[Courtesy of Don Stackhouse, djaerotech "at" erinet.com, June 1999 - as posted on RCSE]
Ah, yes, summer on RCSE, and it's time again for folks who mistrust what they don't understand to blame V-tails for everything from La Nina to the Y2K problem.
Yiu Kwong Chan writes:
>V tail must move two servo to achieve either or both functions...As a result, more battery juice wasted into the motors inertia. <
Theoretically true for pure rudder movements, or purer elevator movements. However, in the real world it's fairly rare to have only one or the other without some activity about both axes. In all our years of working with both V-tails and conventional tails on aircraft that use the absolute minimum battery size in order to save weight, we have seen no significant difference in battery life per charge between the different types of tails. Whether or not the difference truly exists in theory (which is depends on your point of view), it certainly doesn't exist in proactice to any measurable extent.
>Adding to that, some kinetic juice wasted into roll force each time rudder vector is called upon. More to that, rudder roll in adverse direction. <
Once again, theoretically true, but negligible in actual practice. If you had an extremely large tail on an extremely short moment arm it might become significant, but a tail like that would be undesireable for other reasons. Conventional tails and T-tails also have adverse roll, although not as much as a V-tail. In any case, for any reasonable combination of wing span and tail span the effect isn't significant.
>To combat that, consider coreless drive servo. As to the adverse roll, consider inv-V tail. <
Inverted V-tails tend to have worse steady state performance in turns, plus they can be very tricky from a structural standpoint. If you have to beef up the tail surfaces and tail boom enough for the stabilizers to act as landing gear struts, you get a very heavy tail assembly. One possible solution is the double tail boom with the inverted V-tail running between them, like in Mark Allen's "Avenger" HLG, but then you have two tailbooms instead of one, plus some additional structural requirements in the wing to support them. It usually becomes a losing proposition.
>I admit am attracted to V tail, but I cannot find much advantage weight for it against the others. When it comes to 50-50 decission, I would drop V tail very easy for a full flying stabilizer. <
Full-flying stabilizers have other problems. Besides the fact that one of the most highly loaded joints in the tail assembly is now also a moving part (that's spelled h-e-a-v-i-e-r), in many cases an all-flying stab must also be larger. If the size of the tail is determined by lift force requirements ( the high pitching moments from large wing flaps with large deflections might be one example), then the camber-changing properties of a conventional stab-elevator could give it the necessary lift force from less area than the fixed-camber all-flying tail.
The advantage for the upright V-tail in models is usually primarily structural. A conventional tail tends to drag the stabilizer through the grass on landing, hooking tips and causing massive bending loads on the tailboom. The T-tail avoids this, but it places a large mass (the stabilizer) at the end of a long moment arm (the fin). The fin itself must also be beefed up to support this extra mass. This causes high torsion loads on the tailboom in any sort of groundloop, and high bending loads in the tailboom in any sudden stops, such as a "lawn dart" landing.
The V-tail avoids both of these problems. It does a good job of keeping most of the tail clear of the grass, but it keeps its center of gravity low, close to the tailboom. In addition, it only has two panels to be joined instead of three, which saves some more structural weight.
>From an aerodynamic standpoint, the V-tail concentrates approximately the same total area into only two surfaces instead of three, which means that those two surfaces will have either better span or better Reynolds numbers or both. It also has fewer inside angles creating interference drag than either the conventional or T-tail. On the down side, there can be some interference between the two panels in yaw. This is only a significant factor for fairly high amounts of tail dihedral. Since the required tail dihedral is related to aspect ratio, this would be more of a problem in high aspect ratio models such as open class ships. In relatively low aspect ratio models such as typical HLG's, we've found this interference issue to be insignificant. In extreme cases of very high aspect ratios, it might be appropriate to use a "Y" tail ( a V-tail with a small ventral fin) to keep the required tail dihedral low enough to hold the destructive interference down to an insignificant level.
BTW, the reason why tail dihedral is related to wing aspect ratio can be found in the formulas for tail volume coefficients. The way a V-tail distributes its authority between vertical tail effects (yaw) and horizontal tail effects (pitch) is determined by the tail's dihedral. The required vertical effects are related to the semispan of the wing, while the horizontal tail requirements are related to the mean aerodynamic chord. As aspect ratio (the ratio of span to average chord) increases, the need for vertical tail (in comparison to horizontal tail requirements) increases.
Chris Kaiser writes:
>The trade-offs seem to be that the V-tail can be built lighter, even allowing for the larger surfaces and longer tail moment required, whereas the conventional tail gives better low-speed handling, especially in yaw. <
Handling is not inherent to the type of tail. A properly designed V-tail will have almost exactly the same area AND handling as the equivalent conventional tail with the same tail moment. As I mentioned above, you also have to be careful of interference between the tail panels if your wing aspect ratio is very high, but this effect is usually small, and in any case there are ways to deal with it. Our HLG's have some of the best low-speed handling around. In most cases (except for the most recent versions of the Monarch and Wizard, because there simply wasn't enough market demand for the conventional tail to continue providing for it) they are available with both styles of tail. There is essentially no difference in stability and handling between the conventional tail and V-tail versions.
Long tail moments improve dynamic stability in both pitch and yaw, regardless of tail type. Of course it's possible to go too far with this, and develop handling problems in turns, as we have seen in some other recent HLG designs.
The biggest difference between V-tails and conventional tails (including T-tails) is that too many designers simply don't understand how to properly size a V-tail. They design a tail that is too small or the wrong proportions (such as using the "projected area" method for size, then just assuming that the included angle between panels should be the old standard of 110 degrees), and then blame the basic V-tail concept when it doesn't meet their expectations, rather than recognizing that their detail design method is flawed. If conventional tails had the same ham-fisted design techniques imposed on them, they would get a bad reputation too!
There are a number of ways to size a V-tail. Most of them (except for the common-but-wrong "projected area method") will give you pretty much the same numbers, and pretty much the same results. I won't go into those methods here (I've done that enough already many times before), but if you want to get into the details, I've discussed some of them, including the simpler ones, in the "Design" section of the "Ask Joe and Don" department on our website. You can also use these methods for converting a V-tail to a conventional tail.