3D Printed 50mm F-104 Starfighter

telnar1236

Elite member
After a fair bit of time messing around with 80mm and 64mm EDF versions of the F-104 Starfighter, I have decided to revisit a light weight 50mm version that could be flown at a decently large park. Using a combination of 3D printing in LW PLA for the fuselage and vertical stabilizer and balsa and plywood plate construction for the wings and horizontal stabilizer I think I can keep the weight down around 500 grams which should give me a stall speed just under 20 mph. I also want the design to be modular to allow for different wing configurations and channel setups since the stubby scale wings of the starfighter might not be for everyone and not everyone wants the complexity of a 6-channel aircraft. To save weight, all channels will be controlled by a single servo, and the goal is for as many control linkages to be internal as possible. On top of that I plan to design a slightly modified fuselage to make a U-2 since the early versions of the U-2 were essentially heavily modified starfighters and the changes to the CAD should be pretty easy once the starfighter is finished. This is likely a long project and could take months.

The end goal of this project is a flyable F-104 (and possibly also a U-2) model for me, a set of usable free STL files for anyone interested, and to have this thread as a guide that explains some of my design process and how to build the F-104 for anyone interested without going into too much technical detail.

My design is 39.5 inches long not counting the scale pitot tube and has a projected wing area of 130 square inches (slightly larger than scale). This seems big for a 50mm EDF, but because of how long and thin the F-104 is, it actually works out about right. As previously mentioned, the target weight is between 500 and 600 grams.

It is planned to fly on a 3S 1300 mAh battery and a 40A ESC, although peak current draw should never exceed 35A. The EDF unit (I think from ChangeSun, but I'm not sure of the brand; here is the link: Amazon.com: 50mm EDF 4900KV 3S Maximum thrust770g,Brushless Motor, Applicable to RC Jet Aircraft : Toys & Games ) is rated to 770 g of thrust, but looking at RCGroups, it looks like it generates closer to 600g. A lot of brands test their EDFs at a constant peak voltage for the rated battery pack (e.g. 12.6 V for 3S) and scaling down thrust using the fan affinity laws for a more realistic supply voltage gives about 600 g as well, so that seems to be what happened here.

Here are a few screen captures of the CAD. A couple of features to note are the NACA inlets used to supplement the small scale inlets, the ESC cooling using fan air instead of ambient air which should be more reliable, the internal routing for control rods for the rudder and elevator along the top of the fuselage and the large internal space in front of the main gear bay which will hold all the servos and the receiver. Some people might also notice how big the rudder is. This is scale to the real F-104, and is necessary to allow control with a torque tube and internal control arm.
Half View Second Step of CAD.PNG

Half View 2 Second Step.PNG

NACA Duct.PNG

The NACA inlet design is probably familiar to anyone who has followed FliteTest for a while. It offers less drag and better airflow than most other conformal inlet designs. The ramp in the inlet is a key feature that I haven't seen used very much in RC planes because of the added complexity, but it is a key part and contributes a lot to better efficiency.
 

telnar1236

Elite member
Here is a detail of the control linkages for the all-moving horizontal stabilizer and the rudder. The two bell cranks for the horizontal stabilizer are positioned so that up elevator is given by putting all the linkages in tension which should increase responsiveness. The rudder uses a pretty conventional torque tube system using a carbon fiber spar and the only real detail to note is the small space requiring a tiny control horn. While I'd like all of these parts to be 3D printed, the rudder control horn may need to be laser cut plywood.
Tail Control Linkages.PNG
 

telnar1236

Elite member
I was able to do a bit more and design the control linkages for the wing control surfaces. They are all connected using 3mm carbon fiber torque tubes which are in turn connected to internal control horns. The leading-edge flaps and trailing-edge flaps are connected to the same servo and actuate together. The ailerons are then actuated using a second servo. By connecting the leading-edge and trailing edge flaps to opposite sides of the same servo, they will both move in the correct direction without requiring a complex set of bell cranks or two separate
servos.
Wing Torque Tubes.PNG

Aileron LE-TE Flap control horns.PNG

Unfortunately, the leading-edge flap torque tube runs a bit too far forward, so it extends through the inlets and required a small internal fairing to contain the control horn. However, I didn't expect the inlets to be very efficient because of the scale shock cones and small size, and the cheater inlets are meant to be able to support pretty much the full requirements of the fan, so I am not too concerned about any small losses of efficiency the control linkage causes.
LE flap linkage.PNG
 

telnar1236

Elite member
I designed the main gear mount yesterday around a build box of the mechanical retract unit. The only real difficulties in this design were ensuring it would be strong enough to support the main gear during landing and checking clearance for the pushrods for the aileron and flap torque tubes. The gear mount is the most heavily constructed part of the whole airplane with the plate to which the retract units screw being 2.4mm thick.
Main Gear Mount Full.PNG

