Why twin tails?
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Why twin tails?
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.
Thanks
I understand why such things are sometimes added to increase stability - Shuttle 747 comes to mind.
Thanks
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--
PA
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--
PA
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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.
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.
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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. 
The Lockheed 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
The Lockheed 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
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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.
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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.
Airfoil
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.
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It has to do with shedding vortices from the wing root and single tail would be ineffective due the surrounding airflow
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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.
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.
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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.
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.
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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?)
Airfoil
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?)
Airfoil
Last edited by airfoilmod; 2nd Jun 2008 at 02:31.
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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.
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.
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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.
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.
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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.)??
Airfoil
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.)??
Airfoil
Last edited by airfoilmod; 2nd Jun 2008 at 03:50.
Twin Tail drag
Actually,
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
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
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.. 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 ...
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the F-16 fly rings around the F-18
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
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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.