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

[FAI-F1D]

Ok gents, I'm about to go down a rabbit hole. After doing a fair bit of research, it looks like the Folkerts Sk.4 is the best choice for how I want to approach the air race.


17" Sk.4 built by fellow FAC'er George Bredehoft

I somewhat idolized this plane over the years. It's a somewhat obscure model in my disciplines, and I never encountered one in my time in Flying Aces Club mass launch events. For my own work I got most of my contest successes with a Chambermaid, which is a very smooth flying design when built as a rubber powered model. I always wanted to try the Sk-4, and now I finally have the chance, albeit in a completely different context, so here goes!


The Sk.4 was built as a replacement for the Sk.3 after its fatal crash and incorporated a larger engine, different wing structure, and a new tail design. It was initially plagued with flutter problems, but these were resolved. Unfortunately the engine broke a valve during a qualifying run and the fatal crash ended the Folkerts series of air racers.


This model will be built from foamboard, but I'm taking a different approach from the norm. If it's gonna be scale, it's gonna actually be scale. As such, wherever foam can't make the scale cut, something else goes in there. I'll tweak the fuselage cross sections for ease of construction, but scale outlines will largely prevail from nose to tail. I know there's a bonus for retractable landing gear, but I'm not going there. The gear aren't going to be present, and besides, a small (34" span) racer has no business attempting a landing on scale gear. That's a recipe for disaster.

Because of humidity concerns and because I like the lifespans of my models to be measured in decades, all the paper will be stripped off and replaced with light glass cloth. I'm keeping the design in a format where regular foamboard with paper can be used, though, so others not comfortable with this technique can still attempt the design.

One word of warning: this is not a simple model, nor is it a slow one. It there's a question as to whether you can built it the way it's presented, then you need to be looking at something else. This is a challenging build and it will be very fast and very unforgiving. Don't take excessive risks with your safety or your wallet.

On to the build...

It's got sweet lines:

If you can't tell, this is going to be a pretty curvy fuselage. A pain in the neck to draft, but worth it in the end.

The wings will borrow heavily from nerdnic's beveled outline methods, but with a few tricks of my own. A rather beefy carbon spar will enter the mix. Servos will be buried in the wing, and I'll probably go with either heavily streamlined linkages or a top drive setup if appropriate doublers can be put into the aileron faces. One diversion from scale is that I've linked the flaps and ailerons to go full span. More control=better, plus I can put the servos farther inboard and so reduce roll inertia and flutter tendencies.


I think this wing should actually be a very easy build compared with the rest of the plane. Tail surfaces, same way. Cut them from DTFB, peel off the paper, sand to shape, add carbon spars, and glass it. Done. And we're going rudderless just to avoid an extra failure model that provides no benefit at speed.

 

[HilldaFlyer]

Following - with intent of learning new heretic, I mean, building skills.
Hoping to see some fiberglass bagged wings... and composite fuselage.

 

[Mid7night]

AWESOME! Looking forward to seeing this unfold. I'll do my best not to steal incorporate too many of your build methods in my SK3.

It looks so similar, but I can immediately pick out the subtle differences too. I love it!

 

[FAI-F1D]

Following - with intent of learning new heretic, I mean, building skills.
Hoping to see some fiberglass bagged wings... and composite fuselage.

Tempting, but I'm going to try to keep things fairly reasonable. I may just go with Joshua Orchard's methods on most of this. My main concerns are keeping the front end together and keeping the wing reasonably stable. A DTFB wing will stay decently stiff, mind you, so long as it doesn't get compromised by bad handling or humidity (and top drive ailerons are a no-no on DTFB). I'm pretty sure though, that a scale nose cannot exist on this model without either severely limiting the power or glassing the front end. I'm going with a traditional behind the firewall motor mounting, so glass it is. I'll probably play nice and use the FT firewall, but it'll be cut to a more rounded outline to fit within the profile of the nose.

AWESOME! Looking forward to seeing this unfold. I'll do my best not to steal incorporate too many of your build methods in my SK3.

It looks so similar, but I can immediately pick out the subtle differences too. I love it!

I'm loving the parallel build sequence of two very similar airframes. Yours is definitely a beginner friendly setup that should be very straightforward to get in the air (and probably quite fast), whereas I'm just going nuts over here. Hopefully other participants can glean from each and come up with hybrids that even further refine the whole thing.

 

[AkimboGlueGuns]

They're both going to be pretty awesome! I can't wait to see them both together for comparisons, as well as in a race. Honestly, I can't wait to see the crazy designs that people come up with for wings due to the speculated limited power setup.

 

[FAI-F1D]

So here's a proposed power setup using specs similar to those of an old EDF motor that's sitting in my box. I'll start out with this setup and go from there. On a scale model there will be enough drag to stall the inner bits of the blades during acceleration, so it should make an absolutely fabulous noise in that flight regime and hopefully be fairly quiet at full speed.

 

[Mid7night]

Nice! Soooo...basically you're powering it with a 2-blade unducted fan? If your fuse scale is similar to mine, that 5.5" prop won't be much taller than the body itself. Should scream like the dickens though!

 

[FAI-F1D]

Nice! Soooo...basically you're powering it with a 2-blade unducted fan? If your fuse scale is similar to mine, that 5.5" prop won't be much taller than the body itself. Should scream like the dickens though!

I reckon that's one way of looking at it. It's an el cheapo pylon setup until (if?) I get the model trimmed out. That prop actually works really well on actual pylon models, for what it's worth.

