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Feb 24, 2021, 12:02 PM
Barney Fife, Vigilante
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Build Log

Razor F3K F5K - New Project from Tom Siler

So I started this project about mid summer and back burnered it for a while, but it's time to get this one in the air. It's a F3K / F5K project with a high aspect ratio. Initially I wanted to build a "XXLite" replacement, modernized for current materials and techniques for F5K wing loading minimums. As I put that to the computer, it morphed quite a bit... Originally I believe the XXLite was based on the mid span and tip foils of the Zone V2 and was targeted at around 200g.
Changing the target minimum weight to 180 from 200g really dictated some tweaking of the wing design. Once the wing changed, it made more sense to just start over with a clean design and remove any thought barriers. Though it started as an F5K project, I decided to see if I could get a F3K version to stay together through the launch phase. After building the first wings, I'm certain that these are significantly overbuilt for F5K still, and can easily withstand the forces of normal F3K launches.

So here is a little information on it...
  • It's a 1.5m glider with aero designed for a flying weight of 180-230g.
  • Initially (like my other projects) it will be a two piece fuse with one piece molds made later if I like it.
  • Being a F5K design, the first models will be built as V-Tails with traditional cross tails made for the F3K version.
  • A bagged prototype precedes the mold creation to ensure the design is viable prior to tool creation.
  • Since commercially available tailbooms are far too heavy for the target weight ranges, I chose to mold my own booms to control the layup.
  • Unlike most of my designs, a 4 in the pod is pretty much required as the root of the wing is less than 8.5mm thick.
  • With the advent of "big flaps" in the F5J world, much of my aero work has concentrated on hingelines between 36 and 40% of chord, rather than the traditional 30% hinge. I carried some of this work over into this design, and the hingeline chosen here was 40%.
  • The wing is based on Synergy profiles initially, and modified slightly to correct lift distribution. Most of the aero changes were at the root to mid sections.
  • KST X06 servos are targeted for two of the four positions, with X08s or BlueBird A10s driving flapperons. The F5K version MAY be able to function well on four X06s.
  • Extensive use of 3D printing will assist in the creation of tools, templates, accessories, and even molds for small parts.

So here's what I have currently:

Wing - root chord 128mm tapering to 65mm at the tip, using a 3P bagged prototype. Because of the extremely small chord and profile, along with the target weights, a new design strategy needed to be employed. This wing uses vertical shears similar to my flapperon facing method on either side of the flapperon hinge, and either side of the front and rear bolts, essentially "boxing" two spars that are full span. Because of this, the core is cut into several thin strips and "faced" then placed back together to go in the bag. This process is a pain in the rear, but do-able, and should be a much easier task with molds. I have built two test wings so far, destroying the first in testing. and the second built with smaller doublers etc. The first test wing was 96g out of the bag, and the second was 92. I am fully confident that I can take another 3-5g out of this wing by molding instead of bagging, landing in the mid 80s. Flapperons were cut at 40% leaving a very small tip, but I decided to go ahead and NOT trileron the tips and see how this holds up. The flying prototype wing was 94g after trimming and finishing.

Fuselage - The fuselage was shortened both in boom length and in nose length from the "standard" F3K fuse... and my first pull was 18g for the pod (with canopy) and 8g for the boom. I felt that these were both overbuilt so I reduced the layup and built another set. The flying prototype boom was 7g and the pod 15g. They're probably both still overbuilt for F5K but I'll drive for weight savings on the next one. The fuse is roomy enough for most gear, and has a 3D printed firewall (12mm four hole spacing) and 3D printed two-tier servo tray that staggers the pushrods and pull strings vertically so they don't interfere with each other. Initially I chose to do flapperon drive outside of the fuselage, but the fuse walls are vertical at that point, allowing for internal linkages down the road as the project progresses.

Tails - V-Tails were sized in accordance with my own design goals, which are a little larger than most of the prominent sizing models out there. They're bagged now and the first set weigh approximately 9g total using 30g carbon with large doublers. I believe a gram could probably be extracted pretty easily there and we will see how things go with the first molded sets... I expect 8g tail sets.

Airframe only - 126g including a 4g printed servo tray.

