This is just another "weight and balance" problem, like loading the aircraft
with fuel, people, and baggage.
While I agree that it is reasonable to ignore the resolution of forces, i.
e., the niceties of sines and cosines, (and the nuisance of using calculus
for an accurate solution) you can not assume that the entire mass of the
stick is concentrated at a point at the top end of the stick, when in fact
it is uniformly distributed along the entire length of the stick (assuming
constant wall thickness and diameter - a reasonable assumption). Even that
is literally correct only for the original straight stick; the later "S"
shaped stick has two welds that add a small amount of mass and displacement
of the c. g. (which can safely be disregarded). Not only must the sticks be
parallel, but they must be vertical (in the fore and aft plane) to start.
You can, however, consider the entire mass to be on axis at the mid-point
along the length of the stick. Ergo, a lever arm of one-half the stick
length is used to calculate the torque required to displace the stick. Once
the torque is known then the force "at the hand" is calculated using the
(almost) full length of the stick; almost, because the hand is also not
concentrated at a point at the extreme end of the stick but about one-half
the hand's width below.
Note that this technique will determine static friction (that which must be
overcome to initiate movement) not the lesser value of dynamic friction
(which is what it takes to continue movement).
Best regards,
Rob Housman
A070
-----Original Message-----
On Behalf Of Grahamclk@aol.com
Sent: Sunday, November 19, 2000 3:18 AM
Subject: Born free ?
Having asked a nearby builder to measure his stick friction using the string
and weight method- he reported that he couldn't - the stick fell over when
displaced !
This can't be strictly true as the system is the classical mechanics
"conditionally stable" setup, but it does point to a simple way of
measuring
your stick friction without anything elaborate .
You merely displace the stick carefully sideways until it does fall.
Ignoring the niceties of sines and cosines it turns out that the the
friction
force (at the hand) is given by the displacement achieved divided by the
length of the stick times the weight of one stick.
As you will not get as much as 6" extreme displacement this technique will
then work for friction up to around 1/3 of the stick weight of say 300 gm or
11oz.
It can easily be extended by increasing the stick weight articificially,
either by adding a weight on the top or within, taking due acccount of it
not
being at the stick c.of.g.
Clearly some frills can be added to take account of the boss at the bottom
and a grip at the top, but these will be a constant if you just want to see
how your waggling and Brasso are working. The sticks must be accurately
parallel too.
I would be grateful for any measurements taken with and without the wings in
place, as I can't put the wings on mine in its hibernation habitat. Also
whether the bearings throughout are greased or oiled.
A final thought - the autopilot servo could in principle be used to do the
waggling either by direct electrical input or a gps input in simulation
mode.
However one might have to worry about the continous rating limit.
Graham Clarke G-EMIN
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