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Feb 16, 2020, 11:02 AM
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Build Log

Dragon F5J - Scratchbuilt 3.8m Thermal Rig

Hello all,

with my 2m E-Res project (link) firmly in assembly phase and maiden just a month or so out, it is time to think about the next project. So I set myself the goal of developing a plane that jumps the F5J wagon and joins the crowd of highly specialized, very able thermal duration ships.

The project shall introduce a few novel manufacturing techniques to this workshop, and maybe serve as inspiration to fellow builders, as well as collecting the valued advice of the many experienced builders and fliers on these forums.

This is most likely a multi year project, given the limited amount of time available, and a number of other projects running at the same time. Seeing as the build flows along, I might take a stepped approach and first go with a new fuselage to my existing Element F5J wing before adding a wing of my own making.

To conclude this introduction, this build project is much inspired by the works of Curtis Suter (Supra), Tom Siler (Savage), Marc Pujol (Genoma) and the many more professional and recreational builders who share their journeys of developing designs of their own, and contribute to the advancement and proliferation of state of the art thermal model building.

Blue skies,
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Feb 16, 2020, 11:03 AM
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Dragon F5J
Span: 3800mm
Length: 1830mm
AUW: depending on layup and build strength, starts from 1300g
Wing area: ca 0.85m2
Tail: V-tail, possibly a +-tail later

Bagged carbon fiber skin over CNC wire cut cores. Based on prior Anzu F3K wing bagging experience. Spar of carbon caps and wrapped core (new for this builder). Three-panel wing with 1500mm span center panel of constant chord 240mm, flaps. Tip panels of 1150mm each, tapering to ca 135mm, with ailerons.
Airfoil: not yet decided, Syner-J is a strong candidate.

Hollow molded using balloon technique based on F3K experience. Three parts: pod (ca 700mm) and ovalized boom (ca 1000mm), and tail adapter (ca 150mm). Molds to be built from positive plugs made on 3d printer (new for this builder).

V-tail configuration, bagged carbon fiber over cut cores. Field dismountable, standard ruddervator flaps.

Now, off to the drawing board!

Blue skies,
Last edited by Dotcom; Feb 16, 2020 at 11:25 AM.
Feb 22, 2020, 11:07 AM
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Feb 22, 2020, 11:11 AM
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Here is the current state of affairs.

Just a pencil sketch after numerous iterations throughout last year.
The wing panels have very round numbers in terms of dimensions. The tail still looks a bit large-ish. Before committing to cutting the wing, I plan to verify proportions and performance in XFLR5, but my knowledge of the software is very limited, so this will be a lengthy process.

First building steps will anyway be with the fuselage plug.

Blue skies,
Feb 22, 2020, 01:20 PM
Camber down and screw it up.
gdouglas's Avatar
Looks like an F5J ship.

Wow! 1850mm (72.8 inches) overall length? That seems very long. The Explorer BF 4.0m V-tail's fuselage is 1740mm long (68.5 inches) .

Have you put any thought into making it a 4 piece wing, with a 2 piece fuselage? I think this is the future of F5J as more people travel to contests and the 4/2 config makes it SO much easier to transport or ship.

Feb 22, 2020, 04:55 PM
Dark Side of the Red Merle
Curtis Suter's Avatar
Nice project. Certainly following your progress.
XFLR5 is a great program. I would also suggest Sailplane Calc as it will provide suggested tail sizes for the tail moment arm that you choose. It is better than guessing.

Feb 23, 2020, 07:50 AM
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The current wing idea is three piece as it should fit into my DLG box which offers space for max 1500mm span wing segments. The box fits just into a station wagon trunk with the rear seat down. Hence the center panel at 1500mm has shorter flaps (730mm) than the ailerons at 1150mm.
But I figure a center wing joint would add ca 80g to the total weight (40g joiner, 20g joiner tubes, 20g root ribs and reinforcement) therefore with the 3.8m wing I thing I will leave it at 3 segments.

An additional point I considered, the 3.8m span at the shown partition fits just barely onto a sheet of 1m x 3m carbon fabric.

I might add a new two piece center section later at a slightly larger span of 1700mm to complete a 4m wing.

Two piece fuselage is a strong option. I find the Volo split design a real smart solution with the tail servos in the boom part.

Above drawing is really just a TLAR (that looks about right) which for the tail, it doesn't. Current F5J ships have 10% to 11% horizontal tails (with respect to wing size), depending on tail length and wing aspect ratio. My drawing has 13%, so this tail would maybe fit to a full 4m wing.

Thank you Curtis for the link to the Sailplane Calc, I didn't know it and will certainly have a look.

Blues skies,
Last edited by Dotcom; Feb 23, 2020 at 09:33 AM. Reason: typo
Mar 04, 2020, 04:08 PM
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The first drawing was a bit refined to have a slightly larger aspect ratio and resized tail.
The design will need to be confirmed by XFLR5 or similar, but this will take me some time.

In the meantime, the drawing is sufficient to determine the components CoG and estimate the masses in order to complete a weight and balance sheet. This will then be used to determine where to put the components like fuselage servos, drivetrain components, ballast capability etc.

Next will then be design of some small test pieces for the fuselage to establish the process of printing a positive plug serving the creation of negative molds.

Blue skies,
Mar 26, 2020, 04:29 PM
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For the fuselage master, which will be 3d-printed from PLA, some advance work is completed.
The master will consist of several sections to be mounted on a square profile. It will then be sanded smooth, filled and painted to serve as master for negative molds.

To establish the drawing to printing process, and determine the cutout for the profile, three test samples were made. They have elliptical cross section, with the large one measuring 38x45mm. This is barely enough to fit a 3s1800mAh battery. I will make the actual fuselage a slight bit larger. The small ones measure 16x20mm, about the minimum to fit around the profile which has 8x8mm.

Thanks to my friend Marco for printing the pieces!

Next: drawing the fuselage.

Blue skies, Andy
Sep 13, 2020, 07:31 AM
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This project is a bit in limbo at the moment...
As for all my builds, I established a mass breakdown to determine components location and check CoG. This is needed before committing to a fuselage design just to avoid bad surprizes later.

As seen in the attached table, I come out at ca 1335g AUW with a 'normal' wing layup of Spreadtow 60 in the center panel and 40 in the tips. I assume a 100g motor and 3s850mAh battery. Issue is, at these component weights, the design ends up tail heavy and it would need ca 65g of ballast in the nose. There are a few reasons for that:
- tail servos and RX is in the tailboom which splits from the pod just behind the wing. This greatly simplifies splitting the fuse and leaves room for ballast under the wing without any conflict with the tail linkages
- the wing has a straight spar in order to facilitate construction (joiners are straight in the spars), resulting in a CoG of ca 75mm where most commercial planes have some sweepback of the tips and CoG in the 90-110mm range

Next I will check what lengthening the nose and shortening the tail a bit would look like. If in any way possible, I'd like to avoid measures such as putting the wing servos in front the spar or putting the fuse servos in the pod.

Blue skies, Andy

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