F-104 Starfighter profile plane

[Closus]

Hi all,
After some other successful scratch builds, I decided to tackle something quite challenging - that is, the legendary F-104 starfighter!
I wanted to build a small, hand-launchable, prop-powered profile plane version of the iconic jet. I tried to keep the proportions mostly scale, but enlarged the wings and horizontal stab by about 15% in all dimensions. The plane uses only two 9g servos - one for the elevator, one for both ailerons, as well as an Emax 1306 motor and a 3S 1000mAh pack.
My first prototype had a few not-so-successful, but insightful test flights today. the initial version was built as a pusher, and I have now switched to a mid-prop configuration to get some airflow over the elevator even at zero airspeed. This was the main problem on today's test flights - the plane flew decently well, but as soon as the nose went up more than 30 or so degrees, all control was lost due to the lack of airflow.
Hoping V2 flies better, will test tomorrow

Key difference from my first prototype:
- Actual wing profile (drooped leading edge) instead of flat wing
- Lowered horizontal stab to move it into the prop's airflow
- Hinged instead of full-flying elevator
- Much smaller ailerons (roll rate was extreme!)

All the parts are lasercut on my small Flux beamo laser cutter. Once/if I get a decently flying version, I'll post the plans here.

Feedback? Ideas? Suggestions?
Stay tuned for updates tomorrow! I hope V2 will at the very least provide some valuable inputs on what can be improved further. A first test while hanging the plane from the CG, and giving throttle at zero airspeed showed a promising amount of pitch control, at least.

Size:
92cm length, 43cm wingspan. Uses about 1.5 sheets of flite test foam board.
AUW:
V1: around 280g
V2: around 250g

Pictures:

Original Sketchup prototype:

Finished V1 (pusher):

Sad state post-maiden with field repairs:

Redesigning for mid-prop:

Cutting parts for V2:

Finished V2 prototype:

 

[telnar1236]

That looks great! I'm a big fan of the plane, so I'll most likely build one once you post the plans. Deep-stall is a known problem of F-104s and the best way to avoid it is simply not to exceed about 15 degrees angle of attack. The CG on the F-104 also likes to go farther forward then you'd expect, at maybe 10-15% of the chord from the LE of the wing at the root, at least for non-profile models. I don't know how heavy the weights are in the nose, but it looks like the original version might have been a bit tail heavy and having the forward CG helps mitigate pitch-up.

 

[Closus]

That looks great! I'm a big fan of the plane, so I'll most likely build one once you post the plans. Deep-stall is a known problem of F-104s and the best way to avoid it is simply not to exceed about 15 degrees angle of attack. The CG on the F-104 also likes to go farther forward then you'd expect, at maybe 10-15% of the chord from the LE of the wing at the root, at least for non-profile models. I don't know how heavy the weights are in the nose, but it looks like the original version might have been a bit tail heavy and having the forward CG helps mitigate pitch-up.

Thank you! Keeping the nose down on V1 was simply impossible (probably in part due to being tail heavy) - and my goal for this profile plane is that handling should be at least a little easier than a proper jet
I did end up flying V2 today, and it was WAY better! Longest flight was about a minute long - it has very challenging handling characteristics, but flies pretty well and fast (and I say this as someone who regularly flies turbine jets, lol). The airflow from the mid-prop definitely helps maintain pitch control. The key issue now were:
- Sudden snap rolls
- Orientation issues due to no markings
- Somewhat unpredictable elevator control; there was way too much wobble/slack here
- The tail end got wobbly after a few rough landings, so more reinforcement was needed

So, the obvious thing to do was building V3!

I know, it‘s very pretty I‘ll build a good looking one once it flies well. The underside now has yellow/red/green decals for maximum visibility.

Key changes:
- Properly engineered full-flying elevator (4mm carbon tune through lasercut wood parts)
- carbon reinforcement on the belly, so the prop is no longer the lowest part of the plane. Also adds a ton of torsional rigidity to the rear of the plane
- slightly increased aileron size again
- added 5deg of UP to the entire horizontal stab, since this seemed to be the amount of trim needed for level flight
- moved battery forward by 2.5cm
- longer carbon reinforcements in the fuselage

These changes result in a MUCH more rigid plane, at about 20g weight penalty.
Sadly, due to the heavier horizontal stab, I still think it needs 20g of weights in the nose for proper CG (but I may be able to remove some).

Here are the three versions built so far:

I‘m now convinced I can turn this into a useable model, albeit for advanced pilots only. More time will then be spent making the build easier and adding helper lines, scale details etc. to the plans.
It‘s definitely not a quick build though, so expect 2-4h build time at the very least (depending on if you can lasercut the plane in one piece or need to split it up like me).

 

[telnar1236]

Thank you! Keeping the nose down on V1 was simply impossible (probably in part due to being tail heavy) - and my goal for this profile plane is that handling should be at least a little easier than a proper jet
I did end up flying V2 today, and it was WAY better! Longest flight was about a minute long - it has very challenging handling characteristics, but flies pretty well and fast (and I say this as someone who regularly flies turbine jets, lol). The airflow from the mid-prop definitely helps maintain pitch control. The key issue now were:
- Sudden snap rolls
- Orientation issues due to no markings
- Somewhat unpredictable elevator control; there was way too much wobble/slack here
- The tail end got wobbly after a few rough landings, so more reinforcement was needed

So, the obvious thing to do was building V3!
View attachment 232707
I know, it‘s very pretty I‘ll build a good looking one once it flies well. The underside now has yellow/red/green decals for maximum visibility.

