On 04/12/2010 08:06 PM, Peter Zutrauen wrote:
> I was taught that on an inherently stable aircraft (operating within its
> CG envelope) with a tail, the tailplane is always exerting a downward
> force in cruise to counteract the tendency of the wing to twist forward
> in the airflow due to the lift vector. As opposed to a canard where
> the forward wing provides lift, making it a more efficient design.
I agree with this.
> I'm perplexed - all the photos I've ever looked at of Europas in cruise,
> the trim tab is *up* (as others have noted), thus the flying elevator is
> by definition providing *lift* to the tail.... a contradiction to my
> teachings.
I'm stating again: The position of the trim tab is NOT an indication of
the trim setting. The position of the trim tab is determined by the tail
plane... as you can easily see when you move the tailplane up or down.
The trim setting merely affects the tailplane position where the trim
tab is lined up with the tailplane. For the same relative position of
the trim tab to the tailplane, the whole tail plane has to move to a
different position if you change the trim setting.
The confusing part is that this position, where everything is in
balance, is not when the trim tab is lined out perfectly with the
tailplane. It is always somewhat UP, again regardless of trim setting.
The tailplane itself is designed as aerodynamically neutral: which means
that the tailplane won't favor a certain position by itself. Place it in
the airflow in any position you like... it won't rotate to a different
position.
This is where the trim tab comes into the equation. It artificially
makes the tailplane favor a certain position in the air stream.
The only thing I'm troubled with is that the stable rest position of the
tailplane plus trim tab is not when they are perfectly lined out in
relation to each other.
Maybe, just maybe, this may have something to do with the fact that the
hinges are on the upper side of the tailplane, rather than in the
center. The forces acting on the lower side have more leverage than the
forces acting on the upper side, hence the stable position of the trim
tab where all forces are cancelled out, is slightly up.
The more I think about this, the more I think that this is the reason
why we always see a trim tab up position.
Any comments about this hypothesis?
Frans
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