Hi Pete - yes=2C it is totally valid to think of it that way. With the
trim tab set to neutral=2C the tailplane is designed to maintain a negative
angle of attack sufficient to balance the negative pitching moment of the
wing at typical "in the pattern speeds". This angle corresponds to the mat
ching fillets molded into the fuselage sides at the tail.
At low to moderate airspeeds=2C the tailplane needs a sizeable negative a
ngle of attack to create enough downforce at the tail to force the leading
edge of the wing up to it's required angle of attack. Without the tail pus
hing down thus raising the wing leading edge up=2C the wing becomes lazy an
d does'nt want to fly=2C trying to reduce the angle of attack. This is tha
t negative pitching moment that all non symmetrical airfoils have. This is
easily noticed in a stall situation=2C with nearly all the UP elevator bein
g required to hold the high angle of attack.
Now when we want to go fast=2C requirements start to change. We want the
wing to assume a very low angle of attack because as the speeds go up=2C t
he lift
on the wing goes up by the SQUARE of airspeed. Reducing the angle of attac
k
reduces the lift of the wing and also the induced drag. But since we are g
oing
much faster=2C the wing is creating the same amount of lift sufficient to m
aintain a
steady cruising altitude. But at this higher airspeed the tailplane is sti
ll trying to maintain the high negative angle of attack it was designed to
maintain at low to moderate airspeeds. So now we have to do something to r
educe the extremely high downforce the tail is trying to generate. This is
achieved by reducing the negative angle of attack of the tailplane. We n
eeded a large negative angle for takeoff and pattern work but now we need a
much smaller negative angle.
To reduce this negative angle of attack we then apply DOWN trim which rai
ses the trimtab UP=2C which then forces the trailing edge of the tailplane
DOWN=2C which then reduces the negative angle of attack to a much smaller v
alue=2C reducing the downforce generated by the tailplane. It is still pro
ducing a large amount of downforce though as the high airspeed is creating
a lot of force with only a small negative angle of attack (the airspeed squ
ared thing).
The aircraft COULD have been designed to maintain this high speed slightl
y negative angle of attack with neutral trim tab to minimize drag. However
if this had been done the design would have required HUGE trimtabs trimmed
DOWN at lower speeds to then force the trailing edge of the tailplane UP t
hat would then create a large negative angle of attack on the tailplane req
uired to balance the wing at lower airspeeds.
Date: Tue=2C 13 Apr 2010 07:08:34 -0400
Subject: Re: Europa-List: Effects of Flaps and Ailerons on Pitch Trim
From: peterz@zutrasoft.com
Further to my previous post=2C if one were to optimize the airfoil of the f
lying tail to minimize drag (which I had incorrectly presumed Don D had don
e)=2C then the tailplane would have had a very slight inverted camber=2C ca
lculated such that no trim-tab offset would have been required at the "targ
et" cruise speed.
Cheers=2C
Pete
On Tue=2C Apr 13=2C 2010 at 7:00 AM=2C Peter Zutrauen <peterz@zutrasoft.com
> wrote:
Hi Frans etal=2C
OK=2C ok=2C I believe I have figured out what's going on with the upward t
rim tab in cruise.
The when the anti-servo trim tab is *trimmed* out of the neutral position
=2C 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)=2C I had thought this configuration was provi
ding *lift* to the tail. It was this simplistic interpretation which was c
onfusing me. My bad. :-P
So=2C how does the following sound to all the aerodynamicists out there? W
hen one considers the tailplane&trim tab combination as a "symmetrical airf
oil wing with flap"=2C then moving the trimtab off-center is effectively a
dding camber to the tailplane 'wing' and making the airfoil no longer symme
trical. In fact=2C moving the trimtab *up* results in an inverted camber (t
raditional wing upside down)=2C thus generating a downward lift vector - an
d the stars are again all in proper alignment.
The faster one cruises=2C then the more camber is required to provide more
downforce to counter-act the main-wing lift pitch=2C thus more trim tab.
I was/am not speaking of the anti-servo function of the tab=2C which I agre
e fully is driven in the same direction as the stick to augment/provide the
(otherwise negligible flying-tail) stick forces to give proper speed-relat
ed feedback and make the tailplane less prone to flutter - that function is
easy to visualize.
Cheers=2C
A happy that things now make sense again=2C
Pete
A239
On Mon=2C Apr 12=2C 2010 at 8:28 PM=2C Frans Veldman <frans@privatepilots.n
l> wrote:
On 04/13/2010 01:19 AM=2C Peter Zutrauen wrote:
> that was my understanding as well - but when trimmed for cruise=2C
> 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=2C the trim tab moves up even more. It *always* goes back to the
same stabile position=2C 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=2C it drives the
tailplane up=2C this will take the trim tab up even more=2C 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=2C
Pulling back on the stick is not the equivalent. If you pull on the
stick=2C the trim tab won't revert to its stabile aerodynamic position...
and that is why you need to keep pulling on the stick=2C to counteract the
forces on the trim tab.
> to have the tailplane produce
> the required downward force on the tail?
Actually=2C it is not a trim tab=2C but an anti-servo trim tab. It works
differently.
> All the pics I've seen show the trimtab portruding *up*=2C ie=2C trim
> foward stick..... no?
The *only* conclusion you can draw from the trimtab protruding up=2C 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=2C 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=2C 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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