View Full Version : Why twin tails?

james ozzie
31st May 2008, 22:58
Is there a simple technical explanation for the idea of 2 vertical stabilisers (or even 3) at the ends of the tailplane instead of one bigger central element? I would think it adds load to the tailplane structure but I don't see any advantage. Seemed more common on the heavy piston aircraft of WWII era and I can't think of a modern design with this feature.

I understand why such things are sometimes added to increase stability - Shuttle 747 comes to mind.


Pugilistic Animus
31st May 2008, 23:23
In general there are two limiting directional control conditions for an airplane---

in a SE it is the spin---

in ME it is critical engine failure---

basically, due to structural reasons---i.e weight/strength ratios sometimes it is easier to satisfy this requirement with two tails---in the case of the Shuttle carrier---I believe it is also partially due to interference???---at least I do remember reading that somewhere--:)


31st May 2008, 23:29
Also placing the fin/rudder directly in the propwash helps increase direction control in low airspeed/high thrust situations.

1st Jun 2008, 00:19
All valid concerns so far. However, the critical control case for single engine aircraft is crosswind landings. Unless you're in the aerobatic category, there's no requirement to recover from a spin.

But the driving factor for most H-tails (or E-tail in the case of the Connie) is simply reduction in overall aircraft height. Now that hangars that will accommodate 737s and larger are pretty common, you don't see multiple verticals all that often on modern designs. And the the exceptions that do pop up are usually Navy aircraft that have to fit below a carrier deck (F-18, E-2, V-22).

The only oddballs I can come up with are the F-15, F-22, MiG-29, and SU-27. I think in these cases, the amount of power involved means that an enormous tail would be needed in case of an engine out on takeoff. Therefore, it might actually be lighter to use twin tails since the bending moment on a single one would be extemely high.

Oh yeah, and then there's the A-10. The reason for the H-tail there is actually to help reduce the IR signature from the ground. So you can see, there's a variety of reasons why you might go that route. But in general, the fewer tailfeathers you have, the lighter the aircraft and lower the drag. That's the theory behind the V-tail at least.

Pugilistic Animus
1st Jun 2008, 00:22
True that Gr8, but I only fly a plane that's spin capable, otherwise it's simply too, too dangerous:}

1st Jun 2008, 00:28
In the A-10 case, survivability through redundancy was a significant design issue. The ship was designed to lose a significant chunk of tail, or have control cables to one side shot out, and still be flyable. :ok:

The Lockheed 10 Electra (http://en.wikipedia.org/wiki/Lockheed_L-10_Electra) originally was designed with a single tail, but "Kelly" Johnson's wind tunnel work at University of Michigan brought about the twin fins seen on many Lockheed aircraft

1st Jun 2008, 00:41
Are BAES capable of building an aircraft with a small Fin?, as they seem to fit the biggest Fins available on most aircraft they are involved in.

1st Jun 2008, 15:47
The only oddballs I can come up with are the F-15, F-22, MiG-29, and SU-27. I think in these cases, the amount of power involved means that an enormous tail would be needed in case of an engine out on takeoff. Therefore, it might actually be lighter to use twin tails since the bending moment on a single one would be extemely high.

Actually, on the aforementioned aircraft the twin tail is there for different reasons. All these airplanes incorporate low aspect ratio wings, that generate vortex lift, which enables to achieve extremely high values of AoA and CL, resulting in excellent maneouverability (though at a cost very high drag). The twin tail is required fo this a/c configuration to maintaon directional stability. It has to do with shedding vortices from the wing root and single tail would be ineffective due the surrounding airflow. Don't remember exactly why, maybe someone that flew one of those beasts might ellaborate...

1st Jun 2008, 17:34
Most Heavy, High Thrust A/C (fighters) are short-coupled. This describes how close the empennage is to the CG. Keeping the mass compact allows for awesome manouverability (req. #1), but disallows directional authority or short moment arm. Installing the tail feathers way back allows for greater leverage on the mass, and allows for smaller and lighter tails, not a plus when shakin' and bakin'. In other words, now we need two big strong VS, Rudder puppies to wrestle the explosive energy around a point.
Hence the F-15 example. To a lesser extent a payoff in redundancy and permissible structure loss (anyone see the Eagle fly home with ONE WING missing entirely???

