Airplanes: Why aren't V-Tails more common?

Factual Questions
1

Most airplanes we see have a typical vertical rudder and horizontal stabilizer setup.

Some planes, however, have a V-tail setup. Get rid of the the three-bits above (vertical rudder and two horizontal stabilizers) and replace it with two stabilizers in a V-shape. In this configuration they do the job of the usual setup with less overall surface area and less drag.

There is no shortage of examples of these. You see them on small jets all the way to super-high performance military jets of various types.

So, clearly this is a viable design. Why don’t we see more of them? I mean, they are not exactly uncommon but they are certainly in a minority of most planes we see.

2

There are disadvantages to V-tails, such as the need for a longer fuselage or other additional side area to reduce yawing, and stronger structure to handle higher aerodynamic loads (the Beech 35 had problems with this in the original design).

There is normally a specific advantage for use of a V-tail in an aircraft design. The small jet you link to uses a V-tail to keep the tail surfaces out of the jet exhaust from the top-mounted engine, while the YF-23 and F-117 use the slanted tail surfaces to reduce radar returns from most angles.

3

Nailed it in one.

Stealth is all about sacrificing aerodynamic and structural performance on the altar of low observability. It makes the aircraft worse as an aircraft even as it makes it better as a war machine.

The Bonanza was one of the few production aircraft that had a V-tail mostly for the hell of it. And they retained it for years as a signature marketing feature, rather like the reverse-swept tail on the Mooney series of lightplanes. It doesn’t do anything except look distinctive.

Even the Bonanza eventually gave up on the V-tail.

The big problem with V-tails is that you don’t actually gain the advantages the OP suggests: “… with less overall surface area and less drag.”

Now let’s learn a bit about conventional 3-surface tails:

The tail of a conventional aircraft is sized by something called “tail volume” which is the planform area of the tail times the length of the lever arm from the center of gravity. If you’ve got a long aft fuselage a relatively small tail has enough leverage to do the job. A shorter aft fuselage on an otherwise identical airplane would require the tail to be larger to exert the same leverage.

The fixed parts of the tail serve two orthogonal purposes: stability in pitch by the horizontal surfaces and stability in yaw by the vertical surface(s). Every airplane is inherently nose heavy and the purpose of the horizontal tail is to push down continuously to keep the airplane stable. Given a fixed fuselage size, the amount of down force required drives the area of the horizontal tail.

The vertical tail’s size need is completely different. In cruise flight there is no need for any side force; zero is plenty. It’s only in disturbed flight that you need some side force to restore the yaw state to stable. That’s assuming balanced propulsion; we’ll discuss unbalanced propulsion below. A single engine jet will always have balanced propulsion. A single prop will not due to torque and P-factor, so some amount of side force will be continuously required. But it’s a small fraction of the horizontal down force required on the same airplane.

Having 2 horizontal tails surfaces and one vertical surface, each of similar size is a decent starting point to then refine the design. But by the time the refinement is done, the vertical can be anywhere between 0.75x to 1.5x the size of a single horizontal surface. Thy’re similar; they’re not the same.

That’s stability. Next up is control.

The movable part of the horizontal tail is what gives the ability to maneuver in pitch. And again the length of the lever arm is part of the equation. The fixed+moving parts of the horizontal tail form an airfoil. By deflecting the movable part (the elevator), you alter the amount of lift force (downward pointing, but lift nonetheless). How big the elevator is and how much it can move are driven by how aggressively the airplane will need to maneuver.

The movable part of the vertical tail is what gives the ability to maneuver in yaw. By deflecting the movable part (the ruder), you alter the amount of lift force (leftwards or rightwards pointing, but lift nonetheless).

How big the rudder is and how much it can move are driven mostly by propulsion imbalance. A high powered single prop airplane will need a larger rudder to offset the dynamic torque and P-factor as the engine throttles up & down and the airspeed changes vs a low-powered single prop airplane. In a multi-engine airplane, prop or jet, the biggest concern is engine failure, and especially during takeoff when airspeed is slow and engine power is high. That’s the worst case for propulsion imbalance vs available aerodynamic power to offset the imbalance.

The business of aircraft design is to size each of these things as small as gets the job done, but no smaller. A too-big structure will weigh more and will have a larger surface area (“wetted area”) which is one contributor to drag.

