So, let me see if I've got this right: 1 - The "new" V-tail should have the same surface area as a conventional tail; therefore, it will have the same wetted area and same surface drag as a conventional tail?

From : Don Stackhouse

Same whetted area, yes, same drag, not necessarily. Concentrating the same otal area into 2 larger panels instead of 3 smaller ones means that the V-tail panels will have more span, more chord (and therefore better Reynolds numbers) or both. For various reasons, both of these can reduce drag.

In addition, a V-tail has fewer corners than a conventional tail, and those corners tend to have angles that are more obtuse, all of which reduces interference drag.

Having identical areas, the "new" V-tail will have the same weight as a conventional tail, therefore offering no weight savings?

Once again, not necessarily. In my experience, the loads that really size the structure tend to be landing loads. A conventional tail has to tolerate the very high loads on the stabilizer and tailboom that result from snagging the tips of the stab on grass, rocks, etc. on landing. Skegs, "X" tails (straight tail Monarchs, Maple Leafs, etc.) can help this, but now you have a tail with FOUR corners to generate interference drag. A T-tail has a very heavy piece of primary flying surface (the stab) mounted on top of a long moment arm (the fin), which means that the joint between the fin and the stab, the joint between the stab and the tailboom, and the fin and tailboom themselves must all be very strong (that's spelled h-e-a-v-y) to withstand the twisting forces resulting from the wing or tail catching on something during a groundloop. A V-tail has only two joints to reinforce and fillet instead of three or more, does not tend to catch its tips on things during the landing slide, and has a relatively low C/G (in comparison to a T-tail), which minimizes loads on the tailboom during groundloops. In addition, some types of tail attachment arrangements (such as in the new Monarch and Wizard fuselages) allow the tailboom to act as part of an aerodynamic and structural fillet, which helps minimize weight and drag.

The benefits of a V-tail are not so much in the basic concept; they're more in the accumulated small benefits the V-tail makes possible in the execution of the details.

Mr. Ayers, Joe Wurts discussed your questions about the aerodynamics of multiple angled surfaces, so I won't re-discuss that here. I will add that the "constant total area" philosophy is based on the need to be able to develop the same total control authority from SIMULTANEOUS application of rudder and elevator. There's a very elegant mathematical proof of this theory by the great full-scale sailplane designer Stan Hall in an issue of "Soaring" magazine in the early 70's, which also specifically refutes the "projected area" theory. Sorry, I don't recall the specific issue, but I'll keep an eye out for it the next time I'm sorting through my back issues (I NEVER throw aviation magazines away!). As Joe Wurts has pointed out in previous discussions, the "constant total area" theory does not consider the effects of interference between the two panels during a rudder input.

Joe and I have some differences of opinion on this particular issue's significance; although, as Joe says, the effects of this degrade the force generated by one half of the tail (which is why V-tails often need rudder dfferential to prevent unwanted pitch response to a rudder input), they can also enhance the force generated by the other panel. I suspect that there is some overall degradation, although I think it's probably not as severe as Joe believes, but it's a tough one to resolve with any certainty.

Maybe when I get my wind tunnel operating I can investigate it.

OTOH, the "constant total area" theory also doesn't consider the beneficial effects of extra panel span and/or chord. In any case, from what I've seen, in combination these factors seem to be fairly small and to some extent mutually cancelling. In actual practice the "constant total area" approach seems to give consistently good results. The Chrysalis, and all of the various versions of the Monarch and Wizard HLG's prior to the latest (sorry, the newest Monarch fuselage doesn't lend itself well to conventional tails) have included a conventional tail option. There is little or no difference in handling and stability between the two options (which have nearly the same total area), although the V-tail does tend to weigh a little less.

Regarding the question in another post about whether the writer should use the same total area for his V-tail if he is only planning to use it for elevator, the answer is yes. Even though he is not planning to make any rudder control inputs, the tail must still generate the same stabilizing forces in yaw, so the same area is still necessary.

I have always assumed that the V-tail's undisputable extra drag upon actuation was truly offset by the benefits of a lower surface area and lower weight, if not, why were the top racing planes being fitted with these small V-tail feathers?

