nuckolls.bob(at)aeroelect
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 Posted: Wed Mar 17, 2010 6:28 am Post subject: RV ballscrew flap actuator performance |
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At 06:49 AM 3/16/2010, you wrote:
| Quote: | --> AeroElectric-List message posted by: "Lapsley R. and Sandra E. Caldwell" <lrsecaldwell(at)earthlink.net>
Fred
It would be interesting if you could remember if you could remember id the position of the flaps about blowing the fuse was fully up or down.
Also keep in mind that there have been at least two versions of the Flap drive mechanism.
As to the clutch, if you operate the flaps without the engine running you can hear the flap clutch slip at the fully up or down position (if you don have any limit switches installed). |
The "slip clutch" on commonly used ball-screw actuators
are a bit deceiving. When one holds an unmounted actuator
in hand, the ball-nut seems to magically "disconnect" and
spins freely at each end of travel. This type of nut
features non-circulating balls held in a cage. The grooves
in the nut are not matched to the screw (helical) but
are instead circular raceways. See:
http://tinyurl.com/ylkv85l
When the ballscrew is working in the normal mode between
stroke limits, the cage is driven by the balls and rotates
at 1/2 the velocity as the nut. But at the end of travel,
the cage hits a stop pin which locks it to the screw.
The balls now slip in their circular raceways.
When the nut-screw junction is not axially loaded, the
'freewheeling' effect is quite pronounced. One gets
the impression that there's a near total disconnect
for energy from the motor. However, when the actuator
is loaded (like at end of travel for extending flaps),
the load shedding to the motor is not so pronounced.
In fact, when operating the actuator at it's full
load capabilities (which is probably many hundreds
of pounds) there IS a sharp INCREASE in load on the
motor as the actuator hits the stop.
Yes, the 'slip clutch' does a nice job of replacing
limit switches in terms of setting absolute limits
on system stroke but there are electrical considerations
driven by how much the current spikes at end of travel.
Obviously, retracting flaps has very low force on the ball-
nut at up-limit. Further, exercising flaps on the
ground suggests that the end-of-travel event
for flaps extension is pretty benign too. AND IT
MAY BE no big deal in flight . . . if real force
is say 200 pounds out of a 1000 pound rated ball-screw.
The point is, until somebody puts the gauges on
it to go find out, we can debate the DESIGN of
flap system circuits for a long time. I suspect
the extend-limit spike is rather benign given that
many folks have told us they're flying with 10A
or even 5A fuses to protect the motor circuit.
Just be aware that until you actually fly the
airplane to load the flaps -AND- depending on how
long you hold the flap switch in the DOWN position
after the system reaches the limit is what sizes
the fuse.
It would be interesting and most useful if we
could get some in-flight current measurements
on extend-limit current draw with the airplane
flying at the IAS limit (top of the white arc)
for full flap extension.
Bob . . . [quote][b]
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