Now to V-tails:

Because of the need for continuous downforce in flight, the two tails are constantly generating lift outwards/downwards. If we assume a 45 degree tail angle for simplicity, only 70% of the lift generated is doing any good vertically. so we need to generate 1.4x as much total force to get the full 100% we need going downwards. All of the force in the lateral direction is wasted; just the two tails pushing in opposite directions.

Aerodynamic drag comes from several sources. Wetted area is just one. Drag due to lift is the biggee. And with this design we’re creating 1.4x as much lift as we need and therefore 1.4x as much drag. And because we need more total lift than we’d need for a conventional 3-tail, we need the surfaces to be larger = heavier = more wetted area as well.

Just as the two V-tails are less efficient in generating pitch forces up & down, they’re identically less efficient in generating yaw forces when needed. So the same 1.4x problem applies.

I mentioned up above that in a properly optimized 3-tail design, the vertical and horizontal tails are each sized independently for their respective and very different missions.

In a V you need to design the two tails for the larger of those two very different needs. Which means they’re oversized = extra heavy = extra wetted area for the lesser of the two needs.

A variable you can adjust to alter the mix of horizontal & vertical forces is the angle between the tails. The YF-23 needs large pitch forces to maneuver aggressively, and doesn’t need a lot of yaw power because the worst case propulsion imbalance is small due to the engines being nearly on the centerline. So they canted the tails more horizontally. etc.

Bottom line: there’s a lot more to the tradeoff than just “2 tails means 2/3rds the weight & surface area drag vs 3 tails so that’s an obvious win.”

4

A fantastic response. Thanks!

The SDMB should have a Hall of Fame answer section and this should go in there.

5

I think this is a trifle misleading. Drag due to lift (aka induced drag) is proportional to the inverse square of airspeed. So it’s big when flying slowly and low when flying fast.

Since many aircraft (e.g. airliners) spend a high percentage of their flight time at high airspeeds, it would make sense to accept design features that included higher induced drag if they offered meaningful form drag advantages (form drag being proportional to the square of airspeed, and thus high at high speeds).

So I believe it’s the other disadvantages of the V-tail that explain its relative scarcity.

6

Just a moment, LSLGuy…

7

Not to be a noodge but is this true for tail draggers?

8

On the ground you’re right. Tail draggers are tail heavy. But that’s by dint of installing the main landing gear farther forward, not by altering the balance of the overall aerodynamic machine.

In flight both tail draggers and nose-gear airplanes are subject to the very same balance situation. I wasn’t explicit, but I was referring to the in-flight case, not the on-the-ground case. Absent magic flight controls while in flight aircraft with either type of landing gear will be nose heavy and the horizontal tail provides downforce to provide stability.

If a generic airplane, be that a Cessna or a Boeing, suddenly magically had the horizontal tail disappear and be replaced by an equivalent weight of ballast at the attachment point, the airplane would promptly and violently pitch nose down and do an outside loop. Net of overspeeding, over-G-ing or arriving at ground level before bottoming out of the loop.

9

Yes. The CG is somewhere along the chord of the wing, regardless of whether the aircraft has conventional gear, tricycle gear, or skids. Airplanes need airflow over the wings to fly, and the tail surfaces need enough airflow to control pitch. Suppose you’re loaded beyond the aft limits of the envelope and you stall. You need to get the nose down.

10

Is that what they mean when they (things I read) say “minimal control speed”? Kinda like a ship…its rudder needs some speed to work.

11

When is the final exam? :slight_smile:

12

Missed the edit window:

Also, are there different speeds for the elevators and rudder and ailerons or is the plane designed to make them all work at the same speed?

Put another way, is it possible for the wings to work but the elevators piss-off at some speed (or vice-versa)?

13

The typical use of Vmc = Vee-min-control is in relation to rudder and only rudder. With the exception of very high-powered singles like WWII fighters, it’s defined only for multi engine airplanes and refers to the slowest speed at which the rudder has enough power to offset the worst case engine power asymmetry.

For some light twins the Vmc is higher than the power-on stall speed. Which means you could lift off normally, be flying under control for the firt 10-15 seconds, then have an engine quit and be too slow to prevent the airplane from rolling over on it’s back & killing you. Even full rudder & aileron against teh offset power won’t be enough to save you.

A lot of people died before they all figured out there’s that no-mans land and so after liftoff in a light twin it’s more important to accelerate up above Vmc than it is to climb immediately.

