1919 AVRO biplane scratch build in foam board

1919 AVRO 539B Biplane

UPDATE 16/01/2015 - Here's the plans for this build; 1919AVRO539b

Though the plane appears here as a tail dragger it was originally designed and built as a seaplane to compete in the 1919 Schneider Trophy event held in the UK at Bournemouth. Given my present flying skills, building it with floats would be a disaster waiting to happen - so I stuck with the wheels.

 It all starts with a plan…. courtesy of the interweb. 

Next – how big to build it? So that I could make the wings in a single piece I re-scaled and printed the drawing to make use of the longest edge of my A1 foam board sheet (841mm). Using the Baby Blender for comparison - the wing chord of the AVRO 539 is less, but the wing span is greater. Doing the sums it turns out the wing area is almost identical - so the wing load will be very similar to the Baby Blender.
 

This wingspan decision dictated all the other sizes. I cut out the pieces from the scaled print-outs and transferred the outlines and hinge detail etc. to the foam board.
 

I used the plan and elevation to help lay out a typical FT style fuselage – top and two sides. To create the gradual curve on the rear of the fuselage I took the top panel all the way to the tail.
 

Here it is with all the basic components added – tail, servos, power pod, turtle deck formers and top wing support. I added a double-layer cross piece at the front to rest the undercarriage against. Note – the servos are mounted well forward to help with balance.

The top wing support is screwed into 4 tongue depressors – 2 glued on the top and 2 trimmed ones on the inside. 

The wing support frame is made-up of two identical pieces of bent wire that are bound together with thread and CA glue.

I hoped the frame would be stable enough to support the wing but it was a bit too springy, so I added thin bracing wires to create stiff triangles. The top ‘skid shaped’ wires are for attaching the wing elastics. 

Knowing the chord and wingspan, I was able to lay out a typical BB2/Cruiser type wing. The only addition to the basic design are anchor points for the struts. I described them as ‘brace’ points in this drawing. This is the bottom wing, which isn’t as wide as the top wing – that wing goes right to the edge of the foamboard.

After I cut this first wingtip, I used the offcut as a template to draw the other three. 

I use spare servo arms as the attachment points for the struts. These poke through holes I cut in the foam, and are glued on the inside of the wing with a short length of skewer through the end to spread the load. The bit that sticks out has useful holes for fitting wire to. I didn’t intend these to be too structural, just help maintain the spacing between the wings - and they’ll definitely add character.

The upper wing is flat and the lower wing has dihedral. To make-up the lower wing I bent a piece of 2mm wire to the correct dihedral angle and embedded this in my wing spar. This made assembly much simpler as the spar helped to produce the correct dihedral as the wing was assembled. The wire will add some strength, but was mostly just an aid to assembly.
 

The aileron servos were sunk into the wing during assembly. 

The bottom wing centres using a pad glued to the top of the wing, which fits between the fuselage sides. The back rests up against a stiffener running across the fuselage.  

The top wing centres using this rectangular pad, which fits inside the wire frame. 

I used thin wire to make-up the struts that space the wings. I left the ends at the top a bit longer to make it easier to get them in and out just now. 

Here’s the tail detail. I added a strengthener inside the fuselage, and included a little cut-out to help stiffen the rudder keel. I also added a steering tail skid. 

Here’s a test fit of the paper templates for the turtle deck. I later included a hole above the servos to avoid the need for a removable section. 

The undercarriage is held on by elastics. At the back I use an extra loop in the wing elastics and at the front a dedicated elastic/skewer. The wheel axle is sprung by passing it through two twisted prop saver rings. The landing gear frame is made-up from two mirror-image parts bound together using thread and CA. 

I like the overall look of the plane - here it is in comparison to the original.

Almost there - the whole plane has been sprayed with clear matt varnish and is ready to maiden. In this picture you can see the servo access opening hidden under the top wing. I blanked out the wheel spokes with foamboard and card for a better look.  

Foam radiator and headrest fitted - just the windscreen to add to the cockpit, and a final paint job.

I'm very pleased with how this plane has turned out; compact, classy and stylish.  

Here she is with a headrest, windshield, paintjob and a pilot. Those are bigger wheels on the front, which have mostly allowed 'proper' landings. I filled-in the spokes with foamboard using the method from my "Olde-style wheels for olde-style planes." article. You might notice the nose is slightly remodelled. I had a prop-saver fitted, and the prop got knocked off-centre without me noticing - next time I powered-up I 'shaved' the front end down. A little outline plate cut from pizza base foam has neatend it up a bit.

You can see the little beast fly if you follow the Flite Test links below.

Cheers to all in the Flite Test community, all those shared ideas and enthusiasm have helped make this build possible. alibopo.

UPDATE; the plane had a 1700KV 28A motor to start with, but current draw was quite high with the 9x3.8 prop I was using. Time in the air wasn't really that good. I tried a smaller prop, 8x4, to allow the motor to use its revs to produce the thrust - hoping for more efficient use of the motor - but I think too much thrust was lost pushing air against the blunt nose. A bigger prop pushes more air out beyond the blunt nose, so not so much thrust is lost. With all that in mind I decided to try a slower revving motor, better suited to turning a bigger prop. I went for an EMAX GF2215/20 1200KV which theoretically draws about 20A with a 10 x 4.7 slow flyer prop. I only had 9 x 4.7 prop around, so tried the plane with that. It flew fine, with plenty of get-up-and-go for the take-off. I'm guesstimating my max current draw from this 'under-propped' setup is somewhere around 15-16A. This should give me much better flight times, though I've still to get a chance to confirm this. Last outing, with the new motor and prop, shortly after I got the plane in the air the wind picked up and I was happy (relieved) just to get it down without mishap!