1 Introduction
Hi everybody! In this thread we want to introduce our new F3K project! Right now, it is work in progress, but a lot has already happened and we like to share our progress with you. We are still looking for a name for the aircraft, maybe you have some great ideas?

1.1 Who we are
We are the AkaModell from Munich, a group of students from Munich who work on RC aircraft and drone projects. We take part in student competitions like the Air Cargo Challenge, where we finished 2nd out of 28 teams in the 2017 edition of the Air Cargo Challenge. We do all the design and build ourselves and we usually work with MATLAB, CATIA V5, XFLR5, XFOIL, MSES/MSIS.
Since 2002, the AkaModell Munich has developed some F3K models. However, they were hardly used at contests, as our pilots rather focused on F5D pylon racing in the past. The F5D model “Batleth” was quite successful in the mid to end 2000’s. Some of the HLG designs were also experimental and not meant to enter international contests. Our HLG models are “Genius” (2002), “Climax” (2004), “Bachelor” (2012).

1.2 Motivation for a new project
Five years have passed since our last HLG design and technology made huge progress. Light and strong NCFs, 1s LiPo setups, super strong mini servos and full foam core technology have revolutionized F3K. None of our old designs can really profit from these advances – so it’s time to make a new airplane!

1.3 Goals
Of course, the build shall be according to the F3K FAI regulations. We have learned a lot from the past designs and we can learn even more from current competition models. We want to create a high-performance model, which is easy to build at the same time. The “Bachelor” had many difficult elements in the build and a lot could go wrong. This shall not happen anymore. We want to apply all modern building techniques like full Rohacell core to get an airplane at about 250g AUW. As our cnc-milling machine is only 800mm long, we decided to make a two-piece wing design, like the two-piece Snipe or Vibe. The airplane is not meant to attack the leading F3K models in competition, but we expect high performance from our design, so taking part in a competition should be fine.

1.4 Who I am
I am Alex and kind of the project leader. I just graduated my Master studies at TUM with my thesis about automated low Reynolds number slotted airfoil design and optimization with MSES. Right now it is time for me to pass some know-how to our newer members and this is my personal motivation for this project. Besides the fact that I will get a nice HLG 😉.

2 New design
Before getting into details, find a sneak peek in the images! The renderings are an excerpt of our WIP and many details are not realized in CAD yet or may change. But it is enough to show what we are up for.

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Stay tuned for more information and details! I hope to update this thread as often as possible. Please note that this project is not intended for commercial sale. It is likely that we will provide design data such as airfoils and an XFLR5 model at some point.

looks good ! good luck !

2.1 Short history
Here some images: The fleet with the three planes are the “Genius” model, with different empennage configurations. Back at that time it was common to build the wing shell sandwich from glass fiber with balsa wood core.

The blue/green single airplane with the large winglets is our “Climax”. It is a rather experimental configuration with T-tail and four-flap-wing. The winglets serve as throwing peg here. As far as I know was thermaling excellent, but the throwing heights were never competitive enough. The large winglets generated too much drag during launch. And the winglet trailing edge was rather sharp, which is not good for the thrower’s fingers.

With the “Bachelor” (yellow) we hoped to overcome the drag issue by shrinking the winglet to the size of a throwing peg. This is the only design I can answer all kinds of questions as I worked on this airplane at the beginning of my studies. We reused the climax airfoils with about 8.5% thickness at the wing root to 6.5% thickness at the tip. Like the “Climax”, the “Bachelor” is great for flying in thermals, but it cannot keep up with modern DLG’s because of one simple fact: The airplane is too heavy. With about 300 g AUW, you can hardly beat modern 230 g designs in sink rate and agility. The wing is built in shell construction with 1 mm Rohacell. To save some money and weight, we made most of the planes with a 60 g/m² IMS spread-tow D-box and the rest with 27 g/m² g glass fiber. The aileron servos are in the wing. The yellow wing in the picture is 176 g. The tails are full core, a good set has about 15 g. So, overall the build is rather heavy compared to modern full core designs.
The molded winglet/throwing peg design adds a lot of weight at the wing tip, resulting in high momentum around the height axis. The best launches were about 48 m. The “Bachelor” fuselage is hard to build and it needs some training to make a nice fuselage. A lot of sharp edges! But there is one thing that we really like about the “Bachelor” fuselage: We inserted a 1.5 mm steel wire between the carbon layers in the tail boom. After curing, we removed the steel wires and we were left with integrated pushrod channels for 1 mm carbon rods. I did a lot of destructive testing with different layups, 0/90 spread tow finally worked out to avoid some severe buckling at the region right behind the trailing edge. Despite the little problems with the build, the “Bachelor” was very popular among our new members and some guys built one. Nowadays, we use it for launch training in the spare time between lectures and hang gliding.

Hi Alex ,

looking really good ! good luck for your project and if you need any help from a " good pilot " I will enjoy to help you as I could

2.2 Design ideas
We assembled our new concept learning from our old builds and numerous other competition DLGs. Some of our basic thoughts:

- full foam core wings and tails, cnc-milled cores and negative production molds. To achieve a high-quality build, this is the only way to go. Working with positive construction techniques requires experience and training and the aerodynamic quality of the leading edge is rather poor.

