Several customers contacted us, wishing to buy a folding prop like the one fitted on the Graal. To satisfy this request, we have been considering for a while to make the folding hub, independently of the Graal production. It seems that it brings more complications than profit.
Drawings set is now freely available. No commercial product based on this set can be sold. No technical support.
Folding propeller hub drawings are licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.
Download link: hub.zip.
This hub is designed to use propeller blades from Ivoprop (Ultralight model). Those prop have a specific design, allowing to adjust the pitch, when engine is shut off. This folding hub, which substitutes to the original one, still features this adjustment. As the two blades have separate setting, careful attention is required to ensure the same pitch setting on the two blades.
Ivo props also have the peculiarity of being sold at full lenght (72") while allowing you to cut them down to the required size (48" minimum).
Caution ; There are two version of the blades, with differents pitch range. You should check which one you need and if you can find a pitch and diametre configuration for your usage.
The CAD image shows the design modification comparing to the original hub of the Graal (right picture). The specific shaft drive of the Graal, as built on the prototype, does not feature a propeller mounting plate, as it's usual on standard engines. The public version accommodates such mounting. Also it has been slightly reinforced to have a wider use range.
The main structural constraint comes from centrifugal force on the blades but torque peaks can be a source of failure. So it's safer to limit the use of this hub to engine power around 50 hp. In any case, careful check must be made at all rotational speed to ensure there is no harmonic vibration. On the Graal prototype there is a transmission damper at the end of the shaft so there was no problem. There is still a potential risk for other fitting.
To insert the blade ends into the hub, they must be slightly cut out. Odds for this modification are given in the drawings. This cutting has been approved by Ivo, the designer of the prop. You should not drill or cut the blades otherwise. It could dammage the internal structure and lead to a propeller failure.
Notice: There is no commercial relationship between Ivoprop and Graal-aéro. We bought, for our prototype, a standard prop to a local reseller, without special discount. For more information, look at their web site.
On the Graal, the hub is assembled without folding springs. The air drag is sufficient to fold the prop. Free standing blades allow opening the prop by hand, on ground, before starting the engine. For several reasons, it must be needed to use folding springs. The design of such strings is included in the plans, but they have not been tested (not even made). They might need some adjustment.
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