Prop adaptor for large props

[Andy Boylett]

At East Kirkby we managed to break a 24" prop when landing. We found a hole for one wheel just as a gust of wind caught the plane. Anyway, this caused a new prop to be fitted. Now, because we fly electric we do not use spinners in order to allow full airflow through the electric motors. Whilst the motors can be sorted out so that they have enough cooling with a spinner fitted, it is just much more convenient just to do without. Now the prop that broke was half an inch thicker than the one we replaced it with. This meant that the large shaft nut, which was previously flush with the end of the shaft, was now set in and the shaft protruding........so next flight Mr very eagle eyed scrutineer quite rightly spotted it and said not good enough, needs a round end.

BMFA rules recommend that there should be a round or domed prop nut or spinner, which is what I have now mannufatcured for this prop adapter. However, the shaft end that this radiused prop-nut now covers up was a flat end of 12mm diameter....was this not safer than a round end?

Here is the newly modifed large electric prop adapter, hand just to show size....

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The domed end is fitted using the same technique as a large spinner that has a screw through onto the end of the shaft - usually an adapter on the shaft. However, on these large electric motors the shaft already has an M4 threaded hole in the end (used during the manufacturing process) so the scrrew goes straight through....

IMG_3111.JPG (32.51 KiB) Viewed 4133 times

IMG_3112.JPG (30.83 KiB) Viewed 4133 times

[Dave Lowe]

If memory serves me right, it should have a 7mm radius on the dome to comply with the regs. the one in the picture does not look that big.

[Alan Cantwell 1131]

that request is no longer valid, the 7mm measured tangently etc went a while ago--but do you know why it was invented in the first place??

[Andy Boylett]

The 7mm was the original BMFA figure. It came about because they had issue with there being sharp points such as 5mm motor shafts, that could in theory puncture someone if it hit them. I think that just saying rounded was not good enough because you could always get someone who would say pin head is rounded!! Hence the radius. In the latest BMFA rules it just says rounded again, no dimension is included.

The one I have made above is approx 7mm radius, 14mm diameter, but the end is slightly flattened.