- We want a two-piece wing because of our milling machine size limitation. We know it will add some weight and complexity, but we are convinced it is more practical in our case. With the two-piece design in mind right from the beginning, we can design molds with specialized wing root geometry.

- monocoque fuselage with small lid-type canopy. The “Bachelor” had a slip-over canopy (sorry, I am not sure how it is called in English…) and it is an extra part to build, heavier etc…

- As some our students cannot afford something like super small and light MKS or KST servos, we need a little more space in the front fuselage. Also, many guys of us want to fly with a FrSky vario for training. 1s or 2s LiPo batteries should be possible.

- All servos shall be in the fuselage. It is simpler to replace a broken wing or upgrade from first glass fiber (tests) to expensive and final carbon fiber layup wings. Also, wings for strong and light conditions could be swapped quite easily that way.

- The fuselage design shall avoid sharp edges. This means we renounce all transitions from fuselage to the wing that require roving fillings, microballoons or carbon powder filling. Usually, fillets are heavy and increase the risk of having cavities. Also, the wing shall not have fairings to the fuselage. All in favor of weight and fiber bending radii. One day we want try to use out-of-autoclave prepreg, but it’s hard to get some in the right material. Perhaps someone here has good connections?

- We really like the way the ailerons are connected to the servos in the Vortex 3. We try to implement this solution for our design. I think we will 3D-print the servo frame from carbon reinforced filament… If this does not work, we will fall back on the solution the Snipe 2 offers.

- Although we like the integrated pushrods in the fuselage tail boom of the “Bachelor”, we think that spring loaded tails with string connection to the servos are more lightweight. The elevator shall sit under the fuselage, so pulling the string is pulling the elevator.

- The throwing peg will be a classical solution, no winglets this time! Right now, we favorize the push-through type (like Steigeisen, FW5, Stream NXT…). But it is not fixed yet. Maybe you have some feedback from your DLGs to share and convince us of the best solution!

- We do not want to use gluing templates, so everything should be molded directly.

- The elevator will be screwed onto a little pylon, so it can be detached for transport. We plan a hexagonal tail boom ending and we want to insert an Allen key space holder while building the fin. Like this, we get a fitting between fin and tail boom. Then we can slide on the fin and it should align with the boom correctly. Maybe like this it is also possible just to fix the fin with tape and make it detachable. I will show some pictures as soon as it is constructed in CAD.

- Some thoughts on aerodynamics: The airfoils tend to be very thin (more later. We have our own, new airfoils…). Flaps nowadays are already set in cruise flight mode to 1° or 2° to get a little more camber. The flap setting shall not obstruct the lift distribution shape. So, the airfoils on the whole wing will be aligned to have the flap at 70% chord. More on our wing layout later, it is the result of a custom programmed optimization tool.
This text will be supplemented by pictures once I introduce the detail solutions step by step. I hope our ideas are clear to understand. Please do not hesitate to ask if I formulated something unclear!

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Thanks to the people who are interested in the project! As none of us students has years of experience in the contest scene, we appreciate your contributions. I am sure that it is possible to meet some of the European guys here in the future personally at some events.
You can also visit the AkaModell München facebook page , where you can find some of our past projects. For example, you can have a look at our 5.5 meter monster from the Air Cargo Challenge 2017 with a unique slotted flap wing design.

coming up ;-)

Hi Alex,

concerning fuselage forms and aileron/flap connection... Did you have a look at the GO-mini and BAMF models from armsoar?

My experience is that almost nobody swaps wings on models... So the advantage of 4 servo's in the nose might be a better weight distribution. Wingservo's can be simpler to install then 4 in the nose. In that case only the thinkness matters. I heard good comments about and switched to these graupner servo's after bad experience with MKS:
https://www.hoelleinshop.com/SPECIAL...=68112&p=68112
Price is reasonable! (you can have them with metal gears also)
If you put these in the wing, you only need two D47 in the fuselage (smaller)

For the tail: the most wanted 'quality' here is lightness! detachable horizontal is OK, but I don't see much advantage in a detachable vertical. So, if an oval fuselage us easier to build and you can have the matching oval fitting on the tail, you only have to be a bit attentive when you glue it.

It seems like you almost 'appologize' for a 2 piece wing? I read several topics on RC groups, where there is a demand for 2p wings...

Success!

Hi biesmans,

right now I only have following models in my reach: Steigeisen, Snipe, Blaster2, TweagleTD. The rest I can only look up online.
Ah now I see how the aileron actuation is done at the Go-Mini. Is there a fairing like at the Flare? We also discussed having the aileron horns outside, but we do not want to build an extra fairing. And we prefer the carbon in the fuselage as straight as possible, it's more comfortable to build.

Yeah maybe nobody will switch the wing at his off-the-shelf plane but we have some experimenting with foam cores ahead of us ;-)

Currently some guys have the MKS DS450 in the Bachelors, great servos, little heavy, but we got them a little cheaper from mttec as sponsoring ;-)
The Graupner servos are definitely an affordable option. The fuselage is large enough.

