A bigger DH Venom

[Simon Chaddock]

A bit of tortuous story but I recently managed to restore my Depron Folland Gnat.

I thought its 50 mm EDF had failed so I bought a replacement. As it turned out it was a just a poor at the junction on one of the long magnet wires, the EDF is right at the back, where it is soldered to the motor wire.

This left me an unused 50mm EDF. It irritated me just sitting on the bench.

Some 8 years ago I built a Depron Venom using a lightweight AEO40mm EDF.

Not relevant at the time but it ended up sub 250g and still flies very nicely.

So my thought was a 25% bigger Venom using the spare but much more powerful and heavier 12 blade QX 50mm EDF.

First task was to print an inlet duct in LW-PLA along with a short exhaust tube for the EDF in PLA.

The inlet is made up of 3 'vase' printed parts simply butt glued together. A 'bandage' of heavy weight tissue glued on with PVA covers the joint. The exhaust tube is also a vase printed single wall item. The inlet has a constant area of 1.2 times the FSA. The exhaust nozzle is 90% of the FSA.

As a rigid structure the Venom fuselage and wing roots will be built around it. The formers will support the duct to prevent the flatter area collapsing under reduced inlet pressure.

So far so good but it gets harder from this point.  

Edited July 13, 2023 by Simon Chaddock

[Simon Chaddock]

Rather slow progress but the fuselage formers around the duct are on.

It requires more and more thinking about how to do each step and then hand fitting to get the required final shape.

It is no way to build a plane but the end result is, or should be, very light.

Next is the nose/cockpit section. It will be build separately and glued on to the first former. Well that's the plan.  

Edited July 16, 2023 by Simon Chaddock

[Simon Chaddock]

The nose section built initially as a half shell over the plan then lifted, the other half planked and glued on.

The cockpit area is left open as the battery and ESC will go in there and hopefully counter the considerable weight of the EDF right at the back!

A fuselage underside will be completely skinned as no access is required from there.

Edited July 18, 2023 by Simon Chaddock

[Simon Chaddock]

For some reason this has been a slow build. The underside of the fuselage is fully planked and the wing root framing added.

For the same access reason as for the fuselage only the underside of the roots are skinned.

A single broad plank has been added to the upper side to allow the outer wing panels to be glued in place, once they have been built!

They are next.

 

,

 

[Simon Chaddock]

At last some tangible progress.

A 25% size increase certainly makes it look bigger.

Just positioned to see what it looks like.

It will be glued on when the aileron servo and its long wire are fed through the wing. Not difficult as the wing has no ribs or spar just Depron shear webs to create the wing section.

It relies entirely on the 3mm Depron skin for all bending and torsional stiffness.   

 

 

[Simon Chaddock]

Some real progress. The wings are on.

Excessive length aileron servo wire as I don't know where the Rx will end up. The wires can always be shortened.

The tail booms are really tricky to get right and are very flexible until they are complete.

Each boom will have 5 printed formers, all slightly different. Each boom is slightly oval but will be built with 3mm thick flat sides and 2mm planked top and bottom sheeting.

Hopefully with a bit of sanding the end result will be a sort of oval.

  

Well that's the plan.

[Futura57]

I think it's amazing what you are doing with Depron Simon. I've always seen it as a throwaway material for chuckies and quick boxy prototyping. I'm not sure I could keep such a well crafted Depron model looking half decent for long as I'm prone to inflicting hangar rash on my models. I don't doubt the reduced airframe weight results in really good performance. Is that why you are fond of using it?

[Simon Chaddock]

Futura57

I have long been surprised at the suspect structural design in many conventional builds. Much of the material is at nowhere near its stress limit so adds weight meaning more stresses for the other components that are more highly loaded.

An  example is the use of pultruded carbon fibre tube to reinforce a foam structure. The performance of the two materials is so different means the foam is doing little structural work other than maintaining the desired airflow shape. It is a simple and workable solution but overall the foam is likely to be heavier than the carbon so an efficient structural use of material it is not.

I discovered Depron by accident when I wanted to modify/repair my first RTF RC 'foamie'. It flew well enough but I was disappointed by its solid foam wings. In my view heavy for their strength. A key issue for a plane powered by a brushed motor and HiMh batteries.

Depron, particularly the original type, was remarkably stiff, strong and cheap material. It was available in big thin 3mm sheets. Why not build a wing out of it using a conventional ribs and a full skin?

