Further to my previous post, if one were to optimize the airfoil of the
flying tail to minimize drag (which I had incorrectly presumed Don D had
done), then the tailplane would have had a very slight inverted camber,
calculated such that no trim-tab offset would have been required at the
"target" cruise speed.
Cheers,
Pete
On Tue, Apr 13, 2010 at 7:00 AM, Peter Zutrauen <peterz@zutrasoft.com>wrote:
> 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
>>
>>
>>
>>
>>
>
--
"A man is not old until his regrets take the place of his dreams. "
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