Vertical stabilizers

In article ,
Dallas wrote:

Speaking of below 250 knot aircraft, does it really matter much what shape
they make the vertical stabilizer?

They seem to come in all kinds of shapes... de Havilland favored
elliptical, Cessna favors a swept back trapezoid, Mooney swept forward...
etc.

Aerodynamically speaking, what difference does it's shape make? I guess
sweeping back creates a bit less drag than straight vertical. But other
than that, we're not generating induced drag because it's not generating
lift. It seems like area should be the only consideration.

Speaking as an aeronautical engineer: The answer is: No difference.

Selection of tail shape is mostly a matter of aesthetics and
salesmanship, as long as the proper amount of area is available.

I remember when Cessna changed from unswept to swept vertical fins and
remarked that they were seeking to increase its critical Mach number,
even though they were operating at only Mach 0.2-0.3, when sweep doesn't
apply until about Mach 0.6 and above.

--
Remove _'s from email address to talk to me.

On Mon, 22 Feb 2010 17:21:21 -0700, romeomike wrote:

Just askin', but is that always true? In a turn, even if coordinated, is
there not a force against the vertical stab causing it to act as a keel
to help maintain lateral stability?

(Disclaimer: I don't claim to be an aeronautical engineer, but I like the
subject.)

To me, coordinated is coordinated. Sure, there are micro disruptions that
try to produce a yaw that the vertical stabilizer is called upon to resist,
after all, that's why it's there.

But, in a coordinated turn, the goal is to balance the pressure on both
sides of the stabilizer.

Have you ever seen those short strands of yarn some glider pilots tape to
the front of their canopies? The goal is to keep the strand straight down
the centerline during a turn which can only be accomplished if there is
equal airflow on either side of the hull.


--
Dallas

On Wed, 24 Feb 2010 12:57:46 -0500, Orval Fairbairn wrote:

Speaking as an aeronautical engineer: The answer is: No difference.

Gracias... burning question put to rest.
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Dallas

Dallas wrote:


Have you ever seen those short strands of yarn some glider pilots tape to
the front of their canopies? The goal is to keep the strand straight down
the centerline during a turn which can only be accomplished if there is
equal airflow on either side of the hull.



Actually, yes, I have a comm rating in gliders, but that string is on
the wind screen,
not sure if it reflects the forces on the vertical stab. I don't know,
maybe someone does.

Dallas wrote:
Have you ever seen those short strands of yarn some glider pilots tape to
the front of their canopies? The goal is to keep the strand straight down
the centerline during a turn which can only be accomplished if there is
equal airflow on either side of the hull.

Being a glider pilot, I beg to differ:

First, to keep the yaw string centered in a turn, one must apply some
amount of rudder. A coordinated turn is a turn around all three axes.
The required amount of rudder depends a lot on the glider type and of
course the bank angle.

Second, the optimal position of the yaw string in a turn is *not*
centered, but slightly outside, because the cockpit is situated in front
of the turn radius. (A second reason is that for various reasons you
want the glider to slightly slip into the turn, hence the yaw string to
point slightly to the outside, but that's another story altogether.)

And third, the situation at the windscreen is not the same as the
situation at the rudder.


All that said, the lateral forces at the fin are pretty small during a
coordinated turn. But coodinated flight is only part of the story.

On Sun, 28 Feb 2010 13:54:54 +0100, John Smith wrote:

Second, the optimal position of the yaw string in a turn is *not*
centered, but slightly outside

Ok.. the spam can pilot stands corrected.

Funny, I tried the string thingie on my Cessna and almost crashed trying to
keep it centered.

:-/
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Dallas

Dallas wrote:
On Sun, 28 Feb 2010 13:54:54 +0100, John Smith wrote:

Second, the optimal position of the yaw string in a turn is *not*
centered, but slightly outside

Ok.. the spam can pilot stands corrected.

Funny, I tried the string thingie on my Cessna and almost crashed trying to
keep it centered.

:-/

Is your Cessna a twin?

In a single, the string does not work with the fan just in front of it.

A twin is a different story, see e.g.
http://www.airliners.net/photo/Embry-Riddle-Aeronautical-University/Diamond-DA-42-Twin/1413056/L/&sid=d969e71238e1e84442331da93fb3cb59

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Tauno Voipio

On Feb 27, 2:08 pm, Dallas wrote:

To me, coordinated is coordinated. Sure, there are micro disruptions that
try to produce a yaw that the vertical stabilizer is called upon to resist,
after all, that's why it's there.

But, in a coordinated turn, the goal is to balance the pressure on both
sides of the stabilizer.

The rudder/fin produces a lateral force to cause the coordination and
will generate a tip vortex. Otherwise, why would we need the vertical
surface?

Dan

On Sun, 28 Feb 2010 21:37:05 +0200, Tauno Voipio wrote:

In a single, the string does not work with the fan just in front of it.

Oh... so that's what the problem was..

:- )

(I was kidding :- )

--
Dallas

On Feb 28, 5:38*pm, wrote:
On Feb 27, 2:08 pm, Dallas wrote:

To me, coordinated is coordinated. *Sure, there are micro disruptions that
try to produce a yaw that the vertical stabilizer is called upon to resist,
after all, that's why it's there.

But, in a coordinated turn, the goal is to balance the pressure on both
sides of the stabilizer.

The rudder/fin produces a lateral force to cause the coordination and
will generate a tip vortex. Otherwise, why would we need the vertical
surface?

Dan

I am clearly missing something here. I use the control surfaces to
roll into a turn, then return them to neutral and trim away the back
pressure once the desired bank is established in a coordinated turn. I
don't think in that case the airplane knows it's in a turn. The G
vector in normal to the wings and the fuselage centerline, but we
will of course be experiencing more than 1 G.

I won't bother posting to the other topic of interest, but most of us
who are rated simply file IFR and don't worry about cloud separation,
except for the concern that someone not flying on an instrument flight
plan may be poking around. Solid IMC is
somewhat better in that sense.

By the way, if on an instrument flight plan in the US, just ask center
to allow you to cruise within an altitude range, and then if it's
granted you can play in and out of the clouds: it is priceless. Once
out of congested areas in the east that permission is almost always
given, the controller will understand -- he or she will be green
with envy, but will understand.