In mid-September, we finished the new blades and did a final trim on the trailing edge, then mounted them on the static test rig and took it outside to test the blades. (The new 3.66 m (12 ft) diameter prop is too big to operate inside the building. The old 3.05 m diameter prop (10 ft) just fit inside.)
Once we mounted the blades, we took it outside and tested it. We found one issue right away, the chord of this blade is so huge that at the nominal angle of attack, the back of the blade was hitting the chain, so we had to de-pitch by about a degree to make it work (we had calculated the prop shaft length to make sure there was ample toe clearance, but hadn't considered the prop hitting the chain).
The blades seemed to work well at light loading, and once everyone was in place, I ramped up the power and one blade spar immediately snapped near the root. We don't have a power meter on this setup, but I would guesstimate it was only around 200 or 250 W when the prop spar gave way, much lower than our 750 W desired load level (1000 W with safety factor).
This prop had been built using a carbon fiber ski pole in place of the custom-designed prop spars that we have on order, so that we could get the new 3.66 m prop built sooner instead of waiting for a couple of months. These ski poles were slightly heavier than our designed spars, but also slightly smaller diameter. We were hoping they were going to be strong enough, but weren't sure (and also didn't know the fiber composition and ultimate strength of the fiber/epoxy combo used).
On inspection after the break, it looked like the ski poles were made of 4 layers (possibly 5 -- it's hard to tell) -- one unidirectional axial layer, two torsion layers, and one completely circumferential layer that would add almost nothing to the bending strength. (in contrast, the prop spars we designed have spar caps of high strength carbon fiber, with varying number of layers from root to tip).
Since the break was a relatively clean one near the root, we can repair these blades by putting a carbon or aluminum support shaft inside, and then several more layers of carbon on the outside, tying at least one layer into the inner prop skins so they can carry some of the load (we need to add more layers anyway to add about 2.5 mm to the OD to make it fit with the prop hub designed for our custom prop spars).
We'll do that repair in a week or two so these blades are usable, but we also accelerated the manufacture and shipment of the proper prop spars that we custom-designed. They are now being air freighted ahead of the shipment of the rest of the parts that is going by sea and will take about a month to arrive (Ed. Note: The prop spar parts actually arrived on Wednesday). Sometime after we receive these, we'll build a new prop of the new design with the smaller chord size (still TBD if the first version of this will be a built-up style or a foam and fiberglass construction like this one was).
Video of the break:
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| The finished blades, with adapter sleeves on the end to fit into our currrent prototype static rig drivetrain |
Once we mounted the blades, we took it outside and tested it. We found one issue right away, the chord of this blade is so huge that at the nominal angle of attack, the back of the blade was hitting the chain, so we had to de-pitch by about a degree to make it work (we had calculated the prop shaft length to make sure there was ample toe clearance, but hadn't considered the prop hitting the chain).
The blades seemed to work well at light loading, and once everyone was in place, I ramped up the power and one blade spar immediately snapped near the root. We don't have a power meter on this setup, but I would guesstimate it was only around 200 or 250 W when the prop spar gave way, much lower than our 750 W desired load level (1000 W with safety factor).
This prop had been built using a carbon fiber ski pole in place of the custom-designed prop spars that we have on order, so that we could get the new 3.66 m prop built sooner instead of waiting for a couple of months. These ski poles were slightly heavier than our designed spars, but also slightly smaller diameter. We were hoping they were going to be strong enough, but weren't sure (and also didn't know the fiber composition and ultimate strength of the fiber/epoxy combo used).
On inspection after the break, it looked like the ski poles were made of 4 layers (possibly 5 -- it's hard to tell) -- one unidirectional axial layer, two torsion layers, and one completely circumferential layer that would add almost nothing to the bending strength. (in contrast, the prop spars we designed have spar caps of high strength carbon fiber, with varying number of layers from root to tip).
Since the break was a relatively clean one near the root, we can repair these blades by putting a carbon or aluminum support shaft inside, and then several more layers of carbon on the outside, tying at least one layer into the inner prop skins so they can carry some of the load (we need to add more layers anyway to add about 2.5 mm to the OD to make it fit with the prop hub designed for our custom prop spars).
We'll do that repair in a week or two so these blades are usable, but we also accelerated the manufacture and shipment of the proper prop spars that we custom-designed. They are now being air freighted ahead of the shipment of the rest of the parts that is going by sea and will take about a month to arrive (Ed. Note: The prop spar parts actually arrived on Wednesday). Sometime after we receive these, we'll build a new prop of the new design with the smaller chord size (still TBD if the first version of this will be a built-up style or a foam and fiberglass construction like this one was).
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| Testing the new propeller |
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| After the break |
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| The lucky (unbroken) prop blade |
Video of the break:
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