V Tail
 

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.

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

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/jou...afe_v-tail.jpg

.... weight

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 :ok:

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

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

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

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

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.

Other control factors
 

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!

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.

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)

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 ..

John
 

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.

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!)

Cheers lads.

madman
 

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 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.