Confluence

An impeller powered evolution of the Stingray

Ongoing project

After the success of the Stingray, and after seeing gliders with “pop-up” impellers gliders becoming popular, I decided I wanted to try something like that also. But I did not want a glider with the impeller extended most of the time when flying around at lower levels.

Regarding the name : the Cambridge Dictionary explains the word confluence as “a situation in which two things join or come together”. I thought this describes well this combination of glider and impeller system.

In March of 2018 I started sketching. By July I was at revision 5. Often, something that I’m happy with at the start starts to feel wrong after some time so I took a time-out at this point. When I revisited this around the end of the year I still liked what I saw, so I started with the construction.

The wing and stabilizer are basically the same as for the Stingray, except that this time the fuselage includes fairings so the root is slightly different. For the wings I decided to solve this the easy way by cutting the root from wings molded in the Stingray tools. For the stabilizers I decided to make new molds, trying out a new (for me) method.

The fuselage is of course completely new and a new mold needed to be made, and that is what I started with.

Fuselage mold

This followed basically the same process as for the Stingray, main difference being that I had a new milling machine and now was able to 3D-mill the shape. Basic steps were

• glue foam blocks on a stable central construction of glass laminate and aluminium tube
• machine the foam blocks slightly undersize
• cover this with a couple of layer of glass fabric
• apply a thick layer of resin filled with microballoons
• machine the surface to final shape

This time I included fairings for the wing and stabilizer in the plug. I spent quite a lot of time getting those right and making sure that the edges remained sharp and accurate. The pictures below show the main steps and some details. Some learnings were made again on how (not) to do things next time.

Fuselage

The fuselage is a hybrid glass/carbon laminate. Mounting supports for the impeller and the elevator servo were bonded in place before the two halves were joined.

Propulsion system

Propulsion is provided by an 80mm impeller. I decided on the Ejets JF80-V3 with 700-69-2250 motor. This will hopefully provide at least 3kg of thrust on 6S. Battery will be 6S-5000mAh. Controller is a YGE 135 LVT.

The outlet is relatively standard except that it is not completely circular at the exit point. I hope this will not cause too much losses. It is also angled slightly down to adjust for the offset thrustline. The inlets consist of movable flaps in the fuselage side that have been partially cut free from the fuselage laminate. The uncut segment at the front serves as the hinge. the intention is that the inlets can be closed again when the impeller is not running. The inlets are operated by a 180degree servo. In an attempt to “un-sharpen” the edge of the inlets I made 3D-printed pieces with a small radius at the front and bonded these to the inside of the fuselage. The inlet area is slightly less than 100% of the FSA.

I have not included full ducting from the inlet to the impeller, only a circular guide tube with a partial inlet lip in front of the impeller. The molds for the ducting have been 3D-printed as a trial. The surface was then smoothed with filler primer and coated with a 2K-paint for finish. This worked out very well, certainly worth repeating for this type and size of items. The shapes of the inlet and outlet don’t release from a single part mold so I decided to laminate on the molds, and then cut the laminates on one edge to be able to remove them. The cuts were then “repaired” on a purpose printed support tool. The positioning lips that slide over the ends of the impeller were also molded on during this “repair” step.

Due to the location of the impeller I could not use the standard mounting tabs. Luckily the impeller is supplied without mounting tabs so I could construct my own. Final installation in the fuselage was quite nerve wrecking because access to the bolts is marginal and visibility of what I was doing basically non-existent.

Rudder

The rudder is a separate part because the fuselage shape does not allow to use one of the sides as a hinge. As I had read recently about others machining plugs/molds in plexiglass I decided to give this a try also for the rudder mold. I machined the plugs out of plexiglass and laid up the molds on those. I must say the process worked very well for me, except for one thing. I had used transparent plexiglass, which turned out to make the grinding and polishing more difficult because it is difficult to properly see the surface. This could be improved a bit by painting the rear surface of the plexiglass black. Two rudders were laminated because the first one ended up heavier than I liked. The second one was still not to my satisfaction but I decided to see first how the center of balance would work out before going to the effort of making a third one. The hingepoints are machined from glassfibre plate. Small bushings from plastic tube are glued in to reduce wear and friction with the carbon hinge rod. The rudder remains removable. The hinge rod is inserted from the bottom and locked in place by a short M3 bolt that is bonded on. Finally the rudder arm for the closed loop system is recessed and bonded in the bottom of the rudder.

