Vertical stabilizers

[Dallas]

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.


--
Dallas

[[email protected]]

On Feb 18, 2:02*pm, 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. *
>
> --
> Dallas

I don't know that sweep reduces drag. It just looks faster.
Sometimes it appears swept because it's tapered and the trailing edge
is vertical. And a tapered vertical tail can be made lighter and have
less "tip" drag. Cessna's swept tail supposedly helps hold the nose up
a little in a turn if the rudder is deflected, but it would be a
minimal contribution, I think.

The old rounded tails were easy to make with small steel tubing
and fabric; lots harder with sheet metal. Rounded shapes were popular
in those old days--just look at the cars of the '40s. Angular shapes
were popular in the '60s and '70s, just like the cars of the day, and
airplanes were angular, too. Which mostly means that style sells
airplanes more than functionality.

Dan

[[email protected]]

On Feb 18, 7:42*pm, "Morgans" > wrote:

>
> * * *I agree, at the speeds involved, sweep is all for style, not function.
>
> That said, I have heard many people say that their old straight tail Cessnas
> were faster than the later ones with the swept tail, even with the later
> ones having bigger engines. *So is it the tail, or some other changes? *I
> don't know, but it makes you wonder.
> --
> Jim in NC

Those old Cessna were faster before they stuck the back window into
the mix. The fairly sharp drop behind the cabin caused some drag,
especially on the 150. Without the window the top of the fuselage was
much straighter.

Dan

[John Smith]

>> Aerodynamically speaking, what difference does it's shape make?

I'm no aerodynamicist, so I can't offer hard numbers, just some thoughts:

As the stabilizer is a wing like any other wing, its shape will matter
as the shape of any wing.

Aerodynamics in coordinated flight is only part of the story.
Particularly spin recovery is crucial, which depends highly on the
airflow on the rudder, which in turn depends on a lot of things, e.g. on
the the shape of the horizontal stabilizer and the horizontal rudder
position.

Aesthetics certainly matters, too.

[Robert M. Gary]

On Feb 18, 1:02*pm, 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. *
>
> --
> Dallas

Obviously the Mooney design is faster.

[Frank Stutzman[_3_]]

Robert M. Gary > wrote:

> Obviously the Mooney design is faster.

I had one owner explain to me that the tail on the Mooney was indeed
faster as the tip vortices ended up comming off the bottom of the
vertical stablizer. The air down there was already churned up by
the fuselage and so less air was disturbed than otherwise would be.

Personally the fastest thing would be to get rid of the vertical stabilizer.
Hmmm, now what kind of a plane would do that....

--
Frank Stutzman
Bonanza N494B "Hula Girl"
Boise, ID

[John Smith]

> Personally the fastest thing would be to get rid of the vertical stabilizer.
> Hmmm, now what kind of a plane would do that....

Horten, for example: http://www.youtube.com/watch?v=GjXr5w3M4mc

[Peter Dohm]

"John Smith" > wrote in message
...
>> Personally the fastest thing would be to get rid of the vertical
>> stabilizer.
>> Hmmm, now what kind of a plane would do that....
>
> Horten, for example: http://www.youtube.com/watch?v=GjXr5w3M4mc

And, of course, the B2 Spirit: http://www.youtube.com/watch?v=IdbpMOWGYGk

[Dallas]

On Sat, 20 Feb 2010 00:46:03 +0000 (UTC), Frank Stutzman wrote:

>I had one owner explain to me that the tail on the Mooney was indeed
> faster as the tip vortices ended up coming off the bottom of the
> vertical stablizer.

But a vertical stabilizer is not going to generate a tip vortice.
(disclaimer: as far as I know)

The vortices are generated by unequal pressure on opposing sides of an
airfoil. A vertical stabilizer in coordinated flight would have equal
pressure on both sides of the surface, thus no vortice.

--
Dallas

[romeomike]

Dallas wrote:
A vertical stabilizer in coordinated flight would have equal
> pressure on both sides of the surface,


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?

[Orval Fairbairn[_2_]]

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.

[Dallas]

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

[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.
--
Dallas

[romeomike]

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.

[John Smith]

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.

[Dallas]

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.

:-/
--
Dallas

[Tauno Voipio[_2_]]

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>

--

Tauno Voipio

[[email protected]]

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

[Dallas]

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

[a[_3_]]

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.

[[email protected]]

On Feb 28, 5:21 pm, a > wrote:

> 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.

Are you using any rudder in the established turn? If not, you either
have a very tame airplane, or the turn isn't quite coordinated.

Introducing bank causes the airplane to move sideways, and the fin and
rudder resist sideways movement so that the nose is forced to change
direction. Therefore, there is lateral force being generated by the
vertical surfaces and a vortex will be produced.

Dan