OK Pete - the tailplane is ALWAYS pushing down at all airspeeds but at high
er airspeeds the wing is at a very low angle of attack. As the airspeed go
es up=2C the
aerodynamic forces on all control surfaces (ailerons=2C rudder=2C tailplane
) go up by the
SQUARE of airspeed. This is why you cannot give full control deflection at
VNE=2C the
control surfaces will depart the aircraft. So when the wing goes to a very
low angle
of attack=2C the tail must also. The tail is still flying at a negative an
gle of attack but
it is very small. However this small negative angle of attack is generatin
g a lot of
positive CM because of the high airspeed. To get this slightly negative an
gle of attack
at a high airspeed requires a lot of force and this is generated by the UP
position of
the trimtab forcing the trailing edge of the tailplane DOWN. The trim tab
controls the
angle of attack of the tail (always negative but less negative at high spee
d)=2C the
anti servo system keeps the tail at the set angle of attack. Other factors
come into
play here such as the angle of relative airflow from the wing downwash. Th
e tail
may look as though it has a positive angle of attack but because of the win
g downwash=2C the relative airflow is actually hitting it from above. This
angle becomes less at high speeds but the forces go up with the square of
airspeed.
at high speeds
Date: Mon=2C 12 Apr 2010 19:19:06 -0400
Subject: Re: Europa-List: Effects of Flaps and Ailerons on Pitch Trim
From: peterz@zutrasoft.com
Hi Glenn=2C
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 t
railing edge of the tailplane *up* to result in the equivalent of pulling b
ack on the stick=2C to have the tailplane produce the required downward fo
rce on the tail?
All the pics I've seen show the trimtab portruding *up*=2C ie=2C trim =
foward stick..... no?
Still confused=2C
Pete
A239
On Mon=2C Apr 12=2C 2010 at 2:41 PM=2C Glenn Crowder <gcrowder2@hotmail.com
> wrote:
The airfoil of the wing of any aircraft with a non symmetrical section will
produce a negative pitching moment (-cm) around the aerodynamic center of
the wing. This means that without the tailplane producing an equal but op
posite moment the acft will pitch over into a vertical dive. The all flyin
g tailplane on the Europa has no fixed angle of incidence so to produce an
opposing positive torque moment to the wings negative one the trim tab must
be held out of plane. At higher speeds then the tailplane must provide gr
eater downforce then requiring even more trim tab offset. We have to pay a
small drag penalty to achieve a level trim condition in cruise flight. T
his penalty can be reduced somewhat by loading the acft towards the aft cg
limits requiring less trim tab offset reducing the downforce the tailplane
must produce to maintain level flight.
The tailless flying wing derives it's pitch stability due to the outer por
tions of the wing being reflexed up to produce that necessary positive pitc
hing moment but also introduces that small drag penalty again. There is no
free lunch in aerodynamics! Every desireable aerodynamic quality (like pi
tch stability) has a price that must be paid.
Glenn
Fred Klein <fklein@orcasonline.com> wrote:
>
>On Apr 12=2C 2010=2C at 6:24 AM=2C david miller wrote:
>
>> I think that for most efficient flight the incidence of the wing and
>> tail should be such that at cruise there is no up or down trim on
>> the elevator or all flying tail.
>
>Fascinating discussion...as a non-aerodynamicist=2C I can't resist the
>temptation to add a comment...
>
>I've always understood that the elevator or an all-flying tail
>(horizontal tail plane) has a "job"...namely=2C to exert a force=2C either
>up or down=2C in order to balance the lift of the wing and stabilize the
>flight path of a normally configured aircraft (i.e.=2C not a canard
>type). To do this "job" requires "work"=3B the tail plane is never
>neutral. And w/ our Europae...with it's trim tabs acting in
>opposition to the all-flying tail plane thru a direct mechanical
>connection...whenever the tail plane is doing its job (at whatever
>angle of incidence)=2C the trim tabs will be doing their job as well
>(that of counterbalancing forces on the tailplane)=2C and will
>necessarily be raised or lowered w/ reference to the tailplane.
>
>Of course if this explanation is correct=2C one might ponder the
>aerodynamics of my tail-less biplane...pix below.
>
>Fred
>
>
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