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Cox Engine of The Month
January-2022
Mark Boesen's

Twin Needle Thermal Hopper with a Cage Mount.



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Post  balogh Tue Oct 06, 2020 1:36 pm

Below is the magnified picture of a COX 4.5x4 prop hub that says: Do not use on RR-1, Thermal Hopper and Space Bug.

COX prop limitation on use Screen31


While the latter 2 are distinctly different antique engines with unique crankshafts , the RR-1 is just a modified reedie..

Does anyone know why the use of this prop is restricted like that?

COX International, selling this prop, offers it for use on any COX 049 engine... Huh...
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Post  Mudhen Tue Oct 06, 2020 3:19 pm

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Last edited by Mudhen on Thu Nov 18, 2021 9:15 am; edited 1 time in total
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Post  sosam117 Tue Oct 06, 2020 3:32 pm

@Mudhen wrote:My guess is it’s a throwback to pre-nylon days.
Until 1956, except for Tornados, Cox used Kaysun polystyrene propellers. Very brittle, subject to creeping, and dangerous. When Cox began manufacturing its own props, it still used a semi-rigid composite before moving to nylon. Its brittleness also was dangerous for high performance engines, thus the warning. After switching to nylon, it seems there’s no value amending the molds when few peeps would notice the warning.

You would be surprised to see a nylon/plastic prop as it is spinning it is also flexing and twisting.
Like the turbine blades on a wind generator, whose blades flex up to over 12 feet when turning.
Wind pushing on the blades push them back to move them.
If you had access to a high speed camera you would see our nylon/plastic props flexing while running.
A friend of mine worked for a helicopter manufacturer and showed my a video of the helicopter's blades (in flight) and how they were flexing and twisting. After so many hours the blades are retired and new ones installed. It was an eye opener!
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Post  balogh Tue Oct 06, 2020 4:21 pm

Thanks Mudhen it is funny that the warnings became obsolete but still survived in the mold! I also assumed it was performance related but wondered why use on  TeeDees was not restricted...now I know: because when the warning was cast on the prop hub, TeeDes did not exist yet i.e. before 61 or so.
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