Probably my last big Depron plane

[quorneng]

FoamyDM
Thanks for the comments and suggestions.
Unfortunately where I fly (a community soccer pitch area) is rough with relatively long grass so a hand launch is the only option. Even if the grass was 'lawn mowed' smooth I doubt there would be enough thrust to overcome the wheel resistance to achieve flying speed. The up side of this situation is I am pretty well practised at hand launching.

The fuselage former spacing has been chosen so that with a curved 3mm Depron skin there is adequate torsional and bending rigidity in each panel. With limited thrust weight is my biggest driver so everything has to be 'just enough'. The only area of concern is the deep but thin top and bottom elements of the formers that might 'buckle' under a point load. You will notice I have already put cross bracing along the top between the formers. Similar might be done along the bottom.
At some point soon I will have to start considering its 2.5 m swept wing. Compared to the enormous breadth and depth of the fuselage it will be 'thin' indeed.

 

[quorneng]

The forward section of the fuselage is built in exactly the same way as the middle one.

Note the change in the fuselage section towards the cockpit.
The complex bits are the 'cut back' on the lower edge where the nose door fits and creating the cockpit which is 'perched' high up over the nose door.

I did consider making the door lift up but the complexities of the geometry required confirmed a simple plug in would be quite difficult enough.
Starting to think about the wing it seems sensible to have it in 3 pieces.
A one piece centre section that carries all the motors and nacelles with removable outer wing panels.
The AN 124 has a highly tapered wing so the outer panels combined have only 45% of the area of the centre section so the removable joints will not be carrying a big load.
The joint would have to be angled to be perpendicular to the 25 degree wing sweep.

To get an idea of the problems involved I made a simple test joint using the principle of a fixed glass fibre tube in one half that slides into holes in specially reinforced printed ribs in the other.

The glass fibre tube is slightly tapered and printing can achieve remarkably high accuracy so the fit could be arranged to be just 'tight' when pushed right home but still be easily separated.
This is not going to be a simple or quick build.

 

[Piotrsko]

Been my experiences with variable geometry wings that flight loads tend to freeze moveable parts into stasis.

 

[quorneng]

The AN124 uses a TSAGI 12 % wing section.

As this will be flying at much slower speeds I reduced the under camber reflex just a bit.
Some serious printing. The rib 'set' for the RH centre section.

Bed size limitations means the bigger ribs are printed in 3 parts so there are 28 pieces in all.
The wing anhedral means the LH and RH sections have to be built independently and then permanently joined.
The wing will have a single spar with 2mm hard balsa flanges incorporated as part of the Depron wing skin. This means the spar will be the full wing depth.

 

[quorneng]

The RH centre section ribs glued to the bottom wing skin.

The 2 mm depron skin includes a 2 mm hard balsa flange glued within it. A 3 mm Depron shear web is then glued on to it between the wing ribs. The front part of the wing skin has been 'formed' to follow the profile of the wing ribs.
Note the double ribs that will secure the motor pylons and the box shear web where the outer wing panel joining tube is inserted through the 3 outer ribs.
Only when the top spar and the front wing skin is in place with the wing be rigid enough to be handled.

 

[Timmy]

The RH centre section ribs glued to the bottom wing skin.
View attachment 182230
The 2 mm depron skin includes a 2 mm hard balsa flange glued within it. A 3 mm Depron shear web is then glued on to it between the wing ribs. The front part of the wing skin has been 'formed' to follow the profile of the wing ribs.
Note the double ribs that will secure the motor pylons and the box shear web where the outer wing panel joining tube is inserted through the 3 outer ribs.
Only when the top spar and the front wing skin is in place with the wing be rigid enough to be handled.

It looks really good! How are you gluing the 3D printed parts to the foam?

 

[The Hangar]

@quorneng such a cool project, and a really cool build style! I'll be following for sure.

 

[quorneng]

Timmy
Everything is glued with UHU POR which is a Depron safe glue.
I have found that with a glue area equal to the area of Depron the joint, even to PLA, is as strong as the Depron itself.
With such a complicated build POR has a hidden advantage in that a joint can usually be carefully separated even after several days. Useful to correct any mistakes.
In fact POR only reaches the point where the Depron is likely to tear rather than the separate after at least a week.

A bit more progress. The leading edge Depron sheeting added to the RH wing panel.

It is now plenty rigid enough to handled. The bag contains the rib parts for the other side.
It is difficult to judge the size in this picture. The root rib is 450 mm (17.5") long.
The small holes in the 'solid' ribs are for the aileron servo wires to run through. Obviously not a problem for the rest of the ribs.
The wing is so thick at the root my intention is that the 4 ESCs, the flight battery and possibly the rx will all be in the wing.
The only external connection required will for the elevator and rudder servos and they will be in the tail surfaces so that huge fuselage will have absolutely nothing in it.
Well that's the plan.

