator cables.
With the fuselage set up on the bench pack the tail up to 1 degree positive incidence and glue the required faired-off packing on to top of fuselage. Drill mounting holes and insert the two 4BA bolts through from the bottom and epoxy the bolt heads to the plywood. Sheet the fuselage from the nose former to the aft centre section former and blend the sheeting out along stringers to prevent a step where the sheeting ends. Add the nose block, then carve and sand to shape. Add 1/4 x 1/4 in. hard balsa to the top of the longeron in the cockpit area.
If you have chosen the two piece wing version now fair the underside of the centre section to the fuselage and add the curved fairing at the rear. Do not sheet the top nor fix the wing tongue socket, that is fitted later when the wings are complete. Sand the centre fuselage and give it a coat of sealer on the outside to add strength to the nose area. I laid a single layer of chopped strand glass fibre matt in the cockpit and nose, using a small stippling brush to get it into the corners. The tow hook release on a K-18 is directly in front of the monowheel so we have to use a little modelling licence and mount it throught the 1/4in. sheet filler pieces at the side of the keel.
Finally, prior to covering, fit a I / l6in. plywood plate running from the underside of the nose and stopping just short of the mono-wheel. Make up a canopy frame from two layers of 3/16 x I/l6in. spruce for front and rear frames, and side members from 3/8 x 1/8in. spruce. Hinge the frame on the starboard side of the cockpit and fit a canopy lock on the port side
Wing Construction - three piece
Having got this far, check to see if your bench length will let you build an entire wing half in one go, if so I recommend it for obvious reasons. Lay the ribs on the plan at the respective locations and divide them into the required sections. If the mainspar widths are at maximum wood tolerances then cut accordingly. Cut out areas in the ribs for airbrakes, if fitting the commercial type, then finally drill the ribs for locator dowels and control cables.
Now assemble the ribs to the mainspar, false leading edge and the base of trailing edge, packing up at the front to suit undercamber. Add the trailing edge stiffener pieces between the ribs, fair these pieces off and add the top trailing edge. Build the aileron into the wing and cut it out later when sheeting - don't forget to mark it! When all the open structure is completed add the control cablec and airbrakes.
Add the washout by blocking up with a tapered spruce wedge. Sheet the top of the wing; complete the bottom sheeting after fairing-off the aileron spars. Place the wing back on the board the right way up and replace the washout wedge in position, place weights on the top of the wing and leave it until the glue has completely cured.
Repeat this procedure for the opposite wing leaving the root ribs out to allow aluminium joiner plates to slot on to the mainspar. Having separated the inner and outer panels from each other proceed by joining the two inner panels by bolting and gluing up to form a good joint.
Add spacer blocks for the rear wing nuts. Add the front and rear fairing formers and if room will permit add the front wing locator dowels. Assemble the centre portion of the wing to the fuselage and build up fairings to match the fuselage sections.
Cut out access bays on the underside of wings for servo bays. At the joints of outer and inboard wing panels, controls are joined with electrical terminal blocks connectors. Wing joiner dowels are locked in place by means of self tapping screws. For two piece wings the method of construction is similar and should not provide the average builder with difficulties.
Two piece wing only
With wings complete the final operation on the centre section must now be completed.
Assemble wing blade joiner and 8swg tubing into the centre section. Holes should be a slack fit: you need plenty of space for this operation! With fuselage set up so that the main longeron is level, assemble the wing pieces onto the blade joiner and with a straight edge ensure that the wing incidence is zero. Pack up the joiner and 8 gauge tube until this is obtained Check the dihedral angle and spot glue with epoxy both joiner and tubes. Remove the wings and complete the assembly of the wing joiner, insert the servo trays and complete the wing fairings. Cut out the hatch areas and arrange a lip and screw fixing, fair off and sand.
A tapered symmetrical section is used so taper the trailing edge spar and elevator leading edge prior to assembly. Cut out the ribs using the templates shown and proceed with assembly. Centre blocks need to be hard balsa with 0.8mm ply facing. Ensure spars are equally supported on both sides, then add ribs and rib gussets. Add the top pieces and hinge point hardware blocks. Remove the structure from the plan and ensure that the edges are bevelled to suit the rib contour. Sheet the top and bottom with the sheeting overlapping the trailing edge. Fair off the elevator hinge gap, add the leading edge, cut the hinge slots and sand to shape. Cut slots in the leading edge of the elevator and rebates in the trailing edge for hinges. Assemble the elevator and epoxy the hinge pins into position. Add the control horn mounted in a hardwood insert.
Cut the false leading and trailing edge spars, and shape the fin ribs from templates shown on the plan. Drill the bottom two ribs to take 8swg wire dowel. Cut ply reinforcement for the bottom rib and cut a location rib from 1/ /l6in. plywood or l/32in. aluminium. Assemble the fin over plan and fit hinge point blocks. Cut the hinge slots in the rudder leading edge but take care with the top one because the spar section at this point is thin sheet on both sides of the fin overlapping the spar to semi-fair the rudder hinge gap. Insert the horn into the rudder with hardwood block, add the trailing edge of the fin and sand to shape.
With the three piece wing version, access will have to be made for extension connectors passing through the main bulkhead and top deck to connect to wing servos on assembly.
Two piece wings will have two servos permanently installed in the centre section. Receiver and airborne battery are placed behind the cockpit dashboard with rudder and elevator servos either side of the keel just in front of the main bulkhead. The switch is mounted on the cockpit dashboard which is screw mounted to a former in a similar manner to full-size. Make a pilot's bucket seat from ply and balsa, arrange for a mounting just above the rudder and elevator servos. As for the remainder of the cockpit detail well this is up to the individual
If possible obtain a 1/4 scale full length pilot. Balance the aircraft at exactly I / 3 of the wing chord back from the leading edge, the model should be in a slightly nose down attitude.
My model was covered in tissue but with the model being used more on the slope than on the flat this as a big mistake: my second model is to be covered in nylon Ensure that all structures are sealed and sanded prior to covering in nylon. Use a close weave light-weight nylon. Three coats of dope plus sanding sealer should give the desired finish to the skin.
On the wings do not use full strength dope otherwise the result will be a wavy trailing edge. K-18 full size gliders sold in this country all have a basic white finish, some are true white, some off white. I used (British Leyland) white primer and 'Old English' white for finish, easily obtainable from most Leyland agents. Colour trim on fuselage, fin and wings is either orange, gold, blue or green, with the Schleicher motive ASK-I8 in the same colour as the trim, mounted on the port side of the cockpit.
My first flight with the K-18 was made from Ivinghoe Beacon in rather windy conditions. Having gone there by myself I was grateful to find someone to help. With such a large fuselage to get hold of it's nigh impossible to launch it by oneself. Because of the high wind there was no need to run with the model, just a gentle push with a nosedown attitude was all that was required to make a dream become reality.
Penetration was good, picking up speed in a slow descent and gently easing back with a little trim created a beautiful climb out to approximately 500ft above the Beacon. Turns with rudder and aileron and a touch of elevator were flat, like the real thing a dive followed by a good fast pull out and a bootful of rudder produces a great stall turn.
I continued to fly for approximately 3/4 of an hour and can honestly say that I have never enjoyed any flight as much as I did that one with the K-18
Since that day the original model has completed some 48 flights of varying duration and has flown well in thermals. From the flat she tows up well, although the first time nearly ended in disaster due to a loose tailplane. However, since then she has always performed the way her designer intended.