nuckolls.bob(at)aeroelect
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 Posted: Sat Mar 13, 2010 10:10 am Post subject: Flap motor draw for Rv-7A (Corrected) |
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--> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>
Some private emails on the flap motor thing
have prompted some further expansion on the
topic of powering and sizing motors . . .
Anyone know how many amps the flap motor draws on an RV-7A? I
don't see it listed on the motor itself or on any of the
documentation I've got and Van's doesn't know offhand either.
Up until now, we've only discussed the capabilities
and performance of DC motors as described in their
performance data. This discussion says NOTHING about
what the motor is expected to do as-installed in
the airplane.
Just because a motor has some particular nameplate
rating does not automatically set the operating point
as installed. Looking at the data plots I published
yesterday . . .
http://www.aeroelectric.com/Pictures/Curves/Speed-Torque-Current_Curve.jpg
. . . suppose your flap system extension from 0-10
degrees required only 4 oz in of torque from the
motor. During this portion of the stroke it would
draw about 2A and run at 6800 rpm. Assume from 10-
20 degrees, air loads are building on the panels
and the current averages 6 oz in. Now current rises
to about 2.5 amps and speed drops to 6600. From
20-30 degrees, the air loads are at max. Torque might
rise to 10 oz-in at end of stroke with the current
surging up to 3.5A
This may be typical of what an RV or many other
OBAM aircraft demand of their flap extension systems.
This does not amount to much power. Indeed if we
compare the effort required to extend flaps by hand
with a "Johnson Bar", the horsepower developed during
a 30 pound pull through a arc length of 2 feet over
say 10 seconds really isn't much. The motor's task
is no greater except that SOME flap systems may
incorporate some form of worm gear reduction which
can have an efficiency of 30% or worse.
But even then, the power needed to extend flaps
is small. A HP is 550 lb-ft of work per second.
Our 60 lb-ft in two seconds is only 30 lb-ft/second
or 1/20th HP. A really lousy mechanism efficiency
might push that up to 1/5th HP.
In the hypothetical extension scenario I cited
above, electrical power into the flap extension
system peaked just before the haulted at the
fully extended position. 3.5A x 14v = 49 watts.
49w / 746w/hp = 0.06 HP
If these numbers are in the same reality ball park
with the system in an RV, then it's easy to see
how motor ratings on the side of the motor can
be VERY misleading with respect to real performance
numbers. Van's has had no particular reason to
go get the real numbers. In the airframe/power-plant
integrator's world, detail of electrical system
performance become useful ONLY when things are not
working and some investigation is launched to fix
the problem. Even then, the investigators may come
out of the exercise only having UPsized some
obviously undersized component (motor, breaker,
attach hardware, etc) and may STILL be uninformed
as to the real numbers.
I hope this exchange has shed some light on the
difficulty of answering questions like the one
that started this thread. Unless somebody has
taken the time and tools to go measure the performance,
the answer's will always be shallow and tend toward
the conservative. Certainly, wiring and fusing the
flap motor cited in the data plots at something
like 10 A and commensurate wire sizing will eliminate any
possibility of nuisance tripping.
But understand that just because there's a 10 or
15A breaker protecting your hangar-mates flap
circuit doesn't say diddly-doo about what that
system really demands in terms of electrical
energy.
A pretty sharp cookie once opined,
"If you want to know, measure! When you can measure what you are
speaking about, and express it in numbers, you know something
about it, but when you cannot measure it, your knowledge is of
a meager kind." - Lord Kelvin 1824-1907
So after all this 'discussion' we're still unable
to answer the original question. Anyone got an
RV and a data acquisition system who would be
will to gather some real flight test data?
Bob . . .
[quote][b]
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