Yes for the fin we would do it like the Go-Mini for example, but with a hexagonal joint. Of course rather glued than detachable.

I expected some critical questions regarding 2-piece DLG wings ... I'm happy to hear some people have the same ideas. I think we have found a nice solution for the installation of the wings - I will make an extra post on the wing joint and installation.

Originally Posted by biesmans

Hi Alex,

concerning fuselage forms and aileron/flap connection... Did you have a look at the GO-mini and BAMF models from armsoar?

My experience is that almost nobody swaps wings on models... So the advantage of 4 servo's in the nose might be a better weight distribution. Wingservo's can be simpler to install then 4 in the nose. In that case only the thinkness matters. I heard good comments about and switched to these graupner servo's after bad experience with MKS:
https://www.hoelleinshop.com/SPECIAL...=68112&p=68112
Price is reasonable! (you can have them with metal gears also)
If you put these in the wing, you only need two D47 in the fuselage (smaller)

For the tail: the most wanted 'quality' here is lightness! detachable horizontal is OK, but I don't see much advantage in a detachable vertical. So, if an oval fuselage us easier to build and you can have the matching oval fitting on the tail, you only have to be a bit attentive when you glue it.

It seems like you almost 'appologize' for a 2 piece wing? I read several topics on RC groups, where there is a demand for 2p wings...

Success!

have a look at these aswell , to thick for a wing servo but an excellent servo at a great price!

https://www.hoelleinshop.com/SONDERA...=68112&p=68112

Originally Posted by Alex Guffler

...
Ah now I see how the aileron actuation is done at the Go-Mini. Is there a fairing like at the Flare? We also discussed having the aileron horns outside, but we do not want to build an extra fairing. And we prefer the carbon in the fuselage as straight as possible, it's more comfortable to build.

Yeah maybe nobody will switch the wing at his off-the-shelf plane but we have some experimenting with foam cores ahead of us ;-)
...

If you want a simple aileron connection: I 've seen a picture of a STRIKE 2 once, where the builder had installed the horn of the flaps on 2 or 3 mm distance of the fuselage. Then there was a curved slot in the side of the fuselage that followed the travel of the horn. So the pushrods were on the inside, the horns on the outside. A magnet kept the rod from falling in the fuse during wingchange. He trusted the fit of the pushrod in the horn for not coming detached... (if I remember well...) You can use a drop of hot-glue on the end of the wire, if you don't trust the fit.

Go-mini has no fairing...

I’m sure this is one of the most interesting details of the glider. Here I have some images. The yellow ones were made earlier to show to my team members and only show the concept. The renderings with the green canopy show the actual wing root design, but no joiner installed yet! For now please add the joiner from the pictures with the yellow design to the green ones in your imagination 😉 .
We think that this kind of wing root can be built in the molds quite well. Of course, the wing halves need to be built separately. The joiner pocket could be built directly wet-in-wet with an insert when closing the wing. In the other wing, the joiner is glued directly during mold closing.

We are still about to calculate the spar layup, so the thickness and length of the joiner are also not fixed yet. Work in progress ;-)

I like the technicallity of the idea... Joint and fixation in one.
I'm no designer but I can think it's not easy to make. When my son left school and started in engineering, I advised him to think as per the KISS principle...
Keep It Simple, St.p.d...
When you swing the plane around, all the force of the weight of the fuselage is now distributed on the spot where you make the wing weaker. Probably you can calculate it to make it strong enough. Or you can 'slide' the joint to the non-throwing wing e.g. on the aileron line. (and still use youre CNC machine...)

I don't want to sound negative, and as I said I'm not a designer, just want to let you know there are other options!

It's easy enough when all molded.

The graupner servos are good and reasonably priced. Not ideal however for a 4 in pod design where the KST X08 form factor clearly wins.

Nice project Alex!
This is an idea I've been thinking about a lot since my first DLG many years ago… I'm glad to see someone developing a strong 2-piece wing concept. A couple thoughts…. It might be beneficial to have a separate spar vs. molded in as nobody wants to crash; but if it does happen, being able to replace it would be a nice. Also regarding the spar; a square or rectangular tube (with light foam/filler) would add some strength without too much weight. It could also be a spot for ballast as we do with many of the larger 2-3M models and allow for swapping minute changes in dihedral angles if still developing them.

+1 for KST X08. Lots of power for sub-micro size. However Emax has some excellent micro and flat wing servos that have worked well for me in the past on smaller projects like the ES9051, ES9251, ES3302 and ES3352. Another option would be the good old Dymond D47 & D60 which are also available under other names such as Tahmazo TS-1002 & TS-D6011.

May your progress be in great orders of magnitude,
-8th

-8th, that concept doesn't make a lot of sense. The spar in a DLG cannot be molded separately from the wing, at least I don't know a good way. Are you suggesting the use of a spar joiner tube/rectangle instead of the method proposed, so the dihedral angle can be changed?