Scratch Built Balsa F-104

[telnar1236]

A few days ago I decided I wanted to build an F-104 EDF. Because I don't have too much space to fly, it needed to be as light as absolutely possible, which meant balsa and plywood construction. At this point I've designed everything in CAD and laser-cut and assembled the fuselage. The fuselage formers and some internal decks are cut from plywood while the stringers and airfoil formers are balsa. The internal duct is made out of resin impregnated painters tape.

Some specs (I used metric because it's easier to do the math with):
1.1 m length, 47 cm wingspan (not quite scale, but it will help the plane slow down and you won't notice the difference)
Hyperflow 56 mm fan w/ 8.6 N (880 g) thrust, 40 A ESC
Airfoil will be Clark Y for wings, NACA 16009 for the vertical stab, flat plate for the horizontal stab
On paper top-speed in SLF is about 38 m/s (85 mph) which is about what I'm aiming for
On paper stall-speed is about 10.5 m/s (23.5 mph) which is a bit fast, but not too bad for an F-104.

 

[SOB Racing]

What laser setup are your running?

 

[telnar1236]

3W laser on a 3 axis router. 200mm/min with 3 passes for the plywood. Before cutting I soak the balsa wood to keep it from catching fire then 200mm/min and 5 passes on the balsa. Max duty cycle.

I think this is it on Amazon but I got it used so I'm not totally sure.

Amazon.com: Upgrade Version 2 in 1 3000mW Engraver CNC 3018 Pro GRBL Control DIY Mini CNC Machine, 3 Axis Wood Router Engraver with Offline Controller + CNC Router Bits + ER11 Extension Ro

 

[Ryan O.]

3W laser on a 3 axis router. 200mm/min with 3 passes for the plywood. Before cutting I soak the balsa wood to keep it from catching fire then 200mm/min and 5 passes on the balsa. Max duty cycle.

I think this is it on Amazon but I got it used so I'm not totally sure.

Amazon.com: Upgrade Version 2 in 1 3000mW Engraver CNC 3018 Pro GRBL Control DIY Mini CNC Machine, 3 Axis Wood Router Engraver with Offline Controller + CNC Router Bits + ER11 Extension Ro

Cool, that's cheaper than I though it would be. Has your experience with it been good?

 

[telnar1236]

Cool, that's cheaper than I though it would be. Has your experience with it been good?

It's been great so far. Since I got it, the only maintenance it's needed has been some WD40. You need to be decently handy to assemble it since the instructions are awful and have some steps in the wrong order. The laser cutter could probably be used even if you have no prior experience. It might take a bit of trial and error, but it's not hard. Mostly you just need to know how to set the X and Y zeroes so you have enough travel. For anything but engraving you definitely want some prior experience with CNCs and machining for the mill head, especially if you're doing aluminum.

It comes with the LaserGRBL software, which you can also get for free and that lets you convert image files into G-code. You can use Microsoft Paint to count the pixels and then scale just about any plan available online based on that from screenshots or take flat-pattern CAD and do the same.

 

[telnar1236]

The wings are cut, assembled, and installed. A central spar running through the center if the inlet duct helps provide structural rigidity. Everything still needs some sanding.

 

[2jujube7]

The wings are cut, assembled, and installed. A central spar running through the center if the inlet duct helps provide structural rigidity. Everything still needs some sanding. View attachment 185928 View attachment 185929

Haha tiny wings. So yittle.

 

[telnar1236]

Haha tiny wings. So yittle.

And they're still about 15% bigger than scale

 

[rockyboy]

And they're still about 15% bigger than scale

Yikes! All the lift surface of a penguin!

 

[Monte.C]

And they're still about 15% bigger than scale

Wow.

 

[telnar1236]

With the holidays, I have had some more spare time. The tail is installed and I have started the covering process. I'm also working on a design for servo-driven retracts. I may be getting too complicated here, but I decided I want retracts, so they are happening. Probably add about 80g to the AUW.

