Our first rotor blade was supposed to be nothing more than a test for the process of building other rotor blades. However we were curious whether it could hold up to aerodynmaic forces so this blade could create thrust for our test rig.
Our test rig’s goal is to obtain values for aerodynmaic forces of our rotor and duct. For that we need a rotor with the correct geometry. Our first test rotor is very heavy and has a weak point at it’s root. Therefore we were suspicious whether it could hold up in our test rig.
However, we were proven wrong: It was able to carry four times the load which it endures in our test rig. Actually we didn’t have enough containers of water to simulate aerodynamic forces, so we filled up extra large containers which fit 12 kgs of water each.
For hovering, Ephemeron’s whole rotor will create 25 kgs of thrust. Our rotor blade for our test rig has proven capability for loads beyond 100kgs.
Beside the work on the duct mould, we also had to build a rotor.
We need a selfmade rotor, because its shape is not available at the market and important for our usecase. Similar to our duct, the first step was to built a mould. For the first tests, we decided to make the mould rather cheap, so we printed the mould with our 3D printer. Its length of roughly 700mm would not fit in our printer, so we had to split the mould in several parts. After roughly 50 € of material cost and 2 week of printing time, the parts were finished. Unfortunately, the surface was not that smooth to use it directly, so we grinded the mould once, filled the gaps between the parts with some spatula and taped the inner surface with PTFE.
To test our mould we used an easy and fast way of laminating the rotor. Carbon fibre as toplayer and cotton flocks mixed with epoxy resin as core material. To make it resistance against problems and force during the demoulding process we used a rather strong layer structure with mostly unidirectional fibres along its length. After making a template, we cut the carbon fibres. Then we lay them one by one down in our mould and laminated them with epoxy resin. After doing this with both halves, we thicken up the resin with cotton and spread it on the bottom half. The last step was to put both halves together and fixed them with screw clamps.
The next day we demould the rotorblade. It was quiet difficult, because we used to much of the core material, so that it squeezed out everywhere. After that we grinded the edges, which are the results of a splitted mould. We were very happy about the surface and decided to rethink our production process.
We reliazed that the mould is much undersized. In order to get a near end shape, we decided to fill the big gap with gypsum, because it is cheap and easy to handle. To get it stiff and vacuum safe we laminated another ply of fibreglass with epoxy resin. Now it was time to apply spatula to reach the final shape. After several times of grinding the surface and applying spatulas we managed to fill all pores and scratches.
Then we painted the duct mould in black to get a smooth gloss finish.
Having planned all day for work on our duct mould, the whole team gathered to prepare and conduct laminating with glass fibres and epoxy based resin. As previously planned, we couldn’t make use of our hot-wire-cutter as our foam was to resistant to it and styrofoam on the other hand would not have survived a pressure differential greater than minus ~0.2 bar. To achieve good compaction at least -0.8 bar would be desireable.
For that reason we bent strips of foam around the existing foam to approximate our final mould and it’s gaps were filled with milling flakes. Then we split work between actually laminating woven glass fibre layers ontop of the foam, mixing resin and cutting glass fibre layers aswell as “peel ply” which will allow spatula to adhere to our laminate after curing.
The glass-fibres will give our mould enough strength and rigidity to withstand enormous forces from vacuum. This will be necessary when finally laminating our duct in order to use the pressure differential to really press our laminate into the mould.
Next week we will start applying spatula which will be shaped by our milled duct-profile.
All day until late in the evening we began setting up our workshop and started building a mould for laminating a carbonfibre-reinforced-plastics (cfrp) duct for aerodynamic tests. Later we will use the same mould to do an infusion of our final duct made from sandwich cfrp.
Due to its size and to achieve a nearly perfect circle we use a cnc-milled aluminium profile which is fixed to an arm, rotating around the mould’s centre. Initially foam was cut roughly and tommorow we will add a layer of styrofoam which will be hot-wire-cut to approximate the shape of the duct. Then spatula will be shaped by the aluminium profile to accurately model the mould. Ultimately glass-fibre-reinforced plastics will be laminated ontop to provide enough strength to the mould.
We are excited to see how far we can get during this weekend.