Russell Kent wrote:

Mark Wrathall asked:

How long do you think before people get seduced by the idea of a

constant

speeding prop, and build a cheap electronic governer to drive pitch

changes into

this system? If you programmed enough hysteresis into the controller,

the

solenoid system should be fine, but if the prop tries to hold perfect

set rpm,

it wouldn't last long.

Paul Lamar responded:

I don't know Mark but if they insist they will have to put up with the

same

brush life as a Hoffman or MT electric prop.

I disagree.  I think it would be a matter of setting customer

expectations: the

electronic controller would only adjust the prop pitch when the RPM

setpoint was

more than some small number of RPMs away (10, 15, 25).  Then engage the

solenoid

and adjust to exactly the RPM setpoint (+/- 2 RPMs, say).  Further, the

brushes

would need to remain engaged for a short period of time thereafter

(perhaps 60

seconds).

During climb & descent: the system engages and remains engaged due to

constantly

changing throttle position or altitude (combined with 60 second "stay

on" timer).

During cruise: the system engages rarely (if at all) once cruise

altitude and

throttle position are reached.

This would probably be more wear&tear than the "pilot controlled RPM" as

the

system is designed today, but it would be nowhere near as much wear as a
continuously engaged brush system like Hoffman or MT.

So tell the customer it will maintain prop RPM +/- 25 (or whatever the

"small

number of RPMs" is set to), rather than the +/- 2 RPMs that Hoffman or

MT are

capable of.  The performance difference seems negligible, while the

wear&tear

difference seems significant.  Of course, one would need to make certain

the

solenoids were rated for such a duty cycle.

Russell Kent

OK, y'all got me wondering.  Just how bad is this brush wear problem?
There's plenty of industrial applications out there with brushes that last a
loooooong time.  And even when they wear out, it isn't any big deal to
replace them.  Cheap too.

DC motors, commutators, slip rings (which are much easier on the brushes)
are an old technology that is pretty well established.  As Chrissi pointed
out earlier, need to have as low a linear speed as possible (small
diameter), a wiper for contaminated conditions, and then you need to let the
magic of copper chemistry build up a resilient "tarnish" layer that protects
all.  Also need to have enough spring load to keep the brushes from
chattering from vibration without grinding them into the ring.

Tom Kendall

We don't really know. It is all rumor and hearsay.

Paul Lamar
 
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