On 05/12/2010 01:03 PM, Peter Zutrauen wrote:
> As the tailplane is a symmetrical airfoil, my confused mind asserts
> that in cruise (since the horizontal stab by definition must be
> providing a downward force) your tailplane must have been at a negative
> angle of attack, and since the combination anti-servo/trim tab was in a
> neutral position then I would have expected a steady stick force would
> have needed to be applied....
This is an *anti servo trim tab*, if it is not in line with the
tailplane, it puts a force on the tailplane so that the tailplane
rotates until it is in line with the anti servo trim tab again.
You can easily see what I mean by exercising the trim in whatever
position, and then carefully with your hands on the anti servo tab, try
to push it in line with the tailplane again. You will see that the
tailplane rotates during this process, and that you will always end up
with the anti servo tab in line with the tailplane.
This is true for *any* speed, *any* CG, etc, except when the pilot
exercises some force on the stick. Any force on the stick gets into the
equation and disturbes this balance.
If you trim for a specific speed, all you do is put the stick in a
specific position, and then with the trim button, line up the anti servo
tab with the tailplane, to keep the tailplane at its new position. You
will always end up with the anti servo tab in the same relative position
of the tailplane.
The real confusion is that there are people flying around with the anti
servo tab not in line with the tailplane. This means they are either
pulling or pushing slightly on the stick, or that the tailplane is not
balanced out properly.
About the latter: I disconnected everything from the tailplane when
balancing it out. If you leave the stick connected, you won't get a
proper balance and the anti servo tab will not find its equilibrium in
line with the tailplane.
Next time we do this, I will vary the speed, trim for each speed, and
make a picture again. I would be really surprised if at any speed the
anti servo tab will not remain in its same relative position to the
tailplane. (Of course at every speed, the whole stack of tailplane plus
anti servo tab will rotate to a specific position... but with the anti
servo tab always in line with the tailplane. This is what you do if you
trim: lining the anti servo tab up with the tailplane in its new position).
Now, for your questions (although they don't matter):
> - were you in stead-state constant-altitude cruise?
Yep. (We were in close formation so we wouldn't dare to fly anything
than a very stabilized attitude).
> - where was your trim set at?
At 1/3 from the nose down limit.
> - was your stick force = 0?
Of course 0.
> - what was your cruise speed?
About 110 Knots.
> - what was your weight and balance (with fuel at that point in time)?
Two adults, about 20Kg of bagage, and about 40 liters of fuel.
Frans
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