INTERAVIA SL-90

by Graeme Hunter

Plan No. 698

Chamfer S2 at the front edge to butt against F4 then cut at rear so S2 overlaps F5 by 1.5mm. Fit FS3 brace in position before S2 is glued to S1 otherwise it is impossible to get FS3 in place – as I found out! Now glue S3 in place. Elevator and rudder push rods should be installed now (I used closed loop cables on rudder and separate ny-rods for each elevator half). The fin is best installed while the fuselage is pinned down to ensure it is properly vertical. Fuselage top sheeting and WM is now glued in place. Hint: even though the fuselage has lots of room access is difficult through the wing mount. If desired a hatch could be cut around the rear top window or the front windscreen could be made removable. Engine Pod Engine and mount combination should be chosen at this point in case engine pod sides (T2) need to be lengthened or shortened. Remember to make R & L sides as this sets right and left thrust and also down thrust. Amounts shown on plan prove to be sufficient during test flying. Remember also to offset engine mount to left of bulkhead centre line to ensure that engine crankshaft exits the cowl in the centre. Assemble pod then glue in place in fuselage. (The engine I used was a new in box YS-53 FS provided by my friend Shane. The engine was run for one hour on a test stand before being fitted to the aircraft.) Front Canopy The canopy was made from a block temporarily glued in place and shaped then removed and hollowed out and refitted. Fuel proof everything forward of F3 before gluing this on as it is easier to do at this point. Cowl The cowl is made from soft 12mm sheet temporarily glued in place and shaped, then removed and cowl mounting blocks fitted. When building the cowl make sure there is plenty of area for cooling air to exit. The left radiator area was blocked off as air going in here does nothing to cool the engine except it could create an air dam which will very quickly cook your engine (I learnt this a long time ago from bitter experience). The exit area should be about 2.5 times (or more) the intake area.

Wing
Wing is conventional built-up spar and rib construction, fully sheeted. It could be made using balsa covered foam core if desired. A ply template was made to the airfoil section and this was used to cut the ribs out by pinning the sheet to the cutting board and cutting around with model knife. You could also use the “Sandwich Method”.
The centre section was assembled over the plan including the dihedral braces. Then removed and wing blocks and top/bottom sheeting added.
Hint: make sure holes are drilled for wing bolts and the holes for flap/aileron leads are cut into ribs before adding sheeting.

The right and left wing panels were assembled over the plan. The wing bottom skins were made sufficiently large enough to go from leading edge to aileron trailing edge. The skin was pinned to the building board and the wing panel was glued in place. The flap/aileron LE and riblets were added at this point. A slot should be cut in the first 3 ribs between the spars to allow for the dihedral brace. At this point strut mounting hardware should be added, I deviated from the method shown on the plan although it would work just as well except it made it a little more difficult to finish around. A ply block was glued in place at the position shown, over the bottom spar and a T nut (6-32) was fitted. The same method was used for the mounting point in the fuselage with the strut being screw on from below the fuselage. I also made the elevator support struts removable for the same reason. Make up wing top sheeting, it only needs to be wide enough to go from the false leading edge to the rear spar. The flap/ailerons were cut from the rest of the wing and the top sheeting added to these. The wing panels are now glued to the centre section. The method I used was to first tack glue a block at the tip of each panel to hold that end the correct distance off the board, then the centre section was pinned down flat and the panel epoxied to it. When dry the other panel was added.

The temporary block is now removed and one wing panel pinned down while supporting the other, (no wash out was used), and glue top wing skin on; repeat for the other panel. Leading edge is now added and shaped, the wing tips are also added and shaped. The ailerons and flaps are now fitted to each panel and hinged, then separated from each other.
The wing struts are made from two pieces of spruce epoxied together with a strip of brass sheet sandwiched in-between at each end. When dry these were sanded to a streamlined section then holes were drilled through each end and toothpicks were glued through these to ensure the strips do not pull out as the struts are functional.
The flap and aileron servo trays were now made and fitted in position.
Hint: standard size servo’s are a tight squeeze so smaller ones could be used.

Finishing My preference was to tissue and dope then paint, but to save time while various friends were searching the net for colour schemes and markings I decided to use one of the paintable iron-on films. This would allow me to use whatever colour scheme may come to light. As it turned out we were unable to locate any information re markings and colours so the aircraft was painted all over with aluminium coloured high temperature engine enamel. The windows were black sticky back film. A red star was added to each side of the rudder and two thin red stripes were placed down each side of the fuselage. The plan designation (SL-90) were added to the wings - one set on top left wing panel and the other on the bottom right. This completed the colour scheme and in flight looks good.

Flight Testing
With the aircraft finished and ready to fly it was balanced and control throws were set up as follows :-

At the invitation of Brian Winch (Airborne Engineer) flight-testing was carried out at his club field and he also took the first flight pictures. On arrival at the field we found it was open and flat and the weather conditions hot, dry and calm. After assembling the aircraft a series of ground shots were taken. It was now time for the first flight. On taxiing out the first glitch appeared, full up elevator would not stop the aircraft nosing over (I had added some lead to the nose to ensure the CG was in a safe position). The undercarriage was bent forward and the second attempt took place. We got to the strip OK this time but on take-off enough up elevator was not held in and the aircraft nosed over again (bugger!!!!). On the next attempt full elevator was held in until a fair bit of speed was gained then eased off and the plane ran along tail-up (looked great) and gently lifted off and climbed out smoothly. A couple of circuits were made while minor trim corrections were made to enable straight and level flight (did not need much). The aircraft was then brought in low, slow and close so that the flight pics could be taken.

At height the stall was tested and with low throttle and full elevator the plane just mushed straight ahead (did not drop a wing, unless rudder was applied which lead to a gentle spiral dive). The flaps were also tested; these proved to be effective with the travel available. If applied with too much airspeed the plane zoomed gently then settled into a slight nose-down attitude and slowed down. If applied at normal approach speed the plane just assumed the slight nose down attitude with no zoom. The landing was no problem and with the flaps down it occurred at a very low airspeed on the main wheels and rolled out perfectly. On the next flight the aircraft’s aerobatic capabilities were tested –loops, rolls and inverted flight were accomplished easily but it would not spin.

The lead was removed from the cowl and more flights followed (still won’t spin with the rudder travel used). The CG as shown on the plan combined with the suggested control throws should result in a smooth flying aeroplane. The YS-53 provided sufficient power while not being overpowered. Those present at the test flights were impressed with its stability and performance.
All in all a very enjoyable aircraft to fly.