FF2017 Int. Racers: Folkerts Speed King SK-3 "Jupiter"

[Mid7night]

This will be the development/build log for my entry into the "FTFF 2017 International Air races: Design-Off".

I considered several different possible aircraft, but at the last minute converged on the stubby SK-3 racer:



I was drawn to this airplane because of its sleek slender fuse, tiny raked tails, and even tinier main wing. Even at 1/6 scale, this model will only have a 33" wingspan. I haven't began any scale design or modeling yet; for now I'm just gathering reference data and forming a plan of attack. Stay tuned!

Full scale specs:
Length: 21ft
Span: 16ft-8in



Image source, and additional background info here

1/6 scale dimensions:
Length: 42"
Span: 33.3"

Another model-scale reference:



EDIT (11/26/16): Added beta1 PDF plans
EDIT (11/27/16): Updated beta2 PDF plans
EDIT (12/22/16): Added Dropbox links and build instruction PDF


PDF Instructions

Total Package (plans + instructions)

Plans alone (same as posted in the thread)

 

[rockyboy]

Excellent choice! With the smaller wing size and higher stalling speed, are you thinking flaps to help with landings?

 

[Mid7night]

Excellent choice! With the smaller wing size and higher stalling speed, are you thinking flaps to help with landings?

Honestly hadn't even thought that far through, but that's an excellent point. And with points going towards scale appearance and build elements, I think flaps will be in order. Also gonna have my hands full coming up with simple, not-too-hard-to-build retracts.

 

[nerdnic]

This will be a great little racer. Yeah good luck with those retracts, haha. The vstab looks like it will be too small at RC scale. Your first ref image is really nice with all the former sections, should make this one easy to get scale lines.

Will be watching!

 

[Mid7night]

Started prelim modeling tonight ...

 

[rockyboy]

Very neat - are you using Sketchup?

 

[Mid7night]

Very neat - are you using Sketchup?

Nope, it's a program called MOI3D. It's not free, but for all its functions and capabilities it's actually a really good deal. There's no other program, that I know of, that can do NURBS surfaces and solids and boolean functions, for $300. I'd be happy to tell you more about it, but I'd rather not in this thread.

 

[earthsciteach]

That's a beautiful model! That's a lot of engine in that thing!

 

[mrwhiskers]

Hey Mid7Night, I have a suggestion for you if you decide to install retracts: What if you were to install retracts from an F-16 model of similar size and mount them backwards?

 

[rockyboy]

Honestly hadn't even thought that far through, but that's an excellent point. And with points going towards scale appearance and build elements, I think flaps will be in order. Also gonna have my hands full coming up with simple, not-too-hard-to-build retracts.

I saw this excellent DIY Retracts article by HildaFlyer the other day that might be helpful

http://flitetest.com/articles/retractable-landing-gear

 

[Mid7night]

Great suggestions guys! I've seen that tutorial by HildaFlyer, definitely gave me ideas before.

I'm gonna stay away from specifying parts from other kits, I want this to be a self-contained build. With them being so close together in the middle-bottom, I've got some ideas on how to actuate them both with one servo. We'll see....

 

[Mid7night]

More progress on the body and tails ... pretty much there. The fuse will be typical FT fold-up style, with both front and rear turtle decks. Now I've got to sort out the wing.

 

[Mid7night]

Most modeling on the body and wings is now done. I've spotted in places for the aileron servos, as well as potential flaps. Elevator and rudder servos shouldn't be hard to locate, so now I turn my attention to the tiny mountain I've created for myself - the retractable landing gear.

 

[rockyboy]

Looks great! Very interested to see what you come up with for retract design - if weight allows I'll be looking to add them to my racer too.

It might be worth putting together a prototype without the extra weight of retracts to see how she glides and flys too. From what I've read, a lot of scaled down models need adjustments need to be made to the tail surface sizes, or some extra fuse length added in to get good flight characteristics without adding dead balancing weight.

 

[Mid7night]

First cut at the plans are up. I'll be plotting them tomorrow and starting the first test build. The first one will be sans-gear, as rockyboy suggested. I've worked out how the motion of the struts should be to fold into the body, but I haven't sorted out how to simply drive the mechanism from a common servo.


EDIT: Just realized I missed a minor thing....the HOLES FOR THE WING. >_< details, details....

 

[FAI-F1D]

Wow. Very nicely done so far. You definitely have better drafting skills than me!

 

[Mid7night]

Finally got to cut out and start assembling some parts last night. Already I've found a few mods/improvements I want to add to the plans. The lower fuse needs two former reinforcements, and I may rethink where the fuse breaks are. Right now they're not bad, but after assembling one I want to look at the model again with different eyes for potential improvements.

So that's the basic fuse box glued up, next I'll probably tackle the wing. I would like to make it thinner at the tip, but I also like the simplicity of having the aileron servo right there for direct drive. Maybe I'll partially embed the servo in the upper surface too; that'll buy me another 3/16" thickness reduction.

Good times!



Not pictured is the actual scoop-surface of the chin scoop, which I did get glued on as well. Just forgot to snap a pic of it.

 

[Mid7night]

Building continues! I didn't get to finish the wing last night, but I did get the front turtle-deck and canopy on, as well as the tails. The wing spars are glued on, but I didn't join or form the upper surfaces.

For the front turtle-deck and canopy, I cut them out of foam instead of paper because I wanted a bit more strength in those areas. The nose/canopy region is a popular handling point for carrying/manipulating, and with paper over the top I just foresaw it getting crumpled. I think this gives it a better look too. I'm also considering cutting a hatch in the front/top somewhere, so I wanted to have foam over the top for added strength there as well. Forming note: To get the foam to curve nicely- remove the inner paper and gently form the foam around a mailing tube (or similar round implement). That way there is a bit of pre-form curve to it before you go to glue it on the top deck.

In the wing design, there are two crease lines where the spars are for a traditional FT-fold wing, but I always had it in my mind that I would go a more EA-route and peel the inner paper off the upper surface and just curve it over the spars.

 

[Mid7night]

We have wings!!

Actually, all the big bones are blocked out, including servos for elevator, ailerons and flaps (I'm leaving rudder out for now). Also the rear turtle deck is still off until I actually connect all the servos and test everything. So I put it on the scale to see what the damage is .... 12oz! I'm in shock at how light it is, I might actually just take it to the slope for its maiden before even putting a motor on!

The other thing that keeps striking me is how BIG the fuse is, considering how TINY the wings are! If I can get it goin fast enough for those stubby things to work, this thing is gonna be FAST!

 

[Mid7night]

Got all the servos in tonight, and got it all hooked up. Even flaps! Actually, the flaps were extremely straightforward to get working. The fiddly bit was when I decided I wanted a pitot tube on the wingtip to measure airspeed, specifically how to thread the pressure tubes down the wing and into the body cavity.

Oh yeah, forgot to mention; there's an underwing body cavity now. It's the perfect place to mount the battery, and allows some movement to dial in the CG....it just involved cutting a gaping hole in the belly. >_< I'm gonna work on a more elegant access method, but for now I'm working with what I've got.

All things considered, this first experimental build is coming along nicely. I guess it's time to order a motor!