To ‘V’ or not to ‘V’

That is the question… But don’t worry, unlike Shakespeare’s Hamlet I’m not having an existential crisis, just pondering one of the mysteries of aircraft configuration: Why aren’t we all flying aircraft with V-tails?

vtail-fig1-sv11.png

Figure 1 – V-tailed ultralights are out there, like this SV11 for example

The history of aircraft design is littered with innovations which at the time of their inception were heralded as being ‘game-changing’ or even ‘revolutionary’. However, with the definite exception of the jet engine, the vast majority have failed to live up to their promises. This shouldn’t come as a surprise. Combine over-enthusiastic engineers excitedly pursuing a novel idea, with a marketing mentality keen to make grand attention-grabbing claims and it’s easy to see where the hype comes from. But whilst you can fool people, you can’t fool nature, and many a promising idea has fallen foul of the laws of physics.

I’d argue that V-tails fall into this category. On paper they have a huge amount of promise and they turn up quite regularly on UAVs and jet-fighters. However, in the ultralight and GA world they remain something of a curiosity; but why is this?

‘V’ Good

Theoretically V-tails have a lot going for them, especially in the drag department. Firstly a V-tail reduces both wetted and frontal area. The theory goes that the two diagonally mounted aerofoils of a V-tail can perform the same job as the three separate fins in a conventional empennage, but with a smaller combined area. Hey Presto – smaller area, less drag. But wait, there’s more! Because a V-tail only has two fins, there is one less intersection between surfaces and one less wing tip too, so you get a bonus reduction in interference and tip drag as well. Clearly, if you are chasing speed, a V-tail is the way to go.

Vtail Fig2 - Equivalent

Figure 2 – Scale comparison of equivalent conventional and V-tails

Next on the list of V-tail benefits is control. Conventional tails can be subject to “Rudder Lock”, a phenomenon where large yaw angles, such as those occurring during a spin, generate massive aerodynamic forces on the rudder, pinning it hard over with more force than the pilot’s legs can overcome. Obviously this is an undesirable trait, and one which should be avoided if possible (a requirement for certified aircraft and certainly recommended elsewhere!). V-tail geometry limits the aerodynamic forces on the control surfaces during a spin, providing some resistance to rudder lock.

V-tails have two other potential control benefits – based on the V configuration raising the tail surfaces relative to the fuselage. Firstly the V-tail is less exposed to ground effect, meaning it won’t suffer from the same loss of elevator effectiveness conventional horizontal tails experience when close to the ground, i.e. when flaring for landing or raising the nose for take-off. Secondly, a raised position places the centre-of-pressure of the control surfaces above the centre-of-gravity of the aircraft. The benefit here is that you get greater pitch–up authority (albeit at the cost of reduced pitch-down authority) at large control surface deflections. This is because the drag generated by the deflected control surfaces creates a supplementary pitch-up moment in addition to the primary pitch–up due to control surface lift.

Vtail Fig3 - Elevator

Figure 3 – V-tail pitch control

Vtail Fig4 - Rudder

Figure 4 – V-tail yaw control

‘V’ Bad

So that’s the good points wrapped up, but what about the bad stuff? First-up, there are some drawbacks to combining the rudder and elevator functions. In aircraft that have manual flight controls, (i.e. pretty much all ultralights), aerodynamic forces acting on the controls get fed directly back to the pilot. For pilots used to conventional aircraft this can make for some unexpected interactions between control forces, notably when large amounts of trim are applied, or when applying large amounts of ‘rudder’ input such as sideslipping for a cross-wind landing. Control forces are not the only problem, there is also potential for the controls themselves to interact, such as increased drag from large rudder inputs causing some secondary pitch-up effect.

The next problem is also control related. Mechanically combining conventional stick and rudder control inputs to give differential control surface movement for rudder, and coincident movement for elevator, requires a mechanical mixer assembly. This is not only adds weight but represents a complex mechanical linkage which is also a single point of failure for the control system, effectively putting the elevator and rudder control “eggs in one basket”. Trim can also be an issue. Providing a trim system on the pilot side of the mixer assembly is relatively straightforward, but removes the benefit of having a trim system which is independent of the primary controls. A separate trim system, on the other hand, will provide redundancy (required if an aircraft is to meet FAR Part 23), but is yet more complex and heavier to implement.

On the subject of weight, you might imagine that having less and “smaller” surfaces would produce a weight saving. Somewhat surprisingly this turns out not to be the case. Whilst there is inevitably some saving from the reduced overall surface area, each V-tail fin is doing duty as both horizontal and vertical tail and so tends to be larger in area than any single conventional tail surface. The end result is greater aerodynamic loads, which in turn require stronger and thus heavier structure, giving up significant weight benefit.

The final drawback for V-tails is adverse roll. We are all familiar with adverse yaw, the tendency for the nose of an aircraft to yaw away from the direction of bank when rolling (caused mainly by a difference in drag on each wing due to aileron deflection). The usual piloting response to adverse yaw is to apply rudder to counteract the yawing moment, but with a V-tail the act of applying rudder to counteract the yaw generates a rolling moment which tries to roll the aircraft out of the turn, i.e. adverse roll.

It’s Not Wrong, It’s Just Different

There are a few aspects of V-tails that don’t fall into the realm of advantage or disadvantage; they are just differences that need to be considered. One of these is dihedral effect. A V-tail, by definition, has a lot of dihedral and this supplements the dihedral effect of the main wing. This tends to make the aircraft more laterally stable, but also makes it more prone to Dutch Roll. With a conventional tail the solution would be to increase the directional stability by increasing the vertical tail area. This isn’t an option for a V-tail, as reducing the ‘V’ angle to give more ‘vertical’ tail area also increases the dihedral effect and so doesn’t fix the problem.  In fact the usual solution is a ‘Y-tail’ which adds a small fixed vertical tail surface to improve directional stability.

Finally, a claim often made is that V-tails are easier/cheaper to manufacture – as there is one less fin and one less control surface to build. This simplicity argument is certainly true for servo-controlled systems, but for manual systems it’s not so clear cut, having to be balanced against a control system which is significantly heavier, more complex and costly.

V. Ugly?

In summary, V-tails do have their place. If you have jet wash or water spray to avoid; or are fanatical about minimising drag, they may just be the way to go. V-tails make even more sense if your craft is unmanned or fly-by-wire – thereby avoiding the control feedback quirks. However, for an Ultralight, I don’t really see the point. As a comparison between the Sonex and Waiex demonstrates, there is no real performance or weight difference to be had between the two tails. In the end it really comes down to aesthetics, so if you like the look, why not? Just don’t expect miracles in the performance department.

2 thoughts on “To ‘V’ or not to ‘V’”

  1. Most of the claimed drag benefits are just that–claims. Any realistic analysis of tail volume shows that V tails have no drag benefit over conventional tails in terms of stabilizing effect per unit drag. The intersection drag benefits are trivial at best.

    –Bob K.

    Liked by 1 person

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