In effect it would be a hollow stressed skin structure with just shear webs where the spar would normally go. The result was only 70% of the weight, just an strong with better aerodynamics too. The performance and endurance of the RTF improved proportionally.😀

I had used complex balsa planking to create 3 dimensional shapes on some planes so I had not fears doing the same with Depron.

 

From then on as a designer/builder it was simply a case of satisfying my curiosity to find out what I could do with Depron that made the maximum use of its properties.😉

[Futura57]

What then are your preferred coverings for Depron which don't add too much weight but give a durable enough finish? What are your covering / finishing plans for this Venom?

[Simon Chaddock]

Futura57

I am afraid I have found no coverings that don't add weight so I design and test before flight how strong a structure really is. My favourite non destructive test is to pick the fully loaded plane up by its wing tips. For a straight wing design this imposes a bending load at the wing root equivalent to pulling at least 4g. Adding additional weight equal to the planes weight at the fuselage gives the equivalent of pulling 8g.

My sub 250g Depron Super Cub supported by it wing tips and loaded with a 500g steel block at the fuselage to give something like a 12g test.

   

 Note it is all Depron except the balsa wing struts which are fully load bearing!    

 

Original Depron has a hard skin anyway so only a light coat of foam safe paint was all that was required.

The more modern "Depron" type materials have a more open surface and are rather softer so I use acrylic enamel. This provides quite an abrasive resistant surface and of course works on compound curved surfaces.

As far as hanger rash is concerned the very lightness of the models does give them a bit 'ding' resistance. Deep marks can always be light weight filled and repainted. 

One advantage of hollow foam skin structures is they locally "crumple" leaving the remaining airframe undamaged. It is then a case of cutting back to a sound surface, build a replacement part and glue it on.

As an example my all Depron Fairey Delta EDF  in Jul 2014 looked like this.

 

The result of a crash in October 2014

 

By January 2015 it looked like this.

And it still does today.

As a material It may not be everybody's "cup of tea" but it is amazing what can be achieved.

    

[Simon Chaddock]

The basic boom structure goes on.

To improve rigidity and to make sure it all lines up correctly the tail plane is added. Lots of eyeball required!

The top of each boom then uses long and narrow planks (6 each side) in 2mm Depron.

When the boom planking is complete it will hopefully be rigid enough to be sanded.

That is my daughters dog who was 'visiting' for the day. Looked too comfy to disturb.

 

[Futura57]

I sure hope those booms are strong enough for those little cartwheel landings. No carbon rods then?

[Simon Chaddock]

Carbon in the booms? They would then just rip out of the wings.

An un reinforced boom should just break off, hopefully close to the wing root, ready to be glued back on.

[Futura57]

Well I guess a flesh wound is preferable to broken bones 🤔

[Simon Chaddock]

Slowly coming together.

The wing root top skin complete, the fins added and the booms complete and sanded to profile as well as the control surfaces connected to their servo.

The control surface horns are printed.

The aileron linkage viewed from the underside.

 

An all glued installation. The aileron servo has been inserted and glued to the underside of the upper skin and a variable thickness panel glued onto the servo to give a flush underside. The horn is also glued to the aileron surface. My usual totally 'built in' installation. 😉

 

[Mike Chantler]

I Like the simple rubber securing bit on the link, is that heat shrink?

[Simon Chaddock]

Mike

No its silicone wire insulation that is a tight fit. Once I am happy with the installation the silicone retainer is more permanently secured with a tiny dab of super glue on the end of the wire. 

[Mike Chantler]

Sounds like a neat solution 👍

[Simon Chaddock]

Nothing very dramatic but the first test run of the QX 50mm in the Bigger Venom.

 https://www.youtube.com/watch?v=vu0FsDlBA

With a 1500mAh 3s it draws 18a delivering 196W. It also blows the door shut

[Futura57]

1 hour ago, Simon Chaddock said:

Nothing very dramatic but the first test run of the QX 50mm in the Bigger Venom.

 https://www.youtube.com/watch?v=vu0FsDlBA

With a 1500mAh 3s it draws 18a delivering 196W. It also blows the door shut

You just unlocked a poltergeist 😜 

[Eric Robson]

13 hours ago, Futura57 said:

You just unlocked a poltergeist 😜 

Banshee? 

[Nick Cripps]

Surely it's a Ghost?

[Eric Robson]

 D H Goblin

[Nick Cripps]

Goblin was in the Vampire

[Simon Chaddock]

The Venom used the D H Ghost.

A development of the Goblin it was in the same class as the RR Nene but was considerably more successful. Except for the 55 Nene sold to the USSR which went on to power the MIG 15 and were produced tens of thousands.  

 

The first coat of scarlet red.