4 Blade Propeller Problem ?

[Phil Jeffery]

Our gliding club operates a Rallye tug fitted with a Hoffman four blade
propeller. On repeated occasions it has had to be remove for rectification
to serious cracking of the fibre sheathing in the hub area. This can be
after as little as 150 hours flying over a 6 month period. Also, it is not
unique having occurred with a second bought principally to avoid grounding
the tug. This is in direct contrast to the four blade Hoffman unit fitted
to our Pawnee which has given several years of trouble free service.

Is anyone aware of operators having similar problems with the fibre covering
on Hoffman four blade propellers and, if so, has a solution been found?

[stephanevdv]

Is the engine the same in both aircraft? My club used to have a Rallye
4-cilinder 180 hp with a 4-blade Hoffmann prop. I don't recall having
that fibre sheathing problem, but we did have problems with the
starter motor, and it was said the light weight of the prop was the
culprit, giving less inertial effect in comparison to the original,
heavier two-blade prop, and causing abrupt changes in angular
velocity. Perhaps the same phenomenon is causing excessive stress at
the propeller hub. If your Pawnee has a 6-cilinder engine, it's likely
to have more inertia...

[Gary Emerson]

stephanevdv wrote:
> Is the engine the same in both aircraft? My club used to have a Rallye
> 4-cilinder 180 hp with a 4-blade Hoffmann prop. I don't recall having
> that fibre sheathing problem, but we did have problems with the
> starter motor, and it was said the light weight of the prop was the
> culprit, giving less inertial effect in comparison to the original,
> heavier two-blade prop, and causing abrupt changes in angular
> velocity. Perhaps the same phenomenon is causing excessive stress at
> the propeller hub. If your Pawnee has a 6-cilinder engine, it's likely
> to have more inertia...
>
And a 6 cylinder engine will have a higher frequency due to more
cylinders firing. If there happened to be a natural frequency of the
prop that happened to match with the firing frequency of the engine that
might make for some high stresses at the hub.??? Just pondering out loud...