This is actually a 3/4 size Ballerina which is a 53" conventional balsa/ply "Sport aerobat" that comes in at about 70 oz.
At 3/4 size (40" span) it will be small enough to be transported and stored as a 'one piece' plane which saves complication and weight.
In addition at 40" with a thick (14%) wing section I am confident it will be strong enough to be built almost entirely in 2 mm Depron!
First a perspex template for the wing ribs.
The wing is all Depron apart from the wing spar flanges which are just 1 mm sheet balsa.
The wing is built up on the lower wing skin.
To give the maximum possible depth to the spar the flanges are flush with the out skin surface. The inside surface of the spar flange is built up with 2 mm Depron. This does require 1 mm deep slots to be cut to match the rest 2 mm Depron wing skin.
Once the leading edge top skin back is added the wing is plenty rigid enough to be handled.
Note a Depron shear web has been added between the ribs at the front and rear of the spar flange to in effect create a tapered Balsa/Depron/balsa box spar.
Next the aileron is cut out.
Note the tiny 'direct drive' aileron servo. Its servo arm is glued directly to the aileron so the servo also acts as the inboard hinge. There is no external linkage at all.
Finally a 6 mm Depron leading edge is carefully sanded to shape.
The servo arm is not pinned but simply glued, actually with a thin film of Gorilla glue, and clamped until it has set. The glue foams slightly so builds up around and through the servo arm holes ensuring a secure key.
Glue may not sound adequate but you have to remember the aileron itself is built up with only 2mm Depron and nothing else!
It will be a 'dual aileron' set up so the travel of each servo can be adjusted individually from the Tx and most likely with some differential action as well.
The fuselage 'bits' cut out in 3 mm Depron.
Construction is admirably simple as the major part of the fuselage is parallel.
The top decking is 'planked' in 2 mm Depron.
The fuselage underside sheeting is also 2 mm.
The original Ballerina had the undercarriage fixed to the wing which required significant local reinforcing. To do the same with a Depron wing would be proportionally heavier so the undercarriage is fixed to the fuselage with large acetate side plates to spread the load.
The wire is fairly thin so it gives quite a soft and springy undercarriage which also reduces the shock loading.
The tail wheel is mounted in a similar way.
A cheap and cheerful 1200kV Emax 2822 with a 9x4.7 SF gives about a pound of thrust but only draws 10.2A (120W) from a 3s LiPo.
As I intend using a 1500mAh the battery will not be worked hard.
Sitting on it wheels.
The 2.25" (57 mm) wheels may look quite chunky but they too are made of Depron with acetate centres. Each main wheel only weighs 2.8 g!
One advantage of such a lightweight construction is it only requires modest power.
As Depron is such a good insulator the 20A ESC has had its flat plate heat sink replaced with substantial 'fingered' one positioned so the tips of the fingers protrude though the bottom of the fuselage.
The 5 g elevator and 3.7 rudder servos are mounted right at the back and orientated so the servo arm axis matches the control surface. Being so close to their respective surface means only a short direct link is required.
The tail plane, elevator, fin and rudder are all made from 2 mm Depron skins over a tapered Depron shear web to give a symmetrical cross section.
The elevator halves are joined by a substantial glass fibre tube which also acts as the centre hinge pivot.
The outboard hinges are home made acetate 'pin' hinges that are glued into slots cut into the Depron.
The result is rigid and very free moving so making maximum use of the limited servo torque.
Nearly time to start to put it all together as only then can the required battery position be determined.
Not too hard to assemble. Just as case of gluing the bits on and making sure it was all 'square'.
To keep the weight down just the movable surfaces are painted the rest left natural Depron.
With the spinner and prop in place the battery could be positioned to give a 28% CofG.
The overall construction is so light the battery actually goes very close to the CofG
I rather prefer a 'push in' type battery installation as it is light but securely retains it for any type of aerobatic manoeuvre yet still allows it to slide forward in the event of sharp arrival!
With the nose top decking added and a bit more paint it is complete.
It weighs 14 oz (396 g). With a 9x4.7 SF prop the motor draws 10.2 A giving 120W & 137W/lb.
An Orange 7 channel gyro Rx sits in the cockpit although for the maiden the gyro was off.
The hand launched maiden proved remarkably easy.
The soft springy undercarriage really does its job on the rough grass.
About the only thing it really needs a coarser pitch prop as even at full power it went no faster when put into a dive.
Nigel Thanks for the compliment.
The Petiterina has proved to be a remarkably docile flyer to the point that I felt the need to set the fail safe very carefully as with the gyro 'on' it flies absolutely straight and it glides remarkably well.
This video demonstrates both its low speed and its considerable reserves of power.
With modest power and a relatively high drag air frame it is no "speed merchant" but it is nevertheless great fun to throw around in a very modest space.
Its very low minimum cruise power prompted me to try for an 'endurance' flight. With a new 1500mAh 3s it flew for just over 60 minutes before the LVC cut in!
The final 'tweek' was to substitute a 9x6 SF in place of the 9x4.7 to give a slightly wider speed range. Coupled with modest elevator/throttle Tx mix it can now go from a slow glide to full throttle without any drama.
I am very tempted to use the same all Depron construction technique on a 'full size' 52" Ballerina and to add retracts as well!
Your direct servo to ailerons was an idea I toyed with for my Lancaster. To try and operate the rudders. Only problem there was the servo would need to be cut into the rudder as it would not be at one end.
I have also considered using threaded rod into the servo output spline instead of an arm. The rod being used as an axle to fix hinge points to.
But its your use of thin depron and still getting strength from structure.
Yes a direct servo installation really only works on a 'barn door' type control surface and even then there has to be sufficient wing thickness at that point to house it.
The only real issue I have with a direct servo is its very directness!
I first used it on my special slow flying 'endurance' plane where reducing the parasitic drag was of utmost importance rather than having a high degree of control.
In most of my 'normal' applications the control surface moves through a lesser angle than the servo arm which results in a modest mechanical advantage so making the best use of the available servo torque. An important attribute when using micro servos on largish planes.
Still thinking about it but this is what the underside of a Ballerina with retracts might looks like.
A bit 'Havard' like but it keeps the retract geometry very simple.
About the biggest worry is it would be difficult to match the 'springy' undercarriage of the Petiterina which would greatly increase the shock load on the Depron wing.
Just to complete the story...
The Petiterina is great fun to fly. Slow and easy but with huge reserves of power (unlimited vertical!) Flown gently it also has a remarkable 60 minute endurance.
With a big 'thick' wing retracts should not be too difficult to do and at the full 53" span size it should handle the extra weight easily.
So built in virtually the same manner as the Petiterina (all Depron apart from the spar flanges) a "Ballerina with retracts". Not a lot of difference apart from size.
Same prop and motor, same direct drive ailerons (but this time with 5g servos), the same Lemon stabilised Rx and of course the retracts - including the tail wheel!
At 22 oz all up with an 1800 mAh 3s the wing loading is the same as the smaller, lighter Petiterina but its power loading is a bit lower - no longer unlimited vertical - but being bigger it appears to fly even slower than the Petiterina.