 

[2jujube7]

With the holidays, I have had some more spare time. The tail is installed and I have started the covering process. I'm also working on a design for servo-driven retracts. I may be getting too complicated here, but I decided I want retracts, so they are happening. Probably add about 80g to the AUW. View attachment 186931

Is that a really small FT LongEZ on the table? Does it fly?

 

[telnar1236]

Is that a really small FT LongEZ on the table? Does it fly?

That's exactly what it is. It's just a chuck glider with some hot glue in the nose, but it works great.

 

[telnar1236]

Once you get to this point, everything comes together pretty fast. The airplane is covered except for the parts I need access to for finishing installation of the gear. Main landing gear is installed and works great. At this point, there isn't too much left to finish up; control surfaces, nose gear, canopy, remaining covering, and servo protectors for the ailerons remain. Then for a paint scheme.

 

[Monte.C]

Once you get to this point, everything comes together pretty fast. The airplane is covered except for the parts I need access to for finishing installation of the gear. Main landing gear is installed and works great. At this point, there isn't too much left to finish up; control surfaces, nose gear, canopy, remaining covering, and servo protectors for the ailerons remain. Then for a paint scheme. View attachment 187134 View attachment 187136

Man I've been wanting to see this plane for a long time. To see who has the guts to build one and make it fly. Can't wait for the maiden!

 

[telnar1236]

Man I've been wanting to see this plane for a long time. To see who has the guts to build one and make it fly. Can't wait for the maiden!

Thanks for the kind words. Not sure if it's guts or being dumb enough to try and turn a lawn dart into a park flyer. I'll try to catch the maiden on video one way or the other.

 

[BoredGuy]

Thanks for the kind words. Not sure if it's guts or being dumb enough to try and turn a lawn dart into a park flyer. I'll try to catch the maiden on video one way or the other.

This is literally a lawn dart even when it doesn’t crash

 

[Crawford Bros. Aeroplanes]

This is literally a lawn dart even when it doesn’t crash

It works on the same principle as orbit- it's perpetually falling but is going so fast that it keeps missing the ground.

 

[telnar1236]

The F-104 is ready for her maiden flight. I made some changes to the design to reflect some issues I had or foresaw. The biggest is no landing gear. It ended up at about 110g which was twice what I was aiming for, so I ripped it out and fully covered the bottom. I also added a cheater inlet for the fan. The two main inlets are plenty on paper, but for hand launching, I want as much static thrust as I can get. In addition, I removed the ventral fin for belly landing. The final change was the addition of vortex generators in front of the ailerons. No flaps means I need to get to a higher angle of attack for landing and I don't want to lose aileron control without having a rudder. The lack of the ventral fin will also make her more prone to a stall spin, so delaying the stall is doubly valuable. She still needs to be painted and canopy needs work to make it look good, but she will fly (or crash) in her current state.

 

[telnar1236]

Good news and bad news on the F-104. I finally got the chance for a test flight and she does fly, and exceptionally well! The VGs do their job so you retain aileron control even close to a stall, and she slows down surprisingly nicely for her small wings. However, I now understand why the phrase "banking with intent to turn" was coined for the F-104. The high wing-loading means that I have flown 70mm EDFs that can turn in a smaller space and the lack of a rudder is very noticeable. More unfortunately hand-launches are a real problem. The launch for the maiden went smoothly and the flight went well, but on the second hand launch, I didn't quite get enough airspeed and she tip-stalled into the ground so much of the nose was destroyed and the rest of the airframe suffered some minor damage. I'm considering rebuilding the whole design with more weight savings and scaled up for a 64mm edf to reduce the wing-loading and give me room for gear. I selected the 55mm EDF because of its small profile and because I wanted an easy to transport plane, but a 64mm setup on the same battery weighs less while achieving almost 300g more thrust, and scaled up plane would give me the wing area and thrust to add all the features I missed in this one.