Now you know why Goose died. spins are nearly always fatal in fighter A/C. Roll Rate?? awesome. G load? Incredible. Yaw? What's Yaw??!! Also Art Scholl. Can anyone think of a more Short coupled A/C than a Pitts? G-B doesn't count.


Brian Abraham
2nd Jun 2008, 01:33
It has to do with shedding vortices from the wing root and single tail would be ineffective due the surrounding airflow
These aircraft are capable of reaching extreme angles of attack where a single tail would be blanketed by the fuselage. As you say the vortices being shed by the strakes are used to energise the airflow over the twin fins and hence retain control authority.

2nd Jun 2008, 02:23
the vortices being shed by the strakes are used to energise the airflow

Looking at the F18 (and the others presumably follow a similar pattern) at higher alpha, the LEX (leading edge extension) generates a very intense shed vortex which is aligned with the fin. Plenty of photographs about which show the shed vortex.

2nd Jun 2008, 03:08
The B-24 was originally designed with a single, tall, vertical stabilizer. It went into production with the twin vertical stabs due to pressure to move to production early...which consolidated did using existing tooling and design from the Model 32 flying boat.

The US Navy later put the airplane back as God intended it by ordering the original tail as the PB4Y-2.

It flew better, too.

2nd Jun 2008, 03:20
And believe me I understand the implications of the following, a minor challenge to two of my three most esteemed aerodynamic fonts.

At High angles if attack, sure enough, the VS/Rudders are "hooded" from symmetric flow. This directional sincerity is falling off as the Chines on either side of the forebody start to shed flow rather than direct it. Without these chines, the fuselage would wobble uncontrollably side to side as very high AoA is approached. The Chines do not "energise" the flow to enhance aft controlled Yawing moment, instead, they provide the stability needed to Replace that lost by the "shaded" VS/Rudder.

The concept was practicalized in the development of the F-16, an aircraft fought for and supported in every respect by Colonel John Boyd USAF. At Nellis in the early seventies, Boyd developed a method to shed energy in gobs and milliseconds while gunfighting with his students in Fighter Weapons school, the Air Force corollary and predecessor of "Top Gun".
As described in Robert Coram's Book, "Boyd, The Fighter Pilot Who changed The Art Of War", Boyd would allow a student on his six as both accelerated to ~450 knots. After allowing the student to close, Boyd "flat-plated" his F-100, presenting a 90 degree AoA ( Zero IAS, Zero Lift) as his aircraft shed energy so quickly, and decelarated, that the bogie flew past and got hosed. It was this Flat Plating, that led to the Hard chines on the Falcon, and later, articulating strakes, that gave the Fuselage longitutidinal stability while at AoA of 70 or more degrees.

I am aware of a patent involving the science behind this work, I'll look for it. ( Brian, the F-16 does pretty ok with its single tail. no?)


2nd Jun 2008, 03:22
Multiple vertical stabilizers.
If we look back in time, the original design of the DC-4 (C-54 for you AirForce folks) had a triple tail...and with good reason.
The original DC-4 design was much larger than what it turned out to be, so a triple-tail was thought to be slightly more efficient. Prevented 'fishtailing'.
Only one was built.
The DC-4 was down-sized, so a single tail was used.
This info comes directly from the assistant engineering project manager at the time, a close personal relation.
When the DC-6 came along, the single tail was retained (triple-tail briefly considered, but rejected).
'Fish-tailing' was prevented due to a slight wing twist, however the same planform was used (DC-7) as well.
Also info directly from a close personal relation, now the chief engineering project manager.
B377 Stratocruiser.
This airplane, developed from the B-29 bomber (however different engines/enlarged fuselage, but the same basic wing) had a very high single tail.
So much so it would not fit in many hangars of the day.
So, Boeing developed a folding vertical stab...presto, problem solved.