 

[Timmy]

Timmy
Everything is glued with UHU POR which is a Depron safe glue.
I have found that with a glue area equal to the area of Depron the joint, even to PLA, is as strong as the Depron itself.
With such a complicated build POR has a hidden advantage in that a joint can usually be carefully separated even after several days. Useful to correct any mistakes.
In fact POR only reaches the point where the Depron is likely to tear rather than the separate after at least a week.

A bit more progress. The leading edge Depron sheeting added to the RH wing panel.
View attachment 182273
It is now plenty rigid enough to handled. The bag contains the rib parts for the other side.
It is difficult to judge the size in this picture. The root rib is 450 mm (17.5") long.
The small holes in the 'solid' ribs are for the aileron servo wires to run through. Obviously not a problem for the rest of the ribs.
The wing is so thick at the root my intention is that the 4 ESCs, the flight battery and possibly the rx will all be in the wing.
The only external connection required will for the elevator and rudder servos and they will be in the tail surfaces so that huge fuselage will have absolutely nothing in it.
Well that's the plan.

How are going to do the ailerons?

 

[quorneng]

Timmy
It will use the same process I have used on my most of my other Depron planes. The wing skin is initially cut out including the ailerons and then the area of the aileron carefully cut out. These are then used as the skins to build up the ailerons individually with ribs like little wings. Hopefully you will see this when I get round to making the AN124 outer wing panels.

 

[quorneng]

The outer wing panels are surprisingly simple.
The RH panel with just two printed ribs to hold the joining tube and two full depth shear webs fixed to the lower 2 mm Depron skin.

There is no spar as such all the loads are carried by the Depron skin.
The 5g servo looks almost lost and is completely hidden. The servo wire has to be extended to reach the root before the top skin can be added.

The aileron is simply cut out and the open edges 'closed' with 2 mm Depron.
Note the two small front and back locating pins to keep the wing inline with the centre section.
The RH wing assembled to get an idea of what it looks like.

It looks odd to have no spar on the outer panel but the centre section does also have to carry the weight of the motors.
It also shows the significant taper between the wing root and the tip.

Now I have just got to do it all over again for the LH wing and hope I can keep it identical.

 

[quorneng]

Before spending too much time building the plane's structure it was prudent to build and test the nacelles and wing pylons.
They are not true EDFs but actually racing drone motors with 3x3.5 four blade props inside a scale nacelle.
A CAD image of a prototype nacelle.

After much experiment and many hours of printing the final nacelle design with the motor and prop in place.

Although the actual nacelle is the same for all four the pylons are slightly different inboard and outboard.
The first nacelle on a thrust test stand.

On a 4s at 7A it delivers 8.5 oz (245 g) of thrust.
The LH wing with its nacelles in place.

Not fixed at this stage. They will only be 'permanent' when the whole centre section is structurally complete.
If I had known it was going to take this much work I probably would never have started!

 

[BoredGuy]

What motor and prop are you using?

 

[quorneng]

BoredGuy
The motors are Emax 2205 with a racing drone 3x3.5 four blade.
The motors were on offer for four, 2 CW and 2 CCW and the same with the props. All for less than the price of a single cheap 76 mm EDF! All four together are no heavier than the EDF either.
From my point of view the attraction of such a motor/prop combination is it is quite efficient for the thrust they produce. Sort of midway between a conventional prop and a true EDF.

 

[Pepo]

This is the way!

 

[quorneng]

The two halves of the centre section apart from being simply glued together will be reinforced with a thin printed 'plate' top and bottom.

The plate is printed in such a way that the individual print filaments follow the sweep angle.

It is also tapered in thickness centre to tip.
The wing 'dry' assembled showing its full 2.6 m span on the dinning room table.

With a wing skin of just 2 mm Depron with no carbon or hardwood reinforcing just a 1 mm balsa top and bottom spar flange and only on the centre section.
This could all end in tears.

 

[BoredGuy]

You said previously that you would get about 1kg thrust in total, isn’t that way too little for a plane this size?

 

[quorneng]

The thrust maybe low for its size but hopefully not for its weight. Best guess at this stage is about 1.7 kg.
Apart from the bulk of the fuselage the aerodynamics are actually pretty good and the low wing loading means it will fly relatively slowly.
I never suggested success was guaranteed.

 

[quorneng]

With the major structural issue of the wing solved I needed a break so back to the fuselage.
The nose and mid sections joined.

And with the nose door in place.

There is nearly as much fuselage still to go on the rear!
The view inside with the nose door removed.

A ridiculous amount of room really as about the only thing it will have the power to carry would be a few party balloons.

 

[the rc project]

With the major structural issue of the wing solved I needed a break so back tot he fuselage.
The nose and mid sections joined.
View attachment 183471
And with the nose door in place.
View attachment 183472
There is nearly as much fuselage still to go on the rear!
The view inside with the nose door removed.
View attachment 183474
A ridiculous amount of room really as about the only thing it will have the power to carry would be a few party balloons.

Helium filled balloons? Lol