I wanted a model for winter flying, the requirement being that it should fit in the car in one piece, be tough enough for rough-field abuse and be easy to hand-launch by the pilot. I quickly decided that it would also need to suit the O.S. 25SF that I'd just picked up on eBay. Oh, and be reasonably attractive!
Browsing my books and photos looking for inspiration, I was reminded about a very nice Cassutt-style model that I once had, which I duly sold to one of my clubmates. Anyway, I eventually came to the conclusion that something along those lines would be ideal and finally decided on a model based on the Monnett Sonerai. This, for those whove not heard of it, is an American home-built Formula V racer that's been campaigned in various configurations: shoulder wing, low wing, tail dragger u/c and trike u/c. The Sonerai II is a two-seater that's stressed to ±6G for aerobatics, albeit with only the pilot on board.
Now then, do note that my version of the Sonerai is only based on the full-size. In other words I looked at the three-views and then drew a model that matched the shape so that it looked roughly right. As I call it: 'stand way off' scale! This does mean that you're not duty-bound to produce a scale finish and so can create your own colour scheme. Construction is very simple, and quick. I took my time with the prototype but it still went together relatively quickly.
The wings are needed at an early stage in the fuselage construction, so I built them first. There's no dihedral so they can be constructed in one piece, flat on the board.
The spars and t.e. cap strips aren't joined in the middle, but out towards the tip. Use 36" material lengths and make a scarf joint at least 1" long on the end of this.
These joints should be staggered, i.e. the bottom spar will be joined at one end of the wing and the top spar at the other end. The l.e. sheet reinforces the joint. Since these joints are positioned towards the tip they take much less load than if they were in the middle, and you also save on material.
Right then, pin down the lower l.e. sheet, which will be joined under one of the 1/2" R1s. Prepare and pin down the t.e. cap strip followed by the rib cap strips. Pin and glue the lower centre-section sheet into position, remembering to leave the gap for the servo bay. Glue and pin down the lower spar, fit all the ribs, then add the t.e. and leave to dry. Time to fit the top spar, remembering to stagger the joint. Chamfer the bottom of the 1/8" sheet l.e. to match the ribs, and glue this in place. Fit the sheet infill at the centre where the wing bolt goes through, and add the scrap blocks at the hinge locations.
Carefully now... bring the lower l.e. sheet up to the l.e. and glue in place. I used lots of scrap t.e. stock to wedge the sheet in place, gluing it to the l.e. with aliphatic resin and then running cyano' down each rib.
Fit the 1/16" sheet webs to the front of the spar, and then glue the top t.e. cap strip into position. Time to fit the top l.e. sheet, joining it, as mentioned, on the opposite side to the lower sheet.
As you'll see from the plan, good old traditional push-rods and bell cranks are used to transfer the servo movement out to the ailerons, and now's a good time to install both. At the servo location you can slide the link wire (from the output arm) and the two aileron pushrods into a single piece of brass tube, then solder the whole lot together. The connection is a little tricky to set up with the wing still on the board, but it's worth the effort.
Moving on, glue in the 1/8" liteply servo mounting plate; I fitted Radio Active servo brackets to this, and recommend that you do the same. Now add the upper centre-section sheet and top cap strips, and when these have dried, your perfectly flat wing can be removed from the board.
Ailerons can be shaped from 3/8" sheet and temporarily hinged whilst the pushrods from the bell cranks out to the control surface are made up and installed. Once this has been done, deck the underside of the bell crank bays with 1/64" ply to facilitate a neat and easy covering job around the slot. The wingtips are made from 1/2" sheet and should be fitted before final trimming of the ailerons.
Start off by cutting out all the parts, and then begin the assembly by gluing the doublers to the sides with impact adhesive. Prepare F1 by fitting the 'T' nuts that will accept the engine mount fixing bolts, noting that F1 has a hole for the tank neck.
Lay one fuselage side down and glue F1, F2, F3 and F4 into position. When dry, fit the other side, making sure it's perfectly aligned. When the glue has set, pull the sides in at the rear and join with the scrap infill before adding formers F5 to F8. Fit the triangular stock behind F1 and add the 1/4" ply pieces for the undercarriage mounting. Now add the 1/8" ply wing bolt plate to the fuselage, not forgetting the ply sockets that hold it firmly in place. Fill the space above this with scrap sheet to provide a suitable seat for the centre-section trailing edge.
Turning our attention back to the wing for a minute, drill the holes in the wing for the l.e. dowels, marking their positions through the holes in F2. Make sure that they're perfectly aligned before drilling. This is a critical operation, so take your time.
Okay, locate the wing and carefully drill down through the wing bolt location. Fabricate the retaining nut plate using scrap balsa as a leveller, then glue it in place and bolt down the wing. Leave to dry before removing the wing once again. Fit the snake outers, the tailplane platform and tail skid mount.