2nd Jun 2008, 04:16
Looking at the F18 (and the others presumably follow a similar pattern) at higher alpha, the LEX (leading edge extension) generates a very intense shed vortex which is aligned with the fin. Plenty of photographs about which show the shed vortex.

Yup. So much so that they had a big problem with veritcals cracking well before their expected service life was up. The fix is that strake on the LEX that looks bolted on as an afterthought, since it pretty much was.

As for airfoil's comments, I'd be interesting to read more about it. I always thought the strake/chine/LEX was just a massive VG to help keep the airflow "stuck" to the top of the fuselage and hence over the rudder(s). I don't claim to be an expert in high-alpha aero, so I could be wrong.

2nd Jun 2008, 04:40
Grtz. As 90 degree AoA is approached, the A/C undergoes a schizophrenic change. What you would swear is Yaw has become Roll and vice versa. Especially at low speeds, a "Yaw" (side to side "hdg." change) actually describes more of a Roll, relative to the airflow. The Roll effect is less corrupted but undergoes the same challenge. To extend the time spent in such a regime, power can be added, up to full burner. If you have seen an F-18 "Duck walk" down the Runway at seventy knots and 70 feet Alt. with an AoA of 70 degrees That's what I'm talking about, an exaggerated attitude that relies on the Strakes to prevent the Nose from flopping off on either side of the flight path. This manouver is probably 70 per cent ballistic, and you can imagine the VS/Rudders are basically hitchhiking.

Consider the strakes "fingernails in velvet"

I won't claim the strakes have no effect on VS authority in High A, but if that is true to the extent my two colleagues have proposed, why then does the F-16 fly rings around the F-18 with one less engine and one less VS/R ?? (And is equally adept in flight when the VS is simply Dead Weight.)??


Three Wire
2nd Jun 2008, 05:24
if I can recall my history correctly the first to put twin tails on a fighter were the Russians. They were into anything that reduced supersonic drag and saved structural weight.

The F14 introduced twin tails into western design for the same reason, but compare the fin caps with its opponent the Mig 29.

The LERX were designed in to keep airflow attached at high alpha and the strakes added to the LERX to stabilise the vortex which was cracking the structure. Russian structures were a little more substantial and did not suffer as badly.

Just my two bobs worth, Three Wire :cool:

2nd Jun 2008, 08:53
.. which is why the Canadians and Aussies have a sub-industry Industry in the IFOST program ... one doesn't get something for nothing and the cost of high alpha control is that the back bits get shaken about something wicked ...

Brian Abraham
2nd Jun 2008, 10:52
the F-16 fly rings around the F-18
Really? Let me connect some dots for you. Google “RAAF” and “Fighter”, then Google “RAAF” and “Central Flying School”. Note where the CFS is located against my address. There’s a bunch of F-18 types here who would like to benefit from your insights before they attend their next “Red Flag”. ;)
May interest you that one of the two initial design iterations of the F-16 had twin fins. I suspect that they found the vortex from the strakes gave adequate directional control by energising the airflow on each side of the fin, without the vibration and fatigue issues suffered by the twin fins. The 15, 14 and 18 probably needed twin fins due to the width of the aft fuselage, being twin engine.
This manouver is probably 70 per cent ballistic
You might explain what you understand by the term "ballistic".

FE Hoppy
2nd Jun 2008, 12:06
I think I'm right in saying that sticking the verticals on the end of the horizontal increases the effectiveness of the horizontal to some small degree a-la endplates. Not sure if this was a design reason at the time though.

2nd Jun 2008, 14:18
Dear Sir- Hyperbole and flight test are joined at the Hip. My comments paraphrase the Fly off results accurately. My understanding of the knock on the F-18 had to do with its resemblance to prior A/C (F-5) and the F-16's superiority in agility and response.

The strakes themselves create higher lift by planting their vortex on the wing root, preventing earlier Stall, especially at high AoA.They improve longitudinal stability by supporting the Forebody (nose) and prevent it from wallowing, again at high angles of attack. I haven't found your direct linkage re: strake to VS.