Main Gear Mount External.PNG


The space made for the aileron and flap control horns is more than enough for the control surfaces to rotate a full 90 degrees, so it should be plenty.
Ail-Flap-Gear Clearances.PNG
 

telnar1236

Elite member
The next steps were to design in mounts for the aileron flap and gear servos. The Flap servo is mounted flat and linked to the trailing and leading edge flaps so that they deflect together with the leading edge flaps deflecting less. The aileron servo is mounted vertically aft of the inlets and just forward of the gear bay. Finally, the landing gear servo (not pictured) is mounted in a mirror position to the flap servo and offset forward for clearance.
Flap and Aileron Linkages.PNG


Because I have finished all the symmetrical components of the main airframe structure, I was finally able to mirror everything to end up with a model that actually looks like an F-104.
Mirrored Structure.PNG
 

tomlogan1

Elite member
The next steps were to design in mounts for the aileron flap and gear servos. The Flap servo is mounted flat and linked to the trailing and leading edge flaps so that they deflect together with the leading edge flaps deflecting less. The aileron servo is mounted vertically aft of the inlets and just forward of the gear bay. Finally, the landing gear servo (not pictured) is mounted in a mirror position to the flap servo and offset forward for clearance.
View attachment 229448

Because I have finished all the symmetrical components of the main airframe structure, I was finally able to mirror everything to end up with a model that actually looks like an F-104.
View attachment 229449
You are making Kelly Johnson proud! May he rest in peace.
 

tomlogan1

Elite member
The next steps were to design in mounts for the aileron flap and gear servos. The Flap servo is mounted flat and linked to the trailing and leading edge flaps so that they deflect together with the leading edge flaps deflecting less. The aileron servo is mounted vertically aft of the inlets and just forward of the gear bay. Finally, the landing gear servo (not pictured) is mounted in a mirror position to the flap servo and offset forward for clearance.
View attachment 229448

Because I have finished all the symmetrical components of the main airframe structure, I was finally able to mirror everything to end up with a model that actually looks like an F-104.
View attachment 229449
I'm in on this! Have a 50mm EDF waiting.
 

telnar1236

Elite member
I'm in on this! Have a 50mm EDF waiting.
Glad to hear it! The first STL files and G-code should be posted within the week, but all the preliminary files may need to be adjusted a bit for clearances and easier printing.

If you don't mind me asking, what EDF unit are you planning on using? Having a better idea what thrust class power system different people might use should help inform my design a bit since 50mm EDFs can produce anywhere between less than 200 grams and almost a kilogram on the upper end and system and battery weights can widely vary.
 

telnar1236

Elite member
Ruilogod HL5008 2427 5800KV Motor EDF 50mm Ducted Fan for RC Aircraft Airplane
Any idea how much thrust that produces? 5900 KV is insanely high for a 3s EDF, so I'd be worried that it was designed for 2s and I've had a bad experience with the 40mm EDF from that manufacturer not producing adequate thrust on 2s. But that fan will fit the currently designed mount based on the dimensions available online.
 

telnar1236

Elite member
this is the actual one I have, ADF50-300L 5800 Kv
Looks like a good EDF. Surprisingly high KV motor, but I'm sure the designers know what they're doing. And yeah, it fits the mounts designed and looks like it should have enough thrust if the specs online are right (minimum thrust for the 4-channel version will be about 300 grams and for the 6-channel version will be 400 grams so even if the estimate is way too high it should be pretty much fine).
 

cosmocop

Member
I had the exact same AEO fan years ago. I was pretty new to the hobby still and wanted a powerful small edf. I can't say I had a good experience (my rotor exploded in flight) but maybe your luck will be better. I advise you to ensure the fan is well balanced. Also, it has pretty high current draw for 3s (5800KV plus the high blade count) so you'll probably want to use a 1300mah battery.
 

telnar1236

Elite member
Worked great in my L-39, ran it off a 4s 1300 mah
Wait, you used that thing on 4s? What ESC did you use? I'm beginning to suspect that 5800 KV number is just inaccurate. But if it functions, it doesn't really matter if the manufacturer is making up data.

Also, for the F-104, a 1300 mAh 4s pack might be a bit heavy in terms of wing loading, but is still probably workable
 

Ratcheeroo

Legendary member
Wait, you used that thing on 4s? What ESC did you use? I'm beginning to suspect that 5800 KV number is just inaccurate. But if it functions, it doesn't really matter if the manufacturer is making up data.

Also, for the F-104, a 1300 mAh 4s pack might be a bit heavy in terms of wing loading, but is still probably workable
40a ESC.
 

telnar1236

Elite member
OK, I'm going to guess that EDF has a closer to 4500 KV motor. Obviously blade pitch has a significant effect too, but it sounds like that EDF draws about the same current as the one I have and generates a bit less thrust, and the one I have has a nominally 4900 KV motor. Not the first time a manufacturer has given the wrong info for a cheap EDF, and it sounds like a perfectly good unit so long as it is balance right.