V Tail

[jaymak]

Does anyone know if there is definite advantage of having a V-Tail over a single ventral fin?

Three possibilities I've heard are; to reduce height for small areas (F-14 when stored in an aircraft carrier); to increase surface area without increasing aspect ratio (enough/too much lateral static stability already); or to reduce radar reflections (ie. F22 - the more angles on the aircraft that are the same, the greater the reduction of the scatter of the rader return).

Anyone heard otherwise?

Cheers.

[BOAC]

Less form drag and weight when 2 surfaces provide the same control forces as 3?

[B2N2]

Drag reduction is what I've always understood.
Two surface area's vs. three surface area's on a conventional tail

http://www.volpe.dot.gov/infosrc/journal/30th/images/safe_v-tail.jpg

[john_tullamarine]

.... weight

[ehwatezedoing]

F-14/22's are twin tails.

The Bonanza or the French built Fouga Magister are V tails or butterfly tails which combine the tasks of the elevators and rudder.

Both are different concepts.
The best explanation I can come up with is there:
V-tail - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/V-tail) :ok:

[wildpudding]

Reduced interference drag also due to the angles between the control surfaces being larger

[Graybeard]

Like they say about the V-tail Bonanza: "Anything that kills all those doctors and lawyers can't be all bad."

[henry crun]

Are you sure you meant to say a "a single ventral fin" ?

[Captain Smithy]

As has been stated, less drag and weight. Unfortunately there are disadvantages - trying to combine both rudder and elevator functions can be difficult and there is more stress on the tail section when the aircraft experiences yaw and pitch.

Looks good in my opinion though. I always thought that the old V-tailed Bonanzas were among the nicest-looking GA aircraft.

Twin tails on the other hand, a la F-14, F-22, SU-27 etc. can allow bigger rudder area and also reduces the aircraft's weight. Also if you get one tail shot off, you still have another one to help get you home :ok:

Smithy

[jxk]

The Cirrus Jet uses V tail in a neat way:
YouTube - Cirrus Design's The-Jet, Cirrus Jet Rollout & First Look (http://uk.youtube.com/watch?v=nAU0U1__9wM)

[Bullethead]

One other advantage of having twin tails on a fighter is in the event of a really high speed ejection the crew go between the fins rather than the alternative.

Regards,
BH.

[flyboy2]

The V-tailed BE35 Bonanza had other factors not mentioned here:-

The tail fish-tailed so much, that the passengers became air-sick.
To assist, the factory eventually installed a yaw-damper!

With a cross-wind from the left, it ran out of " rudder" during take-off,
so much that one could depart off the runway!

[Mad (Flt) Scientist]

Less form drag and weight when 2 surfaces provide the same control forces as 3?

Not really. For two surfaces to provide the same forces as three, they have to be 50% larger. There's no magic extra force available with the surfaces inclined relative to a notional "aircraft vertical".

If there were really such a significant efficiency improvement, you can be damned sure there'd be a lot more V tails around - no-one's going to ignore a "free" performance improvement.

[Mad (Flt) Scientist]

Reduced interference drag also due to the angles between the control surfaces being larger

Only in the case were the "v" is "wide", which is only the efficient case when the classical "htail sizing" requirement dominates the "vtail sizing" requirement component. Otherwise you end up with, in effect, an oversized horizontal tail (inefficient) or undersized vertical (potentially limiting or even dangerous)

[john_tullamarine]

One other advantage of having twin tails on a fighter

I suspect that the main reason is for high alpha manoeuvring with alignment of vortices shed from up front. Mind you, that has its own problems eg FA18 fatigue ..

[airfoilmod]

I remember a similar discussion re: the 18's chines and vortex mgmnt.

Bonus for the canted tails? Additional pitch stability; off vertical, there is an up/down vector to augment the elevators. On the cat, the rudders are both "in" (pigeontoed) to aid rotation at the bow. The 18 is a very pitchy machine.

The cant also aids stability in the roll, think dihedral.

[Mad (Flt) Scientist]

One other advantage of having twin tails on a fighter

I suspect that the main reason is for high alpha manoeuvring with alignment of vortices shed from up front. Mind you, that has its own problems eg FA18 fatigue ..

You also get a nice freebie with tailoring the directional stability term (Nv or Cn-beta) with Mach number, too. At low M the twin fins aerodynamically interfere, reducing their effect, and as m increases the shock cones shield the fins from each other, increasing their effectiveness just as you start to need increase to compensate for the body destabilising effect increasing. It's not necessarily a reason in itself, but it helps tip the scales (and avoids having Tornado-sized fins!)

[jaymak]

Cheers lads.

[airfoilmod]

Any idea at what point the twins uncouple? I'm getting ~.40M.

[Mad (Flt) Scientist]

Any idea at what point the twins uncouple? I'm getting ~.40M.

Its going to be aircraft dependent, because the shock from the LH fin has to pass aft of the Rh fin (and vice versa).

