L188 Electra

[Bat_Hog]

Now I have my Anet A8 3D printer, I thought I needed a new project. I've been on a couple of turbo prop airplanes and always liked the idea of a multi engine airliner model. IC engines can be a problem, but modern electric motors are powerful and cheap. So I decided to do an L188 Electra, the commercial version of the P3 Orion.

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Design principles: Needs to be big enough so that the wheels are suitable for our grass fields. Not too big that I can't get it in the car!
there are several examples on the web, but I will probably go with the one featured in Air Crash Investigation. A propeller came off and struck the underside of the fuselage, severing engine control and trapping control cables. The crew fought for hours to fly the crippled airplane and finally landed successfully against all odds. So for size I am basing it on my Avios Bush Mule, which is twin engine. From plans (not brilliant 3 views) I ended up with a scale 1:19. This gives a wingspan of 1600mm and fuselage diameter of 190mm. I am designing the parts in Blender 2.79 (it's awesome and free!)
Progress: designed and printed the propellers and spinners. Tested them on the Bush Mule. printed the rudder. prototype for the gear doors, prototype for the cockpit and nosecone. prototype for the front gear oleo. also printed some fuselage bulkheads. I have the servoless retracts (set of 3 from Hobby king for £20 !). I have the 4 motors and escs, same as on the Bush Mule. The wiring for 4 motors is interesting. you have the outer two on a separate battery and channel, and all but one positive is cut to the receiver.
on with designing the engine nacelles.

[Bat_Hog]

Had a bit of a setback. My main PC threw a wobbly and wiped out a memory stick, with all my 3D files, in the process.

But these things are always better when you redo them. Found some better reference pictures for the actual example I am building.

Reeve Aleutian Electra

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I have 3D printed the Rudder. It came out really well, picture to follow. It is the first real production part.

Now I am designing the Engine Nacelles. They are the hardest things I have ever designed. See the picture.

This is hard to model

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On the outside they are a combination of 3 curved surfaces plus the intersection with the wing. When I get the outside shape right, I have to hollow the shape and design the internal structure. There has to be a slot through for the main spar, room for the main wheels and retracts, cooling vents, fire wall to bolt the engine to. Then the thing has to be split into separate pats for printing, the gear doors split off and removable access panels created. And at the end of that it has to be strong and light.
Blender pictures to follow.

[Bat_Hog]

I'm back on the forum now (thanks Colin).
Thought I'd post a progress update.
The design of the parts is harder than the actual 3D printing. For the cockpit I did about 60 file saves, until I was happy with the design.
Actually have quite a few parts ready to go:
Cockpit, nosecone, front landing gear, front lights, gear doors.
Rudder
4 off propellers and spinners, with motors and ESCs.
The 2 inner engine nacelles, front part anyway. main landing gear.
port leading edge wing root (Random!)

Still doing the design in Blender which is now up to version 2.81. As you can see from the picture, it is modelled in flat surfaces (Quads) and then just before exporting as an STL I apply a Subsurface Modifier. If I've done it right and retained all the edges that have to remain "Sharp" then I get a beautiful smooth finish.

[Bat_Hog]

It's been a while since I've updated the progress on the Lockheed Electra L188. I've not been idle during lockdown and the design and build is really coming on. About 90% of the design is complete. I am on Blender version 2.83 now and it is an excellent tool for scale design and export for the 3D printer.

As you can see, some of the parts have gone through several design changes.

ElectraDesign2.png (485.84 KiB) Viewed 6244 times

One bit I did rethink was the Tail plane. Originally I had four pieces, two on each side. But this meant setting the dihedral angle would be difficult.
So I redesigned it in five sections, with the centre section having the correct dihedral built in. I've printed it and it came out perfect. Over 680mm in width.

I printed the nose section with the nosewheel gear doors built in. In this way they are guaranteed to line up, I can fit the hinges and cut the doors out afterwards. Well it was a good idea but the doors would not open passed the curvature of the nose section. Doh! So I had to "Dremel" a curved edge to the top of the doors. That was fun. They work fine now and an Arduino is used to sequence the undercarriage and doors.

NoseGearDoors.png (468.47 KiB) Viewed 6244 times

Any printed parts that I alter rather than reprint, I go back into Blender and change the design to suit. So that when the next crazy person prints this thing, the parts will fit!

Another area causing problems is the mechanism for extending the flaps. I have gone through several ideas and none work well so far. At the moment I am using a screw drive out of an old CD player. Works fine but is underpowered. I need to get this sorted so I can glue the wing parts together. At the moment it is like a giant kebab with all the parts slid on to the aluminium wing spar. here it is loafing on the sofa.

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As far as 3D printing the parts. All the big pieces are done, apart from two tail sections. Then it is down to bits like Main Gear Doors, Nacelle Covers, Servo Mounts, etc. i will soon be able to lay the whole thing out. When I get her up on her wheels I'll post some more pictures.

The guys in the club have been great when I've needed design assistance. There was the question of which direction the Props should rotate on a four engined aircraft. the general consensus, after much really interesting discussion was option 1 below. Out of interest, on full sized 4 engined aircraft, Lancaster, Electra, they all rotate the same way!

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Also I have spent a lot of time uploading the .STL and Cura .3MF files to GitHub and Thingiverse, so that if anyone, crazy enough, with enough filament, can print their own Lockheed Electra!