Flite Fest 2017: Folkerts Sk.4 -- no compromises!

[FAI-F1D]

Alright, moving on from the rudder linkage thing for a while, let's look at some more progress.

I finished the cockpit decking and it came out fairly nice. Could be a little better, but with some fill around the wing junction it'll look fine. I need to get an instrument panel in there at some point and see about blacking out the cockpit interior. Also the nose is finished and ready for a firewall. You can also see here that I'm almost finished sanding the fuse. Just a few items on the aft fuse and it'll be ready for glassing.


I sanded up a block of balsa for a canopy mold, stuck it in a 2 liter coke bottle, and heat shrunk it over the stove. Came out nicely with just a tiny touch of crazing at the very top. I think that'll end up mostly covered up.


Looks great in place. The rear canopy will just be flat acetate sheet. I'm just loving this overall look.


This, folks, looks like an airplane. A properly fast one. I finally am feeling some real confidence that this bird will be a success and that it'll actually get finished and flown in time.

 

[Craftydan]

TLAR

Is the plan to mount the cockpit/canopy/hatch to the wing, or are you planning to latch it on somehow?

 

[FAI-F1D]

TLAR

Yes. Yes it does.

Is the plan to mount the cockpit/canopy/hatch to the wing, or are you planning to latch it on somehow?

The canopy/deck assembly is permanently mounted to the wing. I originally planned to have it be a separate assembly but decided to take the easy route. And it has the added benefit of few failure modes.

 

[willsonman]

Looking good!

 

[planebreaker2000]

Looks really good!

 

[wilmracer]

Awesome! Can't wait to see this one scooting around

 

[Aviator08]

I agree with everyone else. Great looking aircraft. Looking forward to the flight video.

 

[FAI-F1D]

We're coming to the end of things on this model which are relevant to a pure foamboard build, but there's still plenty of relevant stuff in this post.

As mentioned before, this model's design depends upon using a structural foam nose opening. This allows the user to have a scale, clean nose and provides capacity for very large motors (up to 36 mm diameter). In order to provide good structural integrity, I used foam safe CA to bond glass cloth inside the nose opening before CAing a plywood firewall in place. You could probably turn down an FT firewall for this task. The nose is sanded to fair things in nicely.


I'll drill the mounting holes later...

At this point I used 3M-77 to attach glass cloth to the entire fuselage and the cockpit decking. It was all wetted out with Ez Lam and allowed to harden for the next 24 hrs before I gave it a good sanding.


Time for hinges!!!


I cut the ends of the ailerons free to provide clearance


And a 1/8" slot all the way down gives us aileron travel. VERY IMPORTANT: The top and bottom surfaces are debonded all along the full hinge length, so you have to inject hot glue into the gap in the aileron face and in the wing trailing edge, otherwise the ailerons will delaminate at high speeds and/or the wing will flutter since it's no longer a closed torque tube. This is true regardless off whether you do a foamboard build or glass your bird. Welcome to speed wings.


Now for the H stab. I used a file to grind some of the epoxy out of the kevlar along the hinge line.


Next a v notch gets cut just like you would on any other foamboard design. See, some foamboard concepts do carry over! Note, you will want to go with a V notch on the ailerons if you do the pure foamboard version because the paper doesn't have the stiffness of kevlar.


I've started painting the model...it's not going well because I've got expired spray paint that doesn't want to dry. Or spray very well. Not good.

Also, red spray paint does a lousy job of covering up carbon spars. I do have a trick for that...

 

[FAI-F1D]

I can't remember if this was answered before, but I weighed the airframe before painting and ended up around 385 g before painting. I think this puts us at 400 g before installing electronics.

I sunk the servos into the wing and rigged up linkages. I taped over the openings, then laid red tissue on top and taped it over. Came out pretty nice, IMO. The aileron control horns are 1/32" plywood drilled to suit.