In something like jets, Vmc is well below stall or liftoff speed so we can’t get into that particular pickle. Instead jets, and some highpowered prop twins, have a different limitation, Vmcg Vee-min-control-ground. In the case of the 737, at max thrust on one engine you can’t keep it straight on teh runway using any/all of the controls much below 100 knots. If you’re slower than that and one quits, you need to promptly bring the other engine to idle power or you’re going offroading. At more our more typical less-than-max takeoff power, the speeds are correspondingly lower and the problem less critical.


As a more general rule, you always want the controls to be effective even if the wing is not. If you ran out of elevator power with a not-yet-stalled wing you'd be along for the ride. OTOH, if you've stalled the wing but the elevators are still effective, you can use them to point the airplane where it needs to go (usually down) to recover speed, reduce AOA, etc., to return to normal flight.

A counterpoint to that is that if the elevator is still effective that implies it has/had the power to drive you into and keep you in a stall. One of the ideas behind the Ercoupe and of some modern canard designs is to make the airplane so-called “stall-proof” by ensuring the elevators don’t have the control power to move the wing into stall. It’s not a 100% stall-proofing, but it greatly reduces the ways an inattentive pilot can get into trouble.

So overall you’re sort-of right there’s a similarity between steerageway in a boat/ship and the various control-related V-speeds in aircraft. But it’s a pretty tenuous similarity.

14

I have seen videos where someone flying a powerful single-engine turbo-prop say they have to mash on the right-rudder pretty hard to keep center-line and counter the engine’s torque during takeoff.

Is that different?

15

Not really different. Same general idea.

Given a powerful enough single prop engine, it can create a lot of torque that translates into significant rolling/yawing tendency. Even low-powered trainers have enough torque effect the pilot needs to make counter-rudder inputs; just not real large ones.

So in a powerful single it takes a lot of rudder during the takeoff roll to keep the airplane from driving off the left side of the runway. They don’t have a Vmcg Vee-min-control-ground because the rudder & nosewheel steering is powerful enough that even from a standing start you have enough control power to counteract the torque of full power from the engine.

There may not be much extra rudder authority left at that point, but that’s OK because as the speed builds the rudder will get more powerful while the engine torque stays the same. And because we’re talking about a single engine airplane, the torque vs. rudder situation won’t suddenly get worse if a/the engine quits.

Now lets’ go farther down the takeoff run to just short of lift-off. …

There could be an issue where the airplane wing will fly at a slower speed than the rudder & ailerons would need to have enough power to maintain wings level & track against the torque & P-factor once the aircraft weight on the wheels isn’t helping prevent rollover. If you did lift it off at that extra slow speed you’d uncontrollably roll on your back pretty quickly. So don’t do that.

I’m not familiar enough with all the certification regulations to say for sure whether they have to solve that issue by making the rudder and ailerons powerful enough to counteract full torque all the way down to stall speed or whether they can solve the problem by simply specifying a takeoff speed that’s faster than Vmca Vee-min-control-air.

As with the case of the light twins in my earlier post, sometimes the best (read “most affordable for the manufacturer”) fix isn’t hardware, but rather pilot procedures.

16

This 1-minute video appears to be a case in point.

The comments (mostly in English, some in Portuguese) indicate that the plane lost power in one engine shortly after take-off, then the pilot tried to return to the runway and land. As the plane descends toward the runway, it abruptly capsizes and goes splat.

Myself, having never heard of Vmc until I read the comments on this video about a year ago, found it necessary to google it. Here is a very thorough discussion that I found:

Vmc discussion from Langley Flying School.

17

And yet, even sailplanes (not usually notable as war machines) have also been built with V-tails. Here is a German (I think) Libelle “Salto” model:

Additional discussion here.

(ETA: It seems that nearly all modern sailplanes, as least all that I’ve seen, are always T-tails.)

18

A consequence of this is that the further aft your centre of gravity is (within limits), the less downforce you’re tailplane has to generate, therefore it produces less drag and requires less thrust which means lower fuel burn. If you are really keen on minimising fuel costs you load for an aft CofG. I’ve yet to work for a company that took their cost minimisation strategy to that extreme though.

19

Here’s an Electric aircraft with a V-tail. ICE-aircraft are nose heavy because the engine(s) are up front. E-aircraft have relatively light motors; their weight is in the battery. The battery can be distributed throughout the fuselage, so they don’t suffer from default nose-heaviness. They can get away with a V-tail if it reduces the total amount of power needed.

20

The nose heaviness has nothing to do with where the engine(s) are. The airplane MUST be statically nose heavy to be stable.

