Outsize in gliders and one of the most consistent performers amongst the l946 British designs is Roy Yeabsley’s ten-foot Sunspot.
You can build it in just over a week and the total outlay is only a fraction of that needed for a petrol model. The ultra-light wing loading (weight is only 2,75 lbs.) rules out major crackups due to high-speed contact with cars and other obstacles on the deck. Exceptional stability enables you to keep flying when other jobs are grounded by rough weather.
As for performance – the model has practically been all over the country on a solo goodwill trip! Best official flight at the time of writing is 20 min 10 sec on a cloudless day – the model disappearing out of sight overhead dead above the launching point. On a 300 foot towline, consistent non-thermal flights of three minutes are turned in every time.
A dethermalizer is essential for this design. One of the parachute variety is used on the original model and is released by means of a slow-burning fuse instead of the usual timer.
Pin down the longerons on to the side view of the fuselage and add the spacers in the usual way. If you have to join strip for the longerons, make a lap joint midway between the wings and tailplane.
Build the other fuselage side on top of the one already on the plan and when this is quite dry, take them both up and sand them to exactly the same profile shape before slicing them apart with a razor blade.
The two side members are initially joined by the top and bottom side spacers at the wing position. Loop rubber bands over them to avoid springing apart again then join together at the tail and bind with thread. Add all the intermediate top and bottom spacers, then pull together at the nose and add the remaining spacers forward of the wing. The nose block is now roughly shaped, cemented in place and then sanded to continue the smooth fuselage curves.
Three tow hooks are fitted, front hook for testing, the centre for general flying and rear for calm weather, and all of them actuate to the towline. Two cross pieces of 1/16 in. sheet are fitted to take the central hardwood beam ” A “, to which the pivoting hooks are attached. When the hooks have been bound and cemented in place, a piece of fishing thread is tied to them and passed through to the tail where it is temporarily anchored.
The incidence block is carved from medium balsa, but it is cemented in place until final adjustments have been made on the field. Mould the canopy, using one of the patent liquids now on the market and likewise leave off until the incidence block is finally cemented to the fuselage. Well cement the wing and tailplane retaining dowels in place. Sheet the nose with f /8 in, sheet where indicated and attach the bamboo landing skid.
FIN AND RUDDER
Lay out the outline for the upper portion, using 1/4 in. sheet for the L.E. and 1/8 in. for the T.E. When dry, remove from the plan and add the 1/8 in. sheet ribs. Then make the movable rudder by cutting the T.E. and adding the small riblets as shown. Use two pins for the pivot points. Cement to the fuselage noting that the L.E. passes through to the lower fuselage spacer and fill in with scrap between the fuselage and the first rib. Leave enough room for the tailplane to be pushed through easily. Sew and cement the 18 gauge wire to the bottom rib of the rudder tab. This is tensioned on the port side by a rubber band which holds on port rudder all the time the model is flying. The thread from the top hooks is fastened to the other side of the wire, so that when tension is applied by the towline, the rudder is pulled straight for a straight tow up. A piece of I /32 in. ply (stop ” x “) prevents the rudder from being pulled past the central position by towline pressure.
Use 3 /8 in. sheet for the lower fin, with 3 /8 in. square central spar continuing up through the fuselage. Taper the outline to a streamlined section.
First, make the tongue boxes from 1/8 in. sheet and 1/4 in. S.Q. sides and insert them into the 1/2 in. deep slots provided in the first four wing ribs. Then join the 1/2 in. by 1/8 in. lower spars at the dihedral break and cement the keepers on either side. Cut out all the remaining ribs and start the assembly of both panels to ensure that they are both identical. Pin the spars down on to the plan and cement the ribs on the inner portion-then pull out the pins and tilt the spars so that the outer tip portions are flat on the plan and attach the outer ribs. Add the 1/8 in. balsa dihedral keepers to the 1/4 in. square and 1/2 in. by 1/8 in. upper spars, then slot them into the upper notches cut in the ribs.
With the inner panels flat again, attach the leading and trailing edges, then tilt over and add the outer lengths and the sheet tips, The outlines are aII roughly carved to shape before assembly and then finished after cementing in place. Note that all ribs are notched (one quarter) | in. into the trailing edge and that 1/8 in. gussets are added at the root ribs and dihedral breaks. The leading edge is covered with 1 /32 in. sheet to ensure a smooth airflow over the wing. Use Scotch tape to keep the sheeting in place whilst the cement is drying. Lastly, face the two root ribs with 1/16 in. ply and check all the cemented joints carefully to make certain that none have come loose.
Cut the plywood tongue from 1/4 in. ply and cut out 6 similar ribs-two from 1/8 in. and four from 1/16 in. sheet. Thread the ribs on to the tongue and then add the three spars as you did on the wing. Check that the outer ribs line up with the root wing ribs, by attaching the wings to the center section. If this is O.K. cement on the leading and trailing edges and cover the nose with 1/16 in. sheet.
If possible use a single piece of hard 1/2 in. by 1/8 in. balsa for the spar steaming to achieve the bend at the center section. All except the 1/8 in. center ribs are cut from 1/16 sheet.
Pin down the spar over the plan and cement all the ribs in place, next add the 1/4 in. square leading edge and the 1 in by 3/16 in. trailing edge. Lastly, cut out and cement the tips in place, sheet the center section and finish off with a fine piece of sandpaper.
The original model is covered with ordinary tissue for lightness, but a sturdier model will result if petrol model covering is used instead. Each wing panel will need about six separate pieces of covering and be certain that the paper sticks to the under the cambered portion of the ribs.
Only use clear dope or the total light will go up by several ounces. Use four coats on the fuselage, three on the wings and two on the tailplane.
Before you take the model out on the flying field, check that the flying surfaces are quite true and unwarped. Put on plenty of rubber bands to keep the wings and tailplane firmly in position.
We suggest that you take the model out on a fairly calm day for the first time, as it is impossible to be really certain if you have trimmed correctly in gusty weather. When launched gently from shoulder level into wind a long flat slow glide with a gentle curve to the left should result. And slight stalling or nose down conditions can be counteracted by varying the amount of weight carried in the ballast box. To avoid carrying too much dead weight in the form of ballast, the wing position can be altered slightly, but keep it as near to the position shown on the plan as possible. It should not be necessary to alter the angular setting of the wing and tailplane-careful construction will ensure that they are identical with the settings shown.
- Aeromodeller magazine, issue: February 1948
- Plan hosted on Outerzone