Using this nifty page: Oblique Shock Waves (http://www.grc.nasa.gov/WWW/K-12/airplane/oblique.html) there's a calculator thing for oblique shock angle. If we ignore any standing shock, and assume the fin has a leading edge wedge angle of say 10 degrees, we can get a relationship between the freestrem (upstream) MAch and the shock angle.

At M1.4 and below it's a 90 degree shock. At M1.6 it's at about 50 degrees, and at M2.0 its about 40 degrees. So in this case there's no advantage for a low Mach a/c (say a F-16 fixed intake type, perhaps) but for a M2+ aircraft there's a significant decoupling of the fins. Exactly when they fully decouple depends on the fin geometries relative to each other.

[airfoilmod]

I think there is uncoupling and true structural aerodynamic independence. At low Mach, the speed at which air becomes compressible, as I recall is ~ 269 knots. Below that value, air is considered inviscid, and is free to interfere with solids located close by (twin tails). I was taught that interference (airframe) is similar to Ground effect, only at altitude. Shock waves are less mysterious (to me) and since they are frequently visible succumb more easily to explanation.

[firefish]

Very interesting thread this is.

But back to the Bonanza - wasn't the V-tail merely a sales promotion thing to make the aircraft stand out (kinda like for the Arrow that used a T-tail for a couple of years)?

Another reason for using a V or a double fin design rather than a conventional type would be size (height).

[beardy]

The Fouga Magister was derived from a glider post WW2 (part of the Marshal plan I believe, to develop a jet trainer.) The glider had a conventional tail, however, Fouga decided to mount a single jet engine on top of the fuselage; the tail was in the efflux! The simple solution was to move the tail surfaces into a V shape - et voila. Having done that subsequent iterations found twin engines buried in the wing roots, but the tail remained.

[barit1]

With a cross-wind from the left, it ran out of " rudder" during take-off, so much that one could depart off the runway!

A lot of high-powered taildraggers have this issue - and the remedy is opening the throttle a bit slower! Of course, if you've never flown a taildragger... :rolleyes:

[jackharr]

There was a successful glider, the Schemp Hirth SHK in the 1970s with a V-tail (a few are still around today). One of the greatest problems with the SHK was explaining to others how the tail worked.

Stick back - both tails up. Rudder left - both tails left. Rudder left, stick back simultaneously - the surfaces move, well....in a most confusing manner. The explanation was best ended at this point: "The glider will probably spin"

[captjns]

A lot of high-powered taildraggers have this issue - and the remedy is opening the throttle a bit slower! Of course, if you've never flown a taildragger... :rolleyes:

Thank god!!!!:ok::ok::ok: we were exempt from high x-winds in Alaska and Canada. But wait!!!:eek: we floaters were totally exempt from cross winds. Now as for the Bonanza... the beautifuel tall natural blond babe in the right seat blew the x-wind off the planet, while her boy friend blew wind out of his xxxx!:E Hey... maybe that's what accounted for the great shorth field performance in those gusty x-wind conditions:E.

[ChristiaanJ]

Couldn't resist posting this one...

http://img.photobucket.com/albums/v324/ChristiaanJ/12060002w.jpg

1968....

Design exercise at university, basic spec was for a business jet.

So I went all-out for a general-purpose alternative to the LearJet and Mystère 20 of the day.

Less wing sweep, rough field u/c, engines out of the way of FOD (inspired by the A-10), and a V-tail as a consequence.
Nicked the eyelet windows from the SabreLiner, and big airbrakes from the F-104 (but inspired by the beaver tail of the F-28, more instant power during a GA).

Had some trouble defending my choices.... I graduated OK, but somehow my design never flew....

Pity, I still like the look.

CJ

[Pilot DAR]

Hmmm,

I don't want to sound unkind, but, an engine sheds a turbine wheel, and takes out both the other engine, and the flight controls for the tail as well...

Yaw the plane, and the separated air off the engine spoils the airflow over half the tail...

I'm happy you graduated, I hope you're still innovating, the world needs thinkers!

Cheers, Pilot DAR

[barit1]

Pilot DAR speaks the truth:

don't want to sound unkind, but, an engine sheds a turbine wheel, and takes out both the other engine, and the flight controls for the tail as well...

However, in this respect the plane could still be certified - but only because turbine wheels are certified to be "prime reliable" and thus should never fail.

Except maybe once in a while... :ugh:

And then of course there's N60NA (http://libraryonline.erau.edu/online-full-text/ntsb/aircraft-accident-reports/AAR75-02.pdf). Not a turbine wheel, but #3 eng. fan blades flew everywhere, including the #1 gearbox and the #2 inlet.

[ChristiaanJ]

Pilot DAR is right of course on both counts.

However, un uncontained turbine failure would affect any other aircraft with rear-mounted engines in a similar manner, and has done so in the past.
I suspect the Warthog has some added armour in the nacelles to minimise the risk.

As to partial blanking of the tail in yaw, I agree!
May well have been one of the criticisms of the design during the presentation, but I no longer remember... it's over forty years ago after all.
On the A-10 is was solved differently.

CJ