For the reasons discussed above, the benefits are there, but not in the places you were looking for them. The racing planes you refer to are getting less stability if their tails are truly that much smaller (including the effects of tail moment arm). As long as the resulting stability is adequate, that isn't a problem, but if so, this also means that their old conventional tails were bigger than necessary.

By the way, even my newest state-of-the-art RTF racing planes came equipped with V-tails, with much smaller total areas than similar-sized planes with conventional tails; and, all sporting the same 110 degree angle.

Which means they had smaller tails. Tail size is to some extent a subjective call, and some folks are willing to put up with less stability and C/G range than others. You also don't mention if those smaller tails had longer moment arms. Moment arms (either too much or too little) can be an even more important factor in stability and handling than tail area.

Personally, I feel that conventional tails handle better than V-tails, and exhibit better yaw stability,...

If you've been using the old "projected area" method, I can see why you feel that way.

Are we to believe that the leading designers in our hobby are grossly misinformed about the optimal configuration of their models?...

I'll leave it to the thousands of Monarch, Wizard and Chrysalis owners all over the world to say whether I qualify as a "leading designer in our hobby". I will say that a lot of the other leading designers also do not use the projected area method.

P.S.: Let's hope that no one will claim that there is a perceptible performance and handling difference between otherwise identical V-tails, one set at 109 degrees and one set at 110 degrees! That type of non-qualifyable minutia always sets off my "bullshit detector", particularly when the claimant's plane has visible gobs of epoxy accenting clumsy repairs, exposed fiberglass "bandages" on cracked tail booms, or other tangible problems which are being ignored in favor of a focus on techno babble.

None of our kits use a 110 degree angle, although it's entirely possible than there could be one someday purely by coincidence. I prefer to measure tail dihedral in degrees from horizontal, just like you measure dihedral of a wing. We determine through both analysis and flight test what angle is best, and when it comes to things like the balance between dutch roll and spiral stability, it can be quite sensitive. The various designs we've studied need angles anywhere from the high twenties to the low forties.

Look at it another way: a one degree change in tail angle per side (which is two degrees in your method of measurement) on a typical HLG tail is roughly equivalent to one square inch of additional fin effect. I would agree with you, a single degree is probably not that significant, but two or three could be. Not big, but depending on how discriminating a pilot you are, it can be noticeable. BTW, we test tail designs on new, carefully built prototypes, and "visible gobs of epoxy accenting clumsy repairs, exposed fiberglass "bandages" on cracked tail booms, or other tangible problems" are not normally a factor in our case.

In general, a higher aspect ratio wing will need a higher V-tail dihedral, while a lower aspect ratio wing will need less. This is because the need for fin effect is more related to span, while the need for stab effect is more related to chord. As the wing's aspect ratio increases, the need for stabilizer area decreases (less wing chord) while the need for fin effect stays the same or increases (span is still the same or increasing). This means that the tail dihedral (which controls the ratio between stab effect and fin effect from a V-tail) generally needs to increase as the aspect ratio increases.

Do all of your conventional tails require exactly the same ratio between fin area and stab area? Of course not. The same is true of V-tails.

You can fudge and guess at a tail design (instead of analysing and testing), using crude rules of thumb that may not necessarily fit the model you're designing (like the 110 degree angle theory) or even have fundamental flaws (like the projected area theory). However, if you choose to do that, please don't go blaming the overall concept ("Personally, I feel that conventional tails handle better than V-tails, and exhibit better yaw stability") for the natural results of taking design shortcuts.

Now, a final word about RCSE in general. I'm finding that I really don't have the time to get involved in technical discussions on this forum. I've put a restriction on myself to avoid doing so. Any technical input I make these days seems to result in someone acrimoniously demanding large amounts of further input from me, often requiring me to divulge data that may be proprietary. With this in mind, please excuse me if I don't respond any further to this thread. I'm desperately trying to finish some new toys for all of you in time for Christmas!

In closing, I'd like to wish all of my fellow "colonials" a happy Thanksgiving. Please try to not eat too much turkey, save some room for the pumpkin pie. For the rest of the world that doesn't celebrate our holiday,please take a moment anyway to consider all that you have to be thankful for!

Don Stackhouse
DJ Aerotech