Stabilizer

For the stabilizer I went one step further and decided to skip the plug and directly mill the molds in plexiglass. This time I bought some black material which helps a lot with the visibility during grinding and polishing. In order to avoid ending up with rounded off leading edges after the polishing, I milled the centerline surface only after the finishing. This meant I had a sacrificial edge of around 1mm high that was going to be machined away later anyway. At the root a separately machined block was inserted. This was necessary otherwise it would not have been possible to obtain a sharp edge at the root. This all worked out very well, but I think that in future I will go back to machining a plug and laminating the mold because the plexiglass surface is very easily scratched. But for a prototype like this it is more than sufficient.

A prototype stab was made first to test the process. The outer layer for this was 65gr/m² carbon fabric. Everything worked out well, but it was a bit heavy of course. The real stab laminates were made with 30/gr² carboweave as the outer layer, 25gr/m² glass fabric for the inner layer and with 1.2mm foam core inbetween. As for the Stingray, the joiner is a 6mm square carbon tube. I did fill it up partially this time, not sure if this was really necessary.

The elevator servo is installed directly below the stab. The piece of foam you can see in the picture is to make sure that the pushrod is always “loaded” in the same position.

Wing

The wing is basically the same as for the Stingray. The only difference is that the Stingray fuselage has no wing fairings and the Confluence fuselage does, so the root of the wing needs to take that into account. To avoid building new molds for this prototype, I decided to bond in thick wooden root ribs so that I can cut the root to the correct shape afterwards.

The skin layup is :

• paint (UP-Vorgelat)
• carbon fabric 65 gr/m²
• core material 1.2mm thick
• glass fabric 72 gr/m²
• sparcaps from carbon UD 100 gr/m²

On the upper surfaces I added a layer of 25 gr/m² glassfabric before the carbon fabric to improve the surface quality. I did not bother with this for the lower surface, although I will consider it for the next set of wings, the visual appearance is much better.

Adjusting the root to fit the wing was quite some effort. A straightedge clamped to the installed joiner served as a reference to define the cutting angle. I made the first cut with some material to spare so that I would be able to adjust if needed. After this first cut I was able to mount both winghalves on the fuselage with the joiner and the anti-rotation pins to mark the final cutting position parallel to the fuselage. All cutting was done manually with a japanese saw that I had bought specifically for this. It turned out to be the perfect piece of equipment. Final adjustment was made by an iteration of test fitting and grinding until I was happy with the result, and scared that I would screw it up if I went any further.

The rudders are hinged on the bottom. To scratch the hingeline on the underside and to cut the laminate on the upper surface I made a couple of simple templates that align with the trailing edge.

I then added the rudder leading edge “lips”. This was done by applying a self-adhesive tape at the leading edge to serve as a flexible mold, applying a mixture of resin and microballoons on this and then bending the tape to a suitable shape during cure. After cure, the tape is removed again and the forward edge of the resulting “lip” is cleaned up by grinding. This was the first tine I tried it and as usual practice makes perfect, meaning that there is room for improvement.

Servo’s for the flaps are KST X10, for the ailerons KST X10 mini. In the picture you can see that the lower mounting lugs of the aileron servos have been cut off. This is only because the servos come out of another model where the available space was too small for them.

Now was a suitable moment to assemble the model once to fuel my motivation to go forward into the last phase of installation of all the equipment :

After this small intermezzo, all equipment was installed and the wiring and connections finished.

And then, end of October 2020, the big day came. Final mass is 4.2kg making hand launching the model a big no-no, so I had also constructed a starting cart. With the experience of the Stingray I already had the correct control throws and CofG position so that gave some initial confidence.

The first take-off was quicker than I expected as the model leaped into the air in a steep angle. Once the motor was switched off at height, the model behaved much better. I had incorporated a downthrust angle of 4°, but that was clearly not enough. Everytime I used the motor, a lot of down elevator was needed. Despite the wing loading of 93gr/dm² the first flight lasted 10 minutes, using only one minute of motor time. Due to the thrust angle error the first flights were used to set the trims for the different flightmodes and to do some aerobatics without motor. The model behaved very similar to the Stingray.

Motor and inlet controls are combined on the same switch so that the inlet opens slowly (in 1.5 seconds) as the impeller spools up, and closes slowly when shutting down. This worked flawlessly as far as I can tell. The motor power setting can also be adjusted with a separate slider.

In order to determine the correct thrust angle I printed a number of angled extensions that I could tape to the outlet. I ended up finally with a thrust angle of 12°. The model now behaves neutral in the speed flightmode.

In glider configuration the model is completely silent, no turbulence (?) noises you can sometimes hear with certain models. I assume this means that the inlets close properly. All in all I am very happy with the end result.