I don't know about you but I always find sheeting the fuselage quite satisfying, and were going to start the process right now. So, working on the underside, apply the balsa cross-grain leaving a gap where the undercarriage wires go. Within the fuselage you'll notice a ply plate that reinforces the wing bolt access hole; glue it in now. Oh, and while you're at it, add F4a.
Re-fit the wing and then glue F2a, F3a and F4b. Fit the 1/4" square spine and remove the wing.
Add the rolled sheet over F1 and F2, using wood that bends easily across the grain. Cut these pieces slightly over length and glue them to the fuselage sides, at the same time gluing two further pieces of sheet to the sides for the rear decking. When the glue's dry, wet the outside surfaces with water and apply heat whilst bending the sheet to fit the formers. Trim the front decking sheet to meet the spine and then glue in place. The rear sheet is left slightly higher than the formers so that it can be sanded back before gluing on the 1/4" sheet fuselage capping.
The cockpit area over the wing is made in the same way. Referring to the plan side view, make and shape the bottom of the sheet until it fits the wing. Cut away the area where the cockpit opening is, and glue the sheet upright to the wing. Again, wet, heat, and trim to fit the spine.
When making the cowl, fit the engine then glue the nose ring and spinner back-plate together with a 1/16" scrap spacer in-between. Mount this unit on the engine and build up the cowl to the nose ring. When this is dry remove the engine and fit scrap triangular stock to the corners. Shape the cowl accordingly.
For the blocks that go either side of the fin, start by spot-gluing scrap 1/4" sheet in place of the tailplane and fin, then spot-glue pieces of 1/2" sheet each side, shaping the whole lot to match the fuselage. The parts are then separated to leave two blocks that will fit perfectly. Don't fit the tail until the model's been covered.
Carefully drill through the 1/4" ply undercarriage plate and vertical 1/4" ply pieces for the top of your pre-bent legs. Oh, and incidentally, do use decent piano wire for the undercarriage; I don't know who made the stuff I was using but it wasn't much better than wet spaghetti! Finally, here, remember to cut away the bottom sheet where the undercarriage clamps will go.
Whilst construction of the apple cheeks is fairly obvious, the canopy probably isn't! Start by carving a block to match the shape, sand it smooth, then fit it inside a plastic lemonade bottle. Wedge the pattern at an angle so the centre of curvature is in contact with one side and then shrink the bottle with a heat gun. It really is that easy!
COVERING & INSTALLATION
This full-size Sonerai in the USA has an attractive colour scheme. |
I wrapped the prototype using Solarfilm Supershrink polyester. This is my favourite covering material and is easy to use; it doesn't burn through if you're over-enthusiastic with the heat and it doesn't go slack in the sun.
Trim was achieved using Solartrim, and if you want to duplicate my colour scheme, the secret is to make a card template for one of the stripes and cut several (note that the blue one is wider!). A second template, of course, is needed for the stripes in the outer portion of the flag.
When it comes to general fitting out, there's just enough room for a 4oz fuel tank forward of the wing, whilst standard servos are comfortably placed as shown on the plan. The Rx and battery sit between F2 and F3.
Check that the C of G is as shown on the plan; although depending on the engine used you may need a little lead under the tailplane.
The usual winter wait for a decent flying day was frustrating, but it eventually arrived. A fine, fairly sunny and calm day in January. It was even reasonably mild! Our club rotates around flying sites and this session coincided with our visit to the old wartime airfield, which meant that I could fly from concrete.
The take-off was nice and straight with no tendency to swing, after which the Sonerai climbed away in a positive manner. One or two clicks of trim saw her flying straight and level, whereupon I was ready to try her out.
I already had confidence in her as I put her into the first roll which was very smooth and axial. It was clear that I had a very nice model on my hands, and I had no hesitation in taking her through some stock manoeuvres; loops, bunts, square loops and inverted were all very easy to fly.
Flick rolls are fast and positive, and spins start and stop instantly. At no time did she try and bite. Everything was smooth and positive, a real confidence booster for anyone learning aerobatics. Stuart Pickett, who flies my models whilst I take the pictures, had enough faith to make low level passes within a minute of taking over, and some of them were very low indeed!
The glide is fast, smooth and flat, so there are no problems with dead-stick landings, either. Low speed handling is impeccable, with no tendency to drop a wing. SUMMARY
Here's a model that will go in most cars in one piece, even my Aixam microcar! It can be hand-launched solo once trimmed out and it'll do any aerobatic manoeuvre that you could ask of it. What more can anyone want?
Name: SoneraiAircraft type:
Semi-scale sportsDesigned by:
43'' (1092mm)Fuselage length:
35'' (890mm)Wing area:
3.1sq. ft.All-up weight:
3 lb 8ozWing loading:
18oz / sq. ft.Control functions:
Aileron, elevator, rudder, throttle
Rec'd motor: .25 - .30 two-stroke