It has been great reading your experiences with the Woodcomp.
It is possible for most electric props to put you in a position where a
failure in the electrical circuit can cause what they used to call a
"runaway prop" situation. The point where the fine pitch stop is
reached by the controller or manual settings or a failure of one or
both.
My experience with the Airmaster prop is being cited here...
The fine mechanical stop on the Airmaster is typically 10 degrees of
pitch. For a 914 equipped aircraft, this means you can achieve 5800 RPM
with very little throttle movement, because the prop is absorbing almost
no torque. This can put the aircraft in a position where you are
producing insufficient thrust to continue flying in anything but slow
flight. This failure is well spelled out in the Airmaster propeller
operating handbook.
One owner had this malfunction because of a self inflicted switch error
by going manual operation thinking he was in automatic operation. He
took off and flew. When coming in to land in cruise setting, he
reduced the power to idle and the prop controller commanded fine pitch,
(The prop will adjust the pitch in auto to try to get the RPM up to the
5000 rpm, the cruise setting commands, which means the prop went to the
fine stop). As he approached the field he set the prop controller to
Takeoff and inadvertently hit the manual switch. When he added power,
the rpm shot to 5800 in a heartbeat. The prop did as commanded,
nothing, as it is in manual. Even at 5800 rpm the plane would only
barely stay airborne. He luckily set down and called me. We went over
the switches and behold he found the problem.
If an electric prop controller fails, especially at low altitude, a fine
pitch/runaway prop condition puts you in a position, where only your
skill and cunning, cool airmanship, and knowledge of your propeller and
aircraft can save you and or your plane. (Good Job Frans!)
The Airmaster has a dedicated manual control which bypasses everything
to give you electric pitch control and a mechanical stop. Propellers,
such as the Woodcomp, with Beta (reversing) do not have mechanical
stops, only electric. A good manufacturer will insure the prop has a
double backup and clear instructions on operations because the prop
limit switch or controller failure can cause the prop to go into reverse
or bypass an electrical stop, causing an over rev or worst case, reverse
thrust in flight.
A number of aircraft were lost in the 30's because of runaway props.
Consequently reputable manufactures make sure they have manual backups
and mechanical stops to prevent aircraft loss.
Thanks for your running comments because we all could use a review of
our prop manuals.
Bud Yerly
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