electric flight uk - British Electric Flight Association - Jan Bassett's

Fixing Günther Propellers
by Nick Fitton
My first encounter with 2.3mm shafts was via the TwinJet. I was sceptical about
the recommended method of propeller attachment (push it on) so I phoned a highly
experienced EF guru, who shall remain nameless. He told me that a bit of spit on
the shaft and a good push was all that was needed. Indeed, he told me that he had
never had a Günther propeller come off, saying that he must have spit like epoxy!
Thus assured I flew my TwinJet with delight until one day a propeller came off at
exactly the wrong moment, i.e. the instant it left my hand at launch. I couldn't
hold the resultant asymmetric thrust and the clutching hand which is gravity
took over. Not much damage but a lot of annoyance.
I wanted to be convinced that pushing propellers on was sound practice, but my
fathers oft repeated dictum "A man convinced against his will remaineth
unconvinced still" was never far from my mind. I tried bonding the propeller on
using thin cyano, being careful to remove all traces of oil from the shaft and not to
bond the shaft to the motors front bearing. But still propellers flew off occasionally,
fortunately at non-critical times.
My next cunning plan was to drill a pin hole right through the black plastic insert
and allow a blob of gap filling cyano to flow into the insert before pushing it on to
the shaft - you have to do this quickly otherwise the cyano starts curing before the
propeller is fully home. I was convinced this would solve the problem, but there
was no improvement.
I tried the same with epoxy, poking it into the insert with a match, but whilst this
seemed better than cyano, it was only better in the sense that the propeller stayed
on longer before letting go.
As a last resort I tried high shear strength adhesive, one of those specifically
designed for fixing pinions to shafts in geared applications - and I have never had
a failure in this application. Again I was unsuccessful. Presumably these adhesives
are not designed for metal/plastic bonding, which comes as no surprise.
I persevered, but I knew in my heart that gluing propellers on was not the right
path - as Taoists would put it, it was a "path without a heart", thus one to be
abandoned. It takes a lot of pressure to force a Günther propeller on to the shaft.
I was sure that what was happening was that the black plastic sleeve was relaxing
over time, weakening its grip on the shaft.
In addition, propeller acceleration/deceleration forces would constantly test the
integrity of the bond line, as would the occasional ground contact. Cyclic thermal
stresses may play a malign part too.
Thus I was forced to conclude that all glued methods must eventually fail, and I
moved to mechanical type adapters. Here are my findings:
44 E.F.-U.K.