 

[FAI-F1D]

Next, in goes the elevator servo. The elevator panels are split, so two pushrods are needed. Scratch built control horns again.


I cut up a servo control horn and drilled it for the two pushrods. Yes, I know, there are much better, more rubust methods. They happen to cost money and I just spent a bunch on pest control on my farm, so the model budget is dead for a couple months.


Another view of the twin control rod setup.


And all closed back up (with tape, so I can get back in there should the need arise).

 

[FAI-F1D]

So yeah, we have an airframe now. With working servos, and a wing that actually bolts smoothly in place.

ESC and motor tonight. Perhaps it'll fly tomorrow. Perhaps. The trimming setup is rated at 500 W and nearly 90 mph with an APC 6x4. I'll move up to an APC 5.5x6.5 once it's trimmed, which will make it, well, faster.

 

[rockyboy]

Looking excellent! And with some motor/prop ratings like that, it's a good thing you went with red on the color scheme for long distance visibility.

 

[wilmracer]

I'm eager to see this thing go. Good luck on the maiden!

 

[FAI-F1D]

Got the 2836-3000 kv outrunner mounted without issue after soldering everything on a Skywalker 80A esc to suit the ultimate power system goals. That APC 6x4 looks tiny, but it moves a pile of air (and howls like a tornado). A spinner is on the way. It just doesn't look right without one.


The cockpit framing still needs to be added, then it'll start looking pretty nice.


There's a clear air path all the way back. I need cooling outlets though. Fortunately there is a way to do so which is pretty scale.


Battery hatch is held on with magnets. It's very secure since I used magnets on both sides.


We're ready to fly!!!!!! I've got the radio programmed up...just waiting for this evening to fly it. I'll probably load some ballast into the nose to bring the CG forward a bit more just in case. It'll probably fly fine as is, but Ben's experience with the SK3 has me a little paranoid there even though I've seen free flight Folkerts racers fly with the CG pretty far aft. Anyway, unless something crazy happens, I'll be putting the speed prop on it after a few quick circuits with this prop to get things trimmed out.

 

[Mid7night]

Looks great!! Can't wait to see you get it in the air!

And, yeah... a little more nose weight on maiden won't hurt.

 

[AkimboGlueGuns]

Does the prop even clear the air intake? You might be getting ram air thrust from the outlet hole.

 

[FAI-F1D]

Well folks, I don't have the video uploaded to YouTube, but of you visit the J&H Aerospace page on Facebook you'll see video footage of the maiden flight and the second flight. The model tracked fairly well but was nasty at low speeds whenever the motor was running (glide is fine). The P factor (torque yaw, not to be confused with torque roll) was pretty nasty. I finally decided that I wouldn't install the speed prop until that's fixed, so this model does have a working rudder now, and hopefully tomorrow will involve less adrenaline than today did.

Interesting facts:
1. The CG doesn't need to be as far forward as I'd thought. The model is over stable in pitch, and when it gets up to speed, it's on rails in yaw too.
2. Pretty sure that motor is more than 3000 kv. It's crazy hot, but it wasn't even coming down warm, so I can dump more power into it.
3. Having a magnetic batter hatch and no cooling outlets isn't a good idea. The fuselage pressurises and blasts the hatch free.
4. With cowlings like this one, use lots of fiberglass. Had a rough landing (ground littered with pinecones) and it bashed the chin area quite badly.

All in all an absolute rocket of an airplane. Very stable at speed, and no hint of tip stalling. It's heavy, but the glide is good. I'm going to enjoy flying this airplane.

 

[Mid7night]

Congrats on the maiden(s)!! Glad yours actually got in the air the first time.

I'm curious, where did you put your CG for the first maiden?

Looking forward to flying a fast Folkerts formation someday.

 

[wilmracer]

Awesome. Video looked good! It will definitely be a contender

 

[willsonman]

Looks good and I think you can easily get more speed out of her. The air frame looks very solid. Oh, and lastly, Told you so about the rudder.