Hi Frans etal,
OK, ok, I believe I have figured out what's going on with the upward trim
tab in cruise.
The when the anti-servo trim tab is *trimmed* out of the neutral position,
then the 'zero stick force' angle of incidence of the tailplane wrt to the
airflow is changed. When the trim tab is protruding *up* (thus driving the
trailing edge downward), I had thought this configuration was providing
*lift* to the tail. It was this simplistic interpretation which was
confusing me. My bad. :-P
So, how does the following sound to all the aerodynamicists out there? When
one considers the tailplane&trim tab combination as a "symmetrical airfoil
wing with flap", then moving the trimtab off-center is effectively adding
camber to the tailplane 'wing' and making the airfoil no longer symmetrical.
In fact, moving the trimtab *up* results in an inverted camber (traditional
wing upside down), thus generating a downward lift vector - and the stars
are again all in proper alignment.
The faster one cruises, then the more camber is required to provide more
downforce to counter-act the main-wing lift pitch, thus more trim tab.
I was/am not speaking of the anti-servo function of the tab, which I agree
fully is driven in the same direction as the stick to augment/provide the
(otherwise negligible flying-tail) stick forces to give proper speed-related
feedback and make the tailplane less prone to flutter - that function is
easy to visualize.
Cheers,
A happy that things now make sense again,
Pete
A239
On Mon, Apr 12, 2010 at 8:28 PM, Frans Veldman <frans@privatepilots.nl>wrote:
>
> On 04/13/2010 01:19 AM, Peter Zutrauen wrote:
>
> > that was my understanding as well - but when trimmed for cruise,
> > wouldn't the anti-servo/trim-tab be required to portrude *downwards* to
> > drive the trailing edge of the tailplane *up*
>
> Read again what you just said. If the trailing edge of the tailplane
> moves up, the trim tab moves up even more. It *always* goes back to the
> same stabile position, regardless of trim setting. The only difference
> is that this stabile position is achieven by a different tailplane
> incidence. This is how it works: you trim it down, it drives the
> tailplane up, this will take the trim tab up even more, counteracting
> what you just did... and the nett effect is that the tailplane's
> trailing edge has moved up. This is how a anti-servo tab works.
>
> > to result in the
> > equivalent of pulling back on the stick,
>
> Pulling back on the stick is not the equivalent. If you pull on the
> stick, the trim tab won't revert to its stabile aerodynamic position...
> and that is why you need to keep pulling on the stick, to counteract the
> forces on the trim tab.
>
> > to have the tailplane produce
> > the required downward force on the tail?
>
> Actually, it is not a trim tab, but an anti-servo trim tab. It works
> differently.
>
> > All the pics I've seen show the trimtab portruding *up*, ie, trim
> > foward stick..... no?
>
> The *only* conclusion you can draw from the trimtab protruding up, is
> that its stabile position is upwards relative to the tailplane. This
> will remain the same for all trim settings. If you change the trim, the
> only nett effect is that the tailplane will move to a different
> incidence... but the trim tab will remain at the same position. Play
> with it on the ground, until you see what's happening.
>
> Frans
>
>
|