Key changes:
- Properly engineered full-flying elevator (4mm carbon tune through lasercut wood parts)
- carbon reinforcement on the belly, so the prop is no longer the lowest part of the plane. Also adds a ton of torsional rigidity to the rear of the plane
- slightly increased aileron size again
- added 5deg of UP to the entire horizontal stab, since this seemed to be the amount of trim needed for level flight
- moved battery forward by 2.5cm
- longer carbon reinforcements in the fuselage

These changes result in a MUCH more rigid plane, at about 20g weight penalty.
Sadly, due to the heavier horizontal stab, I still think it needs 20g of weights in the nose for proper CG (but I may be able to remove some).

Here are the three versions built so far: View attachment 232708

I‘m now convinced I can turn this into a useable model, albeit for advanced pilots only. More time will then be spent making the build easier and adding helper lines, scale details etc. to the plans.
It‘s definitely not a quick build though, so expect 2-4h build time at the very least (depending on if you can lasercut the plane in one piece or need to split it up like me).

I like the scheme . It's interesting that you mention that it has problems with snap rolls. I had a similar issue with a small foam board EDF F-104 years ago, so it seems to be a recurring issue with models of the aircraft. I've never had that issue with larger F-104s, so I assumed it was a construction issue on my part, but with you experiencing the same problem it seems to be something more than that. What's strange is that my larger F-104s are extremely stable except for the pitch up problem so it might be a rigidity thing or something to do with full span ailerons.

Edit: Also, looking at your pictures, the hinge line for the horizontal stabilizer looks like it's very far back, so that might contribute to poor pitch control. As a general rule, the hinge line should be just a bit forward of 25% MAC line for full flying control surfaces to avoid flutter and large moments.

 

[Tench745]

As a wild guess, this is a pretty small wingspan, so the loss of lift at the wingtip is probably pretty significant, especially at higher angles of attack. Combine that with a relatively sharp leading edge and I would expect stalls to be very abrupt.

 

[Closus]

Thanks for the feedback guys!
Regarding the elevator: good point, I didn‘t know exactly where the ideal point would be. Flutter should be a non-issue at these speeds, and since the elevator assembly is pretty overbuilt, but you never know. V2 actually had good pitch control, and V3 moved the hinge line forward by about 2cm. The reasoning behind the further backwards hinge line is that up elevator will displace the front of the horizontal stab further downwards and thus into the prop’s airflow. But V3 will show whether that actually made a difference
Regarding the wings, I was hoping to tame those tendencies with the (quite significantly) drooped leading edges. If the issue persists, I will surely move towards a proper flat bottom airfoil.
A change I forgot to mention was that the amount of anhedral in the wings was significantly lowered for V3 as well (from 12 deg per wing to just 4-5), which should hopefully also help with roll stability.

I will keep this thread updated with how V3 does

 

[Tench745]

Thanks for the feedback guys!
Regarding the elevator: good point, I didn‘t know exactly where the ideal point would be. Flutter should be a non-issue at these speeds, and since the elevator assembly is pretty overbuilt, but you never know. V2 actually had good pitch control, and V3 moved the hinge line forward by about 2cm. The reasoning behind the further backwards hinge line is that up elevator will displace the front of the horizontal stab further downwards and thus into the prop’s airflow. But V3 will show whether that actually made a difference
Regarding the wings, I was hoping to tame those tendencies with the (quite significantly) drooped leading edges. If the issue persists, I will surely move towards a proper flat bottom airfoil.
A change I forgot to mention was that the amount of anhedral in the wings was significantly lowered for V3 as well (from 12 deg per wing to just 4-5), which should hopefully also help with roll stability.

I will keep this thread updated with how V3 does

Look forward to the updates!
I'd be interested to see what difference (if any) ironing the leading edge makes.

 

[Closus]

Well, here's the promised update

V3 did fly, a little better and more precise than V2. But it still had very unfavorable roll characteristics and tended to enter an unscheduled descent whenever elevator was applied while at a roll angle above a few degrees

So, the logical thing to do was building V4!
V4 marks the largest difference to the previous prototypes:

• Much larger! We scaled the previous version by 1.5, resulting in a length of about 1.4m. The weight only increased by a factor of 1.5 as well (to about 440g), but with the wing area increasing by the square to 2.25x, wing loading is reduced by about a third.
• An actual airfoil for the wing. Seems like there's no way to make a true profile starfighter actually fly well, so we built a simple flat-bottom airfoil wing with the traditional flite test building technique.
• More carbon reinforcements: There is now a central 10mm carbon wing spar (which is much larger than needed), along with more carbon along the bottom of the fuselage. This results in a very stiff plane, which can be picked up from the nose without bending a bit!
• The motor is an Emax 2205 2600kV with a 6x4 prop, which provides a 2:1 thrust ratio, much better than the previous version.
• Slightly reeingineered the elevator assembly according to suggestions provided here, moving the pivot line much further forward.
• Because we wanted to get in a test flight on the same day, the tiptanks were left out for now

And the result is:
The plane flies amazingly well! It's very stable and smooth in turns, pitch control is good, and the very low wing loading is definitely noticeable.
IMO, the sum of the changes has made the plane almost too easy to fly, so it would be quite reasonable to sacrifice some stability/low speed ability for greater agility and speed - that's what the next version will be aimed at. We'll also possibly be making it a bit smaller again.
We've compiled a list of about 20 changes for the next version, which will also be aimed at making it easier to build, since build time has increased to 5-6 hours now.

 

[Closus]

I did some cleanup and added tiptank to the V4 prototype Hoping to test fly again soon. I also had to swap the motor, since the bearings were shot. Plans for V5 are also in progress, along with various improvements such as ball bearings on the horizontal stab, as well as easier buildability...