 

[FAI-F1D]

I have to confess that I've decided I don't like designing foamboard airplanes, especially in a form someone else can reproduce. There's been way too much head scratching as to how to make certain items work in a way that doesn't involve trial and error. That said, I've got enough here that someone else could follow my build with photos provided to guide them through. I might add skin templates later, but they'll be scanned in from the actual parts I cut for the build.

So here's where we are:


Next step is to export to a reliable format and print off a set of plans. Then the foamboard starts getting cut out and the fun begins.

 

[Tench745]

I have to confess that I've decided I don't like designing foamboard airplanes, especially in a form someone else can reproduce. There's been way too much head scratching as to how to make certain items work in a way that doesn't involve trial and error.

Right!?

 

[HilldaFlyer]

I have to confess that I've decided I don't like designing foamboard airplanes, especially in a form someone else can reproduce.

I agree. As for me, I just hope my plans are close enough to get someone close. It takes me about three rounds of design, print, cut, build, adjust plans, repeat to get it to a point where it is repeatable. For this build challenge, I'm doing one round and follow me if you dare.

 

[rockyboy]

I'm finding the opposite experience. I'm having a ball doing the computer design side, taking notes for plan adjustments, and gearing up for all the things that will come out of the maiden flight. Maybe we need to find a way to team up on the design cycles

 

[Mid7night]

I always enjoy the computer design side...but that IS my job, and something I've enjoyed for years. I also enjoy the building, but I live in such a small place (1bed condo, no garage), setting up a workspace and then cleaning up when I'm done takes half again as long as the actual building!

I also don't have the best track record with hand-launch-maidens, so I ALWAYS dread that moment because I just know it'll end in repairs. But in the computer, everything is as it should be!

 

[FAI-F1D]

For this build challenge, I'm doing one round and follow me if you dare.

Now there's a thought!

 

[Mid7night]

Since our planes are relatively similar, a few things to watch out for on yours that I've experienced on mine...

The big nose appears to be destabilizing in yaw - Vertical tail may need upsizing to compensate.
Lower cowl 'ramp' under the motor seems to be an area of positive pressure (under power) creating a 'lifting' effect forward of the wing, necessitating a further-forward CG than you may think.

You may want to rethink your omission of a rudder...

 

[FAI-F1D]

Ben, thanks for that info. Mine will have a fairly scale cowling, but I'm going to enlarge the tail surfaces to compensate for the you mentioned. I'm still going to omit the rudder, but it'll be straightforward to implement it if needed. I think the full span ailerons should help.

 

[FAI-F1D]

So I procrastinated away at this...

Last weekend I get everything traced out on three sheets of DTFB. The wings had to be split apart to fit, and each fuselage segment had to be separated. Worked out ok I reckon, since it took 2 1/2 sheets, but of course that last half sheet will be taken up with turtle decks and such.


You need a long, sharp blade to make speed wings. I did a terrible job. Fortunately no one will see it once it's closed up.


Here I've joined the lower wing surfaces.


And another view of the overlap. The seams will get glued, and then I'll remove all that paper.

 

[FAI-F1D]

I peeled the paper off where the wing spars would be laid in, and removed the paper on the upper (glued) surface of the spars. These are then glued in place, and then the paper is removed. Notice that the spars are glued to the upper surface with the exception of the center section. Next, the spars are slotted for the actual spars--carbon tubes.


All the remaining paper on the inside of the wing, and then the carbon spars are glued in. As mentioned, there is no paper on any part of the inner surface of the wing. Even if you're not going to glass the wing, this is necessary to provide strong good bonding surfaces and to let the wings curve smoothly into a semi symmetrical airfoil.


Glue in place, wing about to be closed. Notice that I glue the LE and spar first, join the trailing edge, and finally step through the tips. The TE and tips are a pain in the neck for me, a devoted balsa builder.
Also, notice the carbon strip embedded in the leading edge.

 

[FAI-F1D]

Here we have the wings closed up. Notice that a slot has been cut through the center spar foam all the way to the paper on the bottom. This will allow dihedral to be set. Also it's important to notice that the spars are installed offset so that they can overlap. This is very important to get the necessary strength in this wing. It is extremely thin and will carry a lot of G load.


Now I've set the dihedral by gluing the spar stubs so that they contact the bottom surface paper.


This is extremely important: if you are building this model and planning to leave it a pure DTFB build, do not use hot glue for the spar join! I'm going to skin that lower surface with glass and epoxy, so I can get away with using hot glue as it'll just be in compression against the glass.


The center section is now closed. All of the joints in the center section are glued because at this point the paper is about to stop holding all that together.


The ailerons will need their front edges sealed together once they are cut. For me, I'll have the benefit of facing them with fiberglass. For y'all who don't do that, facing them with hot glue will work fine.

 

[FAI-F1D]

The fuselage boxes are pretty mundane. You just build them out. They get complex later. There needs to be some plywood in the front of the fuse to make room for an adequately sized rubber motor and plenty of cooling paths.

Here we have something that looks a bit like an airplane. Notice that the very front of the fuselage is nowhere to be seen. That'll be carved from a block of foam later and glassed thoroughly because I'm doing a front mount motor setup.


The tail surfaces will be glassed later. Nothing terribly interesting there.

This is not a small airplane. It's gonna look nice, too.


For those who want to know, there is now no paper on this airplane except on the inside of the fuselage. I figured it at least couldn't do any harm in there.