For an airplane with the engine(s) mounted aft the designers slide the wing a little aft as well to maintain the nose heaviness. If the engines are in the middle like on the wings, the wings are nearer the middle of the fuselage. And if the engine is up front then the wings are closer to the front of the airplane.

In all 3 cases the balance point (center of gravity = CG) is forward of the center of lift = CL = where in effect the airplane is suspended from while the wing is carrying it through the sky.

Absent fancy fly-by-wire computerized flight controls, if instead the CL is in front to the CG the airplane swaps ends and flies (briefly) ass-first before plummeting from the sky. Think like shooting an arrow with the feathers at the front, not the back. Not good.

The key thing I said above is “static balance”. If the airplane is sitting on the ground and you hooked a crane to the CL & hoisted, the airplane would end up dangling roughly vertically with the nose pointed at the ground.

In flight there’s one more factor; the tail downforce we started this lecture series with. In motion, the wing lifts upwards effectively all at the CL and the weight pulls downwards effectively all at the CG. And the tail pulls downwards at the end of its long lever arm. The tail is sized by design, adjusted periodically via pitch trim, and controlled continuously by the elevator position to maintain an exact dynamic balance between nose & tail heaviness so the airplane tends to go straight, neither pitching upwards nor downwards. The airplane may be level, climbing, or descending, but if it isn’t changing pitch, the nose-down forces (CL vs CG) and the nose up force (tail downforce) are in balance.

The magic here is this affords dynamic stability. If, e.g. the airplane is disturbed by turbulence nose down, it will begin to speed up sliding downhill. Which will increase the amount of tail downforce which will alter the balance between up & down forces and cause the nose to rise “automatically”. No high tech gizmos required. And of course the same applies to an un-commanded pitch up.

The bottom line is that for constant power, weight distribution, etc., once properly trimmed the airplane will gently “hunt” nose-up & nose-down to maintain an approximately straight path and approximately the same speed with no fancy technology and no pilot intervention. How stable the airplane inherently is or isn’t is all part of the design trade-off. Airliners and trainers = lots; aerobatics and fighters not so much.

By the pilot moving the elevator, that dynamic balance can be upset slightly & briefly so the force imbalance causes the nose to rise or fall to begin climbing or descending or to level off from a climb or descent.

All of which introduces Richard’s excellent point a couple of posts ago. You can alter the static balance of an airplane by how you load it. Put the heavier stuff at the front of the cargo compartment or cabin interior and the airplane is more nose heavy = tail must work harder all the time = more stable but with more drag from the extra tail effort and the extra wing effort to offset it. Conversely, put the heavier stuff at the back of the cargo compartment or cabin interior and the airplane is more tail heavy = tail works less hard all the time = less stable = but with less drag from the reduced tail effort and the reduced wing effort to offset it.

If you overdo this loading-for-balance too far nose-heavy, the tail may run out of authority when slow and you either can’t lift off or more likely end up in a dive you can’t recover from. If you overdo this loading-for-balance too far tail-heavy, the stability becomes so minimal the airplane wanders off in pitch continuously and it takes high pilot effort to control; it’s like balancing a floor broom vertically in your palm. If the stability is even less the airplane diverges and you can’t prevent it from swapping ends.

Both of those are bad outcomes. So there’s a limited range of permissible loading to ensure the CG stays forward of CL by a range the tail can manage.

As to the Alice and other non-traditional aircraft …

There is a lot of ferment in airplane design right now. Multiple small engines and computerized everything bring in a lot of new possibilities. Providing dynamic stability via tail downforce is tried and true, dating from the timeframe of the Wright Brothers. But it isn’t efficient.

The UAL232 accident demonstrated that you can use power to control pitch. In the case of Alice they may well be using differential power on the tail vs wing electric motors to provide some artificial stability which lets them reduce the tail size. There are lots of other efforts afoot to use non-traditional control systems to achieve the same goals of stability and control via different means. Thrust vectoring, differential power, morphing structures, and surface blowing are a few terms to Google if interested.

Another up and coming idea is air vehicles that are halfway between a drone quadcopter-like machine and an airplane. The quadcopter part is used for vertical or near vertical takeoff and the airplane part is used for cruise to get higher speeds and vastly improved efficiency. There are lots of ways to integrate all the bits so, say, the rotors deliver cruise stability and control while the wing carries the weight and there’s no tail anywhere.

The next 20 years will see some amazing air machines.