The Murphy Builders Site

Wayne

We put 28 hours on the 3 planes last week and Jim made the
comment that
I always had right rudder on while taxiing. Never noticed it
myself. P
factor might account for that, or lousy alignment, a draggy
brake, or
the same unrelenting crosswind... They kept making excuses why I
should
lead the circus so Tom and Jim saw more of my tail then I did of
theirs
but apparently I usually have some twist on in that direction ;)

I was trying really hard to promote some brain activity without
annoying
anyone but here is what I sent to Bob:

Static wise (airplane not moving) there is only one force that
acts
vertically up directly on the center of the wheel. That is the
weight of
the airplane. There is simply no way that force can twist the
tailspring
if the airplane is not moving.

When you use a one sided fork most folks correctly envision
bending
forces in the one sided fork but not how it cancels out. For
example if
the one sided fork is 2" sideways from the center of the wheel
and there
is 50 lbs of weight on the wheel then there is a torque of 2" x
50 lb =
100in-lb WITHIN the side fork. That is balanced by the fact that
the
center spindle that steers the tailwheel is also 2" away from
the side
fork. ie. 2" x 50 lb in the opposite direction exactly
cancels that
torque such that there is zero twist on the tailspring. The key
is that
the pivot is also centered on the wheel so zero twist on the
tail spring
no matter how far away the one side fork arm is from the wheel.

Actually similar to a strung bow. In our case the weight on the
tail is
analogous to the tension on the bow string.

What gets interesting is tailwheel shimmy. Any sideways forces
from the
wheel do twist the spring. Every time the spring untwists it
turns the
wheel back but the wheel overshoots and twists the spring in the
other
direction and so on. Each cycle gets larger and the shimmy can
get
severe. It still makes no difference whether the wheel has a
single or
two side fork. However a flat spring like we use is not very
resistant
to twisting. That may be part of the reason that MAM went to
such a
stiff aluminum spring. An attempt to make it torsionally stiffer.
Anyway there may well be issues with the Maule tailwheel. I have
no
experience with it. I just maintain that there is no fundamental
reason
to avoid a one sided fork design.

Tim
As per the above paragraph I'm suspicious that fatique
cracking from
high cycles of obvious or subtle shimmy/twisting like you saw is
a
factor in the tail cone cracks. Stab flexing in turbulence or
even from
prop pulses may be a bigger factor. I'm a firm believer in
doubling the
rear part of Fus-30 as recommended by Wayne and others. I have
done some
close in formation flying on other Rebels and never noticed tail
flexing
in flight but if it is turbulent I don't get in close or watch
the tail.
With the small hard tailwheel I sometimes had some oil canning
type
noises from the tail while taxiing but not with the pneumatic
tire now.

Ken

Wayne G. O'Shea wrote:

LOL.. see what you got yourself into Ken ! Me to me

to... no don't bother..

I know first hand that the fus-30 cracks in the right rear

corner.. thanks

to the left sided fork twisting the TS assembly in that rotation!

;O)

Wayne

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