Pixel story

A few years ago F Cahour proposed this variable incidence wing glider . Pixel story was born !

Nowadays, Pixel is still produced and sold by Airtech : Pixel kit à construire

This glider is famous by its simplicity:

• 2 wings without ailerons nor flaps

• one fixed V tail

It means that this glider can be assembled in less than 5 minutes and carried into a backpack.

Counterpart of this simplicity : its a "variable incidences wings glider" with a very good tendancy to fly... and some difficulty to land !

A friend of mine gave me a Pixel fuselage, so I had to finish it !

Pixel specifications

• Wing Span : 1.6m

• dihedral : 1°

• root profile : SB96V, chord 180mm

• tip profile : SB96VS, chord 130mm

• tail profile : NACA009, chords 90mm/80mm

• tail length : 260mm

• tail angle : 110°

These dimensions were entered into PredimRC spreadsheet and gave these results:

Note that I have "rounded" the tip of the wings and approximated this into PredimRC with a short tip panel...

Important figure is the position of the Center of Gravity which is at 86.4mm from the leading edge with 1% static margin.

Mass centering must be done with this figure in mind...

To double check, I also entered the dimensions into newer Franck Aguerre's Gemini aerotools program.

And of course results were totally similar !

Building the wings and the tail

Wings

I use my program WiHoWi to design the wings and produce the Gcode to cut it into extruded polystyrene foam.

This was indeed the very first "real wing" designed and cut with WiHoWi and HoWiGS !

Now is the tricky part... These wings have to be reinforced and must be "inserted" into the fuselage with a strong rotary axis to allow "variable incidence".

So I designed a 3d printed part to be glued into the wing and which will guide a 8mm carbon rod. 

Dihedral angle is included into this part. Wing profile is fully respected so that you just have to cut a slot and glue it in place.

As you can see carbon spar (3mm x 1mm rectangular section) can be added and glued to reinforce the wing stiffness (one on the top skin the other on the bottom one)

All this has to be cut and glued into the wing root before fiberglassing

Here is a closeup of the 3d model

A piece of piano Wire (2mm diameter) will be used to move the wing, while a small 1mm hook will help to attach the two wings together with elastic band.

The position of the axis is tricky and is explained into PredimRC "servos" tab.

These values have been used to modelize the real implantation of the servos into the fuselage

And here are the critical values to respect :

• nose to leading edge : 240mm

• leading edge to rotary axis : 92mm

• rotary axis to servos pin : 60mm

• wing angle of incidence : 1.6°

• wing rotation : +- 8°

While this piece printed, the carbon spars added, you can fiber glass the wing. 

I used:

• 2 pieces of 80g/m2 fiber at the leading edge

• 2 pieces of 80g to cover the carbon spars and the 3d printed part

• 1 single piece of 80g to cover the rest of the wing

Here is a picture with the wing into the vacuum bag :

the carbon spar is 250mm long

Vacuum is regulated during all the curing time of epoxy. That is around 18 hours. I used my vacuum pump built around an old fridge compressor

Do the same for the second wing, You can now mark the fuselage (use the provided template) to see were to cut the slots, and cut them

Be sure to align precisely the wings and to respect the 1.6° incidence angle of the root profile. 

When done, print the servos holders and glue them in place into the fuselage

servos are Corona 8kg metal gears ones : DS538HV. 

They are a little heavy but strong enough to sustain the heavy loads induced by the variable incidence...

The "wing servo pin" is attached to the servo itself using two 2mm ball joints :

A piece of 2mm piano wire is glued between the two joints (positionned at 90°)

And the two servos are fixed by the top cover and positionned face to face like on this picture.

Once this is done your pixel is almost finished !

tail

I did the same for the tail:

• cut them with WiHoWi/HoWiGS

• glue 2 pieces of 2mm piano wire to insert the tails into the tail boom

• fiber glass

Centering

As I used High Voltage servos (and receiver) I could use two 18650 LiIon cells to provide power both for the Rx and for the servos without any voltage regulator.

The battery is hidden at the very tip of the nose as well as the Rx. Doing this allows a perfect centering of the plane following PredimRC Center of Gravity position and without adding any lead ! 

total weight is 790g which is correct for 1.6m wingspan. Wing loading is still very low at around 31g/dm2

Now you just have to tune the motions for "ailerons" and "elevator"

• elevator: +/- 13mm -

• aileron : +15mm to top and -10mm to bottom (with 30% expo)

Here is the finished glider (well wings not yet painted !)