Hi Frans,
I understand your thesis (thanks again! :-).
Your thesis assumes that the tailplane is aerodynamically balanced wrt to
the torque-tube position, and the only aerodynamic component which is
stabilizing it and keeping it from flutter is the anti-servo tab.
I question this, as the fulcrum is ahead of the center of the tailplane.
Maybe my skepticism is unwarranted? Is the fulcrum in the center of the
tailplane aerodynamically? Without the anti-servo tab, would the tailplane
flutter by design? I doubt this is the case, as (I believe) the flettner
strips were added to eliminate the 'dead zone' of the anti-servo tab
(bringing it out of the boundary layer) to eliminate a trim-hunting issue,
not flutter (someone with some historical knowledge please do chime in!).
I would expect/hope that the tailplane would remain aerodynamically stable
without the anti-servo tab, and that the tab is there only to provide for
progressive stick forces, and trim.
The "still curious",
Pete :-)
On Wed, May 12, 2010 at 8:54 AM, Frans Veldman <frans@privatepilots.nl>wrote:
>
> 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
>
>
|