This is from memory, like I said before, I don't use Wiki, and if I am confident, I'll write it. I think those who run to "the book" insult the respect of their counterpart, as well as their own. I stand to be corrected at all times, and welcome wholeheartedly new information that improves my knowledge. Both A/C make my Heart go thump, they are both wicked cool, this isn't about whose mates fly what.

2nd Jun 2008, 20:02
In general, I agree. End plates, strakes, VG, Fencing, etc. Shepherding Airflow (Winglet) creates benefits in lessening drag, increasing manouverability, and reducing Stall speed or increasing control effect. "Ducting" airflow can get extremely complex. Intake efficiency, washout, "Flex-Wing"; it remains a bad idea to add anything to an A/C that won't, (can't) perform. There are limits, mine begin with the appearance of the first equation. I admit I am mathaverse. If I understand something, I don't need to teach it or prove it. "Coanda Effect", Blown Wing, Vectored thrust?
OK, pull chox.

Brian Abraham- By Ballistic, I mean Energy Assisted (Created) "Flight"
Watching an F-18 Waddle down the Runway, CLEAN, at 40 knots and AoA in the neighborhood of 70 degrees, at an Altitude of 50 feet with both pipes roaring and burning two parallel ruts into the runway isn't strictly speaking Aerodynamic Flight. Neither is an F-15 at Mach 1 and vertical, its wings producing Zero Lift (Unloaded). On the Eagle in that config., those wings aren't wings at all, more like Fins, (As in Rocket Fins). Ballistic. If I'm wrong, ok, I'm listening.


2nd Jun 2008, 20:28

In a much simpler response to your post, yes, you are correct that an H-tail increases elevator effectiveness through endplate effect. However, the size of the horizontal is rarely a driving factor in selecting this design (the wings are always much longer, right?).

However, endplate effect is one of the factors in using a T-tail. It allows for a shorter vertical, and if the tail is swept, a smaller horizontal as well since it places the aerodynamic center of the horizontal farther away from the CG.

And I'm not sure who brought up the F-16's twin tailed flyoff competitor, but it's also of interest to note that the YF-17 was a highly modified F-5 that eventually became the F-18 after some navalization (for lack of a better word).

Mad (Flt) Scientist
2nd Jun 2008, 22:46
One reason for having twin vertical tails on a supersonic fighter (not yet mentioned I believe) relates to the way the shocks behave and the interference effects between the fins.

Basically, at subsonic Mach numbers the two fins interfere with each other, in the manner of a biplane's two wings, which reduces their effectiveness as lifting surfaces (and thus their directional effectiveness). At a suitably supersonic speed, however, the two fins essentially move out of each others' direct interference, and begin to work like two truly independent surfaces.

It so happens that this kind of effect - where the directional stabilising effect gets more pronounced at higher/supersonic speeds - is just what the doctor ordered to overcome the increasing directional instability one finds due to the increasing effectiveness of the forward fuselage.

The alternative is to design a truly massive single fin sized for the high Mach number case and then cope with the excessive directional stability at low Mach numbers by other means.

2nd Jun 2008, 23:24
That's not necessary. What's in the short pipeline are articulating strakes.
Your suggestion that the forebody, in gaining stability at high Mach introduces instability aft is a good point. At high speed the A/C can swap ends unless the Tail continues to anchor the aft and the front. The new strakes will be retractable above Mach 1. The A/C test bed for this feature, along with flex-foil is the F-18, none other. Why are the tails canted on the F-18? Mainly for two reasons. One, the "interference" you identify. The directional surfaces are in each others "ground effect" unitl a certain speed, and the effect leaves the foil from top down, relieving the thinner portion at the tip of very high load due to flutter. Second, each surface provides angular lift due to its departure from the vertical, similar to a winglet in that regard. Also, the Rudders can both be turned in (pigeon-toed), providing down force aft to increase rotational authority off the CAT. The F-18 is a treasure of aerodynamic demonstration, and will continue to be for a good long while.