Gear - The flying prototype has a T-Motor 1507(17g), 3D printed spinner / hub / prop nut(4g), Propeller(3g), Speedix ES-20 ESC(5.6g), a 2S 350mah 80C battery with XT30s(20.6g), 4ch rx (5g), a Zespus magnetic switch(4g), two KST x06 (6.4g ea) servos on the tail, and two KST X08(8.2g ea) servos on the flapperons. This gave a total gear weight of approximately 89g. I was able to shave 6.1g by shortening wires and removing connectors, etc... so gear was roughly 83g.

With balancing, epoxy, pushrods, etc the AUW is 223g, which is above the target weight but well within the designed range of 180-230g. Honestly I didn't even think I would be able to get a F5K much under 240 so I consider it a win for a prototype.

So... here are some pictures of the construction process. I'm looking for a name for the project, and "NXT big thing" has already been suggested but I'm looking for something simpler.
Last edited by tom43004; Mar 03, 2021 at 04:10 PM. Reason: Title change and more pics
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Feb 24, 2021, 12:06 PM
ThomasLee's Avatar
Tom, thanks for sharing, looks like a really interesting project to follow.
Also very happy to see those printed jigs, well done.
Feb 24, 2021, 12:32 PM
Registered User
ReidRoberts's Avatar
Oumuamua. (Oh-mua-mua)

Inter-stellar object that passed through our solar system a couple years back. It accelerated as it departed, defying what we know about such objects. A Harvard astrophysicist believes it is alien technology.

Cons: you’re going to have to explain the name at every contest. That will get old fast.

Pros: people are going to get distracted arguing different acceleration theories; solar sails, hydrogen bursts, etc. They’ll lose contest focus.

As with all your designs and projects, I want one.

Feb 24, 2021, 12:41 PM
a.k.a. Bob Parks
bbbp's Avatar
FVK since it is an F5K model with a Vee tail and V is 5

Feb 24, 2021, 12:52 PM
Barney Fife, Vigilante
tom43004's Avatar
Thread OP
Originally Posted by bbbp
FVK since it is an F5K model with a Vee tail and V is 5

Wasn't FVK the company that made the Graphite? Sounds very familiarish.
Feb 24, 2021, 01:08 PM
Registered User
mlachow's Avatar
Come on. Just call it Soary McSoarFace. You know that is what everyone will vote on.
Latest blog entry: Is-4 Jastrzab Build
Feb 24, 2021, 02:11 PM
F3B and F3K
RetoF3X's Avatar
Originally Posted by tom43004
Wasn't FVK the company that made the Graphite? Sounds very familiarish.
FVK was indeed a company in Germany, which was distributing various sailplanes, including models from Vladimir. The owner has now entered retirement.

I agree with Michael L. , Soary McSoarFace.

Good luck with the project!

Last edited by RetoF3X; Feb 24, 2021 at 02:17 PM.
Feb 24, 2021, 02:38 PM
Registered User
Feb 24, 2021, 03:05 PM
Time for me to Fly...
Mr. Wiz's Avatar

Something quick and easy to say and spell out on the wing....
Latest blog entry: The latest and greatest
Feb 24, 2021, 03:20 PM
Held by Grace
BDOG1's Avatar


Looks Great Tom
How about : Osprey FVK
Feb 24, 2021, 03:30 PM
Have Fun and Just Fly!
l shems's Avatar
Interested in the large flaperons. Does that already theoretically give better results?

Verstuurd vanaf mijn SM-A415F met Tapatalk
Feb 24, 2021, 04:23 PM
Oleg Golovidov
olgol's Avatar
Great project, Tom.
Always pushing the boundaries.
Looking forward to seeing this on the flying field!
Very cool.
Feb 24, 2021, 06:02 PM
Duane, LSF IV
Wazmo's Avatar
Heisenberg (for the Breaking Bad fans?)
Planck (might be confused for a sloper?)
Bazinga (for the Big Bang Theory fans?)
Pteron (feather, Greek)
Feb 24, 2021, 07:27 PM
Registered User
ReidRoberts's Avatar
Duane had the best Higgs Boson jokes..


The Pangolin
Feb 24, 2021, 07:41 PM
Duane, LSF IV
Wazmo's Avatar
Do these Bosons make me look fat?

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